Hackers Underworld 2: Forbidden Knowledge/Hackers Underworld 2: Forbidden Knowledge.iso/PHREAK/FONEWORX.TXT

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Subject: How Phones Work
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Date: 5 Aug 91 06:07:47 PDT (Mon)
From: Julian Macassey <[e-mail]>
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Status: RO


Dear Patrick,
as requested, here is my introductory article on phones:

----------cut and slash at will -------------------------------


                    UNDERSTANDING TELEPHONES

                               by

                     Julian Macassey, N6ARE

                         First Published
                               in
                       Ham Radio Magazine
                         September 1985

            Everybody has one, but what makes it work?

     Although telephones and telephone company practices may vary
dramatically  from one locality to another, the basic  principles
underlying the way they work remain unchanged.

     Every  telephone consists of three  separate  subassemblies,
each capable of independent operation.  These assemblies are  the
speech  network, the dialing mechanism, and the ringer  or  bell.
Together, these parts - as well as any additional devices such as
modems,  dialers,  and answering machines - are attached  to  the
phone line.


The phone line

     A  telephone is usually connected to the telephone  exchange
by  about three miles (4.83 km) of a twisted pair of No.22  (AWG)
or  0.5  mm  copper wires, known by your phone  company  as  "the
loop".   Although  copper  is  a good  conductor,  it  does  have
resistance.   The resistance of No.22 AWG wire is 16.46 Ohms  per
thousand  feet  at 77 degrees F (25 degrees C).   In  the  United
States,  wire resistance is measured in Ohms per  thousand  feet;
telephone  companies describe loop length in kilofeet  (thousands
of  feet).   In  other parts of the  world,  wire  resistance  is
usually expressed as Ohms per kilometer.

     Because  telephone apparatus is generally considered  to  be
current   driven,  all  phone  measurements  refer   to   current
consumption, not voltage.  The length of the wire connecting  the
subscriber to the telephone exchange affects the total amount  of
current   that  can  be  drawn  by  anything  attached   at   the
subscriber's end of the line.

     In  the  United States, the voltage applied to the  line  to
drive  the telephone is 48 VDC; some countries use 50 VDC.   Note
that telephones are peculiar in that the signal line is also  the
power  supply line.  The voltage is supplied by lead acid  cells,
thus  assuring a hum-free supply and complete  independence  from
the electric company, which may be especially useful during power
outages.

     At  the telephone exchange the DC voltage and  audio  signal
are  separated  by  directing  the  audio  signal  through  2  uF
capacitors and blocking the audio from the power supply with a 5-
Henry choke in each line.  Usually these two chokes are the  coil
windings  of  a  relay  that switches  your  phone  line  at  the
exchange;  in the United States, this relay is known as  the  "A"
relay (see fig.1).  The resistance of each of these chokes is 200
Ohms.

     We can find out how well a phone line is operating by  using
Ohm's  law  and  an  ammeter. The DC  resistance  of  any  device
attached  to the phone line is often quoted in telephone  company
specifications  as  200  Ohms; this will vary  in  practice  from
between  150 to 1,000 Ohms. You can measure the DC resistance  of
your  phone  with an Ohmmeter. Note this is  DC  resistance,  not
impedance.


     Using  these figures you can estimate the  distance  between
your telephone and the telephone exchange.  In the United States,
the telephone company guarantees you no lower current than 20  mA
-  or  what is known to your phone company as a  "long  loop."  A
"short loop" will draw 50 to 70 mA, and an average loop, about 35
mA.  Some countries will consider their maximum loop as low as 12
mA.  In practice, United States telephones are usually capable of
working  at  currents  as  low as 14  mA.   Some  exchanges  will
consider your phone in use and feed dial tone down the line  with
currents  as  low as 8 mA, even though the telephone may  not  be
able to operate.

     Although  the telephone company has supplied plenty of  nice
clean  DC  direct  to your home, don't assume  you  have  a  free
battery  for your own circuits.  The telephone company wants  the
DC resistance of your line to be about 10 megOhms when there's no
apparatus  in use ("on hook," in telephone company  jargon);  you
can  draw no more than 5 microamperes while the phone is in  that
state.   When  the phone is in use, or "off hook," you  can  draw
current, but you will need that current to power your phone,  any
current you might draw for other purposes would tend to lower the
signal level.

     The  phone  line has an impedance  composed  of  distributed
resistance, capacitance, and inductance.  The impedance will vary
according  to the length of the loop, the type of  insulation  of
the wire, and whether the wire is aerial cable, buried cable,  or
bare  parallel wires strung on telephone poles.  For  calculation
and specification purposes, the impedance is normally assumed  to
be 600 to 900 Ohms.  If the instrument attached to the phone line
should  be of the wrong impedance, you would get a  mismatch,  or
what  telephone  company  personnel refer to  as  "return  loss."
(Radio  Amateurs will recognize return loss as SWR.)  A  mismatch
on telephone lines results in echo and whistling, which the phone
company  calls "singing" and owners of very cheap telephones  may
have  come  to expect.  A mismatched device can, by the  way,  be
matched  to  the phone line by placing resistors in  parallel  or
series  with  the line to bring the impedance of  the  device  to
within the desired limits.  This will cause some signal loss,  of
course, but will make the device usable.

     A  phone  line  is balanced feed,  with  each  side  equally
balanced  to ground.  Any imbalance will introduce hum and  noise
to the phone line and increase susceptibility to RFI.

     The  balance  of the phone line is known to  your  telephone
company  as "longitudinal balance."  If both impedance match  and
balance  to ground are kept in mind, any device attached  to  the
phone  line  will perform well, just as the correct  matching  of
transmission  lines and devices will ensure good  performance  in
radio practice.

     If  you  live  in the United States,  the  two  phone  wires
connected  to your telephone should be red and green.  (In  other
parts  of the world they may be different colors.)  The red  wire
is  negative  and  the green wire is  positive.   Your  telephone
company  calls the green wire "Tip" and the red wire "Ring".  (In
other parts of the world, these wires may be called "A" and "B".)
Most installations have another pair of wires, yellow and  black.
These wires can be used for many different purposes, if they  are
used