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[aprssig] Looking for 9600 baud equipment

jimlux jimlux at earthlink.net
Fri Oct 3 13:28:50 UTC 2008


Message: 16
Date: Thu, 02 Oct 2008 23:21:44 -0700
From: Scott Miller <scott at opentrac.org>
Subject: Re: [aprssig] Looking for 9600 baud equipment
To: TAPR APRS Mailing List <aprssig at lists.tapr.org>
Message-ID: <48E5B9F8.1050400 at opentrac.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

 > > It is possible to make a pair of 56k phone line modems work over a 
radio
 > > link.  I've seen it done at UHF (but not done it myself!)  The link 
used
 > > was real time analogue both ways, and had at least a 3k audio 
bandwidth.

Not at 56k, I don't think.  I'm pretty sure 56k requires a direct
T-carrier connection on one end.  They should only be able to do 33.6k
peer-to-peer.

--- That's because of the transmit power limit on the subscriber line, 
so you can't necessarily get enough SNR to support 56kbps.  If you have 
a "Data Access Arrangement" (or whatever the modern equivalent is) that 
allows actually setting the audio levels, you could get the 56 kbps.
--- One of tradeoffs for allowing you to plug your own part 68 gear into 
the phone jack at home is a lower limit on the sent audio level of part 
68 gear, so that you don't exceed the limits.



  That's still pretty good, but that's only going to happen
when you've got a good solid link and no mobile flutter.  It's also not
useful for a half-duplex link.

--- This is the primary reason why standard dialup highspeed modems are 
not suited to the over-the-air application.

Jim, W6RMK




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