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What is the difference between an Ethernet hub and switch?
Although hubs and switches both glue the PCs in a network
together, a switch ismore expensive and a network built with
switches is generally considered faster thanone built with hubs.
Why?
When a hub receives a packet (chunk) of data (aframe in Ethernet
lingo) at one of its ports from a PCon the network, it transmits
(repeats) the packet toall of its ports and, thus, to all of the
other PCs onthe network. If two or more PCs on the network tryto
send packets at the same time a collision is saidto occur. When
that happens all of the PCs have togo though a routine to resolve
the conflict. Theprocess is prescribed in the Ethernet Carrier
SenseMultiple Access with Collision Detection (CSMA/CD)
protocol. Each Ethernet Adapter has both a receiver and a
transmitter. If theadapters didn't have to listen with their
receivers for collisions they would be able tosend data at the same
time they are receiving it (full duplex). Because they have
tooperate at half duplex (data flows one way at a time) and a hub
retransmits datafrom one PC to all of the PCs, the maximum
bandwidth is 100 Mhz and thatbandwidth is shared by all of the PC's
connected to the hub. The result is when aperson using a computer
on a hub downloads a large file or group of files fromanother
computer the network becomes congested. In a 10 Mhz 10Base-T
networkthe affect is to slow the network to nearly a crawl. The
affect on a small, 100 Mbps(million bits per scond), 5-port network
is not assignificant.
Two computers can be connected directly togetherin an Ethernet
with acrossover cable. A crossovercable doesn't have a collision
problem. It hardwiresthe Ethernet transmitter on one computer to
thereceiver on the other. Most 100BASE-TX EthernetAdapters can
detect when listening for collisions isnot required with a process
known as auto-negotiation and will operate in a full duplex mode
when it is permitted. The result is acrossover cable doesn't have
delays caused by collisions, data can be sent in both
directions simultaneously, the maximum available bandwidth is
200 Mbps, 100 Mbpseach way, and there are no other PC's with which
the bandwidth must be shared.
An Ethernet switch automatically divides the network into
multiple segments, acts asa high-speed, selective bridge between
the segments, and supports simultaneousconnections of multiple
pairs of computers which don't compete with other pairs ofcomputers
for network bandwidth. It accomplishes this by maintaining a table
ofeach destination address and its port. When the switch receives a
packet, it readsthe destination address from the header information
in the packet, establishes atemporary connection between the source
and destination ports, sends the packeton its way, and then
terminates the connection.
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Picture a switch as making multiple temporary crossover cable
connections betweenpairs of computers (the cables are actually
straight-thru cables; the crossoverfunction is done inside the
switch). High-speed electronics in the switchautomatically connect
the end of one cable (source port) from a sending computer tothe
end of another cable (destination port) going to the receiving
computer on a per
packet basis. Multiple connections like this can occur
simultaneously. It's as simpleas that. And like a crossover cable
between two PCs, PC's on an Ethernet switch donot share the
transmission media, do not experience collisions or have to listen
forthem, can operate in a full-duplex mode, have bandwidth as high
as 200 Mbps, 100Mbps each way, and do not share this bandwidth with
other PCs on the switch.
Hubs, switches, and routers are all devices that let you connect
one or morecomputers to other computers, networked devices, or to
other networks. Each hastwo or more connectors called ports into
which you plug in the cables to make theconnection. Varying degrees
of magic happen inside the device, and therein lies thedifference.
I often see the terms misused so let's clarify what each one really
means.
Ahub is typically the least expensive, least intelligent, and
least complicated of thethree. Its job is very simple: anything
that comes in one port is sent out to theothers. That's it. Every
computer connected to the hub "sees" everything that everyother
computer on the hub sees. The hub itself is blissfully ignorant of
the databeing transmitted. For years, simple hubs have been quick
and easy ways to connectcomputers in small networks.
Aswitch does essentially what a hub does but more efficiently.
By paying attentionto the traffic that comes across it, it can
"learn" where particular addresses are. For
example, if it sees traffic from machine A coming in on port 2,
it now knows thatmachine A is connected to that port and that
traffic tomachine A needs to only besent to that port and not any
of the others. The net result of using a switch over ahub is that
most of the network traffic only goes where it needs to rather than
toevery port. On busy networks this can make the network
significantly faster.
Arouter is the smartest and most complicated of the bunch.
Routers come in allshapes and sizes from the small four-port
broadband routers that are very popularright now to the large
industrial strength devices that drive the internet itself. Asimple
way to think of a router is as a computer that can be programmed
to
understand, possibly manipulate, and route the data its being
asked to handle. Forexample, broadband routers include the ability
to "hide" computers behind a type offirewall which involves
slightly modifying the packets of network traffic as theytraverse
the device. All routers include some kind of user interface for
configuringhow the router will treat traffic. The really large
routers include the equivalent of afull-blown programming language
to describe how they should operate as well asthe ability to
communicate with other routers to describe or determine the best
wayto get network traffic from point A to point B.
A quick note on one other thing that you'll often see mentioned
with these devices
and that's network speed. Most devices now are capable of both
10mps (10 mega-bits, or million bits, per second) as well as 100mbs
and will automatically detect the
