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Chapter 2 The Physical Layer cn ch2 1 The Theoretical Basis for Data Communication • • • cn ch2 Fourier Analysis Bandwidth-Limited Signals Maximum Data Rate of a Channel 2 Bandwidth-Limited Signals A binary signal and its root-mean-square Fourier amplitudes. (b) cn– ch2 (c) Successive approximations to the original signal. 3 Bandwidth-Limited Signals (2) (d) – (e) Successive approximations to the original signal. cn ch2 4 Bandwidth-Limited Signals (3) Relation between data rate and harmonics. cn ch2 5 Guided Transmission Data • • • • cn ch2 Magnetic Media Twisted Pair Coaxial Cable Fiber Optics 6 Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP. cn ch2 7 Coaxial Cable A coaxial cable. cn ch2 8 Fiber 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. cn ch2 9 Transmission of Light through Fiber Attenuation of light through fiber in the infrared region. cn ch2 10 Fiber Cables cn ch2 (a) Side view of a single fiber. (b) End view of a sheath with three fibers. 11 Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources. cn ch2 12 Fiber Optic Networks A fiber optic ring with active repeaters. cn ch2 13 Fiber Optic Networks (2) A passive star connection in a fiber optics network. cn ch2 14 Wireless Transmission • • • • • cn ch2 The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission 15 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. cn ch2 16 Radio 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. cn ch2 17 Politics of the Electromagnetic Spectrum The ISM bands in the United States. cn ch2 18 Lightwave Transmission Convection currents can interfere with laser communication systems. cn ch2A bidirectional system with two lasers is pictured here. 19 Communication Satellites • • • • cn ch2 Geostationary Satellites Medium-Earth Orbit Satellites Low-Earth Orbit Satellites Satellites versus Fiber 20 Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage. 21 cn ch2 Communication Satellites (2) The principal satellite bands. cn ch2 22 Communication Satellites (3) VSATs using a hub. cn ch2 23 Low-Earth Orbit Satellites Iridium (a) The Iridium satellites from six necklaces around the earth. (b) cn ch21628 moving cells cover the earth. 24 Globalstar cn ch2 (a) Relaying in space. (b) Relaying on the ground. 25 Public Switched Telephone System • • • • • Structure of the Telephone System The Politics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and Multiplexing Switching cn ch2 26 Structure of the Telephone System cn ch2 (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. 27 Structure of the Telephone System (2) A typical circuit route for a medium-distance call. cn ch2 28 Major Components of the Telephone System • Local loops • Trunks • Digital fiber optics connecting the switching offices Switching offices cn ch2 Analog twisted pairs going to houses and businesses Where calls are moved from one trunk to another 29 The Politics of Telephones The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it. cn ch2 30 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. cn ch2 31 Modems (a) A binary signal ch2 (b)cnAmplitude modulation (c) Frequency modulation (d) Phase modulation 32 Modems (2) cn ch2 (a) QPSK. (b) QAM-16. (c) QAM-64. 33 Modems (3) (a) cn ch2 (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. (b) 34 Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL. cn ch2 35 Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation. cn ch2 36 Digital Subscriber Lines (3) A typical ADSL equipment configuration. cn ch2 37 Wireless Local Loops Architecture of an LMDS system. cn ch2 38 Frequency Division Multiplexing cn ch2 (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel. 39 Wavelength Division Multiplexing Wavelength division multiplexing. cn ch2 40 Time Division Multiplexing The T1 carrier (1.544 Mbps). cn ch2 41 Time Division Multiplexing (2) Delta modulation. cn ch2 42 Time Division Multiplexing (3) Multiplexing T1 streams into higher carriers. cn ch2 43 Time Division Multiplexing (4) Two back-to-back SONET frames. cn ch2 44 Time Division Multiplexing (5) SONET and SDH multiplex rates. cn ch2 45 Circuit Switching cn ch2 (a) Circuit switching. (b) Packet switching. 46 Message Switching cn ch2 47 (a) Circuit switching (b) Message switching (c) Packet switching Packet Switching Acncomparison of circuit switched and packet-switched networks. ch2 48 The Mobile Telephone System • First-Generation Mobile Phones: Analog Voice • Second-Generation Mobile Phones: Digital Voice • Third-Generation Mobile Phones: Digital Voice and Data cn ch2 49 Advanced Mobile Phone System (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used. cn ch2 50 Channel Categories The 832 channels are divided into four categories: • Control (base to mobile) to manage the system • Paging (base to mobile) to alert users to calls for them • Access (bidirectional) for call setup and channel assignment • Data (bidirectional) for voice, fax, or data cn ch2 51 D-AMPS Digital Advanced Mobile Phone System cn ch2 (a) A D-AMPS channel with three users. (b) A D-AMPS channel with six users. 52 GSM Global System for Mobile Communications cn ch2 GSM uses 124 frequency channels, each of which uses an eight-slot TDM system 53 GSM (2) A portion of the GSM framing structure. cn ch2 54 CDMA: Code Division Multiple Access • • • • • • • • • Allow each station to transmit over entire frequency spectrum all the time. Multiple simultaneous transmissions are separated using coding theory. Colliding frames may not be totally garbled. There are techniques to separate signals sent by different senders. Similar to a party where different conversations use different language. Extract desired signal and reject others as random noises. Each bit time is subdivided into m short intervals called chips, typically 64-128 chips per bit. Each station is assigned a unique m-bit code or chip sequence. To send a bit 1, a station sends its chip sequence. To send a bit 0, a station sends the complement of its chip sequence. For m=8, A is assigned 00011011. A sends 00011011 as bit 1, and 11100100 as bit 0. cn ch2 55 Simple Analysis of CDMA • • • • Assume 1 MHz band for 100 stations Use FDM, one station has 10kHz and 10 kbps (assume 1 bit per Hz) Use CDMA, one station has 1MHz, and 1Mchips per seconds. If CDMA uses less than 100 chips per bit then CDMA will be more efficient. cn ch2 56 CDMA Coding Theory •Let’s use bipolar notation +1 for binary 1 (chip signal), -1 for binary 0. •A bit1, A send 00011011 or (-1 -1 -1 +1 +1 -1 +1 +1). •Let S be the m-chip vector for station s and S for its negation. m •Two chip sequence S and T are orthogonal if S•T=0. 1 S T • if S•T=0 then S•T =0 m s t i 1 • • All chip sequences must be pariwise orthogonal. • S• S =-1 • CDMA Example • Let S=A+B +C, S•C=(A+B+C)•C=A•C+B•C+C•C=0+0+1=1 cn ch2 57 i i CDMA – Code Division Multiple Access (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions cn ch2 (d) Recovery of station C’s signal 58 Walsh-Hadamard Matrix for Orthogonal Spreading Sequence a) The Walsh-Hadamard Matrix provides the Orthogonal Chip Sequences of length n=2m. Replacing 0 with signal -1 and 1 with signal +1. b) It can be recursively constructed. W1=[0]. W2n= where contains complement elements of Wn. cn ch2 59 CDMA Sending Channel 1: 110 -> +1+1+1 -> (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1) Channel 2: 010 -> -1+1-1 -> (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1) Channel 3: 001 -> -1-1+1 -> (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1) Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1) Chip Sequence c1: (-1,-1,-1,-1) c2: (-1,+1,-1,+1) c3: (-1,-1,+1,+1) c4: (-1,+1,+1,-1) cn ch2 60 CDMA Receiving/Decoding Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1) Channel 2 Sequence: (-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1) Correlator Output: (-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1) Integrated Output: -4, +4, -4 Binary Output: 0, 1, 0 cn ch2 61 Power Control/Assignment • • • For the CDMA to work, the power levels of signals from all stations should be the same (or within certain tolerance level) when received by the receiver. A good heuristic: Each mobile station sends signal with the power level inverse of that received from the base station. The base station can tell mobile station to increase/decrease its power. cn ch2 62 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.) cn ch2 63 Cable Television • • • • • cn ch2 Community Antenna Television Internet over Cable Spectrum Allocation Cable Modems ADSL versus Cable 64 Community Antenna Television An early cable television system. cn ch2 65 Internet over Cable Cable television cn ch2 66 Internet over Cable (2) The fixed telephone system. cn ch2 67 Spectrum Allocation cn ch2 Frequency allocation in a typical cable TV system used for Internet access 68 Cable Modems cn ch2 Typical details of the upstream and downstream channels in North America. 69