Date: Sun, 25 Jun 1995 14:06:36 -0400 (EDT) From: Network Coordinator <nc@ai.net> To: dennis <dennis@et.htp.com> Cc: Poul-Henning Kamp <phk@freefall.cdrom.com>, freebsd-hackers@freebsd.org Subject: Re: FreeBSD as a router Message-ID: <Pine.BSF.3.91.950625140122.9643A-100000@aries.ai.net> In-Reply-To: <199506251650.MAA26983@mail.htp.com>
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> 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. If you are sending 1 packet, yes, it is taking 200 microseconds insteads of 100, so yes thruput is cut in half compared to A->C directly, but the maximum thruput on each line is STILL 100mbps, and if box B could handle it, it could be sending 100 megabits on both wires at the same time. When you are talking about Sending A->B->C you are adding 100 microseconds latency to the arrival of the first packet, and it will take 100 microseconds longer to recieve the last packet, but if you are sending a burst that lasts 2000 seconds [lets say] 100 microseconds latency is less than .000001% "thruput degradation." You would still be getting 100 megabits performance so long as the latency is not significant when compared to the length of the data stream. > 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. As an ISP, we have set companies up with faster than 40kbaud verified thruput on a DS0 line with a router involved. I know the concept you are trying to discuss, but you are confusing the effect of latency on thruput. -Jerry.
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