William Stallings Data and Computer Communications Chapter 4

William Stallings Data and Computer Communications Chapter 4 Transmission Media

Overview z Guided - wire z Unguided - wireless z Characteristics and quality determined by medium and signal z For guided, the medium is more important z For unguided, the bandwidth produced by the antenna is more important z Key concerns are data rate and distance

Design Factors z Bandwidth y. Higher bandwidth gives higher data rate z Transmission impairments y. Attenuation z Interference z Number of receivers y. In guided media y. More receivers (multi-point) introduce more attenuation

Electromagnetic Spectrum

Guided Transmission Media z Twisted Pair z Coaxial cable z Optical fiber

Twisted Pair

Twisted Pair - Applications z Most common medium z Telephone network y. Between house and local exchange (subscriber loop) z Within buildings y. To private branch exchange (PBX) z For local area networks (LAN) y 10 Mbps or 100 Mbps

Twisted Pair - Pros and Cons z Cheap z Easy to work with z Low data rate z Short range

Twisted Pair - Transmission Characteristics z Analog y. Amplifiers every 5 km to 6 km z Digital y. Use either analog or digital signals yrepeater every 2 km or 3 km z Limited distance z Limited bandwidth (1 MHz) z Limited data rate (100 MHz) z Susceptible to interference and noise

Unshielded and Shielded TP z Unshielded Twisted Pair (UTP) y. Ordinary telephone wire y. Cheapest y. Easiest to install y. Suffers from external EM interference z Shielded Twisted Pair (STP) y. Metal braid or sheathing that reduces interference y. More expensive y. Harder to handle (thick, heavy)

UTP Categories z Cat 3 yup to 16 MHz y. Voice grade found in most offices y. Twist length of 7. 5 cm to 10 cm z Cat 4 yup to 20 MHz z Cat 5 yup to 100 MHz y. Commonly pre-installed in new office buildings y. Twist length 0. 6 cm to 0. 85 cm

Near End Crosstalk z Coupling of signal from one pair to another z Coupling takes place when transmit signal entering the link couples back to receiving pair z i. e. near transmitted signal is picked up by near receiving pair

Coaxial Cable

Coaxial Cable Applications z Most versatile medium z Television distribution y. Ariel to TV y. Cable TV z Long distance telephone transmission y. Can carry 10, 000 voice calls simultaneously y. Being replaced by fiber optic z Short distance computer systems links z Local area networks

Coaxial Cable - Transmission Characteristics z Analog y. Amplifiers every few km y. Closer if higher frequency y. Up to 500 MHz z Digital y. Repeater every 1 km y. Closer for higher data rates

Optical Fiber

Optical Fiber - Benefits z Greater capacity y. Data rates of hundreds of Gbps z Smaller size & weight z Lower attenuation z Electromagnetic isolation z Greater repeater spacing y 10 s of km at least

Optical Fiber - Applications z Long-haul trunks z Metropolitan trunks z Rural exchange trunks z Subscriber loops z LANs

Optical Fiber - Transmission Characteristics z Act as wave guide for 1014 to 1015 Hz y. Portions of infrared and visible spectrum z Light Emitting Diode (LED) y. Cheaper y. Wider operating temp range y. Last longer z Injection Laser Diode (ILD) y. More efficient y. Greater data rate z Wavelength Division Multiplexing

Optical Fiber Transmission Modes

Wireless Transmission z Unguided media z Transmission and reception via antenna z Directional y. Focused beam y. Careful alignment required z Omnidirectional y. Signal spreads in all directions y. Can be received by many antennae

Frequencies z 2 GHz to 40 GHz y. Microwave y. Highly directional y. Point to point y. Satellite z 30 MHz to 1 GHz y. Omnidirectional y. Broadcast radio z 3 x 1011 to 2 x 1014 y. Infrared y. Local

Terrestrial Microwave z Parabolic dish z Focused beam z Line of sight z Long haul telecommunications z Higher frequencies give higher data rates

Satellite Microwave z Satellite is relay station z Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency z Requires geo-stationary orbit y. Height of 35, 784 km z Television z Long distance telephone z Private business networks

Broadcast Radio z Omnidirectional z FM radio z UHF and VHF television z Line of sight z Suffers from multipath interference y. Reflections

Infrared z Modulate noncoherent infrared light z Line of sight (or reflection) z Blocked by walls z e. g. TV remote control, IRD port

Required Reading z Stallings Chapter 4