American Telecom AM64/128A manual Appendix C In Service Test

AM64/128A User Manual Appendix C - In Service Test

It must be remembered that this error count is unidirectional only. There can be no looping of the envelope alignment bits. This fact can be used to advantage in determining the location of noise on a link. Consider an example where two BBM’s are connected together. BBM1 indicates a high error rate, and BBM2 indicates a low error rate. It can then be inferred that the noise is worse close to BBM1, (Noise has the greatest effect where the signal is weakest, the signal from BBM2 is weak at BBM1. The noise has little effect on the transmitted signal from BBM1, and hence the signal arriving at BBM2 is error free).

The use of alignment bits for error monitoring is the prefered use of In Service Testing, however there are several user rates where this method will not work (because there are no alignment bits transmitted in the frame structure, see previous section).

The comms channel error monitoring can be used at all line rates. The disadvantage of using this technique however, is that the ratio of comms bits monitored to data bits is low; i.e. error rates have to be high, or tests have to be very long, to accumulate any significant data.

Only one of the four comms bits per frame forms part of a 8 bit multi frame word, (the comms framing word) which can be monitored for errors. This gives the equation for the bit error rate to be:

Comms Error Rate x 108

Bit Error Rate (BER) = _____________________

User Data Rate x Time

There is a complication with the comms framing word.The word is originally transmitted from the master BBM; when it gets to the slave it is monitored for errors, and then re-transmitted (in its errored form) back to the master. The comms error display has thus a different meaning on the master and on the slave. The master display is the error count around the loop, and the slave display is the error count in the single direction only.

There is a further method of error monitoring that is included in the master BBM only. These errors are termed ‘Transmitted envelope errors’. This technique can allow the master BBM to determine the location of noise on the line, with no help from the slave display. It is thus useful if access to the slave is difficult or inconvenient. The envelope alignment errors counted by the slave is encoded into the comms channel and sent to the master. As the ratio of comms bits to envelope bits is fairly low, and provided the over all error rate is fairly low (not many comms errors, i.e. bit error rate less than 10-3), the master can expect to receive a reasonable account of the envelope alignment errors as counted by the slave. The master BBM then has available (1) the envelope alignment errors that it has received, and (2) the envelope alignment errors that the slave has received. The relative performance of either end of the link can thus be determined. It should be stressed that this error information in the comms channel is obviously subject to error itself and so accuracy is very limited with short tests and high error rates.

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