Switches are data link layer devices that, like bridges, enable
multiple physical LAN segments to be interconnected into a single
larger network. Similar to bridges, switches forward and flood
traffic based on MAC addresses. Any network device will create some
latency. Switches can use different forwarding techniques-two of
these are store-and-forward switching and cut-through switching. In
store-and-forward switching, an entire frame must be received
before it is forwarded. This means that the latency through the
switch is relative to the frame size-the larger the frame size, the
longer the delay through the switch. Cut-through switching allows
the switch to begin forwarding the frame when enough of the frame
is received to make a forwarding decision. This reduces the latency
through the switch. Store-and-forward switching gives the switch
the opportunity to evaluate the frame for errors before forwarding
it. This capability to not forward frames containing errors is one
of the advantages of switches over hubs. Cut-through switching does
not offer this advantage, so the switch might forward frames
containing errors. Many types of switches exist, including ATM
switches, LAN switches, and various types of WAN switches.
Asynchronous Transfer Mode (ATM) switches provide high-speed
switching and scalable bandwidths in the workgroup, the enterprise
network backbone, and the wide area. ATM switches support voice,
video, and data applications, and are designed to switch fixed-size
information units called cells, which are used in ATM
communications. Figure 4-3 illustrates an enterprise network
comprised of multiple LANs interconnected across an ATM backbone.
Figure 4-3 Multi-LAN Networks Can Use an ATM-Based Backbone When
Switching Cells
LAN switches are used to interconnect multiple LAN segments. LAN
switching provides dedicated, collision-free communication between
network devices, with support for multiple simultaneous
conversations. LAN switches are designed to switch data frames at
high speeds. Figure 4-4 illustrates a simple network in which a LAN
switch interconnects a 10-Mbps and a 100-Mbps Ethernet LAN. Figure
4-4 A LAN Switch Can Link 10-Mbps and 100-Mbps Ethernet
Segments
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