[MLB-WIRELESS] Cat5 max length

[Ben Anderson]

ethernet collision domains are based on the minimum length of the packet,
and the propegation delay of the cable...

basically one station cant finish sending a packet before all other stations
have received it...

a switch bypasses this problem by storing all packets in memory, and
retransmitting them whenever the destination port is free.  a hub doesnt
store the packets it just blindly amplifies to all ports, an so doesnt deal
with the aforementioned collision domain problem.  Example:


A ------------- hub -----------B

A transmits packet, the signal gets to the hub, A stops transmitting
packet...  start of packet hasnt yet got to B...  so B starts transmitting,
and because A is already finished transmitting its packet, A has no way of
figuring out that B's packet has collided with it's and needs to be
retransmitted.

Hopefully that example makes sense to most of you.

100mbit ethernet as an aside works by having a minimum packet size roughly
ten times larger than 10mbit ethernet, so the minimum 'transmit' time is the
same as 10mbit ethernet, and hence have the same collision domain radius.
Without this 100mb ethernet would have a max radius of 12.5m...  this is
also interesting as it means that as packets get small on 100mbit ethernet,
it performs like 10mbit ethernet (though there are some modern aggregating
methods that can store multiple packets and tx them in one tx zone --
though this technique is much more common in gigabit ethernet.

Later,
Ben.

> How does collision domains have to do with distance?
> and why not a hub, after all it is a multi port repeater...? and they
> regenerate and retime the signal (presuming its powered of course)
>
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