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Re: HF Radio Email



Hello,

Perhaps this is not the appropriate place to speak deeply about it.
However some comments here may be useful for all and us:

----

We are precisely working in this subject, an email system for HF links.
We use sound cards as modems for conventional voice HF radios, so the
system will be cheap.  The system is designed for subnets with a server
and several clients. We have not yet performed tests on real channels but
we think we could achieve between 1000 and 2000 bps, of real user-data.

We are now using the following protocol/application structure:

Postfix: As MTA for ruting messages trough a BSMTP transport.
       Probably any other MTA can be used.

Bsmtpd: Batched SMTP transport. It packs messages in compressed batches.

Uucp: Unix-to-Unix Copy. This level sends the mails compressed by bsmtp.
      The batching is performed in the moment of a successfull uucp connection.
      We are using the 'y' protocol because it is suitable for half-duplex and
      error-free links (the channel is NOT error-free, but that's ax25's duty).

AX25: This protocol is used to reliably transfer uucp y packets and provaides 
      medium access control. The standard Linux AX-25 implementation was modified
      the have better throughput.  The most important is the implementation of an efficient
      selective reject system (SREJ). Just lost packets are resent (some changes in
      timeout's implementation has been mandatory). A DAMA-Master (for the server) has
      been also programmed (with new specifications, does not follows traditional
      DAMA). This improves efficiency when two or more clients want, at the same time,
      to communicate with the server.

Soundmodem: This package from Thomas Sailer allows to use as modem any sound card
      supported by linux's kernel. Then, input/output jacks from the sound card go
      directly to the radio. A PTT circuit is needed:
      (http://www.baycom.org/~tom/pcf/ptt_circ/ptt.html). Soundmodem has several
      different modulations (AFSK, FSK, PSK, ...). The best for HF is "newqpsk".

Newqpsk: This module was developed by Pawel Jolocha (SP9VRC) for the DSP560002.
      Fortunately, Tomi Manninen did a great job translating it from DSP's assembler
      to C, so now it can be used in Linux. We think newqpsk still needs some improvement
      (anyone interested?), but with the exception of bad links, it is a good option.

Moreover, inside Newqpsk, strongest Forward Error Correction was added
      (FEC), based on convolutional Turbo Codes (a good tutorial:
      http://www.ece.arizona.edu/~ryan/turbo2c.pdf). The TC implementation
      that I am using was made in 1998 by Mathys Walma and I have made
      some little improvements: puncturing system to allow variable rate,
      and a crc tail to allow faster decoding. (There is one problem here,
      We have tried to contact M. Walma but his mail is not active,
      so we cannot guarantee he allows his code to be released under
      GNU license).

      Although it may be more than one client, with DAMA, we can assure that only one is
      active at a time. This allows to make Point-to-Point communications and
      adaptative FEC (redundancy,  thus speed, is changed depending on the current
      link quality).

We think that in less than two months we well have all of this stuff
working and better documented, but we are sure that the "newqpsk" could
be improved, so anyone with knowledge in signal processing is welcome.

ps: For people who know spanish we have a working document with a more detailed
    description here:

    http://www.ehas.org/trabajo/hf/HFModem.doc

    This document is by no means complete, so people who wants to use the entire
    system might need some help (in fact the interface between UUCP and AX25 levels
    needs some simple but specific programs: uucall, ...).

On 09 de march 2002, Randy and Wanda Potratz & Family wrote:

> Has any one on this list ever tried to set up and run a HF radio 
> email system?  I have some questions on configuration for such a 
> project here in Niger, West Africa.

-- 

Arnau Sanchez Sala 
arnau@gbt.tfo.upm.es

Joaquin Seoane Pascual
joaquin@dit.upm.es

Visit http://www.ehas.org for a background of our work
(unfortunately only in spanish).




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