Definition, specification and resources to support ISO/IEC 8878:1992 “Information technology – Telecommunications and information exchange between systems – Use of X.25 to provide the OSI Connection-mode Network Service”.

This standard provides mapping between two types of elements obligatory for a protocol supporting the CONS (Connection-Mode Network Service):

1. abstract primitives and parameters of the CONS;
2. real elements of the protocol.

The mapping is provided by the X.25 Pocket Layer Protocol (PLP) and through the use of the Virtual Calls (VC) services. This process is based on exchange of the packets between data terminal devices (DTE) by means of the Virtual Calls.

The X.25 protocol still remains important as it is well-known by its possibility to fix the bugs of the package exchanges in extremely short terms. Despite it is one of the oldest packet-switched services created, it is still widely used in countries with poorly developed telecommunication infrastructure.

This system gives possibility to choose better way of mapping, if more than one available, in terms of the X.25 VC services. Frequently, there are several ways of mapping for different combinations of end systems and Network Layer Relay systems.

The Standard gives user 2 sets of procedures of mapping available to select with three classes of implementation in each. Both these procedures are appliable as to an end system operation as to half the operation of a Network Layer Relay. It is important to mention that different halves of the Network Layer Relay operation can relate to different classes of implementation as well as to the same classes.

The tree types of implementations used are:

1. system-1984 implementation;
2. system-1980 implementation;
3. compatible implementation.

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