TRC FUNCTIONS
The primary functions of a TRC are to perform transcoding and to perform rate adaptation.
Transcoding
As previously explained, the function of converting from the PCM coder information (following A/D conversion) to the GSM speech coder information is called transcoding. This function is present in both the MS and the BSS.
Rate Adaptation
Rate adaptation involves the conversion of information arriving from the MSC/VLR at a rate of 64 kbits/s to a rate of 16 kbits/s, or transmission to a BSC (for a full rate call). These 16 kbits/s contains 13 kbits/s of traffic and 3 kbits/s of inband signaling information. This is an important function. Without rate adaptation the links to BSC’ would require four times the data rate capabilities. Such transmission capabilities form an expensive part of the network. By reducing the rate to 16 kbits/s, it is possible to use one quarter of the transmission links and equipment. In some GSM systems, the TRC contains units, which perform transcoding and rate adaptation. These hardware units are called Transcoder and Rate Adaptation Units (TRAUs). All TRAUs are pooled, meaning that any BSC connected to the TRC can request the use of one of the TRAUs for a particular call. The TRC also supports discontinuous transmission. If pauses in speech are detected, comfort noise is generated by the TRAU in the direction of the MSC/VLR.
TRC IMPLEMENTATION
The TRC is implemented on the AXE platform consisting of standard APZ and APT subsystems and the following APT subsystems:
BSC Subsystems
TRC hardware configuration
The hardware that is specific to the TRC is:
• Transcoder and Rate Adapter Unit (TRAU)
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