The original telephones system was designed for full-duplex analog communication of voice signals. Today, the telephone system is still primarily used for voice, but it employs mostly digital techniques, not only in signal transmission but also in control operations.
The telephone system permits any telephone to connect with any other telephone in the world. This means that each telephone must have a unique identification code—the 10-digit telephone number assigned to each telephone. The telephone system provides a means of recognizing each individual number and provides switching systems that can connect any two telephones.
The Local Loop
Standard telephones are connected to the telephone system by way of a two-wire, twisted pair cable that terminates at the local exchange or central office. As many as 10,000 telephone lines can be connected to a single central office (see Fig. 18-1). The connections from the central office go to the “telephone system” represented in Fig. 18-1 by the large “cloud.” This part of the system, which is mainly long-distance, is described in Sec. 18-2. A call originating at telephone A will pass through the central office and then into the main system, where it is transmitted via one of many different routes to the central office connected to the desired location designated as B in Fig. 18-1. The connection between nearby local exchanges is direct rather than long distance.
The two-wire, twisted-pair connection between the telephone and the central office is referred to as the local loop or subscriber loop. You will also hear it referred to as the last mile or the first mile. The circuits in the telephone and at the central office form a complete electric circuit, or loop. This single circuit is analog and carries both dc and ac signals. The dc power for operating the telephone is generated at the central office and supplied to each telephone over the local loop. The ac voice signals are transmitted along with the dc power. Despite the fact that only two wires are involved, full duplex operation, i.e., simultaneous send and receive, is possible. All dialing and signaling operations are also carried on this single twisted-pair cable.
A basic telephone or telephone set is an analog baseband transceiver. It has a handset that contains a microphone and a speaker, better known as a transmitter and a receiver.
It also contains a ringer and a dialing mechanism. Overall, the telephone set fulfills the following basic functions.
The receive mode provides:
- An incoming signal that rings a bell or produces an audio tone indicating that a call
is being received
- A signal to the telephone system indicating that the signal has been answered
- Transducers to convert voice to electric signals and electric signals to voice
The transmit mode:
- Indicates to the telephone system that a call is to be made when the handset is lifted
- Indicates that the telephone system is ready to use by generating a signal called the
- Provides a way of transmitting the telephone number to be called to the telephone
- Receives an indication that the call is being made by receiving a ringing tone
- Provides a means of receiving a special tone indicating that the called line is busy
- Provides a means of signaling the telephone system that the call is complete
All telephone sets provide these basic functions. Some of the more advanced electronic telephones have other features such as multiple line selection, hold, speakerphone, call waiting, and caller ID.
Fig. 18-2 is a basic block diagram of a telephone set. The function of each block is described below. Detailed circuits for each of the blocks and their operation are described later when the standard and electronic telephones are discussed in detail.
The ringer is either a bell or an electronic oscillator connected to a speaker. It is continuously connected to the twisted pair of the local loop back to the central office. When an incoming call is received, a signal from the central office causes the bell or ringer to produce a tone.
A switch hook is a double-pole mechanical switch that is usually controlled by a mechanism actuated by the telephone handset. When the handset is “on the hook,” the hook switch is open, thereby isolating all the telephone circuitry from the central office local loop. When a call is to be made or to be received, the handset is taken off the hook. This closes the switch and connects the telephone circuitry to the local loop. The direct current from the central office is then connected to the telephone, closing its circuits to operate.
The dialing circuits provide a way for entering the telephone number to be called. In older telephones, a pulse dialing system was used. A rotary dial connected to a switch produced a number of on/off pulses corresponding to the digit dialed. These on/off pulses formed a simple binary code for signaling the central office.
In most modern telephones, a tone dialing system is used. Known as the dual-tone multifrequency (DTMF) system, this dialing method uses a number of pushbuttons that generate pairs of audio tones that indicate the digits called.
Whether pulse dialing or tone dialing is used, circuits in the central office recognize the signals and make the proper connections to the dialed telephone.
This unit contains a microphone for the transmitter and a speaker or receiver. When you speak into the transmitter, it generates an electric signal representing your voice. When a received electric voice signal occurs on the line, the receiver translates it to sound waves. The transmitter and receiver are independent units, and each has two wires connecting to the telephone circuit. Both connect to a special device known as the hybrid.
The hybrid circuit is a special transformer used to convert signals from the four wires from the transmitter and receiver to a signal suitable for a single two-line pair to the local loop. The hybrid permits full-duplex, i.e., simultaneous send and receive, analog communication on the two-wire line. The hybrid also provides a sidetone from the transmitter to the receiver so that the speaker can hear her or his voice in the receiver. This feedback permits automatic voice-level adjustment.
Standard Telephone and Local Loop
Fig. 18-3 is a simplified schematic diagram of a conventional telephone and the local loop connections back to the central office. The circuitry at the central office is discussed in greater detail later. For now, note that the central office applies a dc voltage over the twisted-pair line to the telephone. This dc voltage is approximately 248 V with respect to ground in the open-circuit condition. When a subscriber picks up the telephone, the switch hook closes, connecting the circuitry to the telephone line. The load represented by the telephone circuitry causes current to flow in the local loop and the voltage inside the telephone to drop to approximately 5 to 6 V.
The amount of current flowing in the local loop depends upon a number of factors. The dc voltage supplied by the central office may not be exactly -48 V. It can, in fact, vary many volts above or below the 48-V normal value.
As Fig. 18-3 shows, the central office also inserts some resistance RL to limit the total current flow if a short circuit occurs on the line. This resistance can range from about 350 to 800Ω. In Fig. 18-3, the total resistance is approximately 400 Ω. The resistance of the telephone itself also varies over a relatively wide range. It can be as low as 100 Ω and as high as 400 Ω, depending upon the circuitry. The resistance varies because of the resistance of the transmitter element and because of the variable resistors called varistors used in the circuit to provide automatic adjustment of the line level.
The local loop resistance depends considerably on the length of the twisted pair between the telephone and the central office. Although the resistance of a copper wire in the twisted pair is relatively low, the length of the wire between the telephone and the central office can be many miles long. Thus the resistance of the local loop can be anywhere from 1000 to 1800 V, depending upon the distance. The local loop length can vary from a few thousand feet up to about 18,000 ft.
Finally, the frequency response of the local loop is approximately 300 to 3400 Hz. This is sufficient to pass voice frequencies that produce full intelligibility. An unloaded twisted pair has an upper cutoff frequency of about 4000 Hz. But this cutoff varies considerably depending upon the overall length of the cable. When long runs of cable are used, special loading coils are inserted into the line to compensate for excessive roll-off at the higher frequencies.
The two wires used to connect telephones are labeled tip and ring. These designations refer to the plug used to connect telephones to one another at the central office. At one time, large groups of telephone operators at the central office used plugs and jacks at a switchboard to connect one telephone to another manually.
The tip wire is green and is usually connected to the ground; the ring wire is red. Many telephone cables into a home or an office also contain a second twisted pair if a separate telephone line is to be installed. These wires are usually color-coded black and yellow. Black and yellow correspond to ring and tip, respectively, where yellow is ground. Other color combinations are used in telephone wiring.
In Fig. 18-3, the circuitry connected directly to the tip and ring local loop wires is the ringer. The ringer in most older telephones is an electromechanical bell. A pair of electromagnetic coils are used to operate a small hammer that alternately strikes two small metallic bells. When an incoming call is received, a voltage from the central office operates the electromagnetic coils, which in turn operates the hammer to ring the bells. The bells make the familiar tone produced by most standard telephones.
In Fig. 18-3, the ringing coils are connected in series with a capacitor C1. This allows the ac ringing voltage to be applied to the coils but blocks the 48 V of direct current, thus minimizing the current drain on the 48 V of power supplied at the central office.
The ringing voltage supplied by the central office is a sine wave of approximately 90 Vrms at a frequency of about 20 Hz. These are the nominal values because the actual ringing voltage can vary from approximately 80 to 100 Vrms with a frequency somewhere in the 15- to the 30-Hz range. This ac signal is supplied by a generator at the central office.
The ringing voltage is applied in series with the -48-V dc signal from the central office power supply. The ringing signal is connected to the local loop line by way of a transformer T1. The transformer couples the ringing signal into its secondary winding where it appears in series with the 48-V dc supply voltage.
The standard ringing sequence is shown in Fig. 18-4. In U.S. telephones, the ringing voltage occurs for 1 s followed by a 3-s interval. Telephones in other parts of the world use different ringing sequences. For example, in the United Kingdom, the standard ring sequence is a higher-frequency tone occurring more frequently, and it consists of two ringing pulses 400 ms long, separated by 200 ms. This is followed by a 2-s interval of quiet before the tone sequence repeats.
The transmitter is the microphone into which you speak during a telephone call. In a standard telephone, this microphone uses a carbon element that effectively translates acoustical vibrations into resistance changes. The resistance changes, in turn, produce current variations in the local loop representing the speaker’s voice. A dc voltage must be applied to the transmitter so that current flows through it during operation. The 48 V from the central office is used in this case to operate the transmitter. The resulting ac voice signal produced on the telephone line is approximately 1 to 2 Vrms.
The receiver, or earpiece, is basically a small permanent-magnet speaker. A thin metallic diaphragm is physically attached to a coil that rests inside a permanent magnet. Whenever a voice signal comes down a telephone line, it develops a current in the receiver coil. The coil produces a magnetic field that interacts with the permanent-magnet field. The result is the vibration of the diaphragm in the receiver, which converts the electric signal to the acoustic energy that supplies the voice to the ear. As it comes in over the local loop lines, the voice signal has an amplitude of approximately 0.5 to 1 Vrms.
The hybrid is a transformer-like device that is used to simultaneously transmit and receive on a single pair of wires. The hybrid, which is also sometimes referred to as an induction coil, is really several transformers combined into a single unit. The windings on the transformers are connected in such a way that signals produced by the transmitter are put on the two-wire local loop but do not occur in the receiver. In the same way, the transformer windings permit a signal to be sent to the receiver, but the resulting voltage is not applied to the transmitter.
In practice, the hybrid windings are set up so that a small amount of the voice signal produced by the transmitter does occur in the receiver. This provides feedback to the speaker so that she or he may speak with normal loudness. The feedback from the transmitter to the receiver is referred to as the sidetone. If the sidetone were not provided, there would be no signal in the receiver and the person speaking would have the sensation that the telephone line was dead. By hearing his or her own voice in the receiver at a moderate level, the caller can speak at a normal level. Without the sidetone, the speaker tends to speak more loudly, which is unnecessary.
Automatic Voice Level Adjustment
Because of the wide variation in the different loop lengths of the two telephones connected to each other, the circuit resistances will vary considerably, thereby causing a wide variation in the transmitted and received voice signal levels. All telephones contain some type of component or circuit that provides automatic voice level adjustment so that the signal levels are approximately the same regardless of the loop lengths. In the standard telephone, this automatic loop length adjustment is handled by components called varistors. These are labeled V1, V2, and V3 in Fig. 18-3.
A varistor is a nonlinear resistance element whose resistance changes depending upon the amount of current passing through it. When the current passing through the varistor increases, its resistance decreases. A decrease in current causes the resistance to increase.
The varistors are usually connected across the line. In Fig. 18-3, varistor V1 is connected in series with resistor R1. This varistor automatically shunts some of the currents away from the transmitter and the receiver. If the loop is long, the current will be relatively low and the voltage at the telephone will below. This causes the resistance of the varistor to increase, thus shunting less current away from the transmitter and receiver. On short local loops, the current will be high and the voltage at the telephone will be high. This causes the varistor resistance to decrease; thus more current is shunted away from the transmitter and receiver. The result is a relatively constant level of
transmitted or received speech.
Note that a second varistor V3 is used in the balancing network. The balancing network (C3, C4, R2) works in conjunction with the hybrid to provide the sidetone discussed earlier. The varistor adjusts the level of the sidetone automatically.
The term dialing is used to describe the process of entering a telephone number to be called. In older telephones, a rotary dial was used. In more modern telephones, pushbuttons that generate electronic tones are used for “dialing.” The use of a rotary dialing mechanism produces what is known as pulse dialing.
Rotating the dial and releasing it causes a switch contact to open and close at a fixed rate, producing current pulses in the local loop. These current pulses are detected by the central office and used to operate the switches that connect the dialing telephone to the called telephone. While most telephone companies still support pulse dialing, most dial phones have been long retired. Pulse dialing is no longer widely used.
Although some dial telephones are still in use and all central offices can accommodate them, most modern telephones use a dialing system known as TouchTone. It uses pairs of audio tones to create signals representing the numbers to be dialed. This dialing system is referred to as the dual-tone multifrequency (DTMF) system.
A typical DTMF keyboard on a telephone is shown in Fig. 18-5. Most telephones Use a standard keypad with 12 buttons or switches for the numbers 0 through 9 and the special symbols * and #. The DTMF system also accommodates four additional keys for special applications.
In Fig. 18-5 numbers represent audio frequencies associated with each row and column of pushbuttons. For example, the upper horizontal row containing the keys for 1, 2, and 3 is labeled 697, which means that when any one of these three keys is depressed, a sine wave of 697 Hz is produced. Each of the four horizontal rows produces a different frequency. The horizontal rows generate what is generally known as the low group of frequencies.
A higher group of frequencies is associated with the vertical columns of keys. For example, the keys for the numbers 2, 5, 8, and 0 produce a frequency of 1336 Hz when depressed.
If the number 2 is depressed, two sine waves are generated simultaneously, one at 697 Hz and the other at 1336 Hz. These two tones are linearly mixed. This combination produces a unique sound and is easily detected and recognized at the central office as the signal representing the dialed digit 2. The tolerance on the generated frequencies is usually within ±1.5 percent.
When solid-state circuits came along in the late 1950s, an electronic telephone became possible and practical. Today, all new telephones are electronic, and they use integrated circuit technology.
The development of the microprocessor has also affected telephone design. Although simple electronic telephones do not contain a microprocessor, most multiple-line and full-feature telephones do. A built-in microprocessor permits automatic control of the telephone’s functions and provides features such as telephone number storage and automatic dialing and redialing that are not possible in conventional telephones.
Typical IC Electronic Telephone
The major components of a typical electronic telephone circuit are shown in Fig. 18-6. Most of the functions are implemented with circuits contained within a single IC.
In Fig. 18-6, note that the TouchTone keypad drives a DTMF tone generator circuit. An external crystal or ceramic resonator provides an accurate frequency reference for generating the dual dialing tones.
The tone ringer is driven by the 20-Hz ringing signal from the phone line and drives a piezoelectric sound element.
The IC also contains a built-in line voltage regulator. It takes the dc voltage from the local loop and stabilizes it to provide a constant voltage to the internal electronic circuits. An external Zener diode and transistor provide bias to the electric microphone. The internal speech network contains a number of amplifiers and related circuits that fully duplicate the function of a hybrid in a standard telephone. This IC also contains a microcomputer interface. The box labeled MPU is a single-chip microprocessing unit. Although it is not necessary to use a microprocessor, if automatic dialing and other functions are implemented, this circuit is capable of accommodating them.
Finally, note the bridge rectifier and hook switch circuit. The twisted pair from the local loop is connected to the tip and ring. Both the 48-V dc and 20-Hz ring voltages will be applied to this bridge rectifier. For direct current, the bridge rectifier provides polarity protection for the circuit, ensuring that the bridge output voltage is always positive. When the ac ringing voltage is applied, the bridge rectifier rectifies it into a pulsating dc voltage. The hook switch is shown with the telephone on the hook or in the “hung-up” position. Thus the dc voltage is not connected to the circuit at this time. However, the ac ringing voltage will be coupled through the resistor and capacitor to the bridge, where it will be rectified and applied to the two Zener diodes D1 and D2 that drive the tone ringer circuit.
When the telephone is taken off the hook, the hook switch closes, providing a dc path around the resistor and capacitor R1 and C1. The path to the tone ringer is broken, and the output of the bridge rectifier is connected to Zener diode D3 and the line voltage regulator. Thus the circuits inside the IC are powered up, and calls may be received or made.
All modern electronic telephones contain a built-in microcontroller. Like any microcontroller, it consists of the CPU, a ROM in which a control program is stored, a small amount of random access read-write memory, and I/O circuits. The microcontroller, usually a single-chip IC, may be directly connected to the telephone IC, or some type of intermediate interface circuit may be used.
The functions performed by the microcomputer include operating the keyboard and an LCD display, if present. Some other functions involve storing telephone numbers and automatically redialing. Many advanced telephones have the capability of storing 10 or more commonly called numbers. The user puts the telephone into a program mode and uses the TouchTone keypad to enter the most frequently dialed numbers. These are stored in the microcontroller’s RAM. To automatically dial one of the numbers, the user depresses a pushbutton on the front of the telephone. This may be one of the TouchTone pushbuttons, or it may be a separate set of pushbuttons provided for the purpose. When one of the pushbuttons is depressed, the microcontroller supplies a preprogrammed set of binary codes to the DTMF circuitry in the telephone IC. Thus the number is automatically dialed. Other features implemented by the microcontroller are caller ID and an answering machine.
Previously called an answering machine, this feature is implemented on most electronic phones. The microcontroller automatically answers the call after a preprogrammed number of rings and saves the voice message. In older answering machines, the message was recorded on a tape cassette. But in modern phones, the voice message is digitized, compressed, and then stored in a small flash ROM ready for a replay. The outgoing message is also stored there.
Caller ID, also known as the calling line identification service, is a feature that is now widely implemented on most electronic telephones. To make use of this service, you must sign up and pay for it monthly. With this feature, any calling number will be displayed on an LCD readout when the phone is ringing. This allows you to identify the caller. The caller ID service sends a digitized version of the calling number to your phone during the first and second rings. The data transmitted includes the date, time, and calling number. Data is transmitted by FSK, where a binary 1 (mark) is a 1200-Hz tone and a binary 0 (space) is a 2200-Hz tone. The data rate is 1200 bps.
There are two message formats in use, the single-data message format (SDMF) and the multiple-data message format (MDMF). The SDMF is illustrated in Fig. 18-7.
One-half second after the first ring, 80 bytes of alternating 0s and 1s (hex 05) is transmitted for 250 ms followed by 70 ms of mark symbols. These two signals provide initialization and synchronization of the caller ID circuitry in the phone. This is followed by 1 byte describing the message type. This is usually a binary 4 (00000100), indicating the SDMF.
This is followed by a byte containing the message length, usually the number of digits in the calling number. Next, the data is transmitted. This is the date, time, and the 10-digit phone number transmitted as ASCII bytes with the least significant digit first. The data format is 2 digits for the month, 2 digits for the day, 2 digits for the hour (military time), 2 digits for the minutes, and up to 10 digits for the calling number. For example, if the date is February 14, the time is 3:37 p.m., and the calling number is 512-499-0033, the data sequence would be 021415375124990033. The final byte in the message is the checksum that is used for error detection. The checksum is the 2s complement sum (XOR) of all the data bytes not including the initialization and sync signals.
If the calling number is outside the calling area, the system will display an O on the LCD rather than the calling number. Furthermore, a caller may also have his or her number blocked. This can be done by setting it up with the service provider in advance or by dialing *67 prior to making the call. This will cause a P to be displayed on the LCD instead of the calling number.
A more advanced data format is the MDMF. It is similar to the SDMF but includes an extra field for the name of the calling party plus additional identification bytes.
Most telephones are connected by way of a thin multiwire cable to a wall jack. A special connector on the cable, called an RJ-11 modular connector, plugs into the matching wall jack. Two local loops are available if needed.
The wall jack is connected by way of wiring inside the walls to a central wiring point called the subscriber interface. Also known as the wiring block or modular interface, this is a small plastic housing containing all the wiring that connects the line from the telephone company to all the telephone wires in the house. Many houses and apartments are wired so that there is a wall jack in every room.
Fig. 18-8 is a general diagram of the modular interface. The line from the telephone company usually passes through a protector that provides lightning protection. It then terminates at the interface box. An RJ-11 jack and plug are provided to connect to the rest of the wiring. This gives the telephone company a way to disconnect the incoming line from the rest of the house wiring and makes testing and troubleshooting easier. All the wiring is made by way of screw terminals. For a single-line house, the green and red tip and ring connections terminate at the terminals, and all wiring to the room wall jacks is connected in parallel at these terminals.
If a second line is installed, the black and yellow wires, which are the tip and ring connections, are also terminated at screw terminals. They are then connected to the inside house wiring.
Connections on the RJ-11 connector are shown in Fig. 18-9. The red and green wires terminate at the two center connections, and the black and yellow wires terminate at the two outside connections. Most telephone wires and RJ-11 connectors have four wires and connections. Some cables have only two inner wires. With four wires a two-line phone can be accommodated.
Virtually all offices and most homes now have two or more telephones, and many homes and apartments have a standard telephone jack in every room. This permits a single phone to be moved easily from one place to another, and it permits multiple (extension) phones. However, the ultimate convenience is a cordless telephone, which uses two-way radio transmission and provides total portability. Today, most homes have cordless units.
Cordless Telephone Concepts
A cordless telephone is a full duplex, two-way radio system made up of two units, the portable unit or handset and the base unit. The base unit is wired to the telephone line by way of a modular connector. It receives its power from the ac line. The base unit is a complete transceiver in that it contains a transmitter that sends the received audio signal to the portable unit and receives signals
transmitted by the portable unit and retransmits them on the telephone line. It also contains a battery charger that rejuvenates the battery in the handheld unit.
The portable unit is also a battery-powered transceiver. This unit is designed to rest in the base unit where its battery can be recharged. Both units have an antenna. The transceivers in both the portable and the base units use full-duplex operation. To achieve this, the transmitter and receiver must operate on different frequencies.
Fig. 18-10 shows simplified block diagrams of the base and portable units of a typical cordless telephone. Both the base unit and the handset contain an embedded microcontroller that controls all operations, including the keyboard and display. A high percentage of cordless units also contain a caller ID function and may contain a voice mail feature. An analog-to-digital converter translates a received voice message to digital; it is compressed by the microcontroller and then stored in a flash memory connected to the microcontroller.
The FCC has set aside four primary frequency bands for cordless telephones: 43 to 50 MHz, 902 to 928 MHz, 2.4 to 2.45 GHz, and 5.8 GHz. The older analog phones used 25 assigned duplex frequency pairs in the 43- to the 50-MHz range. In the 902- to 928-MHz ISM band, there are more channels, but the number depends upon the technology used. The 2.4-GHz band has up to 100 wide channels where many spread spectrum signals can exist concurrently and channels are determined by a pseudorandom code. The 5.8-GHz band is the most recent addition with plenty of spectrum space for multiple channels. The phones are programmed to automatically seek a channel pair with no activity and minimum noise.
Cordless Phone Features, Capabilities, and Limitations
The frequency range defines the three basic classes of cordless telephones available today, but there are other considerations. Here is a summary of the three basic types.
The simplest and least expensive cordless phones use the 43- to the 50-MHz range. They are analog phones using frequency modulation. The transmitter output power is limited to 500 mW, and this, in turn, limits the transmission range to a maximum of about 1000 ft, depending upon the environment. The FCC created these limitations deliberately to reduce the amount of interference with nearby cordless telephones as well as the many wireless baby monitors and toy walkie-talkies using the same frequencies. While some 43- to 50-MHz phones are still available, for the most part they have been replaced by the newer digital phones.
Although these older phones work well enough, they are susceptible to noise and their range is limited. If higher quality and longer range are desired, phones in the 900-MHz, 2.4-GHz, or 5.8-GHz range can be used.
Three types of 900-MHz phones are available. These are analog, digital, and spread spectrum. The analog phones use FM. Although they can transmit over a longer distance, they are still susceptible to noise. A digital 900-MHz phone is also available. It uses Gaussian FSK (GFSK) modulation. The best 900-MHz phones use a direct- sequence spread spectrum (DSSS). With a power of up to 1 W, the transmission distance is a maximum of about 5000 to 7000 ft, depending on the environment and terrain. Both types of digital phones are highly immune to noise.
The newer and perhaps the best cordless phones use DSSS in the 2.4-GHz or 5.8-GHz bands. Their maximum range is nearly 7000 ft, and they are virtually immune to local noise. Although these phones are far more expensive, they offer the highest-quality sound and the greatest reliability.
For the most part, cordless phones in the United State have used proprietary designs rather than those conforming to a particular standard. Since the phones are only intended to work in a home or small office setting and there is no requirement that the phone interoperates with other cordless phones, any technology will work as long as it meets the FCC’s frequency and operating mode guidelines. The situation is different in Europe where standards for cordless phones have existed for many years. The newest standard created by the European Telecommunications Standards Institute (ETSI), called Digital Enhanced Cordless Telecommunications (DECT) has now been approved for use in the United States. DECT works in the 1.8- to 1.9-GHz band for U.S. use.
The DECT phones are digital, using Gaussian FSK modulation. Instead of using frequency-division duplexing (FDD) with two channels, DECT uses only a single channel and time-division duplexing (TTD). In a single channel, time-division multiplexing permits 12 users per channel. Typically 10 channels are available. The raw data rate is 1.152 Mbps. The latest version of the DECT phone is 6.0, and these phones are available in the United States.
Most of us take telephone service for granted, as we do other so-called utilities, e.g., electric power. In the United States, telephone service is excellent. But this is certainly not the case in many other countries in the world.
When we refer to the telephone system, we are talking about the organizations and facilities involved in connecting your telephone to the called telephone regardless of where it might be in the United States or anywhere else in the world. The telephone system is called the Public Switched Telephone Network (PSTN). You will sometimes hear the telephone system referred to as the Plain Old Telephone Service (POTS). A number of different companies are involved in long-distance calls, although a single company is usually responsible for local calls in a given area. These companies make up the telephone system, and they design, build, maintain, and operate all the facilities and equipment used in providing universal telephone service. A vast array of equipment and technology is employed. Practically every conceivable type of electronic technology is used to implement worldwide telephone service, and that continues to change as Internet calling is known as Voice over Internet Protocol (VoIP) grows.
The telephone, a small but relatively complex entity, is nothing compared to the massive system that backs it up. The telephone system can connect any two telephones in the world, and most people can only speculate on the method by which this connection takes place. It takes place on many levels and involves an incredible array of systems and technology. Obviously, it is difficult to describe such a massive system here. However, in this brief section, we attempt to describe the technical complexities of interconnecting telephones, the central office and the subscriber line interface that connect each user to the telephone system, the hierarchy of interconnections within the telephone system, and the major elements and general operation of the telephone system. Long-distance operations and special telephone interconnection systems such as the PBX are also discussed. VoIP is introduced.
Most telephones are connected to a local central office by way of the two-line, twisted pair local loop cable. The central office contains all the equipment that operates the telephone and connects it to the telephone system that makes the connection to any other telephone.
Each telephone connected to the central office is provided with a group of basic circuits that power the telephone and provide all the basic functions, such as ringing, dial tone, and dialing supervision. These circuits are collectively referred to as the subscriber interface or the subscriber line interface circuit (SLIC). In older central office systems, the subscriber interface circuits used discrete components. Today, most functions of the subscriber line interface are implemented by one or perhaps two integrated circuits plus supporting equipment. The subscriber line interface is also referred to as the line side interface.
The SLIC provides seven basic functions generally referred to as BORSCHT (representing the fi rst letters of the functions battery, overvoltage protection, ringing supervision, coding, hybrid, and test). A general block diagram of the subscriber interface and BORSCHT functions is given in Fig. 18-11.
The subscriber line interface at the central office must provide a dc voltage to the subscriber to operate the telephone. In the United States, this is typically 248 V dc with respect to ground. The actual voltage can be anything between approximately 220 and 280 V when the phone is on the hook, i.e., disconnected. The voltage at the telephone drops to approximately 6 V when the phone is taken off the hook. The large difference between the on-hook and off-hook voltages has to do with the large voltage drop that occurs across the components in the telephone and the long local loop cable.
The circuits and components that protect the subscriber line interface circuits from electrical damage are referred to collectively as overvoltage protection. The phone lines are vulnerable to many types of electrical problems. Lightning is by f r the worst threat, although other hazards exist, including accidental connection to an electric power line or some type of misconnection that would occur during installation. Induced disturbances from other sources of noise can also cause problems. Overvoltage protection ensures reliable telephone operation even under such conditions.
When a specific telephone is receiving a call, the telephone local office must provide a ringing signal. As indicated earlier, this is commonly a 90-Vrms ac signal at approximately 20 Hz. The SLIC must connect the ringing signal to the local loop when a call is received. This is usually done by closing relay contacts that connect the ringing signal to the line. The SLIC must also detect when the phone is picked up (off-hook) so that the ringing signal can be disconnected.
Supervision refers to a group of functions within the subscriber line interface that monitor local loop conditions and provide various services. For example, the supervision circuits in the SLIC detect when a telephone is picked up to initiate a new call. A sensing circuit recognizes the off-hook condition and signals circuits within the SLIC to connect a dial tone. The caller then dials the desired number, which causes interconnection through the telephone system.
The supervision circuits continuously monitor the line during the telephone call. The circuits sense when the call is terminated and provide the connection of a busy signal if the called number is not available.
Coding is another name for A/D conversion and D/A conversion. Today, many telephone transmissions are made by way of serial digital data methods. The SLIC may contain a codec that converts the analog voice signals to serial PCM format or converts received digital calls back to analog signals to be placed on the local loop. Transmission over trunk lines to other central offices or toll offices or for use in the long-distance transmission is typically by digital PCM signals in modern systems.
Recall that in the telephone, a hybrid circuit (also known as a two-wire to the four-wire circuit), usually a transformer, provides simultaneous two-way conversations on a single pair of wires. The hybrid combines the signal from the telephone transmitter with the received signal to the receiver on the single twisted-pair cable. It keeps the signals separate within the telephone.
A hybrid is also used at the central office. It effectively translates the two-wire line to the subscriber back into four lines, two each for the transmitted and received signals. The hybrid provides separate transmit and receive signals. Although a single pair of lines are used in the local loop to the subscriber, all other connections to the telephone system treat the transmitted and received signals separately and have independent circuits for dealing with them along the way.
To check the status and quality of subscriber lines, the phone company often puts special test tones on the local loop and receives resulting tones in return. These can give information about the overall performance of the local loop. The SLIC provides a way to connect the test signals to the local loop and to receive the resulting signals for measurement.
The basic BORSCHT functions are usually divided into two groups, high voltage, and low voltage. The high-voltage parts of the system are the battery feed, overvoltage protection, ringing circuits, and test circuits. The low-voltage group includes supervision, coding, and hybrid functions. In older systems, all the functions were implemented with discrete component circuits. Today, these functions are generally divided between two ICs, one for the high- voltage functions and the other for the low-voltage functions. However, single-chip SLIC BORSCHT ICs are now available.
Whenever you make a telephone call, your voice is connected through your local exchange to the telephone system. From there it passes through at least one other local exchange, which is connected to the telephone you are calling. Several other facilities may provide switching, multiplexing, and other services required to transmit your voice. The telephone system is referred to as the public switched telephone network (PSTN). The organization of this hierarchy in the United States is discussed in the next sections.
The central office or local exchange is the facility to which your telephone is directly connected by a twisted-pair cable. Also known as an end office (EO), the local exchange can serve up to 10,000 subscribers, each of whom is identified by a four-digit number from 0000 through 9999 (the last four digits of the telephone number).
The local exchange also has an exchange number. These are the three additional digits that make up a telephone number. Obviously, there can be as many as 1000 exchanges with numbers from 000 through 999. These exchanges become part of an area code region, which is defined by an additional three-digit number. Each area code is fully contained within one of the geographic areas assigned to one of the regional operating companies. These companies are called local exchange carriers, or local exchange companies (LECs).
The LECs provide telephone services to designated geographic areas referred to as local access and transport areas (LATAs). The United States is divided into approximately 200 LATAs. The LATAs are defined within the individual states making up the seven operating regions. The LECs provide the telephone service for the LATAs within their regions but do not provide long-distance service for the LATAs.
Long-distance service is provided by long-distance carriers known as interexchange carriers (IXCs). The IXCs are familiar long-distance carriers, such as AT&T, Verizon, and Sprint. Long-distance carriers must be used for the interconnection for any interLATA connections The LECs can provide telephone service within the LATAs that are part of their operating region, but links between LATAs within a region, even though they may be directly adjacent to one another, must be made through an IXC.
Each LATA contains a serving, or point-of-presence (POP), office that is used to provide the interconnections to the IXCs. The local exchanges communicate with one another via individual trunks. And all local exchanges connect to a LEC central office, which provides trunks to the POP. At the POP, the long-distance carriers can make their interface connections. The POPs must provide equal access for any long-distance carrier desiring to connect. Many POPs are connected to multiple IXCs, but in many areas, only one IXC serves a POP.
Fig. 18-12 summarizes the hierarchy just discussed. Individual telephones within a LATA connect to the local exchange or central office by way of the two-wire local loop. The central offices within a LATA are connected to one another by trunks. These trunks may be standard baseband twisted-pair cables run underground or on telephone poles, but they may also be coaxial cable, fiber-optic cable, or microwave radio links. In some areas, two or more central offices are located in the same building or physical facility. Trunk interconnections are usually made by cables.
The local exchanges are also connected to a LEC central office when a connection cannot be made between two local exchanges that are not directly trunked. The call passes from the local exchange to the LEC central office, where the connection is made to the other local exchange.
The LEC central offi ce is also connected to the POP. Depending upon the organization of the LEC within the LATA, the LEC central offi ce may contain the POP.
Note in Fig. 18-12 that the POP provides the connections to the long-distance carriers or IXCs. The “cloud” represents the long-distance networks of the IXCs. The long-distance network connects to the remote POPs, which in turn are connected to other central offices and local exchanges.
Most other long-distance carriers have their own specific hierarchical arrangements. A variety of switching offices across the country are linked by trunks using fiber-optic cable or microwave relay links. Multiplexing techniques are used throughout to provide many simultaneous paths for telephone calls.
In all cases, the various central offices and routing centers provide switching services. The whole idea is to permit anyone telephone to directly connect with any other specific telephone. The purpose of all the different levels in the telephone system hierarchy is to provide the interconnecting trunk lines as well as switching equipment that makes the desired interconnection.
The connections between central offices, central offices, and LEC and POPs are digital and use the T1 and T3 multiplexing schemes described. The transmission method in long-distance is fiber-optic cable using protocols known as the asynchronous transfer mode (ATM), the synchronous optical network (SONET), and the optical transport network (OTN). These systems are described.
Signaling refers to the process of setting up and disconnecting calls on the network. Signaling uses digital packets to perform all of the various operations necessary to establish a connection and tear it down. Typical functions include billing, call management (such as call-forwarding, number display, three-way calling, and 800 and 900 calls), and routing of calls from one point to another. The signaling system used in the United States and other parts of the world is called Signaling System No. 7 or SS7.
Besides the T1, T3, and other connections between central offices and other facilities, telephone companies have built a separate signaling network made up of digital links that run at 56 kbps or 64 kbps. Faster connections also exist at rates of 1.536 kbps and 1.984 kbps. These links carry the digital packets with control words that tell the system what to do when a call is made.
In addition, a formal protocol has been established using the familiar OSI model. There are definitions for the four kinds of layers: physical (1 layer), data link (2), network (3), and application (7). The SS7 protocol is standardized by the ITU-T and is designated Q.700. Multiple versions of SS7 exist around the world.
Private Telephone System
Telephone service provided to companies or large organizations with many employees and many telephones is considerably different from basic local loop service provided for individuals. Depending upon the size of the organization, there may be dozens, hundreds, or even thousands of telephones required. It is simply not economical to provide each telephone in the organization with its own separate local loop connection to the central office. It is also an inefficient use of expensive facilities to use a remote central office for intercompany communication. For example, an individual in one office often may need to make an intercompany call to a person in another office, which may be only a few doors down the hall or a couple of floors away. Making this connection through the local exchange is wasteful.
This problem is solved by the use of private telephone systems within a company or organization. Private telephone systems implement telephone service among the telephones in the organization and provide one or more local loop connections to the central office. The two basic types of private telephone systems are known as key systems and private branch exchanges.
Key systems are small telephone systems designed to serve from 2 to 50 user telephones within an organization. Commercially available systems usually have provisions for 6, 10, 12, or 50 telephones.
Simple key telephone systems are made up of individual telephone units generally referred to as stations, all of which are connected to a central answering station. The central answering station is connected to one or more local loop lines known as trunks back to the local exchange. Most systems also contain a central electronic switching unit that makes all the internal and external connections.
The telephone sets in a key system typically have a group of pushbuttons that allow each telephone to select two or more outgoing trunking lines. Phone calls are made in the usual way.
Private Branch Exchange
A private branch exchange, or PBX, as it is known, is a private telephone system for larger organizations. Most PBXs are set up to handle 50 or more telephone interconnections. They can handle thousands of individual telephones within an organization. These systems may also be referred to as private automatic branch exchanges (PABXs) or computer branch exchanges (CPXs). Of the three terms, the expression PBX is the most widely known and used.
A PBX (see Fig. 18-13) is, in effect, a miniature complete telephone system. It provides baseband interconnections to all the telephones in an organization. All the telephones connect to a central switching system that makes inter-company connections as well as external connections to multiple trunk lines to the central office.
Like the key system, the PBX offers the advantages of efficiency and cost reduction when many telephones are required. Interoffice calls can be completed by the PBX system without accessing the local exchange. Furthermore, it is more economical to limit the number of trunk lines to the central office, for not all telephones in the organization will be attempting to access an outside line at one time.
The modern PBX is usually fully automated by computer control. Although no operator is required, most large organizations have one or more operators who answer incoming telephone calls and route them appropriately with a control console. However, some PBXs are automated so that the individual user’s telephone whose extension is the last four digits of the telephone number can be called directly from outside.
As you can see from Fig. 18-13, the PBX is made up of line circuits that are similar to the subscriber line interface circuits discussed earlier. The matrix is the electronic switch that connects any phone to any other phone in the system. It also permits conference calls. The trunk circuits interface to the local loop lines to the central office. All the circuits are under the control of a central computer dedicated to the operation of the PBX.
An alternative to the PBX is known as Centrex. This service, normally provided by the local telephone company, performs the function of a PBX but uses special equipment, and most of the switching is carried out by the local exchange switching equipment over special trunk lines. Its advantage over a standard PBX is that the high initial cost of PBX equipment can be avoided by leasing the Centrex equipment from the telephone company.
Today, as more companies adopt VoIP systems, the older style PBX systems are gradually disappearing in favor of an equivalent system that uses VoIP standards. These systems attach to the company’s LAN system that typically uses Ethernet to connect phones to a base or key unit for distribution and calling features such as voice mail and PBX-like answering capability. Most of these functions are implemented in software with a server dedicated to this function.
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