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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