transit topology should be selected carefully. When the Nb interface between MGWs is implemented on top of an IP backbone there is no need for a transit layer. In fact, during the call establishment the MSSs controlling the origin and destination MGWs decide which origin and destination IP addresses will be used by the origin and destination MGW for the call. As the IP backbone offers routing functionalities, this is all that is needed by the MGW to set up the call. All the MGWs in the network can be considered to have direct connections and the architecture to be fully meshed. This allows a maximum of two MGWs to be used in each call scenario (i.e. one MGW on the mobile originating and the other on the mobile terminating part of the call). When the Nb interface is implemented on top of an ATM backbone there could be a need for a transit layer. As mentioned above, during call establishment, the origin and destination MSSs decide which MGW to use for the user plane. The user plane is transported on AAL2 links and the establishment of a channel for the call between the MGWs is handled by the MGWs using the AAL2 signalling. The AAL2 signalling uses dedicated AAL2 signalling links that must be configured and maintained. In large networks the configuration and maintenance of fully meshed links (both for AAL2 signalling and the user plane) between MGWs can become quite complex. In such cases a transit layer should be considered. The transit MGWs will interconnect several groups of fully meshed MGWs. Each group of fully meshed MGWs may include one or several MSS areas. There are delay considerations to take into account when planning a layer of transit MGWs. Each MGW element on the user plane path adds some delay to the transmission of user traffic. The main sources of delay in the network are transmission delay, packetisation/buffering delay, codec delay and propagation delay. As a general rule, delay is a critical parameter for real-time traffic and should be kept at a minimum. The maximum delay for voice traffic as d