LAPD-based terminal adaptation for ISDN

An approach to terminal adaption for attachment of existing data terminals to integrated-services digital networks (ISDNs) that is based on the link-access protocol for the D-channel (LAPD) is presented. This terminal adaption mechanism has been incorporated in the text of CCITT draft recommendation V.120. A set of functional objectives for terminal adaption is presented; these relate to connectivity, failure isolation, and ability to take advantage of ISDN capabilities. The technique used in the LAPD-based approach, which relies on the encapsulation of the user´s data stream with the information field of LAPD frames, is briefly described. The use of CCITT recommendation Q.931 procedures, operating over a logical channel that is multiplexed at the LAPD frame level within the end-to-end physical connection, is then discussed. The Q.931 procedures control the setup of one or more logical channels carrying user data, and also control the negotiation of terminal adaption parameters for each such channel. The properties of LAPD-based terminal adaption are reviewed in the context of the stated functional objectives. It is concluded that this mechanism fulfils all the objectives, and in fact does so by taking advantage of the capabilities of ISDN, LAPD, and Q.931.<>