Wireless Networks Ivan Marsic Rutgers University 1 ISO

Wireless Networks Ivan Marsic Rutgers University 1

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 Wi. Fi • IEEE 802. 3 Ethernet • PPP (modems, T 1) 1: Physical MAC • Radio spectrum • Infrared • Fiber • Copper 2

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

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

Waves 5

1 KHz 1 MHz LF 30 KHz 1 GHz (AM radio) MF 300 KHz UV Visible Infrared EM Spectrum Allocation 1 THz X rays 1 PHz 1 EHz (SW radio) (FM radio - TV) (TV – Cell. ) HF VHF UHF 3 MHz 300 MHz ISM 902 MHz Gamma rays 928 MHz Cordless phones Baby monitors (old) Wireless LANs 2. 4 GHz 2. 4835 GHz IEEE 802. 11 b, g Bluetooth Microwave ovens Freq. SHF 30 GHz Freq. 5. 785 GHz Freq. 3 GHz UNII 5. 725 GHz IEEE 802. 11 a Hiper. LAN II 6

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

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

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

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

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

Modulation 12

Modulation—PSK 01 010 011 135 90 10 90 270 0 00 11 0010 0011 0100 0001 1000 1101 1010 1111 000 225 270 315 101 (a) 0110 45 180 0111 (b) (c) 110 00 01 135 45 225 10 315 11 13

Amplitude Example 2. 1 3 bits 110 100 010 Time 14

Gaussian r. v. and Q-function 15

Effect of Noise on Signal 16

Probability of Error for 2, 4 -PSK 17

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

Signals as Vectors s(t) Example 3 -bit message: 5 V 1 0 1 t T A three-bit signal waveform (a) p 1(t) ( 1, 1, 1) (1, 1, 1) p 2(t) s 1(t) ( 1, 1, 1) (0, 0, 0) ( 1, 1, 1) p 3(t) s 2(t) 0 T 2 T 3 T (b) s 3(t) Orthogonal function set (Basis vectors) (c) (1, 1, 1) (d) 19

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

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

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

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

Wave Interactions (a) (b) 24

Interference & Doppler Effect (a) (b) 25

Multipath Fading (1) 26

Plane-Earth Model (a) (b) 27

Delay Spread (a) (b) 28

Discrete-time Delay Model 29

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 30

Error Probabilities 31

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

Coordination Problem 33

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

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” ) 35

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

Deterministic Schemes Static multiaccess schemes: TDMA and FDMA 37

Poisson Arrivals Model 38

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

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

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

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

ALOHA and Slotted ALOHA State Diagram 43

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 Time Slots (m = ) i 1 i 2 44

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

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

ALOHA System Model (2) 47

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=GP 0, where P 0=prob. packet avoids collision • No collision => no other packets in the same slot: 48

Efficiency of ALOHA’s S-ALOHA: In equilibrium, arrival rate = departure rate: = Ge G Max departure rate (throughput) = 1/e 0. 368 @ G 49 = 1

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=GP 0=Ge 2 G • Max throughput 1/2 e 0. 184 @ G = 0. 5 • Less efficient than S-ALOHA, but simpler, no global time synchronization i 50

Markov chain for S-ALOHA 51

Instability of Slotted ALOHA 52

Carrier Sensing (CSMA) • Listen before talk (unlike ALOHA, where talk when you need to) 53

CSMA/CD 1. Wait until the channel is idle. 2. When the channel is idle, transmit immediately and listen while transmitting. 3. In case of a collision, stop the packet transmission, and then wait for a random delay and go to step 1. IEEE 802. 3 (Ethernet) 54

Basic CSMA Protocols CSMA Protocol Transmission Rules Nonpersistent If medium is idle, transmit. If medium is busy, wait random amount of time and sense channel again. 1 -persistent If medium is idle, transmit. If medium is busy, continue sensing until channel is idle; then transmit immediately. p-persistent If medium is idle, transmit with probability p. If medium is busy, continue sensing until channel is idle; then transmit with probability p. 55

CSMA Protocols State Diagram 56

Nonpersistent CSMA 57

Efficiency of CSMA protocols (a) (b) 58

Delay vs. Arrival Rate 59

Hidden and Exposed Terminals Range of A’s transmissions Hidden Terminal Range of B’s transmissions Exposed Terminal 60

CSMA/Basic Atomic Exchange 61

CSMA/MACA Atomic Exchange Busy Sender Idle contention period IFS (2 ) Time RTS Data Packet IFS CTS Busy Receiver Vulnerable period = Busy Covered Station Vulnerable period = RTS + IFS + Access to medium deferred Busy Hidden Station Access to medium deferred 62

RTS/CTS Exchange (1) 63

RTS/CTS Exchange (2) 64

RTS/CTS Exchange (3) 65

Components of 802. 11 LANs Ad hoc network does not have distribution system nor access point 66

IBSS and Infrastructure BSS Independent BSS (IBSS) Infrastructure BSS 67

Extended Service Set (ESS) 68

802. 11 Network Services Service Provider Description Distribution Service used by stations to exchange MAC frames when the frame must traverse the DS to get from a station in one BSS to a station in another BSS. Integration Distribution Frame delivery to an IEEE 802 LAN outside the wireless network. Association Distribution Used to establish a logical connection between a mobile station and an AP. This connection is necessary in order for the DS to know where and how to deliver data to the mobile station. Reassociation Distribution Enables an established association to be transferred from one AP to another, allowing a mobile station to move from one BSS to another. Disassociation Distribution Removes the wireless station from the network. Authentication Station Establishes identity prior to establishing association. Deauthentication Station Used to terminate authentication, and by extension, association. Privacy Station Provides protection against eavesdropping. MSDU delivery Station Delivers data to the recipient. 69

States and Services 70

802. 11 Interframe Spacings 71

Basic 802. 11 Transmission Mode 72

802. 11 Protocol State Diagram – Sender 73

802. 11 Protocol State Diagram – Receiver 74

Example: Infra BSS Assume Station A has a single packet to transmit to B 75

Timing Diagrams Timing of successful frame transmissions under the DCF. Frame retransmission due to ACK failure. Frame retransmission due to an erroneous data frame reception. 76

Backoff Mechanism The backoff mechanism of 802. 11 MAC. The Frame* transmission time includes the RTS/CTS exchange and the MAC layer ACK. CP: Contention period. 77

RTS/CTS Transmission Mode 78

802. 11 MAC Frame Format 79

802. 11 Performance Analysis 80

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Transmission Example 82

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802. 11 Protocol Architecture 802. 11 MAC 802. 11 FHSS 802. 11 DSSS 802. 11 a OFDM 802. 11 b DSSS 84

802. 11 PHY Frame Using DSSS 85

IEEE 802. 11 b BER vs. SNR 86

IEEE 802. 11 b Throughput vs. SNR 87

W-LAN Transmission Rates 11 Mbps 8 % of coverage area 1 Mbps 47 % of coverage area Lucent ORi. NICO 802. 11 b outdoors, no obstruction—ideal conditions! Low probability of having good link!! 88

Asymmetry 89

Receiver-Based Autorate MAC Protocol 90

IEEE 802. 11 b Channels NOTE: The 12 channels in 802. 11 a do NOT overlap 91

Power Conservation 92

Comparison of 802. 11’s Standard 802. 11 a Number of channels Interference Bandwidth 802. 11 b 802. 11 g Power consumption Range/penetration Upgrade/compatibility Price http: //www. nwfusion. com/techinsider/2002/0520 wlan/0520 feat 1. html 93

Route Discovery in DSR (1) 94

Route Discovery in DSR (2) Broadcast Tx Represents a node that has received RREQ for H from C 95

Route Discovery in DSR (3) 96

Route Discovery in DSR (4) 97

Route Discovery in DSR (5) 98

Route Discovery in AODV (1) 99

Multihop Throughput Challenge: more hops, less throughput Links in route share radio spectrum Extra hops reduce throughput Throughput = 1/2 Throughput = 1/3 100

Cellular Hierarchy 101

Hybrid Wireless Networks Infrastructure + MANET (a) (b) (c) 102

Community Mesh Network 103