mobile ad hoc networks manets neil tang 02 02 2009 n.
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Mobile Ad Hoc Networks (MANETs) Neil Tang 02/02/2009

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  1. Mobile Ad Hoc Networks (MANETs)Neil Tang02/02/2009 CS541 Advanced Networking

  2. Outline • Network Architecture • Characteristics • Applications • Major Concerns • Challenges • Ad Hoc Routing • DSR CS541 Advanced Networking

  3. C B E A D B C E A D Network Architecture CS541 Advanced Networking

  4. Characteristics • Multihop wireless network • Unrestricted mobility • Dynamic node membership • Various physical layer techniques, e.g., directional antenna, cognitive radio CS541 Advanced Networking

  5. Applications • Battle-field communications • Emergency communications • Transportation system CS541 Advanced Networking

  6. Major Concerns • Mobility: link breakage • Power consumption • QoS • Scalability • Security CS541 Advanced Networking

  7. Challenges • MAC protocol design (802.11 DCF): directional antenna, cognitive radio • Routing • End-to-end QoS support: mobility andintra-flow interference. • Multicast/Broadcast Routing CS541 Advanced Networking

  8. Ad 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: CS541 Advanced Networking

  9. On-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. CS541 Advanced Networking

  10. 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. CS541 Advanced Networking

  11. Route Discovery • The source node broadcasts a RREQ (request) message to request a path to the 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. CS541 Advanced Networking

  12. A-B-D-G A-B-D-G A-B-D-G A-B A A-B-D A-C-E A A-C-E A-C-E A-C Route Discovery Route request for A->G Red – RREQ, Green - RREP B G D A H E C F CS541 Advanced Networking

  13. Route 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. CS541 Advanced Networking

  14. RERR RERR Route Maintenance B G D G A Route Cache (A)G: A, B, D, G F: C, E, F H E C F CS541 Advanced Networking

  15. Route 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. . CS541 Advanced Networking

  16. Route Optimization • Reflect Shorter Route: A node cansend 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 CS541 Advanced Networking