Date: Mon, 04 May 1998 12:03:57 -0400 From: "Louis A. Mamakos" <louie@TransSys.COM> To: jbryant@unix.tfs.net Cc: thz@tuebingen.netsurf.de (Thomas Zenker), freebsd-current@FreeBSD.ORG Subject: Re: Infrared ? (a simple experiment for laptop owners...) Message-ID: <199805041603.MAA04864@whizzo.TransSys.COM> In-Reply-To: Your message of "Mon, 04 May 1998 10:46:03 CDT." <199805041546.KAA06041@unix.tfs.net> References: <199805041546.KAA06041@unix.tfs.net>
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> Did anyone see my last posting on this topic? > > These ports are ideal for AX.25! HDLC/LAPB type protocols are well > documented. IP can be tunneled easily, as a matter of fact there are > half-duplex TCP/IP networks all over the world using these protocols. It may be true that AX.25 "works" (for some definition of working) over a half duplex channel, but that's no reason to cripple another medium with badly though-out network architecture. LABP protocols were intended to be used over full duplex, point-to-point links. Early experimentation took advantage of USRT parts and Bell 202 modems that happened to be available. Unfortunately, it persists today, more than a decade later. It is astonshing to me, as a networking professional, that you would make a deliberate choice to misapply one kind of technology into a completely different problem domain. Sure, as an experiment... AX.25 does a very simple CSMA channel access algorithm, but there is no collision detection done (or possible). AX.25 exhibits worse than ALOHA behavior in the face of channel congestion, mostly due to the hidden terminal problem. (That is, even though you listen before you transmit, you might not be able to hear another station transmitting, yet you'll interfere with his transmission.) The original implementations didn't do an exponential backoff (and I suspect many still don't) which produces a very entertaining congestive collapse of loaded channels. There have been some attempts to improve the scheme - using full duplex (split frequency) repeaters to remote the hidden terminal problem, or a distributed polling scheme to arbitrate channel access, but none of them have been adopted because of the inertia of the existing deployed base. Thus, we have doomed ourselves. > If anyone is interested, if I remember correctly, I have a copy of the > AX.25 protocol available at my web page [see .sig]... I can also > supply the LAPB docs. Implement an AX.25 if you want to talk to the existing RF systems, but you can do much better than this starting with a clean slate. AX.25 was an experiment that escaped from the lab, and now the Amateur Radio community is stuck with it. There's no reason to repeat these mistakes with a solution for Infrared wireless links. Louis Mamakos WA3YMH To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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