admitted into theVPN

For the purpose of dynamic bandwidth control, a VPG-based VPN can be compared to anATM network in which the link size can be varied. Therefore, controllers in the customerdomain operate on two views of the network (Figure 4). The view on the left side of Figure 4shows a network of end-to-end VPs which connect a set of CPNs. The view on the right showsa VPG network, which connects the same set of CPNs. The relationship between VPs andVPGs defines the mapping between both views.The VP admission controller, which participates in call setup and release in the enterprise network, operates on the left view. The controller decides whether a call can be admitted into theVPN, based on the VP capacity, its current utilization and the admission control policy. TheVP admission controller always ensures that enough capacity is available, such that cell-levelQOS can be guaranteed for all calls that are accepted. The controller runs on the time scale ofthe call arrival and departure rates (seconds or below). There can be one VP admission controller per VP, or one for a set of VPs. The VPG controller operates on both views. Depending onthe state of the VPs (in particular, traffic statistics and VP size) and the control objectives, itdynamically changes the amount of VPG bandwidth allocated to associated VPs. This controller enables customers to exploit variations in utilization among VPs that traverse the sameVPG, allowing bandwidth between VPs of different source-destination pairs to be shared without interacting with the provider. In order to guarantee QOS, the sum of the VP capacities mustbe less than or equal to the capacity of the VPG link. The controller runs on a time-scale ofseconds to minutes. The VPN controller operates on the right view. It is the only controllerwhich interacts with the provider, and it runs on the slowest time scale of all the controllers(minutes or above). The VPN controller dynamically negotiates the bandwidth of the VPGlinks with the provider, based on traffic statistics and control objectives (e.g., minimizing theVPN cost), while observing the customer's QOS requirements.