duce the number of trunks between X and Y, still keeping the BH traffic intensity at 34.25 erlangs. We would thereby increase efficiency on the X-Y route at the cost of reducing grade of service. With a modification of the switch at X, we could route traffic bound for Y that met congestion on the X-Y route via switch Z. Then Z would route that traffic on the Z-Y link. Essentially this is alternative routing in its simplest form. Congestion would probably only occur during very short peaking periods in the BH, and chances are that these peaks would not occur simultaneously with peaks Direct route Figure 4.4 Simplified diagram of the alternative (alternate) routing concept. (Solid line represents the direct route, dashed lines represent the alternative route carrying the overflow traffic from X to Y). TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK Total traffic Overflow Directly routed circuits Overflow traffic Figure 4.5 Traffic peakedness, the peaks are carried on alternative routes. of traffic intensity on the Z-Y route. Furthermore, the incremental load on the X-Z-Y route would be very small. The concept of traffic peakedness that would overflow onto the secondary (X-Z-Y) is shown in Figure 4.5. 4.2.4.2 Efficiency Versus Circuit Group Size. In the present context a circuit group refers to a group of circuits performing a specific function. For instance, all the trunks (circuits) routed from X to Y in Figure 4.4 make up a circuit group irrespective of size. This circuit group should not be confused with the "group" used in transmission engineering of carrier systems. 6 If we assume full loading, we find that efficiency improves with circuit group size. From Table 4.1, given a grade of service of 1 in 100, 5 erlangs of traffic require a group with 11 trunks, more than 2:1 ratio of trunks to erlangs, and 20 erlangs requires 30 trunks, a 3:2 ratio. If we extend this to 100 erlangs, 120 trunks are required, a 6:5 ratio. Figure 4.6 shows how efficiency improves with group size. 4.2.5 Quantifying Data Traffic D