Lecture 5-7 Communication Technologies and their Applications

Transcription

1 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 Magnetic media Twisted pair Coaxial cable Fiber optics Unguided transmission media Radio transmission Microwave transmission Infrared and millimeter waves Lightwave transmission 2 1

2 Twisted 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 less crosstalk and better-quality signal over longer distances more suitable for high-speed computer communication 100MHz Also cat 6 (250MHz) and cat 7 (600MHz) (a) Category 3 UTP. (b) Category 5 UTP. 4 2

3 Coaxial 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 3

4 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. 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 10 (transmitted power/received power) Attenuation of light through fiber in the infrared region. 8 4

5 Fiber 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 5

6 Fiber Optic Networks A fiber optic ring with active repeaters. 11 Fiber Optic Networks (2) A passive star connection in a fiber optics network. 12 6

7 Fiber 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 7

8 Electromagnetic 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 Frequency (Hz) and Wavelength (λ) 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. Fundamental relationship between f, λand c: f*λ= c when f in MHz and λin meters, f*λ= The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. 16 8

9 Electromagnetic 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 9

10 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. 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 10

11 Politics 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 11

12 The 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 (micro) cells can be used 24 12

13 AMPS: Channels and Call Management Uses FDM The 832 full-duplex channels 832 simplex transmission channels from MHz 832 simplex receive channels from MHz 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)

14 GSM (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

15 CDMA 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 , 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 15

16 Public 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

17 Structure 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

18 Modems (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation 35 Modems (2) (a) QPSK. (b) QAM-16. (c) QAM

19 Modems (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

20 Digital 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

21 Cable Television Community Antenna Television Internet over Cable Spectrum Allocation Cable Modems ADSL versus Cable 41 Community Antenna Television One-way system from headend to the users An early cable television system

22 Internet over Cable Two issues in providing Internet access The use of one-way amplifiers move to two-way amplifiers Single cable shared by many houses splitting long cables and adding more fiber nodes Hybrid Fiber Coax (HFC) System 43 Internet over Cable (2) The fixed telephone system

23 Spectrum Allocation Internet over Cable (5-42MHz for upstream, MHz for downstream) asymmetry Analog modulation used downstream: 6/8MHz channel modulated with QAM-64/QAM-256; 6MHz/QAM-64 36Mbps, net = 27Mbps; net w/ QAM-256 = 39Mbps. upstream: QPSK because too much noise from terrestrial microwaves, etc. 2bits/baud; asymmetry larger than what bandwidths alone would suggest! Headend also upgraded Frequency allocation in a typical cable TV system used for Internet access (North America) 45 Cable Modems Data Over Cable Service Interface Specification (DOCSIS) Typical details of the upstream and downstream channels in North America

24 ADSL vs. Cable Both always on Both fiber in the backbone, but differ in the edge ADSL uses twisted pair Cable uses coax Effective capacity ADSL: specific numbers e.g., 1Mbps downstream, 256Kbps upstream (but consistently only 80%) Cable: unpredictable (since dependent on the number of other active users on the user s cable segment) Increase in users ADSL: unaffected Cable: either per-user performance drops, or incurs additional cost (for splitting cables and adding fiber nodes) Distance DSL performance dependent on distance to the end office Cable performance unaffected by increased distance to the fiber node or headend Other factors: Availability ADSL point-to-point, so more secure than cable ADSL (telephone system) more reliable generally 47 Wireless Local Loops Architecture of an LMDS system