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SRL: A Bidirectional Abstraction for Unidirectional Ad Hoc Networks. Venugopalan Ramasubramanian Ranveer Chandra Daniel Mosse. Introduction. Links in an ad hoc network could be unidirectional. Many Ad hoc network routing protocols are not designed to handle unidirectional links (TORA).

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srl a bidirectional abstraction for unidirectional ad hoc networks

SRL: A Bidirectional Abstraction for Unidirectional Ad Hoc Networks.

Venugopalan Ramasubramanian

Ranveer Chandra

Daniel Mosse

introduction
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).
noise source of one way link

C

E

A

B

D

Noise: source of one-way link.
  • Transient unidirectional links.
    • Go away when noise subsides or nodes move.
asymmetry in transmit power

C

B

A

Asymmetry in Transmit Power
  • Topology Control Schemes: Sensor Network
  • Heterogeneity of hardware: Home Network

C

B

A

problems due to one way links

A

B

C

RTS

MSG

X

CTS

CTS

MSG

MSG

Problems due to one-way links.
  • Collision avoidance (RTS/CTS) scheme is impaired
    • Even across bidirectional links!
problems due to one way links6
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.
problems for routing protocols
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.
distribution of bidirectional connectivity
Distribution of Bidirectional Connectivity.

200 random topologies. Probablity of one-way link = 0.25

reverse route for one way link

C

B

A

Reverse route for one-way link
  • Let A  C be a one-way link.
  • C  B  A is a 2-hop reverse route.
observations from analysis
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.
srl sub routing layer
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.
internals of srl
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.
reverse distributed belmanford algorithm

C

A; 1; C

C; 2; B

B; 1; A

A

B

A; 2; C

C; 1; B

Reverse Distributed Belmanford Algorithm.

Reverse Route: C  B  A

Update Message Format: Source; #hops; First Hop

rdba contd
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.
srl periodic updates
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.
optimization 1 dynamic srl
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.
optimization 2 on demand dsrl
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.
services provided by srl
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.
simulation aodv over srl
Simulation: AODV over SRL
  • AODV is adapted on top of SRL.
    • Use reverse routes for RREPs and RERRs.
    • Uses SRL’s 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
simulation setup
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.
conclusions
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!