A Quality of Service (QoS)-enabled computer or application uses Resource Reservation Protocol (RSVP) to send a request to the Policy Enforcement Point (PEP) for resources. RSVP is a Layer 3 protocol in which Layer 3 is the network layer of the Open System Interconnection (OSI) reference model. Layer 3 protocols operate on data packets and rely on the lower layers to deal with hardware and OSs, which makes RSVP independent of the particularities of hardware and OSs.

When the RSVP signal, which the end system sends, reaches the routers, the routers communicate with a QoS server, as in the PEP and Policy Decision Point (PDP) model. If the PDP approves of the request, the routers allocate network resources. In asynchronous transfer mode (ATM), the routers perform this allocation through the creation of a virtual circuit (VC), and in ATM 4.0, multiple types of VCs are possible. Permanent VCs (PVCs) emulate a point-to-point line in a network that provides dependable and near-absolute network resources. However, PVCs are inflexible. Network administrators must preset PVCs, thereby reserving the network resources for PVCs. Oversubscribing networks with PVCs is a dangerous proposition because if everybody tried to use their PVC simultaneously, the demand would overtax network resources and QoS guarantees would collapse. The alternative is the switched VCs (SVCs), which routers can set up on demand (e.g., in response to an RSVP signal).

You can map RSVP flows to VCs in a few different ways. You can map RSVP flows one-to-one, in which each flow receives a VC. The PDP and PEP can also aggregate several flows into one VC, which reduces the network resources required to set up and maintain individual VCs. Of course, the QoS is of slightly lower quality because different flows fight it out in one VC. Aggregating many flows into single VCs isn't a solution; although the router has fewer VCs to maintain, it must process RSVP signals for each individual flow, communicate with the PDP about each flow, then package each flow into a VC.