rick (12/13/82)
An open letter to Dave, WB7RDI,
Saw your draft proposal for a packet radio network on
the net. Here are a few comments/criticisms/suggestions:
o you might consider using delay modulation coding
instead Manchester. Delay modulation has a minimum
at dc (as opposed to a null for manchester), yet
with some minor waveform shaping it should pass
the ac coupled radios without much distortion.
o Should you decide continue using Manchester, take a
look at the Zilog 8530 SCC. It has a Manchester
encoder/decoder on chip. The receiver is not
quite optimal in the sense that it uses digital
techniques (digital pll) to regenerate the clock,
however, it should provide adequate results in your
application. As you might expect, the chip provides
on board baud rate generators, bit stuffing, selectable
error detection polynomials, and universal microprocessor
interface (cost: ~$38).
o I was somewhat puzzled by your trying to evaluate
the CRC polynomials in software. Most SCC's provide
CRC generation. Even if your application requires
software generated error detection (cost reasons?),
there are checksums which are better suited to the
sort of arithmetic available on microprocessors (CRC
polynomial evaluation is long division, ie a serial process).
Take a look at IEEE Trans. on Communications,
January, 1982 pp 247-251 for an article describing
ones complement checksums. The claimed fraction
of errors which pass undetected is .001538%, versus
.001526%for CRC's. The ones complement technique is
not as good with double bit errors, however.
o I would strongly suggest that you consider using AX.25
packet format instead of your home grown format. You might
consider building a standard AX.25 network FIRST (something
stable for others to use), then set up a gateway to your
experimental network.
o Some other papers in Transactions on Communications which
I found useful are:
- L Ehrman, PF Maloney, "A Multimode Modem for Digital
Radio", IEEE Trans. on Comm., Vol COM-27, pp 1786-1793,
Dec 1979.
This paper addresses upgrading Military FM Radios to
digital service. The radios are similar in characteristics
to existing Amateur FM equipment. They investigate various
modulation schemes including MSK, Manchester, and Delay
Modulation.
- JS Ma, "On the Impact of HDLC Zero Insertion and Deletion
Link Utilization and Reliability", IEEE Trans. on Comm.,
VOL COM-30, pp 375-381, Feb 1982.
This paper discusses the impact of bit stuffing on
Link error performance.
o Take a look at MSK! The references are too numerous to
list here, just take a look a T Comm over the last few
years. You will still need a baseband coding scheme which
inserts clocking info, however, it will buy you even more
spectral efficiency.
Rick Spanbauer WB2CFV
..!allegra!sbcs!rick