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From: seanp%undrground@amix.commodore.com (Sean)
Newsgroups: comp.dcom.telecom
Subject: Re: Questions About the GTE Airfone
Message-ID: <15371@accuvax.nwu.edu>
Date: 11 Dec 90 00:26:48 GMT
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> >The transmitter section of the setup is IMPRESSIVE. Everyone could
> >concievably use thier phone at the same time, as there are 400
> >channels available for use.

> I doubt very much that the airplane is able to transmit on 400
> channels, or even 56 channels, at once.
 
If they weren't able to do it, then they would limit the number of
phones that were placed aboard the plane. If, however, four planes on
the same runway all tried to use all of thier phones, a SERVICE
UNAVAILABLE message would be generated, much like cellular.

> >894 - 896 MHz (5 KHz spacing). Assignment is done in the same
> >manner as cellular. It picks an available frequency from what the
> >ground station tells it. AM mode is used for modulation.

> AM on 5KHz spacing?  Are you sure?  I would guess companded SSB, just
> based on the age of the system, but I don't know.
 
Yes, AM is used on the following ranges (Newer AIRFONES use 6 KHz space)
     849 to 851
     894 to 896
     899 to 901
and  944 to 946 
  
And, for future reference, it is COMPANDORED. As in, AMPLITUDE
COMPANDORED SINGLE SIDEBAND. Really, this technology would be
unrealistic for air use. With the ever changing conditions, ACSSB
would be extremely difficult to maintain a tuned carrier. This was the
case when United Parcel Service considered implementing ACSSB. The
company selling them the radios concluded that that technology would
require too much operator control and tuning, thus requiring more
in-depth training about the radios. And this was for GROUND use, AIR
is even worse.

> > When you insert your credit card, the information (number, exp.
> > date, etc. ) is sent via 300 baud signal to a ground station. (REAL
> > TIME). The ground station then confirms or denys usage, and proceeds
> > from there.  Actually, the handset is not released until then card is
> > approved.

> Is the data encrypted? It sounds like a massive security leak,
> broadcasting sensitive information about a card like that. I know all

> > 894 - 896 MHz (5 KHz spacing). Assignment is done in the same
> > manner as cellular. It picks an available frequency from what the
> > ground station tells it. AM mode is used for modulation.

I never attempted to decipher it. I would imagine it should be, but
stranger things have happened!

> scanner column, and reported several 470 MHz band frequencies where
> you can pick up air phone calls. I've found some there.

I have yet to hear of AIRFONE signals in that band. Perhaps you are
thinking of the 415.7 MHz frequency for government VIPS? If not, tell me
what frequencies and modes, please.


Sean