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.

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