8 Nb Interface User Plane (CN-CN)
of a BICC-based CS Core Network (NbF)

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

8.0 General

The data structures exchanged on the Nb interface of a BICC-based Circuit Switched Core Network are symmetrical, i.e. the structures of the sent and received data frames are identical.

The Nb-Interface is defined in a BICC-based Core Network in two different variants, a) for the ATM bearer with Nb-framing, b) for the IP bearer with Nb-framing. The Nb-framing and the use of PDU Type 0 for the speech payload is common for both versions of the Nb-Interface.

These two versions also share the principle of "Nb_Init" by PDU Type 14 messages, where the Nb-Interface is initialized on User Plane level.

8.1 Frame structure on the Nb UP transport protocol

8.1.0 General

Delivery of erroneous SDUs for EVS-coded speech on the Nb-Interface shall be set to: "YES" in a BICC-based Circuit Switched Core Network. Erroneous speech frames may be used to assist the error concealment procedures.

Therefore, PDU Type 0 (with payload CRC) shall be used for the transport of EVS-coded speech on the Nb interface.

8.1.1 Initialisation

The initialisation procedure is used for support mode. At the initialisation, several parameters are set by the CN. The initialisation procedure for the Nb Interface is described in TS 29.414 [6].

The EVS implementation shall use the "Support mode for predefined SDU sizes, version 2", see TS 25.415 [4].

8.1.2 Time Alignment Procedure

The handling of Time Alignment on the Nb Interface is described in TS 29.414 [6]. The Time alignment procedure shall be dismissed in case of TrFO.

8.1.3 SID Frame Generation

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

8.1.4 Rate and Mode Control and CMR-Only frames

For EVS the PDU Type 14 is not used on the Nb interface for Rate Control. Instead, every PDU Type 0 on the Nb interface (with Speech, SID or CMR-Only content) contains the active EVS-CMR. CMR-code-point NO_REQ shall not be used.

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.

8.2 Mapping of the EVS bits

8.2.1 Mapping for EVS Speech and SID frames

The mapping of the bits between the generic EVS frames and the PDU Type 0 for the Nb Interface is identical to the mapping on the Iu Interface for PDU Type 0.

8.2.2 Mapping for EVS-CMR

The mapping of the EVS-CMR to the PDU Type 0 for the Nb Interface is identical to the mapping on the Iu Interface.

8.3 Frame handlers

Nb PDU Frame handling functions are described in TS 29.414 [6].