450 lines
23 KiB
Plaintext
450 lines
23 KiB
Plaintext
The Telephone System
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A Basic Overview
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By:
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EVIL INCARNATE
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WARNING:
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YOU NEED SOME ELECTRONIC EXPERIENCE TO READ THIS...BUT IF YOU KNOW ***REAL***
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BASIC ELECTRONICS, YOU'LL SLIDE. BUT YOU CAN'T BE AN ULTRA-DWEEB EITHER, YOU
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MUST (READ ****ABSOLUTELY MUST****) HAVE AN IQ OF GREATER THAN ROOM TEMPERATURE.
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The telephone arrived as a practical instrument over a century ago in
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1876 , an outgrowth of experiments on a device to send multiple telegraph
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signals over a single wire. Alexander Graham Bell, a native of Scotland,
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while conducting electrical experiments spilled acid on his trousers. His
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sulphurous reaction, the now famous "Mr. Watson, come here, I want you",
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brought Thomas A. Watson on the run not only because of his employer's
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distress, but because the words had been carried by electricity into
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Watson's room and reproduced clearly on his receiving set. The simple
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instrument being tested on Court Street in Boston on March 10, 1876 wasn't
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very practical(the acid was used in the system) but improvement followed so
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rapidly that putting into action Bell's concept of a public telephone
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network - "this grand system", "whereby a man in one part of the country
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may communicate by word of mouth with another in a distant place" - was
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well underway by January of 1878, when the first commercial exchange was
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operated in New Haven. By 1907, one hotel alone(the Waldorf Astoria in NY_
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had 1,120 telephones and processed 500,000 calls per year.
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That concept has grown into an industry in which one entity (AT&T) is
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the largest company on earth, providing over one hundred million telephone
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sets, making a profit several billion dollars per year, and employing over
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one million people.
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Telephone sets perform a surprising number of functions. Here is a
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list of the most important ones:
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1.It requests the use of the telephone system when the handset is lifted.
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2.It indicates that the system is ready for use by receiving a tone, called
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the dial tone.
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3.It sends the number of the telephone to be called to the system. The
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number is initiated by the caller by pressing number keys or rotating a
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dial.
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4.It indicates the state of a call in progress by receiving tones
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indicating the status.
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5.It changes speech of a calling party to electrical signals for
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transmission to a distant party through the system. It changes electrical
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signals received from a distant party to speech for the called party.
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7.It automatically adjusts for changes in the power supplied to it.
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8.It signals the system that a call is finished when a caller "hangs-up"
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the handset.
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Of course for a telephone to be of any use, it must be connected to
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another telephone. In the very early days of telephones, the phones were
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simply wired together with no switching. This became impractical as the
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number of phones increased and the local exchange or central office was
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established to handle the switching and other functions.
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Each subscriber telephone is connected to a central office that
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contains switching equipment, signaling equipment and batteries that supply
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direct current to operate the telephone. Each phone is connected to the
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central office through a local loop of wires called a wire pair(the other
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wires on a phone line are grounds in case of over voltage) One of theses
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wires is called T(for Tip(DUH!)) and the other is called R(for ring) which
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refers to the tip and ring parts of the plug used in manual switchboards.
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Switches in the central office respond to the dial pulses or tones
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from the telephone to connect the calling phone to the called phone. When
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this connection is established, the two telephones communicate over the
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transformer coupled loops using the current supplied by the central office
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batteries.
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INITIATING A CALL
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When the handset of the telephone is resting in its cradle, the
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weight
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of the handset holds the switchhook buttons down and the switches are open.
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This is called the on-hook condition. The circuit between the telephone
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handset and the central office is open; however, the ringer circuit in the
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telephone is always connected to the central office. The capacitor blocks
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the flow of DC from the battery(in the central office). The ringer circuit
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presents a high impedance to speech signals so it has no effect on them.
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When the handset is removed from its cradle, the spring loaded
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buttons come up and the switchook closes. This completes the circuit to
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the exchange and current flows in the circuit. This is called the off-hook
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condition.
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The off-hook signal tells the exchange that someone wants to make a
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call. The exchange returns a dial tone to the called phone to let the
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caller know that the exchange is ready to accept a telephone number.
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SENDING A NUMBER
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Some telephone sets send the telephone number by dial pulses while
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others send it by audio tones.
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DIAL PULSING
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Most telephone sets that use dial pulsing have a rotary dial which
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opens and closes the local loop circuit at a timed rate. The number od
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dial pulses resulting from one operation of the dial is determined by how
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far the dial is rotated before releasing it.
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DUEL TONE MULTI-FREQUENCY
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Some telephone systems use the newer method of using audio tones to
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send the telephone number. These can be used only off the central office
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is equipped to process the tones. Instead of a rotary dial, these
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telephone sets have a push-button keypad with 12 keys, for the numbers 0
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through 9 and the symbols * and the #. Pressing one of the keys causes an
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electronic circuit in the keypad to generate two output tones that
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represent the number.
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CONNECTING THE PHONES
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The central off ice has various switches and relays that
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automatically
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connect the calling and called phones. For now, assume that the connection
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has been made. The actual operation of switching systems will be covered
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in more detail a little later.
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If the called phone handset is off-hook when the connection is
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attempted, a busy tone generated by the central office is returned to the
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calling phone. Otherwise, a ringing signal is sent to the called phone to
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alert the called party that a call is waiting. At the same time, a
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ringback tone is returned to the calling phone to indicate that the called
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phone is ringing(NOT TO BE CONFUSED WITH CALL WAITING WHICH WILL BE
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EXPLAINED IN LATER TEXT PHILES)
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RINGING THE CALLED PHONE
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Early telephone circuits were point to point(not switched), and the
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caller gained the attention of the party at the other end by picking up the
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transmitter and shouting "Hello". This was not very satisfactory, and
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schemes based on a mechanical signaling arrangements were soon invented.
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The one in common use today, called the "polarized ringer" or bell, was
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patented in 1878 by Thomas A. Watson(Mr. Bell's assistant).
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ANSWERING THE CALL
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When the called party removes the handset in response to a ring, the
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loop to that phone is complete by its closed switchook and loop current
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reflows through the called telephone. The central office then removes tex
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ringing signal and the ringback tone from the circuit.
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TALKING
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The part of the telephone into which a person takes is called the
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transmitter. It converts speech (acoustical energy) into variations in an
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electric current (electrical energy) by varying or modulating the loop
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current in accordance with the speech of the talker.
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The part of the telephone that converts the electric current
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variations into sound that a person can hear is called the receiver. The
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signal produced by the transmitter is called by the loop current to the
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receiver of the called party. Also, a small amount of the transmitter
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signal is fed back into the talker's receiver. This is called the
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sidetone.
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Sidetone is necessary so that the person can hear his/her own voice
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from the receiver to determine how loudly to speak. The sidetone must be
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at the proper level because too much sidetone will cause the person to
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speak too softly for the good reception by the called party. Conversely,
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too little sidetone will cause the person to speak so loud that it may
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sound like a yell at the receiving end.
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ENDING THE CALL
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The call is ended when either party hangs up the handset. The
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calling party can hang up the phone for a second, but the called party must
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hang the phone up for twenty seconds for a disconnect. The on-hook signal
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tells central office to release the line connections. In some central
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offices, the connection is released when either party goes on-hook. In
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others theconnection is released when only the calling party goes on-hook.
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BEYOND THE LOCAL LOOP
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Thus far the discussion of connecting two telephones together has
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been limited to local loops and a central office exchange. Most central
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office exchanges can handle up to 10,000 telephones. But what if it is
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required to connect more phones than 10,000 or connect phones in different
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cities or in different states, or in different countries. Over the years,
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complex network of many telephone exchanges has been established to
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accomplish these requirements. Lets look next at how this network is
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arranged.
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THE PUBLIC SWITCHED TELEPHONE NETWORK
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EXCHANGE DESIGNATIONS
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Each telephone exchange in North America has two designations, office
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class and name to describe its function.
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Subscriber telephones are normally, but not exclusively, connected to
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end offices. Toll (long distance) switching is performed by Class 4, 3, 2,
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1 offices. The Intermediate Point or Class 4X offices may interconnect
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subscriber telephones as well as other Class 5 and Class 4 exchanges.
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The ten Regional Centers (Class 1 offices) in the U.S. and two in
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Canada all are connected directly to each other with large-capacity trunk
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groups(see ESS just stops use from 2600 Hz tones LOCALLY, but if you can
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get past the local ESS, and get the 2600 Hz tone past ESS, then there's
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nothing they can do, cause it would cause too much to change ALL THESE
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trunks). In 1981, there were 67 Class 2; 230 Class 3; 1,300 Class 4 and
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about 19000 Class 5 exchanges.
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INTERCONNECTION
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The network is organized like a tree, or rather like a small grove of
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trees, whose roots have grown together. Each exchange is optimized for a
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particular function. A call requiring service which cannot be performed by
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a lower class exchange is usually forwarded to the next higher exchange in
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the network for further processing.
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The regional Center like the base of each tree, forms the foundation
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of the network. The branch levels are the Class 2, 3, 4, 4X and Class 5
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offices. Most offices are connected to more than one other, and the
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interconnections depend on the patters of the traffic arriving at and
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leaving each office.
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The network makes connections by attempting to find the shortest
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path from the class 5 office serving the caller to the class 5 office
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serving the called party. The high usage interoffice trunk groups which
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provide direct connection between offices of equal and lower level are used
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first. If they are busy, trunk groups at the next higher level(called
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final groups) are used. Digital logic circuits in the common control of
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each exchange make decisions based on rules stored in memory that specify
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which trunk groups are to be tried and in what order. These rules, for
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example, prevent more than nine connections in tandem, and prevent endless
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loop connections.
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STRUCTURE
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The supervisory signals used to set up telephone connections and the
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voice signals of the conversations are carried by transmission systems over
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paths called facilities. These systems are divided into three broad
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categories, Local, Exchange Area, and Long-Haul.
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THE LOCAL NETWORK
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The local network is the means by which telephones in residences and
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businesses are connected to central offices. The local facilities are
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almost exclusively wire pairs which fan out like branches of a tree from a
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point called the wire center throughout a serving area. Serving areas vary
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greatly in size, from an average of 12 square miles in urban locations to
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130 square miles for rural areas. An average wire center in an urban area
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will serve 41,000 subscriber lines and 5,000 trunks. The urban exchange
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are generally of higher call carrying capacity than the rural exchanges.
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THE EXCHANGE AREA NETWORK
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The exchange area network is intermediate between the local network
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and the long-haul network. Exchanges are interconnected with exchange area
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transmission systems. These systems may consist of open wire pairs on
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poles, wire pairs in cables, microwave radio links, and fiber optic cables.
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The exchange area network normally interconnects local exchanges and tandem
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exchanges. Tandem exchanges are those that make connections between
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central offices when an interoffice trunk is not available. A tandem
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exchange is to central offices as a central office is to subscriber
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telephone sets.
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THE LONG HAUL NETWORK
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In the long-haul network, local exchanges are interconnected with
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toll(long distance) exchanges. These facilities are normally of high
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capacity per circuit, and consist mostly of cable and microwave radio
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links. In some paths (called routes) which require a great many links,
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such as the backbone links between Boston and Washington, very high
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capacity fiber optic links, each carrying about 4000 voice channels
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simultaneously.
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TYPES OF TRANSMISSIONS
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Spoken messages or voice signals are not the only signals that are
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transmitted on a phone line. In the previous discussion of making a
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connection between the calling telephone and the called telephone, some of
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these other signals were discussed, dial tone, dial pulses or key tones
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used for sending a number, busy tone, and ringback tone. These are for
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control of the switching connections or to indicate the status of the call.
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Such signals are called control signals or supervisory signals. They may
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be tone signals(analog) or ON-OFF(digital) signals. Therefore if one were
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to examine the signals on many local loops, one would find analog voice
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signals, analog tone signaling, and digital ON-OFF signaling. It would be
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a mixture of analog and digital signals.
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ANALOG VOICE TRANSMISSIONS
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Signals that have continuously and smoothly varying amplitude and
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frequency are called analog signals. Speech signals are of this type.
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They vary in amplitude and frequency. Voice frequencies that contribute to
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speech can extend from below 100 Hz to above 6000Hz. However, it has been
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found that the major energy necessary for intelligible speech is contained
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in a band of frequencies between 200Hz and 4000Hz.
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VOICE CHANNEL BANDWIDTH
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In order to eliminate unwanted signals that could disturb
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conversations or cause errors in control signals, the circuits that carry
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the telephone signals are designed to pass only certain frequencies. The
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range of frequencies that are passed are said to be in the bass band. 0 to
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4000 Hz is the pass band of a telephone system voice channel.(a VF channel)
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Bandwidth is the difference between the upper limit and the lower
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limit of the pass band; thus. the bandwidth if the VF channel is 4000 Hz.
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However not all of the VF channel is used for the transmission of speech.
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The voice pass band is restricted to 300 to 3000 hertz. Hence any signal
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carried on the telephone circuit which is within the range of 300 to 3000
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hertz is called an inband signal. Any signal which is not within the 300
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to 3000 Hz band but is within the VF channel is called an out of band
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signal. All speech signals are in band signals. Some signaling
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transmissions are in band and some are out of band.
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VOICE CHANNEL NOISE
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Transmission systems often must operate in the presence of various
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unwanted signals (referred to generally as noise) that distort the
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information being sent. Lightning, thermal noise, induced signals from
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nearby power lines, battery noise, corroded connections, and maintenance
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activities all contribute to degradation of the signal. Analog channel
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speech quality is primarily determined by the absolute noise level on the
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channel when it is idle; that is when there is not speech signal present.
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Speech tends to mask any noise present, but noise in an idle channel is
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quite objectionable to a listener. Stringent standards have been set for
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this idle channel noise in the US network.
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Another type that originates from the voice transmission itself is an
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echo. The primary echo is the reflection of the transmitted signal back to
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the receiver of the person talking. The amount of delay in the echo
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depends on the distance from the transmitter to the point of reflection.
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The effect of the delay on the talker may be barely noticeable to the very
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irritating to down right confusing. Echo also affects the listener on the
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far end, but to a lesser degree. Echoes are caused by mismatches in
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transmission line impedances which usually occur at the hybrid interface
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between a 2 wire circuit and a 4 wire transmission system.
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MULTIPLEXING
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A local loop can carry only one voice channel conversation at a time.
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This is not economical for toll transmission and a method was devised so
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that a transmission path can carry many telephone conversations at the same
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time. This is accomplished by Multiplexing. For analog signals, frequency
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division multiplexing(FDM) is used. In simplified terms, this means that
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several telephone conversations are all sent together over one transmission
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channel, but are separated by their frequency.
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The basic principles of this are a voice signal having frequencies
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within the voice frequency channel bandwidth of from 0 to 4 kilohertz is
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changing the amplitude of another frequency(8,140 Khz) which is called the
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carrier frequency. The 0to 4kHz voice frequency signal is amplitude
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modulating the 8,140 Khz carrier. The information in the voice signal is
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being carried by the changing amplitude of the 8,140 Khz signal and the
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voice frequencies have been translated to different frequencies.
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If different voice signals are placed on different carrier
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frequencies, then many conversations may be multiplexed on one transmission
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path and transmitted to the receiving point. At the receiving point, the
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different conversations can be identified and separated by their unique
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frequency and the original conversation can be recovered from the
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carrier(demodulated) and sent to the called telephone.
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Since each voice channel has a 4 Khz bandwidth, 12 channels require
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48kHz bandwidth. Since the lower frequency in the example is, 8,140 kHz,
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the output multiplexed signal frequency extends from 8140 kHz to 8188 kHz.
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It should be apparent that if the individual voice channel bandpass were
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made larger, the spread in carrier frequency would have to be larger or if
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the number of voice channels to be multiplexed together, the spread in
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carrier frequencies would have to be larger. In technical terms, in
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general, as the number of voice channels to be transmitted, over a
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transmission path increases, the required bandwidth of the transmission
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path must increase.
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SIGNALING TRANSMISSION
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As stated previously, signaling refers to specific signals on the
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transmission line that are used for controlling the connection from the
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calling telephone to the called telephone, or that are used to indicate the
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status of a call as it is being interconnected. The first type to be
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discussed is dc signaling.
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DC SIGNALING
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DC signaling is based on the presence or absence of circuit current or
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voltage, or the presence of a given voltage polarity. The stante of the
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signal indicates on-hook, off-hook, dial pulses, or status of the
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interconnection. These signals are on-off type digital signals.
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On local loops, an on-hook is indicated by an open circuit and no
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current flow. Off hook is signaled by a closed circuit and a continuous
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current flow. Dial pulses consists of a current flow interrupted at a
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specified rate as discussed previously.
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A type of dc signaling called reverse battery signaling is used
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between central offices to indicate the status of the switched connection.
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When the newer end exchange requests service, an idle trunk is seized. A
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polarity oar a given voltage exists on the trunk which indicates to the
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near end that the called phone is on hook and ringing. The far end
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exchange acknowledges and indicates to the near end that the called party
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has answered by reversing the voltage polarity.
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E&M signaling is used for the same purpose on long interoffice and
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short hail toll trunks. This type signaling requires two extra wires in
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the originating and terminating trunk circuits, one for the E lead and the
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other for the M lead. Since separate wires are used for each, the on-hook
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and off-hook states can be signalled from both ends of the circuit. This
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allows signaling to be sent in both directions at the same time without
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interfering with one another. Sometimes two wires are used for each signal
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to avoid noise problems caused by a common ground.
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TONE SIGNALING
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Various tones are used for both control and status indication. The
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tones may be single frequency or combinations frequencies. These are
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analog signals that are either continuous tones or tone bursts. The call
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progress tones are sent by the exchange to the calling phone to inform the
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caller about the status of the call. For example the dial tone which has
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been mentioned previously is a continuous tone made by combining the
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frequencies of 350Hz and 440Hz. The busy signal that tells the caller that
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the called phone is off-hook, is a combination frequency tone that appears
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in bursts of .5 seconds on time separated by an offtime of .5 seconds. The
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receiver off-hook warning signal is separated by an offtime of .5 second.
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The receiver off-hook warning signal is a combination frequency tone of
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four frequencies which os on for .1 seconds and off for .1 seconds. This
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signal is very loud in order to get the attention of someone to hang up the
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receiver that has been left off-hook. All of these tones as well as the
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DTMF addressing tomes discussed previously are in-band signaling.
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---------------------------------------------------------------------------
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This has been a Digital Rodent Syndicate Production by Evil Incarnate.
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Sysops may use this text file as well as long as it is not modified in any
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way. Call these ULTRA-FINE boards:
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(415)524-3649 RatHead Sysop:Ratsnatcher Co:Myself
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(415)524-9951 The Crystal Ship Sysop:Laughing Swede Co:Myself
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(415)527-9444 IHOL Sysop:Yellow Jacket Co:Myself
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(415)649-0416 Metropolis Sysop:Doppy Flisk Co:Ravenman(phew!)
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All of these boards except for metro are 2400 baud and Metro is 1200 baud.
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This text file is simply for informational purposes only, no illigal
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activity is to be derived from this file, and if any illigal activities are
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derived from this file, i am not responsible in any way.
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Leave me mail on ANY of these fine boards about future text files. Next
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one will be on Switching systems..Ever notice how NOBODY talks about
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Crossbar, even though some of us are still on Crossbar? I'll deal with ESS,
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ANI Level 5, tracing, and Crossbar on my next text file.
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