9 Nb Interface User Plane (CN-CN)
of a SIP-I-based CS Core Network (NboIP)

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

9.1 General

The SIP-I-based Circuit Switched Core Network is specified in 3GPP TS 23.231 [3]. The User Plane in this Core Network is further specified in 3GPP TS 29.414 [6]. RTP is specified in IETF RFC 3550 [8].

RTP is used in a SIP-I-based Circuit Switched Core Network as framing protocol at the Nb-Interface (without Nb-framing protocol). The rules for the usage of RTP and RTCP in 3GPP TS 29.414 [6] are applicable in combination with further Codec-specific rules provided in the present specification.

9.2 Frame Structure

9.2.0 General

The RTP framing for EVS is specified in TS 26.445 [12]. For details on the SIP-I-based Nb interface, see clause 9.3.

9.2.1 Initialisation

At the Nb-Interface in a SIP-I-based Core Network, direct RTP packing without Nb-framing is applied. Therefore, the use of PDU Type 0 for the speech payload and PDU Type 14 for Rate Control is not applicable. In addition, the principle of "Nb_Init" is not applicable for a SIP-I-based Core Network.

9.2.2 Time Alignment Procedure

Time Alignment is not specified in a SIP-I-based Circuit Switched Core Network.

9.2.3 SID Frame Generation

The generation of SID frames is determined by the EVS Speech Encoder. The radio subsystem does not influence this timing.

9.2.4 Rate and Mode Control and CMR-Only frames

Every RTP packet on the Nb interface (with Speech, SID or CMR-Only content) contains the active EVS-CMR.

The UE or any node in the speech path (e.g. a MGW during handover) may send CMR-Only frames during speech pauses, if an urgent EVS Codec Mode Request has to be sent.

9.3 RTP Packing for EVS on Nb for SIP-I

The RTP packing for EVS on this version of the Nb-Interface is as specified in TS 26.445[12], with the following additional constraints:

The RTP Payload Type number for EVS for the Nb-Interface is determined by the MSC-S dynamically.

The headerful format shall be used on Nb for SIP-I.

Each RTP packet on Nb shall contain exactly either one Speech or one SID or one CMR-Only frame. RTP redundancy is not applied on Nb.

Every RTP packet on Nb shall contain the active EVS-CMR. CMR-code-point NO_REQ shall not be used.

The active EVS-CMR shall comply with the negotiated EVS Configuration for Nb.

RTCP-APP is not used on the SIP-I based Nb interface.