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WGD070018_G04_02_TT02 - Tutorial 02 Carrier Sense Multiple...

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Tutorial 02 Carrier Sense Multiple Access (CSMA) 1
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Introduction In the IEEE 802 protocols for shared multi-access LANs, the data link layer is divided into two sub layers, as shown next. The upper LLC (Logical Link Control) layer provides a way to address a station on 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. The 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 . CSMA is a network access method used on shared network topologies such as Ethernet to control access to the network. Devices attached to the network cable listen (carrier sense) before transmitting. If the channel is in use, devices wait before transmitting. MA (multiple accesses) indicates that many devices can connect to and share the same network. All devices have equal access to use the network when it is clear. Even though devices attempt to sense whether the network is in use, there is a good chance that two stations will attempt to access it at the same time. On large networks, the transmission time 2
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between one end of the cable and another is enough that one station may access the cable even though another has already just accessed it.
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