LLC – IEEE 802.2 Logical Links Control

The IEEE 802.2 standards for Logical Link Control define a programming interface between that part of the communications software that controls the network interface card (the Media Access Control and Physical Medium Dependent components) and the overlying protocol stack (IP, NetBIOS, NetWare, etc.).

The connection between the network interface card and the rest of the communications system is through a structure called a Service Access Point (SAP, pronounced like the sap from which maple syrup is made. The three letters, when spoken individually, “S”, “A”, “P”, refer to an SQL-based accounting software system, the “SQL Accounting Program”)

The SAP differentiates between communications protocols; there’s a SAP for NetBIOS, another for SNA, another for NetWare, and so on. A programmer can select Type 2 Link Control in which case the frames are given sequence numbers as they pass through the SAP and the 802.2 Link Control layer at the receiver provides an acknowledgement for received frames. This creates a reliable data transfer mechanism at the Data Link Layer. Type 1 it simply provides the differentiation function, with no sequence and acknowledgement process.

This section of the compendium discusses the types of Logical Link Control behavior that you might observe with your protocol analyzer and details how to isolate and describe problems that may occur.