                 HACKING MA BELL
        Another Safe Cracker Production
 
             Basic Telephone Systems
                   Part Two
 
  One of the problems with any current IC is that the frequency
changes if rf gets near it. Many hams are having a hard time mounting
such IC pads on their 2-meter Handie-Talkies. But a solution seems in
sight - Mostek, a large IC company, is coming out with an IC
Touchtone generator which has cheap 3.58 MHz external crystal as
reference, and then produces the tone frequencies by dividing the
3.58 MHz down with flip flops to get the required tone frequencies.
This approach not only promises to be more reliable in the presence
of rf, but should also be cheaper since it would not need the custom
(and expensive) laser trimming of components that the Microsystems
International IC needs to adjust the frequencies within tolerence.
 
  At the other end of the telephone circuit, in the CO, various
circuits are used to decode the digit you dial into the appropriate
signals needed to perform the actual connection. In dial systems,
this decoding is done by relay circuits, such as steppers. This
circuitry is designed for dialing at the rate of of 10 pulses per
second, with a duty cycle of about 60% open, 40% closed. The minimum
time between digits is about 600 milliseconds, although a slightly
greater time between digits is safer since it avoids errors. In
practice, many COs will accept dialing at substantially slower or
faster rates, and often you will see a dial that has been speeded up
by changing the mechanical governor to operate almost twice as fast;
it depends on the type of CO equipment.
 
  Touchtone decoding is usually done by filter circuits which
separate out the Touchtone tones by filters and then use a transistor
circuit to operate a relay. A common decoder is the 247B, which is
designed for use in small dial switchboard systems of the type that
would be installed on the premises of a business for local
communication between extensions. It consists of a limiter amplifier,
seven filters and relay drivers (one for each of the seven tones
commonly used) and some timing and checking circuitry. Each of the
seven relays has multiple contacts, which are then connected in
various serial/parallel combinations to provide a grounding of one of
ten output contacts, when a digit is received. The standard 247B does
not recognize the * and # digits, but can be modified easily enough
if you have the unit diagram.
 
  The 247B decoder is not very selective, and can easily be triggered
by voice unless some additional timing circuits are connected at the
output to require that the relay closure exceed some minimum time
interval before it is accepted. Slightly more complicated decoders
which have the time delays built in are the A3-type and the C-type
Touchtone receivers. Both of these are used in customer-owned
automatic switchboards when a caller from the outside (via the
telephone company) wants to be able to dial directly into the private
switchboard to call a specific extension. The C-type unit is similar
to the 247B in that it has ten outputs one for each digit. The
A3-type does not have output relays, but instead has seven voltage
outputs, one for each of the seven basic tones, for activating
external 48-volt relays.  The A-3 unit is ideal for activating a
Touchtone encoder, which can then be used to regenerate the touchtone
digits if the original input is noisy. This might be very useful in a
repeater autopatch, for cleaning up Touchtone digits before they are
sent to the telephone system.
 
  In addition to the above,there are probably other types of units
specially designed for use in the CO, but information on these is not
readily available. It is also fairly easy to build a Touchtone
decoder from scratch. Though the standard telephone company decoders
all use filter circuits, it is much easier (though perhaps not as
reliable) to use NE567 phase-locked-loop integrated circuits.
 
  An interesting sidelight to Touchtone operation is that it greatly
speeds up the process of placing a call. With a Touchtone dial it is
possible to dial a call perhaps 3 to 5 times faster than with a
rotary dial. Since the CO equipment which receives and decodes the
number is only needed on your line during the dialing time, this
means that this equipment can be switched off your line sooner and
can therefore handle more calls. In fact, the entire Touchtone system
was invented so that CO operation would be streamlined and less
equipment would be needed for handling calls. It is ironic that the
customer should be charged extra for a service which not only costs
the telephone company nothing, but even saves it money.
 
  Another practice which may or may not cost the telephone company
money is the connection of privately-owned extension phones. You have
probably seen these sold by mail order houses and local stores. The
telephone companies claim that connecting these phones to their lines
robs them of revenue and also may cause damage to their equipment.
There are others, of course, that hold the opinion that the easy
availability of extensions only causes people to make more calls
since they are more convenient, and that the companies really benefit
from such use.  The question of damage to equipment is also not
easily answered, since most of the extension phones are directly
compatible, and in many cases the same type as the telephone company
itself uses. Be that as it may, this may be a good time to discuss
such use.
 
  Prior to an FCC decision on telephone company interconnection in
the Caterphone case in 1968, all telephone companies claimed that the
connection of any equipment to their lines was illegal.  This was a
slight misstatement as no specific laws against such use were on the
books. Instead, each local telephone company had to file a tariff
with the public service commission in that state, and one of the
provisions of that tariff was that no connection of any external
equipment was allowed. By its approval of that tariff, the public
service commission gave a sort of implicit legal status to the
prohibition.
 
  In the Caterphone case, however, the FCC ruled that the connection
of outside equipment had to be allowed. The phone companies then
relaxed their tariff wording such that the connection of outside
equipmeny was allowed if this connection was through a connecting
arrangement 'provided by the telephone company' for the purpose of
protecting its equipment from damage. Although this result has been
challenged in several states, that seems to be the present status.
The strange thing is that some telephone companies allow
interconnection of customer equipment without any hassle whatsoever,
while others really make things difficult for the customer.

                ...WHIPPLE

                (and Safe Cracker)

