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Chapter 5
The Medium
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Objectives
Explain why the two wires connecting the
central exchange to a telephone are known
as the local loop.
Explain why twisted-pair copper wire is used
instead of fiber for the local loop.
Know the color code for wires inside a cable.
Understand the principles behind the design
of the local loop.
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Objectives (continued)
Explain revised resistance design.
Explain modified long route design.
Explain when to use revised resistancedesign or modified long route design
principles in designing a local loop.
Explain what a load coil is and why we use
them.
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Objectives (continued)
Explain why loops longer than 18,000 feet
need to be loaded.
Explain what a jumper wire is and what it isused for.
Explain why the serving area for a central
exchange located in the city is kept under 3
miles.
Explain what a carrier serving area (CSA) is.
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Objectives (continued)
Explain why higher-frequency signals are
attenuated more than low-frequency signals
when they are transmitted over a nonloaded
pair.
Explain the differences between feeder,
distribution, and drop cables.
Explain what a SLC-96 TDM system is.
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Objectives (continued)
Explain why we might use SLC-96 instead of
twisted-pair copper wire to serve as the
medium to connect telephones to the central
exchange.
Explain when we would use a loop extender
and/or a voice frequency repeater (VFR).
Explain why the medium for SONET must befiber optic cable.
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The Medium
Every communication system requires a
medium connecting the transmitter to the
receiver.
Wire is an excellent conductor of electrical
energy, which is why it is used to connect
telephones to the central exchange.
The wiring used to connect telephones to thecentral exchange is called the local loop.
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Analog Phone Line Connected
to a Digital Central Exchange
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5.1 Local Loop
The twisted-pair local loop is the weakest linkof the telecommunications network.
It limits our communication to low-frequency audio
signals. It is the most costly portion.
Originally both the local loop and toll loopwire constructed using wire as a facility.
In the telephone industry, the engineeringand construction of these facilities are theresponsibility of outside plant engineers.
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New York City 1890 Open
Wire Used for Outside Plant
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5.2 Introduction of Plastic
Insulated Cable
The use of insulated wires makes it possibleto put many wires in one cable.
They will be electrically isolated from each
other by the insulation surrounding each wire. Copper conducts electricity better than iron,
and the choice of copper allows the use ofsmaller diameter wire.
While shellac and lead were originally used,each wire is now covered with plastic.
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Plastic Insulated Cable
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Plastic Insulated Cable
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5.3 Plastic Insulated Cable
(PIC) Color Code
Colors used in PICs Mate (primary) colors: white, red, black, yellow, and violet
Secondary colors: blue, orange, green, brown, and slate
Color of plastic around each wire composedof two colors Tip wire: wide band of mate, narrow band of secondary
Ring wire: wide band or secondary, narrow band of mate
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Plastic Insulated Cable Groups
Groups of 25 pairs are tied together with
colored plastic strings called binders.
Larger cables are constructed by wrapping
from 100 to 625 pairs in binders.
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Table 5-1 Plastic Insulated
Cable (PIC) Color Code
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5.4 Outside Plant Resistance
Design
Older telephone transmitters needed 23milliamps of current to work properly, with acentral office supply of 48 V.
Need to choose wire thickness to ensure thelongest loop did not exceed 1000/1200 .
Designing a loop to not exceed a statedresistance is called resistance design 26- and 24-gauge wire is used for most loops
19- and 22-gauge wire may be used for longer (7to 20 miles) loops
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5.5 SXS Design Criteria: 27
Milliamps of Loop Current
In the Strowger automatic step-by-step
switching system, the Ring trip relayrequired
27 mA to operate.
Each telephone had about 400 of
resistance.
The Ring trip relay had 350 of resistance.
This would allow for the Tip lead and the Ring
lead to each have 500 of resistance.
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5.6 Transmitter Design Criteria: 23
mA and 20 mA of Loop Current
Ring trip relay design improved: Works with only 20 mA of current
Has resistance of 400
Telephone improved: Works with only 20 mA of current
Has resistance of 400 .
SPC switching system supply increased to
52 V. Todays resistance design allows for a local
loop of 1800 .
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5.7 Revised Resistance Design
Any loop up to 18,000 feet is engineered with
nonloaded cable for a maximum resistance of
1300 .
Loops between 18,000 and 24,000 feet are
engineered with loaded cable for a maximum
resistance of 1500 .
Loaded cable has additional inductance toimprove ability to handle voice over long
loops.
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5.8 Modified Long Route
Design
Used for loops between 18,000 and 24,000
feet
Loop Treatment
Range Extenders (loop extenders)
Boosts voltage from 52 V to 78 or 104 V
Voice Frequency Repeaters (VFRs)
Sets gain so overall power loss of circuit is 4 to 8 dB
Can use 24- or 26-gauge wire
Instead of 19- or 22- with RRD
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MLRD Loop Treatment
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5.9 Serving Area of a Central
Office
The serving area of a central office depends
on the geographic area it is situated in.
All cable pairs terminate on the main distributing
frame (MDF).
When an MDF serves more than 20,000 lines,
jumpers become too long.
Serving area depends on population density. It is cheaper to use more exchanges and
smaller wire in the loop.
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Typical Exchange Boundaries
in a City
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Exchange Boundaries in a
Rural Area
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5.10 Resistance Design: Wire
Gauge Selection
Determine maximum resistance:
Revised Resistance Design: 1500 to 2400
Modified Long Route Design: 2400 to 2800
Choose wire gauge
26-gauge: 42 per 1,000 feet
24-gauge: 26 per 1,000 feet
22-gauge: 16 per 1,000 feet
19-gauge: 8 per 1,000 feet
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5.11 Carrier Serving Area
The outermost customer establishes the
exchange boundaryfor a central office.
The Carrier Serving Area (CSA) is a distant
area of the exchange that can support access
to DS0 digital service and ISDN without
special loop treatment.
12,000 feet for 24-gauge wire 9,000 feet for 26-gauge wire
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Carrier Serving Area
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5.12 Voice Signals (AC) in the
Local Loop
Resistance design of the outside plant
ensures that the dc power loss over the local
loop is within limits.
Voice signals are converted by the
transmitter of the phone into variances in
electric current.
These variances look like ac signals. The signal varies between 27 and 33 mA about a
center point of about 30 mA.
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5.13 Capacitive Reactance
Considerations
A wire pair acts as a capacitor.
The wires of the local loop have a capacitance of
about 0.83 F per mile.
The capacitance allows some voice signal to leakfrom one wire to another.
Capacitive reactance varies inversely with
frequency.
On loops longer than 3 miles, capacitive
reactance causes significant power loss to
signals above 1000 Hz.
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5.14 Loading Cable Pairs to
Improve Voice Transmission
Cable pairs shorter than 3 miles do not
require loading.
Most loaded local loops will use 22- and 19-
gauge wire.
The cutoff frequency of the local loop can be
raised to 3800 Hz by using 88 mH coils and
placing them at 6,000 ft intervals. The cutoff frequency of the local loop can be
raised to 7400 Hz by using 44 mH coils and
placing them at 3,000 ft intervals.
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Load Coils
Spacing coils 3000 ft apart is called B
Spacing, spacing 4500 ft apart is D Spacing,
and 6000 ft spacing is H Spacing.
Load coils are enclosed in one case and
connected to two-cable stubs that will
protrude outside the case.
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Load Coil Spacing
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Loading Cable
Loading a cable flattens out the power loss
for all signals below 3,000 Hz.
The single biggest advantage of using wire
as a medium is the narrow bandwidth of wire
due to mutual capacitance and loading.
Load coils improve the low-frequency
response of a cable, but add more loss tohigh-frequency signals.
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Signal Loss Comparison: Loaded
versus Unloaded Cable
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5.15 Data on the Local Loop
The local telecommunications network was
designed to handle voice frequencies.
Data can be sent over specially designed local
loops and central office equipment.
When modems are used, they must be used
in pairs.
One modem at the transmitting end. One modem at the receiving end.
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Digital Data Signals
Digital data signals can use wire for
transmission, but only for a short distance.
Signals must be regenerated every 1 to 3
miles depending on the speed of the data
transmitted.
ISDN lines use digital subscriber line circuits,
which will work up to 3 miles.
C
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Modems Allow the Connection
of a Data Circuit to the PSTN
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5.16 Power-Loss Design
The amount of power loss that any signal has
over a medium is measured in decibels (dB).
dB = 10 log P2 / P1
dBm = 10 log P2 / 1 mW
For the local loop, this power loss over the
wire between the central exchange and the
telephone should not exceed: 8.5 dB with a signal that is 1000 Hz.
The loss between 500 and 2700 Hz should be
within 2.5 dB of the loss measured at 1000 Hz.
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Twisted-Wire Pair
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5.15 Feeder Cables, Distribution
Cables, and Drop Wires
Feeder Cable provides a direct route to the area ofan exchange it will serve
Main Feeder Cable Cables that leave a central exchange
Contain 1800 to 3600 pairs of wires Branch Feeder
Smaller cables that main feeders are spliced out into
900 to 1800 pairs of wires
Distribution Cable 25 to 400 pairs of wires
Drop Wire (Drop Cable) connects home to a readyaccess terminal or a pedestal
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Feeder and Distribution Cables
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Pedestal
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5.18 Subscriber Carrier
Used in outside plant as a pair-gain device.
Electronic device acting as a multiplexed carrier
Amplitude modulation and frequency division
multiplexing AML-1 used to add one additional customer
AML-8 can add eight customers
Subscriber line carrier (or subscriber loop carrier)
Uses TDM technology
Can handle as many as 96 telephones SLC-96
Si l Ch l S b ib
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Single-Channel Subscriber
Carrier
Ei ht Ch l S b ib
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Eight-Channel Subscriber
Carrier
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SLC-96 Units over a Fiber
5 19 M di f L Di t
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5.19 Medium for Long Distance
Networks
When long distance networks were first
established, the only medium available was
wire: 12 to 24 channels using VFRs
Later, AT&T developed a frequency division
multiplexing (FDM) carrier system
600 channels on coaxial cable
L5E system (1978): 13,200 channels Coaxial cable with 20 pairs, 2 spares, and
repeaters every mile: 10 L5Es
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Long Distance Network Media
AT&T also developed carrier systems that used time
division multiplexing (TDM) technology.
Twisted-pair copper wire: T1 (24 channel), T1-C (48
channel), T-2 (96 channel)
Coaxial cable: T3 (672 channel)
Microwave radio systems were also developed by
AT&T.
These systems could multiplex up to 28,244 circuits. Referred to as line-of-sight transmission
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5.20 Fiber Optic Cable
A fiber optic strand is made by surrounding a
thin fiber of glass with another layer of glass
called cladding.
Fiber optic systems are noise free.
AT&T introduced fiber optic technology in
1979 using graded-index fiber.
A fiber optic strand carries signals in onedirection only.
Dense Wave Division
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Dense Wave Division
Multiplexing
Sprint and AT&T have upgraded their fiber
routes to handle as many as 516,096
channels using Dense Wave Division
Multiplexing(DWDM).
At present, DWDM uses 16 or 32 different
frequencies.
It is estimated that a single-mode fiber canhandle 3 million channels with devices
operating at 250 Gbps.
D W Di i i M l i l i
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Dense Wave Division Multiplexing
(courtesy of Sprint)
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5.21 Fiber Technology
Light waves travel at the speed of light, but
this speed will vary depending on the
medium.
The refractive index, n, is given by the ratio of
the speed of light in a vaccuum, c, to the
speed of light in the medium, v.
Air: 1.003, Water: 1.33, Glass: 1.4 1.9 Snells Law
n1 sin 1 = n2 sin 2
Angles of Incidence
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Angles of Incidence,
Reflection, and Refraction
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Angles at the Critical Angle
A l f I id R fl ti
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Angles of Incidence, Reflection,
and Refraction in Fiber