4.1 Background Information

28.0623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS

Tandem Free Operation (TFO) is intended to avoid the traditional double speech encoding/decoding in MS to MS (GSM), MS to UE (GSM/3G) or UE to UE (3G) call configurations. In the following paragraphs the term "MS" is used for MS and UE, the term UE only if a 3G terminal is explicitly addressed.

In a normal MS-MS call configuration the Speech Signal is first encoded in the originating MS, sent over the Air Interface, converted to A-law or -law ITU-T Recommendation G.711 [13] in the local transcoder, carried over the fixed network, transcoded again in the distant transcoder, sent over the distant Air Interface and finally decoded in the terminating MS (see Figure 4.1-1). In this configuration, the two speech codecs (coder/decoder pairs) are in "Tandem Operation". The key inconvenience of a tandem configuration is the speech quality degradation introduced by the double transcoding. This degradation is usually more noticeable when the speech codecs are operating at low rates.

Figure 4.1-1: Typical Speech Codec Tandem Operation

When the originating and terminating connections are using the same speech codec, it is possible to transmit transparently the speech frames received from the originating MS to the terminating MS without activating the transcoding functions in the originating and terminating networks (see figure 4.1-2). In this configuration, "Tandem Free Operation" is on-going.

Figure 4.1-2: Tandem Free Operation of Speech Codec

The key advantages of Tandem Free Operation are:

– Improvement in speech quality by avoiding the double transcoding in the network;

– Possible savings on the inter-PLMN transmission links, which are carrying compressed speech compatible with a 32 kbit/s or 16 kbit/s or 8 kbit/s sub-multiplexing scheme, including packet switched transmission;

– Possible savings in processing power in the network equipment since the transcoding functions in the Transcoder Units are bypassed;

– Possible reduction in the end-to-end transmission delay.

The major constraint of Tandem Free Operation is that the inter-PLMN transmission links must be transparent to the compressed speech frames. This means that any device located in the transmission path (IPE: in path equipment) between the originating and the terminating transcoders must be disabled, switched-off, or made aware of the TFO situation to keep unaltered any compressed speech frame sent over the transmission path. Examples of such devices are listed in annex B.

The TFO Protocol defined in the present document provides the following services:

– Establishment of a transparent path between transcoders;

– Provision of an In-band signalling link between transcoders;

– Exchange of information on the active speech codec type and supported speech codec types at both ends of the call configuration;

– Codec Mismatch Resolution;

– Establishment and Maintenance of Tandem Free Operation when identical codec types are used at both ends of the call configuration;

– Fast and seamless fall back to Tandem Operation in case of necessary or unexpected TFO interruption (i.e. activation of supplementary services);

– Support for cost efficient transmission.

The present document defines Tandem Free Operation for the different Speech Codec Types used in GSM and GSM‑evolved 3G systems. This includes the GSM_FR, GSM_HR, GSM_EFR and FR_AMR, HR_AMR, OHR_AMR, UMTS_AMR, UMTS_AMR_2, FR_AMR-WB, UMTS_AMR-WB, OFR_AMR-WB, OHR_AMR-WB codec types. However, the procedures used to establish TFO are considered system independent and could be extended to call configurations involving other systems like ISDN phones, speech servers, IP Multimedia or other wireless systems.

For non-AMR Speech Codec Types (i.e. GSM_FR, GSM_EFR and GSM_HR), Tandem Free Operation is fully compatible with the installed equipment base. The feature is fully supported by the Transcoder Units. The additional processing complexity is small compared to the encoding/decoding functions. Other network elements are not affected and possibly not aware of the establishment of Tandem Free Operation.

For the support of AMR Tandem Free Operation in GSM, the BTS and possibly the BSC may be involved in addition to the TRAU.

The resolution of a possible codec mismatch is defined as an optional feature. A codec mismatch occurs when incompatible speech codecs are used at both ends of the call configuration at call set-up. The resolution consists in finding an optimal speech codec on which TFO may be established. For that purpose, other elements in the Radio Access Network (BSS in GSM or RNC in 3G) might be involved.