Wireless Ad Hoc Network
Routing Protocols
CSE 802.11
Maya Rodrig
Ad hoc networking
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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?
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Highly dynamic interconnection topology
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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DV suffers from out-of-date state or generates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
The Protocols
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DSDV, TORA, DSR, AODV
Proactive vs. reactive (on-demand)
Destination-Sequenced Distance
Vector (DSDV)
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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.
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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
No count to infinity
Temporally-Ordered Routing
Algorithm (TORA)
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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.
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When a node discovers a route is no longer valid, it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)
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Packet headers contain the route the packet must
follow
Route Discovery:
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Route Maintenance:
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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
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
DSR Example
Ad Hoc On-Demand Distance
Vector (AODV)
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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.
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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
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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
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Network simulation
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Movement model
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50 wireless nodes moving in 1500m*300m flat
space
Over 200 different scenarios
“Random waypoint” model (pause times: 0, 30,
60, 120, 300, 600, 900 seconds)
Avg speed 10 meters/second
Communication model
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Sending rates: 1, 4, 8 packets/second
10, 20, 30 CBR sources
Packet size of 64 bytes
Metrics
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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
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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
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TORA, DSR, AODV are
on demand
DSDV is largely
periodic
DSR limits overhead
of Route Requests
through caching
Path Optimality
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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)
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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
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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
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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
GPSR versus DSR
Packet Delivery Success Rate
Routing Overhead
Comparison cont.
Path Length
Network Diameter
Choosing Routes
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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
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Link quality distribution is spread out
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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)
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Find paths with fewest expected number of
transmissions required to deliver a packet to
its destination
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Use per-link measurements of delivery ratios in
both directions
Modified DSDV and DSR
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ETX outperforms minimum hop-count
ETX incurs more overhead due to loss-ratio probes
…
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Early protocols assume cooperating
nodes that are willing to forward
packets for others
The role of power in routing protocols