Re-routing Instability in IEEE Multi-hop Ad-hoc Networks Ping Chung Ng and Soung Chang Liew The 4th IEEE International Workshop on Wireless Local Network

Overview Motivation - Re-routing instability AODV with “don’t break before you can make” strategy (AODV_DM) Performance Evaluation Conclusions

Motivation – (1) A string topology

Motivation – (2) Node 4 senses the channel to be busy since node 6 is inside its carrier-sensing range.

Motivation – (3) Node 3 senses the channel as idle since node 6 is outside its carrier- sensing range.

Motivation – (4) At node 4, a RTS frame or a DATA frame sent from node 3 collides with any frames sent from node 6.

Motivation – (5) Node 3 encounters a timeout event and double the contention window size for retransmission.

Motivation – (6) Node 6 transmits successfully and does not notice the collision at node 4.

Motivation – (7) Node 6 uses the minimum contention window size for transmitting the next frame

Motivation – (8) Node 6 “captures” the channel. Although node 3 defers for a longer period before retransmission, the chance of collision at node 4 cannot be reduced. Node 3 fails to transmit after a number of retries, it declares the link as being broken.

Motivation – (9) The routing protocol is invoked to look for a new route. Before a new route is discovered, no packet can be transmitted. Therefore, the throughput drops drastically.

Motivation – (10) There is only route from node 1 to node 7. The routing protocol will eventually “re- discover” the same route again.

Motivation – (11) The breaking and re-discovery of the path results in the throughput oscillations. This phenomenon is called “re-routing instability in IEEE multi-hop ad-hoc networks”.

Motivation – (12)

Motivation – (13)

Motivation – (14) Throughput drops severely for the duration of 1 to 3 seconds. It is not acceptable for real-time applications like video conferencing or VoIP.

Motivation – (15) The routing protocol should continue to use the previous route for transmissions before a new route can be found. AODV routing protocol is chosen as implementation details have been published in IETF RFC [11].

AODV

AODV_DM

Performance Evaluation – Simulation Setup Each node has a droptail FIFO queue which holds up to 500 packets. TCP Reno is used. Throughputs are obtained by averaging over one-second intervals.

Performance Evaluation – Scenarios A single flow in a string topology A multiple flow in a string topology

UDP end-to-end throughput in a 7- node flow

TCP end-to-end throughput in a 7- node flow

Real-break case – Setup

Real-break case – Results

End-to-end throughput versus the number of nodes

Normalized standard deviation of end-to- end throughput versus the number of nodes

Max, Mean and Min end-to-end throughput versus the number of nodes

Multiple Flows

UDP throughputs of two 1-hop flows

Conclusions – (1) Throughput instability problem is mainly due to a “re-routing instability problem”, rather than a binary exponential backoff mechanism. A “don’t break before you can make” modification, which is adopted to AODV, can eliminate the instability problem.

Conclusions – (2) Average UDP and TCP end-to-end throughputs are boosted up. UDP and TCP throughput variations are reduced.