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Wireless Challenges and Signals Outline 18-452/18-750 • Goals and structure of the course Wireless Networks and Applications • Administrative stuff Lecture 2: Wireless Challenges • A bit of history • Wireless technologies • Building a network Peter Steenkiste » Designing a BIG system Carnegie Mellon University » The OSI model » Packet-based communication » Challenges in Wireless Networking Spring Semester 2020 http://www.cs.cmu.edu/~prs/wirelessS20/ Peter A. Steenkiste, CMU 1 Peter A. Steenkiste, CMU 2 Why Use Wireless? What is Hard about Wireless? There are no wires! There are no wires! Has several significant advantages: • In wired networks links are constant, reliable and • Supports mobile users physically isolated » Move around office, campus, city, … - users get hooked » A 1 Gps Ethernet always has the same properties » Remote control devices (TV, garage door, ..) » Not true for “54 Mbs” 802.11a and definitely not for “6 Gbs” 802.11ac » Cordless phones, cell phones, .. » WiFi, GPRS, Bluetooth, … • In wireless networks links are variable, error- prone and share the ether with each other and • No need to install and maintain wires other external, uncontrolled sources » Reduces cost – important in offices, hotels, … » Link properties can be extremely dynamic » Simplifies deployment – important in homes, hotspots, … » For mobile devices they also differ across locations Peter A. Steenkiste, CMU 3 Peter A. Steenkiste, CMU 4 Page 1 Wireless is a shared medium Attenuation and Errors Bob Mary • In wired communication, Bob Mary signals are contained in a conductor • In wired networks error rate 10-10 or less » Copper or fiber » Wireless networks are far from that target » Guides energy to destination » Protects signal from external • Signal attenuates with distance and is signals affected by noise and competing signals • Wireless communication • Obstacles further attenuate the signal uses broadcasting over • Probability of a successful reception depends the shared ether on the “signal to interference and noise ratio” » Energy is distributed in space - the SINR » Signal must compete with many other signals in same frequency band • More details later in the course Peter A. Steenkiste, CMU 5 Peter A. Steenkiste, CMU 6 How Do We Increase Mobility Affects the Network Capacity? Link Throughput Bob Bob Mary Quality of the Easy to do in wired networks: simply add wires transmission depends » Fiber is especially attractive on distance and obstacles blocking the Adding wireless “links” “line of sight” (LOS) increases interference. » “Slow fading” – the signal strength changes slowly » Frequency reuse can help … subject to spatial limitations Reflections off » Or use different frequencies … subject to frequency limitations obstacles combined with mobility can cause The capacity of the wireless “fast fading” Mary network is fundamentally » Very rapid changes in the signal limited. » More on this later Hard to predict signal! Peter A. Steenkiste, CMU 7 Peter A. Steenkiste, CMU 8 Page 2 Implications of Variability How is Wireless Different? in Wireless PHY Layer Wired Wireless • Wireless datalink protocols must optimize • Physical link properties • Physical link properties throughput across an unknown and dynamic are fixed and specified in can change rapidly in transmission medium standards unpredictable ways » Important to understand what causes the changes • Designed for low error • Error rates vary a lot • Wireless “links” as observed by layers 3-7 rates and throughput is and throughput is very will be unavoidably different from wired links fixed and known dynamic » Variable bandwidth and latency • Datalink layer is simple • How do you design an » Intermittent connectivity and optimized for the efficient datalink » Must adapt to changes in connectivity and bandwidth physical layer protocol? • Understanding the physical layer is the key to • Internet was designed • How well will higher making wireless work well assuming low error rates layer protocols work? » High level intuition is sufficient Peter A. Steenkiste, CMU 9 Peter A. Steenkiste, CMU 10 Outline From Signals to Packets Packet • RF introduction Transmission Sender Receiver » A cartoon view » Communication 0100010101011100101010101011101110000001111010101110101010101101011010111001 » Time versus frequency view Packets Header/Body Header/Body Header/Body • Modulation and multiplexing • Channel capacity Bit Stream 0 0 1 0 1 1 1 0 0 0 1 • Antennas and signal propagation • Modulation “Digital” Signal • Diversity and coding • OFDM Analog Signal Peter A. Steenkiste, CMU 11 Peter A. Steenkiste, CMU 12 Page 3 RF Introduction Wireless Spectrum in the US • RF = Radio Frequency » Electromagnetic signal that propagates through “ether” » Ranges 3 KHz .. 300 GHz » Or 100 km .. 0.1 cm (wavelength) • Travels at the speed of light • Can take both a time and a frequency view Peter A. Steenkiste, CMU 13 Peter A. Steenkiste, CMU 14 Cartoon View 1 – Cartoon View 2 – A Wave of Energy Rays of Energy • Think of it as energy that radiates from an • Can also view it as a “ray” that propagates antenna and is picked up by another antenna. between two points • Helps explain properties such as attenuation » Rays can be reflected etc. » Density of the energy reduces over time, distance » A channel can include multiple “rays” that take different paths – “multi-path” effect » Signal strength is reduced, error rates go up • Implications for wireless networks • Relevance to networking? » We can have provide connectivity without line of sight! » Error rates of “wireless” depend on distance » Receiver can receive multiple copies of the signal, which – Also depends on many properties leads to signal distortion » Notion spatial reuse of frequencies » Combined with mobility, it also leads to fast fading – Basis of cellular and WiFi infrastructures Peter A. Steenkiste, CMU 15 Peter A. Steenkiste, CMU 16 Page 4 (Not so) Cartoon View 3 – Time and Point View of Signal Electro-magnetic Signal • Signal that propagates and changes over • Can look at a point in space: signal will change time with a certain frequency and has an in time according to a sine function amplitude and phase » But transmitter can change phase, amplitude, frequency • Can take a snapshot in time: signal will “look” » Think: sine wave like a sine function in space • Relevance to networking? » Signal at different points are (rough) copies of each other » The sender can change the properties of the EM signal • Receiver can observe transmitter’s changes over time to convey information » Receivers can observe these changes and extract the Relevance to information Received Networking? Signal Time (point in space) Transmitter Peter A. Steenkiste, CMU 17 Peter A. Steenkiste, CMU Space (snapshot in time) 18 Communication Sine Wave Parameters Remember: Received • General sine wave Cartoon view Signal Time (point in space) » s(t ) = A sin(2ft + ) • Example on next slide shows the effect of varying each of the three parameters Transmitter Space (snapshot in time) a) A = 1, f = 1 Hz, = 0; thus T = 1s b) Reduced peak amplitude; A=0.5 • Sender changes signal in agree upon way and c) Increased frequency; f = 2, thus T = ½ receiver interprets the changes d) Phase shift; = /4 radians (45 degrees) » “Modulation” and “demodulation” • note: 2 radians = 360° = 1 period • Problem: the signal gets distorted on “channel” » Makes it harder for receiver to interpret changes Peter A. Steenkiste, CMU 19 Peter A. Steenkiste, CMU 20 Page 5 Key Idea of Wireless Space and Time View Revisited 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 “The wireless telegraph is not difficult to understand. The ordinary telegraph is like a very long cat. You pull the tail in New York, and it meows in Los Angeles. The wireless is exactly the same, only without the cat.” Peter A. Steenkiste, CMU 21 Peter A. Steenkiste, CMU 22 Outline Challenge • RF introduction • Cats, really? This is very informal! » A cartoon view » Sender “changes signal” and receiver “observes changes” » Communication • Wireless network designers need more precise » Time versus frequency view information about the performance of wireless • Modulation and multiplexing “links” • Channel capacity » Can the receiver always decode the signal? » How many Kbit, Mbit, Gbit per second? • Antennas and signal propagation » Does the physical environment, distance, mobility, weather, • Modulation season, the color of my shirt, etc. matter? • Diversity and coding • We need a more formal way of reasoning about wireless communication: • OFDM Represent the signal in the frequency domain! Peter A. Steenkiste, CMU 23 Peter A. Steenkiste, CMU 24 Page 6 Time Domain View: Key Parameters of Periodic versus Aperiodic Signals (Periodic) Signal • Periodic signal - analog or digital signal • Peak amplitude (A) - maximum value or strength of pattern that repeats over time the signal over time; typically measured in volts » s(t +T ) = s(t ) • Frequency (f ) – where T is the period of the signal » Rate, in cycles per second, or Hertz (Hz) at which the signal repeats » Allows us to take a frequency view – important to understand wireless challenges and solutions • Period (T ) - amount of time it takes for one repetition of the signal • Aperiodic signal - analog
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