he H.248 message) and partly on the call case. As an example, one call consisting of context and termination reservations, tone connection, speech path through the connection, speech path disconnection and termination releases is expected to require 800-2500 bytes in one direction with ABNF (ASNI binary codes or ASCII) coding. Such a call requires eight transactions and each transaction is packed into one message. There will therefore be 8 X (28 + 20) = 384 bytes of SCTP and IPv4 overhead. For example, if an MGW handles 600 000 BHCA, the total amount of traffic in one direction including the overheads (no L1 overhead) is 1.6-3.9 Mbps. A Interface When the A interface is connected to the MGW, BSSAP signalling traffic is routed through the MGW to MSS using SIGTRAN. The MGW acts as a Signalling gateway for the BSSAP traffic from normal SS over TDM to SIGTRAN. The signalling protocol layers are presented in Figure 4.19. Iu-CS Interface Circuit switched traffic between UTRAN and the core network is carried in Iu-CS interface. This interface is ATM based and carries a user plane using an AAL2 adaptation layer and a control plane using an AAL5 adaptation layer. The control plane consists of the RANAP signalling protocol. In the core network Iu-CS is connected to the MGW. Signalling gateway functionality in MGW relays the control plane to the MSS using the SIGTRAN protocol (see Figure 4.20). The calculation of the RANAP message load depends on the number of subscribers and on the traffic and mobility profile of each subscriber. If no better knowledge or tool is available, the following estimation of one particular user profile can be used. The average number of RANAP signalling messages sent and Core Network Planning and Optimisation BSSAP BSSAP SCCPb Ethernet Ethernet Figure 4.19 MGW as the signalling gateway for BSSAP received by one user per hour is estimated to be 70, or 35 in one direction (1 call, 1.5 SMS, 2 location updates). Assuming an average message length of 70 bytes, two ATM cells are needed to be transmitte