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glen herrmannsfeldt
Guest
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 Posted:
Wed Sep 14, 2005 3:24 pm Post subject:
Re: Why is there a minimum spacing? |
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Dougie! wrote:
| Quote: | the 500 meters part of 10BASE5 actually relates to the Slot time of
Ethernet... Or the maximum time taken to realize a cdollision has
occurred... 2165 nanseconds.
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No, repeaters are allowed between segments.
The 500m limit is mostly due to cable attenuation and the
ability to properly do collision detect over the length of
the cable. Collisions are detected by the DC voltage on
the cable. The threshold must be more than the maximum for a
single nearby transmitter, and, for receive more detection,
less than two at the far end.
The slot time allows for repeaters and FOIRL (fiber) links,
at least traditionally.
-- glen |
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Rich Seifert
Guest
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| Posted:
Wed Sep 14, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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In article <dg855b$ski$1@canopus.cc.umanitoba.ca>,
roberson@ibd.nrc-cnrc.gc.ca (Walter Roberson) wrote:
| Quote: |
The 750 was officially introduced in October 1980, 25 years ago next
month. In an earlier posting, you indicated your thesis was
"some 25 years ago". Coincidence??
|
Not really, except to the extent that: (1) I was doing my graduate work
at WPI in 1976-79; (2) I was working at DEC at the time (and going to
school at night); (3) My job at DEC focused on the 10 Mb/s Ethernet
design and the writing of the original DIX specifications; and (4) DEC
was developing the VAX product line at the same time.
Before going to the Ethernet project, I did some signal integrity work
on the VAX 11/780 (code-named Star). While the VAX 750 (code-named
Comet) and VAX 730 (code-named Nebula) were being designed in the same
building where I worked (an abandoned and converted shopping mall in
Tewksbury, MA), I didn't have much to do with them. My thesis involved
the development of methodology for analyzing signal behavior on long
transmission lines, which came directly from my Ethernet work.
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX
Send replies to: usenet at richseifert dot com |
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James Knott
Guest
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| Posted:
Wed Sep 14, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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Rich Seifert wrote:
| Quote: | By the way, this took a HUGE
amount of computer power, at least by the standards of the time. I
managed to distribute the simulation runs across dozens of VAXen (780s,
the only ones in existence at the time) all around the world, using
DEC's private network.
|
I seem to recall, that when the Intel 386 CPU was announced, it was claimed
to be as powerful as the VAX 11/780. I remember the VAX systems we had,
included some 16 MB memory boards, that had stacked (one chip welded on top
of another) chips, to obtain that amount of memory. |
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James Knott
Guest
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| Posted:
Wed Sep 14, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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Dougie! wrote:
| Quote: | the 500 meters part of 10BASE5 actually relates to the Slot time of
Ethernet... Or the maximum time taken to realize a cdollision has
occurred... 2165 nanseconds.
|
Actually, it's due to signaling limitations. That's why the maximum
distance is different for 10base5, 10base2 and 10baseT. Due to different
cable characteristics, they can support the signal for different distances.
Note that the velocity factor is similar for all three, so timing is not
dependent on cable type. As for the collision distance, a collision has to
be detected within 512 bit times. At 10 Mb/s, that's 51.2 uS (round to 50
uS for convenience). Now, at the speed of light, that signal can travel 15
Km. Since there is also the return trip, the maximum length would be 7.5
Km. Allowing for the cable velocity factor, you're down to somewhere
around 6 Km, which greatly exceeds the distance for any copper segment.
Of course, with full duplex connections, there are no collisions to worry
about. |
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James Knott
Guest
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| Posted:
Wed Sep 14, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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Rich Seifert wrote:
| Quote: | I
managed to distribute the simulation runs across dozens of VAXen (780s,
the only ones in existence at the time) all around the world, using
DEC's private network.
|
I used to support some VAX 11/780s. That was where I first saw ethernet.
However, the first lan I worked on was part of a Collins 8500C system,
which used time division multiplexing on a ring. A device would transmit
individual bytes in it's time slot and the receiving device would listen to
that time slot. IIRC, the high speed "TDX" loop ran at 8 Mb/s and the low
speed "TDM" loop was 2 Mb/s. Each device had relays, which were used to
connect it to one of two rings. There was also a loop sync box, that would
emit a "chirp", whenever a device joined or left a ring. This was in the
late '70s, though (IIRC), the low speed TDM loop originated in the '60s. |
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DHP
Guest
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| Posted:
Sun Sep 18, 2005 8:20 am Post subject:
Re: Why is there a minimum spacing? |
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Henry <me@privacy.net> said
| Quote: | I wonder if anyone can give a definitive answer as to why there is a
minimum spacing specified on (some) ethernet cable. The thick stuff
with markers every 2.5m for example which is 1 bit of delay at 10 MHz.
[No it's not -oops!]
There is some mention of it on various web sites but the reasons for
it are not stated. Maximum lengths etc are simple enough to
understand: you need to be sure that collisions are not late. The only
reason I can think of for specifying a minimum distance is to maximise
the effect of a collision when two MAUs start transmitting at the same
time. Only I can't see that it would. They won't actually start
together. If they're waiting for the line to become free, the last
data going past them will make sure one starts after the other. So the
second will start up at the eaxct moment the first's one's data
arrives. So it will experience a zero time-difference collision. The
first one will have a two bit difference. Even if there's an advantage
in that - which I don't understand -it assumes exactly one 2.5m
section of cable. But the 2.5m is only a minimum: the spec doesn't
require exact multiplesof 2.5m over hundreds of metres! So I'm racking
my brains as to why it was ever specified at all.
|
<...>
Thanks everyone for the ensuing discussion and thanks especially to
you, Rich, for giving the definitive facts behind the case. |
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