X.31
X.31 is an international CCITT specification for the connection of ISDN systems to X.25 networks. X.31 exists in two forms, X31 Case A and X.31 Case B, both of which are described more fully in the Packet Switching section of the ISDN chapter. Here we describe Case B, which is supported by ODEX on most platforms. Case A is not supported by ODEX.
X.31 Case B is an international CCITT specification for the connection of ISDN systems to X.25 networks using the ISDN D-channel. This facility is available in Europe and Japan, but not in the UK.
The 16 kbps D-channel of an ISDN Basic Rate Interface (BRI) is primarily used for signalling between the ISDN and the router, but has some spare capacity which can be used for data communications. X.31 Case B services are offered by some ISDN providers, and give access to an X.25 network through the ISDN D-channel.
X.31 Case B services are primarily for organisations who want to connect to the public X.25 network at relatively low cost.
Transmission rates of a minimum of 9.6 kbps are offered over X.31 link, and the costs of operation are low when compared to the operation via an ISDN B-channel. This makes X.31 Case B a cost-effective solution for low rate data applications such as e-mail or credit card validations.
X. 31 support in DI products
The following products provide X.31 support:
- EPIC
- ODEX Enterprise
- ODEXplus
- ODEX/MVS (though requires protocol converter hardware. ODEX/MVS understands only X.25 so ISDN lines have to be connected to the mainframe via boxes that map X.25 to X.31).
X.32
X.32 is a CCITT specification for a form of X.25 that is designed to operate over a dial-up (rather than leased line) connection to a public data network. It is specifically concerned with options to identify the user to the network.
In effect, X.32 is a special form of X.25, in that your computer may dial the X.25 network as and when you wish to use X.25. Unlike X.28, an X.32 connection allows synchronous X.25 to run over the dialled line. The advantages of this are that, whilst you are connected, you may both make and receive calls and can have multiple simultaneous sessions with the outside world over a single connection.
The X.32 recommendations are specifically concerned with options on identifying the user to the network. As the connection is dialled, theoretically anyone could get into the system and pretend to be this user, with all the attendant security problems. X.32 specifies a set of logon criteria and passwords that ensure the connected user has the authority to become an extension of the X.25 network.
X.25 HDLC (Higher Data Link Control) protocol standards protect the integrity of data over the vulnerable telephone link that connects your computer to the PSE. This means that any error correction is made at a lower level and is no longer the responsibility of the application software. Errors will be found and corrected faster and more efficiently.
Typical line speeds for this type of connection range from 2400bps to 9600bps. X.32 is still not available in many countries and is not currently supported by DI products.