The present document specifies the user plane protocol used between two Media Gateways in the CS core network. Through out the present document this protocol shall be referred to as the Nb UP protocol. The Nb UP protocol is for a large part identical to the Iu UP protocol (see TS 25.415), and only the differences between the two protocols are specified. This specification defines the applicability of the UP, as defined in TS 25.415, for the Nb interface only. Given that the Nb UP uses the same PDU types as the Iu UP, the term IuFP is used to refer to the common framing. For the purpose of the present document, any occurrence of the term "Iu UP" in the corresponding sections of TS 25.415, shall be interpreted as "Nb UP".

The present document specifies the user plane protocol of the bearer used between two MGWs within the BICC-based CS core network, called the Nb UP protocol. The present document assumes the implementation of the split between call control and the bearer transport and control, as specified in TS 23.205, see Figure 1. Note that the present document does not preclude an implementation of a combined MSC Server and MGW.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

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