Date: Tue, 07 Oct 2008 21:24:55 +0200 (CEST) From: sthaug@nethelp.no To: joe@osoft.us Cc: freebsd-stable@freebsd.org Subject: Re: fxp performance with POLLING Message-ID: <20081007.212455.74712524.sthaug@nethelp.no> In-Reply-To: <48EBAB50.5000707@osoft.us> References: <E1Kmq4g-0000UZ-M8@dilbert.ticketswitch.com> <48EBAB50.5000707@osoft.us>
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> 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. You need add Ethernet header (14 bytes) + CRC (4 bytes). This means you have a maximum data rate, assuming 1500 byte MTU, of 12.5M * 1500/1538 = 12.19 Mbyte/s. And for those used to powers of two, M here means one million, not 1048576. Steinar Haug, Nethelp consulting, sthaug@nethelp.no
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