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Wireless Networks

Wireless Networks. Ivan Marsic Rutgers University. ISO OSI Protocol Stack. Protocol at layer i doesn’t know about protocols at i  1 and i  1. Application. 4: Transport. Reliable (TCP) Unreliable (UDP). 3: Network. End-to-end (IP) Routing Address resolution. 2: Link.

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Wireless Networks

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  1. Wireless Networks Ivan Marsic Rutgers University

  2. ISO OSI Protocol Stack • Protocol at layer i doesn’t know about protocols at i1 and i1 Application 4: Transport • Reliable (TCP) • Unreliable (UDP) 3: Network • End-to-end (IP) • Routing • Address resolution 2: Link • IEEE 802.11 WiFi • IEEE 802.3 Ethernet • PPP (modems, T1) MAC 1: Physical • Radio spectrum • Infrared • Fiber • Copper

  3. Infrastructure vs. Ad Hoc (1) (a) (b)

  4. Infrastructure vs. Ad Hoc (2) (c) (d)

  5. Waves

  6. Infrared Gamma rays Visible UV X rays Freq. 1 KHz 1 MHz 1 GHz 1 THz 1 PHz 1 EHz (AM radio) MF (SW radio) HF (FM radio - TV) VHF (TV – Cell.) UHF LF SHF Freq. 30 KHz 300 KHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz ISM UNII Freq. 902 MHz 928 MHz 2.4 GHz 2.4835 GHz 5.725 GHz 5.785 GHz Cordless phones Baby monitors (old) Wireless LANs IEEE 802.11b, g Bluetooth Microwave ovens IEEE 802.11a HiperLAN II EM Spectrum Allocation

  7. Information Source Transmitter Receiver Destination Communication Channel Noise Source Source Data: 0 1 0 0 1 1 0 0 1 0 Input Signal: S Source Data: 0 1 0 0 1 1 0 0 1 0 Noise: N S+N Output Signal: Data Received: 1 1 0 0 1 1 1 0 1 0 Decision threshold Sampling Times: Bits in error Communication Process

  8. Link, channel, repeater, or node Power in Power out Decibel definition

  9. Three dots running at the same speed around the circle in different lanes: Resultant Displacement Time 60 90 30 180 0 60 120 240 300 360 30 90 150 210 270 330 Angular phase of fundamental wave Fourier Series Approximation

  10. Phase space:  / 2 Period (T) Frequency  = 2 / T A Amplitude  [rad / s] A  0 2  Time (t) Phase () A  sin (  t  ) 3 / 2 Phase Space

  11. Communication Channel Modulator Demodulator Transmitter Receiver Error Control Encoder Error Control Decoder Source Encoder (Compress) Source Decoder (Decompress) Information Source Destination Wireless Transmission and Receiving System

  12. Modulation

  13. 01 010 0111 0110 0010 0011 011 001 90 135 0101 0100 0000 0001 90 45 180 180 10 00 100 000 0 225 270 1001 1000 1100 1101 315 270 101 111 1011 1010 1110 1111 (a) (b) (c) 11 110 00 01 135 45 225 315 10 11 Modulation—PSK

  14. 3 bits 110 3 bits 100 3 bits 010 Amplitude Time Example 2.1

  15. Gaussian r.v. and Q-function

  16. Effect of Noise on Signal

  17. Probability of Error for2,4-PSK

  18. Source Data: 0 1 0 0 1 1 0 0 1 0 Input Signal: Noise: Output Signal: Discrete vs. Continuous Channel (a) (b) (c)

  19. Example 3-bit message: 1 0 1 s(t) 5 V t T A three-bit signal waveform p1(t) (1,1,1) (1,1,1) (1,1,1) (1,1,1) p2(t) s1(t) (0,0,0) (1,1,1) p3(t) s2(t) (1,1,1) (1,1,1) 0 T 2T 3T s3(t) Signals as Vectors (a) (b) (d) Orthogonal function set (Basis vectors) (c)

  20. Geometric Representation (a) (b) (c)

  21. N ST SR Signal Space (a) (b)

  22. SR = ST + N O' Noise sphere, radius , centered on ST N h ST O Signal sphere, radius Locus of Error-Causing Signals (a) (b)

  23. Error message 1,1 1,1 Valid message Error message 1,1 1,1 Valid message Error Detection and Correction (a) (b)

  24. Wave Interactions (a) (b)

  25. Interference & Doppler Effect (a) (b)

  26. Multipath Fading (1)

  27. Plane-Earth Model (a) (b)

  28. Delay Spread (a) (b)

  29. Discrete-time Delay Model

  30. Multipath Fading (2) Delay spread (2 components) Flat Fading Direct path (1 component) Doppler spread (2 components) Fast Fading Delay spread (2 components) Frequency Selective Fading

  31. Error Probabilities

  32. Medium Access Control (MAC) • Controls who gets to transmit when • Avoids “collisions” of packet transmissions

  33. Coordination Problem

  34. Collisions Receiver electronics detects collision Receiver broadcasts info about collision (jam)  = total time to detect collision = RTT of the most distant station

  35. Channel State Assumption: There is always at least one station in need of transmission Objective: Maximize the fraction of time for the “Successful transmission” state ( or: minimize the duration of “Idle” and “Collision” )

  36. MUX Receiver Ordering of packets on higher capacity link Ordering of packets on shared medium Multiaccess vs. Multiplexing

  37. Deterministic Schemes Static multiaccess schemes: TDMA and FDMA

  38. Poisson Arrivals Model

  39. txmit txmit Transmitter t t Receiver   < 1  » 1  Parameter  • Ratio of propagation delay vs. packet transmission time Propagation constant  :

  40. Current packet Collides with the head of the current packet Collides with the tail of the current packet tstart Vulnerable period Time tstart  txmit tstart  txmit Vulnerable Period • Packet will not suffer collision if no other packets are sent within one packet time of its start

  41. Packet Arrivals 1 2 3 4 5 6 7 Time ALOHA Departures 1 2 3 4 5 6 7 Packet Arrivals 1 2 3 4 5 6 7 Time Slotted ALOHA Departures 1 2 3 4 5 ALOHA Protocols

  42. Arrivals at Station 1 Time Time Departures Slot 1 Slot 2 Slot 3 Slot 4 Packet Time 1 Receiver k Arrivals at Station k Time Time Departures Slot 1 Slot 2 Slot 3 Slot 4 (a) – Slotted ALOHA (b) – Pure ALOHA Transmission Success Rate

  43. ALOHA and Slotted ALOHA State Diagram

  44. Time Slots i 1 i i 1 i  2 Analysis of Slotted ALOHA (1) ASSUMPTIONS FOR ANALYSIS: • All packets require 1 slot for x-mit • Poisson arrivals, arrival rate  • Collision or perfect reception (no errors) • Immediate feedback (0, 1, e) • Retransmission of collisions (backlogged stations) • No buffering or infinite set of stations(m = )

  45. Backlogged Stations • “Fresh” stations transmit new packets • “Backlogged” stations re-transmit collided packets

  46. ALOHA System Model (1) • In equilibrium state, system input equals system output  = S = GeG

  47. ALOHA System Model (2)

  48. Analysis of Slotted ALOHA (2) • 0 <  < 1, since at most 1 packet / slot • Equilibrium: departure rate = arrival rate • Backlogged stations transmit randomly • Retransmissions + new transmissions:Poisson process with parameter G >  • The probability of successful x-mit: S=GP0,where P0=prob. packet avoids collision • No collision => no other packets in the same slot:

  49. Efficiency of ALOHA’s S-ALOHA: In equilibrium, arrival rate = departure rate:  = GeG Max departure rate (throughput) = 1/e 0.368 @ G = 1

  50. i Unslotted (Pure) ALOHA • Assume: all packets same size, but no fixed slots • The packet suffers no collision if no other packet is sent within 2 packets long: S=GP0=Ge2G • Max throughput 1/2e 0.184 @ G = 0.5 • Less efficient than S-ALOHA, but simpler, no global time synchronization

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