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Transcript
Ad Hoc Network Protocols Project/Thesis Outline Paul J. Fong – October 2002 What is an Ad Hoc Network? IEEE Std 802.11: “A network composed solely of stations within mutual communications range of each other via the wireless medium (WM).” Hacker: Multi-hop self-configuring wireless network Mobile Ad Hoc Networks (MANETs) Military vehicles on a battlefield with no existing infrastructure. A fleet of ships at sea. Emergency workers at an earthquake that destroyed the infrastructure. A gathering of people with notebook computers in an area lacking 802.11. Scientific field networks. The Challenge “We’ll have infinite bandwidth in a decade’s time.” – Bill Gates, PC Mag, 11/11/94. “computer communications will break down into two domains - the fibersphere and the atmosphere.” - George Gilder, Forbes, 3/29/93. Timothy J. Shepard, MIT Thesis “A single-channel radio system can scale to millions or billions of stations within a metropolitan area… “..packets can be transferred to nearby neighbors without any loss due to collisions.” Objectives Discuss IEEE Standard 802.11(b) Analyze scalability of wireless networks Obtain perspective of industry Look at commercial wireless networks Examine wireless layer 3 protocols Origins: Hedy Lamarr UNITED STATES PATENT OFFICE 2,292,387 SECRET COMMUNICATION SYSTEM Hedy Kiesler Markey, Los Angeles, and George Antheil, Manhattan Beach, Cakif. Application June 10, 1941, Serial No. 397,412 Unlicensed Bands IEEE 802.11 – Transmission Modes Infrared: 0.85u or 0.95u, 1 or 2 Mbps Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS) 802.11a: Orthogonal Frequency Division Multiplexing (OFDM) – 54 Mbps/5 GHz 802.11b: High Rate Direct Sequence Spread Spectrum (HR-DSSS) – 11 Mbps/2.4 GHz 802.11g: OFDM with 54 Mbps/2.4 Ghz Basic & Extended Service Sets Roaming Reassociation Response Reassociation Request Roaming Independent Basic Service Set Peer-to-peer No servers Time Synchronization Function (TSF) based on latest time in a beacon No standards for relay 802.11b Channels 14 (22 MHz) frequency channels (11 in US) 3 non-overlapping channels 11 Mbps data rate Spreading Codes Codes used for spreading; not for multiple access Barker Sequence Complementary Code Keying Dynamic Rate Shifting Data Rate Code Modulation Symbol Rate Bits per (Mpbs) Length (MSps) Symbol 1 11 (Barker Sequence) Binary Phase Shift Keying 1 1 2 11 (Barker Sequence) Quadrature Phase Shift Keying 1 2 5.5 8 (Complementary Code Keying) Quadrature Phase Shift Keying 1.375 4 11 8 (Complementary Code Keying) Quadrature Phase Shift Keying 1.375 8 Data rate readjusts for noise and distance 802.11b Range FCC 1w power limit (4w EIRP for fixed) Omnidirectional: 150-300 feet Directional antenna: up to 25 miles Attenuated by walls & glass Hidden Station Problem A is transmitting to B C is out of range of A C transmits to B causing collision 802.11 attempts to solve this problem Exposed Station B is transmitting to A C wants to transmit to D C senses transmission & declines No 802.11 solution Channel Contention Distributed Coordination Function (DCF) uses CSMA/CA (2 modes) Physical Channel Sensing (~Ethernet) Virtual Channel Sensing (RTS/CTS) Point Coordination Function (PCF) Polling by base station Periodic beacon frames No collisions Virtual Channel Sensing: Request to Send Virtual Channel Sensing: Clear to Send Virtual Channel Sensing: Timing A B C Network Allocation Vector RTS DATA CTS D ACK Network Allocation Vector Time Interframe Spacing ACK SIFS PIFS DIFS EIFS Time Short Interframe Spacing PCF Interframe Spacing DCF Interface Spacing Extended Interframe Spacing Virtual Channel Sensing: Fragment Burst A B C D Network Allocation Vector (NAV) RTS Fragment 1 CTS Fragment 2 ACK NAV Time Fragment 2 ACK ACK Point Coordination Function (PCF) Base station polls clients in cell Beacon frames sent for synchronization No collisions within cell Allows guaranteed Quality of Service Scalability Analysis Physical Layer Data Link Layer Environment, Broadcast, Half-duplex Collisions, Bridging Loops Network Layer Scalability, Convergence, Aggregation, Routing Loops Transport Layer Reliability & Error recovery Upper Layers Policies & Services Pitfalls Broadcast medium -> interference Collisions -> retransmissions Loops -> reduced bandwidth Flooding -> broadcast storms Link flapping -> route recalculations Routing updates -> slow convergence Going Exponential “You don’t want to go exponential.” “If you are not careful with a distributed algorithm, you can go exponential in a hurry.” Radia Perlman, NetWorld InterOp, 9/25/1995. Data Link: Wired vs. Wireless Reliable links Few neighbors & links Specialized infrastructure Unreliable links Many neighbors & links Node/LAN/router are packaged Loops: Transparent Bridging A B D C E H F I J G K M L Spanning Tree Protocol (STP) Define spanning tree to each node Recalculate when topology changes Susceptible to link flapping Loops: Source Route Bridging A B D C E H F I J G K M L To find optimal A-M route, explorer packets flood network. Hops put in header: ADIM, ACHM, ACGM, ADHM, etc. 1st explorer to reach M is echoed back. Spanning Tree vs. Source Routing Computed once until network changes Plug and Play End nodes don’t participate Loop-free topology Compute as needed Link flapping Exponential flooding for each src/dest Configuration req’d End nodes must participate “Optimal” routes On demand Variable header size Network Layer: Route Propagation Routers share routing tables Access “routers” have limited functionality Routers execute distance-vector & link-state protocols Network Layer: DVP Scalability Distance Vector Protocols Periodic routing table updates sent to all neighbors Neighbors propagate routes Peer-to-peer Network Layer: LSP Scalability Link State Protocols Updates sent as needed to all routers Less susceptible to link flapping Area border routers Hierarchical segmentation Distance Vector vs. Link State Periodic updates Full distance vector sent to neighbors Slower convergence Peer-to-peer Triggered updates Changes sent to entire area Faster convergence Hierarchical Transport Layer Error recovery at layer 4 assumes reliable links Layer 2 error recovery needed for wireless networks Performance consequences Upper Layer Issues IP address administration Routing policy control Lack plug ‘n play Security IP Address Aggregation Discontiguous Network Very Discontiguous Network Routing Policy Control Routing policies can keep an AS from being a transit Can ad hoc networks enforce policy? Not self configuring Wireless Security Wired Equivalent Privacy (WEP) 40 or 120 bit encryption based on RC4 algorithm “War Driving” www.wardriving.com IEEE response: 802.11i will have better security 802.1x extends authentication servers Industry: Reliable Links No need for layer 2 error recovery (X.25, SNA) Error recovery takes place at layer 4 for performance Industry: Limited Hops 1 hop to wired access point is the norm Wireless bridging but no wireless routing Industry: 1-Layer LAN Segments Switches Buffered I/O Full Duplex I/O No collisions Limited broadcast No shared links Industry: Hierarchical WANs Hierarchical organization: core, distribution, access Each area contains approximately 50 routers Backbone areas Problems isolated to an area Industry: Centralization Centralized network management Centralization vs. Decentralization DNS & DHCP administration IP address administration Wireless Relay Networks: Rooftop Nokia Rooftop “Airhead” aggregation points within cells Hierarchical with a limited number of wireless hops. Wireless Relay Networks: Ricochet Metricom Ricochet 900 MHz spread spectrum: (1)modem – (2)poletop radio – (3)access point Fixed vs. Ad Hoc Reliable links Collision free Few neighbors Efficient infrastructure Address aggregation Centralized services Policy enforcement Unreliable links Contention Many neighbors Node/LAN/router are packaged No aggregation Distributed services No enforcement Desired Ad Hoc Routing Characteristics “Fish-Eye” view Distributed services Fast convergence Link-layer reliability Low overhead Ad Hoc Routing Protocols Geographical routing (GRID) Destination-Sequenced Distance Vector (DSDV) Temporally-Ordered Routing Algorithm (TORA) Dynamic Source Routing (DSR) Ad Hoc On-Demand Distance Vector (AODV) Geographical Routing Longitude & Latitude Grid Location Service Packet forwarded to neighbor nearest to destination Ad Hoc On-Demand Distance Vector (AODV) Related to BellmanFord algorithm On-demand route calculation No periodic broadcasts of routing table Only changes are propagated Route Discovery A Node A wants to send a packet to I Node A broadcasts a B C D E F G H I route request. Route Discovery (2) A B & D propagate B C D E F G H I route requests Duplicate route requests discarded Route Discovery (3) A C, F & G propagate B C D reaches node I E F G H route requests 1st route request I Route Discovery (4) A Node I sends route B C D E F G H I reply Route Maintenance A B C D E F H I Periodic hello messages are sent Node G goes offline Routes for node G are pruned Future Work Evaluate proposed ad hoc routing protocols Select an ad hoc routing protocol for small scale wireless network Demonstrate current industry wireless equipment References Hedy Lamarr: www.hedylamarr.at Nokia Rooftop: www.wbs.nokia.com/download/paper.ht ml 802.11: www.drizzle.com/~aboba/IEEE RF Calculator: www.ydi.com/calc.php TEC Embedded Library: www.tinyos.com Appendices Code Division Multiple Access (CMDA) Code Division Multiple Access Station A (8-bit chip sequence) 1-bit (+A): -1 -1 -1 +1 +1 -1 +1 +1 0-bit (-A): +1 +1 +1 -1 -1 +1 -1 -1 Normalized Inner Product: X * Y = 1/m [(x1)(y1) +…+ (xm)(ym)] Receiver extracts data: A * A = 1 (-A) * A = -1 Walsh Codes (orthogonal chip sequences) for X not = Y: X * Y = 0 CDMA (con’t) 4 stations (orthogonal chip sequences) A: -1 -1 -1 +1 +1 -1 +1 +1 B: -1 -1 +1 -1 +1 +1 +1 -1 C: -1 +1 -1 +1 +1 +1 -1 -1 D: -1 +1 -1 -1 -1 -1 +1 -1 4 stations transmit, receiver hears: A+B+C+D: -4 0 -2 0 +2 0 +2 -2 A+B-C+D: -2 -2 0 -2 0 -2 +4 0 CDMA (con’t) 4 stations transmitting S1=A+B+C+D: S2=A+B-C+D: -4 0 -2 0 +2 0 +2 -2 -2 -2 0 -2 0 -2 +4 0 Extract C: -1 +1 -1 +1 +1 +1 -1 –1 S1 * C = 1/8(+4 +2 +2 -2 +2)= 1 S2 * C = 1/8(+2 -2 -2 -2 -4 )=-1