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Posted by Dominik Neubauer on December 7, 2006, 5:25 am
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IEEE 802.3-2005 refers in sections 9.1 (Repeater unit for 10 Mb/s
baseband networks), 27.1.1.1 (100 Mb/s) and 41.1.1.1 (1000 Mb/s)
to an "overall limit of 1024 stations on a network".
This value's origin, however, is not explained at any place in
the standard; the only rationale I can think of would be the
backoff timing described in section 4.2.3.2.5, with a random
delay between 0 and 2^k (with k=min(retry;10)).
On the other hand, collisions between the backoff timers may
already occur with fewer stations online; and many more stations
might be present on a network unless more than 1024 of them try
to simultaneously start a transmission.
Thanks for any hints,
Dominik Neubauer
PS: Don't worry, the question is purely academic, I will NOT
set up networks with that many stations within a single
segment ;-)
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Posted by Albert Manfredi on December 7, 2006, 10:06 am
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> IEEE 802.3-2005 refers in sections 9.1 (Repeater unit for 10 Mb/s
> baseband networks), 27.1.1.1 (100 Mb/s) and 41.1.1.1 (1000 Mb/s)
> to an "overall limit of 1024 stations on a network".
>
> This value's origin, however, is not explained at any place in
> the standard; the only rationale I can think of would be the
> backoff timing described in section 4.2.3.2.5, with a random
> delay between 0 and 2^k (with k=min(retry;10)).
>
> On the other hand, collisions between the backoff timers may
> already occur with fewer stations online; and many more stations
> might be present on a network unless more than 1024 of them try
> to simultaneously start a transmission.
This is discussed periodically on this NG. I think you're correct. The
limit is not a very credible number, it relates only to the backoff
timer of half duplex Ethernets, and even then it doesn't make a lot of
sense.
Even if there are a total of 1024 different values for backoff timers,
there is no guarantee whatever that two or more nodes in a 1024-node
network will not pick exactly the same value. And more importantly, if
there are far fewer than 1024 nodes in a single Ethernet, it is still
possible that two or more nodes will select the same backoff value
anyway. But I suppose you can say that if there are more than 1024
stations, and they all try to get access at the same time, you are
guaranteed to always have two or more colliding.
It seems clear that max number of nodes in a single network would be a
function of traffic level and traffic patterns, and could be far less
than 1024.
I suppose if you have to pick an arbitrary number which looks large,
1024 is as good as any.
Bert
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