Omer Gurewitz. Omer Gurewitz. Exam 70% Home assignments 30% Prerequisites:

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1 Wireless Networks Department of Communication Systems Engineering at Ben Gurion University Building 37, Room 513 Office hours: Wednesday 11:00-12:00 Content The objective is to understand the latest wireless network design challenges, protocols, and proposed algorithms Cover various topics in the area of wireless and mobile networking Mostly focus on functionalities that are included in the layers above the physical layer Prerequisites: Basic knowledge of computer networks, algorithm design, and probability is expected Topics Wireless communications basics (brief review) MAC protocols (e.g. TDMA, Aloha, CSMA/CA) Recent standards (e.g. IEEE , Bluetooth/IEEE , IEEE ) Wireless networking concepts (ad hoc networks, wireless mesh networks, sensor networks, vehicular networks, etc.) Routing protocols Transport protocols (TCP over wireless, flow and congestion control in wireless) Fairness Cross layer design (joint routing, scheduling, channel allocation, etc.) Advanced topics in wireless communication Reading Material Grading Guidelines Unfortunately, no book exactly covers the course material Recommended textbooks Charles E. Perkins, Ad hoc Networking, Addison Wesley, 2000 Jochen H. Schiller, Mobile Communications, Addison Wesley, 2005 Holger Karl and Andreas Willig, Protocols and Architectures for Wireless Sensor Networks, Wiley, 2005 Exam 70% Home assignments 30% Selected publications Mostly from ACM Mobicom, ACM Mobihoc, IEEE INFOCM, IEEE Trans. Networking, IEEE Trans. Mobile Computing, IEEE Wireless Communications, IEEE Communications 1

2 Some Ground Rules Let s make this educational and enjoyable. Please turn off your cell phone If you must leave the session early, please do so as discretely as possible Please avoid side conversation Feel free to ask or comment Listen to other people s questions and comments Let s begin The Internet The Internet is a worldwide, publicly accessible series of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). If this is new to you, you are probably in the wrong place Internet Protocol Stack Application: supporting network applications FTP, SMTP, HTTP Transport: data transfer between processes TCP, UDP Network: routing of datagrams from source to destination IP, routing protocols Link: data transfer between neighboring network elements Ethernet, WiFi Physical: bits on the wire Coaxial cable, optical fibers, radios application transport network link physical The Layered Reference Model What is New? Application Transport Network Network Network Application Transport Network Data Link Physical Data Link Physical Data Link Physical Data Link Physical Radio Medium Often we need to implement a function across multiple layers. Mobile computers are one of the fastest growing segments of the PC market High-speed wireless LAN products have become easily and cheaply available Low-speed wide area wireless network services are available and spreading quickly Medium-speed metropolitan area wireless network services becoming available and growing The Internet is truly ubiquitous, now with over 489 million total nodes (July 2007) and still growing rapidly 2

3 Growth of the Internet Mobile Computers May change point of network connection frequently May be in use as point of network connection changes Generally less powerful CPU than comparable stationary computers Generally less memory and disk than comparable stationary computers Generally less secure physically than comparable stationary computers Often operate on limited battery power Wireless Networks Generally lower bandwidth than common wired networks Generally higher latency and variability than common wired networks Higher error rates than common wired networks May have higher usage costs than common wired networks Very different transmission propagation characteristics than wired networks More susceptible to interference More susceptible to eavesdropping Mobile and Wireless Networking Existing network protocols are based on assumptions of: stationary computers and wired networks Many existing protocols don t work or work poorly, since: mobile computers don t act like stationary computers, and wireless networks don t act like wired networks These changes open many new areas of research and development Wireless networking standards Wireless Networking Technologies IEEE (Wi-Fi) Bluetooth (IEEE ) WiMedia application transport network Link (MAC) physical SHORT < RANGE > LONG b a/g/n ZigBee Bluetooth WiMedia Wireless Metropolitan Area Network Wireless Local Area Network Wireless Personal Area Network LOW < DATA RATE > HIGH 3

4 Medium Access Control (MAC) Nodes are scattered in a geographic area Need to somehow coordinate the access to channel Transmission time, power, rate, etc. Centralized Managed by an Access Point/Base Station Distributed Guaranteed Random Access (Aloha, CSMA, Ethernet) Requirements Throughput, Quality of Service, Fairness, Energy efficiency IEEE / WiFi Set of standards for Wireless Local Area Networks (WLANs) Define Medium Access Control (MAC) Physical layer Most common g Maximum data rate: 54Mb/s Frequency band: 2.4 Ghz Other variations a,b,e,n Different bands, physical layers, data rates, QoS, etc. Ad Hoc and Infrastructure Modes Ad Hoc Mode The Stations communicate with one another Not connected to a larger network Multihop Network No predefined topology (depends on transmission powers) No Links and Neighbors Infrastructure mode An Access Point connects Stations to a wired network Overlapping Access Points connected to each other Allows Stations to roam between Access Points Wireless networks and applications Mobile Ad-Hoc Networks (MANETs) The standards define the MAC and PHY Several networks have been defined on top of these layers application transport network Link (MAC) physical Tens to hundreds of mobile nodes (PDAs, Laptops, etc.) No need for fixed infrastructure Ease of deployment Main applications: Military, disaster recovery, and public safety Civilian applications: Vehicular Ad Hoc Networks (VANETs) Organization, routing, etc. are done to optimize for QoS (high throughput/low delay) Depleted batteries will be replaced/recharged when needed 4

5 Wireless Sensor Networks (WSNs) Hundreds to thousands of nodes Generally stationary (random locations) Applications: Military, weather/earthquake/fire monitoring, agriculture, building control, Mars exploration Energy conservation is critical Depleted batteries will not be replaced the sensors become useless Energy efficient algorithms that will maximize the lifetime of the application Wireless Local Area Networks (WLANs) Internet Access Point WLANs connect local computers (100m range) Breaks data into packets Channel access is shared (random access) Backbone Internet provides best-effort service Poor performance in some apps (e.g. video) Wireless Mesh Networks (WMNs) Solution for last-mile access Mesh routers Form the backbone Rarely mobile No energy constraints GW Multiple interfaces operating in orthogonal channels Multi-radio Multi-channel Wireless Mesh Networks Several networks are currently deployed Cambridge, Las Vegas, Taipei, Standardization efforts - IEEE s, IEEE , IEEE Design Considerations Different terminologies Common characteristics The classical networking models and protocols do not apply to wireless networks Interference Energy constraints Mobility Frequent topology changes No central authority New Protocols Physical Properties of Wireless Wireless = Waves Makes wireless network different from wired networks Should be taken into account by all layers OSI Model Electromagnetic radiation Sinusoidal wave with a frequency/wavelength Emitted by sinusoidal current running through a wire (transmitting antenna) Induces current in receiving antenna L 2-3 L 1 5

6 Frequency vs. Wavelength All electromagnetic energy (light, radio) travels at the same speed: c = 299,792 km/sec ( meters/sec) Obey it it s the law! In one frequency period, distance traveled is the wavelength: λ = c/f Examples: 1 MHz 300 meters 54 MHz (TV channel 2) 5.5 meters 88.1 MHz (beginning of FM dial) 3.4 meters 900 MHz ISM 30 cm 2.4 GHz ISM 12.5 cm 5.8 GHz ISM 5 cm 30 GHz 1 cm twisted pair 1 Mm 300 Hz Frequencies for Communication 10 km 30 khz coax cable 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 μm 3 THz optical transmission 1 μm 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Radio Licensing Controlled by international and national agreements International Telecommunication Union (ITU): A United Nations organization of about 170 nations Radio Communication Sector holds periodic World Radio Communication Conferences Federal Communications Commission (FCC): Independent agency in U.S. regulates non-federal use Decisions are public, involve periods of public comment Regulations in Title 47 of Code of Federal Regulations Example: 47 CFR 15 is FCC Part 15 regulations (ISM Bands) Public Use Bands Name 900 Mhz 2.4 Ghz 5 Ghz Range Bandwidth 26 Mhz 83.5 Mhz 200 Mhz Wavelength.33m / m / m / 2.4 National Telecommunications & Information Admin. (NTIA): Delegated authority by President to coordinate federal radio use Not all of its decisions are public Frequencies and Regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Cellular Phones Cordless Phones Wireless LANs Others Europe USA Japan GSM , AMPS, TDMA, CDMA PDC 486/ , , , 496, / , 960, TDMA, CDMA, GSM , / , UMTS (FDD) , UMTS (TDD) , CT , PACS , PHS CT2 PACS -UB JCT DECT IEEE IEEE IEEE HIPERLAN , , RF -Control RF -Control RF -Control 27, 128, 418, 433, 315, , Free-space Path-loss Power of wireless transmission reduces with square of distance (due to surface area increase) Reduction also depends on wavelength Long wave length (low frequency) has less loss Short wave length (high frequency) has more loss P L 2 4πD = λ 6

7 Multi-path Propagation Radio waves reflect off different objects Allows radio waves to reach receiver behind something But also creates some fundamental problems for wireless communication Multipath Problems Delay spread: Signals along different paths arrive at different times One symbol (e.g., bit) may overlap with another Worse for higher bit rates Rayleigh fading: Each reflected signal may have a different phase Signal arrivals out of phase cancel each other out Movement creates large random signal strength changes Doppler shift: Movement causes shift in frequency (Doppler shift) Different signal arrivals have different Doppler shifts Can introduce random frequency modulation at receiver Other Path-loss Exponents Behavior of Real Radio Path-Loss Exponent Depends on environment: Free space 2 Urban area cellular 2.7 to 3.5 Shadowed urban cell 3 to 5 In building LOS 1.6 to 1.8 Obstructed by building 4 to 6 Obstructed by factories 2 to 3 The following assumptions are all false: The world is flat A radio s transmission area is circular All radios have equal range If I can hear you, you can hear me (symmetry) If I can hear you at all, I can hear you perfectly Signal strength is a simple function of distance Commonly used in simulation and analysis, though: Sometimes, the results are valid (for some purposes) And sometimes they re not Spectrum and Bandwidth: Shannon Channel Capacity Modulation The maximum number of bits that can be transmitted per second by a physical channel is: W log 2 (1 + where W is the frequency range of the channel, and S/N is the signal noise ratio, assuming Gaussian noise N S ) Objective translate digital data into analog signals (with limited usage of spectrum) Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) 7

8 Digital Modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): Frequency Shift Keying (FSK): t t Phase Shift Keying: BPSK BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK 1 Properties low spectral efficiency robust, used e.g. in satellite systems Q 0 I f c : freq. of carrier R b =B b = 1/T b Phase Shift Keying (PSK): t Phase Shift Keying: QPSK QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK Properties needs less bandwidth compared to BPSK A Q I 01 t Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation it is possible to code n bits using one symbol 2 n discrete levels bit error rate increases with n Q a φ I Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, but different amplitude a and 1000 have same amplitude but different phase Spread Spectrum Technology Spread Spectrum In conventional modulation methods, the overall band-width of modulated signal is minimized so that more channels can transmit simultaneously. Spread spectrum, on the other hand, spread the modulated signal to a wide-band signal. Spread spectrum was developed for military application for its anti-jamming capability. In a multiple access system, spread spectrum techniques, in theory, allows more users to share the same band-width in the same region due to its tolerance to jamming characteristics. 8

9 Benefits of Spread Spectrum Do not interfere with other signals (spread or narrow) Are immune to interference generated by other signals (spread or narrow) present in the frequency band Hard to intercept (CODE) Can be physically co-located License free operation (CEPT TR-1001) Resistance to jamming Narrow Band Spread Spectrum Spread Spectrum Radio A means of sharing access to a radio channel: Avoids interference with other signals Particularly important on ISM bands Originally developed by military to prevent eavesdropping, detection, and jamming Two different types of spread spectrum technology: Frequency-Hopping Spread Spectrum (FHSS) Direct-Sequence Spread Spectrum (DSSS) Frequency-Hopping Spread Spectrum Split total bandwidth into a set of smaller channels Stay on one channel a while, then hop channels Hopping sequence must use all or most channels equally Receiver must be synchronized hopping same as sender Direct-Sequence Spread Spectrum Define an n-bit chipping sequence (known as chips ) Also called a Pseudorandom Noise (PN) sequence For each data bit, send PN sequence or its compliment Chips are sent at n times the data bit rate Receiving DSSS Direct Sequence Spread Spectrum (DSSS) Decode each bit by correlation with known PN sequence: Initialize a counter to 0 For each chip, increment counter if match, decrement if not When done, positive means bit is 1, negative means bit is 0 9

10 FCC Limits on FHSS and DSSS Frequency-hopping spread spectrum: In 900 MHz ISM band: Channel width 500 KHz Time on any channel 400 msec every 20 seconds Number of channels 50 (out of 52 at 500 KHz each) In 2.4 GHz ISM band: Channel width 1 MHz Time on any channel 400 msec every 30 seconds Number of channels 75 (out of 83 at 1 MHz each) Direct-sequence spread spectrum: PN sequence length 10 chips FHSS and DSSS Comparison Frequency-hopping spread spectrum advantages: Simpler to implement Generally uses less power Can hop around narrowband noise Can add overlapping capacity with orthogonal hopping sequences (only partially, under current FCC ISM rules) Direct-sequence spread spectrum advantages: More robust to some kinds of noise and interference Resistant to Rayleigh fading and other multipath effects More secure signal, harder to detect Under current FCC rules, can easily get higher throughput in the same spectrum (e.g., about 2 Mbps vs. about 1 Mbps) Multiplexing A subdivision of the spectrum available for transmission: Can be physical or logical channel Can multiplex multiple transmissions in same spectrum Frequency Division Multiple Access Shift each transmission up to a separate frequency Add all transmissions together and transmit all at once Three basic ways to form a channel: Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) (a form of spread spectrum) Frequency Chan D Chan C Chan B Chan A Time Time Division Multiple Access Divide total time up into frames (repeating) Divide time in each frame up into slots for each transmission Each transmission gets same slot out of each frame Code Division Multiple Access (CDMA) DSSS in which each transmitter uses a different PN sequence PN sequence is known as a code: Codes in use must be approximately orthogonal Other signals just look like noise to your signal Multiple overlapping signals can be decoded by receiver Frequency Chan B No hard limit to number of overlapping channels possible Chan A Example: Qualcomm IS-95/cdmaOne/CDMA2000 digital cellular system (e.g., used by Verizon Wireless and Sprint) Time 10

11 Code Division Multiple Access (CDMA) Power Control for CDMA Code Power control of senders in CDMA is critical: Receiver (base station) regulates sender transmit power All senders must have equal signal strength at the receiver: Senders close to the receiver Senders far from the receiver (inverse square law) Otherwise, noise from near sender would block far sender This is an example of the near-far problem for DSSS Power control happily also saves battery power in senders Main Points The wireless vision encompasses many exciting systems and applications Technical challenges transcend across all layers of the system design Wireless systems today have limited performance and interoperability Standards and spectral allocation heavily impact the evolution of wireless technology 11