1

Scope

The principal use of the XID frame is to exchange data link information between two or more HDLC stations. For the purpose of this International Standard, data link information shall include any and all essential operational characteristics such as identification, authentication and/or selection of optional fu nctions and facilities concerning each station. This International Standard defines a single-exchange negotiation procedure for establishing operational characteristics when either one or more stations are capable of providing multiple selections.

This International Standard provides a means for interchanging the necessary information to establish, at a minimum, a data link connection between two correspondents wishing to commu nicate. It describes the general purpose XID frame information field content and format.

This International Standard defines encoding for information related to the basic HDLC standards only. Mechanisms are provided to permit the general-purpose XID frame information field to be used to negotiate private parameters in a single XID exchange simultaneously with negotiation of the defined basic parameters.

This International Standard does not limit or restrict the use of the XID frame information field from defining other standard formats for use in specific applications.

The following are examples of potential uses of the XID command/respo nse frame interchange:

(a) Identification of the calling and called stations when using circuit switched networks (including switched network backup a pplications).

(b) Identification of stations operating on non-switched networks requiring identification at start-up.

(c) The XID command frame with an individual, group or all- station address may be used to solicit XID response frame(s) from other station(s) on the data link, prior to or following data link establishment.

(d) Negotiation of the Frame Check Sequence (FCS) to be used for subsequent information interchange, by stations that support both 16-bit FCS and 32-bit FCS capabilities.

(e) Convey higher layer information that may be required prior to data link establishment.

(f) Transmission of an XID response frame at any respond opportunity to request an XID exchange to modify some of the operational parameters (for example, window size) following data link establishment.