Anshu, adding more to Varun sharma
TRAU is for speech compression to save resources over air interface but also can help save line costs on terrestrial links.
16kbps fullrate channel – the net bit rate is 13 kbps
8kbps halfrate channel – the bit rate is 6.5 kbps
TRAU is functionally assigned to BSS, it does not matter whether it is independently located (e.g. MSC)
BTS and TRAU both have an interface for payload that is transparent for the BSC. The payload is formatted in TRAU frames [see GSM08.60] then transparently sent over PCM links btw TRAU and BTS in cylces 20ms each direction. Data contained in TRAU frames form the input and output values for channel coding.
TRAU frames sent over traffic channels and not associated control channels.
For fullrate channels every TRAU frame is 320-bits long. Halfrate channels can be possible for 160-bits long and 320-bits.
There are four types of TRAU frame distinguished
1) Speech frame used to transfer voice 260-bits fullrate channel, 112-bits halfrate channel
2) O&M frame transfer O&M data btw BTS and TRAU. O&M 264-bits for full- and halfrate channel.
3) Data frame carries 252-bits payload fullrate and 126-bits halfrate, but carries no voice.
4) Idle frame for quiet times btw BTS and TRAU when DTX is active