An ATM-based network is expected to allow users to negotiate their own service definitions. In this way a user can get a service appropriate to their individual needs.
Some applications require guarantees from the network, on loss or delay or both.
The connection request is refused by the network if accepting it would lead to QOS degradation for one or more currently-active connections which are complying with their contracts. The aim of a preventive CAC scheme is to balance the QOS provided to admitted connections against network utilisation by limiting the number of connections using the network. Accepted connections are provided with a traffic contract and as long as they comply with that contract, their cells are treated as high priority by the network. Cells submitted in excess of their contract may be discarded at the network access point, or may be marked as low priority and discarded if the network is congested. Therefore the network provides no guarantees for low-priority traffic.