File transfer protocol for Radio Links

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What is the most suitable file transfer protocol when using HF,VHF
radio link with a digital modem.

I tried the Zmodem with crash recovery in MS Hyper Terminal to transfer
relatively small files but the transmission fails at the first bad
packet

But I should restart the transfer every time

I think that Zmodem is a full duplex protocol and digital radio modems
operates on a half duplex manner

So Zmodem times out waiting ....

Any advice or suggestion is welcomed

Essam Salah

 

[James Meyer]

What is the most suitable file transfer protocol when using HF,VHF
radio link with a digital modem.

I tried the Zmodem with crash recovery in MS Hyper Terminal to transfer
relatively small files but the transmission fails at the first bad
packet

But I should restart the transfer every time

You don't have to restart the whole transfer for a single failed packet.
Make up your own protocol. Divide the file up into packets that aren't too
long. Add a message number and a packet number to the front of the packet and
calculate a checksum for each packet and send that at the end. Wait for the
receiving end to acknowledge each packet. If the packet gets corrupted, then
the receiving end sends a nak or negative acknowledge. If you don't get
anything back or if you get a nak, then you re-send the packet until it gets
through.

Jim

 

[John Smith]

What is the most suitable file transfer protocol when using HF,VHF
radio link with a digital modem.

I tried the Zmodem with crash recovery in MS Hyper Terminal to transfer
relatively small files but the transmission fails at the first bad
packet

But I should restart the transfer every time

I think that Zmodem is a full duplex protocol and digital radio modems
operates on a half duplex manner

So Zmodem times out waiting ....

Any advice or suggestion is welcomed

Essam Salah

Try Kermit...

http://www.columbia.edu/kermit/

 

[BobG]

Over on avrfreaks, the opinion is that rs232 style NRZ encoding has a
DC bias/average DC level per character that messes up the receiver
threshhold detector. They claim that manchester encoding has a zero dc
level, so long runs of 000 or FFF dont confuse the receiver.

 

[Iwo Mergler]

What is the most suitable file transfer protocol when using HF,VHF
radio link with a digital modem.

I tried the Zmodem with crash recovery in MS Hyper Terminal to transfer
relatively small files but the transmission fails at the first bad
packet

But I should restart the transfer every time

I think that Zmodem is a full duplex protocol and digital radio modems
operates on a half duplex manner

So Zmodem times out waiting ....

Any advice or suggestion is welcomed

Essam Salah

You could try PPP and run any flavour of Internet
protocol over it. They all do checksums and sequence
numbers and TCP based ones (FTP,HTTP,etc) handle lost
packets.

I'm not using MS Windows myself, but I believe somewhere
among the internet stuff you have everything you need
to set up a computer-to-computer PPP link. If not, you
can most definitely download some PPP software for free.

Kind regards,

Iwo

 

[Allan Herriman]

You could try PPP and run any flavour of Internet
protocol over it. They all do checksums and sequence
numbers and TCP based ones (FTP,HTTP,etc) handle lost
packets.

I'm not using MS Windows myself, but I believe somewhere
among the internet stuff you have everything you need
to set up a computer-to-computer PPP link. If not, you
can most definitely download some PPP software for free.

For a (noisy) radio link, it might help to reduce the MTU to perhaps a
few hundred bytes. Normally, this would hurt throughput, as more
packets are needed to send the same information. But when the BER
gets bad (say, 1e-3), the shorter packets will have a much better
chance of getting through than the regular 1500 byte packets.


Regards,
Allan

 

[Wim Ton]

What is the most suitable file transfer protocol when using HF,VHF
radio link with a digital modem.

I tried the Zmodem with crash recovery in MS Hyper Terminal to transfer
relatively small files but the transmission fails at the first bad
packet

You may also think of Forward Error Correction. With an error correction
code (ECC), you can correct a number of bit errors, depending on the code.
Look for keywords like Reed-Solomon, BCH, Turbo codes

It makes the packets larger, but saves you retransmission of a whole packet
if only a few bits are corrupted.

One step further is to use to use an ECC over the datablocks, and then send
a few bits of each datablock in one transmission block , also with ECC. Even
if a whole transmission block it lost, it will only affect a few bits in
each datablock, which then can be corrected. This process is called
interleaving and I have seen it in some military radios.

To increase efficiency of Ack/Nack protocols like ?modem or Kermit, look at
the windowed versions.
I suppose there will be a lot of literature covering an optimimum
combination of packet size and window length for a given error rate.

Wim

 

[Joel Kolstad]

None of the protocols for 'wire line' file transfers will -- typically --
work well over radio links. Of course, if you're close enough and have a
really good signal, you can use whatever protocol you want and you'll be
fine, but as a rule of thumb most on-air protocols are designed with the
expectation of raw bit error rates in the ballpark of ONE IN ONE HUNDRED.
This is orders of magnitude worse than what wired proocols expect;
realistically you're either going to have to resign yourself to very short
distance links or find a protocol aimed at on-air applications. The amateur
radio guys have a wide number to choose from (e.g., Pactor, Olivia, etc. --
avoid the older stuff like RTTY, it gives up tremendous amounts of speed
because it doesn't use error correction).

Although nowhere near as complex as some cell phone protocols (where
carriers are trying to jam as many possible channels into the spectrum they
paid literally millions of dollars for), even the amateur radio protocols
are a lot more sophisticated that Kermit, X/Y/Z-Modem, etc.

---Joel Kolstad

 

[Luhan Monat]

I think that Zmodem is a full duplex protocol and digital radio modems
operates on a half duplex manner

Hi,

Thats the crux of the matter. The best you can do with one-way
communication is to attach checksums to to blocks of data and at least
know when you receive a bad one. Anything more gets real complicated.

 

[Michael Black]

What is the most suitable file transfer protocol when using HF,VHF
radio link with a digital modem.

Track down what they used on AlohaNet in the early seventies in Hawaii.
First packet radio.

A few years later, one of the people connected with it, or maybe it
was just influenced, I forget, had a position with the DOC here in Canada,
and decided packet radio was what amateur radio needed. Hence the
first display of amateur packet radio here in Montreal back in May of 1978.

So look up amateur packet radio. It's been around all these years.

Michael

 

[BobG]

Why not send every byte 3 times and take the one that gets received 2
or 3 times? Of course, that doesnt give much of a clue of what to do if
you send 3 bytes and get 3 different received bytes.... maybe send 3
bytes with parity? Yeah... that might work.... 200% overhead, but
reliable.....

 

[Guy Macon]

Why not send every byte 3 times and take the one that gets received 2
or 3 times? Of course, that doesnt give much of a clue of what to do if
you send 3 bytes and get 3 different received bytes.... maybe send 3
bytes with parity? Yeah... that might work.... 200% overhead, but
reliable.....

Better to send the entire message three times. Radio interference
tends to come in bursts which would wipe out several bytes in a row.

 

[keith]

Better to send the entire message three times. Radio interference
tends to come in bursts which would wipe out several bytes in a row.

Or stir the bits before sending. All ECC methods must be tailored for the
expected failure modes.