Date: Wed, 20 Mar 1996 12:42:17 -0500 From: dennis@etinc.com (dennis) To: "Jordan K. Hubbard" <jkh@time.cdrom.com> Cc: hackers@freebsd.org Subject: Re: ISDN: "modem" or board? (Was: Microsoft "Get ISDN"?) Message-ID: <199603201742.MAA05647@etinc.com>
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>> Ah, sorry Jordan. He's quite correct. >> >> 64kb/sec of _data_ requires 80kb/sec on an async line, because you have >> to _add_ start and stop bits to _make_ it async. > >Sigh. Maybe I'm just dense, but I never saw the part of this >conversation where we were talking about CONVERTING from sync to >async, I assumed that you'd either be talking sync-sync on both sides >or doing async-async and skipping the overhead because you'd never >_get_ 64Kbps of syncronous data from the other end to pad out with >start and stop bits. > >Yes, you and Greg are perfectly correct then and I'm arguing a totally >different and erroneous argument. I'll shut up now and spare -hackers >any more input on ISDN from this end.. :-) I think that its important to note that the original premise was wrong.... the 80k that you receive only yields 64k of actual data bits because the start and stop bits are not data. A 64kbs physical medium can only send or receive 64kbs (its not a difficult concept), The fact that it adds start-stop bits at a higher rate when txing to the PC does not accelerate the data, which cannot come in at more than 64kbs. So the original claim that Mr. Kamp was getting 75000bps throughput on a 64kbs line can only be true if he counts the start-stop bits as data, which they are not. His claim is not possible without compression. Also of note is that sync HDLC has 4 bytes of overhead per frame (8 if you're using PPP), plus IP and TCP overhead if you're relying on FTP statistics, so even a full 64kbs is not possilbe. Dennis ---------------------------------------------------------------------------- Emerging Technologies, Inc. http://www.etinc.com Synchronous Communications Cards and Routers For Discriminating Tastes. 56k to T1 and beyond. Frame Relay, PPP, HDLC, and X.25 for BSD/OS, FreeBSD and LINUX
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