27 Jun
2018
27 Jun
'18
6:18 a.m.
On Jun 26, 2018, at 10:13 PM, Jon Elson via cctalk
<cctalk at classiccmp.org> wrote:
On 06/26/2018 06:20 PM, Eric Smith via cctalk wrote:
It's clear to me: the person making the claim to Jonathan simply didn't know
enough EE to understand that his statement was invalid. No, the terminator placement has
nothing to do with it.
Collision detect is a very simple process: it involves measuring the signal on the wire
and comparing that with what the transceiver is currently sending. If the two differ by
enough, it means another sender is active at the same time. There are no special cases in
this.
Proper collision detection depends on the segment length rule and the minimum packet
length. If you run the numbers, you will see that the minimum packet length and max
segment length were chosen to ensure that a collision between two stations, at opposite
end of a max length cable, will be detected by both senders. If you violate either limit
by a significant amount this will no longer be true, and you've reduced the system to
basically Aloha, which works fine at low load but maxes out at 16% of wire rate, or
thereabouts.
paul
On 06/26/2018 03:15 PM, Grant Taylor via cctalk
wrote:
>> I can only guess that having a terminator too close interferes with or
>> weakens the signal too much in some way.
>
No, I think it may have something to do with properly detecting all
collisions. There are a whole bunch of special cases, where short packets have crossed in
the middle of a segment. This causes a collision at the nodes in the center of the
segment, but the nodes at the ends see their own transmissions without interference.
Possibly, having the terminator too close to (one of) the sending nodes might make this
detection less reliable. Hmmm, but really, anything that goes past the last tap toward
the terminator ought to just DISAPPEAR, so that the length beyond the tap should not
matter.