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 3GPP TS 23.205 [1]@ see figure 1. Note that the present document does not preclude an implementation of a combined MSC Server and MGW.