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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.
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