 becomes an engineered decision; it offers a determin- istic way to respond to changes, on a dynamic basis, to network status. A great ben- efit of ATM is that it provides for real-time traffic management. It enables policing and traffic shaping: It can monitor (i.e., police) the cells and determine, based on congestion, which cell should be dropped (i.e., perform traffic shaping). ATM allows networkwide resource allocation for CoS and QoS provisioning. Again, because it is connection oriented, it looks ahead to the destination point to ensure that each link along the way can deliver on the requested QoS. If it can't, the session is denied. Therefore, ATM also makes possible deterministic transit delay because you can specify and calculate the end-to-end delays, as well as the varia- tions in delays (jitter). This is all administered through multiple QoS levels. Remember that a lot of IP takes place over ATM. Because it is connection ori- ented, ATM gives service providers the traffic-engineering tools they need to man- age both QoS and utilization. ATM's virtual circuits control bandwidth allocation on busy backbone routes. In provisioning a network, the service provider can assign each virtual circuit a specific amount of bandwidth and a set of QoS param- eters. The provider can then dictate what path each virtual circuit takes. Basing these decisions on overall traffic trends reduces the likelihood of network hot spots and wasted bandwidth, and this is why so many service providers turn to ATM to transport IP traffic. However, the service provider has to deal with two control planes-managing both IP routers and ATM switches. Using ATM virtual circuits to interconnect IP routers leads to scaling problems because every router needs a separate virtual circuit to every other router. As the network grows, the number of routes and virtual circuits can increase exponentially, eventually exceeding the capacity of both switches and routers. Network operators can work around this in one of two ways: either they can forg