d that for lower RAB rates (e.g. RAB 32 kbps, RAB 64 kbps) with a lower bandwidth efficiency in the IP transport, the IP-based UTRAN will require more bandwidth than the ATM-based UTRAN on both last mile and backbone links, and therefore will have more overbooking than the ATM transport. On the other hand, for higher RAB rates (e.g. RAB 384 kbps or HSPA) the conclusion will be similar to the results of RAB 128 kbps as shown in the given example, only that the achieved link utilization and overbooking gain is dependent on the bandwidth efficiency of each RAB type in different transport networks. 8.5.4 Summary In section 8.5 overbooking is investigated in detail in both ATM and IP based UTRAN transport networks. It is related to the task of dimensioning the backbone link in the star- structured multi-lub RAN, given the configurations of a certain number of Node Bs located in the RAN and each Node B carrying a certain amount of traffic on an assigned last mile link. Through analyzing the impacts of the number of Node Bs, given traffic profiles, last mile utilizations, and last mile link capacities on the attained optimum overbooking factor at the backbone link by simulations as well as analytically, the following general rules can be summarized for both ATM and IP transport networks: Normally when there are more Node Bs aggregating to the backbone link, the overbooking capability is increased due to a higher multiplexing gain; The overbooking usually shall be reduced when the last mile utilization increases. But if the last mile link already reaches the maximum utilization, the optimum overbooking factor will approach 1 independent of the number of Node Bs in the 8.5 Overbooking of the lub Backbone Link The overbooking can also be dependent on the configured last mile link capacities. Typically, the higher the last mile link bandwidth, the more overbooking is possible. The carried traffic characteristics and their QoS priorities will have considerable impact on the achievable overbooking level. And certainly, the de