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Methods of sharing transmission media CSMA/CD is short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD. This standard enables devices to detect a collision. After detecting a collision, a device waits a random delay time and then attempts to re-transmit the message. If the device detects a collision again, it waits twice as long to try to re-transmit the message. This is known as exponential back off. CSMA/CA is short for Carrier Sense Multiple Access/Collision Avoidance, a network contention protocol that listens to a network in order to avoid collisions, unlike CSMA/CD that deals with network transmissions once collisions have been detected. CSMA/CA contributes to network traffic because, before any real data is transmitted, it has to broadcast a signal onto the network in order to listen for collision scenarios and to tell other devices not to broadcast. Token Passing Scheme This technology is used for token ring systems. Its incorporation along with complimentary fault-tolerance capabilities yield a LAN with a fair amount of sophistication, manageability and reliability. In this channel access method, a small signal called a token regularly visits each device. The token gives permission for the device to transmit if it needs to. If a transfer of data is needed, the device receives a set amount of time to broadcast its data. When it is done, the machine then retransmits the token to another machine giving that recipient permission to transmit, and so the system continues. This mechanism ensures opportunity for all devices to gain access to the LAN. Because of its predictable behavior, token scheme LANs offer the advantage of priorities, where a certain group of devices may have enhanced access to the LAN if warranted. As traffic demand increases on a token LAN, the overall throughput of data rises as well until a point is reached where the networks simply cannot accommodate anymore. Because the throughput characteristics of token LANs are so predictable, and because of the characteristics of traffic demand vs. throughput, these systems are ideal for heavy traffic situations. However, the complexity of such a LAN does come at some cost. Token systems require overhead to carry out their many functions including fault-tolerance. Plus, token ring systems are considerably more expensive than Ethernet systems.
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