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6610-l17- - Ad-hoc Routing 1 Puzzle 10 bags with large...

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1 Ad-hoc Routing
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Puzzle 10 bags with large number (>100) of coins. 9 bags have coins of equal weights (each regular coin weighs 1 unit). 1 bag has defective coins (each defective coin weighs 1.01 units) Using a spring balance, how many weighings do you need to find the defective bag?
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3 Ad-hoc Networks Infrastructure-less wireless networks dynamically formed using only mobile hosts (no routers) Network topology dynamic as all hosts are mobile ! Mobile hosts themselves double up as routers!! Multi-hop paths … Highly resource constrained Extreme case of network mobility…
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4 Applications … Military – digital battlefield Disaster relief Sensor networks Areas with no cellular coverage
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5 Illustration A B C D
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6 Routing Protocols Proactive approaches DSDV (destination sequenced distance vector), LSR (link state routing), OLSR (optimized link state routing) Reactive approaches DSR (dynamic source routing), AODV (ad-hoc on- demand distance vector) Hybrid approaches ZRP (zone routing protocol), CEDAR (Core extraction distributed ad-hoc routing)
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7 Proactive routing protocols… Unsuitable for such a dynamic n/w For example, consider link-state routing that sends out network-wide floods for every link-state change … Even in the absence of any existing connections, considerable overhead spent in maintaining “network state”
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8 Goals Low overhead route computation Ability to recover from frequent failures at low-cost Scalable (with respect to mobility and number of hosts) Robust
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9 Reactive (On-demand) protocols Compute routes only when needed Even if network state changes, any re- computation done only when any existing connections are affected Example: Dynamic source routing (DSR) Ad-hoc On-demand Distance Vector (AODV)
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10 Dynamic Source Routing Based on source routing On-demand Route computation performed on a per-connection basis Source, after route computation, appends each packet with a source-route Intermediate hosts forward packet based on source route
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11 DSR – Basic Operation When higher layer gives DSR a packet to transmit, and there exists no route in the route-cache, route computation is performed Source S floods the network with a RREQ (route request) packet for the destination D Every host I that receives the RREQ packet checks to see if I=D. If not, I adds its identifier to the RREQ packet header and forwards the packet
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12 DSR – Basic Operation (contd.) Each forward is a local broadcast (unlike in a wireline network where a flood would entail multiple local transmissions) When D receives packet, the RREQ message contains the list of identifiers it traversed through D responds to the sender S with an RREP (route reply) message containing the route in the RREQ – How?
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13 DSR – RREP propagation If MAC layer assumes bi-directionality (e.g.?), D has to use the same route (reversed) that is being conveyed to S
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