307 lines
16 KiB
Plaintext
307 lines
16 KiB
Plaintext
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Understanding The Telephone System
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from "Understanding Communications Systems" Chapter 6
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by Don L. Cannon
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and
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Gerard Luecke
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Texas Instruments Publishing, 1984
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In the telephone system, the two-ywa communication is carried by either a
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two-wire or a four-wire system. In the four-wire system on pair of wires is
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connected from the transmitter at telephone A to the receiver at telephone B,
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and the other pair of wires is cnnected form the transmitter at telephone B to
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the receiver at telephone A. In the two-wirere system, which uses one-half of
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the wiring of the four-wire system and therefore is less costly, both
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transmitter and receivers at telephones A and B share the same pair of wires for
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the two-way communication. The power for the communication is provided by a
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battery (or an appropriate power source with battery backup). The battery is
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usually provided at the central office in commercial telephone networks, though
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it can be provided at the transmitter in a private or special purpose four-wire
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system.
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The transmission links in modern telephone systems may take several
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different forms, such as overhead wire, buried cable, microwave links, satellite
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links, and so on. Which option is used for a given communication depends
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laregely on where A and B are located. If A and B are located close to each
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other, the information will be carried over wires on telephone poles or in
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overhead or underground cables. If A and B are located on the same land mass but
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a large distance apart, the information could be transmitted over telephone
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lines, cables, microwave links, or even satellite links). If A and B are
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separated by an ocean, the transmission must be through transoceanic cable,
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radio, or satellite links, since these are the only feasible ways to get
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information from one continent to another. Whichever approach is used, a means
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must be provided for selecting the proper paths over which the information is to
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travel. This requires some way to connect circuits through switches called
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network switching.
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Network switching
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-----------------
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The network switching process identifies the originator and recipient of a
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telephone call and routes the call through suitable transmission links to its
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destination. Requirements: Each telephone in the system must be assigned a
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number to indicate its location in the system. The switching network must
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recognize which telephone is initiating the call and which telephone is to
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receive the call. From this information, it must set up the circuit connections
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for a signal path that will send the information from the sending (calling)
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telephone to the receiving (called) telephone. In commercial telephone systems a
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seven digit number is sufficient to locate individual telephones within a
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metropolitan area. Digits specify which central office supplies the power to the
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telephones that are to be connected and the last four digits determine which
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telephone of a possible 10,000 telephones is calling or is being called. The
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central office recognizes which telephone is calling (initiating the call) when
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the telephone is removed from its hook or cradle. It recognizes which telephone
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is called by the number that is dialed by the calling telephone. If the call is
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to a telephone outside the metropolitan area, an additional three digits are
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used to define the area code or location of the called metropolitan area. This
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would then be a long distance call.
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If central office A has an exchange code of 123 and central office B has an
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exchange code of 456, then a call from telepone number 123-0001 to number
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123-0003 would require a circuit connected by the switched path. If on the other
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hand the telephone at 123-0001 is calling the telephone number 456-0002, the
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switching network would have to set up a differeent switched circuit path. The
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network must provide the switching circuitry allowing for the possibility of
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many calls occurring at the same time within a central office and between
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central offices. Thus, many parallel switch paths must be available.
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Furthermore, the network also provides detection circuits for all of the
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signalling which determines the calling and called telephones and all of the
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line checking required to establish tha the lines and telephones are clear to
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complete the communication. These signalling requirements depend on the
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characteristics of each telephone set and on the signalling sequence required to
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make a telephone call.
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The Handset
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-----------
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The telephone handset contains a microphone, a speaker, switch connections
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to the telephone system, ringing circuitry, and a dial network. The instrument
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used for talking into and listening will be called the subset. It contains the
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microphone and the speaker. When the telephone is not in use the subset rests on
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the cradle which opens the switches denoted SH (for switch hook). These
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switches disconnect the telephone subset from the telephone system. however,
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there is a circuit connection that is maintained to the handset. An
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electromagnet called the ringer solenoid is connected to the telephone line
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wires on the central office side of the SH switch so that the central office can
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ring the telephone with an AC signal when it is called. The telephone wires are
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denoted as the T and R lines, for the terms Tip and Ring, which were related to
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plug connections used in the original manual (operator controlled) switching
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central offices or exchanges.
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With the subset in the cradle (or hanging telephones may be on a hook -
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thus the name switch hook) no DC current flows from the central office 48-volt
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talking battery through the T-R loop or thru the ring solenoid loop because of
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capacitor C. The central office monitors the DC current to determine if the
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phone is idle or busy or is initiating a call. The central office controls the
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signals that can be delivered to a handset through the switches S1, S2, and S3.
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With the phone on the hook, SH is open and only the ringer circuit can function.
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Ringing a Called Telephone
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--------------------------
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The ringing signal is a 20-47 Hz AC signal from the central office. Picking
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up the handset lets DC current flow in the talking circuit, stops the ring, and
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connects the reeceiving telephone to the calling telephone.
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The signals that can be sent are:
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Signal Tone Interrupt Rate Frequencies (Hertz)
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----------- -------------- -------------------
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Dial None 480
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Ringing None 440 modulated with 480
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Busy 60 (called line) 480 modulated with 620
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30 (toll line)
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120 (intra-office)
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Assume that the telephone is idle - the subset is on the cradle or on the
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hook. It is ready and waiting to receive an incoming call as indicated by the
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lack of DC current flow in its T and R lines. If this telephone has been
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selected by the central office to receive a call, S2 will be thrown to connect
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the ringing signal to the T and R lines through the transformer TC. The 110 volt
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AC signal fo 20 to 47 hertz will cause the ringer solenoid to sound the familiar
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telephone ring at the handset. This ringing will continue until the caller hangs
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up or until the telephone is answered by liftig the subset off the switch hook.
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Such action will cause DC current to flow in the T-R loop through the subset.
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the central office circuitry will detect the DC current flow and remove the
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ringing signal by opening S2. It also will throw switch S3 to connect the set to
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the calling transmission path.
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Answering The Telephone
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-----------------------
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As the speaker talks into the subset to answer the phone, the microphone of
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the handset causes the current in the T-R loop to vary to produce electrical
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signals that correspond to the pattern of the speech waveform. The T-R loop
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current through the matching transformer TS and the primary of the transformer
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TC produces the speech signals in the secondary circuit of the transformer TC.
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The secondary of the transformer TC is connected to the calling telephone
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through the circuit path formed by the closing of switch S3 and network
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switching circuits in the central office.
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The direction of the informaton flow through the transformers alternates as
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one person speaks, then the other.
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Now if the handset is considered to be the calling handset, after the
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person calling has dialed and the called telephone owner has lifted the subset
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and spoken into it to answer the telephone (as discussed above), the electrical
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signals representing the spoken word are now the input signal to the transformer
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TC through tthe closed switch S3. The S3 side of the transformer is now the
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primary and the T-R loop side is the secondary. The curreent in the primary
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produced bythe speech signals induce a current in the secondary which excites
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the speaker of the handset through the matching transformer TS and reproduces
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the spoken word of the person answering the called telephone in the ear of the
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person at the calling telephone. As each person speaks at either the called or
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calling telephone, the spoken word is reproduced from transmitter to receiver
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through the completed communications circuit. The system reproduces the spoken
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word in both directions through the same circuit.
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Whenever a person speaks into the microphone of the handset they also hear
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their spoken words in the speaker of the same hadnset. The matching transformer
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TS determines the correct signal level for this signal as well as the correect
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signal level for the signal received from the sending telephone.
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During the above described sequence the dialer sub-circuit is not used, the
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tone signalling circuit is not used, and switch S1 remains open.
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Initiating A Call
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-----------------
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Both the dial network and the tone signal generators are needed to provide
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the proper basic signalling tones between the calling telephone and the central
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office. When the caller lifts the subset off the cradle, DC current flows in the
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T-R loop. This is sensed by the central office and indicates that a call has
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been initiated. The central office locates the line initiating the call and
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places a selected tone signal generator output of 480 hertz on the T-R line by
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closing switch S1 and producing a curreent int he primary of transformer TC.
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This produces a dial tone in the calling hadnset's speaker. Switches S2 and S3
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are open (S2 will remain open, since no ringing current will be delivered to the
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calling telephone). When the caller hears the dial tone, the number of the
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telephone to be called will be indicated as an electrical signal by a rotary
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dial containing switch contacts that opena nd close or by pressing tone dialing
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pushbuttons. The dial tone will be interrupted when the first digit is dialed
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with a mechanical dialer. In the case of the tone dialing system it will be
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replaced with the tone of the button pushed.
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Mechanical Dialer
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-----------------
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If a mechanical dialer (pulse) is used, the dial will be rotated according
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to the digit requireed and allowed to rotate back to its rest position. As it
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does it will interrupt the DC current in the T-R lines to cause a number of
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pulses equal to the number dialed. The dial is mechanically designed to provide
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10 pulses per second. The speaker is usually disconnected by the dial network to
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keep from sounding the dialing clicks. The central office circuits receive the
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pulses and detect the sequence of numbers dialed and stores them. They use these
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numbers to locate the called telephone and to set up an available transmission
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path between the two stations. When the called telephone is located, the central
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office checks the on-hook status of the called telephone, and rings it if the
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telephone is on-hook (idle). It also places a ringing tone throught h closed S1
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switches (S2 and S3 are still open) on the line so it is heard at the calling
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telephone. This ringing tone is developed by modulating a 440-hertz sinusoid
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with a 480-hertz sinusoid. This results in a 920-hertz tone increased and
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decreased in intensity at a 40-hertz rate. The ringing tone continues until
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eithe the called telephone answers or the calling telepone hangs up. If the
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called telephone answers, the central office circuits remove the ringing current
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form the called set, the ringing tone from the calling set and completes the
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transmission path by closing the switches S3.
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Busy Telephone
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--------------
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If the called telephone is busy (off-hook) the tone generator will sound
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one of three possible busy signals to the calling telephone. The busy tone is a
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140-hertz tone generated by modulating a 480-hertz sinusoid with a 620-hertz
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sinusoid and filtering out the 1100-hertz signal (when one sinewave modulates
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another, both the sum and difference frequency sine wave signals result). This
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tone is interrupted at a rate that indicates the reason for the busy signal. A
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signal interrupted at the rate of 60 times per imnute indicates the called line
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was busy. If the interrupt rate is 1/2 of this, the toll line between the
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central offices was busy (full of existing calls.) If the interrupt rate is
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doubled to 120 times per imiute it implies that all intra-office paths are busy.
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Only with the 60 interrupts/minute is the caller sure that the called party line
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is busy.
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Tone Dialing
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------------
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When the handset is a Touch-Tone service telephone, then the dial network
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is more than just switches. It contains tone generating circuits as well. When
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the tone dialing telephone is used, the tone key or pad causes a signal of two
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frequencies to be placed on the line. The frequencies aree indicated by the
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intersection of the frequency lines in the tone matrix of the following diagram:
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697--------1-------2-------3-------!
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! ! ! !
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! ! ! !
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! ! ! !
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770--------4-------5-------6-------!
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! ! ! !
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! ! ! !
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! ! ! !
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852--------7-------8-------9-------!
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! ! ! !
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! ! ! !
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! ! ! !
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941--------*-------0-------#-------!
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! ! ! !
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! ! ! !
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! ! ! !
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1209 1336 1477 1633
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For example, pressing the 5 key causes a 770-hertz and a 1336-hertz tone to
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be sent to the central office (and to the called party if the button is pushed
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during a conversation). Central office circuits that detect and decode the tones
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set up the switching for the communications path just as for the mechanical
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dialer. The use of such tones speeds up the dialing operation and allows command
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and control information to be sent to the called location.
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Bandwidth
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---------
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The frequencies of the tones generated by the tone keys as well as the
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conversattqions of the telephone users must be within the bandpass capabilities
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of the telephone system. the bandwidth of a channel in a typical telephone
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system used for conversations is 4 kilohertz. The bandpass region allowed for
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the voice signals is 200 hertz to 3400 hertz. The tone dialing signals fits into
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this range as do some special control signals at 2400 and 2600 hertz. Other
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control signals form 3400 to 3700 hertz fit into the overall 4 kilohertz
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bandwidth of the telephone channel. This voice channel bandwidth will be an
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important system parameter throughout this chapter. The other important
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parameters of the voice channel are the signalling tones and the DC currents
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that are used by the switching and control circuits of the central offices to
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establish the communications paths between many pairs of telephones over a
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worldwide network. Understanding the basic concepts of the central office
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circuits will help to further understand the telephone communications systems.
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(continued in part II)
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:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-
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The Convent Textfiles BBS 619-475-6187 10 megs 3/1200 baud no pass
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