Download - 4989 2Mb Ins & Maintenence

Learner Guide

04989

2Mb/s Installation and maintenance

principles

Speech Info Channel 1 - 15 ( T/S 1 - 15 ) Speech Info Channel 16 - 30 ( T/S 17 - 31 )

Not Frame Alignment Signal ( Frame 1, 3 , 5 ... 15 ) Signalling Channel ( Frame 1- 15 )

* 1 A Sa4 Sa5 Sa6 Sa7 Sa8 a b c d a b c d

Channel ‘n’ ( 7 ) Channel ‘n’ + 15 ( 22 )

a b c d a b c d

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Multi - Frame ( 2 milli seconds )

Single Frame ( 32 timeslots in 125 micro seconds )

This an in depth 5 Day theoretical and practical course on the 2Mb/s structure

and its associated criteria

A Publication of Telkom SA Limited - Centre for Learning

This page is intentionally blank.

2Mb/s Installation and maintenance principles

Table of Content

Table of Contents

Preface...................................................................................................................... I

General Information................................................................................................. I

Safety, Health & Environment................................................................................ II

Chapter 1 Introduction to 2Mb/s bit structure.........................................1Performance objectives..........................................................................................2

2.048 Mb/s Systems Overview................................................................................3

Alignment.................................................................................................................6

Signalling...............................................................................................................14

CRC-4 Framing......................................................................................................18

Alarms....................................................................................................................22

Signalling...............................................................................................................25

2 Mb/s Impairments...............................................................................................27

Analyzing 2 Mb/s Impairments.............................................................................28

Application 1: In-Service Analysis of Live Traffic..............................................30

The AIS and FAS Distant Alarms.........................................................................32

Code Error analysis..............................................................................................34

Application 2; Out-Of service testing of 2.048 Mb/s Circuits............................39

Analysis of Slips....................................................................................................41

E1 Pulses...............................................................................................................42

Error performance.................................................................................................47

Error Performance Events....................................................................................48

Error performance parameters.............................................................................50

The measurement of the block.............................................................................55

Exercise..................................................................................................................59

Chapter 2 Synchronisation......................................................................60Performance Objectives.......................................................................................61

Introduction...........................................................................................................62

Exercise..................................................................................................................69

Chapter 3 Earthing...................................................................................70Performance Objectives.......................................................................................71

Overview................................................................................................................72

Earthing protection...............................................................................................76

Exercise..................................................................................................................80

Chapter 4 Surveillance.............................................................................82Performance Objectives.......................................................................................83

Overview................................................................................................................84

Course Code: 04989 Draft: 2 2002-06-24 Draft copy of Learner Guide

© This document contains confidential and proprietary information. The dissemination, copying, disclosure, use or the taking of any action in reliance on the contents thereof without the written consent of Telkom S.A. Limited, is strictly prohibited.

Page 1


Table of Content

Exercise..................................................................................................................88

Chapter 5 Installation...............................................................................89Performance Objectives.......................................................................................90

Introduction...........................................................................................................91

Fault localisation...................................................................................................92

Exercise..................................................................................................................93

Chapter 6 Practical Tasks........................................................................94Performance Objectives.......................................................................................95

Task 1.....................................................................................................................96

Chapter 7 Acronyms and Verification....................................................98Acronyms...............................................................................................................99

Verification...........................................................................................................102



Page 2


Preface

Preface

General Information

Gender Neutrality

Gender-specific terms within this document are used for illustrative purposes only and should not be construed to discriminate.

Telkom CFL Information

For more information about Telkom Centre For Learning (CFL), visit our web site at http://www.telkom.co.za/company/cfl/index.shtmlOr call us at +27 12 663 5892 or FreeCall in South Africa 0800 224 224Or fax us at +27 12 663 8683Or e-mail us at [email protected]

Or visit CFL at: CFL Solution Services2001 Lenchen Avenue (South)Private Bag X118Centurion 0046Republic of South Africa

Customer Care

Telkom’s vision is to become a World-Class Communications Company. In 1999, our Chief Executive Officer, initiated the Service Excellence Drive.

All of us within Telkom do not serve external customers directly. However, we all serve internal customers. Identify who your customers are and determine how they will be affected by the service you provide.

Be prepared to commit yourself to Service Excellence by putting your name on any task or problem that comes your way!

Other



Page I

http://www.telkom.co.za/company/cfl/index.shtml


Preface

Safety, Health & Environment

Legislation The Occupational Health and Safety Act, Act no 85 of 1993, compels management and employees of an Organisation to provide for the health and safety of:

persons at work, the use of plant and machinery, the protection of other persons against hazards arising out of

or in connection with the activities at work.

The Environment Conservation Act, Act no 73 of 1989, provides for the effective protection and controlled utilisation of the natural environment.

The National Environmental Management Act, Act no 107 of 1998, provides for the establishment of forums for decision-making on matters affecting the environment and promotes co-operative governance and procedures for co-ordinating environmental functions.

SHE Policies In Telkom, the safety, health of our employees is not only dictated by what the legislation requires. It is also a moral responsibility in which Telkom acknowledges and respects the value of its employees.

Environmental management is also not just a series of law-abiding actions, but predominantly a moral initiative. By demonstrating that we are sensitive to the impact that our operations have on the natural environment and always seeking opportunities to improve, Telkom can position itself as being model Corporate Citizens.

This has been supported by Telkom’s Chief Executive Officer and top management and is documented in our Safety, Health, and Environmental Policies.

SHE I.P.P.

The Safety Health and Environment Incident Prevention Plan (SHE IPP) has been made available on Telkom’s web site.

It clarifies the duties and responsibilities for all employees and advises, by means of guidelines and working procedures, the Company standards that are required in achieving its SHE targets.

Continued on next page



Page II


Preface

Safety, Health & Environment, Continued

Responsibilities Incident prevention is the responsibility of every employee in Telkom.

Compliance to the appropriate Health and Safety and Environmental legislation and the SHE I.P.P. is mandatory and contracted in the Performance Development Management with all Telkom employees.

Every employee must:

support the Company’s SHE Policies and Incident Prevention Plan to ensure that incidents are kept to a minimum.

take reasonable care of him-/herself and of other persons who may be effected by his/her actions.

co-operate with the employer to ensure compliance to the legislation and obeying instructions given in the interest of SHE.

report any unacceptable safety, health or environmental issue to the Health and Safety Representative, Environmental System Responsible Person and/or employer.

demonstrate by a formal knowledge review and job observation, his/her ability in applying the appropriate safety, health and environmental standards and precautions.

be aware of the Emergency Preparedness, First Aid and incident reporting procedures of the workplace. When required, it is also expected that employees participate in emergency evacuation exercises.

This Course Health, Safety and Environmental related issues will be addressed, where appropriate, during this course.

The site at which the course is to be conducted must be inspected for possible hazards. Any unsafe condition or act must be corrected and /or reported in accordance with the appropriate SHE I.P.P. standards and procedures.

All relevant environmental guidelines or work instructions must be adhered to when working with or disposing of paper, printer cartridges, etc.




Page III


Preface

Safety, Health & Environment, Continued

Web Address The main intranet web site for Safety, Health and Environment (SHE) is:

http:/www.emws.telkom.co.za

The following table will assist you to open the guidelines relevant to specific environments.

Step Action

1. Click on the Internet Explorer icon on your Desktop computer

2. In the address field type in http://www.emws.telkom.co.za/

3. Press Enter and wait for the EMWS page to open.

4. With the mouse, point at “Our Services” and select “SHE IPP Part 2 of 3”

5. Under the sub-heading “Guidelines / Procedures”, scroll down to the required environment.

6. Under the column heading “Review Forms”, click on the required attachment.

Example(s):7.

8.

9.



Page IV


Chapter 1 Introduction to 2Mb/s bit structure




Page 1



Performance objectives

Participant, when: asked a question will correctly differentiate between FDM and TDM

shown an illustration, will accurately differentiate between bit and byte interleaving.

asked a question, will correctly explain the difference between MFA and double FA

shown an illustration, will correctly identify the bits associated with a basic frame structure.

Asked a question, will correctly calculate the following:

Bit rate to line

Bit duration

Timeslot duration

Multiframe duration

Bit rate per timeslot

Number of frames per second

Number of multiframes per second

Number of bits per frame

Number of bits per multiframeframe

asked a question, will correctly name the various types of signalling on 2Mb/s circuits

asked a question, will correctly explain the difference between frame alignments with and without CRC.

asked a question, will correctly name the various 2Mb/s impairments.

participating in a simulation, will correctly explain the different alarm types.

asked a question, will accurately draw an outcome for the G703 output pulse

asked questions, will correctly answer the questions on error performance



Page 2



2.048 Mb/s Systems Overview

Introduction Multiplexing involves mixing many different channels together so that they may operate on the same bearer or medium at the same time.

This is achieved by means of either one of two distinct methods. These methods are called FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing).

Frequency Division Multiplexing (FDM)

Frequency Division Multiplexing is the process where different channels are inter-spaced on the same medium, but here the frequency at which each channel operates is different to the others. Simply put, FDM channels operate: On the same medium

At the same time, but

At different frequencies.

Time Division Multiplexing (TDM)

Time Division Multiplexing, on the other hand, inter-spaces different channels: On the same medium

At the same frequency, but

At different time intervals

Considering the processes used to convert a single analogue signal into a digital signal, samples are taken of the analogue signal at a rate of 8000 samples per second. This means that these samples are ‘spaced’ 125s apart. Even once encoded, the 8-bit words formed by these samples are spaced 125s apart, meaning that, during the time interval between two samples, the bearer is not used and, therefore, ‘idle’. During this ‘idle’ time interval, it is therefore possible to utilise the bearer more effectively by sampling more channels and combining these samples into a single PAM (Pulse Amplitude Modulated) signal. (see the diagram on the next page) Should these samples be encoded into 8-bit words, a total of 32 timeslots may be combined to form a 2.048Mbits/s signal. A simple calculation, therefore, reveals that the samples are spaced 3.90625 s apart. (125 s / 32 = 3.90625 s)




Page 3



2.048 Mb/s Systems Overview, Continued

Example of TDM Principle

Below is a sketch highlighting the TDM principle.

Ch 1

Ch 2

Ch 3

Ch 32

3.906s delay

3.906s delay

3.906s delay

8000 Hz clock pulses

3.906s timeslotts

Ch 4 - 31 samples

Exaggerated output

s

TDM

Interleaving Interleaving may be defined as the mixing or combining process used when many digital signals are combined into a single stream. Once digital signals are ready to be combined into a single stream, one of two methods may be used:

Bit interleaving

Byte interleaving

Bit interleaving is when the digital signals are mixed into a single stream on a bit – by – bit basis.

Byte interleaving is when a full 8 bits from each digital channel or signal is mixed or combined into a single stream. (see the diagram below).

Interleaving is done virtually anywhere digital signals are combined into a single stream. All primary multiplexers which conform to ITU-T G.704 use byte interleaving, (30/32 PCM, Diginet, Martis, etc), but any Plesiochronous Digital Hierarchy (PDH) equipment used by Telkom SA uses bit interleaving. This includes all 2/8, 8/34 and 34/140 Mbits/s multiplexers. However, all Synchronous Digital Hierarchy (SDH) equipment uses byte interleaving. The reason for this is basically to simplify multiplexing and demultiplexing of very high bit rate signals.



Page 4






Page 5



2.048 Mb/s Systems Overview, Continued

Bit and Byte Interleaving

The sketch below shows bit and byte interleaving.

Byte Interleaving

Notes



Page 6



Alignment

Alignment As already mentioned, 32 timeslots may be combined or multiplexed into a single bit stream. These timeslots each consist of 8-bit words that are transmitted sequentially or serially along the same bearer or medium, i.e. byte interleaving is used.

At the transmit terminal of a system, the bit stream is synchronised to the sampling frequency, and all the bits occur at exactly pre-determined intervals. This bit stream is transmitted to the receiving terminal of the system, where the information must be demultiplexed and passed on the corresponding timeslot.

However, it is vitally important that the correct 8-bit words are passed to their corresponding channel outlets at the receive terminal. To do this, a point of reference is injected into the bit stream at the transmitting terminal called the frame alignment signal (FAS). The FAS is a fixed 7-bit digital word (#0011011) that is injected into the bit stream as the first timeslot that is transmitted. It is followed by 8-bit words from each of the channels that have been sampled.

At the receive side, therefore, once the FAS has been recognised, the subsequent timeslots are as easily recognised and the system has no difficulty in demultiplexing the digital stream into the appropriate channels.

Frame An E1 frame is specified by ITU-T recommendation G.704. A frame is made up of 32 timeslots within a 125 s period. These 32 timeslots are constituted by one complete sequence of a FAS plus one 8-bit word from each of the channels that are sampled, and are numbered timeslot 0 – 31.

The first timeslot of a frame, timeslot 0, always contains either a FAS (frame alignment signal) or a NFAS (not frame alignment signal). Sequential frames, therefore, alternate w.r.t. the content of timeslot 0, i.e. one contains a FAS and the next a NFAS, and so on.

The remaining timeslots, timeslot 1 – 31, carry speech and signalling information from 30 analogue channels, as in the case of a 30/32 PCM system, or 31 data channels, as in the case if a Diginet or Martis 2Mbits/s signal.




Page 7



Alignment, Continued

The timeslot allocations are as follows for a speech and signalling E1: Timeslot 0 carries FAS / NFAS

Timeslot 1 – 15 carries speech information from channel 1 – 15

Timeslot 16 carries signalling information / MFAS*

Timeslot 17 – 31 carries speech information from channel 16 – 30

The timeslot allocation for a data or Diginet E1 is as follows: Timeslot 0 carries FAS / NFAS

Timeslot 1 – 31 carried data information for channel 1 – 31

(MFAS will be discussed later.)

Frame Alignment Signal (FAS)

As previously mentioned, the FAS word is #0011011. It alternates within alternate frames between the FAS and NFAS. The FAS is transmitted in even numbered frames (0, 2, 4, 6, 8, 10, 12, 14), whereas, the NFAS is transmitted in odd numbered frames (1, 3, 5, 7, 9, 11, 13, 15). All these frames (0- 16) make up a Multi-frame, but this will be discussed in the next few pages. The NFAS is made up as follows:(# 1 A Sa4 Sa5 Sa6 Sa7 Sa8). These alignment signals are used primarily for receiving terminals to correctly demultiplex the incoming digital bit stream to the correct channel or data outputs. The FAS is also used when doing performance monitoring on systems, but this will be discussed in detail later.Should there be no FAS / NFAS in the digital signal received by a system, it would have no way of recognising the correct 8-bit words for each channel, and this would lead to all communication being lost. Two methods of aligning E1 systems are used.

Method 1:- The system searches for a valid FAS (0011011).- Should this be found, it is used as the first ‘marker’.- Now a search is initiated for a ‘1’ in bit position ‘2’ of timeslot 0 of the

following frame (i.e. a simple bit count is done). Should this be correct, it serves as the second ‘marker’.

- Finally, a search is done for another FAS (0011011). If this is correct, the system will be in correct speech / data alignment from the very next timeslot, i.e. channel 1 will be operational w.r.t. speech / data.




Page 8




If any condition is not obtained during the process of alignment, a new FAS search is initiated and the alignment procedure is re-started from the beginning. This will continue until full alignment is achieved. Generally, if all three conditions are met sequentially, frame alignment may be achieved within 253.90625 s from the point where a FAS search was initiated. [(frame time x2) + 3.90625 s]

Method 2:- The system searches for a valid FAS (0011011).- Should this be found, it is used as the first ‘marker’.- Now a search is initiated for the following FAS (0011011), not the NFAS

bit 2. Should this be found, it is used as the second ‘marker’.- Finally, a last FAS (0011011) is sought, after which the system will be in

alignment from the very next timeslot (channel 1).If any FAS is not found during this process, the alignment procedure is re-started from the beginning. This will continue until all conditions are met successfully. Generally, using this method, alignment should be achieved within 503.90625 s, should all the correct conditions be met in one sequence [(frame time x4)+3.90625 s].

Most systems used by Telkom SA make use of method 1, i.e. (FAS – NFAS – FAS)

Not Frame Alignment Signal (NFAS)

An E1 frame is specified by ITU-T recommendation G.704. Every alternate frame has an NFAS in timeslot 0. This occurs in all the odd-numbered frames, viz. frame 1, 3, 5, … 15. The NFAS bits are designated as Si 1 A Sa4 Sa5 Sa6 Sa7 Sa8. The function of each is as follows:Bit 1: Si Reserved for national use.

May be used for CRC error checking.

Bit2: 1 Always set to a 1.

Prevents intentional or unintentional simulations of the FAS.

Bit 3: A This bit is used for a Remote Alarm Indication.

It is an ‘upstream’ indication of alarms, i.e. sent back to the transmitting station.

A ‘0’ indicates no alarm, and a ‘1’ indicates a RAI.




Page 9




Bit 4 – 8: Sa4 – Sa8 These bits are specified by ITU-T G704 as follows: Sa4 – Sa8 may be used in specific point – point applications, or

Sa4 may be used as a message based data link for operations, maintenance and management (4kbits/s), or

Sa5 – Sa7 may be reserved for national use where there are no specific point – point requirements.

When these bits are not used, they should be set to a ‘1’ when crossing international borders.

Multiframe Signal (MFAS)

A multi-frame consists of 16 E1 frames, i.e. it has a duration of 2ms (16x125s). The multi-frame is specifically structured to accommodate all speech information, as well as all the signalling information from each of the 30 channels, within an E1 signal, however, it is also fully compatible with CRC-4 (Cyclic Redundancy Check), but this will be discussed in detail later. Whereas the FAS / NFAS is inserted to align the speech timeslots, the Multi-Frame Alignment Signal (MFAS) is used specifically to align the signalling information within timeslot 16 of each frame to the corresponding signalling ports.

The MFAS signal, (0000XYXX) is always inserted into timeslot 16 of the first frame of a multi-frame, i.e. frame 0. The following frames (frame number 1 to 15), on the other hand, carry signalling information in timeslot 16. Each of these timeslot 16’s, however, carry signalling information for 2 channels simultaneously, i.e. channel ‘n’ and ‘n+15’. This is possible because only 4 bits, designated abcd are necessary to sufficiently pass signalling information per channel. The total bit allocation of signalling timeslots is (abcd abcd). Therefore, frame number 1 carries signalling for channel 1 and 16, frame 2 carries signalling for channel 2 and 17, etc. In this way, all the 30 channels will pass signalling information within a single multi-frame. Below is an example of a multi-frame.Multi-frame alignment can only be achieved once frame alignment has been achieved. The required conditions to achieve multi-frame alignment are as follows: Once frame alignment is achieved, an MFAS search is initiated.

As soon as an MFAS is detected, it is held as the ‘marker’.

Now another MFAS search is initiated. Should this be found correctly in timeslot 16 of frame 0, the signalling ports will be in correct alignment from frame 1 onwards.




Page 10




The ITU-TG.703

A European standard for a digital physical at 2.048 Mb/s is termed as a “E1” signal.The ITU-T G.703 defines the characteristics of hierarchical digital interfaces. It states that for the 2 Mb/s hierarchy, transmission can be bit-sequence independent. In other words, 2 Mb/s and 64 kb/s facilities are clear channel and do not require a particular signal structure to pass through the network.

Although this transparency can be useful for transmission of wideband signals, sending an unstructured signal into the network can have drawbacks.An apparently random signal can’t be monitored in-service for transmission errors, and it is impossible to provide band-width grooming or switching of channels.In the following pages we will discuss the 2048 Mb/s frame structure and the function of the structures bits.

Frame Format G704/G706

Below is a 2Mb/s-frame format as defined by ITU-T G704/G706 standards.

Speech Info Channel 1 - 15 ( T/S 1 - 15 ) Speech Info Channel 16 - 30 ( T/S 17 - 31 )

Not Frame Alignment Signal ( Frame 1, 3 , 5 ... 15 ) Signalling Channel ( Frame 1- 15 )

* 1 A Sa4 Sa5 Sa6 Sa7 Sa8 a b c d a b c d

Channel ‘n’ ( 7 ) Channel ‘n’ + 15 ( 22 )

a b c d a b c d

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Multi - Frame ( 2 milli seconds )

Single Frame ( 32 timeslots in 125 micro seconds )

2 Mb/s frame format




Page 11




2Mb/s Frame Each 2 Mb/s frame contains 256 bits (32 timeslots, each of 8 bits) at a repetition rate of exactly 8 kHz. The first timeslot (timeslot zero, TS0) is reserved for framing, error checking and alarm signals. See drawing below to illustrate TS 1-15.

In PCM31, the remaining 31 timeslots (if common channel signaling, CCS is used) can be used for traffic, either encoded telephone or data signals. This gives 31 timeslots for the payload.

In PCM30, timeslot 16 (TS16) is reserved for channel associated signaling (CAS).

The table below shows significant bits in TS0 of a multiframe.

FRAME SIGNAL SIGNIFICANT BITS IN TIMESLOT ‘0”

NO. TYPE

1 2 3 4 5 6 7 8

0 FAS * 0 0 1 1 0 1 1 T/S 1 - 31

1 NFAS # 1 A Sa4 Sa5 Sa6 Sa7 Sa8

2 FAS * 0 0 1 1 0 1 1


4 FAS * 0 0 1 1 0 1 1


6 FAS * 0 0 1 1 0 1 1


8 FAS * 0 0 1 1 0 1 1


10 FAS * 0 0 1 1 0 1 1


12 FAS * 0 0 1 1 0 1 1


14 FAS * 0 0 1 1 0 1 1

15 NFAS # 1 A Sa4 Sa5 Sa6 Sa7 Sa8 T/S 1 - 31

Timeslots 0-15




Page 12




Calculations Bit rate of an E1= sampling frequency x number of timeslots x bits per sample= 8000 x 32 x 8= 2 048 000 bits/sexpressed as 2.048Mbits/s

Bit duration = 1 / bit rate= 1 / 2048000= 0.000 000 488 281 25 seconds= 488.28125 ns

Timeslot duration= bit duration x number of bits per timeslot= 488.28125 ns x 8= 0.000 000 488 281 25 x 8= 0.000 003 906 25 seconds= 3.90625 s

Frame duration (method 1)= number of timeslots x timeslot duration= 32 x 3.90625 s= 32 x 0.000 003 906 25= 0.000 125 seconds= 125 s

Frame duration (method 2)= 1 / sampling frequency (Hz)= 1 / 8000= 0.000 125 seconds= 125 s

Multi-frame duration= frame duration x number of frames per multi-frame= 125 s x 16= 0.000 125 x 16= 0.002 seconds= 2 ms.




Page 13




Calculations continued

Bit rate per timeslot= number of samples x number of bits per sample= 8000 x 8= 64 000 bits/s= 64 kbits/s

Number of frames per second= 1 second / frame duration= 1 / 125 s= 1 / 0.000 125 s= 8000 frames per second

Number of multi-frames per second= 1 second / multi-frame duration= 1 / 2 ms= 1 / 0.002 s= 500 multi-frames per second

Number of bits per frame= number bits per timeslot x number of timeslots per frame= 8 x 32= 256 bits

Number of bits per multi-frame= number of bits per frame x number of frames per multi-frame= 256 x 16= 4096 bits

Bit rate of a single Sa bit:The number of frames per second = 8000 (see above)Therefore, the number of NFAS signals per second is number of frames per second / 2 (frames alternate between FAS and NFAS)= 8000 / 2= 4000Therefore, a single bit occurs 4000 times per second= 4000 x 1= 4000 bits/s per Sa bit= 4 kbits/s per Sa bit.

Total bit rate of Sa bits combined= 4000 x 5= 20 000 bits/s= 20 kbits/s



Page 14



Signalling

Signalling Signalling is the information that exchanges need to pass to each other so that calls may be set up, controlled and maintained. This includes dialling, metering, clear-back signals etc. There are 3 methods or types of signalling used by Telkom SA. They are, Channel Associated Signalling (CAS), Common Channel Signalling (CCS) and Common Channel Signalling number 7 (CCS-7).

Channel Associated Signalling (CAS)

Channel Associated Signalling: Each channel has a dedicated signalling port / slot

The signalling port is in timeslot 16

Timeslot 16 carries signalling for channel ‘n’ and ‘n+15’, depending on the frame number, as discussed earlier

Therefore, frame 1 carries signalling for channel 1 and 16, frame 2 for channel 2 and 17, and so on.

The table below shows channel associated signalling.




Page 15



Signalling, Continued

Common Channel Signalling (CCS)

Common Channel Signalling: The signalling for all the speech channels is handled by a common

signalling channel

The signalling timeslot doesn’t have to be timeslot 16, but may be any timeslot from 1 to 31 (may be randomly selected)

Common Channel Signalling No7

Common Channel Signalling No7: CCS7 is the same as CCS, except that CCS7 has been specified by ITU so

that different vendors, e.g. Siemens, NEC and Alcatel, can have their equipment may be compatible w.r.t. signalling

Preciously, before CCS7, different vendors were unable to pass signalling information between their respective exchange equipment types.

Advantages of CCS over CAS

Advantages of CCS over CAS: Signalling capacity is greatly increased

Additional speech channels available, therefore the channels are used more efficiently

Exchanges may exchange information that is not related to the speech / data channels, e.g. automatic recall etc.

Notes




Page 16




Example of CAS

Example of CCS



Page 17




Example of CCS7

Limitations of the Standard Framing Format

When the 2 Mb/s frame is used exclusively for PCM voice transmission, the frame alignment criteria is very reliable.However, it has some limitations, particularly for data transmission and on-line performance monitoring. With data transmission, the traffic can inadvertently simulate the frame alignment and non-frame alignment words and false framing is possible. This can have a serious effect on the data.

Performance monitoring of the received signal is limited to checking for errors in the frame alignment signals. This gives a poor indication of errors in the payload as only seven bits in 256 are being checked.There is no way for the remote end to send back this rudimentary error performance data, so only one direction of transmission can be monitored at each location. In an age of increasingly competitive digital leased line services and ISDN (Integrated Services Digital Network), this is inadequate.



Page 18



CRC-4 Framing

Introduction To overcome the limitations of the standard framing format, ITU-T recommendation G.704 specifies the use of a CRC-4 cyclic redundancy check for 2 Mb/s systems. CRC-4 framing provides reliable protection against incorrect synchronization and a means of monitoring bit error ratio (BER) during normal operation. In other words, the CRC-4 framing algorithm is extremely unlikely to be fooled by payload data patterns.On the following four pages we will discuss the CRC-4 frame structure and the function of the bits in the structure.

CRC-4 Multiframe

Each CRC-4 multiframe, which is composed of 16 frames numbered 0 to 15, is divided into two 8-frame Sub-Multiframes (SMF), designated SMF I and SMF II which signifies their respective order of occurrence within the CRC-4 multiframe structure. The SMF is the Cyclic Redundancy Check-4 (CRC-4) block size (i.e. 2048 bits).(See drawing on the next page)The CRC-4 multiframe structure is not related to the possible use of a multiframe structure in 64 kbit/s channel time slot 16.

To enable the receiver to locate the four bits C1, C2, C3, C4 forming the remainder, an additional frame called the CRC multiframe is formed. TheCRC-4 multiframe comprises sub-multiframes I and II, both of which comprise eight normal PCM frames. A CRC multiframe alignment signal is used to synchronize the receiver to this frame. The signal is inserted bitby bit into the first bit position of the NFAS in frames 1, 3, 5, 7, 9 and 11.

The CRC-4 multiframe alignment signal is 001011 and is transmitted in the first bit of the NFAS word. The first bits of frames 13 and 15, called E-bits are used to indicate a negative comparison (that is, data blocks with bit errors) back from the far end to the transmitter. If the E-bit in frame 13 is 0 then thereis an error in sub-multiframe (SMF) I. The E-bit in frame 15 indicates error status from sub-multiframe II in the same way.

Each sub-multiframe in the CRC multiframe consists of eight standard PCM frames, and is 1 ms (8 ´ 125 ms) long.That means there are one thousand CRC-4 error checks made every second.

Please Note The in-service error detection process does not indicate BER unless a certain error distribution (random or burst), to predict the average errors per block is assumed. Rather, it provides a block error measurement.

This is very useful for estimating percentage errored seconds (% ES) which is considered the best indication of quality for data transmission. CRC-4 error checking is very reliable—at least 94% of errored blocks are detected.




Page 19

YV, -0001-01-03,

Page: 8A: Sub-Multiframes (SMF)



CRC-4 Framing, Continued

CRC-4 Multiframe

Below is the CRC-4 multiframe.

Sub-multiframe Frame Bits 1 to 8 of the frame(SMF) number

1 2 3 4 5 6 7 8

Multiframe

I

01234567

C10

C20

C31

C40

01010101

0A0A0A0A

1Sa41

Sa41

Sa41

Sa4

1Sa51

Sa51

Sa51

Sa5

0Sa60

Sa60

Sa60

Sa6

1Sa71

Sa71

Sa71

Sa7

1Sa81

Sa81

Sa81

Sa8

II

89101112131415

C11

C21

C3EC4E

01010101

0A0A0A0A

1Sa41

Sa41

Sa41

Sa4

1Sa51

Sa51

Sa51

Sa5

0Sa60

Sa60

Sa60

Sa6

1Sa71

Sa71

Sa71

Sa7

1Sa81

Sa81

Sa81

Sa8

NOTES

1 E = CRC-4 error indication bits

2 Sa4 to Sa8 = Spare bits

3 C1 to C4 = Cyclic Redundancy Check-4 (CRC-4) bits

4 A = Remote alarm indication

CRC4 Multiframe

Please Note On the following page is the allocation of bits 1 to 8 of the frame.




Page 20




Bit number

Alternate frames

1 2 3 4 5 6 7 8

Frame containing theSi 0 0 1 1 0 1 1

frame alignment signal(Note 1) Frame alignment signal

Frame not containingthe frame alignment

Si 1 A Sa4 Sa5 Sa6 Sa7 Sa8

signal(Note 1) (Note 2) (Note 3) (Note 4)

NOTES

1 Si = Bits reserved for international use. One specific use is described in 2.3.3. Other possible uses may be defined at a later stage. If no use is realized, these bits should be fixed at 1 on digital paths crossing an international border. However, they may be used nationally if the digital path does not cross a border.

2 The bit is fixed at 1 to assist in avoiding simulations of the frame alignment signal.

3 A = Remote alarm indication. In undisturbed operation, set to 0; in alarm condition, set to 1.

4 Sa4 to Sa8 = Additional spare bits whose use may be as follows:

i) Bits Sa4 to Sa8 may be recommended by ITU-T for use in specific point-to-point applications (e.g. transcoder equip-ments conforming to Recommendation G.761).

ii) Bit Sa4 may be used as a message-based data link to be recommended by ITU-T for operations, maintenance and per-formance monitoring. If the data link is accessed at intermediate points with consequent alterations to the S a4 bit, the CRC-4 bits must be updated so as to retain the correct end-to-end path termination functions associated with the CRC-4 procedure (see 2.3.3.5.4). The data-link protocol and messages are for further study.

iii) Bits Sa5 to Sa7 are for national usage where there is no demand on them for specific point-to-point applications [see i) above].

iv) One of the bits Sa4 to Sa8 may be used in a synchronization interface to convey PDH synchronization status mes-sages, as described in 2.3.4.

Bits Sa4 to Sa8 (where these are not used) should be set to 1 on links crossing an international border.

Allocation of bits 1-8 of CRC-4

CRC-4 Remainder

The CRC-4 remainder is calculated on complete blocks of data, and the 4-bit remainder is transmitted to the far-end, using the first bit in the FAS of each even numbered frame (C 1 -C 4 ). At the receiving end, the receiver makes the same calculation and compares its results with those in the received signal.

If the two 4-bit words differ, the receiving equipment knows that one or more errors are present in the payload. Every bit of the block is checked so an accurate estimate of block error rate (or erred seconds) is made while the link is in service.




Page 21




Identifying Alarms and Errors

Another powerful feature CRC-4 framing provides is the local indication of alarms and errors detected at the remote end.

When an errored SMF is detected, E-bits are changed from 1 to 0 in the return path multiframe. The local end, therefore, has exactly the same block error information as the far-end CRC-4 checker.

Counting E-bit changes is equivalent to counting CRC-block errors. Consequently, the local end can monitor the performance of both go and return paths. This can be carried out by the network equipment itself, or by suitable test equipment, monitoring the received 2 Mb/s stream.

In the same way, the A-bits return error alarm signals for loss of frame, loss of synchronization, or loss of signal from the remote end.

The CRC-4 frame structure is the preferred format because of its error detection capability and immunity to false frame alignment (refer to ITU-T recommendation G.706). For systems without CRC framing, in-service testing is limited to checking for errors in the frame alignment word, which provides a poor indication of errors in the payload.

Monitoring code violations in HDB3 is another method of monitoring quality-of-service of live traffic; however, this measurement refers only to the nearest line section or the interface connection between equipment. It can be useful for troubleshooting, but has little application for monitoring overall system performance.

Notes



Page 22



Alarms

Introduction The alarms we are most likely to get are the following: Remote Alarm Indication

Alarm Indicator Signal

Frame sync loss

Multiframe sync loss

Distant multi frame alarm

Remote Alarms

Multiplexes are connected together so the PCM transmission takes place in both directions, it also follows that alarm messages are also transmitted bi directional.

Alarm messages




Page 23



Alarms, Continued

Remote Alarm Indication (RAI)

The NFAS is used to transmit service information. Bit 3 of the NFAS indicates a remote (or distant) alarm.

If bit 3 is Then

0 There is no alarm1 There is one of the following alarms :

Power supply failure

Codec failure

Failure of incoming 2048 kbit/s signal

Frame alignment error

Frame alignment signal bit error ratio > 1 x 10-3

The multiplexer located at B continuously monitors the incoming FAS for bit errors. The FAS is received in timeslot 0 of alternate frames which is every 250s or 4000 times /second.

If the result of the bit error measurement of the FAS is < 1x 10-3 ,transmission is undisturbed. The NFAS transmitted to point A will be Si 101111.

When the FAS bit error ratio reaches a value of greater than 1x 10-3 ,correct operation of the transmission link is no longer possible and the receiving miltiplexor goes out of synchronisation. This is indicated by A bit of the NFAS to 1, which results in an alarm, called a Remote Alarm Indicator (RAI), or distant alarm. In this case the NFAS transmitted back to point A is Si 1111111.

Notes




Page 24



Alarms, Continued

Alarm Indicator Signal (AIS)

The multiplexer at point A registers this alarm and then stops transmitting normal speech or data signals and transmits a continuos sequence of 1 ‘s. This causes the multiplexer at B to show an AIS alarm. All the 1 signals maintain the clock recovery mechanism in the regenerators so that resynchronisation can be attempted as soon as the FAS bit error ratio recovers to equal or less that 1 x 10-3.

ITU-T defines AIS as more that 509 1’s in a 512-bit block which is a signal containing less than three 0’s in a 2-frame period. A signal with all bits in state 1 except for the FAS (001101= Three 0’s) is not a valid AIS and should be declared as frame sync loss.

Frame sync loss

Frame sync loss is declared in PCM 30 framing if three consecutive incorrect FAS words are received or there are more than 914 CRC errors.(CRC will be explained in more detail later in this chapter.)

Multiframe sync loss

If the signalling NFAS is lost then multiframe sync loss is declared.

Distant multi frame alarm

If multiframe sync loss alarm is declared in one direction the Y bit in the NMFAS (bit 2) in the opposite direction is set to 1 which results in a distant multiframe alarm.

Notes



Page 25



Signalling

Introduction Signaling is used in networks to set up the connections between the transmitting and receiving ends of a circuit. Both the standard and CRC-4 framing formats can be used to carry signaling information.

There are two methods of carrying signaling information: common channel signaling (CCS)

and channel associated signaling (CAS).

Common Channel Signaling (CCS)

If CCS is used, multiframe alignment is unnecessary. Timeslot 16 is simply used as a 64 kb/s data channel for CCS messages, or it can be turned over to revenue-earning traffic, giving a total of 31 channels for the payload (PCM31).

2.048 Mb/s frame format structure of Channel Associated Signaling (CAS)

Below is 2.048Mb/s-frame format structure for CAS.

Channel Associated Signaling

Once the multiplexer has gained frame alignment, it searches in timeslot 16 for the multiframe alignment signal (MFAS) (0000) in bits 1 to 4. Amultiframe consists of 16 frames, and 0000 in bits 1 to 4 signifies the first of these frames.

The multiframe is only necessary when CAS is used (that is, PCM30). Timeslot 16 contains the information necessary for switching and routing all 30 telephone channels.

The signaling between the near-end and far-end multiplexers takes place using pulse signals comprising 4 bits (ABCD).




Page 26




These are generated by the signaling multiplex equipment. The 64 kb/s signaling capacity of timeslot 16 is divided between the 30 telephonechannels and two auxiliary channels (synchronization and alarm messages), using pairs of 4-bit ABCD signaling words.

Over a complete multiframe, all 30 channels are serviced. Timeslot 16 can accomodate a pair of 4-bit ABCD signaling words. This ensures that all 30 channels are serviced over a complete multiframe.

In-Service Tests

The major benefit of in-service tests is that it allows the user's traffic to flow normally without interruption. This means that error performance statisticscan be collected over a longer period, and with the storage available on modern test sets, weeks of data can be stored and time-stamped for multiplein-service parameters. These might include CRC-4 block errors, FAS errors, HDB3 code errors, E-bits (remote end clock errors, REBE) and alarmhistory.

Long-term monitoring is useful for catching that elusive burst of errors which only seems to occur at the busiest time of the day. It can also help to confirm that the overall quality of the circuit meets specification.The clearest way to display long term measurements is the histogram. Error bursts or periods of degraded performance can be spotted almost instantly.

Timeslot Monitoring

With a framed 2 Mb/s test set, there are other useful checks you can make. You can monitor the individual channels to check if they are carrying voice ordata.

A demultiplexed channel at 64 kb/s can be decoded to provide a voice frequency output.

In addition, it is possible to demultiplex timeslot 16 and display the 30 ABCD channel associated signaling words. This will highlight any permanentlyidle or ‘stuck bits’. It is also advisable to check that a channel is idle before taking it out of service.

Multiplexer Performance Monitoring

A fully framed 2 Mb/s test signal also allows the operation of alarm and performance monitoring functions within the multiplexer to be verified.Errors can be sent continuously or in burst mode to verify the network element’s repor-ting functions. The network element’s framing circuits canbe fully tested by generating a loss of frame test signal; while the backward reporting functions of the network element can be verified by monitoring the A and E bits.



Page 27



2 Mb/s Impairments

Introduction There are four main causes of 2.048 Mbps impairments namely: Faulty equipment

Improper Connections

Environmental factors

Data Characteristics

Faulty Equipment

Any piece of 2.048 Mbps equipment can cause errors when the components fail or operate outside of specifications.

Errors, which can signal faulty equipment, include code errors, bit errors, FAS (frame) errors, excessive jitter, and slips. For instance, code errors can occur due to faulty clock recovery circuitry in span repeaters. These errors occur as the equipment becomes older and begins to drift out of specifications.

Improper Connections

Transmission errors are created by improper connections or configurations. For example, intermittent errors can occur when component or cable connections are loose, and timing errors can occur when improper or conflicting timing sources are connected together.

Dribbling errors are often caused by loose or unconnected shield ground cables and by bridge taps.

Further, upon installation, the circuit may not work at all due to mislabelled pin-outs on terminating cable blocks and to flip-flopped wires i.e. transmit-to-transmit as opposed to transmit-to-receive. These errors are typically discovered upon circuit installation and possibly during circuit acceptance when tests are performed end-to end.

Environmental Electrical storms, power lines, electrical noise, interference, and crosstalk between transmission links can cause logic errors, FAS (frame) errors, CRC errors in addition to code errors.

Typically, these conditions cause intermittent, bursty errors, which are some of the most difficult to locate.

Data Characteristics

Data characteristics, such as repetitive patterns, can force equipment to create pattern-dependant jitter and code errors. These errors may not exist when testing the transmission path with standard pseudorandom patterns.



Page 28



Analyzing 2 Mb/s Impairments

Techniques and Measurements

To analyse a 2.048 Mbps circuit’s performance and to isolate the causes of degraded services, the test set must perform many measurements in different scenarios.

There are four typical scenarios where 2.048 Mbps testing is required, namely: Installation

Acceptance testing

Routine preventative measure

Fault isolation

Installation When installing a 2.048 Mbps circuit, out-of-service testing is very useful in verifying equipment operations and end-to-end transmission quality.

One starts by testing the equipment (such as NTE’s, channel banks, multi-plexers), and then verifying cable connections, timing source selections, andfrequency outputs.

Acceptance Testing

In addition to the test performed during installation, two other tests i.e. stress tests and timed tests should be performed to ensure that the 2.048 Mbps circuit is operating properly with respect to the relevant 2.048 Mbps circuit specifications and tariff.

The equipment may be stressed by verifying the transmission frequency around 2.048 Mbps equipment. The same procedure may be performed end-to-end to stress the entire 2.048 Mbps circuit.

Timed tests with printouts should be performed over a 24- or 48-hour period using standard pseudorandom patterns, which simulate live data.

Routine Preventive Measure

Routine maintenance test are strongly recommended once live data is transmitted across the 2.048 Mbps circuit.

Routine maintenance can alert technicians to degrading service before it disrupts normal operations. In most instances, this involves monitoring the live data for alarms, code errors, FAS (frame) errors, CRC errors, and signal frequency measurements which provide information about the performance of the 2.048 Mbps circuit.These tests should be performed with printouts over a 24- or 48-hour period to detect time specific or intermittent errors.




Page 29



Analyzing 2 Mb/s Impairments, Continued

Fault Isolation Fault isolation is required once service is disrupted due to excessive error rates. This can be performed using both in-service and out-of- service tests.

In-service monitoring provides general information and can be used before out-of-service analysis to localise problems and minimise circuit downtime. By monitoring the circuit at various points, technicians are able to analyse the results and determine where problems are originating.

By performing standard out-of-service tests, such as loopback and end-to-end tests, technicians are able to stress the equipment, find sources of errors, and verify proper operation once the trouble is repaired.

Please Note We will discuss 2 methods of application testing that covers all the below test types: Installation - Application 2

Acceptance testing - Application 2

Routine preventative measure – Application 1

Fault isolation - Application 1 and 2



Page 30



Application 1: In-Service Analysis of Live Traffic

Evaluation of the performance of a 2Mb/s Syatem

The following sections explain how to evaluate the performance of a 2.048 Mbps system using customer data. It is useful: When performing periodic maintenance and when looking for

transmission degradation before it effects service.

When analysing the span for intermittent errors, which are caused by faulty equipment or environmental influences.

For analysis of 2.048 Mbps circuits which cannot be taken out-of-service.

Before out-of-service analysis, to localise the problem and minimize circuit downtime.

To achieve all these benefits, the test set may be configured to monitorthe 2.048 Mbps circuit from practically any 2.048 Mbps access point. The Figure below shows a typical circuit and possible monitoring locations.

Possible 2.048 Mb/s Circuit Monitoring Locations

Below is a sketch showing possible monitoring locations of a 2.048 Mb/s circuit.

Analysis of Alarm and Error Indications (In-Service Testing)

Testing and troubleshooting of a 2.048 Mbps signal requires regular monitoring for alarms and errors.

The monitoring for alarms and errors allows the user to detect and sectionalize transmission lines or equipment problems in a 2 Mbps signal. Errors can also be intentionally injected to see the response of the system.




Page 31



Application 1: In-Service Analysis of Live Traffic, Continued

Common Alarm and Error indications (In-Service Testing)

The Table 3 below highlights some of the important alarm and error indications along with possible reasons and solutions.

Result Reason Possible solutionSIGNAL LOSS Indicates history of receiver

signal loss Check cabling and

connections

Check network equipment

Frame Loss Indicates history of frame synchronisation loss

Check Signal Loss and Power Loss LEDs. If these LEDs are not on, check FAS Distant alarm and AIS alarm

Frame Sync Signal is unframed, or synchronisation to the specified framing has not been achieved

Verify all settings and connections

FAS Distant alarm Indicates remote (FAS Distant) alarm

Check span equipment downstream from present location

Check local TXAIS Alarm Indicates AIS alarm

(Unframed All Ones) Check span equipment

upstream from the present location



Page 32



The AIS and FAS Distant Alarms

2.048 Mb/s Network Alarms

Below are 2.048 Mb/s network alarms.

The AIS Alarm An AIS alarm is an unframed continuous stream of binary ones. However, a signal with all bits except the frame alignment in the 1 state is not mistaken asan AIS.

If the network equipment shown in Figure above suffers a signal or framesynchronisation loss, or receives an AIS alarm at input 1, it transmits the AISalarm at output 1. Hence, the AIS alarm indicates the presence of an alarm indication to the equipment farther downstream (away from the source of the trouble).

Therefore if the test set receives an AIS alarm, this indicates that the troublemust lie somewhere farther upstream in the network. This is illustrated in Figure below.

Detection of 2.048 Mb/s Network Alarms




Page 33



The AIS and FAS Distant Alarms, Continued

The FAS Distant Alarm

The FAS Distant alarm is indicated by setting bit 3 equal to 1 in time slot 0 of the frames not containing the FAS pattern.

If the network equipment shown in Figure above (2.048 Mb/s Network Alarms) suffers a signal or frame synchronisation loss, or receives an AIS alarm at input 1, it transmits the FAS Distant Alarm at output 2. Hence, the FAS Distant alarm indicates the presence of an alarm condition to the equipment farther upstream (back towards the source of the trouble).

Therefore if the test set receives a FAS Distant alarm, this indicates that the trouble must lie somewhere farther downstream in the network. This is illustrated in Figure above (Detection of 2.048 Mb/s Network Alarms).



Page 34



Code Error analysis

Code Errors

Code Error Analysis

The bipolar nature of the AMI signal allows the detection of single (isolated) errors since single errors on the line cause a pulse to be either incorrectlyadded or omitted, which in turn results in two successive pulses of the same polarity. This constitutes a violation of the bipolar coding scheme.

Recall that due to the zero suppression scheme used in HDB3, the signal mayalso contain intentional bipolar code violations representing strings of 4 consecutive zeros. These intentional code violations due to HDB3 must be distinguishing from code violations due to the errors occurring on the 2.048 Mbps line.




Page 35



Code Error analysis, Continued

Since bipolar code violations due to HDB3 follow specific rules, they can berecognised as such by the test set. This constitutes the basis for the code erroranalysis performed by the test set.

A code error is defined as any violation of the bipolar code, which is nota code violation due to HDB3’s zero substitution algorithm. For comparison, an illustration of a code error along side an HDB3 substitution code is shown in Figure above.

It is not necessary to receive and transmit a known pattern to recognise codeerrors. Hence the test set can perform code error analysis on an in-service basis without disrupting the traffic on the 2 Mbps line. To do this analysis, the test set provides the following key result:

Error ExplanationCode Errors Number of code errors detected since the

beginning of the testCode Error Rate Ratio of the number of code errors in the last

test interval to number of bits examined in last test interval

Advantages/

Limitations of Code Error Analysis

Code errors provide an approximate indication of the error performance on ametallic 2.048 Mbps line without the need to disrupt live traffic. Furthermore, they can generally be used to sectionalise problems to the local span in the 2.048 Mbps network. (This will be discussed further under “Correlationof Results and Problem Causes”).

It must be noted, however, that code error analysis has certain limitations. Code errors are useful in identifying local (near end) metallic span and repeater problems.

However they are not a good indication of end-to-end performance since network equipment beyond the local span or non-metallic transmission media (e.g. microwave and fibre) will correct code errors in the far end 2.048 Mbps span.

FAS (Frame alignment Signal) Error Analysis

As we explained in our discussion of the 2.048 Mbps Framing Format, time slot 0 of every other 2.048 Mbps frame contains a fixed 7-bit long FAS pattern




Page 36




When doing in-service FAS error analysis, the test set takes advantage of the fact that even though the data portion of the 2.048 Mbps frame is unknown, the FAS bits contain a known pattern such that the errors occurring on these bits can be detected without disrupting the traffic.

Hence the test set counts a FAS error each time one or more bits in the FASpattern are received in error.

Upon synchronization with the frame alignment signal, the test set automatically provides the following result:

Error ExplanationFAS Errors Number of FAS errors received since the

beginning of the test

Advantages /

Limitation of FAS Error Analysis

FAS errors allow in-service error performance analysis of the 2.048 Mbpscircuit. Under random (Gaussian) error conditions, the FAS error rate will closely approximate the actual error rate if the test is performed over a significantly long period of time.

Moreover FAS errors can be used to isolate problems to network equipment(such as digital cross connect systems and higher order multiplexers) which frame (or reframe) the 2.048 Mbps data.

The limitations of FAS error analysis are threefold.1. Since the FAS pattern takes up only 7 bits for every 512 bits transmitted (2 frames x 32 time slots/frame x 8 bits/time slot = 512 bits), the analysis is performed on a relatively small number of the received bits (about 1.4%). As a result, errors not occurring on the FAS bits will be missed.2. Bursty error condition are far more common than random (Gaussian)error condition.3. FAS errors are corrected by multiplexers and digital cross-connected systems.

Hence, FAS error analysis cannot be used to determine end-to-end error performance in networks where this type of equipment is installed.




Page 37




Advantages/ Limitations of CRC Error Analysis

Most data sequences generate a CRC word, which can be uniquely associated with that particular data sequence. Therefore, CRC errors can detect the presence of one or more bit errors in a submultiframe to a very high degree of accuracy (93.75%) without the need to take the 2.048 Mbps circuit out-of-service.

However, the following limitation of CRC error analysis must be kept in mind.1. A CRC error indicates the occurrence of one or more errors, but not the total number of errors in a submultiframe. Hence, the BER obtained using the formula above will be somewhat lower then the actual error rate if the error rate is so high that there are several errors in the submultiframe.

1 error per submultiframe corresponds to an average error rate of 4.9E-4.

CRCs may be recalculated by network equipment such as digital cross connect systems. Therefore, CRC error analysis cannot be used to determine end-to-end performance in networks where this type of equipment is installed.

Possible Problem Locations




Page 38




Correlation of In-service Results

The table below refers to the above diagram.

Label Results Problem/LocationA Code Errors Local problem. Possibly bad cabling

connections between test set and circuit, corroded “dirty” cable plugs or defective NTE

A,B or C Received frequency offset Frequencies that are out of range may affect jitter tolerance and noise margins, in addition to causing error bursts and slips.

B or C Code Errors, FAS errors or CRC Errors

Local 2.048 Mb/s span problem. Possible faulty repeater, span line noise, crosstalk, poor cabling or defective monitor jacks.

C Code Errors, NoFAS Errors, or CRC Errors

Local 2.048 Mb/s span problem

C No Code errors, FAS errors or CRC Errors

Typically far-end span line problem. Sectionlise firther. Potential for light guide, radio or violation monitor removal equipment in the network



Page 39



Application 2; Out-Of service testing of 2.048 Mb/s Circuits

Introduction The following sections explain how the test set is used to evaluate the performance of a 2.048 Mbps system using pseudorandom data. It is useful:• When installing 2.048 Mbps circuits and verifying end-to-end continuity.• When isolating 2.048 Mbps circuit faults and verifying end-to-end continuity.• When performing acceptance testing which includes timed and stress tests.

Errors found via this analysis may be caused by faulty equipment, improperconnections, environmental influences, or data content. To find these errors, use results such as bit errors, bit error rate (BER), FAS errors, pattern slips, received frequency, error free seconds (EFS), percentage error free seconds (%EFS), etc, which are all measured simultaneously. These results will help in isolating the cause of the problem.

There are basically two methods of performing out-of-service testing: loopback

testing and end-to-end testing.

These methods are addressed in the following sections.

Analysis of Alarm and error indications (OUT-Of- Service Testing)

Testing and troubleshooting of a 2.048 Mbps signal requires regularmonitoring for alarms and errors. The monitoring for alarms and errors allows the user to detect and sectionalize transmission lines or equipment problems in a 2 Mpbs signal. Errors can also be intentionally injected to see the response of the system.

The Table below highlights some of the important alarm and error indications along with possible reasons and solutions.

Result Reason SolutionPattern Sync Test set not synchronised to

the incoming pseudorandom pattern

Check BERT pattern selection and FRM Sync status. If test set in self loop is operating properly, this indicates 2.048 Mb/s circuit problem

Frame Sync Signal unframed, or synchronisation to specified framing has not been achieved

Verify all setting and connections

FAS Distant Indicates remote (FAS Distant) alarm

Check span equipment downstream from present location




Page 40



Application 2; Out-Of service testing of 2.048 Mb/s Circuits, Continued

Analysis of Alarm and error indications (OUT-Of- Service Testing), (Continued)

Result Reason SolutionAIS alarm Indicates AIS alarm Check span equipment

upstream from present location

End-to-End Testing

End-to-end testing is performed with two TTC test sets so that both directionsof the 2.048 Mbps circuit may be analysed simultaneously. Figure above shows the set-up of an end-to-end test. This test method is better then the loopback test since the direc-tion of errors can be found more quickly.

Loopback Testing

Loopback testing is performed with one test set. Figure above shows the set-up of the loopback test. If NTE loopbacks are established to perform the test, it is important to realise that the far end NTE in loopback will affect the result. By design, most NTE’s remove received code errors beforetransmitting the data. This will affect the analysis result, because the near endtechnician will be unaware of code errors occurring on the far end metallic loop and may draw inconclusive results. Furthermore loopback tests cannot identify incorrect timing configurations where the customer premises equipment (connected to the NTE) may not be loop-timed to the network.



Page 41





Page 42



Analysis of Slips

Slips and their Causes

A pattern slip is the insertion of data bits into or from the data stream. Based on the source of the slip and its effect on the network, all slips can be placed on any of the following categories.1. Controlled Slips: Controlled Slips are bit additions or deletions, which do not disrupt frame synchronisation. These slips are typically caused bysynchronisation impairments in digital cross-connect (DCS) equipment. DCSequipment handles buffer overflows or underflows by deleting or repeating entire frames of data. Since data is added or deleted by entire frames, frame synchronisation is not disrupted.2. Uncontrolled Slips: Uncontrolled slips are bit additions or deletions that cause both data and framing bits to be displaced. The misalignment of framing bits typically results in frame synchronisation loss.

Uncontrolled slips are typically from synchronisation problems in equipment,which buffer the entire bit stream such as satellite down link receivers. Since the buffer in this equipment does not distinguish between framing and data bits, buffer underflows or overflows result in the addition and deletion of arbitrary blocks of data.

It should be noted that slips can also result from impairments unrelated to network synchronisation. Low signal level, noise, and excessive jitter can also cause slips.

Interpreting Slip Results

To troubleshoot a problem, which causes slips, pattern slip results must becompared to other test results.

If an occurrence of a pattern slip is associated with a frame loss, it can beassumed that the frame loss is caused by an uncontrolled slip. If a pattern slip occurs without disrupting framing, it can be assumed that a controlled slip has occurred. Categorisation of slips can help identify the cause of the problem.

A better understanding of the underlying problems can also be obtained byconsidering the frequency at which pattern slips occur.



Page 43



E1 Pulses

Introduction A pulse is a brief change of current, light or voltage produced in a circuit to operate a switch or relay which can be detected by a logic circuit.An E1 pulse is a pulse produced by a E1 circuit.In this section we will discuss the: Quality of the pulse

Pulse shape

Specifications of the shape

E1 Pulse Quality

The quality of the E1 pulse is an important factor in clear transmission.

The ITU-T G.703 standard defines the max and min values in the form of a mask.A good pulse shape must fit inside this mask.(Refer to pulse mask figure on the next page.)

See Table below for the specifications for an E1 pulse, as specified in ITU-T G.703, Table 6.

The 2.048 Mbit/s stream is the basic building block for the transmission of signals in the PCM digital hierarchy. Proper internetworking of equipmentalong that signal path requires strict compliance with various standards such as ITU-T G.703,G.704 and so on.

Output signals from such network elements (NE) as multiplexers, regenerators switches and PBX must be within the defined limits. The inputcircuitry of the network elements must, further, be able to compensate for any attenuation or distortion caused by the transmission media.

Then the logic 1s and 0s will be detected correctly; otherwise, bit/code errors will result.

A portable test instrument can be a useful tool to check the overall health of the transmission system and assist in locating the source of problems or defects. At the physical layer, the parameters of interest are bit rate (and its stability), jitter, wander, level, noise, code errors, and pulse shape distortion.

A key test in this area is verification that the signal pulse shape conforms to the ITU-T G.703 recommendation, as illustrated in the pulse mask Figure on the next page.




Page 44



E1 Pulses, Continued

The Mask of the Pulse at 2.048Mb/s




Page 45




ITU-T G.703 for 2.048Mb/s Pulse Mask Specifications

The table below shows the 2.048Mb/s pulse shape specifications.

Pulse Shape (Nominally rectangular) All marks of a valid signal must conform with the mask (see figure 15/G.703) above irrespective of the sign. The value V correspond to the nominal peak value

Pairs in each direction One coaxial pair (see G.703, 9.4). One symmetrical pair (see G.703, 9.4)

Test load impedance Coaxial cable: 74 ohms resistiveOne symmetrical pair: 120 ohms resistive.

Nominal Peak voltage of a mark (pulse) Coaxial cable: 2.37 VSymmetrical pair: 3 V

Peak voltage of a space (no pulse) Coaxial cable: 0+/-0.237 VSymmetrical Pair: 0+/-0.3 V

Nominal Pulse width 244ns

Ratio of the amplitudes of positive and negative pulses at the centre of the pulse interval

0.95 to 1.05

Ratio of the amplitudes of positive and negative pulses at the normal half amplitude

0.95 to 1.05

Maximum peak to peak jitter at the output port Refer to section 2 of recommendation G.823

Importance of the Pulse Shape Measurement

A G.703-compliant pulse-shaped 2 Mbit/s signal, when transported via metallic cable of correct impedance and prescribed length, will not distort beyond the design limits of the receive ports of the NEs. Otherwise, the resultant errors will lead to degraded service to customers and unnecessary repair costs for service providers (such as PTTs).

At the time of new installations or recommissioning of service after repair, the importance of pulse shape analysis and level is widely recognized. However, these measurements have nonetheless been frequentlyomitted because they have been assumed to require the use of an expensive, bulky digital oscilloscope. Current-generation 2 Mbit/s transmission test sets such as the SunSet E-Series alleviate this problem by incorporating pulse shape measurement, verification, and analysis through a method more practical and appropriate for field use.




Page 46




The pulse shape can provide an excellent qualitative indication of both noise and jitter. Averaging tends to smooth the noise riding on the signal. If jitter is present, however, the rising and falling edges will be seen as scattered along the time axis.

Digital measurement of the pulse width rise and fall times, and over/ undershoot values will give additional information on the possible sources of distortion. The results obtained using this technique compare favorablywith pulse shape measurements obtained from testing with a digital oscilloscope.

One example is shown in the figure on the next page. The closeness between the two measurement results will depend somewhat on the actual signal under test, but the correlation is generally reliable.

The method used in handheld BER test sets is intended primarily for field verification of the 2 Mbit/s transmission system, in that it provides a quick and easy-to-interpret result and relies on streamlined, hand-held equipment. While the more detailed or precise measurements obtained with a digital oscilloscope continue to be appropriate for other applications, such as design verification or type testing, the use of handheld test sets provides a powerful and efficient way for field service technicians to ensure layer 1 testing of a 2 Mbit/s link.

If link quality is not restored to the highest level, other services such as GSM, ISDN PRI, and SS7 – which are also transported on 2 Mbit/s links – will function unreliably. This will lead to levels of downtime and service quality which are unacceptable in today's highly competitive market. Taking the time to perform basic pulse shape analysis plays an important role in both the initial and ongoing quality of any service provider's network.

Notes




Page 47




Example of pulse shape measurement screen

Notes



Page 48



Error performance

Introduction There is a move towards providing broadband services to customers, e.g. primary rate access, broadband integrated services digital network (B-ISDN) and digital coded TV transmission. Because 2Mb/s signal is the building block of both PDH and synchronous digital hierarchy (SDH) systems that will transport the broadband services, it is necessary that the performance of the 2Mb/s link comply with the ITU-T recommendation G826.

It is therefore critical to provide a standard for acceptance testing of all 2Mb/s links in accordance with the error performance definitions and limits given in ITU-T Recommendation G.826.

These standards are media independent, block based and suitable for doing in service measurements (ISM).

End-to-end standards for the National network

The national digital path at or above the primary rate shall meet its allocated standards for all parameters concurrently. The path fails to meet the error performance requirements if any of these standards is not met.

Error performance parameters and standards for international constant bit rate digital paths at or above the primary rate should be adhered to in order to ensure high levels of transmission quality. This will ensure that Telkom meets Service Level Agreements (SLA) with Mobile Cellular Operators (MCOs) and hence drastically cutting down on penalty costs.

Notes



Page 49



Error Performance Events

Introduction While doing error testing various results can be achieved ,namely:

Errored Block (EB)

Errored Second (ES)

Severely Errored Second (SES)

Background Block Error (BBE)

Generic defi-nition of the block

ITU-T G.826 Recommendation gives a generic definition of a block as a set of consecutive bits associated with the path; each bit belongs to one and only one block. Consecutive bits may not be contiguous in time.

The table below shows block sizes for PDH ans SDH performance monitoring.

Hierarchy PDH SDH SDH SDHRate (Mbit/s) 2.048 2.240 48.960 150.336Path Type VC-12 VC-3 VC-4Bits/block 2 048 1 120 6 120 18 792Blocks/second 1 000 2 000 8 000 8 000Error Detection Code (EDC)

CRC-4 BIP-2 BIP-8 BIP-8

NOTE – It shall be noted that single bit parity check does not satisfy the error detection probability of 90%.

Errored Block (EB)

A block in which one or more bits are in error.

Errored Second (ES)

A one-second period with one or more errored blocks or at least one defect.

Severely Errored Second (SES)

A one-second period which contains 30% errored blocks or at least one de-fect. SES is a subset of ES.

Consecutive Severely Errored Seconds may be precursors to periods of unavailability, especially when there are no restoration or protection procedures in use. Periods of consecutive Severely Errored Seconds persisting for T seconds, where 2 T 10 (some Network Operators refer to these events as "failures"), can have a severe impact on service, such as the disconnection of switched services. The only way this Recommendation limits the frequency of these events is through the limit for the SESR.




Page 50



Error Performance Events, Continued

Please Note The defects and related performance criteria are listed in the relevant Annexes (B, C or D) for the different network fabrics PDH, SDH or cell-based.

To simplify measurement processes, the defect is used in the definition of SES instead of defining SES directly in terms of severe errors affecting the path. While this approach simplifies the measurement of SES, it should be noted that there may exist error patterns of severe intensity that would not trigger a defect as defined in Annexes B, C and D of ITU-T G.826.

Background Block Error (BBE)

An errored block not occurring as part of a SES.

Notes



Page 51



Error performance parameters

Introduction In this section we will cover the various methods of performance testing namely: Unidirectional path

Bidirectional path

And there possible results namely: Errored Second Ratio (ESR)

Severely Errored Second Ratio (SESR)

Background Block-Error Ratio (BBER)

Errored Second Ratio (ESR)


Background Block Error Ratio (BBER)

Anomalies

Available State Error performance should only be evaluated whilst the path is in the available state.

Criteria for entry and exit for the unavailable state for a single direction

A period of unavailable time begins at the onset of ten consecutive SES events. These ten seconds are considered to be part of unavailable time. A new period of available time begins at the onset of ten consecutive non-SES events. These ten seconds are considered to be part of available time.

During unavailable time, error performance measurements must be inhibited.

The figure below illustrates this definition.

Example of Unavailability Determination of a Unidirectional Path

Below is an example illustrating the above concept.




Page 52



Error performance parameters, Continued

Criterion for a Bidirectional Path

A bi-directional path is in the unavailable state if either one or both directions are in the unavailable.

The figure below illustrates this definition.

Example of the Unavailable State of a Bi-directional Path

Below is an example illustrating the above concept.


The ratio of ES to total seconds in available time during a fixed measurement interval.


The ratio of SES to total seconds in available time during a fixed measurement interval.

Background Block-Error Ratio (BBER)

The ratio of background block errors (BBE) to total blocks in available time during a fixed measurement interval, excluding all blocks during SES.


The ratio of ES to total seconds in available time during a fixed measurement interval.


The ratio of SES to total seconds in available time during a fixed measurement interval.




Page 53




Background Block Error Ratio (BBER)

The ratio of Background Block Errors (BBE) to total blocks in available time during a fixed measurement interval. The count of total blocks excludes all blocks during SESs.

Anomalies In-service anomaly conditions are used to determine the error performance of a PDH path when the path is not in a defect state.

The two following categories of anomalies related to the incoming signalare defined:a1 - an errored frame alignment signal;a2 - an EB as indicated by an EDC.

Notes




Page 54




Defects In-service defect conditions are used in the G.730 to G.750 series of Recom-mendations relevant to PDH multiplex equipment to determine the change of performance state, which may occur, on a path.

The three following categories of defects related to the incoming signal are defined:d1: loss of signal (LOS)d2: alarm indication signal (AIS)d3: loss of frame alignment (LOF)

For the 2 Mbit/s hierarchy, the definition of the LOF defect condition is given in the G.730 to G.750 series of Recommendations. The table below shows a set of parameters and measurement criteria.

Type Set of parameters Measurement criteria1 ESR An ES is observed when, during one second, at

least one anomaly a1 or a2, or one defect d1 to d3 occurs

1 SESR An SES is observed when, during one second, at least "x" anomalies a1 or a2, or one defect d1

to d3 occurs (Notes 1 and 2)1 BBER A BBE is observed when an anomaly a1 or a2

occurs in a block not being part of an SES2 ESR An ES is observed when, during one second, at

least one anomaly a1 or one defect d1 to d3 oc-curs

2 SESR An SES is observed when, during one second, at least "x" anomalies a1 or one defect d1 to d3

occurs (Note 2)3 ESR An ES is observed when, during one second, at

least one anomaly a1 or one defect d1 or d2 oc-curs

3 SESR An SES is observed when, during one second, at least "x" anomalies a1 or one defect d1 or d2

occurs (Note 2)4 SESR An SES is observed when, during one second,

at least one defect d1 or d2 occurs (Note 3)NOTE 1 – If more than one anomaly a1 or a2 occur during the block interval, then only one anomaly has to be counted.NOTE 2 – Values of "x" can be found in B.4.NOTE 3 – The estimates of the ESR and SESR will be identical since the SES event is a subset of the ES event.



Page 55






Page 56




Anomalies and Defects

If, during the installation phase prior to acceptance testing, anomalies or defects (i.e. error bursts, loss of frame, loss of signal) are observed on a regular basis, the cause of such disturbances must be determined and rectified before acceptance testing can commence.

Notes



Page 57



The measurement of the block

Introduction There are various methods that are used when measuring 2Mb/s lines, namely: In-service monitoring

Out of service monitoring

They can be run at different time intervals, namely 15 Minutes

24 Hours

In-service monitoring of blocks

Each block is monitored by means of an inherent Error Detection Code (EDC), e.g. Bit Interleaved Parity or Cyclic Redundancy Check. The EDC bits are physically separated from the block to which they apply. It is not nor-mally possible to determine whether a block or its controlling EDC bits are in error. If there is a discrepancy between the EDC and its controlled block, it is always assumed that the controlled block is in error.

No specific EDC is given in this generic definition but it is recommended that for in-service monitoring purposes, future designs should be equipped with an EDC capability such that the probability to detect an error event is 90%, as-suming Poisson error distribution. CRC-4 and BIP-8 are examples of EDCs currently used, which fulfill this requirement.

Estimation of errored blocks on an in-service basis is dependent upon the network fabric employed and the type of EDC available. Annexes B, C and D offer guidance on how in-service estimates of errored blocks can be obtained from the ISM facilities of the PDH, SDH and cell-based network fabrics respectively.

Out-of-service measurements of blocks

Out-of-service measurements shall also be block-based. It is expected that the out-of-service error detection capability will be superior to the in-service capability described above.

All measurements will be performed out of service using a measuring instrument with a pseudo-random pattern bit sequence (PRBS) 215-1 for 2 Mbit/s, 223-1 for 34 Mbit/s and 140 Mbit/s data rates. CRC-4 measurements are only to be used for in service measurements and not for out of service acceptance testing. Testing will be performed initially over a 15-minute period and, if successful, a 24-hour test period follows.




Page 58



The measurement of the block, Continued

Initial 15 minute test

Initial measurements must be performed over a 15-minute period. During this period, there should be no error or unavailable time (as defined previously). If an event occurs, this step may be repeated up to two more times. If, during the third test, an event occurs, the fault must be localised and corrected and the above step repeated.

24 hour test If the 15-minute test is successful, a 24-hour test should be conducted. This time must include work periods to ensure detection of any external disturbances. During the time period, there should be no unavailable time (as defined previously). At the end of the test period, the results must be compared to the BIS (Bringing Into Service) limits S1 and S2. If the ES and SES are: All less than or equal to their respective S1 values, the system, link or

path is accepted for service.

If one or more of the error parameters is greater than or equal to their respective S2 values, the circuit is rejected and the cause of the fault must be localised and rectified.

If either one or all error parameters fall in between the S1 and S2 limits, the circuit can be re-tested for a further 24 hour period. If still > S1 and

< S2, the circuit must be rejected and the cause of failure located and

rectified.

If an unavailability event occurs during the 24 hour test period or the prescribed error limits are exceeded, testing must be suspended and the fault localised and cleared. If the unavailability is due to fading on a microwave link, this must be substantiated as the cause and quantified against the predicted calculations.




Page 59




Bringing-Into- Service Limits and Conditions

Two limits S1 and S2 are provided for use in BIS testing.

If the performance is better than limit S1, the entity can be brought into service.

If the performance is between the two limits, further testing is necessary.

Corrective action is required if performance is worse than S2.

Notes




Page 60




Performance measurement for a 24Hour Period

The table below shows performance measurements for a 24 hour period.

Performance Objective for 24 Hour PeriodBit Rate 2048 kbit/s

Path Distance(km)

Path Allocation(%)

Errored Second CountRPO BIS S1 S2

100 0.2 7 3 0 6200 0.4 14 7 2 12300 0.6 21 10 4 16400 0.8 28 14 7 21500 1.0 35 17 9 25750 1.5 52 26 16 361000 2.0 69 35 23 471250 2.5 86 43 30 561500 3.0 104 52 38 661750 3.5 121 60 45 752000 4.0 138 69 52 862250 4.5 156 78 60 962500 5.0 173 86 67 1052750 5.5 190 95 76 114

Path Distance(km)

Path Allocation(%)

Severely Errored Second CountRPO BIS S1 S2

100 0.2 0 0 0 0200 0.4 1 0 0 0300 0.6 1 1 0 3400 0.8 1 1 0 3500 1.0 2 1 0 3750 1.5 3 1 0 31000 2.0 3 2 0 51250 2.5 4 2 0 51500 3.0 5 3 0 61750 3.5 6 3 0 62000 4.0 7 3 0 62250 4.5 8 4 0 82500 5.0 9 4 0 82750 5.5 10 5 0 9

Path Distance(km)

Path Allocation(%)

Background Block Error Count RPO BIS S1 S2

100 0.2 52 26 16 36200 0.4 104 52 38 66300 0.6 156 78 60 96400 0.8 207 104 84 124500 1.0 259 130 107 153750 1.5 389 194 166 2221000 2.0 518 259 227 2911250 2.5 648 324 288 3601500 3.0 778 389 350 4281750 3.5 907 454 411 4972000 4.0 1037 518 472 5642250 4.5 1166 583 535 6312500 5.0 1296 648 597 6992750 5.5 1426 713 660 766



Page 61



Exercise



Page 62


Chapter 2 Synchronisation



© This document contains confidential and proprietary information. The dissemination, copying, disclosure, use or the taking of any action in reliance on the contents thereof without the written consent of Telkom S.A. Limited is strictly prohibited.

Page 60



Performance Objectives

Participant, when: Asked a question, will correctly identify among the various factors

affecting synchronisation.

Shown an illustration, will accurately label the synchronisation hierarchy.

Asked a question will correctly explain the various types of reference timing used in the Telkom synchronisation network.



Page 61



Introduction

Overview Network synchronisation is something, which is hardly noticed as long as it works, but many 'mysterious faults' result from improper synchronisation. The problems of network synchronisation have increased with: the introduction of new transport and switching technologies.

the liberalisation in the telecommunication market.

When a network is not synchronised the table below indicates what problems can be encountered.

Type of Service Effect

Voice Possible 'clicking' on the line

loss of signalling data (CCS7).

Handover problems and interruption of calls in mobile phone networks (GSM).

Fax Deletions of 4 to 8 scan lines or reduced throughput, which results in increased transmission time.

Multimedia Video frames frozen for several seconds,

noise burst on audio

Digital data Deletion or repetition of data

reduction of throughput

A reliable and robust network requires the distribution of a high quality synchronisation clock to the entire network. The entire network must be synchronised to the same clock and this requires proper synchronisation architecture. The aim of the synchronisation architecture is to provide the best possible source of timing to all nodes.

Please Note The SDH network will distribute the synchronisation of clocks in the Telkom telecommunication network.




Page 62



Introduction, Continued

What has to be synchronised?

Most of today's digital communication networks are based on PCM (Pulse Code Modulation)-coded signals or SDH signals. Multiplexing is done in the time domain by arranging the 64kbit/s base signals in time slots relative to a reference signal (frame). Switching is done in a combination of space switches and time switches (time slot interchange). Time domain switching or multiplexing requires that the time slots are equal in all equipment’s assembling, disassembling or switching those signals. The time slot procedure needs elastic data buffers to compensate for wander-induced delay variations of the incoming data. Periodic over or underflow of those buffers (equivalent to a frequency difference) results in regular bit slips, which corrupt the integrity of the signal.

Subsequently the following network elements have to be synchronised: ATM switches/cross-connects

SDH network Elements

Voice switches

Data network elements (All digital network elements.)

How can you synchronise a network?

The task sounds simple, operate all equipment that has to be synchronised at the same frequency and a fixed phase relation.Achieving that state is not so easy because the absolute frequency accuracy has to be very high, in order to allow interworking between different networks. The required accuracy is in the range of 1 to the exp-11 (from the nominal value), which can currently only be guaranteed with atomic frequency references.Jitter and wander must be avoided in order to achieve high levels of accuracy.

It is not cost effective to equip each network element with an atomic clock, so the solution is to distribute reference signals with the required accuracy from a master clock. This clock is the Primary Reference Clock (PRC), which distributes reference signals to all network elements to control their internal clocks (slave clocks).

This results in a 'Synchronisation Network' which is logically separate from the communication network, which has its own architecture, special requirements, special equipment, special communication channels (in SDH networks), and maybe a separate management system. Physically the Synchronisation Network uses paths in the communication network and paths dedicated to synchronisation.




Page 63




What is Jitter and Wander

Short-term variations of a digital signal from their ideal position in time are known as jitter or wander.

Jitter is defined in terms of amplitude and frequency. In digital signal variations with frequencies > 10 Hz are known as jitter variations, with frequencies < 10 Hz as wander variations. Jitter causes digital signals that are sampled at constant times to appear not ideal. This can lead to single errors or errors bursts.Below is an illustration of Jitter.

Jitter

Notes




Page 64




The hierarchy levels are as per the drawing and table below.

Clock Type ITU-T Recommendation

PRC G.811Slave Clock (SSU Transit Node) G.812 Type 1 clock and G.823SDH Network element clock (SEC) G.813 and G.8232.048 Mbit/s Tributary G.823

SSUG.812

SSUG.812

SSUG.812

PRCG.811

N SDH networkelement clocks



1st

(k-1)

k th

Worst case network

k=10N= 20 with a maximum

of 60 SDHelements end-to-end

SECG.812

Clock Trail

Synchronisation Hierarchy




Page 65




Hierarchy of Telkom’s network synchronisation

Telkom’s synchronisation model consists of one PRC, up to a quantity of 10 SSU’ s Synchronisation Supply Unit (k-factors) and a maximum number of 60 SDH elements end-to-end, with no more than 20 SDH elements (N-factors) between two SSU’ s.

The clock trail from the PRC is transported to the SSU units using the SDH line aggregate signal. The clock trail must always be traceable back to the PRC.

The performance limits for jitter and wander are based upon the transmission model detailed in the ITU-T recommendation G.803 paragraph 6.2.4.

The synchronisation clocking is based on a master-slave hierarchy with each level of the hierarchy referenced to a higher level.

Transport of synchronisation

The synchronisation clock trail will be transported over the SDH network. Synchronisation occurs over two layers namely: National and

Regional layers

The clock trails for the Transit Node (DPSU sites) distribution are transported on the National Transport network. The regional distribution cascades from the Transit Node centres (TNC) to the Local Node centres (LNC) on the Regional Transport network.

The Main or First Primary Reference Clock (PRC1) is located at Johannesburg Doornfontein (JDF) and JDF is the Main Primary Reference Clock Node. The PRC1 will provide the second priority input to each Transit Node clock situated at all the DPSU sites. The GPS receivers co-located with every Transit Node clock will be the first priority input to the Transit Node Clock. All local nodes within the regional boundaries will reference their clocks from the Transit Node centres via the SDH network. Each region will accommodate at least two Transit Node Clocks situated at the DPSU and high priority sites. See drawing on the following page.




Page 66




TNC

Clock Trail

SD

H

GPS Reference GPS Reference

Regional Layer

National Layer

Cs

PRC

SSU

PRC

Clock Trail

LNCSSU

LNCSSU

LNCSSU

LNCSSU

SDH Network Element (SMA, SLT/ R)

TNCSSU

TNCSSU

PRC

Transport of synchronisation

Telkom’s National layer clock distribution

The National Layer Clock distribution refers to the synchronisation distribution on the National Transport network between the Transit Node clocks. The distribution of the synchronisation is in a hierarchy.

At the top of the synchronisation hierarchy is the PRC1 situated at Doornfontein.The clock distribution of the PRC1 situated at Doornfontein to each Transit Node Clock located in each region is done on the National Transport Layer SDH/WDM network. This is to provide the second priority input to each Transit Node Clock.

Notes




Page 67




Regional clock distribution

The Regional Clock Distribution refers to the synchronisation distribution from the Transit Node Clocks (TNC) located in each region to the Local Node Clocks (LNC) and network elements in the region.All element clocks (internal clock of a network element) within regional networks must be traceable to a Transit Node Clock (PRC).

The TNC (PRC’ s) are located at DPSU nodes within the regional boundaries. There are at least two TNC (PRC clocks (GPS clock referenced)) in each region. The Local Node Clocks (LNC) are located at most DSSU sites.

The synchronisation of the SDH networks must provide maximum robustness and maintain synchronisation under almost all network failure conditions. Where SDH rings are not directly connected to a PRC but a PRC is traceable through a secondary ring then it may be possible under failure conditions that the PRC traceability is lost. Under these conditions the ring should at least have access to an LNC SSU which can provide adequate clocking in a holdover state for a minimum period of 24 hours. Over this period the clocks accuracy must be maintained to at least 1:10-9.

Please Note Where a TNC and LNC clock the same ring, the TNC must always be the 1st priority clock.It is acceptable for the LNC to recover its clock reference from the TNC.

Please Note Telkom 2 Mb/s links should be synchronised from an external clock.This external clock receives its clocking as we discussed earlier in the this chapter.

Notes



Page 68



Exercise

Question 1 Should there be no synchronisation on the voice network what possible effects could be experienced? Possible 'clicking' on the line

Loss of signalling data (CCS7).

Handover problems and interruption of calls in mobile phone networks (GSM).

Question 2 Referring to the clock hierarchy level complete the table below.

Clock Type ITU-T Recommendation

PRC G.811Slave Clock (SSU Transit Node) G.812 Type 1 clock and G.823SDH Network element clock (SEC) G.813 and G.8232.048 Mbit/s Tributary G.823

Question 3 Name the two layers used to provide synchronisation over the SDH network. National and

Regional transport layer

Quote Technology is no place for wimps!!

Dilbert US Cartoonist



Page 69


Chapter 3 Earthing

Chapter 3 Earthing



Page 70


Chapter 3 Earthing


Participants when: given a fill in the blank exercise will correctly name the various parts of a

lightning impulse

asked a question can correctly name the various factors used to increase earthing effectiveness.

Shown a drawing will accurately describe how a protector works.

Asked a question will correctly name the various types of losses that can occur.



Page 71


Chapter 3 Earthing

Overview

Introduction to earthing

While earthing equipment one of the main objectives is to establish a suitable low resistance connection to the general mass of ground.

The importance of ensuring that the grounding system affords low earth impedance and not simply a low earth resistance must be understood.A spectral study of the typical waveform associated with the lightning impulse reveals both a high frequency and low frequency component.

The high frequency is associated with the extremely fast rising “front” (typically < 10 s to peak current) of the lightning impulse while the lower frequency component resides in the long, high energy, “tail” or follow on current, in the impulse. The figure below shows the percentage of less than a specific value. For example, in an 8/20µs-wave shape 10% of the energy is at frequencies less than 2.5kHz, while 90% of the energy is less than 33kHz. For a 10/1000µs-wave shape 90% of the energy is less than 1 or 2kHz.

Percentage of energy content below a given frequency

The grounding system appears to the lightning impulse as a transmission line where wave propagation theory, with the normal rules of reflection and group velocity, apply.

The soil can act as a dielectric, which under high potential stress at the electrode-soil junction can actually breakdown, thereby decreasing the resistivity of the soil during the surge.




Page 72


Chapter 3 Earthing

Overview, Continued

Measurement of earth resistance with conventional low frequency instruments may not provide results, which are indicative of the ground response to a lightning discharge. In complex installations, many earths can be interconnected and the whole network is measured as one.

Lightning first strokes have 1 to 10 µs impulse current rise times while higher dI/dts occurs with restrikes which occur in 75% of lightning discharges. These may have 0.2 µs rise time. In these circumstances, the local earth is subject to the full discharge current before the wavefront has travelled more than 60 metres. This assumes transmission at the speed of light (300 m/µs). The travel distance is less if inductance and capacitance of conductors is considered.The effect of distant earths is substantially negated with these values of dI/dt. They have little effect in determining local voltage rise due to lead inductance and impedance effects

The need to earth equipment properly

Why is it imperative that the equipment is earthed according to work instructions?The answer to this question is best answered by an example as shown below.

For ease of calculation, we assume that a 1kA surge current at 1 microsecond risetime is applied to the telephone cable entering at the distribution frame. The cables each have a 2 micro henry inductance and .01 ohm resistance.Using ohms law we will calculate the voltage across the frame:

V1 + v2 = [ (IR1) + L1 x vi/rt] +[(IR2) X L2 x vi/rt] = [(1000 x .01)+(2 x 10-6 x 1000/1 x 10-6] + [(1000 x .01)+(2 x 10-6 x

1000/1 x 10-6] = 10 + 2000 + 10 + 2000 = 4020 Volt 4020 Volt is the voltage across the equipment if it not be correctly earthed.

Battery chargerWith Batteries

Electronic PABX

L1 R1 L2 R2

Bonding Conductor Bonding Conductor

V1 V2

MAINS INPUT 48V



Page 73


Chapter 3 Earthing




Page 74


Chapter 3 Earthing

Overview, Continued

Methods to increase earthing effectiveness

Below are a number of methods that can be used to increase the effectiveness of the existing earthing. The use of flat tape rather than circular conductors.

For a given cross-section of conductor, this increases the surface area in contact with the ground, and hence increases capacitive coupling and reduces the overall contact resistance. It also reduces the high frequency resistance due to skin effects. Flat tape also tends to have a lower inductance per metre than a circular conductor of equivalent cross-sectional area.

The use of short-length radial conductors bonded at the injection point, rather than a single long length conductor.

This produces the effect of having a number of conductors in parallel.

Terminating radial conductors with vertical electrodes.

This measure is more effective in low to medium soil resistivity.

Using large bending radii when changing the direction of horizontal conductors.

Sharp bends tend to increase the inductance.

The use of earth improving compounds to improve the soil resistivity in the proximity of the conductors.

Lowering the soil resistivity reduces the resistive component of the impedance and hence improves the total impedance.

Earthing of equipment

Below is a drawing indicating a correct method and on the following page a frequently wired incorrect method of providing earth protection

Test and PatchRack

Mobile operatorequipment

Telkom equipmentStation earth

Correct Method




Page 75


Chapter 3 Earthing

Overview, Continued

Incorrect Method

Notes



Page 76


Chapter 3 Earthing

Earthing protection

Lightning Protectors

How does it work?Assume that protector 1 is a 500-800 volt protector.When there is voltage spike or surge protector 1 handles the current up to 800 volt. Should the spike exceed 800 volt protector 2 will take the voltage until it recedes to below 800 volt again

Please Note A “plug” protector can cause a 0,5 db loss.In some cases when removing the protector the fault clears because of this loss. This does not mean that the fault is cleared because the equipment must have protection. Telkom has a specification 1652 that deals with protectors.The important points of this specification has been extracted and put under the heading of “Earthing protection“ on the next couple of pages. The extractions deal with losses and distortion.




Page 77


Chapter 3 Earthing

Earthing protection, Continued

Insertion loss The protecting equipment connected in parallel with an approved 2-wire telephone during a call, or to a device designed for a standard analogue telephone line, shall not reduce the transmission levels by more than 0,25 dB (insertion loss) between 300 Hz and 3,4 kHz, for a source and load impedance of 900 complex.

Longitudinal conversion loss

The longitudinal balance (longitudinal conversion loss) shall be greater than 52 dB between 50 Hz and 3,4 kHz for a source and load impedance as normed by the relevant Telkom specifications (i.e. 900 complex).

Return loss The return loss of the device shall conform to the data in the diagram below, for a source and load impedance as normed by the relevant Telkom specifications (i.e., load of 120 ).

15

22

17

[dB]

[kHz]4 13 2048 3072 8448 12672

FREQUENCY RETURN LOSS

4 to 13 kHz 15 dB13 to 8448 kHz 22 dB8448 to 12672

kHz17 dB

Return Loss vs. Frequency




Page 78


Chapter 3 Earthing


Digital distortion

The distortion at the two bit-rates of 2 Mbit/s and 8 Mbit/s respectively, will be measured by applying a square wave of the corresponding fundamental frequency to the 2-wire circuit terminated with 120 , where once the protective module is inserted, and for the second measurement the protective module is removed. The following formula is applied:

Distortion =|V - V |

|V | x 100%

0, rms D, rms

0, ms

where V0,rms is the rms value of the a segment of the square wave measured during the length of the positive non-zero value, no protective module present, and VD, rms the same measurement in the presence of the module.

The distortion must be less than 5%.

Capacitance The capacitance between either line terminal and the earth terminal, and between the two line terminals of the protector shall be measured at 1 MHz and shall not exceed the values indicated in the table below. Where the impedance being measured by this test is not purely capacitive, it shall be modelled as a resistance in series with a capacitance, and the capacitance thereby obtained shall be less than the values indicated in the table below

Maximum limiting voltage- Devices with Primary and Secondary protection when applying surge with a rise time of 1KV/uS(V)

Static (quasi-DC) breakdown voltage

Maximum Residual output Voltage (V). peak.

Insulation resistance test voltage (V).

Maximum capacitance (nF).

800 20% higher than the peak maximum voltage expected on the protected line; 90V for the primary protection (gas arrestor).

10 100 5



Page 79


Chapter 3 Earthing




Page 80


Chapter 3 Earthing


Please Note For an E1 circuit not carrying power feed current (200V/50mA) the maximum residual (let-through) voltage for both longitudinal (Line-to-earth) and transverse (line-to-line) surges should not exceed 10Vpeak for all test waveforms. The minimum clamping voltage must exceed 6V typically 6.8V. This requirement means a two-stage protection philosophy is necessary. Primary protection provided by a gas discharge tube (GDT) for high energy absorption (typical residual voltage of a GDT is 500 to 800 volts for a 1kV per us rise time). Secondary protection provided by a semiconductor clamping or crowbar device (typical residual voltage of 7 to 10 volts is feasible for very fast rise times).

Protector types

Below is a list of protector types with the associated catalogue numbers.

Catalogue Number Protector

993533 2 Mb/s 10 way protector993537 Strip earth for earth connection for single pair

Kronin protection995359 2 Mb/s single pair protection 993538 Clip earth for earth connection for 10 pair

Krone module to the profile type frame

Protectors with associated catalogue numbers



Page 81


Chapter 3 Earthing

Exercise

Question 1 A spectral study of the typical waveform associated with the lightning impulse reveals both a high frequency and low frequency component.

Question 2 Name four methods used to increase the effectiveness of earthing: The use of flat tape rather than circular conductors.

The use of short-length radial conductors bonded at the injection point, rather than a single long length conductor.

Terminating radial conductors with vertical electrodes.

Using large bending radii when changing the direction of horizontal conductors.

The use of earth improving compounds to improve the soil resistivity in the proximity of the conductors.

Question 3 Use a drawing to illustrate the correct method of effectively earthing equipment.

Test and PatchRack

Mobile operatorequipment

Telkom equipmentStation earth

Question 4 Draw and describe how a lightning protector works.

Assume that protector 1 is a 500-800 volt protector.When there is voltage spike or surge protector 1 handles the current up to 800 volt. Should the spike exceed 800 volt protector 2 will take the voltage until it recedes to below 800 volt again




Page 82


Chapter 3 Earthing

Exercise, Continued

Question 5 Name four types of losses in earthed equipment. Insertion loss

Longitudinal conversion loss

Return loss

Digital distortion

Capacitance

Quote “People prefer to stay with problems they understand rather than look for solutions they're uncomfortable with.”

Anonymous



Page 83


Chapter 4 Surveillance




Page 82




Participants when: asked a question, will correctly explain what PPM is.

Asked a question will accurately explain the difference between intrusive and non intrusive monitoring

Asked a question will accurately explain the difference between Bi directional uni directional circuits



Page 83



Overview

Introduction to PPM

PPM stands for Primary Performance Monitoring.The PPM equipment provides early warning of any degradation of the signal quality or complete loss of a 2Mbit/s link.The PPM equipment will pass on all 2Mbit/s-alarm conditions to the NNOC, therefore providing an efficiently managed network.One of our major customers are the mobile cellular operators, to who up-time is of utmost importance to have our links up and error free.The suppliers of PPM are: DPAM - Channels and MDAQ - Alcatel.

PPM functions The functions of PPM are to: Measure the performance of the 2Mbit/s PDH signals.

Continuously monitoring many 2Mbit ports 24 hours a day without using expensive test equipment, thus:

reducing the number of costly emergency repairs,

save time by rapidly pinpointing the exact fault location

Win business through guaranteed up-time or short time to repair.

Add value: by monitoring alarms such as intrusion, security, fire equipment

malfunction, etc.

Add value through remote control capabilities such as resetting of rectifiers, lasers, base stations, etc.

PPM features Features of PPM is: Low cost, initial investment. Intrusive and non-intrusive performance monitoring based on FAS and

CRC check words. Local and/or remote monitoring. Cluster or stand alone desktop applications. Co-existence with TMN platform.




Page 84



Overview, Continued

SLA Two service level agreements exist between Telkom and GSM.

1. Telkom under takes to provide service 99,95% of the time between switches. This allows a maximun down time of 21 minutes per month. If this time is exceeded, Telkoms customer starts getting a reduction in the monthly rental for that particular 2Mb/s circuit. This will continue until the customer receives the rental of the 2Mb/s circuit free for that particular month.

2. Telkom under takes to provide service 99,5% of the time between distribution points. This allows a maximun down time of 216 minutes per month. If this time is exceeded, Telkoms customer starts getting a reduction in the monthly rental for that particular 2Mb/s circuit. This will continue until the customer receives the rental of the 2Mb/s circuit free for that particular month.

How does PPM work?

The PPM data collection units are remote units used at various sites. Each unit monitors a 2Mb/s circuit and passes on the collected information back to a card that is fitted in a central rack.

The central rack is connected via a TCP-IP connection to TMNET. All alarm and performance-monitoring information is passed on from the PPM equipment to TMNET.

TMNET is a WAN that is used to connect various transmission sites in a region to the RNOC. The WAN is made up of Hubs, routers and a server.

At the RNOC a element manager is used to gather all the information for the various sites in a region. The collected information is passed on to a RTOM (Real Time Operating Machine) were all alarm information is processed.

The processed alarm information from the RTOM is then passed on to a MMI (Man Machine Interface) which converts the information into customer views at the NNOC.(The drawing on the next page illustrates the explanation above)




Page 85



Overview, Continued

PPM




Page 86



Overview, Continued

PPM Management

Management of the PPM equipment can de done at various levels.

The operator at the NNOC has control of all the PPM equipment throughout the country.

A person situated at the RNOC has control of all the PPM equipment within a partial region. This is done via the element manager.

While the maintenance staff at a particular site, has only access to their own equipment.This is done via a serial connection (F-interface) on the equipment.

Unix TMN GatewaySNMP / HPOpenview

NT EM

Region

Sub-networks

Racks

Cards

Monitoring Units

LAN

Serial

ECT

PPM MANAGEMENT



Page 87



Exercise

Question 1 What does acronym PPM represents?PPM stands for Primary Performance Monitoring.

Question 2 What is the primary function of PPM?To measure the performance of the 2Mbit/s signals.

Question 3

Quote “Chance favours the prepared.”

Louis Pasteur



Page 88


Chapter 5 Installation




Page 89




Participants, when asked a question will correctly: list the various steps required to do a 2Mb/s installation according to

Telkom work instructions

explain the various steps required to do a 2Mb/s installation according to Telkom work instructions

name the various steps of fault localisation.



Page 90



Introduction

Please Note When referring to the points mentioned in this chapter please refer to Handout 1. Handout 1 is work instruction “Bring into service testing and maintenance standards for 2048 kB/s digital circuits and leased line “ – XX-ZZ1111

Steps to follow when doing 2Mb/s testing for installation or maintenance

Below is steps to follow when doing a 2 Mb/s installation:1. Wire the circuit.The information that is required can be obtained from your planning office.Make sure all stations are wired. Should a station be a DXC the NNOC can create the connection.

2. Do a B to A direction and a A to B directionCheck Bit Error Rates at PRPS 10-15

3. Check the 2Mb/s Pulse Shape according to the ITU-T G.703 Mask.

4. Check the 2Mb/s Signal level as specified in ITU-T G.703.

5. Check the S1 (15-munute test) and S2 (24-hour test) Performance criteria as specified in ITU-T M.2100.

6. Check the Synchronisation, as per Telkom's Master Plan and policy for synchronisation of the network. (PO-PF3009 refers.).

Please Note Do the above steps according to steps in section 4.3 of the above mentioned work instruction.

Take note of the following settings :

Abbreviation Definition

MFF MultiframeMFA CRC and multiframeDFF No CRC but includes multiframeCAS NFAS TS 16 information signallingCCS No MFA – No allocated signalling



Page 91



Fault localisation

Introduction When attending to a callout there are basic and generic steps that should be followed.The steps are listed below.

Please Note These steps are a guideline to assist you but each situation you encounter might differ from the steps below. Should they differ use your discretion.The steps are divided into 2 sections namely: Gather information and dispatch technician /or resolve the fault.

Dispatched technician to resolve the fault.

Fault localisation steps

Gather information and dispatch technician or resolve the fault

Establish all the relevant details of the circuit.

Determine the physical routing and the endpoints of the route. (Check for SDH and radio links in the route.)

Write down all the relevant information

Write down the condition of the fault, type of fault and exact diagnostics of the fault.

Establish the direction of the fault .(if it is an A-B or B-A)

Determine the section in which the fault occurred.

Establish if it is a manned site or a remote site.

If the fault cannot be resolved remotely, dispatch a technician to pinpoint the fault. Transfer all the gathered information to the technician.

Dispatched technician to resolve the fault. Before going to the site of the fault make sure you are in possession of:

References to enter the site

The correct spares for the equipment you are working on.

The relevant test equipment with the correct software versions.

Please Note The fault must be proved and the equipment must not be reset before proving the fault. Use the required testers to check the condition of the line without breaking or resetting the line.

Record the cause of failure and fix the fault.

Relay the information to the NNOC by using the Core Network Field operations escalation procedure.

Confirm the fault is clear.

Make sure traffic is running correctly and the client is satisfied.

Leave the site in a good running order according to Telkom specifications.



Page 92


Chapter 6 Practical tasks

Exercise

Question 1 Name the steps to follow when doing a 2 Mb/s installation: Wire the circuit.

The information that is required can be obtained from your planning office.

Make sure all stations are wired. Should a station be a DXC the NNOC can create the connection.

Do a B to A direction and a A to B direction

Check Bit Error Rates at PRPS 10-15

Check the 2Mb/s Pulse Shape according to the ITU-T G.703 Mask.

Check the 2Mb/s Signal level as specified in ITU-T G.703.

Check the S1 (15-munute test) and S2 (24-hour test) Performance criteria as specified in ITU-T M.2100.

Check the Synchronisation, as per Telkom's Master Plan and policy for synchronisation of the network. (PO-PF3009 refers.)

Question 2 What is the bit rate of the test signal when brining a circuit into service?PRBS of 215 -1Where must the measurement be taken?At the point of interconnect.

Question 3 Before proceeding to a site what must the dispatched technician be in possession of?References to enter the siteThe correct spares for the equipment you are working on.The relevant test equipment with the correct software versions.

Quote “Concentrate first on doing the right things, then on doing things right. There is nothing so wasteful as doing the wrong things well ”

Peter Drucker



Page 93



Chapter 6 Practical Tasks



Page 94




Participants, when: Participating in a simulation exercise will correctly install and test a

2Mb/s circuit according to Telkom Work Instructions.



Page 95



Task 1

2 Mb/s installation steps

On installing the 2 Mb line do the steps below on your tester:(This is in accordance with your Work Instruction)

Step Action

1 Using the 'Test Configuration' menu set up the tester as required for a framed/unframed, in-service/out-of-service test.

2 Using the 'Test Pattern' menu, select the required/suitable test pattern.

3 Using the 'Other Features' menu, set up the 'Meas. Configuration 1 and 2', 'Error Injection', 'Other Parameters', 'Alarm Generation’ and 'Send Frame Words' options as required.

4 Connect the tester to the appropriate test points or other tester, depending on situation.

5 Using the 'Measurement Results' menu, start the test. Now, using the 'Page-up' and 'Page-down' buttons, scroll through the various screens and(a) after 30 seconds, record the number of errors, depending on the test type and setup, for each of the following:

-Code-CRC-Frame-Multi-frame-Bit-Ebit

(b) allow the test to run for the required time, depending on the necessary BER for this particular test, and using the G826 screen, record the following:

-EB-BBE-ES-SES-UAS-AS

(c) check the 'ALM/SIG' menu for the following error counts and record them:

-LOSS-AISS-LOFS-FALM-MFAL




Page 96



Task 1, Continued

Step Action

(d) check the 'ALM/SIG' menu to measure the level deviation as follows:

- +LVL in dB- -LVL in dB- LPP in dB

(e) check the 'Block Error' menu for the following:-Block Size-# of Blocks-Block Errors-BLK ERR Rate

(f) check the 'Frequency' menu and measure-RCV/Hz-Max/Hz-Min/Hz-CKSLIP- +Wander- -Wander

Use the 'Other Measurements' menu option to check the following:

-Received Data-FAS Words (if applicable)-MFAS Words (if applicable)-Pulse Mask Analysis-Histogram Analysis-Propogation Delay (NB to first

change the test pattern to AIS for this if the test type allows)

6 Now, using the ERR INJ button, inject a few errors and note the incremental counter values for the applicable menus above.

7 Using the 'Other Features' menu, generate a 'FAS Distant' alarm, and check the 'Other Measurements' menu to monitor the 'FAS Words'. What do you notice? Now repeat this test for 'MFAS Distant', 'AIS' and 'T/S-16 AIS' respectively. What do you notice?

8 Now, using the 'Send Frame Words' menu, change the E-bits, NFAS bit 3 and MFAS bit 6 respectively, and note the alarms.Double check these alarms using the 'Alarm Generation' menu and the 'View FAS Words' menu.



Page 97


Chapter 7 Acronyms and verification

Chapter 7 Acronyms and Verification



Page 98



Acronyms

AIS Alarm Indication SignalA signal indicating that a transmission fault was detected at or upstream from the transmitting terminal.

ATM Asynchronous Transfer Mode The more detailed description of ATM is a transfer mode in which the information is organised into fixed length cells. Fixed length cells have very little chance of being corrupted compared with variable length frames, thus allowing for the efficient transport of all kinds of traffic including data, voice and video. The transmission is called asynchronous because the recurrence of cells containing information from an individual user is not necessarily periodic.

BER Bit Error RateThe number of received bits that are in error, relative to a specific number of bits received, such as 1 errored bit per 106 bits.

CAS Channel associated signalling

CCS Common Channel Signalling.A signalling technique in which the signalling information for a line or trunk is transmitted over a dedicated data link, rather than on per trunk or per line basis. CCS networks use the Signalling System 7 protocol to format the signalling transmissions.

CRC-4 An error checking method that treats the bits of a message as a dividend divided by a predetermined divisor. The remainder of the division is transmitted with the message and used to check the message integrity. A 4-Bit Cyclic Redundancy Check remainder, the product of a CRC calculation using a 5-bit CRC divisor polynomial.

DSSU All Digital Secondary Switching Unit (DSSU) in a local exchange area are interconnected.

ISDN Integrated Services Digital NetworkA switched network providing end to end digital transparency where voice, data, and video services are provided over the same transmission and switching facilities.




Page 99



Acronyms, Continued

FAS The transmit and receive ends of the transmission path are synchronized with the aid of the frame alignment signal (FAS = Si0011011) which is transmitted in TS0 of every second frame (that is, frames 0, 2, 4, 6, and so on).

NFAS The “non-frame alignment signal” (NFAS) is transmitted in TS0 of the alternate frames (that is, frames 1, 3, 5, 7 and so on).

ITU-T International Telecommunications Union-Telecommunications.The agency responsible for the international interworking of telecommunication services. The ITU-T is under the authorisation of the United Nations and their interest lies with WAN’s and telecommunications.

LNC Local Node Centre

NE Network Element

PBX Private Branch ExchangeA user owned customer premise located telephone exchange that connects the customer located with the public telephone network.

PCM Pulse Code Modulation.A technique for converting an analogue signal such as voice into a digital bit stream for transmission. In this technique the instantaneous value of the analogue signal is converted into a binary code which is transmitted as a serial bit stream. Applied to telephony the sampling rate for converting analogue voice to digital format is 8000 times per second.

PRC Primary Reference Clock

RAI Remote Alarm Indication

SDH Synchronous Digital Hierarchy.An ITU-T defined hierarchy of optical transmission rates based on fundamental building blocks of 155.520 Mbps. SDH can accommodate SONET and European transport hierarchies.




Page 100



Acronyms, Continued

SMF Each CRC-4 multiframe, which is composed of 16 frames numbered 0 to 15, is divided into two 8-frame Sub-Multiframes (SMF), designated SMF I and SMF II which signifies their respective order of occurrence within the CRC-4 multiframe structure. The SMF is the Cyclic Redundancy Check-4 (CRC-4) block size (i.e. 2048 bits)

TNC Transit Node Centre



Page 101

YV, -0001-01-03,

Page: 8A: Sub-Multiframes (SMF)



Verification

The following Subject Matter Experts have verified the content of this note:

Andrew ScottNetwork Support Manager

Corrie LourensOperations Support Specialist (Advanced)

Peter StoreyOperations Support specialist

Mario MazzoliRadio site manager



Page 102

Download - 4989 2Mb Ins & Maintenence

Top Related