AI6UA

Adventures in Packet Radio (episode 1)

A while ago, I bought a Kantronics KPC-3+ to experiment with packet radio. I never got around to doing anything with, mostly due to not having a useful radio cable for it (always said I’d get around to making one, and never really did). Last month, I picked up a Kenwood TNC cable off eBay, and tonight I figured I’d give it a shot.

What is packet radio?

Packet radio is one mechanism for transmitting data over RF. It works by breaking that data down into chunks (or packets), and sending those chunks out on the radio. The key new piece of this digital mode is a terminal node controller, or TNC. The TNC decodes packets received from the audio coming out of the radio, and encodes packets into the audio going into the radio.

Your radio connects to a TNC, which connects to your computer:

Antenna ←→ Radio ←→ TNC ←→ Computer

With the KPC-3+, and some other TNCs, the computer is optional. There are a couple of packet modem services, such as mail and BBS functionality, as well as keyboard-to-keyboard (K2K) modes. The radio sends and receives packetized data.

You can also operate in connected and unconnected (or UNPROTO) mode; in connected mode, each packet you send expects an acknowledgement packet in reply. In general, it’s better to think of packet radio as sending mail instead of the normal voice transmission (or in K2K mode, like passing notes). The ARRL published a document titled “AX.25 Link Access Protocol for Amateur Packet Radio” describing the AX.25 protocol often used in packet radio. Alternatively, you could use TCP/IP via a KISS modem. To me, using TCP/IP over radio seems like a waste but to each their own.

Some basic configuration

First, I got set up in terminal mode. According to the KPC-3+ manual,

When set to TERMINAL, the full command set of the TNC is available.

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cmd:HELP INTFACE
INTFACE  {TERMINAL | NEWUSER | BBS | KISS | XKISS | HOST | GPS | NET | MODEM}
cmd:INTFACE TERMINAL
INTFACE was NEWUSER

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I can hear you

I was trying to figure out if my setup was working at all, as transmitting didn’t seem to be triggering the PTT and I decided to take a break to work on this writeup. Suddenly, the backlight on the radio came on. I switched over to the minicom session and saw this:

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cmd:W6OAK>BEACON,KLPRC3,KBERR,KJOHN*,KBETH,KBERR,ROSE: <UI>:
This is OAK:W6OAK-5, a packet radio node in Oakland, CA. Mailbox: OBOX.

NE6RD>BEACON,WOODY,KBETH,KJOHN*,KBULN,KBERR,BRKNRG,ROSE: <UI>:
9612+ runs when it runs

KF6ANX-8>ID: <UI>:
KF6ANX-8/R JBOX/B KJOHN/N

W6ABJ-12>ID,KJOHN*,KBETH,KBERR,WOODY: <UI>:
W6ABJ-12/R KBULN/D KBUL/G KBULN/N
cmd:MHEARD
AI6UA      01/11/2003 08:44:26
W6OAK*     01/11/2003 09:05:07
NE6RD*     01/11/2003 09:13:45
W6ABJ-12*  01/11/2003 09:22:36
KF6ANX-8   01/11/2003 09:32:05

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At least I know I can hear traffic. I still haven’t figured out how to get it to transmit.

Transmit… just started working

At some point, transmitting just started working and I’m not clear about what changed. Attempts to connect to other nodes didn’t really work, though. I think that’s because I’m on the keyboard-to-keyboard / mail frequency. I thought that you had to connect to another node to send mail. I still need to figure out how to send mail, but it’s getting late.