Re-routing Instability in IEEE 802.11 Multi-hop Ad- hoc Networks Ping Chung Ng and Soung Chan g Liew The 4th IEEE International W orkshop on Wireless Local Ne twork
Dec 20, 2015
Re-routing Instability in IEEE 802.11 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 – (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 – (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 802.11 multi-hop ad-hoc networks”.
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].
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 topologyA multiple flow in a string topology
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.