27 Jun
2018
27 Jun
'18
11:39 a.m.
On Jun 27, 2018, at 1:45 PM, Eric Smith <spacewar
at gmail.com> wrote:
On Wed, Jun 27, 2018 at 10:59 AM, Paul Koning <paulkoning at comcast.net> wrote:
I can believe that. But the limit for 10Base-5 is 500 meter segment length, 100
transceivers. For 10Base-2, the limits are 185 meters and 30 stations. Note that's
per segment; the whole network can be larger when repeaters are used. But the segment
limits are the ones that relate to the electrical and signal integrity aspects of
collision detect. I hope your 100-node thinwire network wasn't a single segment; such
a config would run only if you're very lucky.
The first Ethernets I saw were at DEC, in engineering buildings. They had long runs of
10Base-5 coax going all over the building, with transceivers for every node and a whole
lot of nodes total. Those were probably pushing the limits.
This reminds me of a research project done at DEC that at one point was discussed as a
possible product but didn't happen: an Ethernet segment mapping device. It was called
"packet voltmeter". The idea was that you'd have one at each end of the
cable (it replaced the terminator). It would build a table of source addresses and packet
signal amplitude. You could then combine the measurements at the two endpoints, plus the
known cable attenuation, to make a physical map (with tap placement) of each Ethernet
node.
The arrival of thinwire with its short segments and star wiring, and especially twisted
pair with point to point wiring, made this idea not suitable for a product. But the
concept was pretty amazing, and it did work.
paul
On Jun 27, 2018, at 12:36 PM, Eric Smith via
cctalk <cctalk at classiccmp.org> 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.
Yes, to avoid false alarms. The purpose of the spacing rule is to ensure that there is
enough signal integrity that you do not get spurious collision indications due to
reflections off the impedance variations along the cable. On a segment with few
transceivers, there is enough margin that the rule doesn't matter. This is why
10Base-2 doesn't have that rule: the station count limit is low enough that it
isn't needed.
Interesting! I won't disagree with what you're saying, since I'm ignorant of
these details, but in my experience 10BASE-2 networks usually had far _more_ nodes on a
network than any 10BASE-5 network I saw. I routinely saw over 100 nodes on a 10BASE-2, but
I never saw more than 20 or so on a 10BASE-5. (There certainly may have been larger
10BASE-5 networks; I only ever saw about a dozen 10BASE-5 networks.)