d; i.e. the bandwidth required for signalling by 50k users, including the ATM layer, 50 000 subscribers X 35/3600 = 486 messages/s 486 x 2 x 53 bytes X 8 bits = 412 kbps. The RANAP signalling is carried from the MGW to the MSS/GCS using SIGTRAN. The calculation of the bandwidth needs is done considering the same number of messages and message size as in the Iu-CS interface and applying the SIGTRAN overhead to each message, i.e. again assuming a 70 byte average message length, 50k users and that each message is carried in its own SCTP message, get (70 + 12 + 28 + 20) X 8 bits X 486 messages/s = 505 kbps in one direction between the MGW and the MSC server. This includes the M3UA (12 bytes), SCTP (28 bytes) and IP (Ipv4, 20 bytes) overheads. Nc Interface The use of the BICC CS-2 is defined in 3GPP release 4. The BICC CS-2 is a bearer-independent call control capability set 2 signalling protocol on the Nc interface for the IP/ATM backbone. It supports a user plane based on the ATM or IP backbone in the Nb interface. Figure 4.21 presents implementation of the control plane protocol stack for the Nc interface. In MSS and GCS, the SIP can be used on the Nc interface as an alternative to the BICC if an IP backbone is used on the Nb interface. In the MSC server, the SIP is used as a tunnelling method for ISUP messages. The use of the SIP is not required in 3GPP release 4 but the implementation can be done in accordance with IETF specifications. SIP requires the domain name server (DNS). The result of a number analysis in the MSC server will be a domain name that represents an address on the target point of call control signalling. The MSC server then sends a query to the DNS to resolve the domain name to an IP address that will be used to route the signalling message through the IP network. Unlike SIP, BICC call control signalling does not require DNS. RANAP RANAP SCCPb SCCPb MTP3b MTP3b SAAL-NNI SAAL-NNI Ethernet Ethernet Figure 4.20 MGW as the signalling gateway for RANAP (NNI, network-to-network interface) 3GPP Release