Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

In the IEEE 802 protocols for shared multiaccess LANs, the data link layer is divided into two sublayers, as shown next. The upper LLC (Logical Link Control) layer provides a way to address a station on a LAN and exchange information with it. The lower MAC layer provides the interface between the LLC and the particular network medium that is in use (Ethernet, token ring, and so on).

The MAC layer frames data for transmission over the network, and then passes the frame to the physical layer interface where it is transmitted as a stream of bits. Framing packages information into distinct units that are transmitted one at a time on the network. If a frame is corrupted during transmission, only it needs to be resent-not the entire transmission.

Individual LAN addresses are defined in the MAC layer. A network interface card such as an Ethernet adapter has a hardwired address that is assigned at the factory. This address follows an industry standard that ensures that no other adapter has a similar address. Therefore, when you connect workstations to an IEEE network, each workstation will have a unique MAC address. Workstations on the same LAN use the MAC address to forward packets to one another. When LANs are connected in an internetwork, higher-level addressing schemes such as IP (Internet Protocol) identify networks and nodes across the internetwork.

The other job of the MAC layer is to arbitrate access to the medium that is shared by all the computers attached to the LAN. If two stations were to transmit at the same time, the data would be corrupted. The primary access methods are carrier sensing (nodes listen for a carrier tone on the cable and may transmit when the line is free) and token passing (nodes must have possession of a token before they can transmit). The following topics discuss various LAN access methods:

• ALOHA (has historical reference)


• CSMA/CD (Carrier Sense Multiple Access/Collision Detection)


• Token Bus Network


• Token Ring Network

Other access methods include DQDB (Distributed Queue Dual Bus), used with metropolitan area networks, and CDPD (Cellular Digital Packet Data), used for wireless communications. Multiplexing allows multiple devices to transmit on a channel at the same time. The channel is divided into time slots, by frequency, or by other methods. See "Multiplexing and Multiplexers."

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