1. Routing in Mobile Wireless Networks Neil Tang 11/14/2008
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2. Outline Wireless Communications and Networks MANET DSR
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3. Wireless Communications
There is no physical link in wireless networks. Signals are transmitted on a certain frequency, propagate in the space and are captured by the intended receiver tuned to the same frequency.
Wireless communication is normally broadcast communication, i.e., all nodes within the transmission range of a particular node can receive the transmitted packets.
Transmissions in a common neighborhood will interfere with each other. If the Signal-to-Interference-Noise-Ratio (SINR) in the receiver is large enough, a packet can be correctly decoded.
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4. Wireless Networks Single-hop wireless networks.
Emerging multihop wireless networks: mobile ad hoc network, wireless mesh network, wireless sensor network.
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5. Mobile Ad Hoc Network (MANET)
Nodes may join and leave the network at any time.
Each node is mobile and its movement is entirely unrestricted.
Each node may forward packets for other nodes in the network. A B C D E B C E A D
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6. Routing in MANETs
On-demand (reactive) routing: Upon arrival of a connection request, the source node floods route discovery messages and find a route. For example, AODV and DSR.
Proactive routing: Nodes flood updates throughout the network whenever the network topology changes and the routes are pre-computed. For example, OLSR.
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, ZRP
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7. 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.
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8. Dynamic Source Routing (DSR)
DSR is an on-demand routing protocol for MANET.
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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9. 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 a 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 a RREP to the source immediately.
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10. Route Discovery Route request for A->G Red – RREQ, Green - RREP B
A-B-D-G G
A-B A
A-B-D D
A-C-E A H E
A-C C F
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11. Route Maintenance
A link-layer hop-by-hop ACK is usually used for reliable transmissions. For example, 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 initiates a new route discovery.
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12. Route Maintenance B RERR G D G A H E C F CS440 Computer Networks
Route Cache (A) G: A, B, D, G F: C, E, F H E C F
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13. 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; When G receives RREQ [A,B,D,E] destined for some node F, G learns route [G,E,D,B,A] to A. So A node usually organize 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 route better than its solution.
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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14. Route Optimization
Reflect Shorter Route: A node may find that route may be shortened by excluding some intermediate node(s), it will then sends 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 from any host for the same destination.
Piggyback Data on RREQs
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