<<On Fri, 25 Feb 2000 01:13:51 -0800 (PST), "Rodney W. Grimes"
<[EMAIL PROTECTED]> said:
>> [I wrote:]
>> quite right. In a CSMA/CD medium access protocol, like that used by
>> Ethernet, the actual capacity of the link is always(*) somewhat less than
>> 100%; the exact value depends on the precise parameters of the
>> transmissions at both ends.(**)
>> (*)In non-trivial conditions; i.e., when actual work is being done.
>> (**)I've heard numbers between 70% and 95%.
> And the major set of parameters that effect the higher side of this
> number are MTU(Maximum Transmission Unit) and IFG (Interframe Gap)
> and the protocol overhead of what ever proto you are using.
Nope. Those two values are defined by the protocol, and are not
parameters at all. (It's only a parameter if it could conceivably be
varied in an experiment.) The relevant parameters are the
transmission schedules of the end stations, which are of course
dynamic in most real-world applications, but not necessarily so.
These can be boiled down into a single number P(coll), which is the
probability that two stations will cause a collision by transmitting
at precisely the same time. Although this seems unlikely, certain
kinds of network protocols can unintentionally synchronize
end-stations to the extent of causing pessimal behavior.
Back in the mists of ancient time, this was considered one of the
major arguments against CSMA/CD protocols and for token-passing
protocols, and many academic papers were written about it. How
horrifying, that your dearly-bought 10-Mbit/s (or even 3-Mbit/s)
Ethernet channel could be wasting 30% of its capacity on collisions!
Nowadays, we recognize that the sort of protocols which show
pathological behavior are a bad idea for lots of reasons, and the
normal operation of TCP and IP over an Ethernet channel results in
significantly better than worst-case behavior.
-GAWollman
--
Garrett A. Wollman | O Siem / We are all family / O Siem / We're all the same
[EMAIL PROTECTED] | O Siem / The fires of freedom
Opinions not those of| Dance in the burning flame
MIT, LCS, CRS, or NSA| - Susan Aglukark and Chad Irschick
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