4 General approach

08.613GPPIn-band control of remote transcoders and rate adaptors for half rate traffic channelsTS

The TRAU shall be controlled by the BTS when it is positioned remote from the BTS. In this case, the speech/data information and TRAU control signals exchanged between the Channel Codec Unit (CCU) in the BTS and the TRAU shall be transferred in frames denoted "TRAU frames".

The TRAU frames may be carried by either 16 kbit/s traffic channels or 8 kbit/s channels. The choice of the traffic channel bit rate or submultiplexing scheme is operator dependent.

The TRAU frames have a fixed length of:

‑ 160 bits (20 ms) when 8 kbit/s submultiplexing is used;

‑ 320 bits (20 ms) when 16 kbit/s submultiplexing is used.

The protocol between the BTS and the TRAU should be the same if the TRAU is positioned either at the MSC site or at the BSC site. In the first case, the BSC should be considered as transparent for 16 kbit/s and 8 kbit/s channels.

The TRAU is considered a part of the BSC when the TRAU is remote from the BTS (see GSM 08.52), and the signalling between the BSC and the TRAU (e.g. detection of call release, handover and transfer of O&M information) may be performed by using BSC internal signals. However, the signalling between the CCU and the TRAU, using TRAU frames as specified in the present document, is mandatory when the TRAU is remote from the BTS.

For data transfer, a conversion function is required, in addition to the conversion/rate adaption specified in GSM 08.20, to adapt ITU‑T V.110 frames to the TRAU data frames. This function constitutes the RAA.

NOTE: If standard 64 kbit/s switching is used in the BSC, multiplexing according to ITU‑T Recommendation I.460 should apply at both sides of the switch.

A possible configuration of the TRAU and the CCU is shown in figure 4.1.

The functions inside the TRAU are:

‑ "Remote Transcoder and Rate Adaptor Control Function" (RTRACF);

‑ "Remote Speech Handler Function" (RSHF);

‑ the RAA function;

‑ the RA2 function;

‑ the transcoder function.

The functions inside the CCU are:

‑ "Transcoder and Rate Adaptor Control Function" (TRACF);

‑ "Speech Handler Function" (SHF);

‑ the RAA function;

‑ the RA1/RA1′ function;

‑ the channel codec function.

The present document does not describe the procedures inside the TRAU and the CCU. The layout in figure 4.1 is only intended as a reference model.

NOTE: This technical specification assumes the DTX handler function to be part of the Transcoder Function.

Figure GSM 08.61/4.1: Functional entities for handling of control
of remote transcoders and rate adaptors