es smaller. Suppose, however, that we ask how much traffic this resource can support at 2 per- cent blocking. Our tables tell us that a resource with 40 channels can serve 31.0 erlangs of traffic at an occupancy of 76 percent. These same tables tell us that a resource with 4 channels can serve 1.1 erlangs of traffic at an occupancy of 27 percent. This means that the same resource with traffic engineering figured into the equation looks 30 times smaller. Under these conditions, the ISDN rate would have to be 30 times the voice channel rate for the ISDN subscriber to pay an equal share. 5 In the CDMA world, means that ISDN requires a share of the channel greater than 10 times the voice call. Serving both types of users, voice and ISDN, requires a more sophisticated analysis. To make visualization easier, let us consider small users who use one unit of resource, big users who use three units of resource, and a total resource of seven units. At any given moment a small call can start, a small call can end, a big call can start, or a big call can end. Each of these events changes the available resource by one or three units one way or the other. It is tempting, but incorrect, to consider eight states, zero through seven units of resource available, as shown in Fig. 23.3. Why is this picture not complete? Because it blurs the distinction between units of resource used for small calls and units of resource used for big calls. Consider state 4 in Fig. 23.3. There are four ways to leave state 4, a small call can start, a small call can end, a big call can start, or a big call can end. The distribution of new calls is indepen- dent of the four units of resource, but the distribution of calls ending depends on whether the four resource units in state 4 are four small calls or one small call and one big call. This is the important distinction lost in this picture. The reality is two-dimensional, and the full state space for two services is a two- dimensional picture, shown in Fig. 23.4. There are 15 states rather than 8 if w