Transcoder and Rate Adaptation Unit, or TRAU, performs transcoding function for speech channels and RA (Rate Adaptation) for data channels in the GSM network. The Transcoder/Rate Adaptation Unit (TRAU) is the data rate conversion unit. The PSTN/ISDN switch is a switch for 64 kbit/s voice. Current technology permits to decrease the bit-rate (in GSM radio interface it is 13 kbit/s for full rate and 6.5 kbit/s for half rate). Since MSC is basically a PSTN/ISDN switch its bit-rate is still 64 kbit/s. That is why a rate conversion is required in between the BSC and MSC...
Transcoding is the compression of speech data from 64 kbit/s to 13/12.2/6.5 kbit/s in case FR/EFR/HR (respectively) speech coding.
Rate adaptation without transcoding allows Tandem Free Operation (TFO), allowing the original encoded speech data to be carried in a 64 kbit/s channel. TFO offers benefits because transcoding can lead to a degradation of speech quality and requires computational resources.
Brief Explanation
For an MS-to-MS call, the transmission path covers the radio access network (RAN) as well as the core network (CN). Since the transmission modes and coding standards are different for RAN and CN, speech data is converted/transcoded at the transition points from RAN to CN. This conversion is performed in the TRAU network element which connects RAN and CN.
13 kbit/s for FR (Full Rate), Redundancy (Channel Coding)= 9.8 kbit/s
=> Gross data rate after adding redundancy = 22.8 kbit/s
= 12.2 kbit/s for EFR (Enhanced Full Rate), Redundancy (Channel Coding)= 10.6 kbit/s
=> Gross data rate after adding redundancy = 22.8 kbit/s (same as FR) TRAU changes the bit rate of 16kbit/s of the BSS side to 64kbit/s of NSS side.
= 5.6 kbit/s for HR (Half Rate), Redundancy (Channel Coding)= 5.8 kbit/s
=> Gross data rate after adding redundancy = 11.4 kbit/s