TUCoPS :: Phreaking Technical System Info :: pprimer.dox

Basic overview of the phone system

                                   The Telephone System
                                     A Basic Overview
                                      EVIL INCARNATE


      The telephone arrived as a practical instrument over a century ago in
1876 , an outgrowth of experiments on a device to send multiple telegraph
signals over a single wire.  Alexander Graham Bell, a native of Scotland,
while conducting electrical experiments spilled acid on his trousers.  His
sulphurous reaction, the now famous "Mr. Watson, come here, I want you",
brought Thomas A. Watson on the run not only because of his employer's
distress, but because the words had been carried by electricity into
Watson's room and reproduced clearly on his receiving set.  The simple
instrument being tested on Court Street in Boston on March 10, 1876  wasn't
very practical(the acid was used in the system) but improvement followed so
rapidly that putting into action Bell's concept of a public telephone
network - "this grand system", "whereby a man in  one part of the country
may communicate by word of mouth with another in a distant place" - was
well underway by January of 1878, when the first commercial exchange was
operated in New Haven.  By 1907, one hotel alone(the Waldorf Astoria in NY_
had 1,120 telephones and processed 500,000 calls per year.

      That concept has grown into an industry in which one entity (AT&T) is
the largest company on earth, providing over one hundred million telephone
sets, making a profit several billion dollars per year, and employing over
one million people.

      Telephone sets perform a surprising number of functions.  Here is a
list of the most important ones:

1.It requests the use of the telephone system when the handset is lifted.
2.It indicates that the system is ready for use by receiving a tone, called
the dial tone.
3.It sends the number of the telephone to be called to the system.  The
number is initiated by the caller by pressing number keys or rotating a
4.It indicates the state of a call in progress by receiving tones
indicating the status.
5.It changes speech of a calling party to electrical signals for
transmission to a distant party through the system.  It changes electrical
signals received from a distant party to speech for the called party.
7.It automatically adjusts for changes in the power supplied to it.
8.It signals the system that a call is finished when a caller "hangs-up"
the handset.

      Of course for a telephone to be of any use, it must be connected to
another telephone.  In the very early days of telephones, the phones were
simply wired together with no switching.  This became impractical as the
number of phones increased and the local exchange or central office was
established to handle the switching and other functions.

      Each subscriber telephone is connected to a central office that
contains switching equipment, signaling equipment and batteries that supply
direct current to operate the telephone.  Each phone is connected to the
central office through a local loop of wires called a wire pair(the other
wires on a phone line are grounds in case of over voltage)  One of theses
wires is called T(for Tip(DUH!)) and the other is called R(for ring) which
refers to the tip and ring parts of the plug used in manual switchboards.

      Switches in the central office respond to the dial pulses or tones
from the telephone to connect the calling phone to the called phone.  When
this connection is established, the two telephones communicate over the
transformer coupled loops using the current supplied by the central office


      When the handset of the telephone is resting in its cradle, the
of the handset holds the switchhook buttons down and the switches are open.

This is called the on-hook condition.  The circuit between the telephone
handset and the central office is open; however, the ringer circuit in the
telephone is always connected to the central office.  The capacitor blocks
the flow of DC from the battery(in the central office).  The ringer circuit
presents a high impedance to speech signals so it has no effect on them.

      When the handset is removed from its cradle, the spring loaded
buttons come up and the switchook closes.  This completes the circuit to 
the exchange and current flows in the circuit.  This is called the off-hook

      The off-hook signal tells the exchange that someone wants to make a
call.  The exchange returns a dial tone to the called phone to let the
caller know that the exchange is ready to accept a telephone number.


      Some telephone sets send the telephone number by dial pulses while
others send it by audio tones.


      Most telephone sets that use dial pulsing have a rotary dial which
opens and closes the local loop  circuit at a timed rate.  The number od
dial pulses resulting from one operation of the dial is determined by how
far the dial is rotated before releasing it.


      Some telephone systems use the newer method of using audio tones to
send the telephone number.  These can be used only off the central office
is equipped to process the tones.  Instead of a rotary dial, these
telephone sets have a push-button keypad with 12 keys, for the numbers 0
through 9 and the symbols * and the #.  Pressing one of the keys causes an
electronic circuit in the keypad to generate two output tones that
represent the number.


      The central off ice has various switches and relays that
connect the calling and called phones.  For now, assume that the connection
has been made.  The actual operation of switching systems will be covered
in more detail a little later.

      If the called phone handset is off-hook when the connection is
attempted, a busy tone generated by the central office is returned to the
calling phone.  Otherwise, a ringing signal is sent to the called phone to
alert the called party that a call is waiting.  At the same time, a
ringback tone is returned to the calling phone to indicate that the called


      Early telephone circuits were point to point(not switched), and the
caller gained the attention of the party at the other end by picking up the
transmitter and shouting "Hello".  This was not very satisfactory, and
schemes based on a mechanical signaling arrangements were soon invented. 
The one in common use today, called the "polarized ringer" or bell, was
patented in 1878 by Thomas A. Watson(Mr. Bell's assistant).


      When the called party removes the handset in response to a ring, the
loop to that phone is complete by its closed switchook and loop current
reflows through the called telephone.  The central office then removes tex
ringing signal and the ringback tone from the circuit.


      The part of the telephone into which a person takes is called the
transmitter.  It converts speech (acoustical energy) into variations in an
electric current (electrical energy) by varying or modulating the loop
current in accordance with the speech of the talker.

      The part of the telephone that converts the electric current
variations into sound that a person can hear is called the receiver.  The
signal produced by the transmitter is called by the loop current to the
receiver of the called party.  Also, a small amount of the transmitter
signal is fed back into the talker's receiver.  This is called the

      Sidetone is necessary so that the person can hear his/her own voice
from the receiver to determine how loudly to speak.  The sidetone must be
at the proper level because too much sidetone will cause the person to
speak too softly for the good reception by the called party.  Conversely,
too little sidetone will cause the person to speak so loud that it may
sound like a yell at the receiving end.


      The call is ended when either party hangs up the handset.  The
calling party can hang up the phone for a second, but the called party must
hang the phone up for twenty seconds for a disconnect.  The on-hook signal
tells central office to release the line connections.  In some central
offices, the connection is released when either party goes on-hook.  In
others theconnection is released when only the calling party goes on-hook.


      Thus far the discussion of connecting two telephones together has
been limited to local loops and a central office exchange.  Most central
office exchanges can handle up to 10,000 telephones.  But what if it is
required to connect more phones than 10,000 or connect phones in different
cities or in different states, or in different countries.  Over the years,
complex network of many telephone exchanges has been established to
accomplish these requirements.  Lets look next at how this network is



      Each telephone exchange in North America has two designations, office
class and name to describe its function.
      Subscriber telephones are normally, but not exclusively, connected to
end offices.  Toll (long distance) switching is performed by Class 4, 3, 2,
1 offices.  The Intermediate Point or Class 4X offices may interconnect
subscriber telephones as well as other Class 5 and Class 4 exchanges.

      The ten Regional Centers (Class 1 offices) in the U.S. and two in
Canada all are connected directly to each other with large-capacity trunk
groups(see ESS just stops use from 2600 Hz tones LOCALLY, but if you can
get past the local ESS, and get the 2600 Hz tone past ESS, then there's
nothing they can do, cause it would cause too much to change ALL THESE
trunks).  In 1981, there were 67 Class 2; 230 Class 3; 1,300 Class 4 and
about 19000 Class 5 exchanges.


      The network is organized like a tree, or rather like a small grove of
trees, whose roots have grown together.  Each exchange is optimized for a
particular function.  A call requiring service which cannot be performed by
a lower class exchange is usually forwarded to the next higher exchange in
the network for further processing.

      The regional Center like the base of each tree, forms the foundation
of the network.  The branch levels are the Class 2, 3, 4, 4X and Class 5
offices.  Most offices are connected to more than one other, and the
interconnections depend on the patters of the traffic arriving at and
leaving each office.

      The network makes connections by attempting to find  the shortest
path from the class 5 office serving the caller to the class 5 office
serving the  called party.  The high usage interoffice trunk groups which
provide direct connection between offices of equal and lower level are used
first.  If they are busy, trunk groups at the next higher level(called
final groups) are used.  Digital logic circuits in the common control of
each exchange make decisions based on rules stored in memory that specify
which trunk groups are to be tried and in what order.  These rules, for
example, prevent more than nine connections in tandem, and prevent endless
loop connections.


      The supervisory signals used to set up telephone connections and the
voice signals of the conversations are carried by transmission systems over
paths called facilities.  These systems are divided into three broad
categories, Local, Exchange Area, and Long-Haul.


      The local network is the means by which telephones in residences and
businesses are connected to central offices.  The local facilities are
almost exclusively wire pairs which fan out like branches of a tree from a
point called the wire center throughout a serving area.  Serving areas vary
greatly in size, from an average of 12 square miles in urban locations to
130 square miles for rural areas.  An average wire center in an urban area
will serve 41,000 subscriber lines and 5,000 trunks.  The urban exchange
are generally of higher call carrying capacity than the rural exchanges.


      The exchange area network is intermediate between the local network
and the long-haul network.  Exchanges are interconnected with exchange area
transmission systems.  These systems may consist of open wire pairs on
poles, wire pairs in cables, microwave radio links, and fiber optic cables.

The exchange area network normally interconnects local exchanges and tandem
exchanges.  Tandem exchanges are those that make connections between
central offices when an interoffice trunk is not available.  A tandem
exchange is to central offices as a central office is to subscriber
telephone sets.


      In the long-haul network, local exchanges are interconnected with
toll(long distance) exchanges. These facilities are normally of high
capacity per circuit, and consist mostly of cable and microwave radio
links.  In some paths (called routes) which require a great many links,
such as the backbone links between Boston and Washington, very high
capacity fiber optic links, each carrying about 4000 voice channels


      Spoken messages or voice signals are not the only signals that are
transmitted on a phone line.  In the previous discussion of making a
connection between the calling telephone and the called telephone, some of
these other signals were discussed, dial tone, dial pulses or key tones
used for sending a number, busy tone, and ringback tone.  These are for
control of the switching connections or to indicate the status of the call.

Such signals are called control signals or supervisory signals.  They may
be tone signals(analog) or ON-OFF(digital) signals.  Therefore if one were
to examine the signals on many local loops, one would find analog voice
signals, analog tone signaling, and digital ON-OFF signaling.  It would be
a mixture of analog and digital signals.


      Signals that have continuously and smoothly varying amplitude and
frequency are called analog signals.  Speech signals are of this type. 
They vary in amplitude and frequency. Voice frequencies that contribute to 
speech can extend from below 100 Hz to above 6000Hz.  However, it has been
found that the major energy necessary for intelligible speech is contained
in a band of frequencies between 200Hz and 4000Hz. 


      In order to eliminate unwanted signals that could disturb
conversations or cause errors in control signals, the circuits that carry
the telephone signals are designed to pass only certain frequencies.  The
range of frequencies that are passed are said to be in the bass band.  0 to
4000 Hz is the pass band of a telephone system voice channel.(a VF channel)

      Bandwidth is the difference between the upper limit and the lower
limit of the pass band; thus. the bandwidth if the VF channel is 4000 Hz. 
However not all of the VF channel is used for the transmission of speech. 
The voice pass band is restricted to 300 to 3000 hertz.  Hence any signal
carried on the telephone circuit which is within the range of 300 to 3000
hertz is called an inband signal.  Any signal which is not within the 300
to 3000 Hz band but is within the VF channel is called an out of band
signal.  All speech signals are in band signals.  Some signaling
transmissions are in band and some are out of band.


      Transmission systems often must operate in the presence of various
unwanted signals (referred to generally as noise) that distort the
information being sent.  Lightning, thermal noise, induced signals from
nearby power lines, battery noise, corroded connections, and maintenance
activities all contribute to degradation of the signal.  Analog channel
speech quality is primarily determined by the absolute noise level on the
channel when it is idle; that is when there is not speech signal present. 
Speech tends to mask any noise present, but noise in an idle channel is
quite objectionable to a listener.  Stringent standards have been set for
this idle channel noise in the US network.

      Another type that originates from the voice transmission itself is an
echo.  The primary echo is the reflection of the transmitted signal back to
the receiver of the person talking.  The amount of delay in the echo
depends on the distance from the transmitter to the point of reflection. 
The effect of the delay on the talker may be barely noticeable to the very
irritating to down right confusing.  Echo also affects the listener on the
far end, but to a lesser degree. Echoes are caused by mismatches in
transmission line impedances which usually occur at the hybrid interface
between a 2 wire circuit and a 4 wire transmission system. 


      A local loop can carry only one voice channel conversation at a time.
This is not economical for toll transmission and a method was devised so
that a transmission path can carry many telephone conversations at the same
time.  This is accomplished by Multiplexing.  For analog signals, frequency
division multiplexing(FDM) is used.  In simplified terms, this means that
several telephone conversations are all sent together over one transmission
channel, but are separated by their frequency.

      The basic principles of this are a voice signal having frequencies
within the voice frequency channel bandwidth of from 0 to 4 kilohertz is
changing the amplitude of another frequency(8,140 Khz) which is called the
carrier frequency.  The 0to 4kHz voice frequency signal is amplitude
modulating the 8,140 Khz carrier.  The information in the voice signal is
being carried by the changing amplitude of the 8,140 Khz signal and the
voice frequencies have been translated to different frequencies.

      If different voice signals are placed on different carrier
frequencies, then many conversations may be multiplexed on one transmission
path and transmitted to the receiving point.  At the receiving point, the
different conversations can be identified and separated by their unique
frequency and the original conversation can be recovered from the
carrier(demodulated) and sent to the called telephone.

      Since each voice channel has a 4 Khz bandwidth, 12 channels require
48kHz bandwidth.  Since the lower frequency in the example is, 8,140 kHz,
the output multiplexed signal frequency extends from 8140 kHz to 8188 kHz. 
It should be apparent that if the individual voice channel bandpass were
made larger, the spread in carrier frequency would have to be larger or if
the number of voice channels to be multiplexed together, the spread in
carrier frequencies would have to be larger.  In technical terms, in
general, as the number of voice channels to be transmitted, over a
transmission path increases, the required bandwidth of the transmission
path must increase.


      As stated previously, signaling refers to specific signals on the
transmission line that are used for controlling the connection from the
calling telephone to the called telephone, or that are used to indicate the
status of a call as it is being interconnected.  The first type to be
discussed is dc signaling.


      DC signaling is based on the presence or absence of circuit current or
voltage, or the presence of a given voltage polarity.  The stante of the
signal indicates on-hook, off-hook, dial pulses, or status of the
interconnection.  These signals are on-off type digital signals.

      On local loops, an on-hook is indicated by an open circuit and no
current flow.  Off hook is signaled by a closed circuit and a continuous
current flow.  Dial pulses consists of a current flow interrupted at a
specified rate as discussed previously. 

      A type of dc signaling called reverse battery signaling is used
between central offices to indicate the status of the switched connection. 
When the newer end exchange requests service, an idle trunk is seized.  A
polarity oar a given voltage exists on the trunk which indicates to the
near end that the called phone is on hook and ringing.  The far end
exchange acknowledges and indicates to the near end that the called party
has answered by reversing the voltage polarity.

      E&M signaling is used for the same purpose on long interoffice and
short hail toll trunks.  This type signaling requires two extra wires in
the originating and terminating trunk circuits, one for the E lead and the
other for the M lead.  Since separate wires are used for each, the on-hook
and off-hook states can be signalled from both ends of the circuit.  This
allows signaling to be sent in both directions at the same time without
interfering with one another.  Sometimes two wires are used for each signal
to avoid noise problems caused by a common ground.


      Various tones are used for both control and status indication. The
tones may be single frequency or combinations frequencies.  These are
analog signals that are either continuous tones or tone bursts.  The call
progress tones are sent by the exchange to the calling phone to inform the
caller about the status of the call.  For example the dial tone which has
been mentioned previously is a continuous tone made by combining the
frequencies of 350Hz and 440Hz.  The busy signal that tells the caller that
the called phone is off-hook, is a combination frequency tone that appears
in bursts of .5 seconds on time separated by an offtime of .5 seconds.  The
receiver off-hook warning signal is separated by an offtime of .5 second.
The receiver off-hook warning signal is a combination frequency tone of
four frequencies which os on for .1 seconds and off for .1 seconds.  This
signal is very loud in order to get the attention of someone to hang up the
receiver that has been left off-hook.  All of these tones as well as the
DTMF addressing tomes discussed previously are in-band signaling.

This has been a Digital Rodent Syndicate Production by Evil Incarnate.
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All of these boards except for metro are 2400 baud and Metro is 1200 baud.
This text file is simply for informational purposes only, no illigal
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Leave me mail on ANY of these fine boards about future text files.  Next
one will be on Switching systems..Ever notice how NOBODY talks about
Crossbar, even though some of us are still on Crossbar? I'll deal with ESS,
ANI Level 5, tracing, and Crossbar on my next text file.


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