- Transcoder
A transcoder is a device which converts the voice channel coding between the
GSM coder and the standard PCM inmobile phone communication systems.Although the Transcoding (compressing/decompressing) function is as standard defined as a BSC function, there are several vendors which have implemented the solution in a stand-alone rack using a proprietary interface. This subsystem is also referred to as the TRAU (Transcoder and Rate Adaptation Unit). The transcoding function converts the voice channel coding between the GSM (
Regular Pulse Excited-Long Term Prediction , also known as RPE-LPC) coder and the CCITT standard PCM (G.711 A-law or u-law). Since the PCM coding is 64kbit/s and the GSM coding is 13 kbit/s, this also involves a buffering function so that PCM 8-bit words can be recoded to construct GSM 20 ms traffic blocks, to compress voice channels from the 64 kbit/s PCM standard to the 13 kbit/s rate used on the air interface. Some networks use 32 kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/s PCM and the TRAU converts accordingly. When the traffic is not voice but data such as fax or email, the TRAU enables its Rate Adaptation Unit function to give compatibility between the BSS data rates and the MSC capability..However, at least in
Alcatel ,Siemens ' andNokia 's architecture, the Transcoder is an identifiable separate sub-system which will normally be co-located with the MSC. In some ofEricsson 's systems it is integrated to the MSC rather than the BSC. The reason for these designs is that if the compression of voice channels is done at the site of the MSC, fixed transmission link costs can be reduced.Video transcoders are also used to convert signal formats. For example PAL to SECAM.
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