RFGSM TRAU: Transcoding Rate and Adaptation Unit Explained
Advertisement
The Transcoding Rate and Adaptation Unit (TRAU) plays a vital role in GSM networks. It’s positioned between the Base Station Controller (BSC) and the Mobile Switching Center (MSC), or sometimes between the Base Transceiver Station (BTS) and the BSC, as illustrated below:
The primary function of the GSM TRAU is to compress and decompress speech data. This is achieved using Regular Pulse Excitation Long Term Prediction (RPE-LTP) encoding, a specific type of speech compression technique.
Speech Compression Capabilities
The TRAU’s compression capabilities vary depending on the channel being used:
- Full-Rate Channel: It can compress speech from 64 Kbps to 16 Kbps. (The net bit rate for full-rate channels is 13 Kbps).
- Half-Rate Channel: It can compress speech data from 64 Kbps to 8 Kbps. (The net bit rate for half-rate channels is 6.5 Kbps).
It’s important to note that the TRAU is specifically designed for speech connections and is not used for data connections.
TRAU Functionality and Interfaces
Functionally, the TRAU is considered part of the Base Station Subsystem (BSS), regardless of its physical location. Both the BTS and the TRAU possess an interface for the payload.
The BSC remains “transparent” to the actual payload being transferred. The payload is structured into TRAU frames, which are sent transparently over PCM links between the TRAU and the BTS every 20 milliseconds.
This process occurs in both the downlink (from network to mobile) and uplink (from mobile to network) directions.
TRAU Frames and Channel Coding
The data within the TRAU frames represents the input and output values used for channel coding. However, this entire functionality is only applied to speech (voice) connections. The compression is not performed on data connections.
The type of connection—whether it’s data or speech—is communicated to the TRAU during the assignment of the traffic channel.
Important Point: TRAU frames are transmitted using the traffic channel and not over the associated control channels.
