6. INSTALLATION OF STM-1/STM-16 ADM AND MADM

OPTICAL FIBRE SYSTEM

The STM-1 ADM equipment available at RTTC Trivandrum is in the

rack sub rack form. The sub rack is first fitted to the rack and the power supply is

extended to the sub rack connection field. The power supply is extended from the

RTTC power plant and its standard value is -48 ± 2V. Before connecting the

power supply to the sub rack. its value is verified using multimeter. The metallic

part of the sub rack is connected to the RTTC ring earth, which has an earth

resistance value of 0.5 Ω.

The ADM-1 sub rack has 3 identical mother boards and each

mother board has 6 slots. Thus altogether 18 slots are available in a sub rack. A

single sub rack can be configured in different ways. For this present link design

the mother board is equipped In the 1.5 mother board configuration i.e. one and

half portion of the mother board is equipped with cards/ modules. The ADM-1

equipment has three types of cards they are

1) OEO card

2) TEX 1 card

3) PS card

The OEO card is also called as aggregate card; its name stands for

optical- electrical- optical card. Its main function is optical to electrical and

electrical to optical conversion. This card houses the optical source and the

optical detector. The ADM card has two ports labeled as S1 and S2 and each

port has separate TX. and Rx. Points. The second port also has separate Tx.

and RX. Points. The OEO card can also cater for 21 E1s. The ADM modules

can de installed in slots 3, 4, 9,10,15,16. Insert ADM-1 module in to any one of

the slots slowly through the guiding portion till a click hears. Before inserting the

module route the optical cables (optical patch cords) through the cable guides

as circular loops. The figure of an OEO card with two optical ports is shown

below in figure 8.

Figure.8 OEO card with two optical ports

TEX-1 card stands for tributary extension card it is a mux/De mux

card. One TEX-1 card can cater for 21 E1 s. For an ADM-1 system it has 2 such

cards. The TEX cards can be installed in any of the following slots 5, 6,11,12,17

and 18.

The PS module can be installed in any one of the following slots i.e. slots

1, 2,7,8,13,14. The PS module is a DC to DC converter which converts the -48 V

DC into + 5V 7A and -5V 3.1A. The DC to DC converter operates at 100 MHz.

The normal operating temperature of the PS is -25 to 55 o C. The nominal

voltage is -48 V to -60 V and the operating range is -36 V to -75 V. After

installing all the required cards in to the appropriate mother boards and switching

on the sub rack it appears as shown below in figure 9.

Figure.9 An equipped ADM-1 Sub rack

For this present link design 3 such ADM-1 sub racks are installed at

the RTTC Transmission lab and connected using the G652 cable in the 2 fibre

ADM-1 ring form to provide the protection switching facility to the ADM-1 SDH

ring.

7. DESIGNING OF THE SDH ADM-1 LINK

The basic steps involved in SDH link design are (1)

power budgeting and (2) system rise time calculations as per ITU

standards; to check whether the link work satisfactorily for its entire life time

with in the specified bit rate.

7.1. Power budgeting

The optical link power budgeting considers the total

optical power that is allowed between the light source and the

photodetector, and allocates this losses to cable loss, splice loss, connector

loss and system margin. The minimum optical power required at the

receiver for its proper working without BER is called receiver sensitivity.

The first step in power budgeting is the finding out of

the laser out put power and the receiver sensitivity of both the S1 and S2

ports. The system has an ALS function (Auto Laser

Shut down) i.e., whenever a break occurs in the cable, the laser will be

automatically shutting down. For testing, the system the ALS function is

disabled and the Tx port is directly connected to the optical power meter.

The wavelength of 1310nm is selected and the Transmitter power of the

ADM-1 for both the ports is measured. Then the trans-fiber (Tx) is

connected to a variable attenuator. Its value is initially set to a high value

say 40 dB. The other end of the attenuator is connected to the receiver as

shown in figure11(a). And the attenuator is decreased slowly till BER of 10 -9

alarm is lightened.

Figure 11(a) Receiver sensitivity measurement

ADM-1

S1 Tx Rx

S2 Tx Rx

Variable attenuator

Now the receive fiber is removed and it is directly

connected to the optical power meter as shown in the figure 11(b)

Figure 11 (b) Receiver sensitivity measurement

And the received optical power is measured. This is

the minimum power required at the receiver for its proper working without

alarm. This is called the ‘Sensitivity’ of the receiver. Again the receive fiber

is connected to the receiver and the attenuator is decreased till the alarm

again appears. This is the maximum input power that the receiver can

handle safely. The range of optical power within which the system can work

safely is called Dynamic range. The experiment is repeated for both the

ports of all the three ADM-1 equipments. The readings obtained in the

above experiment are tabulated below in table -5.

Optical Power Meter Variable Attenuator

ADM-1 Tx Rx

Laser out put, Receiver sensitivity and dynamic range measurement

Particulars Station A Station B Station C Limit (ITU)

Laser out put

S1 S2 S1 S2 S1 S2

-2 -2.3 -2.1 -2.2 -2 -2 -5 to 0 dbm

Receiver

sensitivity

-40.7 -40.9 -40.6 -40.85 -40.6 -40.7 -37 dbm

Dynamic

range

35.03 35.02 36.0 35.2 35.3 35.2 >25 db

Table-5 Experimental values of laser out put, Rx sensitivity and Dynamic

range

In the link design the worst case design is followed i.e. the lowest of

the dynamic range and receiver sensitivity will taken for further design. The

worst case values selected for the link engineering are shown below in

table-6

Dynamic range 35.02

Laser out put -2.3 dbm

Receiver sensitivity -40.6

Table-6 Selected values of laser out put and Rx sensitivity

The dynamic range shows that the receiver will be

working normally with the received power level of -40.6 dbm to -5.58 dbm

(40.6-35.02) i.e.,. The O.F.system can be installed at any point in the route

where the received power is between -5.58 dbm and -40.6 dbm.

7.1.1. Loss calculation

Component/loss parameter Measured loss Limit

Laser out put -2.3 dbm (lowest ) -5 to 0 dbm

Total Connector loss 1 db 0.5 db per

connector

Total cable loss for 80 Km 18 db 0.25 db/km

Total splice loss 4 db 0.1 db per splice

Total loss 25 db

Table -7 Loss calculation details

Normally in the BSNL link engineering design a margin of 6

to 7db will be added to the total loss so as to contribute to the expected

future losses like ageing of the component, future cable faults and future up-

gradation or shifting of the systems, path penalty etc.

Total loss = cable loss + splice loss + connector loss = 25

Margin = 7 db

Receiver sensitivity >= 25+7= -32 db

The output power available at the receiver when this system is connected

to the above spliced cable is -2.3 -32 = -29.7db

ie,. If we are installing the system as such it will be working properly for a

cable length of 80Km. And as per BSNL standard also the system can be

safely installed at any distance less than or equal to 80 Km with suitable

optical attenuator.

In BSNL the optical fiber transmission systems are

classified as Short-haul, Long-haul, Very long-haul, and Ultra long-haul as per ITU

standards. The distance range of the above systems are shown in the table below

Sl no Range of operation Application Nomenclature

1 Up to 2 km Intra-office I

2 Up to 40 km Short haul S

3 40 to 80 km Long haul L

4 80 -120 km Very long haul V

5 120- 160 km Ultra long haul U

Table-8 ITU classification of SDH systems

The optical fiber link designed using SDH O.F system

belongs to Long-haul. And according to ITU-T classification the system

belongs to the class L.1.1 Here the ‘L’ shows that it is a long-haul system

and the first ‘1’shows that the system is operating on 1310 nm and the

second “1” shows cable used is G652 cable (single-mode cable).

However if required the systems can be installed at lower distance by

connecting suitable attenuators in the receive direction.

7.2. Rise- Time budget

The power budget analysis ensures that sufficient power is

available through out the link to meet the application. Rise time budget

ensures that the link is able to operate for a given data rate at specified

BER. All the components in the link must operate fast enough to meet the

band width or rise time requirements.

The rise time of the light source is specified by the manufacturer. The typical

values of the rise time for MLM laser is 0.1-1.0 ns. The chosen fibre must

have low pulse spreading to achieve longer transmission distance. Finally

the receiver rise time should also be as low as possible.

Component Rise time Worst case value

MLM Laser diode (ts) 0.1-1.0 ns 1 ns

G 652 fibre (tf) < 3.5 ps/km 3.5

APD receiver (tr) < 0.14 ns 0.14

Table-9 The rise time data for source cable and receiver

The rise time data for source, cable and receiver supplied by the

manufacturer is shown in the above table-9

The total system rise time is given by Ts= √ ( ts2+tf

2+tr2 )

For 80 Km, tf = 3.5 X80 = 0.28ns

Ts= √ ( 1+0.282+0.142 ) = 1.048 ns

The system band width = 0.7/Ts

= 0.7/1.048 ns = 667.9 Mbps

This rise time budget shows that the selected components

can be used to design a SDH, STM-1 link because the bit rate of STM-1 is

155.52 Mbps.

Now the designed STM-1 ADM equipment link is formed by

connecting the 3 ADM-1 s as explained below. The three ADMs are named

as station A, B and C. Port 1 of station A is connected to port 2 of station B

using two fibre which is the normal path. Port 2 of station A is connected to

port 1 of station C and port 2 of station C is connected to port one of station

B using two fibers. Thus the three ADM-1 equipments are connected as

STM-1 two fibre bidirectional ring. The figure-12 shows the designed two

fibre SDH ring. The system is verified using digital transmission analyzer for

no alarm condition which shows the designed link is working satisfactorily.

The ring structure is used to provide path protection to the E1 streams in

case of link or path failure

Figure 12 SDH ADM-1 Two Fibre ring

To judge the quality of the link the E1 stream of the

system is subjected to ITU-G821 performance analysis.

8. PERFORMANCE MEASUREMENTS

Transmission errors are common in communication

systems due to different reasons like noise, interference, inter-modulation,

echoes, signal fading, equipment limitations, etc.

Though optical fiber medium is considered to be

the best medium, we know that practical media cannot be hundred percent

error free. Furthermore, the tolerance of the

disturbance depends on the type of service carried by the circuit. To check

the quality of the system as well as the medium ITU-T in its guidelines

(G821) recommends a set of tests to be taken. In BSNL a system is

declared as commissioned if and only if the system survives these tests.

The performance parameters to be tested are:

(1)Bit Error Ratio.(BER)

Bit-Error-Ratio is defined as the ratio of the number of bits

received in error to the total number of bits transmitted in a

specified time interval. The BER threshold for the SDH

system analysis is BER should be better than 10-9

(2) Error Seconds(ES)

A second with at least one anomaly or defect is called

Error Second.

(3) Severely Errored Seconds(SES)

Severely Errored Seconds is defined as the errored

seconds with BER greater than or equal to 10-3

(4) Degraded Minutes(DM)

Degraded Minutes is a group of 60 consecutive seconds

after excluding SES, with a BER of 10-6 or worse. Hence a

DM will have at least 5 errors, assuming a data rate of 64

kbit/s.

(5)Available Seconds(AS)

The measure of percentage of time for which the circuit is

available for use in an error free condition is called

Available Seconds.

(6)Unavailable Seconds(US)

If the error activity continues at an excessive level for a

significant period of time ( say 10 seconds or more )then

the circuit is considered to be unavailable. Unavailable

Seconds is a measure of percentage of time the circuit is

not available for use.

The ADM-1 optical fiber link designed for the study

is then subjected to G821 analysis for three hours continuously to ascertain

the quality and stability of the designed link. The E1 streams at the distant

station are looped back and using digital transmission analyser the G821

measurements are made. For G821 measurement connect the Tx tributary

Out put to the DTA 120 Ω Tx. port and Rx tributary to DTA 120 Ω Rx.

port .The readings obtained are tabulated below in table-10. The results

show the designed link has better performance.

Objective Measured value Limit (ITU) Remarks

%SES 0.0000 0.00025 Test taken

With E1 loop

at distant

station

%ES 0.0000 0.576

%DM 0.0000 0.023

Table-10 G821 analysis report

8.1. BER check

Loop optical output and input through a variable attenuator.

Feed PRBs (Psuedo-random Binary Sequence ). HDB3 data at tributary

input and check the BER at the tributary output. Observe for five minutes.

The results obtained is as shown is table-11 below.

Station A B C Limit

Pulse off set BER 10-9 BER 10-9 BER 10-9 BER 10-9

0 ppm 0 0 0 0

+50 ppm 0 0 0 0

-50 ppm 0 0 0 0

Table-11 BER measurement results

The results obtained showed that the link is acceptable as per

BSNL standards.

. To check the self healing property of the SDH ring. One E1 stream is

routed between the Node A and Node B and other one through the station

C. Such that in normal condition traffic will be going through path A and B.

But whenever a path failure occurs ,traffic between station A and station B is

transported via the node C without interrupting the traffic .Thus the ring

structure provides protection facility also.

To check the automatic protection switching the

normal path is disconnected and surprisingly it was found that not even a

single ES was observed, which shows that the system is carrying the traffic

in the protective path under faulty condition of the normal path. Also to find

the switching speed between the normal and protected path one E1is

connected to the SDH analyzer and the normal path is broken. The

observed switching delay was recorded as 45ms which is within the ITU

prescribed limit of 50 ms. Thus this designed link holds with the ITU

Standards.

Figure of an SDH analyzer showing the Protection switching time delay

Next generation SDH configuration

Next Generation SDH unifies and standardises transport infrastructure for

any type of client network packet or circuit oriented network such as Ethernet,

PDH, Frame Relay, ATM etc. The NG SDH we have configured is STM-16 level

with 4 stm-16 port which is useful in configuring the equipment in MADM mode.

Also these nodes have 252 E1 capacity STM-1 port and STM-4 port. The link

design for these ports were also done as explained above and the results were

tabulated.

Port Type Trans. Power Receiver sensitivity

STM-1 -8.15 dbm -39.4dbm

STM-4 -0.5 dbm -35.2 dbm

STM-16 -2.45 dbm -33 dbm

The results shows that as the bit rate increases the receiver sensitivity

decreases this results is as expected because as the bit rate increases the

dispersion effect is more dominant than lower bit rate and hence sensitivity will

be less.

The mapping of STM-1 into STM-16 MADM ring is shown below. The STM-

1 can be mapped in to the MADM-16 either as E1s or as STM-1. From one node

to any other node the STM-1 or E1s can be transported with out disturbing the

classical STM-1.

The mapping of STM-1 in to STM-4 port and then also

we can map in to STM-16 as shown below. Here the STM-1 is mapped to

STM-4 also required number of 2 Mbps can also be mapped in to the

STM-4 and can be dropped as E1 s, STM-1 s or even as STM-4. This is

possible because we have designed the link for STM-1 level, STM-4 level

and STM-16 level. This special feature of NG SDH is highly useful in

telecommunication transmission design. We designed and implemented a

telecommunication network with classical and NG SDH link.

SDH SYNCRONISATION USING A MASTER CLOCK FROM

STM-4/ STM-16

Synchronization

The role of synchronization is the distribution of a main

clock across the a network to facilitate multiplexing the low bit rate channels to

higher bit rate for long distance transmission with out BER and to select the level

of clocks and facilities to be used to time the network. This involves the selection

and location of master clocks for a network, the distribution of primary and

secondary timing through out the network and an analysis of the network to

ensure that acceptable performance levels are achieved. Improper

synchronization planning or the lack of planning can cause severe performance

problems resulting in excessive slips, long periods of network downtime, elusive

maintenance problems or high transmission error rates. Hence, a proper

synchronization plan which optimizes the performance is a must for the entire

digital network.

For synchronization of the SDH network, it has been decided to use the

clock source available through the transit nodes at the major stations. The

synchronisation plan is based upon provision of Synchronization Supply Units

(SSUs) which will be deployed as an essential component of the synchronisation

network which will support synchronised operation of the SDH network. The

architecture employed in the SDH requires that the timing of all the network

clocks be traceable to Primary Reference Clock (PRC) specified in accordance

with ITU Rec.G.811. The classical method of synchronising network element

clocks is the hierarchical method (master–slave synchronisation) which is already

adopted in the BSNL network for the TAXs. This master–slave synchronisation

uses a hierarchy of clocks in which each level of the hierarchy is synchronised

with reference to a higher level, the highest level being the PRC. The hierarchical

level of clocks are defined by ITU as follows :

– P.R.C.

– Slave Clock (Transit Node)

– Slave Clock (Local Node)

– SDH Network Element Clock.

Node Clock type Staus Remark

C External clock

(T3), Clock via P1

and P2 Do not use

Locked to external

clock

Out put not

squelched

B Clock Via P2

highest priority

Locked to SDH Clock

T2

Out put not

squelched

A Clock Via P2

highest priority

Locked to SDH Clock

T2

Out put not

squelched

C External T3 clock

failed

UnLocked Free run mode and

Node B and A clock

are not affected

Architecture for Primary Rate Networks

Table showing the synchronization results

SDH Equipment Clock

Each node is associated with a particular hierarchical level of clock

prescribed above and is referred to as a nodal clock. The SSU is an important

component of this hierarchical master–slave synchronisation network scheme

and of a slave clock belonging to the transit node level or the local node level as

defined in ITU Rec. G.812.

The BSNL, therefore, has decided to go in for 10–20 nos. of SSUs to

provide a clean reference primary source for other stations. These SSUs are

basically high stability filter clocks which eliminate phase transients, jitter and

wander and provide the exact sync. signal needed for every network element.

Here we have tried to synchronize the SDH ring with point to point traffic

as shown below. The master clock was derived from the STM-16/STM-4

equipment and that clock acted as the master source. The result showed that

synchronization with master clock sdh with point to point traffic is successful and

hence the network designed by us is able to be mapped across the national ring.

9. CONCLUSION.

The present day long – distance transmission

systems are mainly optical fiber transmission systems. In BSNL also the

long-distance transmission systems are mainly optical fiber transmission

systems. The present day scenario is that both classical and next

generation SDH systems are jointly working in BSNL network.

In the present work we have designed both

classical and nest generation SDH optical fiber link , its testing as per ITU-T

guidelines and the formation of SDH 2 fibre ADM ring using the state of the

art technology to provide protection switching with the aim of providing

uninterrupted service even after route failure. The STM-1 and STM-16 ADM

and MADM optical fiber link is then subjected to stability tests and protection

switching is also verified using SDH analyzer as per G821 guidelines. Also

as a part of link design, I have done splicing and testing of optical fibre cable

using both 1310nm and 1550 nm windows as per BSNL standard.

Thus as a telecommunication engineer this project

work helped me to study the splicing and testing of optical fibre as per ITU

standards. Also I gained experience in designing an SDH 2 fiber ADM and

MADM ring with protection and measured the system performance as per

ITU guide lines. Also through this project work I got experience in

connecting and mapping E1s, STM-1, STM-4 from SDH to Next generation

SDH, here we recommend more NG SDH in a telecommunication network

because NG SDH can carry classical SDH with out any network

modification and only the end nodes need to be NG SDH. The NG SDH

makes the telecommunication network planning simple and cost effective.

Also we could synchronize both SDH and NG SDH using the master clock;

which shows the possibility of migration from SDH to Next Generation SDH

in a cost effective simple manner with out affecting the data. Thus practical

experiences and knowledge that I gained in designing optical fiber long-

distance transmission system was highly useful and inevitable to any

engineer working in the telecommunication sector. In this sense this project

work was very much useful.

10. REFERENCES.

1 SDH module of Bharat Sanchar Nigam Limited

2. Optical fiber cables and system module- Bharat Sanchar Nigam Limited

3. E1, E2, Transmission modules of Bharat Sanchar Nigam Limited.

4. JTO Training module- Bharat Sanchar Nigam Limited

5. Optical networking & WDM by Walter Goralski. TMH

6. Optical fiber communications by Gerd Keiser, MH

7. Optical fiber communications, Principles and Practice, John M. Senior,

PHI.

8.SDH AC1 Family System Manual.