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Mobile Ad Hoc Networks (MANETs)Mobile Ad Hoc Networks (MANETs)

Neil TangNeil Tang02/02/200902/02/2009

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OutlineOutline

Network Architecture

Characteristics

Applications

Major Concerns

Challenges

Ad Hoc Routing

DSR

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Network ArchitectureNetwork Architecture

A

BC

D

E

A

BC

D

E

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CharacteristicsCharacteristics

Multihop wireless network

Unrestricted mobility

Dynamic node membership

Various physical layer techniques, e.g., directional antenna, cognitive radio

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ApplicationsApplications

Battle-field communications

Emergency communications

Transportation system

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Major ConcernsMajor Concerns

Mobility: link breakage

Power consumption

QoS

Scalability

Security

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ChallengesChallenges

MAC protocol design (802.11 DCF): directional antenna, cognitive radio

Routing

End-to-end QoS support: mobility and intra-flow interference.

Multicast/Broadcast Routing

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Ad Hoc RoutingAd Hoc Routing On-demand (reactive) routing: Upon arrival of a connection request, the

source node floods route discovery messages and find a route for packet forwarding. For example, Ad hoc On-demand Distance Vector (AODV) protocol, Dynamic Source Routing (DSR) protocol.

Proactive routing: Nodes flood updates throughout the network whenever the network topology changes. For example, Optimized Link State Routing (OLSR) protocol.

Hybrid routing: Route discovery is basically conducted reactively but link state update is conducted proactively within a certain range, e.g., 2-hop neighborhood of a node. For example, Zone Routing Protocol (ZRP)

IETF MANET group: http://www.ietf.org/html.charters/manet-charter.html

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On-Demand VS. ProactiveOn-Demand VS. Proactive

On-demand (reactive) routing: Low routing overhead but long route discovery latency.

Proactive routing: High routing overhead especially in the case of high mobility but short route discovery latency.

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Dynamic Source Routing (DSR)Dynamic Source Routing (DSR)

DSR is an on-demand routing protocol for MANETs.

The whole source-to-destination route is included in every data packet

and no routing table is needed for packet forwarding in each node.

Loop freedom is guaranteed.

Large overhead in the packets.

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Route DiscoveryRoute Discovery The source node broadcasts a RREQ (request) message to request a path to th

e destination.

A tuple (SrcID, RequestID) is used to uniquely identify a route request.

A node v receiving the RREQ will, - discard the packet if it is an old or duplicate one - discard the packet if v is already in the route list - send an RREP (reply) packet back to the source through the reverse route if v is the destination. - otherwise, append itself in the route list and re-broadcast the packet.

Both RREQ and RREP will be sent out only once in each node.

If a node has a record in its cache showing how to reach the destination, it can reply an RREP to the source immediately.

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A

B

C

E

D

G

H

F

A

A

A-B

A-C

A-C-E

A-C-E

A-C-E

A-B-D

A-B-D-GA-B-D-G

A-B-D-G

Route DiscoveryRoute DiscoveryRoute request for A->G

Red – RREQ, Green - RREP

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Route MaintenanceRoute Maintenance

A link-layer hop-by-hop ACK is usually used for reliable transmissions. For example, 802.11 DCF supports the link-layer ACK.

A RERR will be sent by the end node to the source node if it detects a link breakage. Nodes along the path will then update their caches accordingly and the source node will initiate a new route discovery.

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Route MaintenanceRoute Maintenance

A

B

C

E

D

G

H

F

G

RERRRERR

Route Cache (A)G: A, B, D, G F: C, E, F

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Route OptimizationRoute Optimization

Route Caching: Each node caches a new route it learns by any means. For example, when A finds route [A,B,D,E,F] to F, A also learns route [A,B,D,E] to E; D forwards data [A,B,D,E,F], D learns route [D,E,F] to F; So a node usually organizes its cache in the format of a shortest path tree with itself as the root.

Avoid RREP Storm Problem: An intermediate node will delay transmitting the route reply for a random period of d. During this period, cancel the route reply if overhearing any packet containing a better route.

Limit the Propagation of RREQ Packets: First, set TTL = 1 for first route request packet. If no route reply is received after some time period, set TTL = maximum for next RREQ. .

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Route OptimizationRoute Optimization

Reflect Shorter Route: A node can send an unsolicited RREP to the source to inform the shorter route.

Improve Error Handling: exponential backoff is used to limit the rate at which new route discoveries are initiated.

Piggyback Data on RREQs