eral levels of traffic control capabilities to be the requirement for meta-signalling procedures to establish, check provided by the ATM-based B-ISDN: and release signalling virtual channel connections. Three types of signalling virtual channel are considered at the user 1. Connection admission control, which are actions taken by the access (those in the network are for further study): network during call set-up phase in order to establish whether the call should be accepted or rejected. Acceptance is on the 1. Point-to-point, one in each direction allocated to each signall- basis that sufficient resources are available to establish the call ing end-point. (at the required QOS) across the whole network without impact 2. Selective broadcast, one allocated to each service profile (re- on the QOS of existing calls. lates to supplementary services: see CCITT Rec. Q.932). It can 2. Usage parameter control, which are actions taken by the net- only apply in the network-user direction. A number of possible work during the information transfer phase of a call to ensure service profile configurations have been identified. that actual cell flows conform to the parameters negotiated 3. General broadcast, which is used for broadcast signalling inde- during connection admission control, thus protecting the QOS pendent of service profiles. of other calls on the network. (Also referred to as the flow enforcement or policing function.) The meta-signalling is carried in a permanent virtual channel Priority control, which is the possibility for the user to assign connection with a standardised VPI and VCI value. different priorities to different traffic flows by using the cell 1.311 illustrates some possible signalling configuration cases: loss priority bit. each describes a possible provision of meta-signalling, signalling Congestion control, which are mechanisms used by the net- and information virtual channels and paths between the customer work in the face of traffic overloads which prevent the network 41/14 Asynchronous Transf