03.533GPPService descriptionStage 2Tandem Free Operation (TFO)TS
5.1 TFO Access Interfaces and Reference Points
The BSS reference points for TFO are described in Figure 1. GSM A interface reference point applies here with the exception (as compared with 08.02) that speech is not in the standard 64 kbit/s PCM format and between TFO TRAUs in-band signalling is used. For speech and in-band signalling the MSC is transparent.
Additional transmission equipments are taken into account. These equipments are all those which can process the PCM signal between the two peer BSS, e.g. echo cancellers, DCME. The BSS to BSS transmission is then considered transparent provided no transmission equipment along the circuit do not modify the 2 LSB or these transmission equipments implement a TFO "transparency mode". A specific signalling allows to remotely control in-band these equipments.
BSS-MSC out-of-band signalling consists in this version of TFO of the normal BSSMAP signalling which carries the list of acceptable speech codec that may be used for the TFO negotiation mechanism.
Use of TFO specific out-of-band mechanism is not foreseen in that version of the specification.
5.2 General In-band Transmission Mechanism for TFO
In TFO establishment mode, one bit out of 16 LSB bits is regularly stolen of the 64 kbits/s circuit. This provides for a 0.5kbits/s channel and has been selected so that the degradation on the speech quality is inaudible.
In TFO established mode and when the FR of EFR codec is used, bit stealing takes place on the 2 LSB bits (bits 7-8) on each PCM sample. This provides for a 16 kbits/s channel that allows to carry vocoded speech in TFO speech frames which are similar to the frames in 08.60. The contents of the 6 MSB (bits 1-6) is normal A/-law information.
In TFO established mode and when the HR codec is used, bit stealing takes place on the LSB bit (bit 8) on each PCM sample. This provides for a 8 kbits/s channel that allows to carry vocoded speech in TFO frames which are similar to the 8 kbit/s frames in 08.61. The contents of bit 1-7 is normal A/-law information.
5.3 High-Level Functions Required for TFO
The high level functions which apply to TFO are described hereafter. Not all these functions need necessarily to be covered by the first description of TFO since some issues may be too complex to solve in an initial stage or at all.
The list of functions which are applicable to the BSS follows:
– signalling a request message (TFO_REQ) on the A interface that the TRAU supports TFO;
– signalling an acknowledgement message (TFO_ACK) that the request message has been received from the peer entity;
– monitoring request and acknowledgement messages on the A interface that the peer entity supports TFO;
– remote control of echo cancellors, DCME, …;
– signalling of current speech codec with acknowledgement message;
– signalling of speech codec capability with acknowledgement message;
– change of current speech codec within the BSS (optional);
– sending of TFO speech frames to the A interface;
– reception of TFO speech frames from the A interface;
– conversion of a flow of TRAU frames into TFO speech frames;
– conversion of a flow of TFO speech frames into TRAU frames.
The following function applies to IPEs that may be along the 64 kbits/s circuit, such as DCME or echo cancellers equipments:
– monitoring of TFO negotiation messages- repetition of LSB bit 8 one time out of 16 (TFO establishment mode, i.e reproduce at the output of the IPE the detected message);
– capability to go full or sub-64 kbit/s transparent mode and to resume normal operation based on TFO negotiation messages
– capability to monitor and alter TFO speech frame control information (optional);
– capability to insert TFO negotiation messages (optional).
No assumption is made currently on the interactions between call set-up as seen by the MSC and TFO. This means that the initial working assumption is that TFO works independently of the Call Control in the MSC, using only a mechanisms internal to the BSS to activate and de-activate TFO operation.