22-01-2013, 03:05 PM
Telephone System
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How telephone works
A telephone uses an electric current to convey sound information from your home to that of a friend. When the two of you are talking on the telephone, the telephone company is sending a steady electric current through your telephones. The two telephones, yours and that of your friend, are sharing this steady current. But as you talk into your telephone's microphone, the current that your telephone draws from the telephone company fluctuates up and down. These fluctuations are directly related to the air pressure fluctuations that are the sound of your voice at the microphone. Because the telephones are sharing the total current, any change in the current through your telephone causes a change in the current through your friend's telephone. Thus as you talk, the current through your friend's telephone fluctuates. A speaker in that telephone responds to these current fluctuations by compressing and rarefying the air. The resulting air pressure fluctuation reproduces the sound of your voice. Although the nature of telephones and the circuits connecting them have changed radically in the past few decades, the telephone system still functions in a manner that at least simulates this behavior. The current which powers your telephone is generated from the 48V battery in the central office. The 48V voltage is sent to the telephone line through some resistors and indictors (typically there is 2000 to 4000 ohms in series with the 48V power source). The old ordinary offices had about 400 ohm line relay coils in series with the line. Here is a simplified picture of typical traditional telephone line interface: to
Network interface details
The telephone has a circuit called network interface (also called voice network or telephone hybrid) which connects the microphone and speaker to the telephone line. Network interface circuitry is designed so that it sends only the current changes the other telephone causes to the speaker. The current changes, which the telephones own microphone generates, are not sending to the speaker. All this is accomplished using quite ingenious transformer circuitry. In theory the hybrid circuit can separate all incoming audio from the audio sent out at the same time if all the impedances in the circuitry (hybrids on both ends and the wire impedance in between) are well matched. Unfortunately, the hybrid is by its very nature a "leaky" device. As voice signals pass from the 4-wire to the 2-wire portion of the network, the higher energy level in the 4- wire section is also reflected back on itself, creating the echoed speech. The because circuit does not work perfectly and you can still hear some of your own voice in the speaker The actual amount of signal, which is reflected back, depends on how well the balance circuit of the hybrid matches the 2-wire line. In the vast majority of cases, the match is quite poor, resulting in a considerable level of signals being reflected back. The signal, which is reflected back, is not always bad and in normal telephone some if it is really intentional by the design. The separation of the received and transmitted audio could be done much better with modern electronics than with old phones, but people who use the telephone prefer to hear some of their own voice back. Radio Shack's "Understanding Telephone Electronics" (copyrighted around 1985 I think) calls this effect side tone and gives the impression that this was indeed intentional in order for the speaker to determine how loud they were speaking with reference to the called party.
TELEPHONE SETS
Telephone sets can originate and receives telephone calls. It is simple in appearance and operation, yet it performs surprising number of functions.
Here is a list of few important functions.
It requires the use of telephone system when the handset is lifted.
It indicates that the system is ready for use by receiving a tone, called the dial tone.
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 dial.
It indicates the state of a call by receiving tones, which indicates the status (ringing, busy etc).
It indicates the incoming call to the called telephone by ringing bells or other audible tones.
It changes speech of the calling party into electrical signals for transmission to a distant party through the system. It changes electrical signals of the distant party into speech for the called party.
It automatically adjust the changes in the power supplied to it.
It signals the system that a call is finished when a caller “hangs-up” the hand set.
Operating conditions of the telephone sets
The telephone set is designed to operate under a wide range of electrical, mechanical and acoustical conditions. Some of the designed parameters are dictated by human factor such as sound pressure levels, hand set dimensions, etc. and some are historical carryovers such as ringing voltage and frequency, and some are dictated by physical properties of the materials used in the telephone sets, such as minimum line current for satisfactory carbon transmitter and relay operations.
DUAL TONE MULTI FREQUENCY (DTMF)
Some telephone sets use the newer method of using audio tones to send the telephone number. They can be used only if the central office is equipped to process the tones. Instead f a rotary dial, these telephones sets have push buttons keypad with 12 keys for the number 0 through 9 and symbol “*” (asterisk) and “#” (octothotpe). Pressing one of the keys causes n electronic circuit in the keypad to generate two output tones that represents the number.
Phone connection
The central office has various switches and relays that automatically connects the calling and called phones. If the called phone hand set if OFF-HOOK when the connection is attempt, 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 ring back tone is returned to the calling phone to indicate that the called phone is ringing.
Ringing the called phone
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” or “AHOY”. This was not very satisfactory, and schemes based on a mechanical signaling arrangements were soon invented. The one is common use today, called the “POLARIZED RINGER or BELL”.
Answering the call
When the called party removes the handset in response to a ring, the loop to that phone is completed by its closed switch hook, and loop current flows through the called phone. The central office then removes the ringing signal and the ring back tone form the circuit.
Talking Transmitter
The part of the telephone into which a person talks is called the “Transmitter”. It converts speech (acoustical energy) into variation in an electric current (electrical energy) by varying or modulating the loop current in accordance with the speech of the talker.
Receiver
The part of the telephone that converts the electric current variation into sound that a person can hear is called the receiver. The signal produced by the transmitter is carried by the loop current variation to the receiver of the called party.
Side Tone
A small amount of the transmitter signal is fed back into the talker’s receiver. This is called the “Side Tone”. Side tone is necessary so that the person can hear his/her own voice from the receiver to determine how loudly to speak. The side tone must be at the proper level because too much side tone will cause the person to speak too softly for good reception by the called party. Conversely, too little side tone will cause the person to speak so loudly that it may sound like, yet at the receiving end.
Ending the call
The call is ended when either party puts the hand set on the cradle. The on-hook signal tells the central office to release the line connection. In some central offices, the connection is released when either goes on-hook.
Pulse dialing
In the conventional telephone set, pulse dialing is accomplished with a rotary dial having ten equally spaced finger holes.
The number of dial pulses resulting from one operation of the dial is determined by how far the dial is rotated before resulting it. The regularly spaced holes in the dial plate and the finger stop make it easy to rotate the dial to the correct amount for each digit. This action winds up a spring, which rotates the dial back to the rest position when it is released. A small governor inside the dial causes is to return at a constant rate of rotation. C cam turned by the shaft through the gear, operates the switch contact S3. Which opens and closes the local loop circuit during the returned rotation of the dial. (The loop is never broken during forward rotation of the dial). Opening the loop circuit interrupts the loop current flow of 20 to 120mA and closing the circuit permits the loop current to flow again. Thus pulse dialing produces a series of current pulses in the loop circuit. One pulse is sent for the digit “1”, two for the digit “2” and etc, up to ten pulses for the digit “0”
SET-UP AND RELEASE OF CALL
Each telephone has a switch that indicates an on-hook or off-hook condition. When the hook is raised, the switch is closed and an approximately 50mA current starts flowing. This is detected by a relay gibing information to the control unit in the exchange. The control unit is an efficient and reliable computer in the telephone exchange. It activates signaling circuits that then receive dialed digits from Asubscriber (we cal a subscriber who initiates a call an A-subscriber and a subscriber who receive a call a B-subscriber). The control unit in the telephone exchange controls the switching matrix that connects the speech circuit through to the called B-subscriber. A connection is made according to the numbers dialed by the A-subscriber.
When a call is coming to the B-subscriber, the telephone exchange supplies a ringing voltage to the subscriber loop and the bell of the telephone starts ringing. The ringing voltage is often about 45v AC with 25Hz frequency, which is high enough to activate the bell on any telephone. The ringing voltage is switched off immediately when an off-hook condition is detecte3d on the loop of subscriber B, and then an end-to-end speech circuit is connected and the conversation may start.
Figure shows the signaling phase on a subscriber loop. When the exchange detect the off-hook condition of a subscriber loop, it informs us with a dial tone that we hear when we raise the hook that it is ready to receive digits. After dialing it keeps us informed whether the circuit establishment is successful by sending us a ringing tone when the telephone at the other end rings. When the B-subscriber answers, the exchange switches off both the ringing signal and the ringing tone and connects the circuit through. At the end of the conversation an on-hook condition is detected by the exchange and the speech circuit is released.