6 Model of operation

23.0533GPPRelease 17Service 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 are using compatible speech codec types. If they don’t use compatible speech codec types they can inform the BSC in order to modify the speech codec used in the MS. If they are using a compatible speech codec type the TRAU starts to insert TFO speech frames in the LSB(s) of the PCM octet present on the A / Nb 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, UMTS…);

– 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 to first synchronise on "transparency patterns" contained in the in-band transmitted TFO messages (negotiation messages TFO_REQ, TFO_ACK) as discussed in clause 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 20 ms. 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 successfully 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 Go transparent command after having reproduced the received Go transparent 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 64 kbit/s should be less than or equal to the delay in normal operation.

These principles should be GSM/UMTS 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 LSBs 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 DCME) should reproduce the speech from the received TFO frame). This implies that the DCME implements the decoding part of some vocoders.

6.4 Modification of speech codec

6.4.1 Introduction

In the speech codec modification procedure BSS informs the peer BSS the speech codec types 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 a compatible speech codec type.

6.4.2 Exchanged parameters

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 a compatible speech codec type is used, the modification is not necessary and the BSS can move to the TFO established mode.

If the speech codec configurations used on both sides are not compatible, each BSS looks at the list of the acceptable speech codec configurations from the other BSS.

There is an implicit rule as to the speech codec configuration that every BSS will try to use based on the current speech codec configuration, the list of locally acceptable speech codec configurations, and the list of remotely acceptable speech codec configuration. Each BSS then tries if it supports it to perform a change of codec configuration 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 configuration. The process goes on until either a compatible codec configuration is used, or no more change of the codec configuration 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.

6.4.3 Change of speech codec configuration 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 contained in the TFO frames is used;

– 16 kbits/s. The synchronisation contained in the TFO frames 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 this SID frame 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, NO_DATA frames are sent to the peer BSS.

Based on this information, the receiving BSS can perform the best action depending on its mode.

6.5.5 Error concealment

The BSS which has received a bad frame in the uplink sends it unmodified to the peer BSS. The peer BSS either performs error concealment or forwards the frame to the MS.

6.5.6 Management of UFE bit

The UFE bit is managed as described in the 3GPP TS 48.060 and 3GPP TS 48.061.

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 clause 6.5.3, the synchronisation loss timer may elapse and TRAU A will revert to normal mode. TFO will then be re-established given TFO establishment attempts are 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).