GSM TRAU-Transcoding Rate and Adaptation Unit
TRAU, referred as Transcoding Rate and Adaptation Unit is located as shown in the figure below between BSC and MSC or between BTS and BSC. The main task of GSM TRAU is to compress and de-compress the speech data.
For speech compression Regular pulse excitation long term prediction (RPE-LTP) encoding is used. It is able to compress speech from 64 Kbps to 16 Kbps, in the case of a full-rate channel (Here, net bit rate with fullrate is 13 Kbps). It will compress speech data from 64Kbps to 8 Kbps in the case of a half-rate channel (Here, net bit rate with halfrate is 6.5 Kbps). TRAU is not used for data connections.
The TRAU is functionally assigned to the BSS, independently of where it actually is located.Both the BTS and the TRAU have an interface for payload. This payload is transparent to BSC entity. The payload is formatted in TRAU frames which is transparently sent over PCM links between the TRAU and the BTS once every 20 ms. This is done in downlink as well as uplink direction. The data contained in the TRAU frames form the input and output values for channel coding. This functionality is only applied to speech (voice) connections. The compression is not applied to the data connections.
The type of connection (data/speech) is communicated to the TRAU during the assignment of the traffic channel. Please note that TRAU frames are transmitted using traffic channel and not over the associated control channels.