[rescue] u2 drive trays

[Dave McGuire]

On Tuesday, August 19, 2003, at 11:29 AM, Curtis H. Wilbar Jr. wrote:
> If the equipment supports 100baseTX full duplex though that would 
> eliminate
> collisions between the two points.  no ???

   Well not exactly.  Yes, the collisions would be gone, but there's 
still a shared medium in the middle: the switching fabric.  There can 
be a lot of contention there.  Visualize nodes A, B, and C connected to 
switched ports running full-duplex.  Nodes A and B both try to send a 
packet to node C at the same time.  Many (most?) switches will use a 
store-and-forward approach to address this issue.

   A fully-switched ethernet is still a shared medium, but the media 
arbitration is handled by a switching fabric rather than detecting 
collisions and retransmitting.  The *medium* is effectively the switch, 
rather than the wire.

   Ethernet is described as a CSMA/CD system...Carrier Sense Multiple 
Access with Collision Detect.  If a node needs to transmit, it looks at 
the wire to see if there's activity...if not, it goes ahead and 
transmits.  If another node happens to do so simultaneously, you get a 
collision...which results in garbled noise on the wire.  Since an 
ethernet interface always sees its own packets, it knows whether 
they've gone out ok or have been stomped on.  In the event of a 
collision, the interface waits a random period of time and attempts to 
retransmit.

   I don't have a background in statistics, so I don't know the correct 
terminology here...but if you visualize this behavior over time, it 
becomes obvious that, for a given amount of data, the more packets that 
are being transmitted results in a higher probability for 
collisions...even for the same amount of data.  Ethernet's tiny MTU 
results in lots and lots of packets.

   CSMA/CD is at the very core of Ethernet's design.  When running a 
fully-switched network with full-duplex ports, you've modified that 
aspect of its operation...it's effectively no longer CSMA/CD.  In fact, 
at that point, I'd hesitate to even call it Ethernet...it's that much 
of a change.

   Not to be a name-dropper, but...oh, hell, I'm definitely being a 
name-dropper.  Everyone knows that Ethernet was designed by DEC, Intel, 
and Xerox (hence DIX).  It was later standardized by IEEE committee 
802.3.  That committe's chairman was an old IBM programmer (he wrote a 
chunk of what became known as OS/360) named Gerry Clancy, Jr.  He runs 
a tiny (four people at the time) software development company in 
Trenton, NJ called Princeton Desktop Systems, inc.  I worked for him 
for about three years doing database programming and desktop publishing 
stuff from about 1988.  I learned a *lot* from him.

           -Dave

--
Dave McGuire                 "You don't have Vaseline in Canada?"
St. Petersburg, FL                     -Bill Bradford