Date: Tue, 07 Oct 2008 13:32:48 -0500 From: Joe Koberg <joe@osoft.us> To: Pete French <petefrench@ticketswitch.com> Cc: antik@bsd.ee, koitsu@FreeBSD.org, freebsd-stable@freebsd.org Subject: Re: fxp performance with POLLING Message-ID: <48EBAB50.5000707@osoft.us> In-Reply-To: <E1Kmq4g-0000UZ-M8@dilbert.ticketswitch.com> References: <E1Kmq4g-0000UZ-M8@dilbert.ticketswitch.com>
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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
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