                                                      311 Stanford Avenue
                                                      Menlo Park, CA 94025
                                                      August 11th, 1981
 
Dear Packet Radio Enthusiast,
 
     Thanks very much for the letter of inquiry which you sent me.  The re-
sponse I've received to the initial publicity about the packet repeater has
been very enthusiastic, and I have been deluged with requests from hams, both
locally and from various points around the country, for more information about
the repeater, for schematics, for listings, specifications, modems, proms,
SDLC chips, Vancouver boards, and for talks at clubs.  Needless to say, all
this activity, plus continuing development on the packet hardware and software
has kept me very busy, and I apologize for the long delay in responding to
your letter.  Let me bring you up to date on what has happened, or is happen-
ing, since the initial announcement of the repeater, which went on the air in
December of 1980.
 
     In the early months of this year, the packet repeater was operating out
of my residence, and was still an experimental machine.  Since then, we have
installed a couple of upgrades to the control software, we have used a better
CPU card, increased the power level, moved the repeater to 700 feet elevation,
and integrated its operation to be 100% compatible with the protocol used by
the Vancouver Digital Communications Group (VADCG).  The repeater has changed
from being a laboratory curiosity to a major Bay Area repeater heard from
Berkeley to south San Jose, and the user community has grown from a couple of
stations to a network of some 30 users.  The packet system here now has a
mailbox on-line 24 hours a day, several on-line personal computers, and net-
work links (courtesy of a commercial packet network) to the other active pack-
et radio centers in Vancouver and Ottawa.  We have also just installed an HF
port on 20 Meters, and are beginning some experiments aimed at establishing
connection with AMRAD in Washington and with equipment located at W1AW.
 
     Most of the original packet radio experiments were done in Canada (in
part due to the Canadians' pioneering communications spirit, and in part due
to less restrictive regulations up there), and three main centers were at
work:  Montreal, Ottawa and Vancouver.  The technology employed by each of
these groups differed, and each approach has its own merits.  My thinking and
ideas very closely paralleled the work started by Doug Lockhart, VE7APU, and I
can best report on what is happening with groups which have adopted HDLC (High
-level Data Link Control) framing as the basis of their protocol.  The HDLC/
SDLC frame is a new, universally accepted standard in the data communications
industry, and Doug and I feel it offers a good starting point on which to
build a packet radio network.  As it turns out, groups in Washington D.C., Los
Angeles, El Paso, Denver, Sacramento, and Hamilton have also taken up this
technology, and it is likely that we already have a sufficient number of
people using this technique that it will become the defacto standard in the
amateur radio community.
 
     It would be impossible for me to completely describe the protocol and
equipment being used in this letter, so I will briefly cover some of the top-
ics and give you some pointers on where to find additional information.  As
you might guess, this is a new area for amateur radio, and tutorial material
and handbooks simply do not yet exist.  Many issues and problems remain to be
discussed, and there is opportunity to make substantial contributions to the
state of the art.
 
     The Protocol - The basis for our technology is the HDLC frame, which is
simply a way of encapsulating a series of bits into a message block.  This
type of message framing offers a high degree of error detection, data trans-
parency, NRZI encoding, and a comprehensive set of standards for its use in a
point-to-point protocol.  Source documents for HDLC/SDLC and protocol are the
following:
            IBM Synchronous Data Link Control, General Information, GA27-3093
 
            This manual is available through your company's IBM comp center
            or over-the-counter at most major IBM offices.
 
            Advanced Data Communications Control Procedures, ANSI X3.66-1979
 
            This is the American standard or HDLC. Write ANSI, 1430 Broadway
            New York, New York, 10018 for current price information.
 
     For easier to get references, most recent books on data communications
have chapters on "bit-oriented-protocols".  These are some sources available:
 
            IEEE Transactions on Communications, COM-28 No. 4, April 1980
            (Special issue on Computer Network Architectures and Protocols)
 
            IEEE Proceedings, Vol. 66, No. 11, pp 1301-1588, November 1978
            (Special issue on Packet Communications Networks)
 
            Technical Aspects of Data Communications by John E. McNamara,
            Digital Press (See Chapter 19 on Bit Oriented Protocols)
 
            Communications Architecture for Distributed Systems by R.J. Cypser
            Addison Wesley (See Chapter 11 on Data Link Control)
 
            NCC (National Computer Conference) Proceedings, 1975,
            AFIPS Press (Many excellent articles on packet radio concepts)
 
     Doug Lockhart's initial implementation of the protocol is a subset of the
IBM SDLC standard.  The enclosed notes document the type of frames used.
 
     HDLC Chips - Quite a few manufacturers are now making HDLC/SDLC VLSI
chips.  Of the half-dozen or so available, only two have all the hardware re-
quired for easy use in an amateur radio environment.
 
The special features required are NRZI encoding and an on-board digital phase-
locked loop for timing recovery.  The two chips which have been used by ama-
teurs are the Intel 8273 and the Western Digital 1933.  The Intel component
data sheet has a brief summary of HDLC framing.  In quanity purchase the Intel
part is about $35, and the Western Digital part is $30.
 
     Repeater Hardware - The repeater hardware is based on STD bus cards.  The
STD bus uses 56-pin  4 1/2" x 6" cards and is very popular for industrial
process control applications. There are now many manufacturers supplying cards
for this bus, and the repeater uses the Z-80/CPU-2 card from Mostek, which
costs about $195. There is no reason why S-100 Z80 cards could not be used for
a repeater.  The STD card is very compact, and does not have unneeded extra
circuitry which is typically found on more versatile personal system CPU cards
A WD1933 chip was breadboarded onto a Vector STD board, and one additional
card to control the transmitter is all that was required.  The software, writ-
ten in PASCAL/Z and assembler fits into two 2716 EPROMS, and 2K bytes of RAM
memory is required.  The repeater can store up to 8 128-byte packets.  The
software and schematics will be available from me for a fee.  Write or call if
you are seriously interested in duplicating this equipment.  The radio is an
FT202 handheld transceiver with a 20 Watt power amplifier.  Since this repeat-
er runs simplex, problems associated with full-duplex repeaters (desense, etc)
are not encountered, and exotic remedies, such as duplexers, dual antennas,
and so forth, are not required.
 
     The repeater serves to increase geographic range due to its advantageous
location, it digitally regenerates the packet, providing all stations with a
uniform signal, it selectively repeats only those packets addressed to it,
allowing the possibility of multiple repeaters on the same frequency (an ad-
vantage instead of a curse!), and its beacon and packet-repeat facilities
allow stations to do full-loopback testing, an invaluable resource in bringing
up new equipment and checking out hardware/software modifications.
 
     The VADCG Terminal Node Controller - Doug's group designed a low cost,
8085 based circuit board which uses the Intel 8273, and which has proven to be
a very easy way to get started in packet radio.  The hardware/software does an
excellent job and many are in use here in SF. Most people ask "why can't I put
this software onto my personal computer?" First, HDLC I/O interfaces are not
commonly found on personal computers.  Next, the VADCG TNC acts as a dedicated
peripheral controller, handling the radio link protocol, and thus off-loads
this responsibility from the home system.  The personal computer can go off
reading/writing its disks while packets are coming in over the air.  Handling
two full-duplex I/O paths at high speeds is a non-trivial programming job and
is above the skill level of many amateurs.  The current program is about 30
pages of assembly language, and is not particularly easy to transport to other
8080/Z80 environments because of the interrupt handling required.  We have
managed to support a wide variety of different home systems with one software
package, and an upgrade to the program requires replacing some EPROMs, not
rewriting the software for 30 different DOS's.
 
Thus, for newcomers I would highly recommend buying a pair of TNC's, as the
cost of the bare board is only about $35.  Total cost fully loaded is around
$200.  The software is available from several sources around the country.  The
TNC could also be programmed to be a packet repeater, with relatively simple
modifications to the existing code.
 
     Modems - The Bell 202 modem is a voice frequency 1200 bits-per-second
modem which is ideal for radio work at 2 meters and which can now be found at
reasonable prices through various surplus outlets, or which can be fabricated
using EXAR chips or op-amps.  This modem has become standard for amateur radio
use at this Baud rate.  At one time I had a good supply of these modems for
$75-$90, but I'm totally sold out at present.  There are as yet no standards
for higher speeds or direct digital modulation of an RF carrier.  This is an
area which needs more experimentation.
 
     Newsletters - Our group does not have a newsletter.  However, the follow-
ing organizations periodically publish very fine newsletters which deal with
packet radio:
                AMRAD, 1524 Springvale Avenue, McLean  VA  22101
 
                VADCG, 818 Rondeau Street, Coquitlam, B.C., Canada,  V3J 5Z3
 
                Hamilton Area Packet Net, Stu Beal, Editor, 2391 Arnold
                Crescent, Burlington, Ontario, Canada,  L7P 4J2
 
     Networking - Most active packet users agree that the real driving force
behind packet is the ability to interconnect geographical areas through net-
works.  Electronic mail, digitized imagery, networking games, computer con-
ferences and other unthought of applications remain to be explored.  The next
year should bring some heavy-duty activity in the areas of defining network
protocols and standards.  A conference to discuss these issues has been arran-
ged by AMRAD, and will be held on October 16th in Gaithersburg, MD.  Try to be
there, if you can.
 
     Thanks again for your interest.  See you on the net.
 
Best regards,
 
Hank Magnuski, KA6M
