Understanding Wireless Networks: Challenges, Capacity, and Differences from Wired Networks, Summaries of Computer science

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CSCE 5520 Wireless Networks and Protocols Wireless Challenges O utlin e • Challenges in Wireless Networking • RF introduction » A cartoon view » Communication » Time versus frequency view • Modulation and multiplexing • Channel capacity • Antennas and signal propagation • Modulation • Diversity and coding • OFDM Wireless is a shared medium • In wired communication, signals are contained in a conductor » Copper or fiber » Guides energy to destination » Protects signal from external signals • Wireless communication uses broadcasting over the shared ether » Energy is distributed in space » Signal must compete with many other signals in same frequency band Bob Mary Attenuation and Errors • In wired networks error rate 10-10 or less » Wireless networks are far from that target • Signal attenuates with distance and is affected by noise and competing signals • Obstacles further attenuate the signal • Probability of a successful reception depends on the “signal to interference and noise ratio” - the SINR • More details later in the course Bob Mary How Do We Increase Network Capacity?  Easy to do in wired networks: simply add wires » Fiber is especially attractive  Adding wireless “links” increases interference. » Frequency reuse can help … subject to spatial limitations » Or use different frequencies … subject to frequency limitations  The capacity of the wireless network is fundamentally limited. Bob Mary Implications of Variability in Wireless PHY Layer • Wireless datalink protocols must optimize throughput across an unknown and dynamic transmission medium » Important to understand what causes the changes • Wireless “links” as observed by layers 3-7 will be unavoidably different from wired links » Variable bandwidth and latency » Intermittent connectivity » Must adapt to changes in connectivity and bandwidth • Understanding the physical layer is the key to making wireless work well » High level intuition is sufficient O utlin e • Challenges in Wireless Networking • RF introduction » A cartoon view » Communication » Time versus frequency view • Modulation and multiplexing • Channel capacity • Antennas and signal propagation • Modulation • Diversity and coding • OFDM From Signals to Packets “Digital” Signal Analog Signal Bit Stream Header/Body Header/Body Header/Body 0 0 1 0 1 1 1 0 0 0 1 Packet s Sender Receiver 01000101010111001010101010111011100000011110101011101010101011010110101 12 Packet Transmissi on C artoon V iew 2 – Rays of Energy 15 • Can also view it as a “ray” that propagates between two points » Rays can be reflected etc. » A channel can include multiple “rays” that take different paths – “multi-path” effect • Implications for wireless networks » We can have provide connectivity without line of sight! » Receiver can receive multiple copies of the signal, which leads to signal distortion » Combined with mobility, it also leads to fast fading (Not So) C artoon V iew 3 – Electro-magnetic Signal 16 • Signal that propagates and changes over time with a certain frequency and has an amplitude and phase » Think: sine wave • Relevance to networking? » The sender can change the properties of the EM signal over time to convey information » Receivers can observe these changes and extract the information 18 Dealing With Bit Errors • Wireless vs. wired links  Wired: most loss is due to congestion  Wireless: higher, time-varying bit-error ate • Dealing with high bit-error rates  Sender could increase transmission power  Requires more energy (bad for battery-powered hosts)  Creates more interference with other senders  Stronger error detection and recovery  More powerful error detection codes  Link-layer retransmission of corrupted frames Sine Wave Parameters a) A = 1, f = 1 Hz, θ = 0; thus T = 1s b) Reduced peak amplitude; A=0.5 c) Increased frequency; f = 2, thus T = ½ d) Phase shift;  θ = ¼ radians (45 degrees) • note: 2 radians = 360° = 1 period Phase Phase is measurement of the sine wave, and it indicates where the wave is in its cycle. It is measured in degrees (0°- 360°) or radians (0-2π) and is denoted with the Greek symbol Phi (ϕ). Amplitude • Amplitude indicates how far the wave deviates from its position of rest. Key Idea of Wireless Communication • The sender sends an EM signal and changes its properties over time » Changes reflect a digital signal, e.g., binary or multi- valued signal » Can change amplitude, phase, frequency, or a combination • Receiver learns the digital signal by observing how the received signal changes » Note that signal is no longer a simple sine wave or even a periodic signal Outline • Challenges in Wireless Networking • RF introduction » A cartoon view » Communication » Time versus frequency view • Modulation and multiplexing • Channel capacity • Antennas and signal propagation • Modulation • Diversity and coding • OFDM C halleng e • Wireless network designers need more precise information about the performance of wireless “links” » Can the receiver always decode the signal? » How many Kbit, Mbit, Gbit per second? » Does the physical environment, distance, mobility, weather, season, the color of my shirt, etc. matter? • We need a more formal way of reasoning about wireless communication: Represent the signal in the frequency domain! Key Property of Periodic EM Signals • Any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases • The period of the total signal is equal to the period of the fundamental frequency » All other frequencies are an integer multiple of the fundamental frequency • There is a strong relationship between the “shape” of the signal in the time and frequency domain Coding Terminology —_- pie—y _J LI sv_ U Signal element: Pulse (of constant amplitude, frequency, phase) = Symbol U Modulation Rate: 1/Duration of the smallest element =Baud rate QO Data Rate: Bits per second Signal Modulation • Modulation: the process of encoding the information from a message source in a manner suitable for transmission • Amplitude modulation (AM): change the strength of the carrier based on information • High values -> stronger signal • Frequency (FM) and phase modulation (PM): change the frequency or phase of the signal • Frequency or Phase shift keying • Digital versions are also called “shift keying” • Amplitude (ASK), Frequency (FSK), Phase (PSK) Shift Keying Signal Modulation Q Quadrature Amplitude and Phase Modulation QO 4-QAM, 16-QAM, 64-QAM, 256-QAM Q Used in DSL and wireless networks Amplitude Q 01 Q ll Q @ @ 2 @ | I 0 1 00 10 @ @ Binary 4-QAM 16-QAM O 4-QAM=> 2 bits/symbol, 16-QAM =4 bits/symbol., ... Numeric example For a communication system, the 4 bits are encoded per symbol and four symbols are transmitted per second. Compute the bit rate of the system  Multiple bits are encoded per symbol to increase the bit rate By substituting the known values, we can find the bit rate of the system to be 16 bits/second. Channel Capacity • Channel capacity is a maximum information rate that a channel can transmit. It is measured in bits per second (bps). Channel capacity is a rough value as measuring takes into account only the whole amount of data transferred but leaves out of account communication quality. • Bandwidth can be considered as a subset of channel capacity term. When bandwidth is measured, the maximum volume of information that can be accurately transferred per unit of time is taken into account. For example, channel capacity may be very high, but low signal quality would make bandwidth low as well. Signal to Noise Ratio • Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission • Typically measured at a receiver • Signal-to-noise ratio (SNR, or S/N) • SNR is a essential parameter that shows signal quality • high SNR means a high-quality signal, low number of required intermediate repeaters • A low SNR means low quality signal, may require further signal processing to recover original signal Shannon’s Theorem Q Bandwidth = B Hz Signal-to-noise ratio = S/N Q Maximum number of bits/sec = B log, (1+S/N) QO Example: Phone wire bandwidth = 3100 Hz S/N = 30 dB 10 Log ,, S/N = 30 Log j) S/N =3 S/N = 10° = 1000 Capacity = 3100 log , (1+1000) = 30,894 bps Amplitude Carrier Modulation NTN > ULL th Lil, Signal Carrier Modulated yoeduenc Carrier Multiplexing Techniques • Frequency-division multiplexing (FDM) » divide the capacity in the frequency domain • Time-division multiplexing (TDM) » Divide the capacity in the time domain » Fixed or variable length time slices Multiple Users Can Share the Ether Different users use Different carrier frequencies Frequency Frequency versus Time-division Multiplexing  With frequency-division multiplexing different users use different parts of the frequency spectrum. » I.e. each user can send all the time at reduced rate » Example: roommates » Hardware is slightly more expensive and is less efficient use of spectrum  With time-division multiplexing different users send at different times. » I.e. each user can sent at full speed some of the time » Example: a time-share condo » Drawback is that there is some transition time between slots; becomes more of an issue with longer propagation times  The two solutions can be combined. Fr eq ue nc y Slot Frame Time Frequency Bands