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The underlying philosophy of controlled maintenance is to prevent network failure.. MAINTENANCE ORGANIZATION 665 The advantage of controlled maintenance is that it concentrates on areas

36 Maintaining the

Network

No matter how much careful planning goes into the design of a network, and no matter how reliable the individual components are, corrective action will always be required in some form or another, to prevent or make good network and component failures, and maintain overall service standards However, attitudes towards maintenance and the organization behind it vary widely, ranging from the ‘let it fail then fix it’ school of thought right through to ‘prevent faults at any cost’ This chapter describes a typical maintenance regime in its philosophical, organizational and procedural aspects

As succinctly stated by ITU-T, the objective of a general maintenance organization is to minimize the occurrence of failures and to ensure that in case of failure

0 the right personnel can be sent to

0 the right place with

0 the right equipment at

0 the right time to perform

0 the right corrective actions

In pursuing these objectives, the wise network operator establishes a maintenance philosophy closely linked with overall targets for network quality and for the

663

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

664 MAINTAINING THE NETWORK

proportion of time that the network is intended to be fault-free (available) In this task

he will take due account of network economics and the most likely causes of failure Networks fail for all sorts of reasons; common examples are

0 cable or connector damage or disturbance

0 equipment overheating

0 electronic component failure

0 mechanical equipment jamming, or other failure

0 mechanical wear

0 dirty (high resistance) relay or switch contacts (a diminishing problem as electro- mechanical exchanges are withdrawn)

0 power supply loss

0 vandalism (e.g to public payphones)

0 software errors

0 erroneous exchange data

0 poor connections between cables or other components (e.g dry soldered joints)

0 interference (e.g due to electromagnetic disturbances, recent standards on E M C , electromagetic compatibility, are designed to ensure that equipment does not cause electromagnetic disturbance and is itself not unduly sensitive to such interference,

i.e is electromagnetically protected)

Each cause of failure has its own cure, but broadly speaking, there are three main

approaches

0 corrective maintenance

0 preventive maintenance

0 controlled maintenance

Corrective maintenance is carried out after the failure has been diagnosed, and it consists of the repair or replacement of faulty components

Preventive maintenance is to eliminate the accumulation of faults Preventive maintenance usually consists of routine testing and correction of working equipment

(as opposed to failed equipment) to prevent degradation in performance before any

failure actually occurs

Controlled maintenance is a more systematic approach, combining both the corrective and preventive methods The underlying philosophy of controlled maintenance is to prevent network failure This is done by using special analysis techniques to monitor day-to-day network performance and degradation, thereby avoiding maintenance work

MAINTENANCE ORGANIZATION 665

The advantage of controlled maintenance is that it concentrates on areas where the customer is likely to benefit most, and it reduces the extent of preventive maintenance and the complications of corrective maintenance When new networks are being designed or extended, or when new capabilities are being added to existing networks, consideration needs to be given to the maintenance philosophy and to the organization,

the maintenance facilities, and the test equipment that will support it The best controlled

mix of both corrective and preventive philosophies depends on the number and nature of problems These, in turn, depend on the overall network structure and the component equipment types So, in the days of widespread electromechanical switches and relays

when a frequent cause of failure was mechanical wear, a good deal of time was spent on preventive type maintenance, oiling the moving parts as it were Nowadays, when hardware faults in modern electronic equipment are relatively rare and software faults often take some to present themselves (the exchange operating fault-free for extended periods between occurrences), a corrective philosophy is adopted Faulty component boards are completely replaced without even attempting a diagnosis, and software faults are debugged as they arise

36.3 MAINTENANCE ORGANIZATION

Real networks are in a constant state of change throughout their lives To match traffic demand, new circuits are continually established between exchanges Established cir- cuits may need to be re-arranged as transmission systems are upgraded or taken down and faulty equipment needs to be repaired, replaced or avoided by a diversion For optimum efficiency the organizations set up to establish these maintenance tasks, and the tools with which they are provided, should be planned in such a way that lifetime costs will be minimized There is a choice, for instance, between paying more at the start for expensive but reliable equipment, or using cheaper equipment and incurring higher ongoing running costs Lifetime cost analysis must include

0 initial cost of equipment

0 cost of spares and test equipment

0 ongoing running and maintenance costs

0 costs associated with periods of lost service

High wages and skills shortages in recent years have weighed the scales in favour of using more reliable equipment and a smaller maintenance workforce Indeed, in some instances the field workforce has been pared to the minimum of two people, one worker plus a stand-in to cover annual leave and periods of sickness Some observers question the sense of this, pointing to the fact that so complex are the devices, so computerized the routine activities and so rare the faults, that the field maintenance staff often do not have the experience to cope

For this reason a comprehensive headquarters maintenance support, technical

support or back-up organization (sometimes called second line support or third line

666 MAINTAINING THE NETWORK

support) is needed in addition to the direct maintenance staff, to perform the following functions

0 To provide detailed equipment and maintenance documentation

0 To provide maintenance training on new equipment and document ‘fail-safe’ maintenance procedures (explaining the general methods to be adopted, and making sure that unintended disturbance to other customers is not caused by maintenance action)

0 To develop and put in place a fault-reporting procedure

0 To repair complicated items of equipment or resolve complex software problems

0 To develop and procure the necessary test equipment

0 To maintain an appropriate store of spare parts and to call for re-design of poor equipment, taking duly into account the failure rate of each item, the number of items in operation, and the actual repair turn around time (e.g repair time, or delivery time for a part not held in stock), and calculating the service and revenue risk if no spare part is available

0 To develop and maintain an equipment identification and inventory scheme for tracking equipment in use and spare equipment either in stock or on order (in addi- tion, maintenance staff in different exchanges need to be able to indicate faulty lines

or circuits to one another)

0 To prepare a list of contact points and telephone numbers through which the maintenance staffs in different maintenance centres may communicate

The direct maintenance workforce is usually collocated with the exchange, some staff being switching experts while others have transport so that they can go out and deal with exterior plant problems The staff located within the exchange provide a ‘control

point’ for new circuit lineups, and for the initial reporting and diagnosis of faults

The number of staff located at any exchange depends on the size, complexity and reliability of the exchange Not every exchange can justify its own on-site maintenance staff, and out-stationed staff may be posted to the exchange either on a regular

preventive maintenance schedule, or simply when there is a fault

A modern practice, aimed at reducing the maintenance workforce, is to leave exchanges unmanned Computer technology and extended alarms allow staff in a single

centralized operation and maintenance (CO&M) centre to monitor and control a number

of different exchanges in real time (giving instantaneous and live control of each exchange) Figure 36.1 illustrates a typical centralized operation and maintenance

scheme

LINING UP ANALOGUE AND MIXED ANALOGUE/DIGITS CIRCUITS 667

R e m o t e

e x c h a n g e s

*

C e n t r a l i z e d

m a i n t e n a n c e c e n t r e

o p e r a t i o n a n d e

M a i n t e n a n c e C o m p u t e r

s t a f f

*

*

-m

D a t a l i n k s t o

a n d c o n t r o l the

monitor

e x c h a n g e s

Figure 36.1 Centralized operation and maintenance

Centralized operation and maintenance (CO&M) can be introduced only when the remote exchanges are computer-controlled, and are designed to be capable of self- diagnosis of faults A datalink back to a computer at the centralized operation and

maintenance centre allows the maintenance staff to monitor the exchange performance, noting any problems and applying any necessary controls Under a CO&M scheme, the

exchanges are designed with duplicated items of equipment, which remain idle until they are activated electronically to take over the function of a failed item The exchange can thus continue to work at full load, while a member of the maintenance team is sent out to the exchange site, to repair the faulty equipment, or to replace it completely by a

circuit-board change

ANALOGUE/DIGITS CIRCUITS

The transmission links between exchanges are commissioned (or lined up) using a two-

stage method as follows First, the lineplant itself is established in sections which are tested and calibrated in turn, and then connected together A number of reference

measurements are made along the entire length to check and calibrate the overall end- to-end performance When the line system as a whole has been established, multiplexing and other terminal equipment is applied to its ends to obtain the individual circuits or groups which may be tested individually The individual circuit testing is necessary because, as Figure 36.3 shows, a real circuit (or group) is likely to traverse a number of line systems, which may interact adversely So although each section in isolation may be within limits, the combination may not be so The calibration of each group and circuit

is thus carried out on an end-to-end basis

668 MAINTAINING THE NETWORK

Figure 36.2 Maintenance workstation The AT&T SESS telephone exchange now includes a video monitor that displays colour-coded diagrams, each of which indicates the status of a certain part of the system A central office technician is checking the status of digital trunks (Courtesy of

AT&T)

LINING UP ANALOGUE AND MIXED ANALOGUE/DIGITS CIRCUITS 669

I I r - - - J I

I l l

670 MAINTAINING THE NETWORK

Figure 36.3 shows four exchanges A, B, C and D, located in transmission centres a, b,

c and d From the inset diagram of Figure 36.3 we see that the exchanges are configured topologically in a fully interconnected manner Each exchange has a circuit to every other exchange but the total of six circuits has been achieved with the use of three line systems only: a-b, b-c and b-d Where the transmission centres are not directly

interconnected by a line system, a circuit has been provided by the concatenation of two line systems, with a jumper wire completing the connection across the intermediate

transmission centre Thus for example the circuit from exchange A to exchange C uses

line systems a-b and b-c, and a jumper wire across transmission centre b

The number and diversity of calibration measurements and adjustments necessary during circuit line-up, and the amount of deviation allowed in the ongoing values depend

on the type of circuit (e.g analogue or digital), and on the use to which the circuit is being put (e.g voice or data) High grade data circuits, for example, have more strin- gent line conditioning requirements than simple voice grade circuits The measurements and adjustments ensure that the circuit conforms with the transmission plan (see Chapter 33) Thus on analogue and mixed analogue/digital line systems and circuits

line-up, measurements will be made of

0 overall loss in signal strength (in dB)

0 amplitude loss/frequency attenuation distortion

0 group delay (particularly if the circuit is to be used for data)

0 noise, crosstalk, echo, etc

0 inter-exchange signalling tests (if appropriate)

Various equipments, including amplfiers, equalisers, filters, and echo controllers, are

then adjusted to bring the line conditions within the set limits, using measuring

equipment as follows

0 signal tone generators (calibrated for precise frequencies and signal strengths)

0 calibrated frequency and signal strength detectors

0 noise meters

0 equipment for inter-exchange signalling or data protocol testing

First, the end-to-end circuit loss is determined by sending a calibrated 1020Hz (or in purely analogue networks, 800Hz) signal of a known strength, and measuring the

received strength of this allows the circuit amplification to be adjusted accordingly

Next, a range of different calibrated signal frequencies across the whole circuit band-

width is sent, and the received signal strengths are again measured This allows the frequency distortion equalizers to be adjusted Then the group delay distortion is corrected using group delay equalizers This equalization is particularly important for high speed modem data circuits, and it is achieved by measuring the relative phase

of different frequencies relative to the 1020Hz (or 800Hz) signal Following this,

HIGH GRADE DATA CIRCUIT LINE-UP 671

psophometric noise checks are conducted, together with tests of inter-exchange signalling systems or of data protocols (e.g X.25, frame relay or IBM’s SNA), and finally a test call is established

Where a pure tone signal of calibrated strength needs to be injected into the digital part of a mixed analogue/digital connection or network, this can be done either using

an anlogue tone sender and an analogue/digital converter, or by the use of a digital

reference sequence ( D R S ) A digital reference sequence is a digital bit pattern cor- responding to a particular analogue signal frequency and strength Such a pattern is easy to store in computer-like memory and is a very reliable means of reproducing an accurately calibrated signal

In high grade data circuits which use modems over analogue or mixed analogue/digital

plant, a number of extra line-up measurements may be necessary, as follows

m weighted noise

m notched noise

m impulse noise

m phase hits and gain hits

m harmonic disturbance (or inter-modulation noise)

m frequency shift distortion

m jitter

Weighted noise is a measure of the noise in the middle of the channel bandwidth Noise frequencies in this range are most likely to cause modem errors Psophometric

(European) and C-messageJilters (United States) are used to measure this type of noise

Notched noise is measured by applying a pure frequency tone at one end and

removing it with a notchJilter at the other; the remaining noise is then measured The

notched noise itself arises from the way in which signals have been digitized or other-

wise processed over the course of the link It is thus similar to quantization distortion,

which was discussed in Chapter 5

Impulse noise is characterized by large ‘spikey’ waveforms and arises from unsup- pressed power surges or mechanical switching noise It is most common on electro- mechanical switched networks

Phase hits and gain hits are intermittent but only moderate and relatively short (less than 200 ms) disturbances in the phase or amplitude of a signal Typically, less than 10 should be recorded in a 15 minute test period Special test equipment is required More

serious gain hits are called dropouts Gain hits are most troublesome in voice use; phase

hits manifest themselves as bit errors in data signals

672 MAINTAINING THE NETWORK

Harmonic disturbance may result from the intermodulation of two different signal frequencies F1 and F2 when passed through nonlinear processing devices New stray signals of frequencies F1 + F2, F1 - F2, F1 + 2F2, etc., are produced This type of disturbance is measured by a spectrum analyzer

Frequency shift is also measured by a spectrum analyzer Frequency shift is obviously

a problem for a data modem if the frequency received differs to such an extent from that sent as to be mis-interpreted It is most likely to occur when a carrier system or other frequency modulating signal processing has been used

Jitter or phase jitter arises when the timing of the pulses on incoming data signal

varies slightly, so that the pulse pattern is not quite regular The effects of jitter can

accumulate over a number of regenerated links, and they result in received bit errors

It can be reduced by reading the incoming data into a store and then reading it back out

at an accurate rate, using a highly stable clock controlled by a phase-locked loop ( P L L )

circuit

l a ) V22 bis modem - perfect

u n d i s t o r t e d s i g n a l -

a p p e a r s a s c l e a n ’ d o t s ’

1 b ) Noisy signal-cloudy pattern of dots

l c ) Signal affected by phase hits-appears as circular streaks

( d ) Signal affected by gain hits-appears as radial streaks

Figare 36.4 Detecting analogue line disturbances using constellation diagrams