--- TIC-TECH message:
Maybe it would be worthwhile to look at how Hubs and Switches differ. They
look identical from the outside but are quite different on the inside and
how they impact network performance.
First of all, you need to know how Ethernet basically works. (Ethernet is
the networking that we use in SSD). Whenever a computer wants to send some
data out on the network, it first listens to the network to see if any
other computer is "talking". If it doesn't detect any network traffic, it
starts to broadcast its own data. But, there is a certain chance that
another computer will also start broadcasting at the same time. The two
signals will collide and both pieces of data will be distroyed. When the
two computers detect the collision, they both back off and rebroadcast
their data at random intervals. This is called a contentious network
system. We use it since it is very easy to setup and keep running. Other
more exacting system, like Token Ring, are much harder to setup and often
if one computer connection goes bad, the whole network goes down.
Ethernet is OK until you get too many computers all trying to talk at once.
So, how many computers will be trying to talk at the same time? For most
of our schools, this means all of the computers anywhere in the building
that are on the Internet. So, this isn't just adjacent computers or
computers in the same room. At Ingraham, this is over 300 computers. So
collisions happen very, very often. The more computers on the same
network, the worse the collisions are and the slower all of the computers
will run since they have to keep rebroadcasting the collided data. (Sound
familiar?) And, how do Hubs and Switches mix into the problem?
Hubs are very unintelligent devices. What they do is take any data
entering it from any port, amplify it, and push it out all of the ports.
So, hubs actually add to the problem of collisions. When you pass a
certain limit of computers on the same network, data traffic becomes nearly
or actually impossible. Also take into account school periods when large
numbers of users are trying to logon, logoff, save their work, or print at
the same time. This is like having a gym full of kids talking together,
each trying to shout over everyone else. Hubs are like giving all of the
kids megaphones so that they can shout louder.
Switches work much differently. Each packet of network data has
information encoded into it telling where the data is bound for. Whenever
data enters a Switch, the switch decodes this information. The Switch also
keeps a record telling which computers are on which ports. So, when a data
packet enters a switch, the switch sends the packet out on just the one
port where that computer is connected. So, instead of everyone trying to
shout over each other as a Hub does, a Switch is more like kids passing
notes to a teacher who passes the note to the correct recepient. It takes
a Switch a little longer to operate, but it more than makes up for the time
to decode the destination and retransmit the data down the right channel
due to reducing the collisions and the need to keep rebroadcasting the data.
At Ingraham, our Business Ed networked labs have a hundred computers in
three rooms. These computers are also tied to over 200 other computers
around the building, all of whom are connected to the Internet. So, all
300+ computers used to send data packets that collided with each other. I
replaced two of the Hubs in one of the labs with switches. So, in that
lab, all 43 computers see only a data cable (actually called a "segment")
with two computers on it, itself and the computer that it is sending data
to. So, those computers can operate much faster and they do NOT send data
packets to any of the other 260 Internet connected computers in the
building or even see that traffic.
So, what is the district giving us for the WAN (and I'm sure the LAN)
wiring is 10/100 autosensing Switches with a lot of ports. As we've said
before, the district can NOT give us enough ports on Switches or Hubs for
all of the jacks that they are adding to the buildings.
So, what happens as you add more and more Hubs? More and more collisions!
And, again, we are not talking about just computers in a particular room.
All of the computers that are connected to the Internet in your building
(eight buildings at Ingraham) are adding to the gridlock. So, while it
seems like a worthy goal to add another room's computers to the Internet,
it also adds to the collisions for all of the other computers. At some
point, it will become worthless to try using any of the computers for
Internet access or anything else.
-Wes
wfelty@gte.net
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This archive was generated by hypermail 2b29 : Mon Dec 04 2000 - 20:47:28 PST