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Chapter 2 The Physical Layer 1 The Theoretical Basis for Data Communication • Fourier Analysis – Any reasonably behaved periodic function can be written as Fourier series. • Bandwidth-Limited Signals – How fast a signal can be transmitted depends on the bandwidth, general meaning of how much information can be carried in a given time period (usually a second) over a communication link, measured mostly by frequency range. • Maximum Data Rate of a Channel 2 Theory of Data Communications • The signal (for example, measured in volts) can be viewed as a) a function of time, g(t), or b) a function of frequency, G(f). • Time-Domain – Let g(t) denote the voltage on a wire at time t. – A signal, g(t), is periodic with period T if g(t+T)=g(t) for all t. – A signal is discrete if it only takes on a finite number of values. – The fundamental frequency is the inverse of the period, f = 1/T, and is measured in cycles per second (Hz). 3 Frequency-Domain Analysis • Any "reasonably-behaved" periodic function, g(t), can be written as a Fourier Series - that is broken up into components with different frequencies. • The time, T, required to transmit a character depends on: a) the encoding method b) the signalling speed or baud rate; that is, how many times per second the signal changes its value (voltage). • Baud rate is not necessarily the same as bit rate. For example, if the values 0, 1, 2, 3, 4, 5, 6, 7 are used in a signal, then each signal value can represent 3 bits. That is 1 baud = 3 bps. 4 Bandwidth-Limited Signals • A binary signal (‘b’ = 01100010) and its root-mean-square Fourier amplitudes. 5 (b) – (c) Successive approximations to the original signal. Bandwidth-Limited Signals (d) – (e) Successive approximations to the original signal. 6 Frequency-Domain Analysis • Below we will only consider 2 voltage levels, so the bit rate is the same as the baud rate. – Let b = bit rate (measured in bits per second (bps)). – Then, it takes 8/b seconds to send 8 bits (one character). – So, T = 8/b, and the fundamental frequency is b/8 Hz. • A voice grade line is an ordinary telephone line and has an artificial cutoff frequency, fc, of about 3000Hz. So, the number of the highest harmonic that can be passed through is 3000/(b/8) = 24000/b. Note the highest harmonic has a frequency that is a multiple of the fundamental frequency (b/8) and the highest harmonic can have a frequency no more than 3000Hz. 7 Bandwidth-Limited Signals Relation between data rate and harmonics. 8 Maximum Data Rate of a Channel • Noiseless channel: Nyquist’s Theorem – If the signal has V discrete levels over a transmission medium of bandwidth H , the maximum data rate = 2H log2 V bits/sec – Example: a noiseless 3-kHz channel cannot transmit binary signals at a rate exceeding 6000 bps (= 2 x 3000 log2 2). • Noisy Channel: Shannon’s Theorem maximum data rate = H log2 (1 + S/N) bits/sec H: bandwdith, S: signal power, N: noise power – S/N (Signal-to-noise ratio), usually measured as 10 log10S/N in db = decibels, is called thermal noise ratio. 9 Physical Interfaces • Physical layer is responsible for the generation, transmission, and receipt of binary data • Generation and Receipt – Conversion of data between binary and analog – E.g. wire: voltage is applied • +V means a 1 • -V means a 0 • 0V means no data 10 Physical Interfaces +V t -V 0 1 1 0 1 0 0 1 11 Physical Interfaces • Errors in physical layer: – Attenuation (reduced signal) – Distortion (wrong signal) • Influences to error: – Type of Media – Bit Rate – Distance • Finally, binary values are passed to Data Link 12 Guided Transmission Data • Magnetic Media • Twisted Pair • Coaxial Cable • Fiber Optics 13 Guided Transmission Data • Magnetic Media: magnetic tape or removable media • Consider an industry standard Ultrium tape – It can hold 200 gigabytes. – A box 60x60x60 cm can hold about 1000 of these tapes. Total capacity is 200 terabytes or 1600 terabits (1.6 petabits). – The box can be sent to anywhere in US in 24 hours. The effect bandwidth is 1600 terabits/86,400 sec, or 19 Gbps. – If it is sent within an hour drive, the bandwidth is increased to over 400 Gbps. No computer network can even approach this. 14 Twisted Pair • Properties – A twisted pair consists of two insulated copper wires. – Why twisted? Countervail the magnetic field – Used in telephone and local area networking – Run several kilometers – The bandwidth depends on the thickness of the wire and distance travelled. • Common types: UTP (Unshielded Twisted Pair) – Category 3: bandwidth of 16 MHz – Category 5: more twists per centimeter, which results in less crosstalk and better-quality signal over longer distance, bandwidth of 100 MHz – Category 6 and 7: 250 MHz and 600 MHz 15 Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP. 16 Coaxial Cable • 50-ohm cable for digital transmission • 75-ohm cable for analog transmission and cable television • 1 GHz • Local area networking and CATV 17 Coaxial Cable A coaxial cable. 18 Fiber Optics • Glass is used instead of copper wires • Light is transmitted instead of electrical current • Components: – Light source – Transmission medium – Detector: convert light plus and electronic signal • Single-mode fiber – Different rays bouncing around at different angle are said to be a multimode fiber. – If the fiber’s diameter is reduced to a few wavelength of light, the light can propagate in a straight line without bouncing, yielding a single-mode fiber. – 50 Gbps for 100 km 19 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. 20 Transmission of Light through Fiber • Three bands are used: 0.85, 1.3, 1.55 μm Attenuation of light through fiber in the infrared region. 21 Fiber Optics • Ways to connect fibers: – Terminate in connectors and plugged into fiber sockets: 10 ~ 20% light lose – Spliced mechanically: 10% light lose – fused • Comparison of fiber optics and copper wire – Advantages: • Higher bit-rates, immune to interference, hard to tap – Disadvantages: • Less familiar technology, unidirectional, easily damaged, expensive interfaces 22 Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers. 23 Fiber Cables • Light sources: LED (Light Emitting Diodes) and semiconductor lasers. • The receiving end consists of a photodiode. A comparison of semiconductor diodes and LEDs as light sources. 24 Fiber Optic Networks A fiber optic ring with active repeaters. 25 Fiber Optic Networks A passive star connection in a fiber optics network. 26 Wireless Transmission • The Electromagnetic Spectrum • Radio Transmission • Microwave Transmission • Infrared and Millimeter Waves • Lightwave Transmission 27 Wireless Transmission • λf = c where λ is the wavelength, f is the frequency, and c is the speed of light, 3 x 108 m/s • Two basic modulation techniques used in spread spectrum signal transmission: – Frequency hopping: The transmitter hops from frequency to frequency. – Direct sequence: The signal is spread over a wide frequency band with specific coding for each channel. • The stream of information to be transmitted is divided into small pieces, each of which is allocated across to a frequency channel across the spectrum. • A data signal at the point of transmission is combined with a higher data-rate bit sequence (also known as a chipping code). 28 Electromagnetic spectrum • LF (Low Frequency, 105 Hz): maritime • MF (Medium Frequency, 106 Hz): AM radio • HF (High Frequency, 107 Hz): radio • VHF (Very High Frequency, 108 Hz): FM radio, TV • UHF (Ultra High Frequency: 109 Hz): TV, terrestrial microwave • SHF (Super High Frequency:1010 Hz): Satellite, microwave • EHF (Extremely High Frequency, 1011 Hz) • THF (Tremendously High Frequency, 1012 Hz) • Higher frequency: IHF?, AHF?, PHF? (Incredibly, Astonishingly, Prodigiously). 29 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. 30 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. 31 Politics of the Electromagnetic Spectrum • Allocate spectrum policies – Beauty contest requires each carrier to explain why its proposal serves the public interest best. – Lottery – Auction • Open band: Frequencies are not allocated but restrained in a short range. The ISM bands in the United States. 32 Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here. 33 Communication Satellites • Geostationary Satellites (GEO) • Medium-Earth Orbit Satellites (MEO) • Low-Earth Orbit Satellites (LEO) • Satellites versus Fiber 34 Communication Satellites • Geostationary Satellites (GEO) – VSAT (Very Small Aperture Terminals): 1-meter antennas, DirecPC • Low-Earth Orbit Satellites (LEO) – Iridium: 66 satellites – Globalstar: 48 satellites – Teledesic: 30 satellites 35 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. 36 Communication Satellites The principal satellite bands. 37 Communication Satellites VSATs using a hub. 38 Low-Earth Orbit Satellites Iridium (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth. 39 Globalstar (a) Relaying in space: Iridium (b) Relaying on the ground: Globalstar 40 Satellites versus Fiber • A single fiber has more bandwidth but is not available to most users. • Satellites are possible for mobile communication. • Satellites are cheaper for Broadcasting. • Satellites can be deployed in places with hostile terrain or a poorly developed terrestrial infrastructure such as Indonesia. • Satellites can be deployed in areas where obtaining the right for laying fiber is difficult. • Satellites is possible for rapid military communication deployment. 41 Public Switched Telephone System • Structure of the Telephone System • The Politics of Telephones (FYI) • The Local Loop: Modems, ADSL and Wireless • Trunks and Multiplexing • Switching 42 Structure of the Telephone System • The PSTN (Public Switched Telephone Network) is the world's collection of interconnected voice-oriented public telephone networks. It's also referred to as the POTS (Plain Old Telephone Service). (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. 43 Structure of the Telephone System A typical circuit route for a medium-distance call. 44 Major Components of the Telephone System • Local loops – Analog twisted pairs going to houses and businesses • Trunks – Digital fiber optics connecting the switching offices • Switching offices – Where calls are moved from one trunk to another 45 The Politics of Telephones • LATA (Local Access and Transport Area) is a geographic area covered by one or more local telephone companies, which are legally referred to as local exchange carriers (LECs). • LEC (Local Exchange Carrier) is a public telephone company in the U.S. that provides local service. Some of the largest LECs are the Bell operating companies (BOCs). • IXC (IntereXchange Carrier) is a company handling inter-LATA traffic such as AT&T, MCI, and Sprint. • A POP (Point of Presence) is a switching office built to handle calls from a LATA. 46 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. 47 The Local Loop: Modems, ADSL, and Wireless • Transmission lines suffer from three major problems: – Attenuation – Delay distortion – Noise • The square waves used in digital signals have a wide frequency spectrum (usually, high frequency) and thus are subject to strong attenuation and delay distortion. 48 Modems The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.49 Modems • The modulation is introduced to solve this problem. – Amplitude: two different amplitudes are used to represent 0 and 1. – Frequency: different tones are used. – Phase: the wave is systematically shifted (45, 135, 225, or 315º). • A modem (modulator-demodulator) is a device that modulates outgoing digital signals to analog signals. 50 Modems (a) A binary signal (c) Frequency modulation (b) Amplitude modulation (d) Phase modulation 51 Modems • The number of samples/symbols per second is measured in baud. • In quadrature phase-shift keying (QPSK), the four angles, usually out of phase by 90°, are used to transmit 2 bits/symbol. The bit rate is twice the baud rate. • QAM-64 (Quadratrue Amplitude Modulation- 64) allows 64 different combinations, so 6 bits can be transmitted per symbol. 52 Modems Constellation Diagrams: (a) QPSK. (b) QAM-16. (c) QAM-64. 53 Modems • To reduce the chance of an error, standards for higher speeds modems do error correction by adding extra bits to each sample. The schemes are known as TCM (Trellis Coded Modulation). • In V.32, 14,400 bps is achieved by transmitting 6 data bits and 1 parity bit per sample at 2400 baud. It uses QAM-128. • In V.34, the modem can run at 28,800 bps at 2400 baud with 12 data bits/symbol or 33,600 bps at 2400 baud with 14 data bits/symbol. 54 Modems (a) (b) (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. 55 Modems • Why are 56 kbps modems in use? – The telephone channel is about 4000 Hz (300 ~ 3400 Hz). – The maximum data rate = 2 x 4000 log2 2 = 8000 sample/sec – The number of bits per sample is 8, one for control purpose, allowing 8000 x 7 = 56,000 bit/sec. • V.90 provides 33.6 kbps upstream and 56 kbps downstream. • V.92 provides 48 kbps upstream. 56 Modems • A connection that allows traffic in both directions simultaneously is called full duplex. • A connection that allows traffic either way, but only one way at a time is called half duplex. • A connection that allows traffic only one way is called simplex. 57 Bandwidth, Baud Rate, Bit Rate • The bandwidth of a medium is the range of frequencies that pass through it with minimum attenuation, usually, measured in Hz. • The baud rate is the number of samples/sec made. Each sample sends one symbol. • The bit rate is the amount of information sent over the channel and is equal to the number of symbols/sec times the number of bits/symbol. 58 Digital Subscriber Lines (DSL) • xDSL is made to work by connecting to a different switch instead of the filter that attenuates all frequencies below 300 Hz and above 3400 Hz. • The xDSL services have been designed with the following goals: – They must work over the existing category 3 twisted pair local loops. – They must not affect existing telephones and fax machines. – They must be faster than 56 kbps. – They must be always on. 59 Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL. 60 Digital Subscriber Lines (DSL) • DMT (Discrete MultiTone) divides the 1.1 MHz spectrum available on the local loop into 256 independent channels of 4312.5 Hz each. – Channel 0: POTS – Channel 1-5: not used – One for upstream and one for downstream control – 32 channels for upstream and rest for downstream • The ADSL standard (ANSI T1.413 and ITU G.992.1) allows speeds of 8 Mbps downstream and 1 Mbps upstream. 61 Digital Subscriber Lines Operation of ADSL using discrete multitone modulation. 62 Digital Subscriber Lines A typical ADSL equipment configuration. 63 Wireless Local Loops • Business practice of a long-distance telephone company for the local phone service: – It must buy or lease a building for the end office. – It must fill the end office with switches. – It must run a fiber between the end office and the toll office. – It must acquire customer. • How is the new local phone company to connect customer telephones and computers in the end office? – Buy the right to lay the new wires. Costly – Buy/lease from other local phone company. Costly – Use the WWL (Wireless Local Loop). 64 Wireless Local Loops • A fixed telephone using a wireless local loop is different from a mobile phone in three ways: – The wireless local loop customer often wants high-speed Internet connectivity. – A directional antenna is needs to be installed. – The user does not move. • LMDS (Local Multipoint Distribution System) is a system for broadband microwave wireless transmission direct from a local antenna to homes and businesses within a line-of-sight radius, a solution to the so-called last-mile technology problem of economically bringing high-bandwidth services to users. • The IEEE 802.16 can be used for wireless local loops standard. 65 Wireless Local Loops Architecture of an LMDS system. 66 Trunks and Multiplexing • Two categories of multiplexing schemes are used to multiplex many conversations over a single physical trunk: – In FDM (Frequency Division multiplexing), the frequency spectrum is divided into frequency bands. For fiber optic channels, WDM (Wavelength Division Multiplexing) is used. – In TDM (Time Division Multiplexing), the entire bandwidth is used for a chunk of time period. 67 Frequency Division Multiplexing (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel. 68 Wavelength Division Multiplexing Wavelength division multiplexing. 69 Time Division Multiplexing • The analog signals are digitalized by a device called a codec (coder-decoder) producing a 7 or 8 bit number. • PCM (Pulse Code Modulation) is a technique to digitalize analog data. – T1 carriers can handle 24 channels multiplexed together. 24 x 8 = 192 bits + 1 bit for framing = 193 bits/frame – Since each analog signal must be sampled 8000 times per second, we must repeat this process every 1/8000 sec = 125 microseconds. – So, the transfer rate on the T1 carrier is: 192 bits / 0.000125 seconds = 1.544 Mbps. • DPCM (Differential Plus Code Modulation) is a method, which consists of outputting the difference between the current value and the previous one, to reduce the number of digitalized bits, 70 Time Division Multiplexing The T1 carrier (1.544 Mbps). 71 Time Division Multiplexing Delta modulation. 72 Time Division Multiplexing Multiplexing T1 streams into higher carriers. 73 SONET/SDH • SONET (Synchronous Optical NETwork) is the American National Standards Institute standard for synchronous data transmission on optical media. • SDH (Synchronous digital hierarchy) is the international standard for synchronous data transmission on optical media. • The goal of SONET: – Possible for different carriers – Unify the U.S., European, and Japanese digital systems – Provide a way to multiplex multiple digital channels – Provide support for operations, administration, and maintenance (OAM) 74 SONET/SDH • Synchronous Optical Network (SONET) – The full specification is larger than this book. – It addresses both the framing and encoding problems. – It multiplexes several low-speed links onto one high-speed link. 75 SONET/SDH • SONET Frame Structure: (Synchronous Transport Signal-1) – 9 x 90 = 810 bytes – The first three columns are reserved for system management information. – The first 9 rows contain the overhead. Overhead has multiple functions: across different links, specify voice channel, concatenation frames. – The remaining 87 columns hold the user data, called the SPE (Synchronous Payload Envelope). The first column is the overhead for the sublayer. – STS-N frame can be thought of as consisting of N STS-1 frames. 76 Time Division Multiplexing Two back-to-back SONET frames. 77 Time Division Multiplexing SONET and SDH multiplex rates. • STS (Synchronous Transport Signal) • OC (Optical Carrier): OC-256 – 13.271 Gbps, OC-768 – 40 Gbps • Synchronous Transport Modules (STM) 78 Switching • Circuit switching – seek out a physical path from sender to receiver. An end-to-end path must be (conceptually) established before data is sent. • Message switching – no path is established in advance. The message is stored in the first switching office and forwarded later one hop at a time. – Example: store-and-forward network – Problem: No restriction of block size • Packet switching – place a restriction on block size, to allow packets to be buffered in main memory at the switching office. – Advantages: Well-suited for interactive traffic • Improved response time and throughput 79 Circuit Switching (a) Circuit switching. (b) Packet switching. 80 Message Switching 81 (a) Circuit switching (b) Message switching (c) Packet switching Packet Switching A comparison of circuit switched and packet-switched networks. 82 The Mobile Telephone System • First-Generation Mobile Phones: Analog Voice • Second-Generation Mobile Phones: Digital Voice • Third-Generation Mobile Phones: Digital Voice and Data 83 Politics and Issues of Mobile Telephones • At first, the U.S. had a single mobile phone system. In Europe every country devises its own system. • Then Europe learned from mistake and standardized on a single system (GSM). By then, the U.S. deregulated the standard. As a consequence, the U.S. has two major and one minor incompatible system. • Mobile phone ownership and usage in Europe is far greater than in the U.S. – A single system for all of Europe – In the U.S. the telephone companies charge the mobile phone owners for incoming call to keep callers from getting nervous about using the telephone. • The widespread use of prepaid mobile phones in Europe (up to 75% in some areas) and Asia. • http://www.gsmworld.com/news/statistics/index.shtml 84 The Mobile Telephone System • Every cellular system digital or analog is comprised of four parts. 1. Cells and cell sites (base stations) 2. Switching station ( mobile telephone switching office, MTSO ) 3. System operator and its local office 4. Cellular telephones 85 The Mobile Telephone System • The heart of the system is made up of individual radio coverage areas called cells. Each cell is a self-contained calling area. • Within the cell, a cell site is strategically positioned as a base station for receiving, sending and routing the radio signals of cellular phone calls. • All cell sites are connected to the Mobile Telephone Switching Office (MTSO). – It provides connection into the Public Switched Telephone network ( PSTN ) - the local telephone company. – It provides other central functions, including call processing, traffic management, and transferring calls as a phone moves between cell sites. 86 The Mobile Telephone System 87 The Mobile Telephone System • Making a call – When a cellular user makes a call from a cellular phone, radio signals are transmitted to the cell site. – The cell site alerts the Mobile Telephone Switching Office (MTSO) switching station. The MTSO, in turn, provides an open channel ( frequency ) and connects the call to the Public Switched Telephone Network ( PSTN ). – The PSTN put the call through to the number to be reached. This process takes the same amount of time that it takes to make a call from a land line phone. 88 The Mobile Telephone System • Receiving a call – Once the MTSO receives a call, it searches for the correct cellular phone by sending out data over the radio waves. – Cellular phones in standby mode continuously scan the radio waves being transmitted by the MSTO. If a phone hears its telephone number, it sends back a signal that informs the closest cell site of its Electronic Serial Number (ESN) and its telephone number (Mobile Identification Number or MIN). – The cell site passes this information to the MTSO, where the ESN and MIN are verified and a channel (frequency) is assigned for the call. – The cellular phone receives the message directing it to tune to the correct voice channel. The cell site makes the voice channel available, and the call is completed. 89 The Mobile Telephone System • Hand-off is the transfer of a call from one cell site to another as the cellular phone moves through the service coverage area. – The cell site warns the MSTO that the mobile's signal strength is falling below a predetermined level. – The MTSO then alerts all cell sites bordering on the first one. They measure the mobile's transmitting signal and report back to the MTSO. – The MTSO, which is programmed to select the site receiving the strongest signal, then switches the call from the weak cell to the strongest cell without interrupting the call. 90 The Mobile Telephone System • Roaming is a service offered by most cellular service providers that allows subscribers to use cellular service while traveling outside their home service area. – When they are outside their home service area and come within range of another cellular system, the ROAM indicator on the cellular phone will light to show that they are in range. – When they roam (operate outside their home system), their cellular phone will seek service from the same type of cellular system as the one they subscribe to at home. But if that type is not available where they are roaming, the phone will try to obtain service from the non-home-type system. A blinking light indicates a non-home-type system. There is an extra charge for calls placed while roaming. 91 Advanced Mobile Phone System (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used. 92 Advanced Mobile Phone System • AMPS (Advanced Mobile Phone System) is the analog system (1G) first developed and used in the U.S. • The AMPS system uses FDM to separate 832 full- duplex channels. – 832 simplex transmission channels from 824 to 849 MHz – 832 simplex receive channels from 869 to 894 MHz – Each simplex channel is 30 kHz wide. • These channels are divided into four categories: – Control (base to mobile) to manage the system (21 channels) – 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 (45 channels) 93 D-AMPS • D-AMPS (Digital-AMPS) is the first digital version (2G) of AMPS. – It uses the 800 or 1900 MHz spectrum. – Each simplex channel is 30 kHz wide. – It is described in IS-54 and IS-136. • It is also known as TDMA (Time Division Multiple Access). – Several physical channels are located by dividing one frequency channel into several time slots. – The advantage of TDMA is that several channels are co- located on one carrier frequency, so there are less transmitters required. 94 D-AMPS Digital Advanced Mobile Phone System (a) A D-AMPS channel with three users. 95 (b) A D-AMPS channel with six users. GSM • GSM (Global System for Mobile communications) is a digital voice or data cellular network used throughout the world. – The European version of GSM operates at the 900 MHz and 1800 MHz frequencies. – The North American version of GSM, called GSM 1900, operates at the 1900 MHz frequency. – Each simplex channel is 200 kHz wide. – Connection rate is up to 9.6K bps – American Personal Communications (APC), a subsidiary of Sprint, is using GSM as the technology for a broadband personal communications service (PCS). 96 GSM Global System for Mobile Communications GSM uses 124 frequency channels, each of which uses an eight-slot TDM system 97 GSM A portion of the GSM framing structure. 98 CDMA • CDMA (Code Division Multiple Access) is a standard using spread spectrum transmission (2G). – The original CDMA standard, also known as cdmaOne and still common in cellular telephones in the U.S., offers a transmission speed of up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. – It operates in the 800 and 1900 MHz bands. – Each simplex channel is 1.25 MHz wide. – It can carry data at rates up to 115 kbps. 99 CDMA • Operation of CDMA: – In CDMA, the input signals are digitized and transmitted in coded, spread-spectrum mode over a broad range of frequencies. – In CDMA, each bit time is subdivided into m short intervals called chips. Typically, there are 64 or 128 chips per bit. – Each station is assigned a unique m-bit code called a chip sequence. – To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, the station sends the one’s complement of its chip sequence. 100 CDMA – Code Division Multiple Access (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions 101 (d) Recovery of station C’s signal Third-Generation Mobile Phones: Digital Voice and Data • Factors which drives the telephony industry: 1. Data traffic exceeds voice traffic. 2. Design a lightweight portable device with versatile functions (telephone, music player, gaming device, digital camera, Web interface, and more). • IMT-2000 (International Mobile Telecommunication- 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.) 102 Third-Generation Mobile Phones: Digital Voice and Data • Two main IMT-2000 proposals (differences in coding methods): – W-CDMA (wideband code-division multiple access) by Ericsson. • W-CDMA can support communications at up from 384 Kbps to 2 Mbps • A 5 MHz-wide channel is used. • UMTS (Universal Mobile Telecommunication System) is the system pushed by the EU. – CDMA2000 by Qualcomm. • CDMA2000 can support mobile data communications at speeds ranging from 144 Kbps to 2 Mbps. • A 5 MHz-wide channel is used. 103 2.5-Generation Mobile Phones: Digital Voice and Data • EDGE (Enhanced Data rates for GMS Evolution) is GSM with more bits per baud. • GPRS (General Packet Radio Service) is a data service that can be layered onto D-AMPS or GSM. – It allows mobile stations to send and receive IP packets. – Each channel is 200 kHz wide. – Data rates of up to 115 kbps 104 Cable Television • Community Antenna Television • Internet over Cable • Spectrum Allocation • Cable Modems • ADSL versus Cable 105 Community Antenna Television • The head end is an amplifier to strengthen the signals. • Cable television was initially called community antenna television. An early cable television system. 106 Internet over Cable • HFC (Hybrid Fiber Coax) system is a system with fiber for the long-haul and coaxial cable to the houses. 107 Cable television Internet over Cable The fixed telephone system. 108 Internet over Cable • Spectrum Allocation – 5 – 42 Mhz: upstream channels – 54 Mhz ↑: downstream channels – A 6 Mhz or 8 Mhz downstream channel is modulated with QAM-64 or QAM-256 for the high quality cable. – With a 6 MHz channel and QAM-64, the net payload is 27 Mbps. – For upstream, QPSK is used because QAM-64 does not work well when there is too much noise. • The head end and amplifier are upgraded to CMTS (Cable Modem Termination System). 109 Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access 110 Cable Modem • A cable modem is a device • How cable modems work? – Ranging: get the distance from the headend to get correct timing to fit in one or more minislots. – Acquiring upstream channel, downstream channel, and minislot assignments: request minislots and wait for the acknowledge from the headend. Otherwise, retry. – Sending packets to request an IP address. – Establishing a secret key between the head-end and modem. – Log in and provide its unique identifier over the secure channel. 111 Cable Modems Typical details of the upstream and downstream channels in North America. 112 ADSL versus Cable • Which is better, ADSL or cable? – Theoretically, coax is hundreds of times more than twisted pair. But, the full capacity is not available for data users (See comment on Page 175). – In practice, ADSL providers achieve about 80% of the bandwidth. Cable depends on how many people are sharing the cable. – Being a point-to-point medium, ADSL is more secure than cable. – The telephone system is more reliable than cable. – Most ADSL providers offer a choice of ISPs (sometimes, required by law). 113
"The Physical Layer"