A LAN/Airplane Performance Parable

	Fortunately, Dale got to ride on a modern airplane.
	(And sent data by Ethernet.)

Ethernet has moved computer data at high speeds across local areas
for more than twenty years.  During that time the original 
10 Base 5 has gotten better and better.  First, we lost those 
cumbersome external transceivers with the advent of 10 Base 2.
Next, it became easier to troubleshoot and administer 
with 10 Base T.  Many other improvements are still being made.

	Unfortunately, the plane caught fire and Dale jumped out.
	(Dale had to share bandwidth on Ethernet with other users.)

Because an entire LAN shares the same data bus, each user
only gets part of the entire transmission capacity.   Specifically,
if there were 99 other users on the same Ethernet with Dale,
all constantly trying to send data, the theoretical maximum 
throughput Dale might ever attain would be 10,000,000/100, or
100,000 bits/second.  Practically, Dale might get a good deal less
due to longer transmissions by other computers, timing accidents,
etc.  Or Dale might get a good deal more if the other computers don't
happen to use their time slots.

	Fortunately, Dale found a parachute and jumped out.
	(And switching saved Dale's LAN performance.)

Just when it looked like Dale's company would have to replace the shared 
Ethernet with some exotic technology, Dale talked them into installing
an Ethernet switching hub, and found they could handle many more packets!  
In fact, while the old shared media hub could handle only one 
10-Megabit bit stream at a time, the switching hub could run 
16 such streams at once!)  In effect, they now had a network 
running at 16 * 10 = 160 Mb/s!  Even better was the fact that
they only had to replace the hub, and so didn't have to spend
money to replace all the network cards in their computers.

	Unfortunately, Dale's parachute failed and Dale was about to die!
	(Mostly, though, Dale didn't have 16 separate streams to switch.)

Because Dale had a dedicated-server operating system, the desktop
computers never talked to each other, but only talked to the
server.  Hence, although the switching hub would have handled
more data, the server could only transmit to or receive from
one desktop ("client") computer at a time, and it was all for 
naught.  Response time was awful, and the boss was having a fit!

	Fortunately, Dale happened to fall out over a haystack.
	(So Dale retrofitted the server with multiple switch connections.)

When performance didn't improve after the new switch was installed, 
Dale called a network consultant, who analyzed the situation and
realized that it wasn't working.  Adding more interfaces to the 
server enabled several clients to simultaneously access it through
the switch, thus taking better advantage of the switch's capacity.
Response time improved dramatically, and the boss was happy again.

	Unfortunately, a pitchfork pointed up from the middle of the haystack.
	(Five bigger servers and fifty clients were coming.)

Dale knew that as soon as this pilot project completed, the network
was to be expanded.  Although they were getting by nicely now, the new
traffic load would undoubtedly bury their network again.  The traffic 
load would go up by at least a factor of four, and no way will the
network handle it, even with switching.

	Fortunately, Dale missed the pitchfork.
	(This particular hub runs Fast Ethernet.)

Dale hadn't paid much attention to speed when the switching hub 
was purchased, but when the administrator checked it out, they
found out that it also runs 100 Base-T, also known as Fast Ethernet.
Thus, the 10/100 switching hub is already in place and ready to go.
All Dale's group will have to do when the time comes is to 
buy and install Fast Ethernet adapters in all the computers
connected to the hub.  That will provide a ten-fold increase in
wirespeed, and although it won't really move data ten times faster,
it improve things enough to handle the new computers.

	Unfortunately, he also missed the haystack!
	(Now there are more applications to pile onto the network!)

The next day, Dale's boss said that they would be adding 
computer telephony and video conferencing ability to everyone's 
computer, too.  Of course, voice and video traffic demand streaming,
that is, a need for a continuous transmission of bits at a high
rate of speed and very high reliability.  This is necessary because
if the audio or video stream is interrupted, even momentarily,
its quality is greatly impaired for the listener or watcher.  Dale
went home early today, and is now thinking about a career change 
or early retirement.

However, after Dale calms down and returns to work tomorrow,
we know that the contest will continue between the traffic and
the network, and that Dale will continue to direct it.  After all,
there's still full duplex and Gigabit Ethernet,
not to mention ATM....

-------- 30 --------

Author Note:  

Carl Shinn is the principal at compuTutor Consulting,
a Denver-based computer networking and training firm.  
Please contact him at cshinn@compututor.com or at 
303.467.2888.