From: jra@law7.DaytonOH.NCR.COM (John Ackermann)
Newsgroups: rec.radio.amateur.packet
Subject: Re: high speed packet radio question
Date: 7 Jan 93 13:31:46 GMT
Organization: NCR Corporation -- Law Department

A
mattc@leland.Stanford.EDU (Matthew William Clarke) writes:

>I read the FAQ, but I don't understand the distinction between a TNC, a packet
>radio modem, and a radio used for packet radio.  Is a TNC the same thing as
>a packet radio modem? (I've used phone line modems since 1981, so I pretty 
>well understand what a modem is, but don't understand what a Terminal Node
>Controller does.) 

What we call a "TNC" (terminal node controller) actually has two pieces
inside:  a PAD (packet assembler-disassembler) that takes the serial
data from your computer or terminal and converts it to packets of data
in ax.25 format.  The output of the PAD is still at digital levels.  The
second part is the modem that converts the digital output of the PAD to
analog tones to modulate a radio.  (I'm referring to the outbound data
stream here; received data is converted from analog to digital by the
modem, and from ax.25 to serial data by the PAD).

The commercially available TNCs generally come with modems for 1200 baud
radio operation, but those internal modems can be bypassed (or sometimes
replaced) with modems for other modulation speeds or methods.  The PAD
will generate packets at various speeds, depending on the clock
frequencies it's fed.  Most TNC PADs will operate at speeds from 300 to
at least 9600 baud, and with some modification will go higher (we
operate 19.2kB radio channels with TNCs by doubling the crystal
frequency in the TNC and using faster components; we bypass the internal
modem).

For 1200 baud operation, just about any radio will work.  At higher
speeds, things become a bit more complex because a) the radios need to
switch from transmit to receive very quickly, and b) the modulation is
more complex and can't be routed into/out of the radio through the mic
and speaker jacks.

>What I am mainly trying to figure out is this:  What type of setup will
>allow high speed operation  >= 9600 baud?   What bands are used by high-speed
>packet radio?  How many people (approx.) operate at these speeds?  What
>are the limitations?

9600 baud isn't "plug and play" but it's not too difficult.  You need a
TNC, a 9600 baud modem (available from several of the TNC manufacturers,
or you can build one yourself for ~$50 -- the most common design is the
"G3RUH" which is compatible with the "K9NG" or "TAPR" designs), and a
radio that you're willing to open up and solder on.  It's legal to run
9600 baud on 2m and up (it may be legal on 6m also, I'm not sure) but of
course you'll want to operate on the same band as other local 9600 baud
users (if any -- >1200 baud operation is still very sparse in many
B
areas); I can't help you with that!

If you're considering <starting> a local high-speed packet network, let
me suggest another alternative.  Kantronics has a UHF radio designed for
packet radio that runs at 19.2kB.  We've built a network using these
radios and, though it still requires an experimenter's touch, we're very
happy with it.  The cost is very competitive with slower systems, and
offers a better baud/buck ratio.  If you're interested in this, let me
know and I can send you via email an article I wrote about our
experiences.

-- 
John R. Ackermann, Jr.         Law Department, NCR Corporation, Dayton, Ohio
(513) 445-2966		       John.Ackermann@daytonoh.ncr.com
Packet Radio: ag9v@n8acv.oh    tcp/ip: ag9v@ag9v.ampr [44.70.12.232]