Description of BS ISO/IEC 10177:1993 1993

This International Standard specifies the method by which a Network-layer interworking unit (IWU) uses the X.25 packet layer protocol specified in ISO/IEC 8208 to support the OSI connection-mode Network service. The specification is expressed in terms of a mapping between the Network internal layer service defined in ISO/IEC 10028 and the Virtual Call (VC) and Permanent Virtual Circuit (PVC) services of the X.25 packet layer protocol.

An IWU to which this International Standard applies interconnects, for any individual Network connection, two real subnetworks, at least one of which is accessed using the X.25 packet layer protocol. When both subnetworks are accessed using the X.25 packet layer protocol, the specification applies independently to the IWU's operation on each subnetwork.

In addition, this International Standard uses ISO/IEC 10028's definition of the relaying functions of an intermediate system to extend its application beyond a single subnetwork point of attachment, to specify the requirements to be met by an IWU as a whole in supporting the OSI connection-mode Network service.

This International Standard covers operation of the X.25 packet layer protocol that is compatible with the 1984 and later versions of CCITT Recommendation X.25, and that is compatible with the 1980 version of X.25 when the Permanent Virtual Circuit service is used; compatibility with the 1980 version of X.25 for Virtual Calls is outside the scope of this International Standard.

To enable use of the PVC service, this International Standard defines a set of subnetwork dependent convergence protocol (SNDCP) procedures. These allow a given PVC to to support one Network connection at a time, but to support different Network connections, with the same or different values for address and quality of service parameters, etc, at different times. The procedures also allow for the possibility of using a PVC in support of other protocols, but not simultaneously with these procedures (apart from transient collision cases, which do not fail unnecessarily).

To evaluate conformance of a particular implementation, it is necessary to have a statement of which capabilities and options have been implemented. Such a statement is called a Protocol Implementation Conformance Statement (PICS), as defined in ISO/IEC 9646-1. This International Standard provides the PICS proforma in compliance with the relevant requirements, and in accordance with the relevant guidance, given in ISO/IEC 9646-2.

This International Standard does not specify any requirements relating to the exchange of routing information or layer-management information between an IWU and other systems.

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