X.25 link access procedure
B. Cosell, Alan Nemeth, David C. Walden · ACM SIGCOMM Computer Communication Review · 1977
We prepared this description of the X.25 link access protocol for a commercial packet-switching network installation in which we participated. Our goal was to provide the network's users with a version of the link access procedures specification which was easier to read and understand than the X.25 specification documents available at the time. No attempt was made to change or improve the X.25 link access procedure; we merely attempted to write the procedure down more clearly. We have written the description in terms of "Nodes" and "Hosts", terms which are in common use in the packet-switching network field. Others desiring to adopt this description may decide to use the CCITT terms "DCE" and "DTE", or still other terms, for the same functional entities. The X.25 specification of its link access procedure includes a detailed description of the mechanisms used to provide bit error detection and data transparency on the link, i.e., a specific 16-bit checksum and a method of inserting and later deleting additional 1-bits which provides the equivalent of the BSC escape character mechanism. We do not view the detailed specification of these aspects of the (presumably hardware) interface as fundamental to the X.25 link access procedure. Both the error control and transparency functions must be provided, but it is easiest to use whichever specific mechanisms are provided in the communications interface line of the vendor of the Host computer rather than requiring the X.25 specification to be used. We were aided in learning the X.25 link access procedure by Marie Rousseau. Our colleague, Alex McKenzie, provided important editorial assistance and guidance regarding the presentation of this description, and by rights he should be considered a co-author.