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Two switching methods

Two switching methods 
4.2.10 This page will introduce store-and-forward and cut-through switching.
The following two switching modes are available to forward frames: 
  • Store-and-forward - The entire frame is received before any forwarding takes place. The destination and source addresses are read and filters are applied before the frame is forwarded. Latency occurs while the frame is being received. Latency is greater with larger frames because the entire frame must be received before the switching process begins. The switch is able to check the entire frame for errors, which allows more error detection.
  • Cut-through - The frame is forwarded through the switch before the entire frame is received. At a minimum the frame destination address must be read before the frame can be forwarded. This mode decreases the latency of the transmission, but also reduces error detection.
The following are two forms of cut-through switching: 
  • Fast-forward - Fast-forward switching offers the lowest level of latency. Fast-forward switching immediately forwards a packet after reading the destination address. Because fast-forward switching starts forwarding before the entire packet is received, there may be times when packets are relayed with errors. Although this occurs infrequently and the destination network adapter will discard the faulty packet upon receipt. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted.
  • Fragment-free - Fragment-free switching filters out collision fragments before forwarding begins. Collision fragments are the majority of packet errors. In a properly functioning network, collision fragments must be smaller than 64 bytes. Anything greater than 64 bytes is a valid packet and is usually received without error. Fragment-free switching waits until the packet is determined not to be a collision fragment before forwarding. In fragment-free mode, latency is also measured from the first bit received to the first bit transmitted.
The latency of each switching mode depends on how the switch forwards the frames. To accomplish faster frame forwarding, the switch reduces the time for error checking. However, reducing the error checking time can lead to a higher number of retransmissions.
This page concludes this lesson. The next lesson will describe Ethernet Switches. The first page will explain the main functions of switches.

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