4Mbit/s FAS errors 61.3.6 In service measurement parameters and Line-code errors standards SONET/SDH BIP-8 (Byte B1) BIP-8 (Byte B3) In service measurements have become increasingly important as (section overhead) Path overhead they allow long term performance monitoring and preventative BIP-8 (Byte B2) maintenance without interrupting customer traffic. In the deregu- (line overhead) (Payload path moni- lated competitive environment, this is very desirable. In service tor e.g. DS-1, DS-3) 61/12 Telecommunication system measurements tion standard bit rates. (See M.550 and ANSI TIM1.3 draft stand- specification. It can also alert the engineer to potential concerns ards for interpretation of these in service measurements.) such as cable degradation. 61.4 Data communications network 61.4.1.2 Multiple attachments measurement By attaching a network analyser and a number of subsidiary nodes Network measurement is no longer restricted to the trusty cable on a single network segment, it is possible to form an even clearer tester and a selection of rudimentary software utilities. Along with picture of the physical behaviour of the network. If a network the wide variety of network installations now in existence, there is adapter board failed in such a way as to intermittently transmit an equally diverse range of measurement tools available, with garbled data onto the network, network measurement equipment features matched to the needs of the many potential users. operating at a higher level would be swamped with meaningless data. Whilst it would be clear that there was a problem with the The simplest tool, the cable tester, uses Time Domain Reflecto- network, there may be few clues as to the source of the disruption. metry (TDR) to check for open or short circuits. Where a problem exists, the tester will indicate the distance from its connection to the However, by analysing the electrical signals at more than one point fault. More sophisticated devices will passively monitor the traffic on the segment, it is possible to qui