This page is a complement to the DPRK-ARQ protocol page. It describes routine habits and conventions followed by the users of the modem, but not part of its protocol.

Link establishment

Typically, the calling station stays on its scheduled, starting frequency through the whole link. When the link speed is insufficient, or the link cannot be established at all, the downstream station will be called to move to another frequency. It is much less common for the calling station to try moving itself to another frequency instead. Frequencies are chosen without respect to other HF users (amateur radio operators, shortwave broadcasters etc.), and more often than not consist of non-random sequences, like sequential or repeated digits, e.g. 13333, 14444, 12345 kHz. Less often, the operators might even try out the same frequency as the calling station, resulting in a simplex link.

Links always begin in 600Bd mode. When a change of modem setting or frequency requires a restart, the calling station is the one sending a shutdown burst before powering down and restarting from the CQ stage. Likewise, at the end of the link, the calling station is also the one sending a shutdown burst to definitively end the link.

Chat messages

Most of the key chat messages consist only of ASCII characters, and are at most a few characters long, possibly so they fit in 1 or 2 packets only - they use radio operator codes such as the "QRU" Q code and the "SK" Morse code prosign. All other discussion takes place in Korean written in Hangul, encoded in KPS 9566 as normally for chat messages.

Phrase Meaning Response Notes
# rt ok sk
I have received
# messages from you,
end of transmission
The same phrase,
except about the other
station's messages
"#" and/or "rt" may be omitted
if the station had not delivered any traffic
qru ok sk
Neither of us has traffic,
end of transmission
The same phrase Used only when there is no traffic,
from neither station on the link
modem ok
I need to use my modem
for something else
"ok" Used when the time constraint on this link
has been or is about to be exceeded
예비 ok
(tr: yebi)
Please switch to
the backup system
"ok" Further meaning unidentified
Modem settings
Let's switch the waveform
from 600Bd to 1200Bd
"ok" when accepting,
"no" when refusing
Refusals are very rare
Let's switch the waveform
from 1200Bd to 600Bd
"ok" when accepting,
"no" when refusing
Refusals are very rare
(tr: gamdo?)
Please turn up your
modem sensitivity
"ok" Might simply refer to the modem's
FSK demodulation threshold
(tr: chullyeog?)
Please turn up your
radio power output
Link setup
I am changing my frequency
to 12222 kHz LSB
"ok" Used by the calling station to notify a
switch away to a frequency different
from the scheduled, starting one

Additionally, on links involving the Pyongyang hub (0xD3), soon after entering data for the first time in the transmission, the Pyongyang operator writes "#ok", where "#" is Pyongyang's message count. The peer station responds with "ok" followed by "#ok", where "#" is the peer station's message count. The Pyongyang operator finishes the exchange by writing "ok". The reason this exchange occurs is unknown, especially as the message count information is readily available in the transmission header.

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