Digital Telecommunications Technology - EETS8320 Fall 2006

1
Digital Telecommunications Technology -
EETS8320Fall 2006

• Lecture 2
• Analog and Digital Telephone and Wireless Sets
• (Slides with Notes)

2
Topics of Lecture

• What are the major parts (or modules) of a wired
  landline analog telephone set? What major parts
  for a digital wireless handset?
• What functions do these parts perform?
• If time permits, we will open and view a wired
  analog telephone set on camera.

3
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)
• Signaling
• 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.

4
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).
• Signaling
• 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.

5
Direct Acoustic Communication
Sound pressure variations at eardrum ultimately
cause nerve signals to the brain, perceived as
sound.
Small variations in air pressure at audio
frequencies, produced by the mouth and throat,
propagate through the air as an acoustic wave.
6
Telephonic Communication
An ideal telephone system (sometimes called an
ortho- telephonic system) reproduces precisely
the same acoustic waveform that the listener
would hear in a face-to-face conversation.
Telephone System
A real telephone system only imperfectly
reproduces the speech (high frequency components
are attenuated, some distortion and delay are
introduced as well).
7
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 rate coding
• Typically 100 to 200 milliseconds is perceptible
• Over 200 ms (from geostationary satellite delays)
  is disturbing to many users
• 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

8
Microphone (Transmitter) -1

• Carbon microphone is most widely used in analog
  wire telephone sets
• 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 and earphone
• 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.

9
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
  pressure

10
Some Microphone Types

• Category
• Variable Resistance
• Electro-magnetic
• Electro-static

Type, (Name, Structure Operation) Fidelity/ Quality Application(s) Audio electrical power output Historical notes
Carbon (Edison-Berliner) Low Analog telephone set. high Western Union used this to negotiate with Bell in ca. 1879
Liquid Transmitter Medium/ high Historically one of the abandoned Bell investigations. High Impractical because liquid evaporates and is corrosive. Variable resistance method probably inspired Edison
Flexible iron disk near coil on permanent magnet. medium Seldom used in telephone. Used in some wireless or intercom units. Low First commercial Bell telephones used this. Some used the same device as a mike or earphone.
Moving coil or velocity microphone. (Movable current-carrying element in magnetic field) Medium/ high Radio broadcast, sound recording. Low. Requires amplifier. 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 polarized rod Medium/ high Landline telephones, primarily proprietary ones, and many wireless sets. Low. Requires amplifier Polarized rod made of plastic or wax.Overall mike is very low cost.
Piezoelectric, crystal mike. Crystal is variably compressed by flexible metal disk. Medium/ high Medium quality recording or public address systems. Some wireless systems. Low. Requires amplifier Many natural crystals quartz, rochelle salts, even table salt have piezoelectric properties.
Condenser mike. Flexible metal disk is one plate of a capacitor. Medium/ high High quality recording systems. Low. Requires amplifier Requires high-voltage power supply.
11
Earphone (Receiver)

• Electromagnetic transducer used almost
  universally ever since Bells 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 users 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.

12
Loudspeaker- Hands Free

• Amplification (sometimes with separate
  loudspeaker) used for hands-free or
  speaker-phone
• Continuous amplification may allow audio feedback
  problems
• 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.
• DSPDigital Signal Processing

13
BORSCHT AcronymFunctions 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 busy/idle status
• 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.

14
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
  equipment.
• 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.

15
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, lead-acid gel-cells, etc.)
  with sufficient reserve power to operate for the
  anticipated duration of outside power failure
• POTSPlain old Telephone Service

16
Subscriber Loop Jargon Analog Subscriber Wire
Pair

• A third wire called Sleeve (C) was used in
  electro-mechanical switches, but not today in
  digital switches.

17
Wireless Battery

• 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 providers
  schedule, delay is typically 5 to 20 seconds.

18
Over-voltage Protection

• 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
  absolutely necessary.

19
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
  voltage-sensitive 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.

20
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 naturally
• 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

21
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 switchs
  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.

22
Wireless Ringing/Alerting

• 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).

23
Caller ID

• 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 originators 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 subscribers.
• If the destination subscriber has subscribed to
  Caller ID service, and the originator did not
  forbid it, the callers 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 originators 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
  destination LIDB.
• Actually just the receive part of a modem (a
  DEM). More info later.

24
Supervision

• 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 frames).

25
Wireless Supervision

• 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 data bits.
• 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.

26
CODEC (Coder-Decoder)

• 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
  handset.
• 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 24
  conversations.
• Lower cost due to use of large scale integration
  allowed the use of a dedicated CODEC microchip
  for each subscriber loop in an economically
  feasible design.

27
CODECs for Wireless

• Wireless systems use several different types of
  CODECs, all presently not waveform coders.
• 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 into
  service.

28
Hybrid Coils

• 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 jargon.
• Also called 2-wire to 4-wire converter
• Permits simultaneous two-way signal power
  transmission on subscriber loop,
• 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

29
Background about Transformers-1

• Prolific American inventor William Stanley made
  the first transformer in 1886. Transformers have
  both power and communication uses.
• Electric current (moving electrons) produces a
  magnetic field in space surrounding the current
  flow.
• Intensity and direction of that field
  mathematically described by a 3-component vector
  B, measured in voltsec/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 childrens toys (of the
  1980s to the present) with parts that can be
  rearranged to make a robot, a truck (lorry) etc.

30
Magnetic Induction
Loop of wire, with small gap, penetrated by
time-varying magnetic field. Field can be caused
by current in the loop itself (self-inductance)
or due to current in other wires (transformer) or
due to a permanent magnet.
Arrows represent magnetic B field. Loop area A is
about ?(D/2)2, where D is diameter of loop.
A voltage Vm will occur here if B is
changing with time
vm -dB/dt A

• We can stack up such loops to form a helical
  coil of wire. Each added turn adds another vm
  volts

31
Background about Transformers-2
g
Graphic schematic symbol
d

• Electrical inductance measured via a unit called
  a henry voltsec/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
  is approximately
• L µ n2 A/g
• Where µ is the magnetic permeability of the core
  material. For air or vacuum µ is 4p10-7
  henry/meter. If iron is used in the core instead
  of air, typical µiron is 1200010-7 henry/meter
• n is the total number of turns of wire (n5 here)
• The cross section area of the core Ap(d/2)2
• g is the length
• For most inductors, the unit millihenry (mHy),
  1/1000 of a Hy, is used. Incidentally, 4p
  12.56636

32
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 waveform results.
• 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.

33
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 both coils
• 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
  secondary winding.
• 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.

34
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 primarycoil 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

i1
Schematic transformer symbol
Left coil has 2 times the number of turns on
right coil.
V22v1 and I2i1/2, So V2/i24R or 40 ?
i2
-
-
v1
v2
R10?
35
Lowest Frequency for Transformer
1
Power/(Nis)2R
i2
Ideal transformer model
i1
Is
-
-
0.5
-
v1
v2
L
R
Nv2
Ni1
frequency
fc

• Transformers dont 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
  (Nis)2R. (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 fcR/(2?p?L),
  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.

36
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 inductance.
• 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.

37
Current and Power Flow
A
B
C
D
v -
- v
Power flow
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 elements.
• 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 wire.

38
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
  de-magnetization (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
  end!

39
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 current
• 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.

40
Telephone Test Capabilities

• Many modern telephone switches have built-in test
  capabilities
• 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 pops)
• 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.

41
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 xs
  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

42
Historical Telephone Schematic
Earphone (receiver)
Microphone (transmitter)
_

Battery

• 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.

43
4-Wire Circuit
Simplified physical 4-wire circuit, as used in
some military telephone systems
_

_

Battery
Simplified diagram dies not show details of
battery feed, dial, ringer, transformer coupling
of voice signals, etc.
44
Historical 2-wire Carbon Mike Circuit
Simple, but inefficient and causes excessive
sidetone in earphones.
Desired Destination Earphone.
Audio frequency sidetone appears at this
earphone.
Common battery Installed at Central Office.
Switching aspects not shown.
Audio input here.
_

Audio frequency power wasted here
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.
45
Hybrid/Induction Coil Directional Coupler
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
Line matching circuit
Microphone signal current (red arrows) divides,
produces canceling effects on secondary winding
Two wires to CO switch.
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.
46
Capacitor (Condenser)
Plate area A sq. meters
d
Graphic symbol. Curved line is the outer plate
in a rolled up capacitor made of flexible
metal foil and plastic sheet dielectric.

• Electrical capacitance measured via a unit called
  a farad amperesec/volt (abbreviated F)
• Capacitance C (in farads) of two metal plates
  separated by an insulating dielectric is
  approximately
• C eA/d
• Where e is the dielectric permittivity of the
  core material. For air or vacuum e is 8.8510-12
  farad/meter. If plastic is used instead of air,
  typical e plastic is 5010-12 farad/meter
• Ais the area of each plate
• d is the dielectric thickness
• For most capacitors, the units microfarad or
  picofarad (µF or pF) are used

47
Capacitor Electric Properties

• Relationship between voltage and current is (for
  ideal non-resistive 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.

48
Telephone Connection with CO Hybrid Coils
telephone set and subscriber loop
CO part
Common battery feed and voice coupling
Transmit signal
_
Hybrid or induction coiland matching network
Hybrid and matching network

Receive signal
Amplifier and D/A converter
48 V battery
Wire loop, up to 10 km
Telephone set (dial, ringer, cradle
switch circuits for loop length level
compensation not shown)
Central office switch equipment. Actual switching
is not shown. Positive battery terminal grounded
to minimize electrolytic corrosion. Audio
frequency voice signals coupled via transformer.
Ringing power, loop current detection not shown.
49
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 purposes.
• 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
  process.
• Unlike a linear electrical resistor, in which
  current is proportional to voltage (Ohms law v
  Ri, 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 Kv2, where K is a constant depending on
  varistor material and size
• Sign correction required in this formula since
  current has same polarity as voltage (current is
  negative when voltage is negative). Using signum
  function symbol i Ksignum (v)v2. Signum
  (v) is 1 for positive v, and -1 for negative v.

50
Varistor Symbol Graph
current
?I
io
Schematic symbol
?V
voltage
vo
Incremental or small signal resistance is re
?V/ ?I. Varies with operating point voltage vo or
current io. Larger io gives smaller re.

• 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.

51
Traditional vs. Modern Telephone Sets

• The previous explanations mostly show traditional
  telephone set structure, based mainly on the type
  500 design by ATT Bell Laboratories in 1948.
  Its relatively bulky by todays 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 craftsperson

52
Integrated Circuit Telephone Sets

• 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 dialing
  signals

53
Telephone Switching

• 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
  electro-mechanical switches
• Including incidental items like intentional
  post-dialing delay and non-symmetrical treatment
  of origin/destination vis-à-vis disconnect (for
  landline switches only modern cellular switches
  disconnect immediately due to either
  participants actions)

54
Historical Switching

• Original 1876 A.G.Bell installations were
  point-to-point hard wired. Examples
• Office to warehouse of same firm (like a modern
  intercom circuit)
• 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 from subscribers
• 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

55
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
  ATT
• Alternating current ringer (low maintenance)
  instead of previous buzzer devices with vibrating
  electric contacts subject to sparking, corrosion
  and deterioration
• Common (central office) battery for dc loop
  current using transformer to couple audio voice
  signal between two telephones in a conversation

56
Switchboard Plug

• Same diameter used today for 1/4 in (6.35 mm)
  stereo headset plug

Insulators
Tip (green positive wire)
Sleeve (only in electro- mechanical switches, no
standard outside-plant color)
Ring (red negative wire)
Note use of red insulation for neg- ative
polarity is unique to the telephone industry.
Other electrical standards (power, electronics,
auto- motive) use red for positive.
Plug Assembly Graphic Symbol
Tip
Ring
Sleeve
Tip
Ring
Sleeve
Socket Assembly Graphic Symbol
57
Supervision Methods

• In traditional telephone jargon, supervision
  describes only the aspects of signaling which
  relate to busy/idle status
• Dialed digit information was historically
  distinct (called signaling)
• In modern cellular/PCS software both things are
  often described by the word supervision
• therefore, be careful about jargon!
• Historical method to get attention of the
  operator was a small hand-cranked AC generator or
  magneto at subscriber end
• 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
  subscribers 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
  1890s operator corps (except in military
  settings) was exclusively female until 1960s.

58
Call Connection

• 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 switches.)
• 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 operators 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 a relay.
• 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.

59
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
  intelligence
• 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)
• BHCABusy Hour Call Attempts, a measure of how
  many call attempts per hour a switch can handle.

60
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 todays
  (2005) level of public telephone traffic using
  operator cord board switching!
• The GSM cellular system can optionally be
  configured to do this.

61
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 switch
• 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 development
• Strowgers 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

62
Schematic Stepper Diagram
Tip, Ring, Sleeve wires from Rank 8, column 7.
Electromagnets and springs activate the
motions of the wiper arm in response to dial
impulses.
6
5
4
7
8
3
9
Rank 0
2
0
Rank 9
Arm
1
Ten places on each circular rank where a
3-contact assembly is located -- not illustrated
in detail.
Axle
Vertical Motion

• Many details omitted here

Rank 1
Rotary Motion
63
Stepper Switching

• 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)

64
Selector Switches

• 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