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Date: Tue, 07 Oct 2008 13:32:48 -0500
From: Joe Koberg <joe@osoft.us>
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To: Pete French <petefrench@ticketswitch.com>
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Cc: antik@bsd.ee, koitsu@FreeBSD.org, freebsd-stable@freebsd.org
Subject: Re: fxp performance with POLLING
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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
joe_at_osoft_dot_us