Editing X.25 (section)

===Relation to the OSI Reference Model===
Although X.25 predates the [[OSI model|OSI Reference Model]] (OSIRM), the [[physical layer]] of the OSI model corresponds to the X.25 ''physical layer'', the [[data link layer]] to the X.25 ''data link layer'', and the [[network layer]] to the X.25 ''packet layer''.<ref name="Friend 1988 230"/> The X.25 ''data link layer'', [[LAPB]], provides a reliable data path across a data link (or multiple parallel data links, multilink) which may not be reliable itself. The X.25 ''packet layer'' provides the virtual call mechanisms, running over X.25 [[LAPB]]. The ''packet layer'' includes mechanisms to maintain virtual calls and to signal data errors in the event that the ''data link layer'' cannot recover from data transmission errors. All but the earliest versions of X.25 include facilities<ref>[http://www.itu.int/rec/T-REC-X.25-199610-I/en/ ITU-T Recommendation X.25], G.3.2 Called address extension facility, pp. 141–142.</ref> which provide for OSI [[network layer]] Addressing (NSAP addressing, see below).<ref>[http://www.itu.int/rec/T-REC-X.223/en/ ITU-T Recommendation X.223], Appendix II.</ref>