
                       Understanding The Telephone System

             from "Understanding Communications Systems" Chapter 6

                                by Don L. Cannon

                                      and

                                 Gerard Luecke

                       Texas Instruments Publishing, 1984




    In the telephone system, the two-ywa communication is carried by either a
two-wire or a four-wire system. In the four-wire system on pair of wires is
connected from the transmitter at telephone A to the receiver at telephone B,
and the other pair of wires is cnnected form the transmitter at telephone B to
the receiver at telephone A. In the two-wirere system, which uses one-half of
the wiring of the four-wire system and therefore is less costly, both
transmitter and receivers at telephones A and B share the same pair of wires for
the two-way communication. The power for the communication is provided by a
battery (or an appropriate power source with battery backup). The battery is
usually provided at the central office in commercial telephone networks, though
it can be provided at the transmitter in a private or special purpose four-wire
system.

     The transmission links in modern telephone systems may take several
different forms, such as overhead wire, buried cable, microwave links, satellite
links, and so on. Which option is used for a given communication depends
laregely on where A and B are located. If A and B are located close to each
other, the information will be carried over wires on telephone poles or in
overhead or underground cables. If A and B are located on the same land mass but
a large distance apart, the information could be transmitted over telephone
lines, cables, microwave links, or even satellite links). If A and B are
separated by an ocean, the transmission must be through transoceanic cable,
radio, or satellite links, since these are the only feasible ways to get
information from one continent to another. Whichever approach is used, a means
must be provided for selecting the proper paths over which the information is to
travel. This requires some way to connect circuits through switches called
network switching.


Network switching
-----------------

     The network switching process identifies the originator and recipient of a
telephone call and routes the call through suitable transmission links to its
destination. Requirements: Each telephone in the system must be assigned a
number to indicate its location in the system. The switching network must
recognize which telephone is initiating the call and which telephone is to
receive the call. From this information, it must set up the circuit connections
for a signal path that will send the information from the sending (calling)
telephone to the receiving (called) telephone. In commercial telephone systems a
seven digit number is sufficient to locate individual telephones within a
metropolitan area. Digits specify which central office supplies the power to the
telephones that are to be connected and the last four digits determine which
telephone of a possible 10,000 telephones is calling or is being called. The
central office recognizes which telephone is calling (initiating the call) when
the telephone is removed from its hook or cradle. It recognizes which telephone
is called by the number that is dialed by the calling telephone. If the call is
to a telephone outside the metropolitan area, an additional three digits are
used to define the area code or location of the called metropolitan area. This
would then be a long distance call.

     If central office A has an exchange code of 123 and central office B has an
exchange code of 456, then a call from telepone number 123-0001 to number
123-0003 would require a circuit connected by the switched path. If on the other
hand the telephone at 123-0001 is calling the telephone number 456-0002, the
switching network would have to set up a differeent switched circuit path. The
network must provide the switching circuitry allowing for the possibility of
many calls occurring at the same time within a central office and between
central offices. Thus, many parallel switch paths must be available.
Furthermore, the network also provides detection circuits for all of the
signalling which determines the calling and called telephones and all of the
line checking required to establish tha the lines and telephones are clear to
complete the communication. These signalling requirements depend on the
characteristics of each telephone set and on the signalling sequence required to
make a telephone call.


The Handset
-----------

     The telephone handset contains a microphone, a speaker, switch connections
to the telephone system, ringing circuitry, and a dial network. The instrument
used for talking into and listening will be called the subset. It contains the
microphone and the speaker. When the telephone is not in use the subset rests on
the cradle which opens the switches denoted SH (for switch hook). These
switches disconnect the telephone subset from the telephone system. however,
there is a circuit connection that is maintained to the handset. An
electromagnet called the ringer solenoid is connected to the telephone line
wires on the central office side of the SH switch so that the central office can
ring the telephone with an AC signal when it is called. The telephone wires are
denoted as the T and R lines, for the terms Tip and Ring, which were related to
plug connections used in the original manual (operator controlled) switching
central offices or exchanges.

     With the subset in the cradle (or hanging telephones may be on a hook -
thus the name switch hook) no DC current flows from the central office 48-volt
talking battery through the T-R loop or thru the ring solenoid loop because of
capacitor C. The central office monitors the DC current to determine if the
phone is idle or busy or is initiating a call. The central office controls the
signals that can be delivered to a handset through the switches S1, S2, and S3.
With the phone on the hook, SH is open and only the ringer circuit can function.


Ringing a Called Telephone
--------------------------

     The ringing signal is a 20-47 Hz AC signal from the central office. Picking
up the handset lets DC current flow in the talking circuit, stops the ring, and
connects the reeceiving telephone to the calling telephone.

     The signals that can be sent are:

Signal Tone          Interrupt Rate            Frequencies (Hertz)
-----------          --------------            -------------------

Dial                 None                      480

Ringing              None                      440 modulated with 480

Busy                 60 (called line)          480 modulated with 620

                     30 (toll line)

                     120 (intra-office)



     Assume that the telephone is idle - the subset is on the cradle or on the
hook. It is ready and waiting to receive an incoming call as indicated by the
lack of DC current flow in its T and R lines. If this telephone has been
selected by the central office to receive a call, S2 will be thrown to connect
the ringing signal to the T and R lines through the transformer TC. The 110 volt
AC signal fo 20 to 47 hertz will cause the ringer solenoid to sound the familiar
telephone ring at the handset. This ringing will continue until the caller hangs
up or until the telephone is answered by liftig the subset off the switch hook.
Such action will cause DC current to flow in the T-R loop through the subset.
the central office circuitry will detect the DC current flow and remove the
ringing signal by opening S2. It also will throw switch S3 to connect the set to
the calling transmission path.


Answering The Telephone
-----------------------

     As the speaker talks into the subset to answer the phone, the microphone of
the handset causes the current in the T-R loop to vary to produce electrical
signals that correspond to the pattern of the speech waveform. The T-R loop
current through the matching transformer TS and the primary of the transformer
TC produces the speech signals in the secondary circuit of the transformer TC.
The secondary of the transformer TC is connected to the calling telephone
through the circuit path formed by the closing of switch S3 and network
switching circuits in the central office.

     The direction of the informaton flow through the transformers alternates as
one person speaks, then the other.

     Now if the handset is considered to be the calling handset, after the
person calling has dialed and the called telephone owner has lifted the subset
and spoken into it to answer the telephone (as discussed above), the electrical
signals representing the spoken word are now the input signal to the transformer
TC through tthe closed switch S3. The S3 side of the transformer is now the
primary and the T-R loop side is the secondary. The curreent in the primary
produced bythe speech signals induce a current in the secondary which excites
the speaker of the handset through the matching transformer TS and reproduces
the spoken word of the person answering the called telephone in the ear of the
person at the calling telephone. As each person speaks at either the called or
calling telephone, the spoken word is reproduced from transmitter to receiver
through the completed communications circuit. The system reproduces the spoken
word in both directions through the same circuit.

     Whenever a person speaks into the microphone of the handset they also hear
their spoken words in the speaker of the same hadnset. The matching transformer
TS determines the correct signal level for this signal as well as the correect
signal level for the signal received from the sending telephone.

     During the above described sequence the dialer sub-circuit is not used, the
tone signalling circuit is not used, and switch S1 remains open.


Initiating A Call
-----------------

     Both the dial network and the tone signal generators are needed to provide
the proper basic signalling tones between the calling telephone and the central
office. When the caller lifts the subset off the cradle, DC current flows in the
T-R loop. This is sensed by the central office and indicates that a call has
been initiated. The central office locates the line initiating the call and
places a selected tone signal generator output of 480 hertz on the T-R line by
closing switch S1 and producing a curreent int he primary of transformer TC.
This produces a dial tone in the calling hadnset's speaker. Switches S2 and S3
are open (S2 will remain open, since no ringing current will be delivered to the
calling telephone). When the caller hears the dial tone, the number of the
telephone to be called will be indicated as an electrical signal by a rotary
dial containing switch contacts that opena nd close or by pressing tone dialing
pushbuttons. The dial tone will be interrupted when the first digit is dialed
with a mechanical dialer. In the case of the tone dialing system it will be
replaced with the tone of the button pushed.


Mechanical Dialer
-----------------

     If a mechanical dialer (pulse) is used, the dial will be rotated according
to the digit requireed and allowed to rotate back to its rest position. As it
does it will interrupt the DC current in the T-R lines to cause a number of
pulses equal to the number dialed. The dial is mechanically designed to provide
10 pulses per second. The speaker is usually disconnected by the dial network to
keep from sounding the dialing clicks. The central office circuits receive the
pulses and detect the sequence of numbers dialed and stores them. They use these
numbers to locate the called telephone and to set up an available transmission
path between the two stations. When the called telephone is located, the central
office checks the on-hook status of the called telephone, and rings it if the
telephone is on-hook (idle). It also places a ringing tone throught h closed S1
switches (S2 and S3 are still open) on the line so it is heard at the calling
telephone. This ringing tone is developed by modulating a 440-hertz sinusoid
with a 480-hertz sinusoid. This results in a 920-hertz tone increased and
decreased in intensity at a 40-hertz rate. The ringing tone continues until
eithe the called telephone answers or the calling telepone hangs up. If the
called telephone answers, the central office circuits remove the ringing current
form the called set, the ringing tone from the calling set and completes the
transmission path by closing the switches S3.


Busy Telephone
--------------

     If the called telephone is busy (off-hook) the tone generator will sound
one of three possible busy signals to the calling telephone. The busy tone is a
140-hertz tone generated by modulating a 480-hertz sinusoid with a 620-hertz
sinusoid and filtering out the 1100-hertz signal (when one sinewave modulates
another, both the sum and difference frequency sine wave signals result). This
tone is interrupted at a rate that indicates the reason for the busy signal. A
signal interrupted at the rate of 60 times per imnute indicates the called line
was busy. If the interrupt rate is 1/2 of this, the toll line between the
central offices was busy (full of existing calls.) If the interrupt rate is
doubled to 120 times per imiute it implies that all intra-office paths are busy.
Only with the 60 interrupts/minute is the caller sure that the called party line
is busy.


Tone Dialing
------------

     When the handset is a Touch-Tone service telephone, then the dial network
is more than just switches. It contains tone generating circuits as well. When
the tone dialing telephone is used, the tone key or pad causes a signal of two
frequencies to be placed on the line. The frequencies aree indicated by the
intersection of the frequency lines in the tone matrix of the following diagram:

697--------1-------2-------3-------!
           !       !       !       !
           !       !       !       !
           !       !       !       !
770--------4-------5-------6-------!
           !       !       !       !
           !       !       !       !
           !       !       !       !
852--------7-------8-------9-------!
           !       !       !       !
           !       !       !       !
           !       !       !       !
941--------*-------0-------#-------!
           !       !       !       !
           !       !       !       !
           !       !       !       !
          1209  1336     1477    1633


     For example, pressing the 5 key causes a 770-hertz and a 1336-hertz tone to
be sent to the central office (and to the called party if the button is pushed
during a conversation). Central office circuits that detect and decode the tones
set up the switching for the communications path just as for the mechanical
dialer. The use of such tones speeds up the dialing operation and allows command
and control information to be sent to the called location.


Bandwidth
---------

     The frequencies of the tones generated by the tone keys as well as the
conversattqions of the telephone users must be within the bandpass capabilities
of the telephone system. the bandwidth of a channel in a typical telephone
system used for conversations is 4 kilohertz. The bandpass region allowed for
the voice signals is 200 hertz to 3400 hertz. The tone dialing signals fits into
this range as do some special control signals at 2400 and 2600 hertz. Other
control signals form 3400 to 3700 hertz fit into the overall 4 kilohertz
bandwidth of the telephone channel. This voice channel bandwidth will be an
important system parameter throughout this chapter. The other important
parameters of the voice channel are the signalling tones and the DC currents
that are used by the switching and control circuits of the central offices to
establish the communications paths between many pairs of telephones over a
worldwide network. Understanding the basic concepts of the central office
circuits will help to further understand the telephone communications systems.

(continued in part II)

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