Transcript Document

EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
EE 489
Telecommunication Systems Engineering
Introduction to Analog Telephony Concepts
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Network Types
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Trade-off Between Switching and Transmission
Costs in Network Architecture
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Trade-off Between Switching and Transmission
Costs in Network Architecture
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Network Types
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Concept of a “Homing Plan”
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
PSTN Hierarchy
Also connected to
submarine cables and
satellite links.
International Exchanges
National Exchanges
Regional Exchanges
Primary Switching
Centres
Local Exchanges, or
Central Offices (C.O.’s)
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
World Numbering Plan
• World is divided into zones, with each zone assigned a single
digit zone code.
• Each country within a zone is assigned a country code (usually
2-3 digits).
– 1st digit is the country’s zone code.
• Regions or “numbering plan areas” (NPAs)within countries
are assigned a 1-4 digit “area code” or “routing code”.
– NPA size and shape driven by numerous factors:
•
•
•
•
Size and shape
Present and future numbering capacity
Political boundaries
Population demographics
• Local exchanges are assigned codes (3 digits in N.A.), followed
by several digits assigned to each phone (4 digits in N.A.)
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
World Numbering Plan (2)
• World Numbering Zones
Code
Zone
1
N.A. & Carib.
2
Africa
3&4
• Sample Country Codes
Code
Country
20
Egypt
231
Liberia
Europe
5
S.A. & Cuba
6
South Pacific
7
Former USSR
8
N. Pac., E. Asia
9
Far & Mid East
0
Spare
9
33
France
351
Portugal
44
UK
55
Brazil
593
Ecuador
60
Malaysia
886
Taiwan
966
Saudi Arabia
EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Network Architecture
Customer
terminals 20%
Outside plant,
cables 29%
Switching
equipment 25%
Multiplexing and
Transmission
equipment 15%
Buildings, land,
other 11%
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Telephone System
• Early telephone system
– Powered by self-contained local battery
– Ringing created by cranking generator
• Today’s telephone system
– Powered through the line by battery at the central office (-48V)
– Circuit is closed when handset is lifted from the cradle (“off
hook”)
• Transmitter – carbon granule microphone
– Air pressure of sound waves impact on diaphragm, varying
pressure on carbon granules
– Resistance of electrical current passing through carbon granules
varies the current (analog)
• Receiver
– Varying electrical current passing through windings on magnet,
moves a diaphragm. Same as in a music loudspeaker.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Telephone System (2)
• “PSTN”, or “POTS” simplified circuit model of any connection:
speech current
coil (ZB)
central battery
The coil is a “transmission bridge coil”
with a high impedance (ZB) preventing the
speech current from shorting out at the
central battery.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Establishing A Call (Conventionally)
1. Calling customer takes phone off hook which closes the circuit to the
C.O. (“looping the circuit”).
2. C.O. detects the “loop” and indicates readiness with dial tone.
3. Calling customer hears dial tone and dials number.
•
The network converts (“translates”) the phone number to a physical
equipment address
4. The network checks on the called party status and decides on a
routing for the connection.
5. If connection possible, the called party is alerted.
–
Large 20 Hz alternating current is applied to line (“ringing current”).
6. “Ring tone” is returned to the caller.
7. The called party picks up the handset and closes his/her loop.
8. Exchange detects second loop and “trips” or stops ringing, then
establishes call.
9. One party opens loop by hanging up, and exchange clears connection.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Subscriber Line Signalling
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Loop and Disconnect Signalling
“pulse dialling” :
• Line is rapidly disconnected and reconnected in sequence with
one pulse for digit value “1”, two pulses for digit value “2”, etc.
• Each pulse lasts 0.1 second.
• Inter-digit pause (IDP) must be >0.5 second.
– If not, current digit may combine with previous digit.
• Ten digit phone number typically takes 6-15 seconds total.
• This is the kind of signalling old “rotary dial” phones produced.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Dual-Tone Multi-Frequency Signalling
DTMF signalling” or “tone signalling”.
• Faster than pulse dialling (1-2 seconds for ten digit number).
– Reduces call set-up time.
• Each digit produced by combination of 2 pure frequency tones.
– Reduces chances of error or interference.
Low Frequency Tone
High Frequency Tone
1208 Hz
1336 Hz
1477 Hz
1633 Hz
697 Hz
1
2
3
spare
770 Hz
4
5
6
spare
852 Hz
7
8
9
spare
941 Hz
*
0
#
spare
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Address Signalling
Dial
Pulsing
Dual-Tone
Multifrequency
(DTMF)
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
DTMF Receiver
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
The Concept and Implications of “twowire” (2W) to “four-wire” (4W)
conversion
Or…why your phone only needs one twisted pair and
why you sometimes hear an echo 
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
2-W to 4-W Conversion
•
For short distances, two-way communication is possible on a single pair
of wires (bi-directional transmission).
•
Problems occur however when amplification (in past) or digital
regeneration (nowadays) is needed.
– Amplifiers or regenerators in the network are uni-directional.
 A Hybrid Transformer is used to convert a 2-wire circuit at the
phone/terminal end to a 4-wire system in the switching network:
Send Amplifier
2-W Line (ZLine)
Balance Network (ZB)
Receive Amplifier
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4-wire portion
EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
2-W to 4-W Conversion (2)
Amplifier or Regenerator
transmit
receive
2-wires
2-wires
Hybrid
2-wires
Hybrid
2-wires
receive
2-wires
2-wires
Amplifier
21
transmit
EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
2-Wire to 4-Wire Conversion
•Any telephone call undergoes 2W-4W conversions:
- from the phone (4W) to the subscriber line (2W)
- from the subscriber line (2W) to the network interface (4W)
“4W”
“4W”
“2W”
“4W”
“2W”
Overall structure of any phone connection
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
2-W to 4-W Conversion (3)
• Balance network has a balance impedance of ZB.
• If ZB=ZLine then half the signal goes to the line and half goes to
the balance network with little or no coupling (reflection) to the
local receiver.
• But by design, we use ZBZLine to create “sidetone”.
– Reflections from the C.O. return to the station set.
– Talker hears his/her own voice.
– Useful because acts (almost subconsciously) as a signal to the
talker that the line is live.
– No sidetone makes the line feel “dead” and unnatural (IP telephony
often sounds like this since there’s no sidetone).
– Today’s electronic phones have a small sidetone network within
them to create sidetone.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
2W to 4W conversion: The “Hybrid Coupler” Circuit
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
2W to 4W conversion: The “Hybrid Coupler” Circuit
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Electronic Hybrid Coupler
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Concept of Hybrid Return Loss
Echo return loss (ERL) = average attention of power reflected at the 2W-4W interface
Singing Return Loss (SRL) = minimum attenuation to reflected power at any frequency
coming back from the 2W-4W interface
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Introduction to the “Subscriber Loop” Plant
• Wire network from the central office to the station sets.
• Largest portion of capital expenditure (50%?) and workforce
requirements (30%-40%?).
• Prime candidate for replacement by optical fibre but costs often
prohibitive.
• Main goal is to design and work with length limits.
– Limited by resistance and attenuation along the line.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Subscriber “Loop” Plant (N. America)
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Subscriber “Loop” Plant (UK)
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Three Main Design Goals/Methods
• (A) (D.C.) Resistance Limit Requirement
– Keep total line resistance below a target level by choosing the
appropriate wire gauges.
– Historically 1300  limit but now ~1700 .
• (B) (A.C.) Attenuation Limit Requirement
– Keep total signal loss below a target maximum level.
– North America usually uses 8 dB maximum loss at 1000 Hz.
– Elsewhere usually uses 7 dB maximum loss at 800 Hz.
• “Uni-Gauge” Design Method
– In principle could mix and match wire gauges in loop makeup to
satisfy (A) and (B) at minimum cost of the copper used.
– Actual practice has been to keep to a single size wire (often 26
gauge) as much as possible (better economically) and add battery
boost, range extenders, amplifiers, or “load coils” as needed.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
More on Resistance Design
• How do we determine the target resistance?
– We need a high enough current at the customer premises to
operate the station set (20mA minimum in North America).
–
–
–
–
Use V=IR, with a known battery voltage of –48V.
48V  20mA x R  R  2400  total
Budget  400  for the battery feed bridge at the C.O.
Budget  300  for other miscellaneous wire resistances (e.g.
subset wiring, etc.).
–  The subscriber loop’s wire resistance must not exceed 1700 .
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
American Wire Gauge (AWG) Data
Loop Resistance Data
AWG
19
22
24
26
28
Diameter
mm
inches
0.910
0.036
0.644
0.025
0.511
0.020
0.405
0.016
0.302
0.012
2-W Loop Resistance
/1000 ft /mile
/km
16.1
85
53
32.4
171
106
51.9
274
168.5
83.5
440
268
132
697
433
1 Wire
/km
26.5
53
84
134
216.5
•N.B.: Each change of 3 gauge numbers is a factor of 2 in wire area (cross section), this a factor
of 2 in resistance / unit length
Loop Attenuation Loss Data (@ 1kHz)
AWG
19
22
24
26
28
Diameter
mm
inches
0.910
0.036
0.644
0.025
0.511
0.020
0.405
0.016
0.302
0.012
2-W Loop Loss
dB/1000 ft dB/mile
dB/km
0.21
1.11
0.71
0.32
1.69
1.01
0.41
2.16
1.27
0.51
2.69
1.61
0.615
3.25
2.03
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1 Wire
dB/km
0.355
0.505
0.635
0.805
1.015
EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Subscriber “Loop” Plant
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Extending Loop Length: Load Coils
• Amplifiers can be used to add gain (7dB each is common).
• “Line loading” by adding inductive coils at fixed intervals.
– Decreases velocity of signal propagation.
– Increases impedance.
– Acts as high frequency filter but generally outside speech band.
•
Transmission line theory says that attenuation is lowest when:
l  g  r c
where l  Inductance
r  Resistance
g  Conductannce c  Capacitance
 We can change l by adding coils periodically along the line.
“Line Loading” reduces attenuation.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Inductive Loading - Lumped “Load Coils”
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Effect on Transfer Function of Twisted Pair
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Example Calculation of Cutoff Frequency
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Inductive Loading - “Load Coils”
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Inductively Loaded Line Notation
Some Cable Conductor Properties:
Example #1:
“19H88” means 19 gauge wire
loaded every 1830 m (H) with
88mH inductors.
Letter
Code
A
B
C
D
E
F
H
X
Y
Spacing Spacing
(ft)
(m)
700
213.5
3000
915
929
283.3
4500
1372.5
5575
1700.4
2787
850
6000
1830
680
207.4
2130
649.6
Example #2:
“26B66” means 26 gauge wire
loaded every 915 m (B) with
66mH inductors.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Psophometric and “C Weighting” Noise Filters
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Example
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Network Loss Planning
• Received Volume Control
– Subscribers must have a received signal level within an appropriate
range.
– i.e. Not too loud and not to quiet.
• Stability or Oscillation Control: “Singing”
– Manage reflections that can result if there’s a poor mismatch of the
2-wire line impedance and the hybrid balance impedance.
– Singing can result.
• Talker Echo
– Talker should not hear his/her own voice reflected back (with a
significant enough delay).
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Volume Objectives
• Reference Equivalent (RE) or Overall R. E. (ORE)
–
–
–
–
A measure of perceived loudness of the signal.
ITU in Geneva used group of telephone users to judge loudness.
Measured by adjusting an attenuator in a simulated network.
Rated “highest tolerable volume”, “preferred volume” and “lowest
tolerable volume”.
– Results showed that attenuator settings of <6dB were too loud and
>21dB were too faint.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Overall Loudness Rating (OLR)
• New standard circa 1990.
• Loss accumulated from speaker’s mouth and listener’s ear.
• OLR = SLR + CLR + RLR
Mouth to
Interface
Loss
Interface to
Interface
Loss
OLR
SLR – Send Loudness Rating
CLR – Circuit Loudness Rating
RLR – Receive Loudness Rating
Interface
to Ear
Loss
Good/Excellent
Poor/Bad
5-15dB
90%
1%
20dB
80%
4%
25dB
65%
10%
30dB
45%
20%
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability
• Long distance connections all have 2-W to 4-W to 2-W
conversion (as do most local connections).
• If there’s a poor mismatch of the 2-W line impedance with the
hybrid balance impedance, signal energy passes across the
hybrid reflecting from one 4-W direction into the other.
Amplifier
transmit
2-wires
2-wires
Hybrid
2-wires
Hybrid
2-wires
receive
receive
2-wires
2-wires
Amplifier
transmit
Reflection
(ZB  ZL)
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability (2)
• Reflection at the hybrid re-inserts the signal with “balance
return loss” (BRL or BS) into the return side of the 4-W loop.
BS  20log10
Minimum return loss seen at
the hybrid in any frequency
in the voice-band
ZB  ZL
ZB  ZL
• Additional 3+dB loss at hybrid when converting 4-W signal to 2W signal, and another 3+dB going from 2-W to 4-W (6db total).
• Total trans-hybrid loss of returned signal: THL  3dB  BS  3dB
THL  BS  6dB
THL  BS  7dB
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Ideal loss
Loss in practice
(~3.5 db splitting loss)
EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability (3)
Net Gain of one side of 4-W loop
(total amplifier gain minus line losses)
G
3dB
BS+6dB
3dB
T
2-W to 2-W total attenuation
T  6dB  G
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability (4)
• Total round-trip closed loop loss (“singing margin”):
m  2( BS  6dB  G)
m  2(T  BS )
• Generally found to be adequate if:
2(T  BS )  6dB
• Otherwise, singing may result.
– out of control runaway oscillation in the loop.
– can continue even after the original impulse ceases.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability (5)
• Loss in a 4-W circuit may depart from its nominal value for a
number of reasons:
– Variation in line losses and amplifier gain with time, temperature,
etc.
– Gain or loss will differ at different frequencies (usually tested at
800 Hz and/or 1600 Hz).
– Measurement errors.
– Circuit errors.
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability (6)
• Define new term for variance of round trip loss:
2
Tot
 2 B2  n(2T )2
S
 B2 
Variance of trans-hybrid loss
S
 T2 
n
Variance of gain or loss in
each 4-W section
Number of 4-W sections
• What if we want only 0.1% chance of singing?
Recall:
3 standard deviations
from the mean is
equivalent to 0.1%
2(T  BS )  3Tot  6dB
Recall:
We have singing if
m = 2(T+BS) < 6dB
3Tot
0.1%
6dB
2(T+BS)
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability (7)
 B  2.5dB
Typical values:
S
2
Tot
 2 B2  n(2T )2
S
 T  1dB
BS  6dB
 Tot  2 B2  n(2 T )2
S
 2  2.52  n(2 1) 2  12.5  4n
2(T  6)  3 12.5  4n  6
2(T  BS )  3Tot  6dB
T  1.5 12.5  4n  3
This is basis for a generally accepted
approximation or rule of thumb for 99.9%
chance of stability (i.e. no singing):
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T  4  0.5n
EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Stability (8)
• Example: We have a long distance circuit with 6 4-W sections.
What is the minimum attenuation (T) required for a 99.9%
chance of not singing?
T  4  0.5n
T  4  0.5  6
T  7 dB
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Echo-Delay Phenomena
•
If the reflection at the hybrid is strong enough, telephone users will
hear it.
•
Talker echo is when talker hears his/her own voice.
•
Listener echo is when listener hears talker’s voice twice.
Talker
Listener
Be+6dB
T
Talker Echo:
Listener Echo:
Loss = Be + 2T
Loss = 2Be + 2T
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Echo-Delay (2)
• Recall Bs:
– Balance Return Loss
– Minimum return loss seen at any voice-band frequency
• What is Be?
– Hybrid Echo Return Loss
– Average return loss in voice-band.
B( f )  20log10
B(f)
Return Loss
at Frequency f
ZB ( f )  ZL ( f )
ZB ( f )  ZL ( f )
Be (echo)
BS (stability)
Frequency
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Why Be and not BS?
EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Echo-Delay (3)
• Subjective annoyance of echo depends on relative echo level
and on the delay.
– The stronger the echo and the longer the delay, the more
troublesome the echo.
One-Way Delay
Loss to Satisfy 50%
10 ms
11.1 dB
20 ms
17.7 dB
30 ms
22.7 dB
40 ms
27.2 dB
50 ms
30.9 dB
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Echo-Delay (4)
• The echo objective is for 99% of all connections to have
acceptable or better echo effects.
• Factors to consider:
• Be = Expected hybrid echo return loss.
• Be = Standard deviation of Be
• T = Nominal 2W-2W loss in connection.
• T = Standard deviation of T
• Ē(t) = E = Expected echo attenuation for delay t at which 50% of
users find echo tolerable.
• E = Standard deviation of E
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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering
Echo-Delay (5)
• For the connection to be acceptable, we want:
M  Be  2T  E  0
E  Be  2T
Mean margin against
objectionable echo
M has a standard deviation:
M
 M2   B2  n(2T )2   E2
e
If we want 99% chance
of acceptable echo in all
connections then:
Recall:
2.33 std. dev.
from the mean is
equivalent to 1%
Be  2T  2.33 M  E
2.33M
1%
0
E
M BBe e22TT  E
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