Wireless Ad Hoc Network
Routing Protocols
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
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LS generates loads of link status change msgs DV suffers from out-of-date state or generates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
The Protocols
DSDV, TORA, DSR, AODV
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
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
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
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When a node discovers a route is no longer valid, it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
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
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
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
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
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
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
Packet Delivery Ratio
DSR and AODV deliver
over 95% of data packet
TORA does well with 20
sources
DSDV fails to converge
Routing Overhead
TORA, DSR, AODV are
on demand
DSDV is largely
periodic
DSR limits overhead
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
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
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
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
GPSR versus DSR
Comparison cont.
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
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
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
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