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TUCoPS :: Phreaking Cellular - Misc. :: mblfon.txt

Build your own mobile (IMTS) phone

  How to Get into the AT&T Network
  by Building Your own Mobile Phone.

     I am going to explain in this  article how you can build your own
mobile phone. If you haven't figured  it out already, you will soon see why
the security man was concerned.
     This article presupposes that you  have a working knowledge of two-way
radio. If you don't possess this  knowledge, get a copy of "The Radio
Amateur's Handbook" (readily available  from libraries and book stores) and
study up on narrow band FM  and  2-Meter transmitters.
     To get everything you will need  in one file, I am reprinting the IMTS
article here:

Signaling Used in IMTS
(Improved Mobile Telephone Service)

     Each mobile telephone channel  consists of two frequencies; one for
the land base station and one for the  mobile phone. The base station uses two
tones for signaling:

Idle   2000 Hz
Seize  1800 Hz

The mobiles use three tones:

Guard       2150 Hz
Connect     1633 Hz
Disconnect  1336 Hz

     The land base station marks the  idle channel by placing the idle tone
on it. All the mobiles search for the  channel with the 2000 Hz idle tone and
lock on to it.
     Each mobile phone is assigned a  standard telephone number consisting of
area code + 7 digits. When a land  customer dials a mobile number, the
idle tone (2000 Hz) changes to seize  (1800 Hz). The number pulsed to the
mobile phone contains 7 digits  consisting of the area code and last 4
digits of the number. The digits are  made up of 50 ms pulses of 2000 Hz
separated by 50 ms of 1800 Hz.
     If there is a mismatch between  the digits sent and the wired ID in the
mobile, the mobile drops off and hunts  for the idle channel. If the number
matches, the mobile will send back an  acknowledgement tone of 750 ms
of guard (2150 Hz). The base station  waits 3 to 4 seconds for this tone. If
not received in that time, the calling  party gets a recording. If the
tone is received, the mobile phone  will ring for up to 45 seconds. Ringing
is composed of 1800 Hz and 2000 Hz  shifting at 25 ms for two seconds then
four seconds of 1800 Hz. When the  mobile phone is picked up it sends a
connect tone of 1633 Hz for 400 ms to  tell the base station it has answered.
When the mobile hangs up, it sends  disconnect, which is 750 ms of 1336 Hz.
When the base receives the disconnect  tone, it will drop carrier for about
300 ms and go off. If it is the only  available channel, it will return to
     Now I will describe what happens  when a call is originated by a mobile.
When the mobile goes off hook, it  sends 350 ms of guard (2150 Hz)
followed by 50 ms of connect (1633  Hz). When the base station hears the
connect tone, it removes the idle tone  and stays quiet for about 250 ms.
It then transmits 250 ms of seize  (1800 Hz). The mobile then sends 190 ms
of guard and starts transmitting the  ID sequence at 20 pulses per second.
The ID is the area code and last four  digits of the mobile's number. The
pulses are marked by 25 ms of connect  (1633 Hz) followed by 25 ms of either
silence or guard tone (2150 Hz). If  the pulse is odd, it is followed by
silence. If even, it is followed by  guard tone. This is used for parity
checking. The interdigit time is 190  ms and will be either silence or guard
tone depending on whether the last  pulse was odd or even. If the last
pulse of the last digit in the ID is  even it will be followed by 190 ms of
guard tone.
     When a number is dialed from a  mobile phone, 2150 Hz is sent
continuously as soon a the dial goes  off normal (when the dial is moved from
its resting position). Dial pulses  representing breaks are marked by 1633
Hz and are sent at 10 pulses per  second. A pulse is 60 ms of 1633 Hz
with 40 ms of 2150 Hz between pulses.
     The most popular mobile telephone  channels are located in the VHF high
band. More cities are equipped with  these channels than any other band.
They are listed below.

Mobile Telephone Frequencies

Channel     Base     Mobile
-------     ----     ------
  JL       152.51    157.77
  YL       152.54    157.80
  JP       152.57    157.83
  YP       152.60    157.86
  YJ       152.63    157.89
  YK       152.66    157.92
  JS       152.69    157.95
  YS       152.72    157.98
  YR       152.75    158.01
  JK       152.78    158.04
  JR       152.81    158.07

     This is a list of the components  you will need to build your own mobile

1. Cassette Tape Recorder.
2. Radio Scanner (Like those used to  receive police calls).
3. Mobile phone dialer (build your  own).
4. Low Power Transmitter (Modified  2-Meter transmitter 1 - 5 watts).

How to Build a Mobile Phone Dialer

     Build a Wien-Bridge oscillator.   These are commonly used in red boxes.
If you don't have a red box schematic,  look up Wien-Bridge in an electronics
textbook. Where you would normally  connect a frequency adjustment pot, use
two multi-turn pots connected in  series. Power for the oscillator will
be supplied by a 9 volt battery.
     Obtain a rotary dial of the type  used on rotary telephones. The dial
will have four wires coming out of it;  two white, one blue, and one green.
The two white wires make a connection  when the dial is off normal (moved from
its resting position). Connect the two  white wires in series with one of
the leads from the 9 volt battery.   The oscillator will be running only
when the dial is moved off normal. It  works like this: Dial is moved off
normal. Circuit is completed between  oscillator and battery. Dial goes back
to resting position. Circuit is opened.
     The blue and green wires go to a  normally closed contact in the dial.
This contact opens once for each pulse  in a dialed digit. For example it
opens three times for the digit "3".   Connect these two wires (blue & green)
across one of the pots in the  oscillator. With the dial in its
resting position, adjust the other pot  for a frequency of 2150 Hz (Guard
tone). Move the dial until the contact  opens and adjust the pot with
the blue and green wires going to it  for a frequency of 1633 Hz (Connect
     When the dial is moved off  normal, power will be applied to the
oscillator, and it will begin running  at 2150 Hz. When the dial is released
the short across the second pot will  be removed each time the contacts open
for a dial pulse. During these pulse  times the frequency will shift down to
1633 Hz. When the dial gets back to  its resting position, power will be
removed from the oscillator. This will  exactly duplicate the dial pulsing
of a mobile telephone.                  
The Transmitter

     Antennae used by mobile phone  base stations are located on high
towers. This allows line-of-sight  transmission to and from the mobiles.
If you are within a few miles of a  base station very little power is
needed to establish contact. 1 to 5  watts should be completely adequate.
The less power you use, the less your  chances of getting caught. More on this
     2-Meter transmitters, used in  amateur radio, operate in the range of
144 to 148 Mhz. With a change of  crystals and a little retuning, you
have your transmitter.

How to use Your Home brew Mobile  Telephone

     With your scanner, locate the  base station frequency which currently
has the idle tone on it. Switch to the  mobile frequency on that same channel
and monitor it with the cassette  recorder running continuously.  What
you want is a clean recording of a  mobile unit broadcasting its ID
sequence. You also want a recording of  the disconnect tone when he hangs
up. Once you have these, rewind the  tape to the start of the sequence. Now
you are ready to make a call.

The procedure For Placing a Call

1. Set your scanner to the base  station frequency with the idle tone
and leave it there. Monitor with  earphones to avoid audio feedback
through the transmitter.

2. Set the transmitter to the  corresponding mobile frequency. Turn it
on and leave it on.

3. Play the taped ID sequence.

4. Use your dial pulser to call the  desired number. If all has gone well,
you will hear your dial pulses in the  earphones. You can use this method to
call one of the special 800 numbers  and whistle off with 2600 Hz; then MF
to anywhere in the world. This  technique will reduce your visibility
on the bill for the ID you are using.

5. When you are ready to hang up, play  the disconnect tone and switch off the

A Few Notes About Your Own Security

     You should use only as much  transmitter power as necessary to
maintain a reliable contact. If you do  much of this kind of experimenting,
the FCC is going to be after you with  direction finding equipment. These use
directional antennae and a process of  triangulation to locate illegal
transmitters. If you keep your power  down, stay mobile, and avoid
establishing a pattern of calling at  the same time every day, it will be
nearly impossible to track you down.

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