Date: Sun, 25 Jun 1995 12:50:53 -0400 From: dennis@et.htp.com (dennis) To: Poul-Henning Kamp <phk@freefall.cdrom.com> Cc: freebsd-hackers@freebsd.org Subject: Re: FreeBSD as a router Message-ID: <199506251650.MAA26983@mail.htp.com>
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>> >Here is the funny shaped conical hat, please go think about what you >> >said for a while <:-). >> > >> I love this "I can demonstrate" stuff. Try the hat on yourself. To get from >> a to b to c requires 2 transmission times at 10mbs (because you have to wait >> for the full frame to arrive, which was what we were talking about) , which >> means that net throughput cannot be greater than 50% of 10mbs. >> > >Why shouldn't I be able to receive the next frame while transmitting the >first one ? > >Recv Frame 1 Recv Frame 2 Recv Frame 3 >------------> ------------> ------------> > Send Frame 1 Send Frame 2 ... > It has nothing to do with receiving while transmitting, it has to do with physical science. Box A ----> Box B (the Ethernet Router) -----> Box C I transmit a frame from Box A to Box B. For simplicity say it takes 100 microseconds to get to point B at 10mbs. I now need to re-transmit the frame to get it to Box C. It takes ANOTHER 100 microseconds to get it to Box C (Assuming no latency). To get from Box A to Box C with a non-specialized controller takes 200 microseconds, or 1/2 the single medium's max throughput. This is the same concept that applies more importantly to WAN communications and the reason for the existance of cell relay to produce higher switching thoughputs. Cells are smaller (53 octets or something for ATM) and forwarding can begin as soon as the first cell is full. This is also exactly why router based networks are slower than switched-protocol networks, because routers lose a transmission time at every hop (although the big guys are starting to do cell switching). This is also why when you chose an internet provider make sure that your provider has high-speed connectivity, because a 56k line to a router with a 56k connection only yields 28k to the "real" net. Dennis
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