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[Ham-80211] Re: ham-80211 Digest, Vol 16, Issue 12

David Ray ac7ub at yahoo.com
Fri Oct 14 17:13:40 UTC 2005

Just an fyi.  PPP compression only compresses headers, and will not buy you much.

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Today's Topics:

1. Re: Aerocomm Modems (Guillaume Filion)


Message: 1
Date: Fri, 14 Oct 2005 09:38:02 -0400
From: Guillaume Filion 
Subject: Re: [Ham-80211] Aerocomm Modems
To: "TAPR Mailing List for Ham Radio Use of 802.11"

Message-ID: <434FB4BA.3070908 at logidac.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Le 05-01-23, à 18:26, Steve Lampereur a écrit :
> Quick summary : 1 watt 900 MHS FHSS, 115.2 kbps $99
> [...]
> It appears that we could set up a DHCP sever and handle this like an
> ISP. That would be truly cool, make it a TCP/IP network.

It looks interesting. If you're using the ConnexLink model (serial)
you'd need a convertion from TCP/IP to serial -- I guess using PPP. I'm
looking into setting a Linux router with gzip compression on both ends
of such a link.

I did a little experiment to get an idea of what the gzip compression
can do. All my mailboxes are stocked in Maildir format, most of them
containing plain text messages. The PC I'm doing this experiment on is
an AMD Athlon XP2000+ (1.3GHz) with 512 MB RAM running Linux Debian
gfk at ali:~$ tar -c Maildir/* > maildir.tar
gfk at ali:~$ ls -lh maildir.tar
-rw-r--r-- 1 gfk gfk 252M Oct 13 21:02 maildir.tar
gfk at ali:~$ time gzip --best maildir.tar

real 1m2.976s
user 0m49.160s
sys 0m2.660s
gfk at ali:~$ ls -lh maildir.tar.gz
-rw-r--r-- 1 gfk gfk 155M Oct 13 21:04 maildir-best.tar.gz
gfk at ali:~$ gunzip maildir.tar.gz
gfk at ali:~$ time gzip --fast maildir.tar

real 0m46.027s
user 0m32.470s
sys 0m3.080s
gfk at ali:~$ ls -lh maildir.tar.gz
-rw-r--r-- 1 gfk gfk 161M Oct 13 21:07 maildir.tar.gz

That gives between 36% and 38% compression. For a 78 Kbps link that
would raise it to about 121 Kbps for plain text. Anyone knows what would 
be the actual throughput for such a link? I think that the PPP
protocol has built-in optional compression.

My mother is paying $25 per month for a 128 Kbps link to the Internet,
if I'm able to set up a PtP link from my house to hers with this setup,
it would pay for itself pretty fast.

BTW, anyone remembers the tree attenuation per meter for 900 MHz?

Guillaume Filion, ing. jr
Logidac Tech., Beaumont, Québec, Canada - http://logidac.com/
PGP Key and more: http://guillaume.filion.org/


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End of ham-80211 Digest, Vol 16, Issue 12

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