SRL: A Bidirectional Abstraction for Unidirectional Ad Hoc Networks'

1
SRL A Bidirectional Abstraction for
Unidirectional Ad Hoc Networks.

• Venugopalan Ramasubramanian
• Ranveer Chandra
• Daniel Mosse

2
Introduction

• Links in an ad hoc network could be
  unidirectional.
• Many Ad hoc network routing protocols are not
  designed to handle unidirectional links (TORA).
• Some handle unidirectional links but are very
  inefficient (DSR).

3
Noise source of one-way link.

• Transient unidirectional links.
• Go away when noise subsides or nodes move.

4
Asymmetry in Transmit Power
C
B
A

• Topology Control Schemes Sensor Network
• Heterogeneity of hardware Home Network

5
Problems due to one-way links.

• Collision avoidance (RTS/CTS) scheme is impaired
• Even across bidirectional links!

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Problems due to one-way links

• Collision avoidance (RTS/CTS) scheme is impaired
• Even across bidirectional links.
• Unreliable transmissions through one-way link.
• May need multi-hop Acks at Data Link Layer.
• Link outage can be discovered only at downstream
  nodes.

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Problems for Routing Protocols

• Route discovery mechanism.
• Cannot reply using inverse path of route request.
• Need to identify unidirectional links. (AODV)
• Route Maintenance.
• Need explicit neighbor discovery mechanism.
• Connectivity of the network.
• Gets worse (partitions!) if only bidirectional
  links are used.

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Average Bidirectional Connectivity

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Distribution of Bidirectional Connectivity.
200 random topologies. Probablity of one-way
link 0.25
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Reverse route for one-way link

• Let A ? C be a one-way link.
• C ? B ? A is a 2-hop reverse route.

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Connectivity with reverse routes.
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One-way links with reverse routes.
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Average Reverse Route Length
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Observations from analysis.

• Topologies generated with asymmetric transmit
  power also produce similar graphs.
• The connectivity follows a long tail
  distribution.
• Reverse routes are short (2 or 3 hops) for most
  one-way links.

15
SRL Sub Routing Layer

• Short reverse routes for one-way links
• Improve connectivity substantially.
• Also decrease route lengths.
• SRL discovers and maintains reverse routes for
  one-way links.
• It provides a bidirectional abstraction to the
  routing protocols.
• Provides services such as reliable transmission
  and link breakage detection.

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Internals of SRL

• Reverse Distributed Belmanford Algorithm
• Distance vector based technique.
• Each node maintains
• Shortest path from other nodes in its locality.
• Periodically neighbor-casts this information.
• Locality of node A
• Set of nodes that can reach A in r hops.
• r is the radius of locality.

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Reverse Distributed Belmanford Algorithm.
Reverse Route C ? B ? A
Update Message Format Source hops First Hop
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RDBA contd.

• Periodic update messages are neighbor-cast
• Source ID Hop Count First Hop
• Sources restricted to locality of radius r.
• r called SRL radius is small (2 3).
• Scalable to large networks.
• No counting to infinity problem.
• Ignore distances bigger than r.
• No Route-loops.
• Use first hop information to check for loops.

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SRL Periodic Updates

• Incremental Updates
• Most recent changes in hop count or first hop.
• Sent periodically at same rate as hello messages.
• Replaces hello messages.
• Complete Updates
• Contains entire data for locality.
• Sent with much lower frequency.
• Random distribution to avoid co-ordination.
• Hello Packets
• Sent when no incremental updates need to be sent.

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Optimization 1 Dynamic SRL

• The SRL radius of each node could be different.
• Each node increases radius until it can find
  reverse routes.
• Radius decreases if reverse routes are shorter
  than the radius.
• Decreases the number of updates that is
  neighbor-cast lower overhead.

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Optimization 2 On-demand DSRL

• Routing protocol requests DSRL to find reverse
  routes for certain one-way links.
• Reverse routes maintained only for the chosen
  one-way links.
• Routing strategy that uses one-way links only
  when route discovery along bidirectional links
  fail.

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Services provided by SRL

• Identification of one-way links (radius 1)
• Routing protocols can avoid them.
• Reverse route forwarding
• Routing protocol uses reverse routes to send
  route replies and route errors.
• Not good for data packets.
• Link breakage detection
• Several protocols rely on lower layers to do
  this.
• Reliable Transmission across unidirectional
  links
• Multi-hop Acks can be used if required by the
  protocol.

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Simulation AODV over SRL

• AODV is adapted on top of SRL.
• Use reverse routes for RREPs and RERRs.
• Uses SRLs link break discovery service.
• Compared with traditional AODV.
• Routes only along bidirectional links.
• Uses black-list to identify unidirectional links.
• Runs on top of IEEE 802.11

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Simulation Setup

• 80 nodes in 1300m x 1300m area.
• 220m nominal radio range (WaveLan).
• 360s total simulation time.
• 300s of data origination.
• 20 random src-dest pairs for each run.
• 50 random topology for each experiment.
• Packet Size random between 64B 1024B.
• Average data rate 1 packet per sec.

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Static Experiments Packet Delivery.
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Static Experiments Average Route Length.
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Mobility Experiments Packets Originated
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Mobility Experiments Packet Delivery.
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SRL Overhead Average Length of Update Packets.
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Conclusions

• SRL increases the packet delivery of AODV by 30.
• The overhead generated by SRL is not very
  significant and can be further reduced.
• The effect of optimizations need to be studied.
• RTS/CTS implementation with SRL would be
  interesting!