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).  Some handle unidirectional links but are very inefficient (DSR).

Noise: source of one-way link.  Transient unidirectional links. Go away when noise subsides or nodes move. A B C D E

Asymmetry in Transmit Power  Topology Control Schemes: Sensor Network  Heterogeneity of hardware: Home Network A B C A B C

Problems due to one-way links.  Collision avoidance (RTS/CTS) scheme is impaired Even across bidirectional links! ABC RTS CTS X MSG

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  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.

Average Bidirectional Connectivity

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

Reverse route for one-way link  Let A  C be a one-way link.  C  B  A is a 2-hop reverse route. A B C

Connectivity with reverse routes.

One-way links with reverse routes.

Average Reverse Route Length

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  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  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. A B C A; 2; C C; 1; B C; 2; B B; 1; A A; 1; C Update Message Format: Source; #hops; First Hop Reverse Route: C  B  A

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  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  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  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  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  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

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.

Static Experiments: Packet Delivery.

Static Experiments: Average Route Length.

Mobility Experiments: Packets Originated

Mobility Experiments: Packet Delivery.

SRL Overhead: Average Length of Update Packets.

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!