[MLB-WIRELESS] Cat5 max length

[Eric Orton]

On Thursday, January 23, 2003, at 11:16  PM, Ben Anderson wrote:

> 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...

its worse, there must be time for the other stations to receive it, and 
for their collision signal to travel back to the sending station before 
it completes transmitting, otherwise it will not know to retransmit.

So the time required to transmit the minimum frame size allowed must be 
something like 2t (where t is the time required for the signal to 
travel the length of the network).

> 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)
>>
>>
>>


--
Eric Orton
Eric.Orton at mineralsprings.net.au


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