f total loopback initial outg ISW initial outg ISW% Traffic distribution of Exchange : Switch internal traffic = Subscribers MSC / LE Other Networks VMS listen IVR Traff 60.0% 48.0% 49.4% 12.0% 40.0% 28.1% 26.7% VMS Forward 24.4% 23.0% Interswitch traffic Other MSC's/LE's Figure 4.6 Example of traffic dimensioning CORE NETWORK PLANNING AND OPTIMISATION 4.3 BASICS OF SIGNALLING 4.3.1 Signalling Points As mentioned in Chapter 1, signalling in the NSS network is SS7. Signalling can be trans- ferred on 64 kbps time slots in PCM. Any point that is capable of sending or receiving the signalling is known as a signalling point (SP). Signalling traffic passes through a signalling transfer point (STP) and reaches its intended destination, known as the signalling end point (SEP). Thus the SP and the SEP are the same. SPs are connected to each other via PCM links. A SP can also act as a signalling control point (SCP). Through this point, access to advanced services can be made, such as freephone numbers. Signalling networks can be of different types. Signalling network indicators give the information on the type of signalling network used. In GSM networks, as SS7 is used for signalling, four different types of networks are possible: NAO, NA1, INO and IN1. The first two are for national networks and the last two for international networks. A SP can support one or more network addresses. It can also support all four networks, but in that case it would have four different SPCs, one for each network. 4.3.2 Signalling Links A signalling link is a logical connection between two SPs. SPs are connected using PCM links (also known as PCM circuits). The purpose of the signalling link is to carry the messages of higher layers. Thus, a signalling link has an ability to define the start and end of a frame structure, locate the frame for initial alignment, maintain the signalling link, detect errors, etc. Usually two signalling links are used between two SPs, for protection purposes. These two signalling links are sent on two physically s