Thicknet/10base5 Test Segment: The Cable is In!

[Eric Smith]

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:
>
>> 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.