eed, in designing VPCs and VCCs, the user should be given information and assistance based on the software's knowledge of the per- formance and capacity characteristics of the network components. 13.3.2 Connection Admission Control Connection admission control protects the network from excessive loads. When a user requests a VPC or VCC, the user specifies the traffic characteristics. The user selects a QoS from the QoS classes that are available for the network. The network accepts the connection only if the resources are available to provide that QoS. Acceptance of the connection creates a "traffic contract" with the user: the user is bound to ensure that the traffic does not exceed the level specified and the network is bound to provide the QoS promised. ATM connections are policed by network operators at the UNI and Network-Node Interface (NNI) according to traffic contracts. If a connection does not adhere to a traffic contract, then action is taken against the cells which violate the traffic con- tract. Such action includes discarding or reducing the priority with which they are tagged. A multiplexer with a built-in traffic-shaping function is used to delay violat- ing cells in a buffer and transmit them to the next node once the cells adhere to the prevailing traffic contract. The Peak Cell Rate (PCR) is the maximum rate at which cells are generated on a connection. CDV expresses the variation in the rate at which cells are generated. The Sustainable Cell Rate (SCR) is the maximum rate of an ATM connection averaged over the duration of the connection. Burst tolerance is the maximum variation in the rate at which cells are generated with respect to the SCR. In short, the SCR and burst tolerance are analogous to PCR and CDV with respect to a throughput average taken over the duration of the connection. Traffic parameters are summarised in Table 13.1. Table 13.1 Traffic parameters. Traffic parameter Source Constant bit rate (CBR) Variable bit rate (VBR) Peak cell rate (PCR) Relevant Relevant Cell delay variati