PCM Channel Analyzer
Why it is very important in Railway system?
The advent and augmentation of digital communication techniques has resulted not only in application of hi-tech methods but also in different performance compliance criteria as compared to the earlier systems. The A/D conversion and vice-versa used here entails complex quantizing, multiplexing and framing techniques, the quality of which governs the performance of the transmission equipments, PCM multiplexers and digital exchanges, which form the basic building block of present-day railway communication systems. The performance of the transmission equipments e. G. STM1, STM4 etc. Can be rated through BER and G. 821 monitoring but the last mile performance at the user interface level e. G. The analog VF parameters need additional quality check-UPS. This is because of umpteen technical reasons some of which are:
1. The channel performance degradation is a highly probable reality in the case of multiplexer related problem at the interface level even if link error performance is quite satisfactory.
2. For long networks, the omnibus channels used for Railway controls acquire significant noise level at each user nodes. This degrades overall S/N ratio which may corrupt the performance of highly selective VF band application e. G. DTMF signaling and data application using modems. In such cases, DTMF tones may not be decoded properly or data stream may be erroneous with garbage even if speech signal is through.
3. The users in the Railway environment are often co-located with HV induction environment, where the induced level of higher order harmonics of traction power is so high or imbalanced that even the isolation transformers frequently used by Railways cannot prevent the contamination of the circuits.
All these warrant a constant vigil on voice parameters not only at the time of commissioning, but also periodically as a part of maintenance in addition to line-up and fault finding. Typical requirements involve VF measurements and recording of PCM parameters like quantization noise and gain response.
The data communication (x. 25, group3 facsimile, v. 24 at PRS) using high-speed modems places significant demand on quality of these PCM channels. The end-to-end measurement of user channels throws light on the total network performance when a network cannot be put to off-line for adjudging link health. Last but not the least, the out of service measurement during various testing and breakdown maintenances is another area where voice channel analyzers find wide application.
