IEEE 802 Model

Another networking model developed by the IEEE further defines sublayers of the Data Link Layer. The IEEE 802 project (named for the year and month it began — February 1980) defines the Media Access Control (MAC) and the Logical Link Control (LLC) sublayers.

As Figure 1.3 shows, the Media Access Control sublayer is the lower of the two sublayers, providing shared access for the computers' network adapter cards to the Physical Layer. The MAC Layer communicates directly with the network adapter card and is responsible for delivering error-free data between two computers on the network.

The Logical Link Control sublayer, the upper sublayer, manages data link communication and defines the use of logical interface points [called Service Access Points (SAPs)] that other computers can reference and use to transfer information from the LLC sublayer to the upper OSI layers. Two protocols running on the same computer would use separate SAPs.

Figure 1.3 Logical Link Control and Media Access Control Sublayers

Project 802 resulted in a number of documents, including three key standards for network topologies:

• 802.3 defines standards for bus networks, such as Ethernet, that use a mechanism called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
• 802.4 defines standards for token-passing bus networks. (The ArcNet® architecture is similar to this standard in many ways.)
• 802.5 defines standards for Token-Ring networks.

IEEE defined functionality for the LLC Layer in standard 802.2 and defined functionality for the MAC and Physical Layers in standards 802.3, 802.4, and 802.5.

Figure 1.4 Project 802 Standards as Related to LLC and MAC Layers

This chapter describes the layered components of the Windows NT networking architecture, beginning with an overall description of that architecture.