HOW TELEPHONES WORK - AN OVERVIEW


This overview is taken from an American magazine and modified for the UK.

The Phone Line

The Exchange Line Circuit

Customer Premises Wiring

The Speech Network

The Dial

The Ringer


The phone line
A telephone is usually connected to the telephone exchange by about three miles (4.83 km) of a twisted pair of No.22 (AWG) or 0.5 mm copper wires, known by your phone company as "the local loop". Although copper is a good conductor, it does have resistance. The resistance of No.22 AWG wire is 16.46 Ohms per thousand feet at 77 degrees F (25 degrees C).

In the United States, wire resistance is measured in Ohms per thousand feet; telephone companies describe loop length in kilofeet (thousands of feet). In other parts of the world, wire resistance is usually expressed as Ohms per kilometer.
In the UK the maximum resistance should be no more than 1000 ohms although modern exchanges will work up to 1200 ohms. This can extended by the use of line drivers, which come in two types. One to extend the signalling parameters and the other for speech.

Because telephone apparatus is generally considered to be current driven, all phone measurements refer to current  consumption, not voltage. The length of the wire connecting the subscriber to the telephone exchange affects the total amount of current that can be drawn by anything attached at the subscriber's end of the line.

Line Circuit
In the United States, the voltage applied to the line to drive the telephone is 48 volts DC; the UK use 50 volts DC and on some modern exchanges the voltage can reach 60 volts. Note that telephones are peculiar, in that the signal line is also the power supply line. The voltage used to be supplied by lead acid cells, floated across a power source, thus assuring a hum-free supply and complete independence from the electric company, which may be especially useful during power outages. Today the modern exchanges have sealed cells interspersed amongst the system racks.

On the older electromechanical telephone exchanges the DC voltage and audio signal are separated by directing the audio signal through 2 uF capacitors and blocking the audio from the power supply with a 5-Henry choke in each line. Usually these two chokes are the coil windings of a relay that switches your phone line at the exchange; in most countries this relay is known as the "A" relay. Modern exchanges use current sensing integrated circuits.

In the United States, the telephone company guarantees you no lower current than 20 mA or what is known to your phone company as a "long loop". A "short loop" will draw 50 to 70 mA, and an average loop, about 35 mA. Some countries will consider their maximum loop as low as 12 mA. In practice, United States telephones are usually capable of working at currents as low as 14 mA. Some exchanges will consider your phone in use and feed dial tone down the line with currents as low as 8 mA, even though the telephone may not be able to operate.

Although the telephone company has supplied plenty of nice clean DC direct to your home, don't assume you have a free battery for your own circuits. The telephone company wants the DC resistance of your line to be about 10 megOhms when there's no apparatus in use ("on hook," in telephone company jargon); you can draw no more than 5 microamperes while the phone is in that state (modern phones draw current for memory storage purposes and last no redial). When the phone is in use, or "off hook," you can draw current, but you will need that current to power your phone, any current you might draw for other purposes would tend to lower the signal level.

A phone line is balanced feed, with each side equally balanced to ground. Any imbalance will introduce hum and noise
to the phone line and increase susceptibility to RFI.

The balance of the phone line is known to your telephone company as "longitudinal balance." If both impedance match and balance to ground are kept in mind, any device attached to the phone line will perform well, just as the correct matching of transmission lines and devices will ensure good performance in radio practice.

Customer premises wiring
In the United States the two phone wires connected to a telephone should be red and green. The red wire
is negative and the green wire is positive. In the USA and other parts of the world the telephone company calls the green wire "Tip" and the red wire "Ring" (after the switchboard plug terminals).

Other parts of the world, refer to these wires as "A" and "B".

Most installations in the USA have another pair of wires, yellow and black. These wires can be used for many different purposes, if they are used at all. Some party lines use the yellow wire as a ground and sometimes there's 6.8 VAC on this pair to light the dials of Princess type phones.  If there are two separate phone lines (not extensions) on the premises, you will find the yellow and black pair carrying a second telephone line. In this case, black is "Tip" and yellow is "Ring."

In the UK the line is presented on a single pair, either "figure of eight" style dropwire or a multicore dropwire with steel strengthening wires. Before 1980 this wiring would have connected to the internal wiring directly and would only be accessible by Post Office technicians. After 1980 the new style plug and socket was introduced and this used three wires to connect each socket together. With the introduction of the Network Terminating Unit it was then possible for householders to legally connect their own wiring.

Click here for UK Plug and Socket working.

The Speech Network
The speech network - also known as the "hybrid" or the "two wire/four wire network" takes the incoming signal and feeds it to the earpiece and takes the microphone output and feeds it down the line. The standard network used all over the world is an LC device with a carbon microphone; most modern phones use discrete transistors or ICs.

One of the advantages of an LC network is that it has no semiconductors, is not voltage sensitive, and will work
continuously as the voltage across the line is reduced.  Many transistorised phones stop working as the voltage approaches 3 to 4 Volts.

When a telephone is taken off the hook, the line voltage drops from 48 Volts to between 9 and 3 Volts, depending on the length of the loop.  If another telephone in parallel is taken off the hook, the current consumption of the line will remain the same and the voltage across the terminals of both telephones will drop.  Bell Telephone specifications state that three telephones should work in parallel on a 20 mA loop; transistorised phones tend not to pass this test, although some manufacturers use ICs that will pass. Although some European telephone companies claim that phones working in parallel is "technically impossible," and discourage attempts to make them work that way, some of their telephones will work in parallel.

Because a telephone is a duplex device, both transmitting and receiving on the same pair of wires, the speech network must ensure that not too much of the caller's voice is fed back into his or her receiver.  This function, called "sidetone," is achieved by phasing the signal so that some cancellation occurs in the speech network before the signal is fed to the receiver.

Users faced with no sidetone at all will consider the phone "dead."  Too little sidetone will convince users that they're
not being heard and will cause them to shout or raise their voice.  Too much sidetone on the other hand causes users to lower their voices and not be heard well at the other end of the line. Most modern headsets have an adjustment to ensure that the user is comfortable with the amount of sidetone being returned.

The Dial
There are two types of dials in use around the world. The most common one is called pulse, loop disconnect, or rotary; the oldest form of dialing and it's been around since the beginning of the century. The other dialing method, much more modern is called Touch-tone, Dual Tone Multi-Frequency (DTMF) or Multi-Frequency (MF) in Europe.  In the U.S. MF means single tones used for system control.

Pulse dialing is traditionally accomplished with a rotary dial, which is a speed governed wheel with a cam that opens and closes a switch in series with your phone and the line.  It works by actually disconnecting or "hanging up" the telephone at specific intervals. The United States standard is one disconnect per digit, so if you dial a "1," the telephone is "disconnected" once.  Dial a seven and you'll be "disconnected" seven times; dial a zero, and you'll "hang up " ten times.  Some countries invert the system so "1" causes ten "disconnects" and 0, one disconnect.  Some add a digit so that dialing a 5 would cause six disconnects and 0, eleven disconnects. There are even some systems in which dialing 0 results in one disconnect, and all other digits are plus one, making a 5 cause six disconnects and 9, ten disconnects.

Although most exchanges are quite happy with rates of 6 to 15 Pulses Per Second (PPS), the phone company accepted standard is 8 to 10 PPS.  Some modern digital exchanges, free of the mechanical inertia problems of older systems, will accept a PPS rate as high as 20.

Besides the PPS rate, the dialing pulses have a make/break ratio, usually described as a percentage, but sometimes as a straight ratio.  The North American standard is 60/40 percent; most of Europe and the UK accept a standard of 63/37 percent. This is the pulse measured at the telephone, not at the exchange, where it's somewhat different, having travelled through the phone line with its distributed resistance, capacitance, and inductance. In practice, the make/break ratio does not seem to affect the performance of the dial when attached to a normal loop. Bear in mind that each pulse is a switch connect and disconnect across a complex impedance, so the switching transient often reaches 300 Volts.

Modern dial phones use a CMOS IC and a keyboard. Instead of pushing your finger round in circles, then removing your finger and waiting for the dial to return before dialing the next digit, you punch the button as fast as you want. The IC stores the number and pulses it out at the correct rate with the correct make/break ratio and the switching is done with a high-voltage switching transistor. Because the IC has already stored the dialed number in order to pulse it out at the correct rate, it's a simple matter for telephone designers to keep the memory "alive" and allow the telephone to store, recall, and redial the Last Number Dialed (LND). This feature enables you to redial by picking up the handset and pushing just one button.

Because pulse dialing entails rapid connection and disconnection of the phone line, you can "dial" a telephone that has lost its dial, by hitting the hook-switch rapidly.  It requires some practice to do this with consistent success, but it can be done.

A more sophisticated approach is to place a Morse key in series with the line, wire it as normally closed and send strings of dots corresponding to the digits you wish to dial.

Touch tone (DTMF), the most modern form of dialing, is fast and less prone to error than pulse dialing.  Compared to pulse, its
major advantage is that its audio band signals can travel down phone lines further than pulse, which can travel only as far as your local exchange.  Touch-tone can therefore send signals around the world via the telephone lines, and can be used to control phone answering machines and computers.

Bell Labs developed DTMF in order to have a dialing system that could travel across microwave links and work rapidly with computer controlled exchanges.  Each transmitted digit consists of two separate audio tones that are mixed together.  The four vertical columns on the keypad are known as the high group and the four horizontal rows as the low group; the digit 8 is composed of 1336 Hz and 852 Hz.  The level of each tone is within 3 dB of the other, (the telephone company calls this "Twist"). A complete touch-tone pad has 16 digits, as opposed to ten on a pulse dial. Besides the numerals 0 to 9, a DTMF "dial" has *, #, A, B, C, and D. Although the letters are not normally found on consumer telephones, the IC in the phone is capable of generating them.

The * sign is usually called "star" or "asterisk." The # sign, often referred to as the "pound sign" or "hash" is actually called an octothorpe. Although many phone users have never used these digits - they are not, after all, ordinarily used in dialing phone numbers - they are used for control purposes, exchange feature codes, phone answering machines, bringing up remote bases, electronic banking, and repeater control.  The one use of the octothorpe that may be familiar occurs in dialing international calls from phones in the United States or on some modern telephone systems. After dialing the complete number, dialing the octothorpe lets the exchange know you've finished dialing.  It can now begin routing your call; without the octothorpe, it would wait for an "end of dialing time out" before switching your call.  In the UK some telephones including Public Payphones use the # key to change the dialing mode.

When DTMF dials first came out they had complicated cams and switches for selecting the digits and used a transistor
oscillator with an LC tuning network to generate the tones. Modern dials use a matrix switch and a CMOS IC that synthesizes the tones from a 3.57MHz (TV color burst) crystal. This oscillator runs only during dialing, so it doesn't normally produce QRM.

Standard DTMF dials will produce a tone as long as a key is depressed. No matter how long you press, the tone will be decoded as the appropriate digit. The shortest duration in which a digit can be sent and decoded is about 100 milliseconds (ms). It's pretty difficult to dial by hand at such a speed, but automatic dialers can do it. A twelve-digit long distance number can be dialed by an automatic dialer in a little more than a second - about as long as it takes a pulse dial to send a single 0 digit.

The output level of DTMF tones from your telephone should be between 0 and -12 dBm. In telephones, 0 dB is 1 miliwatt over 600 Ohms. So 0 dB is 0.775 Volts.

The Ringer
This is the device that alerts you to an incoming call. It may be a bell, light, or warbling tone. The telephone company sends a ringing signal which is an AC waveform. Although the common frequency used in the United States is 20 HZ, it can be any frequency between 15 and 68 Hz. Most of the world uses frequencies between 20 and 40 Hz. The voltage at the subscribers end depends upon loop length and number of ringers attached to the line; it could be between 40 and 150 Volts.

In USA minimum ring voltage supplied is 40Vrms (delivered into a 5 REN load). This is the must detect limit. There is also a minimum must ignore value of 10Vrms. Ringing PBX's will vary greatly, but will generally guarantee to deliver 40Vrms into a 3 to 5 REN load (see user documentation).

Note that ringing voltage can be hazardous; when you're working on a phone wiring ensure that it is disconnected at the network connection point.  The telephone company may or may not remove the 48v DC during ringing; as far as you're concerned, this is not important.  In the UK the 50v is maintained during ringing and most telephone systems will not operate properly if this is removed.

The ringing cadence, the timing of ringing to pause, varies from company to company. In the United States the cadence
is normally 2 seconds of ringing to 4 seconds of pause. An unanswered phone in the United States will keep ringing until the caller hangs up. But in some countries, the ringing will "time out" if the call is not answered. In the UK ring timing goes .4 sec on, .2 sec off, .4 sec on, 2 sec off and then repeats, whilst the "time out" is six minutes.

The most common ringing device used to be the gong ringer, a solenoid coil with a clapper that strikes either a single or
double bell. A gong ringer used to be the loudest signalling device that is solely phone line powered.

Modern telephones now tend to use warbling ringers, which are usually ICs powered by the rectified ringing signal. The audio transducer is either a piezoceramic disk or a small loudspeaker via a transformer.

Ringers are isolated from the DC of the phone line by a capacitor. Gong ringers in the United States use a 0.47uF
capacitor, whilst in the UK the capacitor was 1.8uF. Warbling ringers in the United States generally use a 1.0 uF capacitor. Telephone companies in other parts of the world use capacitors between 0.2 and 2.0uF. The paper capacitors of the past have been replaced almost exclusively with capacitors made of Mylar film. Their voltage rating is always 250 Volts.

The capacitor and ringer coil, or Zeners in a warbling ringer, constitute a resonant circuit. When your phone is hung
up ("on hook") the ringer is across the line; if you have turned off the ringer you have merely silenced the transducer, not
removed the circuit from the line.  On the old hard wired telephone systems in the UK the bells of a multi-bell system were actually shorted out when switched off.

Because there is only a certain amount of current available to drive ringers, if you keep adding ringers to the phone line it will reach a point at which either all ringers will cease to ring, some will cease to ring, or some ringers will ring weakly.

In the United States the phone company will guarantee to ring five normal ringers. A normal ringer is defined as a standard gong ringer as supplied in a phone company standard desk telephone. Value given to this ringer is Ringer Equivalence Number (REN) 1. If you look at the FCC registration label of your telephone, modem, or other device to be connected to the phone line, you'll see the REN number. It can be as high as 3.2, which means that device consumes the equivalent power of 3.2 standard ringers, or 0.0, which means it consumes no current when subjected to a ringing signal. If you have problems with ringing, total up your RENs; if the total is greater than 5, disconnect ringers until your REN is at 5 or below.

Other countries have various ways of expressing REN, and some systems will handle no more than three of their standard ringers. In the UK the maximum REN is 4 - click here for more information .  But whatever the system, if you add extra equipment and the phones stop ringing, or the phone answering machine won't pick up calls, the solution is disconnect ringers until the problem is resolved. Warbling ringers tend to draw less current than gong ringers, so changing from gong ringers to warbling ringers may help you spread the sound better.

The American definition of 1 REN is the ringer power required by one ringer of an AT&T standard 500 series telephone set in single-party configuration (ringer placed ACROSS the line).  In the UK it is the standard 700 type telephone.

Because a ringer is supposed to respond to AC waveforms, it will tend to respond to transients (such as switching transients) when the phone is hung up, or when the rotary dial is used on an extension phone. This is called "bell tap" in the United States; in other countries, it's often called "bell tinkle."  Bell tinkle can also be caused when dialling out using a loop disconnect dial.
While European and Asian phones tend to bell tap, or tinkle, United States ringers that bell tap are considered defective, whilst bell tinkle in the UK is frowned upon.

In the USA, because they only use two wires to connect telephones together, the bell tap is designed out of gong ringers and fine tuned with bias springs. Warbling ringers for use in the United States are designed not to respond to short transients; this is usually accomplished by rectifying the AC and filtering it before it powers the IC, then not switching on the output stage unless the voltage lasts long enough to charge a second capacitor.

In the UK bell tinkle is stopped by the use of a three wire system between telephones.

Telephone ringer classification
In USA FCC regulations need the ringer type to be specified on the device. The possible types are Class A and Class B. Class B ringers will respond to ringing frequencies of between 17 and 68 Hertz while Class A ringers will respond to between 16 and 33 Hertz. Class A devices are those typical old telephone bells and practically all electronic ringers are B type. Nearly all of the devices made to connect to the phone lines today are of the Class B type. The telephone ringer type on your device (if you live in USA) is printed on the FCC sticker on the bottom with a REN number on it. You'll see something like .9B (= REN 0.9 Class B) or 1.0A (= REN 1.0 Class A).

Taken from an article by Julian Macassey, (N6ARE), First Published in Ham Radio Magazine September 1985 and added to by Bob Freshwater to reflect the UK and more up to date equipment.

 

 
 
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Last revised: December 20, 2010

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