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Chapter 5. The Medium. 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.
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Chapter 5 The Medium
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.
Objectives (continued) • Explain revised resistance design. • Explain modified long route design. • Explain when to use revised resistance design or modified long route design principles in designing a local loop. • Explain what a load coil is and why we use them.
Objectives (continued) • Explain why loops longer than 18,000 feet need to be loaded. • Explain what a jumper wire is and what it is used 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.
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.
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 be fiber optic cable.
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 the central exchange is called the local loop.
5.1 Local Loop • The twisted-pair local loop is the weakest link of 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 loop wire constructed using wire as a facility. • In the telephone industry, the engineering and construction of these facilities are the responsibility of outside plant engineers.
5.2 Introduction of Plastic Insulated Cable • The use of insulated wires makes it possible to 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 of smaller diameter wire. • While shellac and lead were originally used, each wire is now covered with plastic.
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 composed of two colors • Tip wire: wide band of mate, narrow band of secondary • Ring wire: wide band or secondary, narrow band of mate
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.
5.4 Outside Plant Resistance Design • Older telephone transmitters needed 23 milliamps of current to work properly, with a central office supply of 48 V. • Need to choose wire thickness to ensure the longest loop did not exceed 1000/1200 Ω. • Designing a loop to not exceed a stated resistance is called resistance design • 26- and 24-gauge wire is used for most loops • 19- and 22-gauge wire may be used for longer (7 to 20 miles) loops
5.5 SXS Design Criteria: 27 Milliamps of Loop Current • In the Strowger automatic step-by-step switching system, the Ring trip relay required 27 mA to operate. • Each telephone had about 400 Ω of resistance. • The Ring trip relay had 350 Ω of resistance. • This would allow for the Tip lead and the Ring lead to each have 500 Ω of resistance.
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 Ω • Telephone improved: • Works with only 20 mA of current • Has resistance of 400 Ω. • SPC switching system supply increased to 52 V. • Today’s resistance design allows for a local loop of 1800 Ω.
5.7 Revised Resistance Design • Any loop up to 18,000 feet is engineered with nonloaded cable for a maximum resistance of 1300 Ω. • Loops between 18,000 and 24,000 feet are engineered with loaded cable for a maximum resistance of 1500 Ω. • Loaded cable has additional inductance to improve ability to handle voice over long loops.
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
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.
5.10 Resistance Design: Wire Gauge Selection • Determine maximum resistance: • Revised Resistance Design: 1500 to 2400 Ω • Modified Long Route Design: 2400 to 2800 Ω • Choose wire gauge • 26-gauge: 42 Ω per 1,000 feet • 24-gauge: 26 Ω per 1,000 feet • 22-gauge: 16 Ω per 1,000 feet • 19-gauge: 8 Ω per 1,000 feet
5.11 Carrier Serving Area • The outermost customer establishes the exchange boundary for 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
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.
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 μF per mile. • The capacitance allows some voice signal to leak from 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.
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.
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.
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 to high-frequency signals.
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.
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.
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.
5.15 Feeder Cables, Distribution Cables, and Drop Wires • Feeder Cable provides a direct route to the area of an 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 ready access terminal or a pedestal
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
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
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
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 one direction only.
