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By
Jeff Bouton
5/12/2002
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Hubs,
Switches, They're All The Same...Right? |
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Not Really... |
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Before we head
on over to the test bench, we'd like to take a small detour
for our less network savvy readers. I'm sure there are
a few of our readers who have heard us use some terminology
that left them scratching their heads a little. So we
thought we'd try to cover a couple of key concepts about
networking to help clarify the basics, as well as
demonstrating the functions of a switch versus a hub.
Don't worry, this is not going to be rocket science, just a
very basic description to help you understand some
key terms and concepts.
When a computer
sends a request, it is comprised of a series of "packets"
which contains the basic details of the request. In
general, the packet contains the sending computers MAC
address, the data itself, and the receiving computers MAC
address. A MAC address is a unique identifier common
to all network cards that acts much like a finger print,
meaning no two networks cards have the same address.
This helps to insure that the request is received by the
intended recipient. A "request" can be a simple print
command, e-mail, file transfer or any other action that
requires that data be sent across the network. On a
fundamental level, all requests are the same, since they all
involve the sending and receiving of "packets."
On a hub-based
network, the packets sent between devices are "broadcast" to
all of the devices connected to the network until it is
received by the intended recipients. A device can be
anything that is connected to the network, whether it be a
printer, computer or other network ready component.
Once the message is received, the recipient sends a
confirmation packet back to the original sender confirming
that the message has been received and that it is waiting
for the next request.
Below is a basic
diagram of how a hub-based network handles a single request.
In this example, Computer "A" is sending a request to
Computer "C." As the request travels across the
network, it is 'broadcast" to all of the devices on the
networks, in this case computer "B" and the Printer.
Since computer "B" and the printer recognize that the
request is not for them, the packet is rejected, while
Computer "C" recognizes the request and accepts it.
The Hub-Based
Network
This is a fairly
rudimentary explanation, but nonetheless, this has been the
norm for many years. The main disadvantage of a
hub-based network, however, is a condition called
"collisions." Collisions occur when two or more
devices send a request at the same time. As the
"packets" from each device are "broadcast" across the
network, they can collide, corrupting the packet, requiring
that it be retransmitted. In general, this is a common
occurrence that has little noticeable effects on the
performance of a small network. Nevertheless, as a
network grows, adding more devices and making an increasing
amount of requests, these collisions can bring a network to
a crawl. In fact, if enough devices are added to the
network, a "broadcast storm" can occur where the hub-based
network is saturated with enough requests that nothing can
reach its intended recipient. This is where the
advantage of switching technology can help.
A "switch" has
one distinct advantage over a hub which can have a
tremendous effect on how well a network performs.
While a hub can only "broadcast" a request for all devices
to hear, a switch has the added ability to direct the
request to the intended device. A switch can analyze a
packet, determining who to direct the request to by scanning
the packet for the MAC address of the desired recipient.
Once the switch determines the destination machine, the
packet is sent directly to the port that machine is
connected to, while leaving the other ports on the switch
free for other traffic.
The
Switch-Based Network
In the diagram
above, computer A is sending a command to computer C, while
computer B is sending a print job to the printer. Both
requests pass through the switch, reaching their destination
at the same time. Once again, this is only a very
basic demonstration, but hopefully it helped some of our
readers gain a better understanding of the basic concept of
networking.
Next we'll hook
up the DynaStack DSR2216 16-Port Switch and transfer a few
files...
Some Benchmarks and The Rating...
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