INTRODUCTION TO TELECOMMUNICATION SHAKEEL AHMAD

ALREADY STUDIED SIGNALS BASICS PCM

TODAY ON Transmission Media for Trunks

Media Types Open Wire Pairs Twisted Pair Wires Coaxial Cable Microwave Links Submarine Cables Satellite Communications Optical Fiber

Open Wire Pairs Used in early years of telephony, more of history now. A pair consists of two open wires suspended on telephone poles. The wires are separated by approximately 1 ft to minimize capacitance. Glass insulators mounted on wooden crossbeams of the telephone pole suspend the wires between poles

Open Wire Pairs Open wire is usually made of steel, coated with copper Steel is used for the strength necessary to withstand varying weather conditions and to withstand the suspension weight of the wire between poles Distance between repaeters upto 50kms No. of channels over a single pair – 12

Open Wire Pairs Frequency range up to 160kHz Disadvantages: Bulky and unsightly Affected by weather conditions i.e. large leakage with insulators Severe cross-talk High radiation at the high frequencies

Twisted-Pair Wires Insulated Pairs of copper wire bundled together Insulated Pairs of copper wire bundled together Individual pairs of wires twisted together to minimize crosstalk Individual pairs of wires twisted together to minimize crosstalk Cables can contain several hundreds of twisted pairs in different gages. Cables can contain several hundreds of twisted pairs in different gages. Laid in cities underground Laid in cities underground

Twisted-Pair Wires Suffer from cross-talk because of pairs being bound closely. Suffer from cross-talk because of pairs being bound closely. Due to the small diameter of the wires, resistance contributes significantly to signal loss Due to the small diameter of the wires, resistance contributes significantly to signal loss

Twisted-Pair Wires Repeaters required every 3 to 6.5 km Repeaters required every 3 to 6.5 km Used for short haul trunks Used for short haul trunks Voice channels over a single pair Voice channels over a single pair Frequency range up to 1MHz Frequency range up to 1MHz

Coaxial Cable Coax is manufactured with a hollow cylindrical copper tube used for the shield Insulating material (dielectric ) separates the conductive tube from the copper center conductor Frequency Range 3-60 MHz Radiation losses and adjacent channel interference are virtually eliminated by coaxial shielding

Coaxial Cable Highly suited for Long haul trunk circuits such as inter-city or interstate routes. Several individual coaxial tubes are often bound together with insulating material and steel reinforcement to produce a high capacity trunk cable Repeaters spacing 3-65kms Each coaxial tube can carry several thousands of voice channels.

The Bell System's L5 coaxial carrier A long-haul trunk that includes 22 coaxial tubes bound together to form a single cable. Total of 108,000 simultaneous two-way voice conversations can be carried by the cable. Ten tubes carry108,000 voice channels in one direction, and 10 tubes are for the opposite direction. Overall system frequency 58Mhz

Microwave Links An alternative to coaxial cables for high- capacity long-haul trunks are microwave radio links Repeater spacing 30-50kms Work on line of sight, therefore repeaters antennas are typically put on towers, hilltops, and huge skyscrapers. Frequency range 3-7 GHz approx.

Microwave Links Affected by weather conditions such as rain resulting in fading Can cause radio interference High velocity of propagation minimizing the time delay No. of channels in thousands per route

Microwave Links Advantages Fewer repeaters are necessary for amplifying signals Underground facilities are not necessary Multiple channels can be transmitted over a single link

Microwave Links Advantages Minimal delay times Minimal cross talk Fewer repeaters mean increased reliability and less maintenance

Submarine Cables Submarine cables are coaxial cables specially designed to withstand the rugged oceanic floor conditions throughout the world First voice-grade cable was laid across the Atlantic Ocean floor in The TAT-l (Transatlantic) Cable System, developed by AT&T, spanned a distance of 2200 nautical miles

Submarine Cables Construction of the cable includes several layers of insulation and armored steel reinforcement surrounding the conductor to protect it from corrosion, temperature changes, and leakage Repeaters are constructed in a similar manner to prevent the damage of internal circuitry

Submarine Cables Important factors The cable must be protected from saltwater corrosion and leakage Suboceanic terrain conditions and ocean depth must be considered Temperature and pressure changes from sea level to ocean floor must be determined

Submarine Cables Important factors The weight of cable material and rate of descent to the oceanic floor are critical parameters. Off-coast trenches must be dug in shallow waters to bury and protect the cable from fishing trawlers and anchors

Submarine Cables Electrical circuits must be environmentally tested at temperature extremes exceeding those of the ocean floor Repeater units must be x-ray tested for faulty welds and leaks Performance tests must be exercised constantly while laying cable to determine immediately the location of a fault

Submarine Cables First generation laid in 1950s 48 voice channels Repeaters at 39 nautical miles Overall bandwidth 164 kHz

Submarine Cables Latest generation Overall bandwidth 28Mhz 4000 Voice channels Now fiber optic cables being installed with half the size, one third the weight and double the capacity of existing coaxial cable

Satellite Communications First satellite Intelsat I (called Early Bird) was designed to handle 240 voice channels (in 1965) Telephone and television broadcast signals are beamed up to the satellite from an earth through the use of a large, highly directive microwave dish antenna that is synchronized to the position of the satellite

Satellite Communications A device called a transponder is used on board the satellite to receive the weak microwave signal, amplify and condition it, and retransmit the signal back to another earth station in a different location on earth

Satellite Communications To prevent the transponder's strong transmitted signal from interfering with the earth station's weak received signal, most commercial satellite links separate, transmit, and receive carrier frequencies by about 2 GHz

Satellite Communications Earth stations typically transmit their signals to satellites on carrier frequencies in the 6-GHz band, ranging from 5.92 to 6.43Ghz (the up-link frequency). The satellite's transponder down- converts these signals to a 4-GHz band, ranging from 3.7 to 4.2 GHz. (the down-link frequencies)

Satellite Communications Geo-synchronous or geo-stationary satellites positioned at approximately 22,300 miles above the equator

Satellite Communications At an altitude of 22,300 miles, 40% of the Earth is exposed. The satellite's antenna is designed to emit a radiation pattern that covers this entire exposed portion

Satellite Communications Satellites positioned in geo- synchronous orbit, 120° apart from each other, can cover the entire surface of the earth Subject to long delays

Satellite Communications Much lower cost per channel than submarine cable for transatlantic communications 600 channels per transponders, 12 transponders per sartellite

Optical Fiber Material composition types (cladding & core) Glass cladding and glass core Plastic cladding and glass core Plastic cladding and plastic core Special protection required, the extent of which depends upon application used. No. of voice channels in thousands

Optical Fiber Not subject to interference or tapping making it secure as well Modern day requirements of several gigabits per sec can be accommodated Repeaters spacing possible over 100 km because of less loss Optic fiber Cables substantially lighter than copper cables with same capacity

Optical Fiber Optic fiber cable has a longer life span than copper because it is more resistant to corrosion Interfacing cost is higher for electronic facilities which are to be converted into optics. Difficult to splice Remote powering is a problem, sometimes metallic conductors are bundled in the cable for the purpose

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