[MLB-WIRELESS] Cat5 max length

[Michael_Florence at dlink.com.au]

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.

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.

Now this is where it gets (only slightly) technical and I search back into the
memory banks of when I was A/NZ sales manager for Microtest Cable testers.

Believe it or not, current travels on a wire at slightly less than the speed of
light. The speed of any cable is measured as an NVP (nominal velocity of
propagation). NVP is the speed of the cable expressed as a percentage of the
speed of light in a vacuum. Most CAT 5 cable has an NVP of 70% - 75%. Let's say,
72% (or, the speed of Craig Mead's Datsun on a Saturday night).

Minimum frame size is 512 bits, each bit takes 0.1 microseconds (roughly the
time it takes a MLB-Wireless user to get a Galaxy dish off a house!) This means
that the minimum frame size takes 51.2 microseconds to complete. Considering
propagation times of various connectors and devices, 51.2 microseconds means
about 153 metres. Multiply this by the NVP of the cable and you get around 110
metres.

100 metres is used because it is safe, a round number and easy to work with.

Probably could re-write this better, however it is early and the coffee has not
kicked in yet!

-Michael





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