TRAU:

Abbreviation for Transcoding Rate and Adaption Unit. Within the GSM network infrastructure, the TRAU is used for the following tasks:
  • Speech Compression: In fullrate configuration, the TRAU achieves a compression rate of 1:4. In halfrate configuration, the compression rate is 1:8. Speech compression is done according to the RPE-LTP algorithm. (RPE-LTP <=> Regular Pulse Excitation - Long Term Prediction).
  • TRAU-Framing: All information in downlink direction is converted into so called TRAU-frames that represent the basic information format within a GSM network. In uplink direction the opposite procedure is performed: TRAU-frames are converted into circuit-switched data.
  • Creation of Comfort Noise: When DTX in uplink direction is enabled, the TRAU is responsible for the creation of comfort noise towards the fixed line user.

The figure below illustrates the different positions of the TRAU. It may be located either between the MSC and the BSC or between the BSC and the BTS. However, most installations deploy the TRAU collocated with the MSC to achieve the highest possible savings in line costs. The figure also illustrates the reason for these savings: Between TRAU and mobile station, 16 kbit/s channels are used in case of fullrate operation, occupying only a quarter of the resources of 64 kbit/s channels.

[2GTS 06.10 / 2GTS 06.60 / 2GTS 08.60]