What is the general principle of X.25?
The general principle of X.25 is that many connections are made over a single physical link. The single link into the data network is maintained at the data link layer, while the separate packet layer maintains the connections or circuits from one DTE to other DTEs attached to the network. The data link layer is maintained between a DTE and its connected DCE, while packet layer circuits are maintained between pairs of DTEs.
Two main types of circuits exists, permanent virtual circuits and Switched virtual circuits. The permanent circuits are set up by configuration in the DTEs at either end of the circuit and in the X.25 network. When a DTE starts running, it can send data on a permanent circuit straight away. In contrast, Switched circuits are not set up when the DTE starts running. Instead a DTE must make a call through the X.25 network to set up the call. A special call request packet is sent to the network, containing the DTE address of the DTE being called. DTE addresses are set up by the network administration and uniquely identify DTEs. The DTE being called must accept the call before data transmission can begin. The call may be refused for a number of reasons. Once the call is in progress, data can be exchanged on the Switched circuit. Either end of the circuit may terminate the call. The DTE is the end user equipment that uses the X.25 network. Two common types of DTE are computers and PADs. The term PAD means literally packet assembly/disassembly but in this context means a terminal server that runs over X.25. A typical PAD has a number of asynchronous ports for supporting terminals and a synchronous port for connecting to the X.25 network. Users of terminals connected to the PAD can call up a remote DTE through the X.25 network.
Computers use X.25 in a number of different ways. Software on the computer can act as a virtual PAD, allowing users logging in to the computer to call a remote DTE through the X.25 network. Different Software on the computer can use X.25 circuits to set up network connections to other computers for mail and general transfer of data. In all cases the general principle is the same, that of two DTEs making a packet layer connection via the X.25 network. For more information about the X.25 recommendation and how computers and PADs use X.25, see CCITT recommendations X.25, X.21 and X.121, and ISO 8208.
The Router supports X.25 as both a DCE and a DTE. An interface on the Router can be configured as either a DCE or a DTE.