4. LAN Internetworking Technologies

IEEE 802 LAN Standards and Logical Link Control

In 1980, the Institute of Electrical and Electronic Engineers (IEEE) began work on the development of LAN/MAN standards: IEEE project 802. Its charter was to develop a set of OSI-RM Layer 1 and Layer 2 protocol standards for LANs and MANs. These standards specified the operational definitions for a collection of logical interfaces that make up the transmission interface that resides on OSI-RM Layers 1 and 2.

Logical interfaces are needed so that different transmission protocols can interact with one another in an efficient manner. Each logical interface defines a “handoff” point that must be able to provide a specific service. Establishment of such a point enables different transmission protocols to hand off upper layer protocol (ULP) data between them without corruption or additional interaction with the ULP. As mentioned in our discussion of the OSI-RM, this point also provides a basis for service enhancement or replacement without affecting the communication process as a whole. To this end, the IEEE's standards development focuses on distinct functional categories. These categories are as follows:

• General Architecture and Internetworking—This category covers bridging, switching, Layer 2 addressing, and management. These standards are developed by the 802.1 subcommittee. The majority of these standards define how OSI-RM Layer 2 transmission protocols will function in an operational and implementation way.

• Logical Link Control (LLC)—LLC provides the software interface between the Layer 2 transmission protocol and the Layer 3 networking protocol. The LLC standard was developed by the 802.2 subcommittee. LLC is used by all the IEEE standards-based protocols and several non-IEEE protocols. It is classified as part of OSI Layer 2 functionality. Nevertheless, it functions as a distinct service interface to manage the actual delivery of data between end-stations. Data link (the term used to describe this function with WAN protocols) or link control is provided differently, depending on the transmission protocol, but the service's function is the same regardless.

• Media Access Control (MAC)—MAC is responsible for the data transmission frame format, data integrity checking, and managing station access to the physical transmission medium (for example, collision detection in the case of Ethernet). For each MAC protocol there is a separate 802 subcommittee. Currently eight different IEEE subcommittees develop MAC standards for physical media and wireless communication. In terms of the LAN, the 802.3 subcommittee develops standards for Ethernet, and 802.5 develops standards for Token Ring. The MAC subcommittee standards encompass both OSI-RM Layers 1 and 2 functionality. The MAC service interface resides in Layer 2, and the PHY service interface—which defines the signaling and encoding used to transmit the MAC interface frames—resides at Layer 1. The MAC protocol is often used to generalize the Layer 2 function because it represents for the most part the common denominator between the PHY and LLC service layers of the data transmission function.

• Security—All security issues and standards dealing with the 802.x standards are developed by the 802.10 subcommittee.

The development and implementation of standards-based technologies assures compatibility between networking products manufactured by different product vendors. The standards development process is a collaborative effort between the IEEE, networking product vendors, and other interested parties. Actually, quite often, vendors develop proprietary implementations of technologies and then offer them as a standard or as an addition to the existing standard. When a new technology is developed, a new IEEE subcommittee is formed to codify a standard. In the event that an innovation is made to an existing IEEE standard, it is handled as a subcommittee's supplemental project. Each supplement is assigned a one- or two-letter designator. For example, the IEEE standard for 100Mbps Fast Ethernet is 802.3u. The IEEE standardization process usually takes a few years, so quite often, there will be competing “proprietary” versions of the same base technology in the marketplace. Although these technologies might provide the same functionality, they are usually incompatible. For more information about the IEEE 802 committee, you can check out their Web site at http://grouper.ieee.org/groups/802/index.html.