6 Model of operation

03.533GPPService descriptionStage 2Tandem Free Operation (TFO)TS

6.1 Overview

The TRAU must first identify each other as TRAU that are TFO capable. Next the TRAUs must check that they’re using the same speech codec. If they don’t use the same speech codec they can inform the BSC in order to modify the speech codec used in the MS. If they’re using the same speech codec the TRAU starts to insert TFO speech frames in the LSB(s) of the PCM octet present on the A interface. These frames contain the speech parameters as obtained from the uplink TRAU frames.

6.2 TFO establishment phase

6.2.1 Sending of TFO_REQ message by a BSS

The TFO_REQ message is sent using bit stealing on bit 8, by stealing one bit out of 16. This allows to have the least possible degradation of the PCM, since the TFO_REQ message is sent even in cases where TFO will not be possible e.g. MS to PSTN call.

6.2.2 Monitoring of TFO_REQ by a BSS

The monitoring of the TFO_REQ message can be performed by a BSS which is capable of TFO.

Upon reception of a TFO_REQ message, the BSS replies with an TFO_ACK message which can contain the following information:

  • – system identification (GSM,…);
  • – the current speech codec;
  • – the list of supported speech codecs;
  • – a random value that allows the detection of loop backs of a given BSS.

6.3 Transparency of transmission equipments

6.3.1 Local disabling

Not used.

6.3.2 Transparency to TFO negotiation messages

IPE which support TFO protocol need t o first synchronise on "transparency patterns" contained in the in-band transmitted TFO messages (negotiation messages TFO_REQ, TFO_ACK) as discussed in subclause 6.2 and in addition track some specific TFO message (Go transparent command, Go to normal) needed to set the transparency mode required in TFO established mode or return to normal operation. Having synchronised to a transparency pattern, the IPE should reproduce at its output the TFO message in order to ensure transparency of the in-band signalling path. Some additional delay with respect to normal operation of the IPE may be introduced by the IPE as far as the in-band signalling information is concerned but such delay should be a multiple of 20ms. No delay should be introduced on other bits.

6.3.3 Transparency to TFO speech frames

As part of the TFO establishment, if TFO can be successufully established (same codec type) then each TRAU sends TFO negotiation messages (Go transparent commands) which indicate to the In Path Equipment (IPE) along the BSS to BSS circuit the type of IPE transparency mode. The TFO speech frame then contains a "keep open" pattern that maintains the operation of the IPE as commanded. That “keep open” pattern is part of the synchronisation pattern of each TRAU frame and does not require hence complete tracking of the TFO speech frames by the IPE (nevertheless the commands within the TFO negotiation messages should be checked continuously). IPE revert to normal operation in case the keep open pattern is absent for [1s] or a new command "Go to normal" is received.

IPE which support TFO protocol set the transparency mode as indicated in the received Gotransparent command after having reproduced the received Gotransparent command at its output. The IPE should simultaneously start to track the keep open pattern. Then the bits corresponding to the "transparent" path are copied in the 64 kbit/s output stream of the equipment. The delay introduced by the IPE on the TFO contained in the transparent part of the 64kbit/s should be less than or equal to the delay in normal operation.

These principles should be GSM independent, allowing other systems to use the same transparency principle.

The behaviour of the IPE regarding the remaining part of the circuit can be either of the following:

– the IPE is speech transparent, meaning that the upper bits in the 64 kbits/s output stream should contain PCM speech, that speech being obtained by running the IPE in its normal way, possibly reproducing the PCM at the input. IPE should keep the same alignment as present at the input between transparent LSB:s and upper bits containing the PCM speech;

– the IPE is non speech transparent, meaning that the IPE may strip the upper bits, if the IPE is the compressor part of an DCME. In such a case the peer IPE (the decompression part of the a DCME) should reproduce the speech from the received TFO frame). This implies that the DCME implements the decoding part of some GSM vocoders;

– half-duplex TFO capable IPE.

6.4 Modification of speech codec

6.4.1 Introduction

In the speech codec modification procedure BSS informs the peer BSS the GSM speech codec versions supported by the sending BSS (within the ongoing call) and tries to align the codecs using implicit rule and BSS internal means to make appropriate handover

The support of the informing and BSS internal modification procedures are optional for a BSS since it depends on the BSS internal capabilities. This means that TFO should be applicable even if the informing and modification are not supported by one of the two peer entities. In this case, TFO will be available if both BSS happen to use the same speech codec version.

6.4.2 Exchanged parameters

6.4.3 The exchanged parameters are the speech codec currently in use and the other possible codecs that can be used within the call Modification procedure

In the TFO request message (TFO_REQ), the BSS informs the fact that it supports TFO.

The possible modification procedure is started when the TFO request message from the peer BSS has been detected. It is not synchronised explicitly on Call Set-up states.

If both BSS receive a TFO_REQ indicating that the same speech codec is used, the modification is not necessary and the BSS can move to the TFO established mode.

If the speech codecs used on both sides are different, each BSS looks at the list of the acceptable speech codecs from the other BSS.

There is an implicit rule as to the speech codec that every BSS will try to use based on the current speech codec, the list of locally acceptable speech codecs, and the list of remotely acceptable speech codecs. Each BSS then tries if it supports it to perform a change of codec via a intra-BSS (normally intra-cell) handover.

When a handover has been performed, each BSS will reply to TFO_REQ by a new TFO_ACK message indicating the newly used speech codec. The process goes on until either the same codec is used, or no more change of codec is possible according to the fixed rule.

There is a timer that protects the TFO establishment phase. The BSS, after the timer has elapsed, stops sending TFO request messages, but continues the monitoring of the peer entity. This is in order not to degrade the communication if TFO cannot be established. To resume the TFO establishment phase, it will be up to the peer entity to send a new TFO_REQ message to initiate a new attempt of establishment/modification of TFO e.g. when a handover has occurred.

6.4.4 Change of speech codec in the BSS

This procedure is internal to the BSS. It can be based on the use of O&M frames on the Abis interface or on the use of a specific TRAU to BSC interface or on some other method.

6.5 TFO operation

6.5.1 End to end delay

The end to end delay is similar to normal MS-MS call in TFO operation within a 20 ms range.(

6.5.2 Synchronisation between TRAUs

The synchronisation depends on the kind of bit stealing used:

– 0,5 kbits/s. There is a specific pattern one bit out of 16 on the LSB;

– 8 kbits/s. The synchronisation of GSM 08.61 is used;

– 16 kbits/s. The synchronisation of GSM 08.60 is used.

Some sub-part of the synchronisation patterns can be understood by transmission equipments as well.

There is no time alignment between TRAUs.

6.5.3 Monitoring in TFO operation

A TRAU in TFO established mode monitors permanently the synchronisation with the peer TRAU entity

In case of loss of synchronisation, a timer is started. When the timer elapses, the TRAU sends normal PCM speech, reverts to non-TFO mode and starts re-establishment procedures.

6.5.4 DTX aspects

Each direction is independent from the DTX point of view. This leads to the following scenarios for the transfer of speech in one direction:

Table 1

Uplink User A

Downlink User B




no DTX

no DTX


no DTX

no DTX

The sending BSS, when DTX is applied on the uplink direction, generates normal speech frames when no SID frame is received

When a SID frame is received in the uplink, the BSS sends a normal speech frame to the peer BSS. The fact that TRAU frame was a SID frame is indicated to the peer BSS.

When uplink DTX is applied and nothing is received from the MS by the BTS, i.e during comfort noise generation, normal speech frames are sent to the peer BSS. The fact that comfort noise generation was applied is indicated to the peer BSS.

Based on these information, the receiving BSS can perform the best action depending on its mode DTX/no-DTX. If DTX is applied, it is felt that the best is to re-use the SID frame that was received by the peer BSS, even if it means shifting in time the sending of the frame (because both radio physical layers are not synchronised).

6.5.5 Error concealment

The BSS which has received a bad frame in the uplink direction builds a normal speech frame which is sent to the peer BSS. the fact that it was a bad frame that has been rebuilt is indicated as well to the peer BSS (BFI indication).

6.5.6 Management of UFE bit

The UFE bit is managed as described in the GSM 08.60 for the GSM EFR and GSM 08.61 for the GSM HR.

6.5.7 Handover management

Different cases of handovers from the TRAU point of view will be encountered :

1) handover with modification of the TRAU on one side or, which is equivalent, no modification of TRAU but change of transcoder DSP;

2) intra-cell handover without modification of transcoder DSP;

3) inter-cell handover without modification of transcoder DSP.

In case 1, TFO will be interrupted and re-established if the newly allocated TRAU (TRAU B) is TFO capable. Indeed the newly allocated TRAU will start in normal mode and, if it is TFO capable, will immediately attempt to establish TFO. This will interrupt the reception of TFO frames at the TRAU in TFO mode (TRAU A). As defined in subclause 6.5.3, the synchronisation loss timer will elapse and TRAU A will revert to normal mode. TFO will then be re-established given TFO establishment attempts performed by the new TRAU.

The management of handover for cases 2 and 3 will depend on the time put to actually switch the communication from one traffic channel to another one in the intra-cell handover case or between base stations in the inter-cell handover case. If the handover is fast enough, i.e. the timers to detect the loss of synchronisation do not elapse, then the TFO will be maintained. In the opposite case there will be a fallback to regular tandem. Attempts to re-establish TFO shall be performed if both TRAUs are still TFO-capable.

6.5.8 Other issues

The bridging of the communication in the MSC can potentially interfere TFO. The conference bridge will prevent TFO operation as long as it is present. The TFO synchronisation pattern should be lost and both TRAUs should revert to non-TFO operation if the bridge is present. TFO operation will start again from the start when the bridge is removed (handover is performed).