On Tue, Jun 26, 2018 at 8:13 PM, Jon Elson <el...@pico-systems.com> 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. 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. > 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.