Date: Tue, 25 May 1999 06:13:52 +1000 (EST) From: Ian Smith <smithi@nimnet.asn.au> To: Tom <tom@uniserve.com> Cc: Dag-Erling Smorgrav <des@flood.ping.uio.no>, blitz@pdq.net, stable@FreeBSD.ORG Subject: Re: ISDN Terminal adapter no work Message-ID: <Pine.BSF.3.96.990525052036.21716A-100000@gaia.nimnet.asn.au> In-Reply-To: <Pine.BSF.4.02A.9905240935480.29433-100000@shell.uniserve.ca>
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[ isdn trimmed, I'm not on it ] 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. Yes, but for smaller values of 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. I expect that's true, though in Australia at least, you pay per 64K for data connections (at a much higher cost), so it's a fair consideration. > 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 > > So a 64k ISDN channel is fair amount faster (about 20%) than two 33.6k > modems. That's what it looks like if you're not running LAPM error correction on your modem link/s. But you most likely are, and while ignoring any data compression, after allowing for LAPM packet overhead and (_synchronous_) bit stuffing, LAPM modems run at about 1.16 - 1.17 times the async rate. 2 x 33.6k modems: 67200 / (10/1.16) ~= 7800 bytes/sec. Which makes 64k ISDN only about 2.5% faster, before any compression. Of course your links between serial ports and modems are most likely async, so (each) need to run at ~3900 x 10 ~= 39000 bps at that rate. Cheers, Ian To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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