eparate PCM links, constituting a signalling link (SL) set as shown in Figure 4.7. The maximum number of signalling links that can constitute a signalling link set is also limited, e.g. 16. These signalling links are usually assigned priorities. If there is a failure, the highest priority link carries the traffic of the failed signalling link. For protection purposes, different routes are designated to carry the signalling from one destination point to another. These destination points are known as destination point codes (DPC). There is also a limit to the number of routes that can be assigned between two DPCs. However, there should be a minimum of two routes for protection purposes. SL Set Figure 4.7 SEP, STP, SL and SL set 4.3.3 Signalling Network Dimensioning Signalling is transferred on 64 kbps time slots on the PCM. High-capacity links use more capacity (i.e. more than one time slot). But as traffic increases, delays will take place in delivering messages. There are land-, equipment- and satellite-based signalling links. The acceptable delays for each of these can be 0.2 Erl, 0.4 Erl and 0.6 Erl respectively. Of these THE INTELLIGENT NETWORK (IN) three values. 0.4 Erl is the performance requirement of the equipment nodes and 0.6 Erl is used for satellite connections. The main aspect to be remembered during dimensioning is to not exceed the recommended traffic load. The process of dimensioning of the signalling network is as follows: dimension the end-to-end traffic (based on capacity assessment) route the traffic work out the transmission required. The end-to-end traffic calculation is perhaps the most tedious as it is the whole basis of signalling dimensioning and planning. The main parameter inputs required for this include call-related parameters (i.e. those relating to successful calls). Short calls are essentially considered in core network planning because they generate large signalling traffic. They also include parameters related to the ISDN user part and telephony user part (i.e. TUP/ISUP). Another