Editing X.25 (section)

==X.25 details==
The network may allow the selection of the maximal length in range 16 to 4096 octets (2<sup>n</sup> values only) per virtual circuit by negotiation as part of the call setup procedure. The maximal length may be different at the two ends of the virtual circuit.
*[[Data terminal equipment]] constructs control packets which are encapsulated into data packets. The packets are sent to the data circuit-terminating equipment, using [[LAPB]] Protocol.
*Data circuit-terminating equipment strips the layer-2 headers in order to encapsulate packets to the internal network protocol.

===X.25 facilities===
X.25 provides a set of user facilities defined and described in ITU-T Recommendation X.2.<ref>[http://www.itu.int/rec/T-REC-X.2/en/ ITU-T Recommendation X.2]</ref> The X.2 user facilities fall into five categories:

* Essential facilities;
* Additional facilities;
* Conditional facilities;
* Mandatory facilities; and,
* Optional facilities.

X.25 also provides X.25 and ITU-T specified DTE optional user facilities defined and described in ITU-T Recommendation X.7.<ref>[http://www.itu.int/rec/T-REC-X.7/en/ ITU-T Recommendation X.7]</ref> The X.7 optional user facilities fall into four categories of user facilities that require:

* Subscription only;
* Subscription followed by dynamic invocation;
* Subscription or dynamic invocation; and,
* Dynamic invocation only.

===X.25 protocol versions===
The CCITT/ITU-T versions of the protocol specifications are for [[public data network]]s (PDN).<ref>[http://www.itu.int/rec/T-REC-X.25-199610-I/en/ ITU-T Recommendation X.25 (10/96)], Summary, p. v.</ref> The ISO/IEC versions address additional features for private networks (e.g. [[local area network]]s (LAN) use) while maintaining compatibility with the CCITT/ITU-T specifications.<ref>ISO/IEC 8208:2000, Fourth Edition, Section 1: Scope, p. 1.</ref>

The user facilities and other features supported by each version of X.25 and ISO/IEC 8208 have varied from edition to edition.<ref name="annexc">ISO/IEC 8208:2000, Fourth Edition, Annex C.</ref> Several major protocol versions of X.25 exist:<ref>[http://www.itu.int/rec/T-REC-X.25/en/ ITU-T Recommendation X.25].</ref>

* CCITT Recommendation X.25 (1976) Orange Book
* CCITT Recommendation X.25 (1980) Yellow Book
* CCITT Recommendation X.25 (1984) Red Book
* CCITT Recommendation X.25 (1988) Blue Book
* ITU-T Recommendation X.25 (1993) White Book<ref>[http://www.itu.int/rec/T-REC-X.25-199303-S/en/ ITU-T Recommendation X.25 (1993) White Book]</ref>
* ITU-T Recommendation X.25 (1996) Grey Book<ref>[http://www.itu.int/rec/T-REC-X.25-199610-I/en/ ITU-T Recommendation X.25 (1996) Grey Book]</ref>

The X.25 Recommendation allows many options for each network to choose when deciding which features to support and how certain operations are performed. This means each network needs to publish its own document giving the specification of its X.25 implementation, and most networks required DTE appliance manufacturers to undertake protocol conformance testing, which included testing for strict adherence and enforcement of their network specific options. (Network operators were particularly concerned about the possibility of a badly behaving or misconfigured DTE appliance taking out parts of the network and affecting other subscribers.) Therefore, subscriber's DTE appliances have to be configured to match the specification of the particular network to which they are connecting. Most of these were sufficiently different to prevent interworking if the subscriber didn't configure their appliance correctly or the appliance manufacturer didn't include specific support for that network. In spite of protocol conformance testing, this often lead to interworking problems when initially attaching an appliance to a network.

In addition to the CCITT/ITU-T versions of the protocol, four editions of ISO/IEC 8208 exist:<ref name="annexc"/>

* ISO/IEC 8208:1987, First Edition, compatible with X.25 (1980) and (1984)
* ISO/IEC 8208:1990, Second Edition, compatible with 1st Ed. and X.25 (1988)
* ISO/IEC 8208:1995, Third Edition, compatible with 2nd Ed. and X.25 (1993)
* ISO/IEC 8208:2000, Fourth Edition, compatible with 3rd Ed. and X.25 (1996)