10 Mb Interface User Plane

26.4543GPPCodec for Enhanced Voice Services (EVS)Interface to Iu, Uu, Nb and MbRelease 17TS

10.1 Overview

TS 29.163 [20] specifies the Interworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networks. Figure 1 of TS 29.163 defines some terms, reused in the present document for the Mb interface. Figure 10.1-1 reprints this Figure 1 of TS 29.163.

Figure 10.1-1: IM CN subsystem to CS network logical interworking reference model

The MGCF (Media Gateway Control Function) is a logical entity, interworking between CS Control Plane and IMS Control Plane. Depending on the result of the Codec Negotiation at call setup, the EVS Configuration for Mb may be identical to the EVS Configuration for Nb, or may be smaller or bigger. As long as both EVS Configurations are TrFO-compatible, transcoding is not required within the IM-MGW. TS 29.163 specifies the Codec Negotiation between CS Control Plane and IMS Control Plane. The MGCF informs the IM-MGW via the Mn-Interface about the selected Codec Types and Codec Configurations for both interfaces. The IM-MGW decides on the necessary interworking.

The IM-MGW (IP Multimedia Media Gateway Function) is a logical entity, interworking the User Plane between the PS-based Mb interface and the CS-based Nb-interface. Figure 10.1-1 calls this Nb-interface "CS channels". If the Codecs on Mb and Nb are TrFO-compatible, then the IM-MGW does not need to deploy transcoding.

The IM-MGW may have to perform repacking. The IM-MGW may have to modify the EVS-CMR. The IM-MGW may have to interwork the various alternatives for Rate Control on Mb with the Rate Control on Nb.

The present clause recommends the EVS Codec Configurations for the Mb User Plane, based on TS 26.445 [12]. Clause 11 describes the interworking between the Mb interface and the Nb Interface.

10.2 RTP packing on the Mb Interface

On the Nb interface, each RTP packet contains either one Speech or one SID or one CMR-Only frame with EVS-CMR in each RTP packet. The UMTS_EVS Codec Configurations on Nb are symmetrical for both speech path directions.

TS 26.445 [12] allows more freedom, how RTP packets transport EVS Speech and SID frames and CMR-Only frames on Mb. If this freedom is used, then repacking is required in the IM-MGW.

The following recommendations for EVS on Mb aim to avoid transcoding and repacking with UMTS_EVS on Nb:

– The EVS Codec Configurations on Mb should be symmetrical for both speech path directions.

– Each RTP packet on Mb should contain exactly either one Speech or one SID or one CMR-Only frame.

– The headerful format should be used on Mb in order to include the EVS-CMR in every RTP packet.

– Every RTP packet on Mb should contain the active EVS-CMR, never NO_REQ (0x7Fh, see TS 26.445).

– DTX should be allowed on the Mb interface in both directions.

10.3 Rate and mode control for EVS on Mb

On the Nb-interface the EVS-CMR provides rate and mode control for UMTS_EVS and for interworking between EVS and AMR-WB. TS 26.453 [19] specifies EVS-CMR in accordance with TS 26.445 [12] in sub-clause A.2.2.1.1. On Nb the EVS-CMR is always transported in every Speech, SID and some rare CMR-Only frames. EVS-CMR is not acknowledged, because this permanent, redundant transmission is a reliable Forward Error Correction.

TS 26.445 specifies four different alternatives how to transport CMR on the Mb interface, depending on the SDP Offer/Answer negotiation.

Mb-alt 1: The EVS-CMR is transported in RTP on Mb in every Speech, SID and some rare CMR-Only frames.

Mb-alt 2: The EVS-CMR may be transported in RTP packets on Mb "on demand", i.e. only when the
EVS-CMR value changes.

Mb-alt 3: The EVS-CMR is forbidden in RTP on Mb and instead EVS-CMR is transported in RTCP-APP
on demand on Mb. RTCP-APP is not deployed on the Nb interface.

Mb-alt 4: The EVS-CMR is forbidden in RTP and RTCP on Mb. Consequently, Transcoding is required in the IM-MGW.

Mb-alt 1 is the preferred and recommended alternative for interworking with UMTS_EVS on Nb.

Clause 11 describes the interworking between these alternatives on Mb and the transport of EVS-CMR on Nb.