The deciding factor in choosing a connection method for communications is usually a question of the trading partner’s requirements. Automotive manufacturers, major retailers, BACS and the Inland Revenue, for example, will dictate which method of communication is to be used. The companies which exchange electronic documents with these bodies therefore have little choice available, unless those bodies allow connections via a VAN.

With a VAN acting as middleman, a company has far more choice, since the VAN can accept communications via one method and forward them via another.

The next consideration, then, is likely to be cost, speed and security of each method.

On the whole, X.25 is very secure, but is slow and very expensive. TCP/IP is cheap and fast but can lack security (unless a secure point-to-point dial-up connection is made or an encrypted VPN tunnel is used). X.25 over ISDN falls in the middle, as it is secure and reasonably cheap.

There are other methods of using X.25, whereby a standard modem connection is made to a local X.25 provider which then makes an X.25 connection on the sender’s behalf. This is called X.28 or, where the modem connection is encrypted, X.32.

Connections can also be made to the X.25 network for free using the D channel of an ISDN line (the D channel usually just controls the two B channels which are used for data). This method, X.31, is currently not available in the UK, but is available in several European countries.

The following sections give more detail about each of these communication standards and related areas. Whichever means of communication is used, the connection method needs to be the same at both ends. To explain why this is so, we begin with a description of the OSI Reference Model.