Subject: Transmission and Switching systems

Topics: 1-Line encoding. 2-PLMN. 3-Blocking Models And Loss Estimation

By:Tanzeel ur Rahman 305-19053Ishtiaq Anwer 305-19064Touseef Iqbal 305-19065Qaiser Shamraiz 305-19055Abid Ali 305-20013Abbas Ali 305-20007

Bs Telecommunication Engineering(Final Semester)

Contents:

Chepter-1 Line Encoding Techniques1.1 Introduction1.2 Line Coding Schemes/ Line Coding Methods

1.2.1 Unipolar1.2.2 Polar1.2.3 Bipolar1.2.4 Multilevel

Chapter-2 PLMN2.1) Introduction:2.2) Access to PLMN services:2.3) Specifications of PLMN:2.4) Objectives of a GSM PLMN:2.5) Management infrastructure:

Chepter-3 Blocking Models and Loss Estimation

3.1 Introduction3.2) Loss Systems

3.2.1) Lost Calls Cleared(LCC):3.2.2) Lost calls returned (LCR):3.2.3) Lost calls held (LCH):

Chepter-1 Line Encoding:

1.1) Introduction: Line coding is also known as digital to digital conversion. This means converting a sequence of data bits (text, numeric, audio, or video) into a digital signal, at the sender end, then recovering the original bit sequence from the signal, at the destination end.

The goal is to increase the data rate (information flow) while decreasing band rate (better utilization of channel BW, cheaper links).

There are many line encoding methods that are in use e.g.

NRZ,AMI,B6ZS,B8ZS,BAMI,HDB3,2B1Q,4B5B,8B10B,Manchester Encoding.

But we discuss only HDB3 2B1Q techniques in detail that are assigned us as presentation topic and other techniques will be discuss literally as a basic how know.

1.2) Line Coding Schemes/ Line Coding Methods:

Figure 1.1 Line Coding

1.2.1)Unipolar:

NRZ (non return to zero) A unipolar is a two-voltage-level signal that typically swings between zero Voltage and + V. Digital data is represented as follows.

‘0’ Bit by a voltage of 0 volts‘1’ Bit by a voltage of + volts

No signal return to zero level at the mid of bit.

Figure 1.2 Unipolar NRZ scheme

1.2.2) Polar:

2 voltage levels are used for digits encoding –ve and +ve.

I. NRZ-L(None Return to Zero leve):

+ve volt encodes 0,

–ve volt encodes 1 Very sensitive to polarity change, if happened, all 0’s become 1’s and vice versa.

II. NRZ-I(None Return to Zero Invert): instead of using voltage level for encoding the notion of transition is used.

At the bit start: transition exists encodes 1

No transition encodes 0 In NRZI encoding digital data is represented as follows.

‘0’ bit by voltage of 0 volts‘1’ bit is represented by 0 volts or +V volts depending on the previous level. If the previous voltage was 0 volts then ‘1’ bit will be represented by +V volts, however if the previous voltage was +V volts then the ‘1’ bit will be represented by 0 volts.

Figure 1.3 Polar NRZ-L and NRZ-I schemes

III. RZ: Return to Zero:

Uses 3 levels of voltage to encode a digit: -ve, 0 , +ve. There is a mid bit transition to return to 0, from whichever level it was before.In RZ encoding the digital data is represented as follows.‘0’ bit by a voltage of 0 volts.‘1’ bit by a voltage of + volts during the first half of bit and 0 volts during the second half.

Figure 1.4 Polar RZ schemes

IV. Biphase:

Combines RZ and NRZ-L. Still there is a mid bit transition where the duration of theBit is divided into two levels one in the 1sthalf of the bit and a different one in the2ndhalf.

Two encodings:i. Manchester Encoding (ME):

Transition in the middle of the bit period.Mid-bit transition serves as clock and data1 = low-to-high transition0 = high-to-low transition

ii. Differential ME:Mid-bit transition is clockData is encoded at the beginning of the bit period0 = transition at beginning of bit period1 = no transition at beginning

Figure 1.5 Polar biphase: Manchester and differential Manchester schemes

1.2.3) Bipolar:

Bipolar Alternate Mark Inversion (AMI).

Use 3 voltage levels –ve, 0, +ve. One digit value (say the 0) is always encoded using the zero voltage level, the other (in this case the 1) encoding alternates between +ve and –ve voltage

Pseudo ternary:

An AMI, with voltage alternation for sequence of 0’s instead of 1’s.

Figure 1.6 Bipolar schemes: AMI and pseudo ternary

1.2.4) Multilevel Schemes:

2B1Q (Two binary, one quaternary):Four levels of voltage signal, each encode 2 bits. (Used in DSL lines) Digital data is represented as follows.

Figure 1.7 Multilevel: 2B1Q scheme

HDB3 (High Density Bipolar 3):

Another coding scheme is HDB3, high density bipolar 3, used primarily in Europe for 2.048MHz (E1) carriers. This code substitutes bipolar code for 4 consecutive zeros according to the following rules:

If the polarity of the immediate preceding pulse is (-) and there have been an odd (even) number of logic 1 pulses since the last substitution, each group of 4 consecutive zeros is coded as 000-(+00+).

If the polarity of the immediate preceding pulse is (+) then the substitution is 000+(-00-) for odd (even) number of logic 1 pulses since the last substitution.

Modification to Bipolar-AMI to eliminate zero strings:

–Replace any 4 zero bits (0000) with:Odd OR Even000+ -00- if previous non-zero signal was +000- +00+ if previous non-zero signal was – –Alternate (odd/even occurrence) between the two–Each replacement causes one code violation

Figure 1.9 HDB3 technique

Any 4 consecutive zeros = i) 000V if # of nonzero pulses after last substitution or is odd making total non zero pulses even ii) B00V of # of nonzero pulses after last substitution or is even making total nonzero pulses even. (ii is assumed initially)

Figure 1.10 Different situations in HDB3 technique

Chepter-2

Public Land Mobile Network:

2.1) Introduction:

Public land mobile network (PLMN) is a network that is established and operated by an administration or by a recognized operating agency (ROA) for the specific purpose of providing land mobile telecommunications services to the public.

A PLMN is identified by the Mobile Country Code (MCC) and the Mobile Network Code (MNC). Each operator providing mobile services has its own PLMN. PLMNs interconnect with other PLMNs and Public switched telephone networks (PSTN) for telephone communications or with internet service providers for data and internet access of which links are defined as interconnect links between providers. These links mostly incorporate SDH digital transmission networks via fiber optic on land and digital microwave links.

2.2) Access to PLMN services:

Access to PLMN services is achieved by means of an air interface involving radio communications between mobile phones or other wireless enabled user equipment and land based radio transmitters or radio base stations or even fiber optic distributed SDH network between mobile base stations and central stations via SDH equipment with integrated IP network services.

2.3) Specifications of PLMN:

A PLMN may be considered as an extension of a fixed network, e.g. the Public Switched Telephone Network (PSTN) or as an integral part of the PSTN. This is just one view-point on PLMN.

PLMN mostly refers to the whole system of hardware and software which enables wireless communication, irrespective of the service area or service provider. Sometimes separate PLMN is defined for each country or for each service provider. Its case is same as that of PSTN. Sometimes it refers to the whole circuit-switched system, or else specific to each country.PLMN is not a term specific to GSM. In fact GSM can be treated as an example of a PLMN system. These days many discussions are going on to form the structure of UMTS PLMN for the third generation systems.

2.4) Objectives of a GSM PLMN:

The general objective of a PLMN is to facilitate wireless communication and to interlink the wireless network with the fixed wired network. The PLMN was specified by the European Telecommunications Standard Institute (ETSI) following-up with their GSM specification. Even as times changed, the GSM PLMN objectives conceptually remained the same.

To give access to the GSM network for a mobile subscriber in a country that operates the GSM system.

To provide facilities for automatic roaming, locating and updating of mobile subscribers.

2.5) Management infrastructure:

Every PLMN organization has its own management infrastructure, which performs different functions depending on the role played and the equipment used by that entity. However, the core management architecture of the PLMN Organization is similar.

Every PLMN Organization:

provides services to its customers; Needs an infrastructure to fulfill them (advertise, ordering, creation, provisioning,...);

Assures them (Operation, Quality of Service, Trouble Reporting and Fixing,...);

Bills them (Rating, Discounting,...).

Not every PLMN organization will implement the complete Management Architecture and related processes. Some processes may be missing dependent on the role a particular organization is embodying. Processes not implemented by a particular organization are accessed via interconnections to other organizations, which have implemented these processes. The Management architecture itself does not distinguish between external and internal interfaces.

For roaming relations the following abbreviations are additionally used:

HPLMN denotes the Home PLMN (the PLMN the customer belongs to). VPLMN denotes the Visitor PLMN (the PLMN the customer is roaming in).

A PLMN requires special security measures because a wireless system is inherently more susceptible to eavesdropping and unauthorized use than a hard-wired system. Smart cards containing user data, encryption/decryption, and biometric verification schemes can minimize this problem.

Chepter-3

Blocking Models and Loss Estimation

3.1 Introduction

The service of incoming calls depends on the number of lines. If number of lines equal to thenumber of subscribers, there is no question of traffic analysis. But it is not only uneconomicalbut not possible also. So, if the incoming calls find all available lines busy, the call is said to be blocked. this is called overflow traffic.

Depending on the way in which Overflow traffic is handled, telecommunication systems can be classified as loss systems or delay systems. The behavior of loss systems is studied by using blocking models and that of delay systems by using queuing models.

The type of system by which a blocked call is simply refused and is lost is called losssystem. Most notably, traditional analog telephone systems simply block calls from enteringthe system, if no line available. Modern telephone networks can statistically multiplex calls oreven packetize for lower blocking at the cost of delay. In the case of data networks, if dedicatedbuffer and lines are not available, they block calls from entering the system. Loss systems include the following Traffic models.Poisson, Erlang B, Extended Erlang B(EEB), Engest, Binomial.

In the second type of system, a blocked call remains in the system and waits for a freeline. This type of system is known as delay system. Loss systems include the following Traffic models. Erlang C, Delay, Crommelin.

Because assigned presentation topic to us is Blocking Model and Loss Systems. So in this section loss system is described and Delay system is not discussed in the section.

These two types differs in network, way of obtaining solution for the problem and GOS.For loss system, the GOS is probability of blocking. For delay system, GOS is the probability of waiting.

3.2) LOSS SYSTEMS

Erlang determined the GOS of loss systems having N trunks, with offered traffic A,with the following assumptions. (a) Pure chance traffic (b) Statistical equilibrium (c) Full availability(d) Calls which encounter congestion are lost.

Pure Chance Traffic, the call arrivals and call terminations are independent random events. If call arrivals are independent random events, their occurrence is not affected by previous calls. This traffic is therefore sometimes called memory less traffic.

In queuing theory, systems in “statistical equilibrium” are those in which the number of customers or items waiting in the queue oscillates in such a way that mean and distribution remain constant over a long period

A system with a collection of lines is said to be a fully-accessible system, if allthe lines are equally accessible to all in arriving calls. For example, the trunk lines for interoffice calls are fully accessible lines.The lost call assumption implies that any attempted call which encounters congestion is immediately cleared from the system. In such a case, the user may try again and it may cause more traffic during busy hour.

The Erlang loss system may be defined by the following specifications.

1. The arrival process of calls is assumed to be Poisson with a rate of λ calls per hour.2. The holding times are assumed to be mutually independent and identically distributed random variables following an exponential distribution with 1/μ seconds.

3. Calls are served in the order of arrival.

There are three models of loss systems. They are:1. Lost calls cleared (LCC)2. Lost calls returned (LCR)3. Lost calls held (LCH)

All the three models are described in this section.

3.2.1) Lost Calls Cleared:

The LCC model assumes that, the subscriber who does not avail the service, hangs up the call,and tries later. The next attempt is assumed as a new call. Hence, the call is said to be cleared.This also referred as blocked calls lost assumption. The first person to account fully andaccurately for the effect of cleared calls in the calculation of blocking probabilities was A.K.Erlang in 1917.

The two most commonly used LCC models are the Erlang B model having infinite sources and Engset with finite sources.

Erlang B Model:

The Erlang B traffic model is based on the following assumptions:

• An infinite number of sources

• Random traffic arrival pattern

• Blocked calls cleared

• Hold times exponentially distributed

The Erlang B model is used when blocked calls are rerouted, never to come back to the original trunk group. This model assumes a random call arrival pattern. The caller makes only one attempt; if the call is blocked, then the call is rerouted. The Erlang B model is commonly used for first-attempt trunk groups where you need not take into consideration the retry rate because callers are rerouted, or you expect to see very little blockage.

The following formula is used to derive the Erlang B traffic model:

Where:

• B(N,A) is the probability of blocking the call.

• N is the number of circuits/trunks.

• A is the traffic load.

LCC Example 1:

Consider a trunk group with an offered load 4.5 erlangs and a blockingprobability of 0.01. If the offered traffic increased to 13 erlangs, to keep same blocking probability, find the number of trunks needed.

Sol. Given data : A = 4.5, B = 0.01From the following Figure or from the Table below,No. of trunks = 10For the increase in load of 13 erlangs, from the figure 8.6No. of trunks required = 21 for same B = 0.01 requiredHence B(10, 4.5) = B(20, 13) = 0.01

LCC Example 2.

A group of 7 trunks is offered 4E of traffic, find (a) the grade of service (b)the probability that only one trunk is busy (c) the probability that only one trunk is free and (d) the probability that at least one trunk is free.

Engset Model:

The Engset model is based on the following assumptions:

• A finite number of sources

• Smooth traffic arrival pattern

• Blocked calls cleared from the system

• Hold times exponentially distributed

The Engset formula is generally used for environments where it is easy to assume that a finite number of sources are using a trunk group. By knowing the number of sources, you can maintain a high grade of service. You would use the Engset formula in applications such as global system for mobile communication (GSM) cells and subscriber loop concentrators.

3.2.2) Lost calls returned (LCR):

In LCC system, it is assumed that unserviceable requests leave the system and never return.This assumption is appropriate where traffic overflow occurs and the other routes are in othercalls service. If the repeated calls not exist, LCC system is used. But in many cases, blockedcalls return to the system in the form of retries. Some examples are subscriber concentratorsystems, corporate tie lines and PBX trunks, calls to busy telephone numbers and access toWATS lines. Including the retried calls, the offered traffic now comprise two components viz.,new traffic and retry traffic. The model used for this analysis is known as lost calls returned(LCR) model. The following assumptions are made to analyse the CLR model.1. All blocked calls return to the system and eventually get serviced, even if multipleretries are required.2. Time between call blocking and regeneration is random statistically independent ofeach other. This assumption avoid complications arrising when retries are correlated to eachother and tend to cause recurring traffic peaks at a particular waiting time interval.3. Time between call blocking and retry is somewhat longer than average holding timeof a connection. If retries are immediate, congestion may occur or the network operation becomesdelay system.Consider a system with first attempt call arrival ratio of (say 100). If a percentage B(say 8%) of the calls blocked, B times λ retries (i.e. 8 calls retries). Of these retries, however apercentage B will be blocked again.Hence by infinite series, total arrival rate is given as

where B is the blocking probability from a lost calls cleared (LCC) analysis.The effect of returning traffic is insignificant when operating at low blocking probabilities.At high blocking probabilities, it is necessary to incorporate the effects of the retruning trafficinto analysis.

3.2.3) Lost calls held (LCH):

In a lost calls held system, blocked calls are held by the system and serviced when the necessaryfacilities become available. The total time spend by a call is the sum of waiting time and theservice time. Each arrival requires service for a continuous period of time and terminates itsrequest independently of its being serviced or not. If number of calls blocked, a portion of it islost until a server becomes free to service a call. An example of LCH system is the time assignedspeech interpolation (TASI) system.

LCH systems generally arise in real time applications in which the sources arecontinuously in need of service, whether or not the facilities are available. Normally, telephone

network does not operate in a lost call held manner. The LCH analysis produces a conservativedesign that helps account for retries and day to day variations in the busy horn callingintensities. A TASI system concentrates some number of voice sources onto a smaller numberof transmission channels. A source receives service only when it is active. If a source becomesactive when all channels are busy, it is blocked and speech clipping occurs. Each speech segmentstarts and stops independently of whether it is served or not. Digital circuit multiplication(DCM) systems in contrast with original TASI, can delay speech for a small amount of time,when necessary to minimize the clipping.LCH are easily analysed to determine the probability of the total number of calls in thesystem at any one time. The number of active calls in the system at any time is identical to thenumber of active sources in a system capable of carrying all traffic as it arises. Thus thedistribution of the number in the system is the poisson distribution. The poisson distributiongiven as