Digital Telecommunications Technology - EETS8320 Fall 2006. Lecture 2 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 wireless handset?
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Analog and Digital Telephone and Wireless Sets
(Slides with Notes)
Basic parts/functions of an analog telephone set:
Basic parts/functions of a digital wireless telephone set:
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 ortho-
telephonic 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).
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.
*DSP=Digital Signal Processing
*POTS=Plain old Telephone Service
*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.
*Actually just the receive part of a modem (a “DEM”). More info later.
*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.
Arrows represent magnetic B field.
Loop area A is about ·(D/2)2, where D is diameter of loop.
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.
A voltage Vm
will occur here
if B is changing
vm = -dB/dt• A
*For most inductors, the unit millihenry (mHy), 1/1000 of a Hy, is used. Incidentally, 4•p= 12.56636
Schematic transformer symbol
Left coil has
2 times the
V2=2•v1 and I2=i1/2,
So V2/i2=4•R or 40
Ideal transformer model
* A relay comprises electromechanical switch contact(s) actuated (on/off) by the magnetic field produced by a separate control current.
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.
Simple, but inefficient and causes excessive “sidetone” in earphones.
at this earphone.
Installed at Central
aspects not shown.
Audio input here.
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.
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
*For most capacitors, the units microfarad or picofarad (µF or pF) are used
Graphic symbol. Curved line is the
outer plate in a “rolled up” capacitor
made of flexible metal foil and plastic
converterTelephone Connection with CO Hybrid Coils
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.
Audio frequency voice signals coupled via transformer. Ringing power,
loop current detection not shown.
“Incremental” or small signal
resistance is re= V/ I. Varies
with operating point voltage vo
or current io. Larger io gives
Sleeve (only in
Note: use of red
insulation for neg-
ative polarity is
unique to the
motive) use red
Plug Assembly Graphic Symbol
Socket Assembly Graphic Symbol
* Boys were replaced by more polite ladies in 1890’s; operator corps (except in military settings) was exclusively female until 1960s.
*BHCA=Busy Hour Call Attempts, a measure of how many call attempts per hour a switch can handle.
Note that modern “feature-rich” PBX, small business key systems, and some PSTN switches now do these things via computer control
*The GSM cellular system can optionally be configured to do this.
Tip, Ring, Sleeve
wires from Rank 8,
springs activate the motions
of the wiper arm in response
to dial impulses.
Many details omitted here
Ten places on
located -- not
The last 3 were mainly used by “independent” telcos in North America. All here except Crossbar and Multi-relay were “gross motion” switches.
Note restored letters
q and z. Otherwise
with North American
& British alpha dial