GSM Transponder

The Transcoder  is required to convert the speech or data output from the MSC (64 kbit/s PCM), into the form specified by GSM specifications for transmission over the air interface, that is, between the BSS and MS (64 kbit/s to 16 kbit/s and vice versa) The 64 kbit/s Pulse Code Modulation (PCM) circuits from the MSC, if transmitted on the air interface without modification, would occupy an excessive amount of radio bandwidth.

This would use the available radio spectrum inefficiently. The required bandwidth is therefore reduced by processing the 64 kbit/s circuits so that the amount of information required to transmit digitized voice falls to a gross rate of 16 kbit/s. 

The transcoding function may be located at the MSC, BSC, or BTS. The content of the 16 kbit/s data depends on the coding algorithm used. There are two speech coding algorithms available and selecting which one to use depends on the capabilities of the mobile equipment and the network configuration. The Full Rate speech algorithm is supported by all mobiles and networks. It produces 13 kbit/s of coded speech data plus 3 kbit/s of control data which is commonly referred to as TRAU data (Transcoder Rate Adaption Unit). The TRAU data on the downlink will be used by the BTS and therefore removed from the 13 k of speech data before transmission on the air interface. the 13 kbit/s of speech data is processed at the BTS to form a gross rate of 22.8 kbit/s on the air interface which includes forward error correction. In the uplink direction the BTS adds in TRAU data which will be used by the transcoder.

 Enhanced Full Rate is an improved speech coding algorithm and is only supported by

Phase 2+ mobiles and is optional in the Network. It produces 12.2 kbit/s from each 64

kbit/s PCM channel. The TRAU data in this case is made up to 3.8 kbit/s to keep the

channel rate to and from the BTS at 16 kbit/s as for Full Rate. As with Full Rate the

TRAU data is used at the BTS and Transcoder.

 For data transmissions the data is not transcoded but data rate adapted from 9.6 kbit/s

(4.8 kbit/s or 2.4 kbit/s may also be used) up to a gross rate of 16 kbit/s for transmission

over the terrestrial interfaces, again this 16 kbit/s contains a 3 kbit/s TRAU.

As can be seen from the diagram opposite, although the reason for transcoding was to

reduce the data rate over the air interface, the number of terrestrial links is also reduced

approximately on a 4:1 ratio.