Fall 2006 Digital Telecommunications Technology - EETS8320 Lecture 2
Fall 2006 Digital Telecommunications Technology - EETS8320 Lecture 2
Technology - EETS8320
Analog and Digital Telephone and Wireless Sets
(Slides with Notes)
Topics of Lecture
• What are the major parts (or modules)
of a wired landline analog telephone
set? What major parts for a digital
• What functions do these parts
• If time permits, we will open and view a
wired analog telephone set on camera.
Analog Wired Telephone Set
Basic parts/functions of an analog telephone set:
• Microphone: converts acoustic waveform (instantaneous
incremental air pressure) to electrical audio frequency waveform
• Earphone: converts electrical to acoustic waveform
• Transmission via wire pair (loop). Directional coupler (hybrid or
induction coils) used toaid in separation odf incoming/ outgoing
electrical power flow to/from the earphone/microphone
respectively. (2-wire/4-wire conversion)
– Dialing via rotary current impulse count or via DTMF (touch tone)
– alerting via ringer or special sound source
• Analog/Digital conversion:
– Analog telephone set (digital conversion at central office switch)
– A/D conversion in the telephone set for ISDN
• Power from central office battery.
• Extra Optional Features: Caller ID, stored number dialing, etc.
Digital Wireless (2G) Handset
Basic parts/functions of a digital wireless telephone set:
• Microphone: converts analog acoustic waveform to analog
electrical audio frequency waveform
• Earphone: converts analog electrical to acoustic waveform
• Analog/Digital conversion:
– A/D conversion in the digital cellular handset
– Digital signal sent via base-mobile radio link typically comprises 50% digitally
coded speech, 50% error protection codes.
• Transmission via radio for cellular. Typically separate radio
frequencies are used for earphone/ microphone signals (FDD).
– Pushbutton dialing via binary coded messages using a separate logical radio
channel than the voice.
– alerting via special sound source (ring tone generator) activated by message
Power from internal rechargeable battery.
• Extra Optional Features: Caller ID, stored number dialing, etc.
more so than most landline tel sets.
Direct Acoustic Communication
Sound pressure variations at
eardrum ultimately cause nerve
signals to the brain, perceived
Small variations in air pressure
at audio frequencies, produced by
the mouth and throat, propagate through
the air as an acoustic wave.
An ideal telephone system (sometimes called an orthotelephonic system) reproduces precisely the same
acoustic waveform that the listener would hear in
a face-to-face conversation.
A real telephone system
only imperfectly reproduces
the speech (high frequency
components are attenuated,
some distortion and delay
are introduced as well).
Imperfect Telephone Speech
• Telephone speech quality is intentionally sub-optimal
– But it does what is needed economically.
• Audio Spectrum is intentionally incomplete
– Typically 300 Hz to 3500 Hz audio spectrum is adequate for
known-language voice communication
– Improved audio bandwidth is nice for music, entertainment, but
providing it is costly and it adds little to voice intelligibility
• Time Delay
– Partly due to physical transmission time; partly due to low bit
– Typically 100 to 200 milliseconds is perceptible
– Over 200 ms (from geostationary satellite delays) is disturbing to
» Less disturbing for one-way broadcasting
• Small amount of “noise” and distortion is tolerated
– Ideally noise is 30 dB “below” (1/1000th) voice power level
Microphone (“Transmitter”) -1
• Carbon microphone is most widely used in analog wire
– Invented by Thomas Edison; Improved by Emile Berliner
– Historically, the Bell “liquid transmitter” was also variable resistance, but
impractical due to use of a liquid.
– Original Bell commercial telephone used electromagnetic microphone
» Some early telephone sets used two identical devices, some used
only one device that the user moved from ear to mouth during the
conversation. Electromagnetic “mike” output was weak.
– Carbon “mike” is sensitive but low fidelity.
» Carbon grain packing is a minor problem.
Microphone (“Transmitter”) -2
• Technical alternatives for modern telephones:
• Electro-magnetic microphone
– Coil of insulated wire carries varying current due to motion of iron disk
(“diaphragm”) near it. Can use either dc coil current or a permanent
magnet inside the coil to establish basic magnetic field.
– Used in early production (1876) Bell telephones.
– Revived (1960s), with transistor amplification correcting the low electrical
power level of the signal
• Electret microphone
– Used in some modern electronic telephone sets, with amplifier
– An electret is a permanently electrically polarized solid (analogous to a
permanent magnet). Conductive diaphragm near an electrically charged
electret surface has varying voltage, responsive to motion caused by air
Some Microphone Types
Type, (Name, Structure
Analog telephone set.
Western Union used this to negotiate with Bell in ca.
Historically one of the
Impractical because liquid evaporates and is
corrosive. Variable resistance method probably
Flexible iron disk near coil
on permanent magnet.
Seldom used in telephone.
Used in some wireless or
First commercial Bell telephones used this. Some
used the same device as a mike or earphone.
Moving coil or velocity
in magnetic field)
Radio broadcast, sound
Fragile. Used where it will not be dropped or shaken.
Most loudspeakers use a moving-col in a magnetic
field as well.
Electret: Metal disk near
end of a permanently
primarily proprietary ones,
and many wireless sets.
Polarized rod made of plastic or wax.Overall mike is
very low cost.
mike. Crystal is variably
compressed by flexible
Medium quality recording
or public address systems.
Some wireless systems.
Many natural crystals – quartz, rochelle salts, even
table salt – have piezoelectric properties.
Condenser mike. Flexible
metal disk is one “plate” of
High quality recording
Requires high-voltage power supply.
• Electromagnetic transducer used almost universally ever
since Bell’s original invention.
– Magnetically induced force from a current carrying coil of wire acts to
flex an iron disk producing sound.
– Similar to mechanism of loudspeakers and radio earphones.
Loudspeakers typically use a very large moving cone of stiffened paper,
mechanically attached to the coil of wire fidtted into a groove near a
permanent magnet, to obtain louder sound waves in air
• Fidelity is relatively good
• Use of same device for earphone and alerting or hands-free
loudspeaker may present hazard of ear injury due to loud
ringing sound if near the ear when ringing.
– Latest gimmick to prevent this is an infra-red beam “proximity detector”
in some Nortel handsets. Automatically lowers earphone volume when
user’s head is nearby.
• A blue grommet where the cord enters the handset on a
public telephone indicates “hearing-aid compatible”
– Intentional external audio-frequency magnetic field.
Loudspeaker- “Hands Free”
Amplification (sometimes with separate loudspeaker)
used for “hands-free” or “speaker-phone”
Continuous amplification may allow audio feedback
– Hollow, reverberating or echoing sounds due to in-room audio
reflections from walls, etc.
– Self-oscillation or squealing audio when reflections are too strong
Hands-free sets have some type of echo canceling
True echo cancellation (generation of a delayed inverse polarity
waveform to cancel the echo) may be accomplished via DSP* or
alternatively in the transmission system in the central office switch.
… or automatic audio switching
– Mute the loudspeaker when there is local microphone audio
– Mute the local microphone when there is audio from distant end.
– Local microphone audio can take priority over distant audio.
*DSP=Digital Signal Processing
Functions in the Tel Set and Switch
• Battery: dc electric power
• Over-voltage protection: not in the telephone set itself
• Ringer: pre-answer alerting in general. May include
caller ID feature signal between rings.
• Supervision: that aspect of signaling which conveys
• Codec: Analog-digital COder/DECoder in a digital
telephone system. Not in analog telephone set itself.
• Hybrid: directional coupler, 2-wire to 4-wire conversion
• Test: modern telephone switches have built-in test
capabilities. Simple analog telephone sets have little or
no internal test-related equipment.
Landline Central Office Battery
• Lead-acid rechargeable batteries in the CO
building provide -48 V dc for subscriber loops
and also to power almost all the electronic
– In telephone practice the + battery terminal is ultimately
connected to the earth/ground. (opposite of vehicle power
and most other dc power systems)
– This can cause surface corrosion (deposition of copper carbonate or
“verdigris”) on the wire but will not “eat away” the copper wire
• “Float” charging circuits rectify commercial ac power (110
or 220/208 V ac) and produce dc
• Battery is main continuous power source, not just as a
back up source.
– Backup (if used) comprises Diesel engine, electric power generator
(on truck in some cases) and fuel.
Landline Battery Functions
• Provides power for loop current supervision
– supervision works via cradle switch (“switch hook”)
• Provides power for dial signals
– Rotary (decadic) dial pulsing
– Touch-tone (dual tone multi-frequency – DTMF – oscillator)
• Provides power for carbon microphone
– Or for amplified electret or electromagnetic microphone.
• Allows basic POTS* telephone service in case of
municipal electric power failure
– Many PBXs have some designated telephone stations which
automatically connect to pre-designated outside analog lines via
relays actuated when local electric power fails.
– Best solution for digital T-1 type PBX trunks or ISDN is overall
customer premises telecom power backup systems (UPS, leadacid gel-cells, etc.) with sufficient reserve power to operate for
the anticipated duration of outside power failure
*POTS=Plain old Telephone Service
Subscriber Loop Jargon
Analog Subscriber Wire Pair
Tip (A) wire
Ring (B) wire
- 48 V
(in quad cable)
Colors in oddcount cables
• A third wire called Sleeve (C) was used in electromechanical switches, but not today in digital switches.
• Batteries in wireless handsets are mostly secondary
(rechargeable) dry cells
– After many years of living with batteries designed primarily for
flashlights (electric torches) and toys, in the 1990s the wireless
market for rechargeable cells got the battery industry to make greatly
improved and smaller cells.
• Electrode choices of exotic metals such as nickel,
cadmium, lithium, etc. produce a light weight repeatably
rechargeable (typically up to 100-200 times) battery.
• Battery “life” (time between needed recharges) is achieved
partly by good system design
– Base wireless system broadcasts a sleep-wake time schedule for
various ranges of e.g. telephone numbers. Handset can then be
automatically internally turned almost completely off (except for a
timer and power control device) for up to 90% of the time when not in
use, and “awake” only 10% of the time.
– All “paging” messages indicative of incoming calls are delayed until
the next “awake” time window for that particular group of handsets.
– Alerting delay depends on service provider’s schedule, delay is
typically 5 to 20 seconds.
• Protect against lightning or “line crossing” with
power (mains) wires
• Lightning arrestors installed at the point where the
outside wire enters the customer or CO premises,
limits over-voltage to 300 volts
– Most arrestors consist of a simple spark gap with sufficient space
between the electrodes so gas between will spark-over (ionize) at ~300 V.
Ionization voltage of enclosed gap sealed in dry nitrogen is more uniform
and not affected by atmospheric pressure or humidity changes. In an
ionized gas many molecules have one ore more electrons removed, thus
leaving a net positive electric charged “Ion.” Moving ions and electrons
carry electric current across the gap to make the spark.
– The insulating (usually ABS plastic) housing of the telephone set is
designed to withstand far more than 300 V
– Despite all of this protection, telephone operating companies urge
subscribers not to make telephone calls during a lightning storm unless
Analog Ringer Parameters
• Early buzzers or chimes were replaced by low frequency ac
ringing signal in late 19th century.
• Ringing frequency and voltage used today mimic the early
hand-cranked magneto generator, originally used for both
subscriber-to-CO and CO-to-subscriber ringing
• Ringing ~90 V ac RMS (about 127 V peak for sine wave)
– 20 Hz frequency (although other frequencies used for selective
ringing on older multi-party lines, etc.)
– Occasional problem: Some PBX or key telephone equipment uses
square (not sine) waveform with same RMS voltage but lower peak
voltage. This waveform will not be detected by some voltagesensitive electronic ringer devices.
• Today many telephone sets use a local audio oscillator
triggered by ringing voltage, and a loudspeaker. Local
oscillator typically produces a ~1-2 kHz waveform with
other higher frequency components as well.
Alerting Audio Requirements
• Alerting audio typically contains power at ~1-2 kHz for
maximum ear sensitivity
– Based on Fletcher-Munson measurements (coming in later lecture)
describing relative ear sensitivity at different audio frequencies
• Also must contain some higher audio frequencies to permit
listener to localize the sound source
– Low frequency audio does not allow listener to perceive the direction of
the audio source accurately.
• Electromechanical metal chime ringer does all of this
• A two-tone component “warbling” audio signal is frequently
used for non-chime sound.
• Ringer current drawn is described by a Ringer Equivalent
Number (REN) according to US FCC Rules Part 68.
– Example: REN 2.0 ringer draws twice the ringing current vis-à-vis a
standard electromechanical ringer.l
Other Ringing Topics
• Ringing cadence
– North American public telephone systems standardize on a 6 sec cycle: 2 sec
ringing and 4 sec silence.
– European systems vary widely. Example: UK uses 4 sec cycle with two ring bursts
in one sec, then 3 sec silence.
• Most public telephone systems do not produce instantaneous
ringing burst(s) at the beginning of a call
– Delayed ringing bursts are synchronized to the cadence for that portion of the
switch’s telephone lines.
– Connect-before-ringing could cause “glare”* and false connections
• “Bell tap” is a jargon term for any false alerting signal (with an
electro-mechanical or an electronic ringer) due to undesired causes:
– Transient changes in loop voltage due to decadic dialing, hanging up handset, etc.
– Lightning pulses or other “foreign” electrical signals
*Glare is a condition due to seizure of both ends of a two-way loop or trunk due to time
delay of the test used beforehand by the seizing equipment to determine that loop/trunk
is idle vs. busy.
• When a wireless handset is “on” but idle, its receiver scans the
assigned range of radio frequencies, seeking an adequately
powerful radio signal having the special signal characteristics that
identify a so-called “paging” channel
– The exact format of the paging channel is different for GSM, TDMA and CDMA
wireless systems, and will be described in a future lecture.
– If/when the radio signal strength of that paging channel fades – usually due to the
handset moving out of the “cell” -- the handset receiver scans again to find the
paging channel of the nearest base antenna cell.
• When an incoming call for that handset occurs, a paging message is
transmitted (subject to the sleep/wake schedule previously
mentioned) on the paging channels in all the cells where the base
system “suspects” that the handset may be located. This is in some
cases all the cells in the city.
• When a handset receives a paging message for itself, it responds
with a “here I am” message, and then is commanded to exchange
furhter messages, typically on a separate radio channel. One of
these is an alerting message, which automatically causes the
handset to “ring” (play a pre-recodrded sound or ring tone).
• A very popular optional service, which helped to “pay”
for Common Channel No. 7 signaling upgrades in the
public telephone network.
– The originating telephone switch sends a digital call setup
message in SS7 format, called the “Initial Address Message”
(IAM), containing both the dialed number and the originator’s
number. This message is sent via a “common” (shared) call
processing data channel, ultimately to the destination switch. If
the originator has specified “private” option, a code is also sent
indicating not to display the number to ordinary destination
– If the destination subscriber has subscribed to Caller ID service,
and the originator did not forbid it, the caller’s telephone number
is transmitted via a modem signal between the first two ringing
bursts. A Caller-ID modem* and display at the destination
telephone displays the caller number.
– If the destination subscriber has also subscribed to caller name
ID, the destination switch also obtains the originator’s directory
listing name from a separate data base called the Line
Information Data Base (LIDB). Each RBOC has its own LIDB. If
the originator is outside the area of the destination RBOC, the
number will display but the name is typically not available in the
*Actually just the receive part of a modem (a “DEM”). More info later.
• Supervision is traditionally that part of signaling which
conveys busy/idle status
– In some systems, the signals for dialed digits etc. are considered
distinct from supervision signals.
– In new fields of telecommunication, such as wireless, “supervision” is
often used to describe all forms of signaling (rather than a subset of all
types of signaling), thus leading to jargon confusion when a traditional
telephone person discusses technology with a wireless person.
• In the analog subscriber loop, dc current flow, controlled by
the cradle switch, indicates supervision status
• In digital transmission systems, this status may be indicated
by digital messages or by means of periodic status bit
values (1 vs. 0) that occur in certain digital time division
multiplexing bit streams in switching or multiplexing
equipment, at predetermined bit locations (like the least
significant bit position in one of each 6 consecutive digital
• The base system of a wireless call determines a call is still
in progress by means of the successful reception of
digital messages and digitally coded speech at an
adequate power level. Error-protection coding used in the
data stream allows evaluation of the amount of erroneous
• An intentional disconnection is the result of pressing the
END button on the handset. This produces a repeated
and acknowledged disconnect message.
– A similar sequence of disconnect messages is used when the other
party ends the call.
• An unintentional disconnect could occur due to a weak
signal or continual excessive data errors for 5 seconds.
• This slide describes GSM methods. Other technologies
differ in certain ways. GSM service providers can
optionally configure their system to automatically
reconnect an unintentionally disconnected call, although
this requires some processing time.
• In most public (analog) telephone installations, the CODEC or
analog-digital converter is in the CO equipment (on a
“subscriber loop card”). The external loop and customer
telephone equipment are all analog
– The details of the CODEC will be discussed later in the course
• Certain integrated services digital network (ISDN) or
proprietary PBX telephone sets have a CODEC in the
telephone set, and transmit digital signals to the CO or PBX
over the subscriber loop. Digital cell phones have a CODEC in
• The cost of a CODEC was an important factor in the initial
introduction of digital end office and PBX switches. Earlier
digital multiplexers (channel banks) used a shared CODEC for
– Lower cost due to use of large scale integration allowed the use of a
dedicated CODEC microchip for each subscriber loop in an economically
CODECs for Wireless
• Wireless systems use several different types
of CODECs, all presently not waveform
• Internal details of various wireless CODECs
will be described in a later lecture.
• Typical net bit rates for these CODECs is from
6 kb/s to 13 kb/s. Although significantly less
than the 64 kb/s used for standard PSTN
waveform coding, the quality of most wireless
CODECs is very close to the PSTN.
• Most parts of a wireless system are designed
to allow new CODECs to be easily introduced
• “Hybrid coil” is telephone industry jargon for a
particular “transformer” type of directional coupler.
The version in a telephone set is also historically called
an “induction coil”
– confusing, since any single coil -- not a multiple winding
transformer -- is also called induction coil in general electrical
– Also called 2-wire to 4-wire converter
– Permits simultaneous two-way signal power transmission on
– … yet separates microphone and earphone signals at the ends
of the 2-wire loop
• Uses a multi-winding structure with a “matching
circuit” that has approximately the same electrical
impedance as the subscriber loop and CO equipment
Background about Transformers*-1
• Prolific American inventor William Stanley made the first
transformer in 1886. Transformers have both power and
• Electric current (moving electrons) produces a magnetic field
in space surrounding the current flow.
– Intensity and direction of that field mathematically described by a 3component vector B, measured in volt•sec/meter2
• When an almost-closed piece of conductive wire is placed in
that region of space, and the magnetic field changes inside
that wire, a voltage appears at the wire ends.
– This induced voltage is proportional to the time rate of change of the
enclosed magnetic field. For a small area wire “loop” all in one plane,
• v = -dB/dt • Area enclosed by wire
– This is one of the ways to determine the presence of the magnetic field
and to measure its rate of change
– The induced voltage can be 2, 3 or more times larger, by wrapping the
wire around the same area 2, 3 or more times.
– A coil of insulated wire can be both the source and the detector for the
magnetic field. Such a coil is usually called an inductor.
*Not to be confused with children’s toys (of the 1980s to the present) with parts that can be
rearranged to make a robot, a truck (lorry) etc.
magnetic B field.
Loop of wire, with small
gap, penetrated by
field. Field can be
caused by current in the
loop itself (selfinductance) or due to
current in other wires
(transformer) or due to
a permanent magnet.
Loop area A is about
·(D/2)2, where D is
diameter of loop.
A voltage Vm
will occur here
if B is changing
vm = -dB/dt • A
• We can “stack up” such loops to form a helical coil of
wire. Each added “turn” adds another vm volts
Background about Transformers-2
• Electrical inductance measured via a unit called a henry =
volt•sec/ampere (abbreviated H)
• (Self-) inductance L (in henrys)* of the tightly wound helical insulated
coil shown, in terms of its dimensions (meters) and material properties
» L = µ • n2 • A/g
» Where µ is the magnetic permeability of the core material. For
air or vacuum µ is 4••10-7 henry/meter. If iron is used in the
core instead of air, typical µiron is 12000•10-7 henry/meter
» n is the total number of turns of wire (n=5 here)
» The cross section area of the core A=•(d/2)2
» g is the length
*For most inductors, the unit millihenry (mHy), 1/1000 of a Hy, is used. Incidentally, 4•= 12.56636
Inductor Electric Properties
• Relationship between voltage and current is
» v= L•(di/dt)
• When the current does not change with time, there is zero
voltage. The ideal inductor has effectively zero resistance for
dc. Real inductors are typically represented for analysis by a
series resistor with an ideal resistance-less inductor.
• Following a short voltage pulse, current continues to circulate
indefinitely in a closed circuit zero resistance inductor (for
example, a “super-conducting” wire inductor)
– An appropriate size and duration negative voltage pulse can restore the
current to zero, or reverse the current direction if the pulse lasts longer.
– A sequence of positive and negative voltage pulses produces an
alternating positive and negative current.
– When a sine voltage waveform is used, a negative cosine current
– The sine wave voltage and current are “out of phase” by 90 deg (1/4 cycle).
Voltage positive peak occurs ¼ cycle before current peak.
– The ratio of the magnitude of the voltage to the magnitude of the current is
proportional to the frequency. That is, an inductor “passes” more current
(has lower impedance) at lower sine wave frequencies.
Background about Transformers
• A transformer comprises two insulated coils
(typically multi-turn coils) surrounding the same
interior space (typically one coil inside the other)
– A time-varying current in one coil will produce a voltage of the
same waveform (proportional to time derivative of the current) in
– The voltages appearing at the two coils will be proportional to
their respective n (number of turns of wire)
• Transformer with equal number of turns are
typically used to couple electrical non-dc
waveforms at same voltage, but to isolate or
separate the dc current flow in the primary and
– Instantaneous polarity of voltage is fixed by the relative
direction of the two windings. A transformer can be used to
produce a signal with same voltage waveform on the secondary
coil as on the primary, but opposite polarity.
Step-Up or Step-Down
• Transformers with unequal number of turns on primary and
secondary coil are used to “step up” or “step down” voltage
– typically power voltages
– Example: in power cords for portable equipment 110 volt ac
“primary”coil produces, for example, 6 volts on “secondary” coil for use
by low voltage device. Ratio of turns N is 110/6= 18.3 in this example.
• Because of change in voltage/current ratio seen via the
coupled coils of a transformer, the apparent resistance (in
general the “impedance”) of a circuit device is modified per
the square of the turns ratio:
Schematic transformer symbol
So V2/i2=4•R or
Left coil has
2 times the
Lowest Frequency for Transformer
Ideal transformer model
• Transformers don’t “work” at dc. What is the lowest useful frequency?
• In this ideal model of a transformer, used with driving current source Is,
and self inductance L, the high frequency power in “load” resistor R is
(N•is)2•R. (N is the coils turns ratio n1/n2.)
• At dc (zero frequency), the power in resistor R is zero since all current
is diverted by the inductor L. At sine wave frequency fc=R/(2L),
resistor power is ½ of its high frequency value. “Half Power Frequency”
is convenient to measure.
• In telephone transformers, fc is typically 300 Hz by design. This is low
enough so speech intelligibility is adequate.
Implications of Large L value
• Inductor value L in previous figure is a representation
of the combination of the primary and secondary coil
self inductance values
• In order to design a transformer that works well at low
electrical signal frequencies, its coils must have a large
– Requires many turns of wire, core material with high magnetic
permeability (iron or ferrite ceramic, etc.), large area A, etc.
• Good power efficiency also requires low wire
resistance (not explicitly analyzed here)
– Requires thicker (larger wire diameter) wires, use of lower
resistance metals (silver, copper, etc.)
• These things make the transformer physically larger,
heavier and costlier
• Every design is a compromise between high efficiency
(100 % coupling of electric power from one coil to
another) and low size/weight.
Current and Power Flow
• Power flow depends on the polarity of both voltage and
current. In the two examples above, current flows from box A to
B in the upper wire and returns from B to A in the lower wire.
The same directions of current flow exist between boxes C and
D. The boxes contain power sources and other circuit
• Due to the opposite polarity of the voltage on the wire pairs in
the AB vs. the CD case, power flow is toward box B but away
from box D.
• For your own education, examine two other cases where the
voltage is the same as the two cases above, but the current
flow is to the left in the top wire and to the right in the lower
Transformer Power Flow
• Even though a transformer with unequal number of turns
on the secondary vs. primary can produce increased
voltage, it does not produce increased power
• The current flow in the winding with the larger number of
turns is inversely proportional to the turns ratio.
• Thus the power flow into the primary (product of primary
input voltage and current) will ideally be the same as the
output power flow from the secondary winding (product of
output voltage and current)
– Real transformers are slightly less than 100% efficient in transferring
power due to the fact that both coils do not always enclose the same
total magnetic field area, and due to power loss in the resistance of
the wires, certain power loss due to cyclic magnetization and demagnetization (hysteresis) of the iron or other core material, etc.
• A transformer is analogous to a lever: The short end of a
lever has high force and small movement, while the long
end has low force and large movement. The work (energy)
transferred (product of force and distance moved) is the
same in at one end of the lever as it is out at the other
Transformer Uses in Telephones
• Multi-winding transformer in telephone set (“hybrid
coil” or “induction coil” together with other
components acts as a directional coupler
– Directs most of the audio frequency power from the microphone
to the CO, rather than to the earphone.
– Directs most of the audio frequency power from the CO to the
earphone, rather than to the microphone
• Simple transformer at CO couples ac speech waveform
between subscriber and switching/ transmission
equipment, without connecting through the dc loop
• “Hybrid coils” multi-winding transformer at CO
separates earphone and microphone audio power into
two separate unidirectional signals.
– Known as 2-wire to 4-wire conversion.
• Many other uses in T-1 transmission lines, ISDN
systems, etc. not described here.
Telephone Test Capabilities
• Many modern telephone switches have built-in test
– Late at night the subscriber loop is switched over (via relay*
contacts on the line card) to a loop tester
– Tests are done for on-hook resistance between wires and from
each wire to ground
– Excessive test current flow (low resistance) indicates problems usually due to moisture in cables, damaged insulation, etc.
– Some trunks can also be tested for idle circuit noise (clicks and
– Problems are often caused by moisture in cables. “Wet” cables
must be dried or replaced. Drying is often accomplished via
infusion of dry nitrogen gas.
• Automatic testing anticipates problems, and levels the
work load for repair personnel
– Built-in test equipment (BITE) is one of the most important
features of modern telecom systems
A relay comprises electromechanical switch contact(s) actuated (on/off) by the
magnetic field produced by a separate control current.
Manual and Automatic Tests
• Craftsperson can dial test numbers
• Ringback numbers in the CO switch allow test
of the ringer (historic example: 550-xxxx
where x’s represent “your own” last 4 digits)
• “Quiet line” allows human audible
assessment of line noise
• Above tests are due to the switching system,
not to the analog telephone set.
• In PBX and special CENTREX telephone sets,
automatic test of each indicator light and
button may be performed
Historical Telephone Schematic
•In this simple two-wire circuit, the battery provides dc current to
generate a static magnetic field in the earphone.
•In the original 1876 Bell installations, the microphone had the same
structure as the earphone (magnetic coil and flexible iron diaphragm) so
the talk direction through it was reversible (microphone/earphone).
•After the 1880s, a permanent magnet was used in the earphone and the
more sensitive Edison-Berliner carbon microphone was used.
•This simple circuit with carbon microphone is now definitely one-way.
•The battery provides current for the carbon microphone.
Simplified physical 4-wire circuit, as used in some military telephone systems
Simplified diagram dies not show details of battery feed, dial, ringer,
transformer coupling of voice signals, etc.
Historical 2-wire Carbon Mike Circuit
Simple, but inefficient and causes excessive “sidetone” in earphones.
at this earphone.
Audio input here.
Installed at Central
aspects not shown.
Audio frequency power from this microphone is
“wasted” in the local earphone and the other mike.
Simplified diagram dies not show all details of battery feed, ringing,
transformer coupling of voice signals, etc.
power wasted here
More efficient, less (not zero) side tone, uses only two wires to CO.
Earphone having permanent
magnet does not need dc
- Secondary winding
- Iron core
- Split primary winding
Microphone signal current
(red arrows) divides, produces
canceling effects on secondary
Current from distant telephone (green arrows) produces same
sense (direction) voltage in secondary, increases audio level.
Simplified diagram of “induction coil” in telephone; many actual details set omitted.
A sq. meters
Graphic symbol. Curved line is the
outer plate in a “rolled up” capacitor
made of flexible metal foil and plastic
• Electrical capacitance measured via a unit called a farad =
ampere•sec/volt (abbreviated F)
• Capacitance C (in farads)* of two metal “plates” separated
by an insulating “dielectric” is approximately:
» C = e•A/d
» Where e is the “dielectric permittivity” of the core
material. For air or vacuum e is 8.85•10-12 farad/meter.
If plastic is used instead of air, typical e plastic is
» A=is the area of each plate
» d is the dielectric thickness
*For most capacitors, the units microfarad or picofarad (µF or pF) are used
Capacitor Electric Properties
• Relationship between voltage and current is (for ideal nonresistive “plates”)
» i = C•(dv/dt)
• When the voltage does not change with time, there is zero
current. The capacitor “does not pass dc.”
• Following a short current pulse, positive charge remains on one
plate and equal negative charge remains on the other plate
– Electrons have moved from the positive plate to the negative plate.
– An appropriate size and duration negative current pulse can restore the
electrically neutral status of the plates, or reverse the charge polarity if the
pulse last longer.
– A sequence of positive and negative current pulses produces an alternating
positive and negative voltage.
– When a sine voltage waveform is used, a cosine current waveform results.
– The voltage and current are “out of phase” by 90 deg (1/4 cycle). Voltage
positive peak occurs ¼ cycle after current peak.
– The ratio of the magnitude of the current to the magnitude of the voltage is
proportional to the frequency. That is, a capacitor “passes” more current
(has lower impedance) at higher sine wave frequencies.
Telephone Connection with CO
telephone set and
Common battery feed
and voice coupling
Telephone set (dial,
ringer, cradle switch
circuits for loop length
Central office switch equipment. Actual switching is not shown.
Positive battery terminal grounded to minimize electrolytic corrosion.
~10 km Audio frequency voice signals coupled via transformer. Ringing power,
loop current detection not shown.
Varistors and their Uses
• A varistor is a simple non-linear silicon electrical device used
in Type 500, 2500 and related telephone sets for several
• Varistors are made by binding together small grains of
impure silicon with a conductive “glue” and fastening on two
wires as “terminals,” then coating with plastic. Typically
made from “scrap silicon” discarded during the zone refining
• Unlike a linear electrical resistor, in which current is
proportional to voltage (Ohm’s law: v= R•i, where v is voltage,
R is constant resistance, and i is current), the current in a
varistor increases more than proportionately
• An empirical approximate formula for the varistor is i= K•v2,
where K is a constant depending on varistor material and
– Sign correction required in this formula since current has same polarity
as voltage (current is negative when voltage is negative). Using signum
i= K•signum (v)•v2. Signum (v) is +1 for positive v,
Varistor Symbol & Graph
“Incremental” or small signal
resistance is re= V/ I. Varies
with operating point voltage vo
or current io. Larger io gives
Three varistors are used in a type 500 telephone set:
One parallel with earphone to bypass high peak voltage audio (from
power crossing or manual switchboard clicks)
Two in parallel with microphone and matching network, to bypass
more microphone audio on short loops (where loop current io is
large) so high microphone audio level is not required at the CO.
Traditional vs. Modern
• The previous explanations mostly show traditional telephone
set structure, based mainly on the type 500 design by AT&T
Bell Laboratories in 1948. It’s relatively bulky by today’s
standards for several reasons:
– Discrete electrical components were used, since integrated
circuits were not available in 1948
– Numerous wiring variations (e.g. multi-party ringer connected
from one wire to ground, etc.) were provided via re-arrangeable
spade-lug tipped wires and brass screw terminals. (Multi-party
service has today almost disappeared in North America.)
– Press-in or machine-screw terminals were used because of
union work rules, which prohibited any tool or instrument more
sophisticated than a screwdriver for an ordinary telephone
Integrated Circuit Telephone
• Today most inexpensive one-piece telephone sets use
an integrated subscriber line circuit (SLC), which
performs the 2-to-4 wire functions of the telephone by
means of unidirectional transistor amplifiers.
• A variable gain amplifier controlled by loop current is
used to compensate the microphone signal level for
different loop lengths (no varistors needed)
• The earphone signal level is automatically controlled
via an adjustable amplifier to prevent overly loud audio
(no earphone varistor used)
• The pushbutton dial can produce either rotary-impulse
or tone signals at will (controlled by an auxiliary
switch), using a digital waveform generator for the tone
• Modern Electronic Digital Switching
software is real-time event-driven:
– The driving events are end-user actions such as
dialing digits, lifting or replacing handset, etc.
• Circuit-switched voice telephone
software mimics the human interface
behavior of historical electromechanical switches
– Including incidental items like intentional postdialing delay and non-symmetrical treatment of
origin/destination vis-à-vis disconnect (for
landline switches only – modern cellular
switches disconnect immediately due to either
• Original 1876 A.G.Bell installations were point-topoint hard wired. Examples:
– Office to warehouse of same firm (like a modern
– Palace to beach-house of the King of Hawaii
• Manual cord-board switching introduced in
Hartford, CT in 1880s.
– Teen-age boys pulled electric wires across the room and
temporarily connected them in response to verbal instructions
– Later developments led to standard cord-board: a desk-like
panel with a retractable cord from each voice connection unit,
and a wall panel in front of the human operator with a socket
for each subscriber (and historically later, a socket for each
trunk line to another switching center)
– Parallel historical development of common battery power and
supervision technology also facilitated the cord switchboard
Other 19th Century Improvements
• Carbon Microphone (Edison and Berliner)
– Permitted loops of up to ~5 mi (8 km) due to greater
transmitted electrical audio power level
• 2-wire “loop,” instead of single wire using earth
conductivity for current return path
– Earth return was previous standard in telegraph systems, but
produced tremendous “cross-talk” for telephones
– Loop greatly improved voice quality and reduced audio noise
– “Invented” by J.J.Carty, later chief engineer of AT&T
• Alternating current ringer (low maintenance)
instead of previous buzzer devices with vibrating
electric contacts subject to sparking, corrosion
• Common (central office) battery for dc loop
current using transformer to couple audio voice
signal between two telephones in a conversation
• Same diameter used today for 1/4 in (6.35 mm)
stereo headset plug
Note: use of red
insulation for negative polarity is
unique to the
electronics, automotive) use red
Sleeve (only in
wire) Plug Assembly Graphic Symbol
Socket Assembly Graphic Symbol
In traditional telephone jargon, “supervision” describes only the
aspects of signaling which relate to busy/idle status
Historical method to get attention of the operator was a small
hand-cranked AC generator or “magneto” at subscriber end
Dialed digit information was historically distinct (called “signaling”)
In modern cellular/PCS software both things are often described by the word
» therefore, be careful about jargon!
Resembled a hand-operated pencil sharpener…
Produced about 90 V ac, at 20 Hz frequency.
Still standard ringing waveform for North America today
Then the common-battery circuit was introduced
Subscriber “switch-hook” closed a current loop and operated a light and/or
buzzer near that subscriber’s socket on the switchboard panel, in response to
lifting the handset.
Operator lifted a retractable plug cord from the desk-top, connecting her* headset
to the subscriber via a voice-frequency transformer
Operator then asked, “Number Please?”
* Boys were replaced by more polite ladies in 1890’s; operator corps (except in military settings) was
exclusively female until 1960s.
• Operator plugged other end of cord circuit into callèd subscriber
socket. (The second syllable of callèd is artificially stressed in telephone
jargon to emphasize the spoken distinction with “call”)
– Outer part of socket and “sleeve” (called “C” wire in European jargon) of plug
carried a voltage when that line was busy. (No C wire in modern electronic
– Voltage (if present) on sleeve produced an audible click in operator
earphone, indicating busy line. If so, operator would advise caller and
abandon the process.
If callèd line is idle, destination cord circuit plug is pressed in, connecting
voice circuit of both telephones
– … and temporarily connecting the operator as well
– Operator presses momentary contact switch to apply 20 Hz, 90 V ac ringing to the
callèd loop. Note that human operator controls ringing cadence.
– When callèd person answers, operator presses a latching switch on desk near the
cords to disconnect operator’s headphone from the cord circuit
– When either participant hangs up, dc loop current from common central office
battery stops, indirectly operating a distinct buzzer and light on the cord board via
– Operator then “tears down” the connection by pulling both retractable cord plugs
from the callèd and calling part circuit sockets. Cords fall back into desk surface
due to weights installed under the desk.
Cord Switchboard Capacity
The number of simultaneous conversations is limited to the
number of cord circuits installed in a cord switchboard
– Each cord circuit is similar to a storage address (byte) in an electronic
switch vis-à-vis capacity
– The BHCA* (call processing) capacity is limited by the attention and
operational speed available from the human operator
Both were improved by providing more operator positions (and
thus more cord circuits)
– Each subscriber loop appeared at multiple wall sockets, each one
within reach of an individual operator position
» Thus a historical need for busy status signal (sleeve or C wire)
» Early example of switch “concentration”
• Operator-handled calls were controlled by human
– Computer controlled (stored program controlled - SPC)
switches merely strive to put back into automatic service
many of the clever things human operators did historically
(example, ring back to originator when initially busy
destination finally becomes available)
*BHCA=Busy Hour Call Attempts, a measure of how many call attempts per hour a switch
Some Human Operator Features
• Call by name (no telephone number required)
– Response to: “Please call the Smith home.”
Wake up calls (at pre-determined time)
Re-connect calls accidentally disconnected*
Notify busy line of incoming call waiting
Set up 3-way (or more) conference call
Connect call to alternate line when subscriber is
away from home (call forwarding)
Note that modern “feature-rich” PBX, small business key systems, and some
PSTN switches now do these things via computer control
• Several experts have calculated that there are not enough
people on earth to support the today’s (2005) level of public
telephone traffic using operator cord board switching!
*The GSM cellular system can optionally be configured to do this.
Strowger Step-by-step Switch
• Almon B. Strowger, a mortician (undertaker) in
Kansas City, KS, invented the first practical
automatic dialing system
– Famous story: fearing that the human operator was directing calls for a
mortician to his competitor, he invented an automatic user-controlled
– First version (installed in LaPorte, IN, circa 1895) used extra wires and
push buttons on each subscriber set
– Rotary dial with impulsive current on the voice wire pair was a later
• Strowger’s manufacturing firm, Automatic Electric,
moved to suburban Chicago, IL.
– Later absorbed by GTE, later moved to Phoenix AZ, now AG
Communication Systems (partly owned by Lucent)
– “Stepper” progressive control switches were manufactured world wide for
many decades as exact replicas
– Electromechanical common-control switches developed by other
manufacturers, such as “panel” and “crossbar” types partially succeeded
steppers in the 1930 - 1960 decades
Schematic Stepper Diagram
Ten places on
located -- not
Tip, Ring, Sleeve
wires from Rank 8,
springs activate the motions
of the wiper arm in response
to dial impulses.
Many details omitted here
Strowger switches evolved into an assembly with a movable wiper
switch “inlet” and 100 “outlets” (wire pairs with “sleeve” wire)
10 contact pairs are arranged in a horizontal arc, selected by rotating the wiper
switch arm. (Also a third “sleeve” wire in addition)
10 such horizontal arc sub-assemblies are stacked and selected via vertical motion
of the axle (actually the first motion is vertical)
Single-motion (rotation only) switch assemblies were also used
“Line Finder” switch (mostly single motion) acts as input
concentrator (“inverse” of selector action)
Wiper arm contacts act as the single outlet
Each line finder single-motion stepper is typically wired to 10 subscriber lines, and
selects a line when that line goes off-hook
» Stepper starts stepping from line to line when any of the 10 lines go off hook,
then stops when correct “off-hook” line is “found”
analogous to operator responding to buzzer and light
Multiple line finders are wired in parallel to the same 10 telephone sets analogous to
multiple operator stations with each having access to the same subscriber sockets.
» Number of simultaneous originating conversations for that particular group of 10
subscribers is limited to the number of line finder switches connected to those
lines. Ten line finders wired to ten subscribers is “non-blocking” with regard to
line finders. (Overall system may still block at later stages…)
Line finder outlet goes through a transformer “cord circuit”
Connected to dial-tone generator until the first dialed digit.
Then the circuit is switched through a chain of two-motion selector stepper switches, with
a “motion” for each digit. Each burst of impulses (dialed digit) produces a rotary or vertical
motion constituting the next stage of the wiper arm selection process
Dial pulses from rotary dial (typically 10 impulses per second, each one approximately 60
millisecond current OFF and 40 ms current ON) are passed around the cord circuit by
special electro-mechanical relays
» A relay employs magnetically operated switch contacts, so that current ON/OFF status in the
contacts mimics the current ON/OFF status in the wire coil causing the magnetic relay action.
Special “slow release” relays hold the line finder so the 60 ms OFF intervals do not cause a
• Rotary Dialing: The subscriber turns the dial to an angle
corresponding to one of the 10 digits, and then releases it. A
spring wound by this action then rotates the dial back to normal
position at uniform speed, producing 1 to 10 brief current
Simultaneously, an “Off-normal” switch contact in the telephone set temporarily shortcircuits earphone so clicking is not heard
» Following a stage of selection motion, a slow release relay is automatically connected into that line
to prevent further disturbance of that particular selection due to the succeeding bursts of dialing
Significant Properties of Stepper Switches
• To add more traffic capacity, install more line finders
and more paralleled selector switches
– This increases parallel path (traffic) capacity through the switch, since
multiple last stage selectors lead to the same destination lines.
» Only one last stage selector can connect at a given time. The sleeve wire is
also connected to each corresponding position on the selectors and is
used to divert the call to a busy signal generator if the sleeve voltage is ON
for that destination line and a call is attempted while destination line is
» A non-blocking Strowger step switch assembly would require 100 last
stage selector switches connected to 100 destination telephone lines, and
similar replication of parallel paths all the way to the originating lines (line
finders, earlier stepper stages, etc.).
• This automatically increases the call processing
capacity (BHCA) of the switch as well
– Each selector is both a traffic path and a part of the digit processing hardware
– When there is a traffic path available to the destination, there is also the
hardware to respond to the succeeding dialed digits.
» A stepper switch assembly “automatically” has enough call
processing capability if it was provisioned with adequate traffic
• Stepper switches are extremely reliable overall, when maintained
– Because of parallel path capability through a large stepper switch, the failure
rate of these switches (when properly maintained) is very good
» Failures affecting only one user amount to only about 1 hour cumulative in 20 years
» Failure of the entire switch is only 1 or 2 minutes in 20 years, and when this occurs
it is mostly due to human error or power supply aspects of the system
• Steppers can be adapted to many improvements
» Touch-tone dialing (by means of a tone-to-pulse converter)
» Computer control has been adapted to steppers to make advanced features available (such as
call waiting, 3-way conference, etc.)
» But speed of connections, basic reliability, power consumption and size are not improved!
• Inter-switch signaling between stepper switches requires
electrical transmission of dialing impulses
– conversion between modern digital signaling (common channel 7) and
impulse switching is feasible, but slow acting
» European version of SS7 signaling allows transmission of one dialed
digit at a time, but North American (ANSI) version does not send dialed
number onward until the “last” digit is dialed.
– several earlier “electronic” but non-digital switching systems still used
electromechanical switching (small relays) and analog transmission
(example: No. 1 ESS), but digital computer central control or stored program
Undesirable Stepper Properties
• Relatively High maintenance
– ‘“Gross Motion” or “Large Motion” wiping contacts
» Require lubrication, cleaning, adjustment, etc.
– Susceptible to corrosion from sparking, air pollution (such as
SO2 in the air, etc.)
• Slow mechanical operation
– Even when tone-to-pulse converters support Touch-tone
• Slow signaling
– Can’t take full advantage of SS7 and other electronic
• Big and bulky
– Digital switches use ~1/50th the floor area of steppers; ~1/10th the
floor space of crossbar switches.
Some Other Historical Electro-Mechanical Switches
Panel (AT&T 1930s through 1950s)
Crossbar (Ericsson and AT&T, 1930s through 1980s)
A horizontally platform with rows and columns of contacts with wipers actuated
by magnetic coils. Gross motion problems, but more compact than Strowger
design. Used only in relatively small switches.
An assembly of rocking contacts attached to vertical and horizontal rotating
actuator axles. Because of relatively small motion and compact size, this was the
heir apparent to the stepper switch in both North America and Europe until
electronic switching appeared.
X-Y (Stromberg-Carlson, 1930s through 1970s)
A huge mechanical “monster” switch using continuously running electric motors
and electrically operated clutches to move wipers vertically and horizontally on a
rectangular wall panel of contacts. High maintenance was a serious problem. Not
Similar to X-Y switch, but platforms had contacts arranged in semi-circles of
increasing radius. More compact than Stepper, but same gross motion problems.
Rocking contact motion, but still rather complex and difficult to maintain.
The last 3 were mainly used by “independent” telcos in North America. All here
except Crossbar and Multi-relay were “gross motion” switches.
Many of these electro-mechanical designs, particularly crossbar,
had separate relay assemblies to count (“decode”) the dial
impulses, completely separate from the switching portion of the
system. These so-called “common control” portions were
analogous to the computer control in a digital switch.
Once the desired destination directory number was decoded, it was
“translated” by special purpose wired logic devices
One method for this was to use magnetic core memory of a special wired type (not
addressable RAM like modern computer memory)
The equipment numbers resulting from the translation were used to select a path
through the switching part of the system.
The result of the “translation” was a code designating the proper
bay, shelf, and switch outlet wire for the internal destination calls,
or the proper outgoing trunk group for outgoing (other switch)
calls. The first non-busy channel in a trunk group was selected by
an appropriate special outgoing trunk switch.
These systems first demonstrated the need for provisioning
separately both sufficient call processing capacity (BHCA) and also
sufficient switching capacity (Erlangs)
• Rotary dial label “0” represents 10 impulses everywhere in the
world (except Sweden, where the dial is labeled 0, 1, 2…9)
– However, touch-tone dials in Sweden use the same digit labels for DTMF
tones as the world standard.
– Impulsive signaling must be converted at Sweden’s international boundaries.
But symbolic signaling (binary digit codes used in SS7, etc.) is the same
• Alphabetic dial labels (2=“ABC”, 3=“DEF”, etc.) were introduced
in New York City in ~1923 when subscribers complained about
“long” 5 digit directory numbers.
– Alphabetic dial labels were introduced in US, Canada, UK, France,
Scandinavia and USSR (three cities only) but not all the same:
» Examples: Q on French dial, Russian (Cyrillic A B... G ...F) letters in Moscow,
– Considered an obstacle to direct international dialing, alphabetic exchange
names were purged from telephone directories in 1960s by international
» The “anti-digit dialing league” and other grass roots groups in the US
opposed all-digit directories in the 1960s.
– Letter labels still appear on the dial in most of these named countries.
Business users highly value so-called “Anagram” numbers such as 1-800FLOWERS, or 1-800-NORSTAR, 1-800-AMERICAn, etc.
De Facto Modern Alpha Dial Labels
Note restored letters
q and z. Otherwise
with North American
& British alpha dial
• Emerged as cellular radio de facto standard
• Alternatively used for composing short alphabetic text