The layered model in Figure 6.11 shows how the LAN functions are placed within the two lower layers of the OSI reference model. The data link layer is divided into two sublayers: the logical link control (LLC) sublayer and the medium access control (MAC) sublayer. The MAC sublayer deals with the problem of coordinating the access to the shared physical medium. Figure 6.11 shows that the IEEE has defined several MAC standards, including IEEE 802.3 (Ethernet) and IEEE 802.5 (token ring). Each MAC standard has an associated set of physical layers over which it can operate.
The MAC layer provides for the connectionless transfer of datagrams. Because transmissions in LANs are relatively error free, the MAC protocols usually do not include procedures for error control. The MAC entity accepts a block of data from the LLC sublayer or directly from the network layer. This entity constructs a PDU that includes source and destination MAC addresses as well as a frame check sequence (FCS), which is simply a CRC checksum. The MAC addresses specify the physical connections of the workstations to the LAN. The main task of the MAC entities is to execute the MAC protocol that directs when they should transmit the frames into the shared medium.
In Figure 6.12 we show the protocol stacks of three workstations interconnected through a LAN. Note how all three MAC entities must cooperate to provide the datagram transfer service to the LLC sublayer. In other words, the interaction between MAC entities is not between pairs of peers, but rather
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