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Routing and Broadcast in a Mobile Ad Hoc Network Professor Yu-Chee Tseng Dept. of Routing and Broadcast in a Mobile Ad Hoc Network Professor Yu-Chee Tseng Dept. of Computer Science and information Engineering National Central University (曾煜棋 中央大學 資訊 程系)

Outline q Introduction to Wireless Networks q Mobile Ad Hoc Network (MANET) q Routing Outline q Introduction to Wireless Networks q Mobile Ad Hoc Network (MANET) q Routing in a Mobile Ad Hoc Network l Review l Fully Location-Aware Routing q Broadcast Storm Problem in MANET q MAC Introduction (IEEE 802. 11 Background) 2

Introduction to Wireless Networks Introduction to Wireless Networks

When You Are Mobile Today q Desperately looking for a computer to check your When You Are Mobile Today q Desperately looking for a computer to check your e-mails q Need to access Internet, WWW Info, etc. q Need cellular phone, airphone, pager, FAX, etc. q Using a laptop to do work while traveling q People of the late 20 th century: Keeping “connected” any time, any where!! 4

Applications of Wireless Communications q Mobile Office/Meeting Room: l with multitude of notebooks, palmtop, Applications of Wireless Communications q Mobile Office/Meeting Room: l with multitude of notebooks, palmtop, PDA, etc. q One who needs to work with customers face-to-face l doctor/nurse l clerk/salespersons l adv. : paperless, less error-prone q q Hospitality: 服務業, 餐廳入座, 代客泊車 Utility: 水公司, 電力公司 l Kansas City: wireless metering system. q q Field work, Field services: always on the road Warehousing/Supermarket: l pricing, order, bar-code input, etc. 5

Wireless Network Models q Wireless LAN: infrastructured q Wireless LAN: ad hoc 6 Wireless Network Models q Wireless LAN: infrastructured q Wireless LAN: ad hoc 6

Wireless Network Models (cont. ) q Cellular: q Diffused: (Infrared) 7 Wireless Network Models (cont. ) q Cellular: q Diffused: (Infrared) 7

Wireless Network Models (cont. ) q Wireless MAN: q Wireless WAN: 8 Wireless Network Models (cont. ) q Wireless MAN: q Wireless WAN: 8

MANET: Mobile Ad Hoc Network MANET: Mobile Ad Hoc Network

MANET q MANET = Mobile Ad Hoc Networks l a set of mobile hosts, MANET q MANET = Mobile Ad Hoc Networks l a set of mobile hosts, each with a transceiver l no base stations; no fixed network infrastructure l multi-hop communication l needs a routing protocol which can handle changing topology 10

Applications of MANET q battlefields (戰場) q nature disaster areas (緊急救難) q fleet in Applications of MANET q battlefields (戰場) q nature disaster areas (緊急救難) q fleet in oceans q historical cites (古蹟) q festival ground (集會) 11

Related Information q IEEE 802. 11 for Wireless LANs l MAC l PHY q Related Information q IEEE 802. 11 for Wireless LANs l MAC l PHY q IETF manet group l to stimulate research in this area l RFC 2503 q Routing Protocols: l unicast – AODV, DSR, ZRP, TORA, CBRP, CEDAR l multicast – AMRoute, ODMRP, AMRIS 12

Research Issues GPS應用 Application Layer WWW 行動教室 TCP/UDP Multicast IP Layer MAC Layer PHY Research Issues GPS應用 Application Layer WWW 行動教室 TCP/UDP Multicast IP Layer MAC Layer PHY Layer Geo. Casst LA Routing Power Ctl multi-code Channel Assignment CDMA 13

Routing in a Mobile Ad Hoc Network (Part I: Review) q q Ant’s Food Routing in a Mobile Ad Hoc Network (Part I: Review) q q Ant’s Food Search Reviews (DSR, ABR, SSR, LAR, TORA)

Ants Searching for Food ? ? ? ? ? ? ? ? ? ? Ants Searching for Food ? ? ? ? ? ? ? ? ? ? ? 15

16 16

Three Main Issues in Ants’ Search q Route Discovery: l searching for the places Three Main Issues in Ants’ Search q Route Discovery: l searching for the places with food q Packet Forwarding: l delivering foods back home q Route Maintenance: l when foods move to new place 17

Protocol 1: DRS (Dynamic Source Routing) q on-demand q Source Routing: l routes are Protocol 1: DRS (Dynamic Source Routing) q on-demand q Source Routing: l routes are denoted with complete information (each hop is registered) q Two major parts: l route discovery l route maintenance 18

Route Discovery of DSR q When a host has a packet to send, it Route Discovery of DSR q When a host has a packet to send, it first consults its route cache. l If there is an unexpired route, then it will use it. l Otherwise, a route discovery will be performed. q Route Discovery: l A ROUTE_REQUEST packet is sent by flooding. l There is a “route record” field in the packet. Ø Each node will append its address to the record. 19

Route Request Route Reply 20 Route Request Route Reply 20

Route Reply of DSR q A ROUTE_REPLY packet is generated when l the route Route Reply of DSR q A ROUTE_REPLY packet is generated when l the route request packet reaches the destination l an intermediate host has an “unexpired” route to the destination q A route is then generated in two manner: l from destination: Ø the route traversed by the ROUTE_REQUEST packet l from intermediate host: Ø the route traversed by the ROUTE_REQUEST packet concatenated with the route in the intermediate host’s route cache 21

Path of ROUTE_REPLY q Which way should be taken by the ROUTE_REPLY? q Two Path of ROUTE_REPLY q Which way should be taken by the ROUTE_REPLY? q Two possibilities: l symmetric path: Ø follow the same route in the reverse order to reach the source l asymmetric path: Ø need to discover a new route to the source by initiating a ROUTE_REQUEST to the source Ø piggyback the discovered route to the ROUTE_REQUEST packet S D 22

Route Maintenance of DSR q When the data link layer encounters a link breakage, Route Maintenance of DSR q When the data link layer encounters a link breakage, a ROUTE_ERROR packet will be initiated. l The packet will traverse in the backward direction to the source. l The source will then initiate another ROUTE_REQUEST. q Maintenance of route cache: l All routes which contain the breakage hop have to be removed from the route cache. S D 23

How to Detect a Link Breakage q Active Acknowledge: l The receiver of a How to Detect a Link Breakage q Active Acknowledge: l The receiver of a packet actively sends an ACK to the sender. q Passive Acknowledge: l The sender passively listen to the receiver’s sending. S R data packets active/passive ACK S R V 24

Protocol 2: ZRP q q ZRP = Zone Routing Protocol A hierarchical approach: l Protocol 2: ZRP q q ZRP = Zone Routing Protocol A hierarchical approach: l zone = the area that a node knows the complete routing information l so routing goes in a zone-to-zone basis 25

Protocol 3: ABR (Associativity-Based Routing) q ABR considers the stability of a link. q Protocol 3: ABR (Associativity-Based Routing) q ABR considers the stability of a link. q Basic Idea: l Each node periodically generates a beacon to signify its existence. l On receipt of the beacon, a node increases the “tick” of the sender by 1. Ø A higher degree means more stability. Ø A lower degree means less reliable. l When a link becomes broken, the node will set the tick of the other node to 0. 26

ABR Outline q Route Discovery: l (similar to DSR) Ø On needing a route, ABR Outline q Route Discovery: l (similar to DSR) Ø On needing a route, a host will broadcast a ROUTE_REQUEST packet. Ø Each receiving host will append its address to the packet. l The “ticks” will be appended in the ROUTE_REQUEST packet. l The destination node will select the route with the highest tick. 7 5 source 8 10 4 destination 27

q Route Maintenance: l On route error, a node will perform a local route q Route Maintenance: l On route error, a node will perform a local route search. Ø in hope of rebuild the path locally. l If the local search fails, a ROUTE_ERROR will be reported to the source local searched zone destination 28

Protocol 4: SSR (Signal Stability Routing) q Observation: l The ABR only considers the Protocol 4: SSR (Signal Stability Routing) q Observation: l The ABR only considers the stability to nodes. q Two more metrics: l signal strength: Ø the strength of a signal Ø provided by link layer l location stability Ø how fast a host moves Ø could be measure by: ü the change of signal strength over a period of time ü location devices (such as GPS) 29

Protocol 5: Location-Aided Routing (LAR) q to limit the area to search for the Protocol 5: Location-Aided Routing (LAR) q to limit the area to search for the route l I will forward the ROUTE_REQ; l J will not forward the ROUTE_REQ. A B D J I Expected zone of D S C Route search zone 30

Assumption of LAR q Location Device is available. l outdoor positioning device: Ø GPS: Assumption of LAR q Location Device is available. l outdoor positioning device: Ø GPS: global positioning system Ø accuracy: in about 20 to 50 meters l indoor positioning device: Ø Infrared Ø short-distance radio, bluetooth, etc. 31

Protocol 6: TORA (Temporally Ordered Routing Algorithm) q q source-initiated protocol provide multiple paths Protocol 6: TORA (Temporally Ordered Routing Algorithm) q q source-initiated protocol provide multiple paths for any source-destination pair l Like water flowing, it goes from upstream to downstream. q for highly dynamic mobile networks 32

Main Idea q q Regard the network as a directed graph. For each destination, Main Idea q q Regard the network as a directed graph. For each destination, a DAG (directed acyclic graph) will be maintained. l Note: There are n copies of DAG’s, each associated with one destination, where n is the number of hosts. l In the following discussion, we only discuss one DAG associated with a destination. q The DAG is accomplished by assigning each node i a height metric hi. l A link from i to j means hi > hj. 33

Full Reversal Method q q A node will update its height to adapt to Full Reversal Method q q A node will update its height to adapt to the change of network topology. Height hi = (valuei, IDi) l a node will change its value to change the direction of a link q Relation: hi > hj if the following is true: l valuei > valuej l (valuei = valuej) and (Di > Dj) l Ex: (5, 4) > (4, 6) l Ex: (5, 4) > (5, 2) l Property: Given any to heights, there must exist a “>” relation between them. 34

q Rule: l Each node other than the destination that has no outgoing links q Rule: l Each node other than the destination that has no outgoing links reverses the direction of all its incoming links. l This means that the node’s height is a local minimum. q This is done by getting a larger height such that the node becomes a local maximum. l MAX{all neighbors’ heights} + 1 a, 5 b, 6 e, 3 d, 4 c, 3 dest, 8 f, 1 g, 2 35

a, 7 b, 6 original b, 6 a, 5 f, 1 b, 6 dest, a, 7 b, 6 original b, 6 a, 5 f, 1 b, 6 dest, 8 b, 6 e, 3 e, 6 d, 4 c, 9 d, 4 g, 5 a, 5 g, 2 a, 5 dest, 8 f, 7 e, 3 dest, 8 c, 9 d, 9 c, 9 d, 4 c, 3 e, 6 dest, 8 f, 4 g, 5 f, 4 g, 2 36

a, 7 b, 10 e, 10 d, 9 c, 9 dest, 8 f, 7 a, 7 b, 10 e, 10 d, 9 c, 9 dest, 8 f, 7 g, 10 a, 11 b, 10 e, 10 d, 9 c, 9 dest, 8 f, 11 g, 10 Eventually, the DAG will stablize. 37

TORA Summary q q There will exist multiple paths leading to a destination. Note: TORA Summary q q There will exist multiple paths leading to a destination. Note: l The above DAG is associated with node dest. l Associated with each node, there is a DAG. q The above scheme is called Full Reversal. l In TORA, more complicated rules are used. Ø Partial reversal Ø Temporally-ordered routing l Height metric 38

Routing in a Mobile Ad Hoc Network (Part II: Fully Location-Aware Routing) “GRID: A Routing in a Mobile Ad Hoc Network (Part II: Fully Location-Aware Routing) “GRID: A Fully Location-Aware Routing Protocol for Mobile Ad Hoc Networks”, Telecommunication Systems (to appear)

Basic Idea q Adopt Positioning Systems l such as GPS receivers l President Clinton Basic Idea q Adopt Positioning Systems l such as GPS receivers l President Clinton ordered to discontinue SA (selective availability) in May 2000 Ø will increase the accuracy by an order q Fully utilize location information: l route discovery l data forwarding l route maintenance q We propose a new protocol called GRID. 40

Observation 1 q Determine route quality based on location information: l passing B is Observation 1 q Determine route quality based on location information: l passing B is better than passing A 41

Observation 2 q Improving the vulnerability and quality of a route based on location Observation 2 q Improving the vulnerability and quality of a route based on location information: l When B moves away, E can work on behalf of B. l When F roams in, using F is more reliable. 42

Comparison of Using Location Information Scheme Route Discovery Packet Relay Route Maintenance DSR AODV Comparison of Using Location Information Scheme Route Discovery Packet Relay Route Maintenance DSR AODV ZRP LAR GRID 43

The GRID Routing Protocol q Partition the physical area into d x d squares The GRID Routing Protocol q Partition the physical area into d x d squares called grid. 44

Protocol Overview q In each grid, a leader will be elected, called gateway. q Protocol Overview q In each grid, a leader will be elected, called gateway. q Routing is performed in a grid-by-grid manner. q Responsibility of gateway: l forward route discovery packets l propagate data packets to neighbor grids l maintain routes which passes the grid 45

Route Search q We can adopt any existing route discovery protocol. q Major features/differences: Route Search q We can adopt any existing route discovery protocol. q Major features/differences: l limit the search range by the locations of source and destination l only gateway will help with the discovery process Ø The more crowded the area is, the more saving. l routing table is indicated by grid ID (instead of host address) 46

Route Search Example route search route reply 47 Route Search Example route search route reply 47

Route Search Range Options 48 Route Search Range Options 48

Routing Table Format q Next-hop routing: l the next hop is identified by grid Routing Table Format q Next-hop routing: l the next hop is identified by grid ID Node S B E F D Destination D D D (2, 2) (3, 2) (4, 2) (5, 3) null Next hop 49

Route Maintenance q Two issues: l how to maintain a gateway in each grid Route Maintenance q Two issues: l how to maintain a gateway in each grid l how to maintain a grid-by-grid route q Special Feature: l longer route lifetime: Ø as long as there is a host in each gateway, a route will be alive Ø more robust l In existing protocols, once a node in the route roams away, the route will be broken. 50

Gateway Election in a Grid q Any “leader election” protocol in distributed computing can Gateway Election in a Grid q Any “leader election” protocol in distributed computing can be used. q Weaker than leader election: l It is acceptable that there are multiple leaders in a grid. Ø less acceptable without leader q Preference in electing a gateway: X l near the physical center of the grid Ø likely to remain in the grid for longer time l once elected, a gateway will remain as so until leaving the grid Ø to avoid ping-pong effect 51

Gateway Election Details BID(g, loc) GATE(g, loc) RETIRE(g, T) 52 Gateway Election Details BID(g, loc) GATE(g, loc) RETIRE(g, T) 52

How to Maintain a Grid-by-Grid Route q Strength: more robust route l mobility-resistant q How to Maintain a Grid-by-Grid Route q Strength: more robust route l mobility-resistant q Problems: l Gateway moves away: Ø The gateway election will find the new gateway. Ø So the route will remain alive. l Source moves away: (see next page) Ø getting closer Ø getting farther away l Destination move away: (similar) 53

54 54

Relationship of Grid Size and Transmission Distance q q r = radio transmission distance Relationship of Grid Size and Transmission Distance q q r = radio transmission distance d = grid size 55

Simulation Model q q Physical area of size 1000 m n = number of Simulation Model q q Physical area of size 1000 m n = number of hosts: 100~300 r=300 m d = grid size l GRID-1: l GRID-2: l GRID-3: q Roaming speed: 30 km/hr, 60 km/hr 56

Route Lifetime q With better route maintenance, our route lifetime is longer. 30 km/hr Route Lifetime q With better route maintenance, our route lifetime is longer. 30 km/hr 60 km/hr 57

Routing Cost (s=30 km/hr) n = 100, 200, 300 (number of hosts) àGRID is Routing Cost (s=30 km/hr) n = 100, 200, 300 (number of hosts) àGRID is better in more crowded area. 58

Delivery Rate q With less routing cost (and thus less traffic load), our packets Delivery Rate q With less routing cost (and thus less traffic load), our packets can be delivered with higher success rate. 30 km/hr 60 km/hr 59

Route Length q Limited by gateway positions, the route length could be longer for Route Length q Limited by gateway positions, the route length could be longer for GRID approach. 30 km/hr 60

Conclusions q A FULLY location-aware routing protocol: l route discovery: by gateways only l Conclusions q A FULLY location-aware routing protocol: l route discovery: by gateways only l data forwarding: by gateway ID, instead of host ID l route maintenance: like handoff in GSM systems q Taking advantage of geometric property of network. l instead of graph property in other approaches q Less routing cost l longer route lifetime, more resilient route l less traffic load 61

The Broadcast Storm Problem in MANETs The Broadcast Storm Problem in MANETs

Storms of Nature 63 Storms of Nature 63

T-Storm in St. Louis 64 T-Storm in St. Louis 64

“Touchdown” of a Tornado 65 “Touchdown” of a Tornado 65

Can Human Create Storms? 66 Can Human Create Storms? 66

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The Storms in the Internet 69 The Storms in the Internet 69

Broadcast Problem q Broadcast: the sending of a message to other hosts l Ex: Broadcast Problem q Broadcast: the sending of a message to other hosts l Ex: Route search in a MANET l Ex: DSR, AODV, ZRP protocols. q Assumptions: l The broadcast is spontaneous. Ø no synchronization Ø no prior global topology knowledge l The broadcast is unreliable. Ø no acknowledgement of any kind ü not to cause more contention ü 100% reliability is unnecessary for some application l No RTS/CTS dialogue. 70

Broadcast by Flooding q A straight-forward approach l A host rebroadcasts the message on Broadcast by Flooding q A straight-forward approach l A host rebroadcasts the message on receiving a broadcast message for the first time. q Broadcast storm problem: l redundant rebroadcasts l contention problem l collision problem 71

Serious Redundancy q Optimal broadcasting vs. Flooding (a) optimal = 2 steps (b) optimal Serious Redundancy q Optimal broadcasting vs. Flooding (a) optimal = 2 steps (b) optimal = 2 steps q Severity of Redundant Coverage. 72

Analysis on Redundancy q Additional Coverage provided by a rebroadcast. l The max. additional Analysis on Redundancy q Additional Coverage provided by a rebroadcast. l The max. additional coverage is 61%. l The coverage is 41% in average. q The expected additional coverage EAC(k)/ r 2 after a host heard a broadcast message k times. 73

Analysis on Contention q When a host broadcasts, its neighbors are likely to contend Analysis on Contention q When a host broadcasts, its neighbors are likely to contend with each other for the medium. l A ==> B, C, D l B, C, D could seriously contend with each other. A q cf(n, k): The probabilities of having k contention-free hosts among n receiving hosts. B C D 74

Analysis on Collision q Higher Possibility of Collision: l Rebroadcasts are likely to start Analysis on Collision q Higher Possibility of Collision: l Rebroadcasts are likely to start at the same time. Ø Backoff window runs out if medium is quiet for a while. l lack of RTS/CTS dialogues l lack of collision detection (CD) if collision occurs l hidden terminal problem 75

Broadcast Storm Problem Summary q Redundancy q Contention q Collision q How to derive Broadcast Storm Problem Summary q Redundancy q Contention q Collision q How to derive an efficient scheme for broadcasting in a MANET? 76

Possible Broadcast Solutions q Probabilistic Scheme q Counter-Based Scheme q Distance-Based Scheme q Location-Based Possible Broadcast Solutions q Probabilistic Scheme q Counter-Based Scheme q Distance-Based Scheme q Location-Based Scheme q Cluster-Based Scheme 77

Probabilistic Scheme q Rebroadcast by “Tossing a Die” q A host always rebroadcasts with Probabilistic Scheme q Rebroadcast by “Tossing a Die” q A host always rebroadcasts with a certain probability P. l When P = 1, this is flooding. l A smaller P will reduce the storm effect. 78

Simulation Parameters q q q no of hosts = 100 transmission radius = 500 Simulation Parameters q q q no of hosts = 100 transmission radius = 500 meters packet size = 280 bytes transmission rate = 1 M bits/sec broadcast arrival rate: 1 per sec. to the whole map: (1 unit = 500 meters) l 1 x 1, 3 x 3, 5 x 5, 7 x 7, 10 x 10 q roaming pattern: random walk l speed: 0~10 km/hr in a 1 x 1 map, 0~30 km/hr in a 3 x 3 map, etc. q IEEE 802. 11 without PCF (point coordination function) 79

Performance of Probabilistic Scheme RE = REachability (in lines) SRB = Saved Re. Broadcast Performance of Probabilistic Scheme RE = REachability (in lines) SRB = Saved Re. Broadcast (in bars) Latency 80

Observation q Reachability: l In smaller maps, a low P is sufficient to achieve Observation q Reachability: l In smaller maps, a low P is sufficient to achieve high reachability. l A larger P is needed in a larger map. q Saved Rebroadcast: l linear with respect to P q Latency: l Interestingly, in smaller areas, broadcast tends to complete in a slower speed. 81

Counter-Based Scheme q If a host has received a broadcast packet > C times, Counter-Based Scheme q If a host has received a broadcast packet > C times, l then do not rebroadcast. q Examples: Addition Coverage l 1 time => 41% l 2 times => 19% l 3 times => 9% l 4 times => 5% l > 4 times, very little extra area 82

Performance of Counter-Based Scheme q We vary C = 2, 3, . . . Performance of Counter-Based Scheme q We vary C = 2, 3, . . . , 6 to observe the performance. l A larger C means more rebroadcast. 83

Observation q Reachability: l C >= 3 can offer a reachability close to flooding. Observation q Reachability: l C >= 3 can offer a reachability close to flooding. q Saved Rebroadcast: l In denser area, there is more saving. In sparser area, there is less saving. q Latency: l Higher latency is smaller area. 84

Distance-Based Scheme q Calculate the distance to the sending host. q dmin = Min{the Distance-Based Scheme q Calculate the distance to the sending host. q dmin = Min{the distance to each sending host} q If dmin < D (a threshold), then do not rebroadcast. q How to find distance: l signal strength l GPS devices 85

Performance of the Distance-Based Scheme q We vary D = 147, 72, 37, 20, Performance of the Distance-Based Scheme q We vary D = 147, 72, 37, 20, 11 to observe the effect. l Smaller D means more rebroadcasting. 86

Observation q Why choosing D=147? l addition coverage = 0. 187, equal to that Observation q Why choosing D=147? l addition coverage = 0. 187, equal to that of C=2 q Reachability: l All look good, close to flooding. q Saved Rebradcast: l not much q Latency: l smaller area has higher latency 87

Location-Based Scheme q q From GPS to obtain the sender’s location. Let (x 1, Location-Based Scheme q q From GPS to obtain the sender’s location. Let (x 1, y 1), (x 2, y 2), (x 3, y 3), . . . , (xk, yk) be locations of senders. l We can accurately calculate the additional coverage of this rebroadcast. No Extra Coverage Some Coverage S 2 S 1 A S 1 S 3 A S 2 88

Difficulty q Involve complicated math to calculate the extra coverage. l A lot of Difficulty q Involve complicated math to calculate the extra coverage. l A lot of calculus! q Approximation: l grid simulation S 1 A S 3 S 2 89

Performance of the Location-Based Scheme q We vary A (addition coverage) from 0. 1 Performance of the Location-Based Scheme q We vary A (addition coverage) from 0. 1 to 0. 01. l Smaller A means more rebroadcast. 90

Observation q Why choosing A=0. 187? l This is additional coverage offered by C=2. Observation q Why choosing A=0. 187? l This is additional coverage offered by C=2. q Best performance over all the above schemes! 91

Modified Location-Based Schemes q Polygon Test: l If a node is within the polygon Modified Location-Based Schemes q Polygon Test: l If a node is within the polygon formed by the locations of senders, then DO NOT rebroadcast. (Fig. (a)) l Otherwise, rebroadcast. (Fig. (b)) l If a host is within the convex, the maximum additional coverage is well below 22%. (Fig. (c)) 92

A Short Summary q Main Concern: l Extra coverage of a rebroadcast q Different A Short Summary q Main Concern: l Extra coverage of a rebroadcast q Different levels of accuracy: l probabilistic, counter, distance, location, polygon q Performance: l Flooding < Probabilistic Scheme < Counter-Based Scheme < Distance-Based Scheme < Location-Based 93

Relationship between Reachability and Saving q Points closer to the upper-right corner are better. Relationship between Reachability and Saving q Points closer to the upper-right corner are better. 94

RE vs. SRB at Larger Maps 95 RE vs. SRB at Larger Maps 95

Conclusions q Broadcast Storm: l a newly identified problem that could affect the performance Conclusions q Broadcast Storm: l a newly identified problem that could affect the performance of MANET l deserve more debate in the future l high severity: Ø redundancy, contention, collision q Solutions: l based on the expected additional coverage of a rebroadcast l probabilistic ==> counter ==> distance ==> location 96

Medium Access Control (MAC) Introduction (IEEE 802. 11 Background) Medium Access Control (MAC) Introduction (IEEE 802. 11 Background)

Radio Nature -- Hidden Terminal Problem q Hidden Terminal Problem: l A is sending Radio Nature -- Hidden Terminal Problem q Hidden Terminal Problem: l A is sending to B. l C is unaware of this fact, and may corrupt A’s transmission. 98

Radio Nature -- Exposed Terminal Problem q Exposed Terminal Problem: l B is sending Radio Nature -- Exposed Terminal Problem q Exposed Terminal Problem: l B is sending to A. l C overhears B’s transmission, and thus is prohibited from sending to D. 99

IEEE 802. 11: RTS/CTS Exchange q q To send, a host must issue a IEEE 802. 11: RTS/CTS Exchange q q To send, a host must issue a RTS (request to send) packet. To receive, a host must reply a CTS (consent to send) packet. hidden terminal exposed terminal 100

IEEE 802. 11: CSMA/CA q CSMA (Carrier Sense Multiple Access): l sense the channel IEEE 802. 11: CSMA/CA q CSMA (Carrier Sense Multiple Access): l sense the channel before attempting to transmit l several packets may collide at the end of previous transmission q CD (collision detection): l abort current transmission once collision is detected l in Ethernet, collision can be sensed at transmitter side l IEEE 802. 3 for Ethernet q CA (collision avoidance): l hard to sense collision while transmission is going on l exponential-backoff + acknowledge + RTS-CTS l IEEE 802. 11 for wireless LAN 101