 national code for the operator (e.g. 12 for an particular operator), etc. Parameters required for call charging include call type (e.g. if the subscriber is using the network for a roaming call or for sending an SMS), whether it is a free or chargeable call, whether the incoming call is from an own-network subscriber or from an external-network subscriber, etc. Signalling in the CS Core Network RANAP signalling exists between the RNC and the MGW (i.e. on the I-cs interface) and is carried to the MSC. The signalling protocol on the I interface is shown in Figure 9.8. RANAP messages are embedded in MTP3SL messages. SS7 signalling network services over the ATM network require signalling AAL (SAAL) to deliver MTP3 signalling mes- sages between network elements. SAAL is responsible for the correct transfer of signalling messages on an ATM-based SS7 signalling link. Each of these sub-layers is considered when doing the traffic calculations and parameter settings. DETAILED NETWORK PLANNING RANAP MTP 3 SAAL NNI Physical Layer Figure 9.8 SS7 signalling on the I interface The signalling load calculation forms an important part of CS core network planning. The factors affecting the RANAP signalling load are the amount and type of traffic generated by the subscribers, and the number of subscribers. Based on these, the bandwidth required for signalling is calculated. When calculating the signalling load towards the MSC (from the MGW; also known as 'SIGTRAN'), overheads must be included. The signalling parameter group consists of signalling points, signalling point codes, signalling links, signalling link sets, etc. Also included are the signalling routing definitions. 9.3.2 Packet Switch (PS) Core Network As shown in Figure 9.4, the core network is quite similar to the one discussed in Chapter 5. Here we will look at two aspects of detailed PS core network planning, namely: connecting the (PS) core network elements to each other (i.e. planning for the interfaces) IP addressing and traffic routing. As with any core network, e