File transfer protocol for Radio Links

Hyperterminal is a POS. I would blame it before I'd blame the protocol.

You could try kermit.

Not sure why that would matter. As long as Hyperterminal is configured to operate in half-duplex mode. Of course I'd be shocked if Hyperterminal actually does work in half-duplex mode.

There's nothing I can think of in the ZModem protocol that requires full duplex. It sounds like hyperterminal isn't properly set up for a half-duplex link (it's quite possible that it can't be).

It depends what you mean by a "digital modem". If you are referring to some ordinary phone line modems, they definitely are not of much help. Any high speed (9k6 and above) require a full duplex connection for the training session.

If by "digital modem" you are referring to some dedicated radio modem that handles the error control internally, the situation is of course different. In this case any protocol should be usable.

I hope you are not offended, but I had a big laugh if someone really tried to run Zmodem over any radio path with a simple modem. The radio path is a very hostile environment, in which ordinary land line protocols do not work very well.

If I remember correctly, Zmodem only sends retransmission requests when the CRC check fails, i.e. it requires a full duplex system. However, older protocols, such as X-modem requires the acknowledge after each block has been transferred, which thus also works in half-duplex systems, such as most radio systems. In a sense, it is quite similar in performance to the AX.25 protocols used by amateur radio operators on VHF and UHF. The throughput of such protocols is also limited due to the Tx/Rx turnaround delays.

However, trying to run AX.25 on HF will just cause a lot of nuisance, due to the high number of retries.

A good protocol intended for radio paths should handle random and burst errors effectively and really assume that at least a few bits in each message is corrupted. Forward error correction is more or less mandatory to handle some random errors without retransmissions (and a severely degraded throughput due to the Tx/Rx/Tx turnaround delays).

In order to avoid burst errors caused by multipath cancellation or impulse noise (lightnings and badly filtered ignition systems), some form of interleaving is typically used. When the interference wipes out a series of bits, after deinterleaving, these errors are converted to random errors, which can be corrected by FEC (ECC).

Do a search for "amateur radio" and "digimode" to get a feeling of what protocols are used on HF. However, it appears as if new protocols are invented every week, so you have to be a bit careful :-).

However, protocols like Pactor has been used on HF for quite a few years.

To get any more detailed suggestions, you should specify more accurately the frequency range you are interested in, what kind of propagation (true line of sight, VHF nearly line of sight, HF ground wave, HF skywave or HF short skip (NVIS).

Alternatively, get an RF modem for each type of environment (which handles the dirty details for the type of operations) with a serial or ethernet connection.

Paul