[MLB-WIRELESS] Cat5 max length

[Donovan Baarda]

On Fri, 2003-01-24 at 10:46, Michael_Florence at dlink.com.au wrote:
> 
> The reason that the Cat5 standard mentions 100 metres is because of Carrier
> Sense Media Allocation/Collision Detection or CSMA/CD. Or more to the point CD.
> CSMA is tha part which listens to the connection, and if no-one is talking it
> transmits (being the youngest of seven, this would have been very handy at the
> dinner table when I was a kid). If CSMA hears a transmit, it waits for a random
> amount of time, listens, and if all is well, transmits.

I thought that the random back-off was only after a collision occurred.
I think any node can transmit as soon as it hears silence, though
different NIC's probably have different ways to look for it.

The random back-off CD stuff is what causes ethernet to degenerate to
50% utilization with lots of busy nodes... half the time is spent
waiting in random back-off.

There are also CA (Collision Avoidance) protocols... these usually
involve nodes reserving frames. One frame is reserved for CD style
negotiation when reserving frames.

> CD (collision detection) comes in when both ends of the line hear nothing and
> then decide to transmit at the same time. At this point both ends of the
> connection tell the other  (to use the dinner table analogy "Hey Pete, I was
> talking here"). At this point both "talkers" wait a random number of time and
> retransmit again.

The analogy I remember was both ends blow a loud rasberry down the line
to make sure everyone knows there was a collision and begins their
random back-off. A polite "Hey, I was talking" might get missed.

i2c has an interesting collision avoidance scheme. It has propagation
times of less that 1 bit transmission time, and the bus is wired-OR.
This means if two nodes start transmitting at the same time, they "talk
in unison" until one speaker talks where the other has silence... at
that point the silent node notices someone else is talking and backs
off, leaving the message undamaged.

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