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VARA, ARDOP and PACTOR
High-speed ARQ data modes.
High-speed ARQ data modes.
VARA, ARDOP and PACTOR are modems for moving real data over HF: email, attachments and other files. They share a design: two stations hold a connection, every block is checked, damaged blocks are sent again, and the speed adapts to the conditions. They are most often the engine under Winlink and radio email.
A weak-signal mode like FT8 survives by being slow and using FEC, with nothing acknowledged block by block. That works for tiny messages, but a file would take forever. ARQ takes the opposite approach: send at a decent speed, check each block, and ask again only for blocks that were damaged.
When a block fails to arrive after repeated attempts, the connection is dropped. Because each block needs a reply, both stations alternate sending and listening, and the speed of switching the radio between transmit and receive counts.
HF paths change by the minute. These modems measure the channel from the replies they get and move between faster, more demanding transmissions and slower, more robust ones. When the path is good, throughput climbs; when it fades, the modem slows down and holds on rather than failing. That is why the fastest levels, such as PACTOR IV in clear conditions, move far more data than keyboard or weak-signal modes, which in turn win when the path is poor.
| PACTOR | VARA | ARDOP | |
|---|---|---|---|
| Runs on | dedicated hardware modem | computer software and sound card | computer software and sound card |
| Design | proprietary | proprietary | openly published |
| Link | two stations only | two stations only | two stations only |
| Used for | HF email and files | HF email and files, plus a VHF/UHF FM version | HF email and files |
Software licensing, supported platforms and speed tiers change over time, so check the current information from each maker before choosing.