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In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. The IEEE has subdivided the data link layer into two sublayers: logical link control (LLC) and media access control (MAC). IEEE has also created several physical layer standards for different LAN protocols.
Logical Link Control (LLC)
The IEEE project 802 models take the structure of an HDLC frame and divide it into 2 sets of functions. One set contains the end user portion of the HDLC frame – the logical address, control information and data. These functions are handled by the IEEE 802.2 logical link control (LLC) Protocol.
Protocol Data Unit (PDU)
Framing LLC defines a protocol data unit (PDU) that is somewhat similar to that of HDLC. The header contains a control field like the one in HDLC; this field is used for flow and error control. The two other header fields define the upper-layer protocol at the source and destination that uses LLC. These fields are called the destination service access point (DSAP) and the source service access point (SSAP).
Media access control (MAC)
MAC sublayer resolves the contention for the shared media. It contains synchronization, flag, flow and error control specifications necessary to move information from one place to another, as well as the physical address of the next station to receive and route a packet.
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