From: barry@dgbt.doc.ca (Barry McLarnon)
Subject: Re: high-speed RF modems on half duplex links
Organization: The Communications Research Centre
Date: Mon, 14 Dec 92 15:28:31 GMT

I posted this last week, but later noticed that my newsreader was quietly
defaulting to local distribution when posting followups :-(, so here 'tis
again:

In <1992Dec8.190356.1748@ultb.isc.rit.edu> cep4478@ultb.isc.rit.edu (C.E. Piggott ) writes:
>I have a datacomm question with regard to high-speed modems like the GRAPES
>(56 kbps) design.  When it comes to actual implementation, if you're using
>these modems for high-speed point-to-point links, I can see where it's
>worthwhile to go to a fast modem.  However, it appears that most of the
>hardware that supports these high-speed devices (including the NOS drivers
>that support DMA) are half-duplex.

>Once you get outside of the engineering labs and the text books, what is
>the real performance gain of high-speed modems in the CSMA environment?
>Even if a radio takes 10 milliseconds to be on frequency (FAST for ham
>equipment), it still takes a modem a finite amount of time to acquire
>the rx signal and then recover clock.  If you are sending "typical" ham
>packets (let's say, for the purpose of this discussion <1k), then you
>are sending for less than 150 milliseconds at 56 kbps.  That's not very
>long.  Can anybody quote how much time it takes for the modem clocks to
>lock and be ready to send data, from the time the squelch opens?

>Is the CSMA environment what these modems are designed for, or are they
>really intended for point-to-point so-called "backbone" links?

We have had a 56 kbps CSMA MAN operating in Ottawa for 3 years, and I can
attest to the fact that the GRAPES modem is an excellent performer in this
environment.  The first step is to put up a full-duplex repeater so that
you have true CSMA rather than aloha.  Although it increases the delay
through the repeater somewhat, we chose to do bit regeneration in the
repeater.  One of the keys to minimizing the time between asserting RTS
and getting good data out the rf end is to arrange that the transmit side
of the modem runs continuously, and only key the external up-converter.
This typically brings the keyup time down to less than 1 ms.  The
demodulator locks up in 3-4 ms.  We use 10 ms txdelay, which is more than
enough time to allow the tx to key up, the repeater rx to lock up, the
repeater tx to key up, and the far end rx to lock up.  This is pretty
small overhead with the packet sizes typically used for file transfers
(>1 kbytes)... of course, telnet is a different story, but it's not bad if
you can keep it in line mode.

We've discussed trying some collision detection schemes, but we're not
convinced it would buy us much in the way of increased performance, and
we're pretty happy with the way it works now.  56 kbps is a nice speed
for CSMA using conventional modulation techniques... if you go much
higher, you run into increased multipath problems (not to mention the
increased overhead penalty).

C'mon in, the water's fine... :-)

Barry VE3JF

-- 
Barry McLarnon                  |  Internet: barry@dgbt.doc.ca
Communications Research Center  |  AMPRnet:  barry@bbs.ve3jf.ampr.org
Ottawa, Canada  K2H 8S2         |  PBBSnet:  ve3jf@ve3jf.#eon.on.can
Newsgroups: rec.radio.amateur.packet
From: dave@fusk.demon.co.uk (David Brooke)