Re: File transfer protocol for Radio Links

From: Wim Ton (wimton_at_blueyonder.co.uk)
Date: 02/18/05 

Date: Fri, 18 Feb 2005 09:29:05 GMT

<esamsalah@gmail.com> wrote in message
news:1108631996.879774.181430@f14g2000cwb.googlegroups.com...
> 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

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