Lecture notes on Computer Communication network

lecture notes in communications in computer and information science,what is computer communications, what are the uses of computer communications give examples pdf free download
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Published Date:23-07-2017
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Computer Communications Lecture 5-7 Communication Technologies and their Applications The Physical Layer • Transport a raw bit stream from one machine to another • Physical media characterized by bandwidth, delay, cost, and ease of installation and maintenance • Guided transmission media • Unguided transmission media – Magnetic media – Radio transmission – Twisted pair – Microwave transmission – Coaxial cable – Infrared and millimeter waves – Fiber optics – Lightwave transmission 2 1Twisted Pair • Oldest and still most common, provides adequate performance, low cost • Two insulated copper wires, typically about 1mm thick, twisted together in a helical form • Can run several kilometers without amplification • But repeaters are needed for longer distances • Can transmit both analog and digital signals • Bandwidth dependent on wire thickness, distance traveled – Several Mbps achievable for a few kilometers 3 Twisted Pair • Two relevant types of cabling (often referred to as unshielded twisted pair – UTP) • Category 3 – two insulated wires gently twisted together – four such pairs typically grouped in a plastic sheath for holding them together and protection – 16MHz bandwidth • Category 5 – more twists per centimeter (a) Category 3 UTP. – less crosstalk and better-quality signal over longer distances (b) Category 5 UTP. – more suitable for high-speed computer communication – 100MHz • Also cat 6 (250MHz) and cat 7 (600MHz) 4 2Coaxial Cable • Better shielding than twisted pair  span longer distances at higher speeds • Two kinds – 50-ohm cable for digital – 75-ohm cable for analog and cable television • Good combination of high bandwidth and excellent noise immunity • Bandwidth (1GHz typical) dependent on cable quality, length, SNR A coaxial cable. 5 Fiber Optics • Achievable bandwidth in excess of 50Tbps • Current signaling rate of 10Gbps limited by electrical-optical signal conversion capability • Optical transmission system (inherently unidirectional) – Light source (light pulse or the absence of it) – Transmission medium (ultra-thin fibre of glass) – Detector (generates an electrical pulse when light falls on it) 6 3Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection. • Multimode fiber: many different rays bouncing at different angles • Single-mode fiber – Very small diameter, acts like a wave-guide, light propagates in a straight line – More expensive – Widely used for longer distances: 50Gbps for 100Km without amplification 7 Transmission of Light Through Fiber • Attenuation of light through glass dependent on light wavelength and physical properties of glass • Attenuation in dB = 10 log (transmitted power/received power) 10 Attenuation of light through fiber in the infrared region. 8 4Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers. • Core diameter – 50 microns in multimode fibers – 8-10 microns in single-mode fibers • Three ways of connecting fibers – Terminate in connectors (10-20 percent attenuation) – Mechanical splices (10 percent attenuation) – Fusion splices (smaller attenuation) 9 Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources. • Photodiode at the receiving end • Typical response time is 1ns • Need to deal with thermal noise 10 5Fiber Optic Networks A fiber optic ring with active repeaters. 11 Fiber Optic Networks (2) A passive star connection in a fiber optics network. 12 6Fiber Optics versus Copper Wire • Compared to Copper, Fiber Optics – Higher bandwidths – Need fewer repeaters  low cost – Unaffected by power surges, electromagnetic interference or power failures, corrosive chemicals in the air – Thin and lightweight  lower installation cost for new routes – Do not leak light and difficult to tap – Less known, more skill required – Can be damaged by bending too much – Inherently unidirectional – Fiber interfaces more costly 13 Wireless Transmission • The Electromagnetic Spectrum • Radio Transmission • Microwave Transmission • Infrared and Millimeter Waves • Lightwave Transmission 14 7Electromagnetic Waves • Electron movement creates electromagnetic waves that can propagate in space or even in vacuum. – Predicted by James Clerk Maxwell in 1865 – Observed by Heinrich Hertz in 1887. λ • Frequency (Hz) and Wavelength ( ) • Electromagnetic waves travel in vacuum at the speed of light (c), regardless of their frequency – In other media such as copper or fiber, speed reduces and is somewhat frequency dependent. λλ • Fundamental relationship between f, and c: f = c – when f in MHz and λ in meters, fλ = 300. 15 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. 16 8Electromagnetic Waves and Data Rate • Amount of information that an EM wave can carry is dependent on its bandwidth. – Few bits/sec at low frequencies, up to 8bits/Hz at higher frequencies. – Examples: coaxial (750MHz, several Gbps), fiber at 1.3 micron band and 8bits/Hz = 240Tbps. • Most transmissions use narrow frequency band for best reception • Exceptions: – frequency-hopping spread spectrum (good against jamming, multipath fading) – direct sequence spread spectrum (in use in 2G/3G and WLANs; good spectral efficiency, noise immunity, etc.) 17 Radio Transmission • Easy to generate • Can travel long distances • Can penetrate buildings easily • Radio wave properties frequency dependent • At low frequencies: – Can pass through obstacles well – Power falls off sharply with distance • At high frequencies: – Travel in straight lines and bounce off obstacles – Absorbed by rain • Interference – From motors and other electrical equipment – Among users 18 9Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere. 19 Microwave Transmission • Above 100MHz, waves travel in nearly straight lines  narrowly focused beam  high SNR • If microwave towers too far apart, repeaters needed • Distance between repeaters goes up roughly with sqrt (tower height) – Repeaters spaced 80km apart for 100m-high towers • Do not pass through buildings well • Multipath fading – Weather and frequency dependent • Above 4GHz, waves are absorbed by rain • Advantages compared to fiber: – No right of way needed – Relatively inexpensive 20 10Politics of the Electromagnetic Spectrum • Spectrum allocation methods: – Oldest: proposal-based (beauty contest) – Lottery – Auction – Unallocated, but regulated (e.g., ISM bands) The ISM bands in the United States. 21 Other Wireless • Infrared and Millimeter Waves – Widely used for short-range communication (e.g., connecting computers to printers, cable replacement, remote controls) – Relatively directional, cheap and easy to build – Do not pass through solid objects – Less interference and more security compared to radio waves – No government licensing required unlike radio and microwave • Lightwave transmission – Has been in use for a long time – Very high bandwidth and low cost – Relatively easy to install – Does not require government licensing – Narrow laser beam is also a weakness – Cannot penetrate rain or thick fog – Affected by convection currents on sunny and hot days 22 11The Mobile Telephone System • First-Generation Mobile Phones: Analog Voice • Second-Generation Mobile Phones: Digital Voice • Third-Generation Mobile Phones: Digital Voice and Data 23 Advanced Mobile Phone System (AMPS) • The “cell” concept – Base station: radio relay – MTSO (Mobile Telephone Switching Office): channel assignment and handoffs (hard/soft) – Neighboring cells use distinct frequencies • System capacity, interference, cell size and “frequency reuse” (a) Frequencies are not reused in adjacent cells; (b) To add more users, smaller 24 (micro) cells can be used 12AMPS: Channels and Call Management • Uses FDM • The 832 full-duplex channels – 832 simplex transmission channels from 824-849MHz – 832 simplex receive channels from 869-894MHz – 30 KHz wide • Four categories: – Control (base to mobile) to manage the system, including mobile registration – Access (bidirectional) for call setup and channel assignment – Paging (base to mobile) to alert users to calls for them – Data (bidirectional) for voice, fax, or data 25 D-AMPS Digital Advanced Mobile Phone System • Digitization and compression of voice • FDM and TDM • Mobile Assisted HandOff (MAHO) (a) A D-AMPS channel with three users (with compression to 8kbps). (b) A D-AMPS channel with six users (with compression to 4kbps). 26 13GSM (Global System for Mobile Communications) • Similar to D-AMPS • But wider channels (200 KHz vs. 30KHz), more users per channel (8 vs. 3) and higher data rate per user GSM uses 124 frequency channels, each of which uses an eight-slot TDM system 27 GSM (2) A portion of the GSM framing structure. 28 14CDMA – Code Division Multiple Access • Chips, chip sequence (code) and orthogonality • Potentially higher effective bandwidth, also wider band (1.25 MHz) Practical limitations • synchronization • noise • non-uniform power levels • knowledge of sender at receiver (a) Binary chip sequences for four stations; (b) Bipolar chip sequences (c) Six examples of transmissions; (d) Recovery of station C’s signal 29 Third-Generation Mobile Phones: Digital Voice and Data • Basic services an IMT-2000 network should provide: – High-quality voice transmission – Messaging (replace e-mail, fax, SMS, chat, etc.) – Multimedia (music, videos, films, TV, etc.) – Internet access (web surfing, w/ multimedia.) • Additionally: worldwide, always-on with quality-of-service (QoS) guarantees • Two main proposals: (1) W-CDMA (Wideband CDMA) or UMTS, and (2) CDMA2000 » Both use direct sequence spread spectrum and 5 MHz bandwidth » Only W-CDMA can interwork with GSM networks » Other differences: chip rate, frame time, spectrum used, method of time synchronization • 2.5G systems – GPRS, an overlay packet network over GSM or D-AMPS – EDGE, GSM with more bits/baud • 4G systems: high bandwidth, ubiquity, all-IP seamless wired-wireless integration, adaptive resource and spectrum management, software radios and better QoS for multimedia 30 15Public Switched Telephone System • Structure of the Telephone System • The Politics of Telephones • The Local Loop: Modems, ADSL and Wireless • Trunks and Multiplexing • Switching 31 Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. 32 16Structure of the Telephone System (2) • Local loops – Analog twisted pairs going to houses and businesses (typically 1-10 km) • Trunks – Digital fiber optics connecting the switching offices • Switching offices – Where calls are moved from one trunk to another A typical circuit route for a medium-distance call. 33 The Local Loop: Modems, ADSL, and Wireless The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs. 34 17Modems (a) A binary signal (c) Frequency modulation (b) Amplitude modulation (d) Phase modulation 35 Modems (2) (a) QPSK. (b) QAM-16. (c) QAM-64. 36 18Modems (3) (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. 37 Digital Subscriber Lines • Frequencies below 300 Hz and above 3400 Hz are not filtered at end office • Local loop capacity dependent on length, thickness and general quality Bandwidth versus distanced over category 3 UTP for DSL. 38 19Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation. • Asymmetric (32 upstream channels, 216 downstream channels) • ADSL standard (ANSI T1.413 and ITU G.992.1): 8Mbps downstream and 1Mbps upstream • Within each channel, sampling rate 4000 baud and V.34 like modulation (12 bits/symbol) • Different channels may have different data rates 39 Digital Subscriber Lines (3) • DSLAM: Digital Subscriber Line Access Multiplexer • NID: Network Interface Device • NID/Splitter nowadays replaced by a microfilter. A typical ADSL equipment configuration. 40 20

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