4 General Approach

08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS

4.1 Background

In case of mobile-to-mobile calls (MS-MS calls) in GSM networks without TFO, the speech signal is encoded within the first mobile station for transmission on the air interface, and decoded within the associated first "transcoder and rate adaptor unit" (TRAU). The PCM samples are then transported within the fixed part of the network to the second TRAU using 64 kBit/s traffic links. This second TRAU encodes the speech signal a second time for the transmission on the second air interface, and the associated mobile station decodes is again. The two Codecs (Encoder-Decoder pair) of the connection are in "Tandem Operation".

This Tandem Operation has several disadvantages:

  • The two consecutive encoding/decoding processes degrade the speech quality more than necessary;
  • The links between the TRAUs within the fixed network need 64 kBit/s, where 16 or 8 kBit/s would be sufficient;
  • The unnecessary encoding/decoding within the TRAUs allocates DSP power.

Tandem Free Operation requires two (back and forth) "transparent" digital channels or paths between the TRAUs. Devices within these paths need to be transparent or to be switched off for the TFO Messages and the TFO Frames. To guarantee this digital transparency with out_of_band signalling is not trivial. Out_of_band signalling is especially not fast enough for fall back to normal operation in case of sudden interruption of the transparency of the links.

This TFO recommendation defines therefore an inband signalling protocol which tests, if:

  • an MS-MS call is given;
  • the paths between the TRAUs are digitally transparent;
  • both TRAUs support TFO;
  • the speech Codecs on both radio legs are identical.

establishes the TFO connection by:

  • commanding the paths to go transparent;
  • bypassing the decoder/encoder functions within the TRAUs.

guarantees a fast fall back procedure for sudden TFO interruption and supports:

  • the resolution of Codec mismatch situations; and
  • the cost efficient transmission within the fixed part of the network.

The Tandem Free Operation is fully compatible with existing GSM equipment. In its basic operation it affects only TRAUs. The additional computational complexity is small compared to the encoding/decoding functions of the TRAUs. Mobile Station, BTS, MSC and other network elements are not at all affected in this basic operation.

In an optional mode, the TFO supports the resolution of Codec mismatch situations, i.e. the situation where the Speech-Codecs at both radio-legs are different. For this, an additional communication channel between TRAU and BSS is necessary and the BSS has to perform a normal local intra cell handover to change the Codec type. That communication between TRAU and BSS is considered as manufacturer proprietary and not handled within this recommendation.

Once TFO functionality is implemented in TFO compatible TRAU equipment, it can be employed also for TFO connections to other systems, like ISDN phones, speech servers, Internet connections or connections to other systems, like UMTS.

4.2 Principle of Tandem Free Operation

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 shall be transferred between the BTS and the TRAU in frames denoted "TRAU Frames" on the Abis respectively Ater interface.

In Tandem Free Operation similar frames, denoted ”TFO Frames”, are transferred between the two TRAUs on the A-interface by inband signalling, i.e. inserting them into the PCM sample bit stream.

In the case of Half Rate speech traffic, these TFO Frames shall be carried by 8 kBit/s traffic channels mapped onto the least significant bit (LSB) of the PCM samples.

In the case of Full Rate and Enhanced Full Rate speech traffic, these TFO Frames shall be carried by 16 kBit/s traffic channels mapped onto the two least significant bits of the PCM samples.

Like TRAU Frames the TFO Frames have a fixed size (and length) of:

  • 160 bits (20 ms) when the Half Rate Codec is used (see 06.20);
  • 320 bits (20 ms) when the Full Rate Codec is used (see 06.10);
  • 320 bits (20 ms) when the Enhanced Full Rate Codec is used (see 06.60).

Prior and parallel to these TFO Frames also other TFO Messages are transferred on the A-interface. TFO Messages conform to the IS_Message Principles described in Annexes A and B.

The TFO protocol between the TRAUs is independent of the position of the TRAUs within the GSM networks.

A possible configuration of two TRAUs is shown in Figure 1, which is intended as a reference model.

Figure 1: Functional Entities for Handling of Tandem Free Operation in MS-MS calls

TFO shall provide a virtually transparent digital channel from Encoder of Mobile A to Decoder of Mobile B and vice versa.