The Address Field comprises two subfields, viz. a Terminal Endpoint Identifier (TEI), and a Service Access Point Identifier (SAPI). See Note 1 on Terminal Endpoint Identifier and Note 2 on Service Access Points below.

It was shown in the description of Logical Terminal Profile that a TEI uniquely distinguishes the logical terminals associated with the physical terminals on the S-Bus. The inclusion of the Service Access Point Identifier, SAPI, is to facilitate an additional addressing requirement in LAPD which takes account of different services which may be supported by the layer two protocol. These include Call Control Procedure calls (SAPI=0), Packet mode communications (SAPI=16), OAM (Operational, Administration and Management) layer management functions (SAPI=63). Thus TEI and SAPI together uniquely identify a logical link which is used for addressing a logical terminal on the S-Bus.

Note 1: This note on TEI should be read in conjunction with Logical Terminal Profile.

Note 2: The Service Access Point, (SAP), is a conceptual interface between the layered protocols of ISDN and is the interface employed to facilitate communication between the layers. This communication is implemented by means of primitives and the use of primitives will be dealt with in another paper. Its addressing function is associated with the value of its Identifier, SAPI. The logical links are multiplexed on the D channel.