Re: File transfer protocol for Radio Links
From: Paul Keinanen (keinanen_at_sci.fi)
Date: 02/18/05
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Date: Fri, 18 Feb 2005 01:26:48 +0200
On 17 Feb 2005 01:13:04 -0800, esamsalah@gmail.com wrote:
>What is the most suitable file transfer protocol when using HF,VHF
>radio link with a digital modem.
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 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
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
- Next message: Dave Hansen: "Re: WinAVR 20050414 Released"
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