HOW TELEPHONES WORK - AN OVERVIEW
|This overview is taken from an American magazine and modified for the UK.
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.
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.
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
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
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
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.
The Speech Network
One of the advantages of an LC network is that it has no semiconductors, is not voltage
sensitive, and will work
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
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
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
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.
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
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.
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.
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.
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
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.
Last revised: December 20, 2010