ups of two hundred and fifty six channels. calling DTE to indicate that the virtual circuit has not been estab- The relevant group and channel number are contained in each lished. Either DTE can clear an established call. packet header. There is a one to one correspondence between DTE and DCE channel numbers. Each virtual circuit can support at least one communication 40.6.3.3 Permanent virtual circuit session, SO in the most extreme (but unrealistic) case a single DTE/DCE link can support 4095 concurrent virtual circuits. The A permanent virtual circuit is a logical connection between two available virtual circuits are utilised to offer two facilities, namely DTEs at extreme ends of a network which is constant for an agreed the Permanent Virtual Circuit (PVC) and the Virtual Call (VC) or period of time. When the lower levels of X.25 are operational, either Switched Virtual Circuit (SVC). DTE can send packets containing data via the network to the other Physical layer standards 40/13 DTE without the need for the Virtual Call establishment procedures signals have balanced interfaces which gives greater noise im- mentioned above. munity. V.36 uses the 37 pin D type connector. 40.7.4 40.6.3.4 Virtual circuit services X.21 operates over eight interchange circuits, and this is shown in A virtual circuit service (connection oriented) is considered superior Table 40.2. to a connection-less service for handling long messages. Virtual circuits are analogous to the telephone service in that messages are delivered in the order in which they are sent. The full destination and source network addresses as well as the information required in Table 40.2 X.21 interchange circuits order to deliver a packet, are only required to establish a circuit. Once a virtual circuit has been established the logical channel numbers allow either of the DTEs on the virtual circuit to send data Circuit Direction Direction to the other. Network congestion is controlled by limiting the inputs to DCE to DTE to a virtual circuit (or controlling the f