Date: Mon, 24 May 1999 19:01:06 +0200 (CEST) From: Bart van Leeuwen <bart@ixori.demon.nl> To: Tom <tom@uniserve.com> Cc: Dag-Erling Smorgrav <des@flood.ping.uio.no>, isdn@freebsd.org, blitz@pdq.net, stable@freebsd.org Subject: Re: ISDN Terminal adapter no work Message-ID: <Pine.BSF.4.05.9905241851250.1785-100000@ixori.demon.nl> In-Reply-To: <Pine.BSF.4.02A.9905240935480.29433-100000@shell.uniserve.ca>
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On Mon, 24 May 1999, Tom wrote: > On 24 May 1999, Dag-Erling Smorgrav wrote: > > > "Craig Harding" <crh@outpost.co.nz> writes: > > > Most TA's act just like very fast modems. > > > > "very" is perhaps not the right word, since an single ISDN channel is > > less than twice as fast as a 33k6 modem. > > Nope. > > First of all, most everyone gets 2B+D ISDN (2 x 64K) anyhow, in fact > 1B+D (64k) isn't available in most areas of the world. > > Second, ISDN is sync, but modems are async. Now some people use various > async emulation protocols over ISDN (like V.120), but that is a complete > waste, especially if all you are doing is PPP. > > Here is what is looks like: > > 64k ISDN channel: 64000 / 8 = 8000 bytes/sec > 2 x 33.6 modem: 67200 / 10 = 6720 bytes/sec > Uusually async modems also support things like mnp4 and 5, v42, v42bis etc. one of the results of this is that the actual transport of data across a pstn or leased line with 2 async modems looks pretty much the same, and also achieves pretty much the same throughput as using 2 sync modems on that same line. Your comparison is only true for a plain async connection, but thats not what 2 async modems do. (and, when testing this on a network that has way more throughput then your async connection, you will plainly see this by achieving approx 110% of the theoretical async throughput, given you use things like a as high as possible dte-dce connection and hardware handshaking) > So a 64k ISDN channel is fair amount faster (about 20%) than two 33.6k > modems. > So... this is only true for a async connection employing 2 plain async modems without any error correction and such, which are almost extinct nowadays. Still there are a lot of advantages when using isdn, and esp. when compared to 2 33k6 connections the fact that its a single connection that can be setup quickly and requires very little overhead to be managed is where your potential performance gains are. Bart To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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