INTRODUCTION TO INTRODUCTION TO TELECOMMUNICATIONTELECOMMUNICATION

INTRODUCTION TO INTRODUCTION TO TELECOMMUNICATIONTELECOMMUNICATION

SHAKEEL AHMADSHAKEEL AHMAD

ALREADY STUDIED

• SIGNALS BASICS• PCM

TODAY ON TODAY ON 21-02-200421-02-2004

TODAY ON TODAY ON 21-02-200421-02-2004

Transmission Media for TrunksTransmission 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 wirebetween 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 Twisted-Pair Wires

Insulated Pairs of copper wire Insulated Pairs of copper wire bundled together bundled together

Individual pairs of wires twisted Individual pairs of wires twisted together to minimize crosstalktogether to minimize crosstalk

Cables can contain several Cables can contain several hundreds of twisted pairs in hundreds of twisted pairs in different gages.different gages.

Laid in cities undergroundLaid in cities underground

Twisted-Pair WiresTwisted-Pair Wires

Suffer from cross-talk Suffer from cross-talk because of pairs being because of pairs being bound closely.bound closely.

Due to the small Due to the small diameter of the wires, diameter of the wires, resistance contributes resistance contributes significantly to signal losssignificantly to signal loss

Twisted-Pair WiresTwisted-Pair Wires

Repeaters required every Repeaters required every 3 to 6.5 km3 to 6.5 km

Used for short haul trunksUsed for short haul trunks Voice channels over a Voice channels over a

single pair 12-120single pair 12-120 Frequency range up to Frequency range up to

1MHz1MHz

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 1956. 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

Thank You