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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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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