1352100599072-report control circuit protection

8/12/2019 1352100599072-Report Control Circuit Protection

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Protection scheme for Control Circuits Page 1

Recommendations on Control Circuit Protection Scheme

Back ground: A two member committee was formed by Railway Board vide letter No. No.2010/Tele/9(1) dated 19.01.2012 with following terms of reference:Identify most suitable protection scheme for control communication circuits includingSCADA. This report has been prepared in compliance to these directives.

1. Introduction and Scope

The control circuits are worked on 64 kbps channels configured in conference modeand multiplexed by Primary Add-Drop Multiplexers (to be referred as PD-MUXhenceforth ) and resultant E1 is carried on STM1 on OFC backbone. This E1 threadextends to the control section boundary/ division boundary. Time-slots other thanthose carrying omnibus control circuits carry point-to-point circuits. The PD-MUXes

are in linear topology, with E1 level ring protection on STM backbone on OFCnetwork at the end of the thread.

The committee has examined protection schemes followed in various PD-MUXes.Also, certain common issues that are affecting reliability/availability of controlcircuits irrespective of protection scheme are discussed. Based on this study, featuresof the most suitable protection scheme taking care of reliability/availability as well areidentified. Since a large number of PD-MUXes of various makes and versions arealready working, ways to implement/migrate to the most suitable protection schemeon various types of PD-MUXes are recommended.

In addition to this, certain other recommendations related to monitoring and periodictesting of control circuits to ensure availability are also included in the report. As IP

based control communication is still under development, recommendations given inthis report are relevant in the present context of Primary Multiplexer based controlcommunication.

2. Protection of control circuits on OFC sections

Ring/ loop protection mechanism is in general implemented in two different ways:

Approach-A : Use of LPC (Loop protection card) and 2 E1s at every station for alltime-slots protection

Approach-B : Ring protection mechanism using spare time slots in working E1

These are discussed below

2.1 Approach-A : Use of LPC and 2 E1s at every station for all time-slots protection

Please refer figure-1


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In this approach, the E1 carrying 30 time-slots and designated as working E1 is takenfrom East-1 port of LPC (Loop Protection Card) of PD-MUX. This is carried overSTM1 Add-drop MUX to next station and connected to West-1 port of LPC card ofPD-MUX. Like-wise the E1 thread continues.

Another E1, designated as protection E1, taken from East-2 port of LPC of PD-MUXis also carried via the same Add-Drop STM1 to next station and connected to West-2

port of LPC of the PD-MUX. Like-wise, the second thread of E1 continues.

From the terminal station, E1 from loop-back port of LPC is carried via protection pathon STM back-bone on different route to the originating station and connected on loop-forward port of LPC.

PD-MUX PD-MUX PD-MUX PD-MUX PD-MUX

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

PCM2 PCM2 PCM2 PCM2 PCM1 PCM1 PCM1 PCM1

Protection on STM backbone via different route

Fig.1 Use of LPC and 2 E1s at every station for all time-slots protection

Station-A Station-B Station-C Station-D Station-E


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In the above figure, STM1 Add-Drop MUXes are not shown to avoid congestion

In case of failure of E1 due to port failure/OFC cut between terminal station andimmediate next station (for example between station-A and station-B) switching in

LPC takes place as shown in fig.2 :

a. At station-A, PCM2-to-E1 and LF-to-E2 connections are interrupted; instead, LF-to-PCM2 is connected in LPC

b. At station-B, W1-to-PCM1 and W2-to-E2 connections are interrupted; instead,PCM1-to-E2 is connected in LPC


LPC

E

2 E

1

W

2 W

1

L

B LF

LPC

E

2 E

1

W

2 W

1

L

B LF

LPC

E

2 E

1

W

2 W

1

L

B LF

LPC

E

2 E

1

W

2 W

1

L

B LF

LPC

E

2 E

1

W

2 W

1

L

B LF



Fig.2 Switching in LPC in case of failure of E1/ OFC cutbetween terminal station and immediate next station



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In case of failure of E1 due to port failure/OFC cut between two intermediate stations,(for example between station-C and station-D) switching in LPC takes place as shownin fig.3 :

a. At station-C, PCM2-to-E1 and W2-to-E2 connections are interrupted; instead,W2-to-PCM2 is connected in LPC b. At station-D, PCM1-to-W1 and W2-to-E2 connections are interrupted; instead,

PCM1-to-E2 is connected in LPC


LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F



Fig.3 Switching in LPC in case of failure of E1/ OFC cutbetween two intermediate stations



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Thus, in this arrangement of using LPC and 2 E1s at every station, all time-slots of theE1-thread are protected. Loop-back protection is end to end on STM back-bone viadifferent route.

General terms are used for description of ports. The above arrangement is used byPUNCOM PD-MUXes.

Features: The scheme has following features.

It offers protection to all the 30 channels. The switching to alternate E1 takes place simultaneously at two stations. (On

either side of the cable cut section). Therefore the switching can be madevery fast.

The protected sections can be subdivided into smaller sections to take care

of multiple cable faults without the use of additional PD-MUX.

Problem of overreach:

In case of multiple cable cuts in a single protected section, the stations on either sideof the isolated section (section isolated by two cable faults) get directly connected.This will cause extension of point to point circuits between these two non-adjacentstations which were meant between adjacent stations only.

Figure 4 shows how station-C and station-C1 get isolated and also over-reach of time-slots from station-B to station-D

Consider time slot 20 is used for working point to point telephone between stations Aand B, and the same time slot is used for working Point to point telephone betweenstation C and D, then in case of faults between station B and C & between stations Cand C1, direct connection between point to point telephones at stations B and D will

be established.

This fact has to be considered if such telephones are being used for train working.Similar problem shall occur on other point to point circuits. Special precaution need to

be taken if on point to point circuits, axle counters channels are worked. To avoid this,ether the time slot or the address setting for modem to modem communication must

be different for axle counters in a sect ion. For example if two axle counters are usingsame time slot 21, then they must not use the same address setting.


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2.2 Approach-B : Ring protection mechanism using spare time slots in working E1

Please refer fig.5

In this approach, all the conference circuits are always protected whereas only those point-to-point circuits get protected which are mapped to spare time slots in E1 for protection purpose. Therefore, in this approach, the protection has to be planned attime slot level for point to point circuits.

This approach is followed in Webfil PD-MUX and Nokia PD-MUX having loop- protection module. In case of older versions of Nokia/ Webfil where automatic loop- back is not supported, protection E1 is kept ready for patching at test-room

PD-

MUX PD-

MUX PD-

MUX PD-

MUX PD-

MUX

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

LPC

E

2 E

1

W

2 W

1

L

B L

F

PCM2

PCM2

PCM2

PCM2

PCM1

PCM1

PCM1

PCM1


Fig.4 Multiple Cuts Scenario : Isolation and overreach problems

Station -A Station-B Station-C Station-D Station-E

PD-

MUX

LPC

E

2 E

1

W

2 W

1

L

B L

F

PCM2

PCM1

Station-C1


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Problem of over reach:

This scheme shall also lead to overreach in case of multiple cable cuts asexplained in case of PUNCOM MUX. However, this scheme allows for

protection of selected time slots only. Therefore the overreach in sensitivecircuits like station to station hot line telephone can be avoided by disablingautomatic protection for those circuits while continue to have automatic

protection for control circuits. In case of applications like axle counter whichrequire automatic protection switching for improved reliability purpose,

judicious time slot allocation can prevent such problem.

3. Common issues affecting control circuit reliability/availability

Irrespective of the approach being followed for protection, certain common issues thatare affecting reliability/availability of control circuits are discussed below.

PD-

MUX P2

PD-

MUX

PD-

MUX

PD-

MUX PD-

MUX


Fig.5 Ring-protection using spare time-slots in Working E1


P

1 P

2 P

1 P

2 P

1 P

2 P

1 P

2 P

1


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3.1 Common E1 thread for adjacent control sections

In some of the sections, the E1 thread carrying control circuits are not coinciding withcontrol section boundaries. For example, in figure 6, A-P is one control section and P-

B is another, but they are worked on common E1-thread from A to B. Therefore, anyfailure of PD-MUX on section A to B is affecting entire thread.

Figure 6 Problem of Common E1 thread for adjacent control sections

3.2 Delay in Protection Switching on PD MUX NMS

Automatic Protection Switching on STM backbone is within 50 msec. However, thereis no laid down time for automatic protection switching for PD MUX resulting in suchtime being in excess of 1.5 second, which may result in failure of axle counters using

point to point voice channel.

3.3 Non-uniformity of PD-MUXes on multi-segment control section

On multi-segment control sections, variety of PD-MUXes are in place. For example,on control sections where spur links take-off or on sections involving junctionstations, different segments of the same control in different directions from the

junction station may have different PD-MUXes : on one segment-Nokia, on another-Webfil; on yet another segment PUNCOM .

Such a situation arises due to commissioning of PD-MUXes on these segments underdifferent works.

Such an arrangement complicates time-slot management on the control section.


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3.4 Inadequate/Improper monitoring of PD-MUX NMS

In some of the divisions, PD-MUX NMS is not located in test room and in somecases, it is not being monitored on 24x7 basis. Consequently, interpretation of alarms

and corrective action, pin-pointing the faulty section are delayed.

3.5 Over-reach of point-to-point circuit in case of PD-MUX failure/switch-off

In case of failure of PD-MUX/switch-off or pass-through of KLM carrying the E1thread on STM1, over-reach of time-slots is inevitable. Consequently, point-to-pointcircuits across stations may be carried to unwanted destination.

4. Features of most suitable protection scheme

Considering the features of the two approaches discussed in section-2 andreliability/availability issues discussed in section-3, features of most suitable

protection scheme are identified as :

1. E1 thread carrying control circuits on work-path shall terminate at control section boundary; it shall not be continued on adjacent control section.

2. However, other omnibus circuits like EC, SCADA etc. are to be patched at VFlevel across control sect ion boundaries and picked-up in the fresh E1 thread.

3. E1 level ring protection on STM backbone on OFC network at the end of thethread shall be via different path.

4. Also, wherever feasible, additional protection path (3 rd path) shall be provided onSTM backbone to ensure availability of circuits in case of simultaneous cuts onWorking Path and Protection Path.

5. Protection has to be automatic through PD-MUX NMS6. Overreach problem is to be avoided in case of PD-MUX failures/switch-off/KLM

pass through on STM17. Overreach problem is to be avoided in case of multiple OFC cuts8. Smaller sections for loop protection to be implemented to the extent possible so

that multiple cuts scenarios have better availability of control circuits9. Uniformity of PD-MUXes on all segments to be ensured on multi-segment control

section

Since there is large installation base of PD-MUXes of various types/versions and nofurther technical developments are taking place in this segment, migration to theabove scheme of most suitable protection is discussed in the next section for allexisting types/versions of PD-MUXes


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5.3 Sections with Coral PD-MUX

As mechanism for loop protection is not available in Coral PD-Mux, external LPMmodule of Webfil or LPC module of PUNCOM can be procured and protectionimplemented as shown in Fig.7. However, as this scheme provides complete time-slot

protection, which can cause overreach of point-to-point circuits, time-slots shall be programmed leaving spare slots to avoid overreach.

6. Summary of recommendations

Considering the different protection schemes followed as described in section-2 andthe issues reliability/availability as described in section-3, the committee recommendsthe most suitable protection scheme with features as described in section-4 and

implementation strategy described in section-5. Accordingly, following action-plans by Zonal Railways, RailTel and RDSO are recommended:

6.1 Action by Zonal Railways

a. All Zonal Railways shall ensure uniformity of PD-MUX equipment on one controlsegment.

b. Instead of single thread E1 covering entire section, additional PD-MUX shall be provided at control circuit boundaries (please refer figure 8). By providingadditional PD-MUX, different E1s can be used for two different control sections.This work should be carried out by the divisions themselves as PD-MUX NMS andchanneling plan are under their control.

Figure 8 Additional PD-MUX at Control Boundary

c. Migration to most suitable protection scheme as described in section 5 to beimplemented as per the make of PD-MUXes.

d. PD-MUX NMS of every control section of the division should necessarily belocated in Test room and be manned round the clock by staff trained in the NMS.


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e. Smaller sections for loop protection may be implemented to the extent possible sothat multiple cuts scenarios have better availability of control circuits

f. PD-MUX and its NMS to be maintained by Divisions only as many point-to-pointcircuits are concerned with safety of train operations.

g. Since it is only E1 level handover to PD-MUX and no scope for channel-wisevisibility / alterations within E1 by RailTel and also SDH NMS is with RailTelwith various options on STM backbone for protection of E1s, Railways mayconsider handing over of STM1 maintenance to RailTel on mutually agreeableterms and conditions.

6.2 Action by RailTel

a. RailTel should provide protection path via SDH backbone from each controlcircuit boundary to test-room. This needs to be provided on a path different thanworking path of the control-E1 thread so that OFC cut shall not affect the

protection E1. b. RailTel NOC should share protection path details of control circuit E1s with

concerned Zonal Railways.c. Wherever feasible, RailTel shall provide alternate path in addition to back-bone

protection path for the control circuits E1s so that availability of control circuitsis greatly improved

d. RailTel should share fault escalation numbers and contact persons to respond incase of control circuit failures. Top most priority shall be given to restoration of

control circuits.e. Monthly testing program of control circuits protection shall be prepared and

implemented considering the number of divisions being served by RailTel-NOC.This can be carried out from Test Rooms by asking for port-down on WP (Working Path) in the E1 thread and checking for protection-E1 availability.

f. RailTel should provide required STM backbone protection when Railways arecarrying out the work of split-up of E1 thread at control boundaries, split-up intosmaller loops etc.

6.3 Action by RDSO:

Telecom Directorate-RDSO should standardize the protection switching time for PDMUX in consultation with Signal Directorate-RDSO and amend the specificationsuitably.

RDSO to issue typical channeling plan for various cases of PD-MUX deploymentonce the present report is accepted by Railway Board.

(P.V.Sreekanth) (Yashpal Singh Tomar)General Manager(NPM)RailTel Director Tele/RDSO

1352100599072-report control circuit protection

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