fxp performance with POLLING

Joe Koberg joe at osoft.us
Tue Oct 7 19:02:16 UTC 2008

Pete French wrote:
>> 1 megabit = 106 = 1,000,000 bits which is equal to 125,000 bytes.
> you are assuming eight bits per byte - but this is a serial line so
> you should use ten bits per byte instead.
> -pete.

That was a rule of thumb in the heyday of async serial lines, which used 
a start and stop bit per byte.

However, ethernet at 100Mbit is 4B5B coded at a 125mhz rate. So the raw 
synchronous data rate really is 12.5Mbytes/s.  Minus the sync preamble 
of 8 bytes per packet and the mandatory inter-frame-gap of 12 bytes 
that's a physical layer rate of (12.5M * (1500/(1500+20))) or 12.34Mbyte/s.

Even in the later days of modems this rule applied less and less, 
because the modulation schemes became synchronous.

Joe Koberg

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