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A switch is sometimes described as a multiport bridge.
While a typical bridge may have just two ports linking
two network segments, the switch can have multiple ports
depending on how many network segments are to be linked.
Like bridges, switches learn certain information about
the data packets that are received from various
computers on the network. Switches use this information
to build forwarding tables to determine the destination
of data being sent by one computer to another computer
on the network. Essentially, a LAN switch creates a
series of instant networks that contain only the two
devices communicating with each other at that particular
moment.
Although there are some similarities between the two, a
switch is a more sophisticated device than a bridge. A
bridge determines whether the frame should be forwarded
to the other network segment based on the destination
MAC address. A switch has many ports with many network
segments connected to them. A switch chooses the port to
which the destination device or workstation is
connected. Ethernet switches are becoming popular
connectivity solutions because, like bridges, switches
improve network performance by improving speed and
bandwidth.
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Switching is a technology that reduces
congestion in Ethernet LANs by reducing the traffic and
increasing the bandwidth. Network switches look nearly
identical to hubs, but a switch generally contains more
"intelligence" (and a slightly higher price tag) than a
hub. Unlike hubs, network switches are capable of
inspecting the data packets as they are received,
determining the source and destination device of that
packet, and forwarding that packet appropriately. By
delivering messages only to the connected device that it
was intended for, network switches conserve network
bandwidth and offer generally better performance than
hubs
In data communications today, all switching equipment
performs two basic operations:
• The first operation is called switching data frames.
Switching data frames is the process by which a frame is
received on an input medium and then transmitted to an
output medium. This consists of the learning process.
The switch gets the first frame (packet of data) from a
Node. It reads the MAC address and saves it to the
lookup table. The switch now knows where to find that
Node anytime a frame is addressed to it. This process is
called learning. This is followed by the bridging
process.
• The second is the maintenance of switching operations
where switches build and maintain switching tables and
search for loops. Learning and flooding continue as the
switch adds nodes to the lookup tables. Most switches
have plenty of memory in a switch for maintaining the
lookup tables; but to optimize the use of this memory,
they still remove older information so that the switch
doesn't waste time searching through stale addresses. To
do this, switches use a technique called aging.
Basically, when an entry is added to the lookup table
for a node, it is given a timestamp. Each time a packet
is received from a node, the timestamp is updated. The
switch has a user-configurable timer that erases the
entry after a certain amount of time with no activity
from that node. This frees up valuable memory resources
for other entries.
Switches operate at much higher speeds than bridges and
can support new functionality, such as virtual LANs.
An Ethernet switch has many benefits. One benefit is
that an Ethernet switch allows many users to communicate
in parallel through the use of virtual circuits and
dedicated network segments in a virtually collision-free
environment. This maximizes the bandwidth available on
the shared medium. Another benefit is that moving to a
switched LAN environment is very cost effective because
existing hardware and cabling can be reused. |
