> On Jun 26, 2018, at 10:13 PM, Jon Elson via cctalk <[email protected]>
> wrote:
>
> On 06/26/2018 06:20 PM, Eric Smith via cctalk wrote:
>> 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.
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
