t be established.9 If we were to establish a grade of service as p = 0.01 per link on a final route, and there were four links in tandem, then the grade of service end-to-end would be 4 X 0.01 or 0.04. In other words, for calls traversing this final route, one in 25 would meet congestion during the busy hour. The use of HU connections reduces tandem operation and tends to improve overall grade of service. Grade of service is the probability of meeting congestion (blockage) during the busy hour (BH). LOCAL AND LONG-DISTANCE NETWORKS Final routes Primary center High-usage routes Secondary center Tertiary center Quaternary center Figure 8.8 A hierarchical network showing alternative (alternate) routing. Note the CCITT nomenclature. The next step in the network design is to lay out HU routes. This is done with the aid of a traffic matrix. A typical traffic matrix is shown in Table 8.1. Some guidelines may be found in Section 4.2.4. Remember that larger trunk groups are more efficient. As a starting point (Section 4.2.4) for those traffic relations where the busy hour traffic intensity was > >20 erlangs, establish a HU route; for those relations <20 erlangs, the normal hierarchical routing should remain in place. National network design as described herein lends itself well to computer-based design techniques. The traffic intensity values used in traffic matrices, such as Table 8.1, should be taken from a 10-year forecast. 8.4 TRAFFIC ROUTING IN A NATIONAL NETWORK 8.4.1 New Routing Techniques 8.4.1.1 Objective of Routing. The objective of routing is to establish a successful connection between any two exchanges in the network. The function of traffic routing is the selection of a particular circuit group, for a given call attempt or traffic stream, at an 8.4 TRAFFIC ROUTING IN A NATIONAL NETWORK exchange in the network. The choice of a circuit group may be affected by information on the availability of downstream elements of the network on a quasi-real-time basis. 8.4.1.2 Network Topology. A network comprises a number