CCITT Recommendation X.25 (1984) and ISO DIS 8208 define facilities that will adequately support the ISO Connection-Oriented Network Service over X.25 sub networks. However, until all public and private packet-switched X.25 networks, and existing X.25
DTEs, conform to X.25 (1984), there is a need for a method of providing the Connection-Oriented Network Service over the X.25 (1980) sub network access protocol.
This ECMA Standard defines a sub network dependent convergence protocol (as defined in ISO 8648) to offer the ISO Connection-Oriented Network Service over an X.25 sub network service. The convergence protocol is applicable in the following cases:
- When the X.25 service does not support the full Network Service (that is when the X.25 service conforms to the 1980 recommendation).
- When the resulting service for a concatenation of X.25 sub networks only conforms to the 1980 recommendations, even when one or more of the sub networks conform to later X.25 recommendations.
- When the X.25 sub network conforms to a CCITT recommendation later than 1980, but when the calling or called DTE only implements the 1980 recommendations.
The Standard specifies a minimal set of procedures that will support the ISO Connection-Oriented Network Service, principally by the use of protocol parameters conveyed within X.25 User Data Fields. The parameter encodings have been aligned to the facility codes of X.25 (1984) as defined in ISO 8208.