wireless ad hoc network routing protocols cse 802.11 maya rodrig
Post on 20-Dec-2015
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Ad hoc networking Infrastructureless networking – mobile
nodes dynamically establish routing among themselves to form their own network on the fly.
Mobile nodes operate as routers Mobile nodes participate in an ad hoc
routing protocol
Why not reuse existing protocols?
Highly dynamic interconnection topology LS generates loads of link status change
msgs DV suffers from out-of-date state or
generates loads of triggered updates Heavy computational burden on mobile
nodes Wireless medium differs in important
ways from wired media
Destination-Sequenced Distance Vector (DSDV) Preserve the simplicity of RIP while avoiding
the routing loop problem Hop-by-hop distance vector Routing table contains entries for every
reachable node Each route is tagged with a sequence
number originated by destination (even numbers)
Routing info is transmitted by broadcast Updates are transmitted periodically and
when there is a significant topology change
DSDV cont. Route R is more favorable than R’ if R
has a greater sequence number or if the two routes have equal sequence numbers but R has a lower metric (hop count)
Broken links are indicated by “” metric and the sequence number of destination is incremented to odd number before broadcast
Temporally-Ordered Routing Algorithm (TORA) Based on a “link-reversal” algorithm Node broadcasts a QUERY packet which
propagates to destination or to node having a route to the destination
Recipient of the QUERY broadcasts an UPDATE packet listing its height with respect to the destination
Each node that receives the UPDATE sets its height to be greater than the height of the neighbor from which the UPDATE came creates a series of directed links from the QUERY originator to the node initiating the UPDATE
TORA cont. When a node discovers a route is no longer
valid, it adjusts its height so that it is a local maximum and transmits an UPDATE
When a network partition is detected, a node generates a CLEAR packet to reset routing state and remove invalid routes
Dynamic Source Routing (DSR) Packet headers contain the route the packet must
follow Route Discovery:
Source node S broadcasts Route Request packet that is forwarded through the network
Destination node D or another node that knows a route to D answers with a Route Reply
Route Maintenance: When the network topology has changed s.t. the route to D
can no longer be used, a Route Error packet is sent to S S can try another route to D from its cache or invoke Route
Discovery again Network interfaces in promiscuous mode nodes
cache overheard route information
Ad Hoc On-Demand Distance Vector (AODV) Combination of DSR (on demand) and DSDV
(hop-by-hop routing, sequ nums) Node S broadcasts a Route Request message
for destination D, including the last known sequence number for D
Node with a route to D generates a Route Reply with its sequence number for D
Nodes that forward Route Request store reverse route back to S; nodes that forward Route Reply store forward route to D
AODV cont. No HELLO messages from neighbor
indicate link is down Nodes that recently forwarded packets
using the failed link are notified via an UNSOLICITED ROUTE REPLY with infinite metric for the destination reinitiate Route Discovery
Simulation Environment Model attenuation of radio waves
between antennas Link layer implements 802.11 standard
MAC protocol DCF Broadcast packets sent only when
virtual and physical carrier sense indicate the medium is clear (no RTS/CTS and no ACKs)
Methodology Network simulation
50 wireless nodes moving in 1500m*300m flat space Over 200 different scenarios
Movement model “Random waypoint” model (pause times: 0, 30, 60,
120, 300, 600, 900 seconds) Avg speed 10 meters/second
Communication model Sending rates: 1, 4, 8 packets/second 10, 20, 30 CBR sources Packet size of 64 bytes
Metrics Packet delivery ratio- ratio between num
packets originated by sources and num packets received at their destination
Routing overhead- num routing packets transmitted during the simulation
Path optimality- difference between the num hops a packet took to reach its destination and the length of the shortest path
Packet Delivery Ratio
DSR and AODV deliver over 95% of data packet
TORA does well with 20 sources
DSDV fails to converge at pause time < 300
Routing Overhead
TORA, DSR, AODV are on demand
DSDV is largely periodic
DSR limits overhead of Route Requests through caching
Path Optimality
Internal mechanism knows the length of the shortest path between all nodes at any time
DSDV and DSR use routes close to optimal
AODV and TORA have a tail
Another Protocol: Greedy Perimeter Stateless Routing (GPSR)
Geography to achieve scalability in wireless routing protocols
Assume bidirectional radio reachability Assume a location registration and
lookup service that maps node addresses to locations
Position of a packet’s destination and positions of candidate next hops sufficient to make correct decisions
Greedy Forwarding
Beaconing algorithm provides all nodes with their neighbor’s positions
Packets are marked with their destinations’ locations A forwarding node makes a locally optimal greedy
choice: next hop is the neighbor geographically closest to the destination
Problem: topologies in which the only route to the destination requires temporarily moving farther in geometric distance from the destination
Planar Perimeters Right-hand rule : when arriving at node x from node y,
the next edge traversed is the next one sequentially counterclock-wise about x from edge (x,y) navigating around the void
Construct planarized graphs to eliminate crossing links from the network without partitioning the network
Choosing Routes
Shortest path is not a good metric choose routes with less capacity than best existing paths
Minimum hop-count routes include links with high loss ratios retransmissions consume bandwidth
Link Behavior in Experimental Networks
Link quality distribution is spread out 30% of link pairs are unusable Best 40% of link pairs deliver 90% of their packets
30% link pairs have asymmetric delivery rate Delivery rates sometimes change very quickly
(averaging not applicable) No good correlation between delivery rate and
radio’s signal strength
We need practical estimates for link quality and ways to combine link metrics into path metrics
Expected Transmission Count (ETX)
Find paths with fewest expected number of transmissions required to deliver a packet to its destination Use per-link measurements of delivery ratios in
both directions
Modified DSDV and DSR ETX outperforms minimum hop-count ETX incurs more overhead due to loss-ratio
probes