karn@mouton (08/30/85)
From: karn@mouton (Phil R. Karn)
It all depends on which section of 97.69 ("Digital Communications") you
choose to operate under. If you're operating internationally, then the code
and speed limits of paragraph "a" apply (300 baud below 28 Mhz; 1200 baud
between 28 and 50 Mhz; 19.6 kilobaud [sic] between 50 and 220 Mhz; 56
kilobaud about 220 Mhz.) You must use Baudot, ASCII or AMTOR under this
part. Everybody assumes that AX.25 is okay, since we're sending ASCII
characters in our synchronous, bit stuffed HDLC frames. We haven't pressed
this point, and nobody has complained.
If you're operating on a domestic path, you qualify for paragraph "c",
authorizing the use of "any digital code" above 50 MHz subject only to the
following bandwidth restrictions: 50-220 Mhz: 20 Khz; 220-1240 MHz: 100 kHz;
1240 Mhz and up: unlimited. Bit and baud rates are left completely
unspecified, although it is impossible to signal at a baud rate greater than
twice the bandwidth without intersymbol interference.
I guess the Commission set up these two separate sections because they
didn't feel that they had the authority to allow the use of "nonstandard
codes" on international QSO's. Yes, there are lots of contradictions and
unnecessary complications in this section, but we're a lot better off with
it than without.
Note that the speed limits in paragraph a are specified in BAUD, not
bits/sec. When they put out their NPRM, the FCC proposed to change "baud" to
"bits/sec", thinking that it didn't make any difference. A number of us
pointed out the distinction in our comments, with the result that the
wording (i.e., "baud") was left unchanged. This means that even over HF you
can experiment with multi-level modulation and run any bit rate you desire,
assuming you have the necessary signal-to-noise ratio and stay within the
baud rate limits (which, as mentioned above, determine the bandwidth anyway.)
The vast majority of packeteers run domestic links, so anything goes as long
as it's above 50 MHz, fits within the allowable bandwidths, and doesn't
involve deliberate encryption. You don't have to worry at all about baud
rates (i.e., paragraph "a") unless you operate packet on HF, or
internationally (over Oscar-10 or across a nearby border).
Phil