From: Bruce Harvey (BruceH@ROUTESCAPE.COM)
Date: Thu Jul 18 2002 - 11:36:00 EDT
I like to picture Ethernet CSMA/CD in this way ...
"The Stadium Scenario"
Imagine you are a 'system.' Imagine you are sitting in a football stadium
with one hundred other 'systems' spaced about equally around the stands.
Each of you is hard of hearing, and each of you can only talk in whispers.
So, you need a megaphone to be heard ... and to listen (acting as an ear
trumpet).
Only one of you can talk at a time and be heard correctly.
You can either talk or listen ... when talking, you can hear _that_ someone
is talking, and that you weren't heard correctly, therefore, but you can't
understand what they said. Other folks will understand only part of what's
said, because they heard two statments for a time.
Some of you are patient, some of you are not, so your waiting time to repeat
your message varies.
Unless you're nosey (sniffer), you don't care about statements spoken to
individuals ... just to yourself or your group or everybody.
"Power" systems have two megaphones ... one for talking and one for
listening ... and can communicate with other "Power" systems at a faster
clip than everybody else, because they can both listen and talk at the same
time. However, they get frustrated talking with "non-Power" systems who
must switch from mouth to ear and back. They don't like to only use one
megaphone, even if it means that they will be more "in sync" with the other
"systems." And when "Power" systems sit down in a stadium with "non-Power"
systems and talk "Power," it's really hard to understand sometimes if you're
a "non-Power" system.
There are three common speeds of talkers ... traditional, speedy, and
downrightzoom ... the second 10x the speed of the first, and the third 10x
the speed of the second. Naturally, in the same stadium, if you want to
make sure everyone is available, you must all talk at the same speed ... the
speed of the slowest talker.
"Remote" users may just send their walkie-talkies (half-duplex) or
telephones (full duplex). But they need to have friends to tell them which
to use, or they won't understand or be understood. Or they need their
attorneys (SysAdmins) to guide them properly. Depends on how smart their
friends are.
I know it's not pertinent to solving the problem, but I saw the opportunity
and jumped. Hope it's useful in explaining to someone.
---------------------------------------
Bruce T. Harvey, Special Projects
bruceh@routescape.com, www.routescape.com
Insight Distribution Systems/COAXIS, Inc.
-----Original Message-----
From: Drew [mailto:apt@DCS-SUCCESS.COM]
Sent: Thursday, July 18, 2002 11:12 AM
To: aix-l@Princeton.EDU
Subject: [aix-l] ONE Last Time! SP2 Node going slowly
Here is my 2 cents, I haven't been following the thread and I know you have
just about had it but...here it goes. Sorry if some of this is sounds basic
but give me a chance.
Let's start with 10baseT and 100baseT.
Ethernet needs to work on CSMA/CD Carrirer Sense Multiple Access with
Collision Detection), which means each device must "check the wire" (or
broadcast domain) to see if any other computer is communicating and if so,
it must wait a random amount of nano-seconds and then try again.
Communication is strictly one way. Only one node at a time communicating in
only one direction at a time. Traffic is ONE way. Each computer (at least
with TCP) must periodically let the other computer know that it has received
packets 1,2,3,4 etc before the other computer will send packets 5,6,7,8 etc.
If this response does not occur then the first computer will try to resend,
and if it has to keep resending, all of a sudden, the transfer will slow to
a major crawl.
Then, out of the clear blue, Full Duplex Ethernet cards are announced and
advertises that it will allow two way communication! What a great thing!
However, how does that work with CSMA/CD? Well, if you are all plugged into
a switch that will allow two way communication (by reducing the broadcast
domain down to only the two nodes communicating, and buffering in transit)
then all is well and the equipment is working as advertised.
But, If you are plugged into a hub that does not allow for two way
communication and/or you have your cards set to full duplex, and maybe a
card or two on the LAN are set to autonegotiate or even 10 baseT, then you
will have major issues as the cards try to negotiate a communication
channel. You can even experience problems with all new cards going 100 full
duplex through a hub by itself.
When I'm setting up a LAN that may have multiple NICs going through a hub, I
usually try to set them all at a common speed and at half Duplex. I only go
full duplex through switches.
Hope this helps
Drew
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