rcuits, or virtual calls, must be dynamically all networks should provide them (although not all provide PVC established between DTEs at extreme ends of a network by the services). Datagrams are now being made obsolete within connec- exchange of command packets between the two DTEs and the tion oriented (CON) X.25 and will be replaced by a connectionless network. (CLNS) version of X.25. A virtual call is a temporary association between two DTEs. It is Recommendations X.25 defines the user facilities for each of the initiated by a DTE sending a Call Request packet to the network on three types of data transmission services. These facilities provide a free logical channel. The logical channel should be the highest free capabilities beyond the basic service, a few examples are: reverse channel available. A Call Request packet must contain either the charging, preventing incoming calls and call re-direction. destination address or a reference to it. When a Call Request packet is received by a network it will send the called DTE an Incoming call packet on the lowest free logical 40.6.3.1 Virtual circuits channel number available at that interface. The called DTE then has the option of accepting or refusing the call. The packet level interface comprises a set of logical channels, each The calling DTE will subsequently receive a response indicating with a unique identifier. Each logical channel can be associated with whether or not the called DTE has accepted the call. If the call is a single virtual circuit. A virtual circuit provides a connection based accepted the virtual circuit is established and enters the data transfer service similar to that of circuit switched services but with the added mode. At this point either DTE can send packets containing data, advantage of statistical multiplexing. via the network, to the other DTE. If however a virtual call cannot For every physical link, the number of possible logical channels be established the network will return a call clearing packet to the is given by sixteen gro