27 Jun
2018
27 Jun
'18
9:36 a.m.
On Tue, Jun 26, 2018 at 8:13 PM, Jon Elson <elson at pico-systems.com> wrote:
On 06/26/2018 06:20 PM, Eric Smith via cctalk wrote:
Collision detection was the reason (or at least _a_ reason) why the spacing
of taps on the 10BASE-5 "thick" Ethernet cable was required to be an exact
multiple of 2.5m. It was never clear to me why this was not also a
requirement for 10BASE-2 "thin" Ethernet.
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.
Yes. I don't recall that that 10BASE-5 had any restrictions on the length
between the last tap and the terminator.
Ethernet trivia: the DIX Ethernet standard (predecessor of IEEE 802.3)
would have used a 20 Mbps data rate, but the available CRC-32 chips didn't
run that fast.