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LLC (Logical Link Control)

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The LLC is part of the data link layer in a protocol stack. The data link layer controls access to the network medium and defines how upper-layer data in the form of packets or datagrams is inserted into frames for delivery on a particular network. The underlying physical layer then transmits the framed data as a stream of bits on the network medium.

The IEEE 802.2 standard defines LLC, which is positioned in the protocol stack as pictured in Figure L-4. Note that LLC resides on the upper half of the data link layer. The MAC (Medium Access Control) sublayer is where individual shared LAN technologies such as Ethernet are defined. Early on, the data link layer contained only LLC-like protocols; but when shared LANs came along, the IEEE positioned the MAC sublayer into the lower half of the data link layer.

Basically, LLC provides a common interface, and provides reliability and flow-control features. It is a subclass of HDLC (High-level Data Link Control), which is used on wide area links. LLC can provide both connection-oriented and connectionless services.

The LLC acts like a software bus, allowing multiple higher-layer protocols to access one or more lower-layer networks. For example, a server may have multiple network interface cards (and an Ethernet and a token ring card). The LLC will forward packets from upper-layer protocols to the appropriate network interface. This scheme allows upper-layer protocols to operate without specific knowledge of the lower-layer network in use.

Note that the MAC layer is responsible for appending the actual physical address of the destination computer to the frame. The physical address is the hardwired address on the network interface card for the destination.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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