Project Title

Distributed Link Scheduling Multiple Access Protocol for QoS in Ad-hoc Networks

Overview

We propose a novel medium access control (MAC) protocol for QoS support in multi-hop ad-hoc wireless networks. The proposed D-LSMA (distributed link scheduling multiple access) protocol uses an extension of the 802.11 CSMA/CA procedure as the basis for a distributed link scheduling algorithm which results in dynamic TDMA-like bandwidth allocation among neighboring wireless nodes without the need for global synchronization. In addition to supporting QoS, the proposed scheduling technique also solves the “exposed node” problem in ad-hoc 802.11, thus resulting in improved throughput in many scenarios. Simulation results from an ns-2 model are presented for a 15-node random ad-hoc network. The results demonstrate significant performance improvements relative to ad-hoc 802.11, with capacity increases typically ˜20% for the example considered. Also, the D-LSMA network is shown to offer far better real-time packet delay and fairness properties than 802.11, particularly under overload and heavy contention conditions.

Publication

Zhibin Wu, Dipankar Raychaudhuri, "D-LSMA: Distributed Link Scheduling Multiple Access Protocol for QoS in Ad-hoc Networks", in Proceedings of IEEE GLOBECOM '04, November 2004. 

Follow-Up

In summary, the performance of D-LSMA was evaluated using an ns-2 simulation model and compared with 802.11 and DCMA. The results show that D-LSMA achieves throughput gains of up to 23% when compared with IEEE 802.11, while providing bounded delay and improved fairness. The performance was also found to be competitive with DCMA and MACA-P, two other recent proposals for improving 802.11
performance in multi-hop scenarios. However, the general performance improvement of D-LSMA is not as high as expected.
Intuitively, if “exposed node” problems are well avoided, the link transmission opportunities shall be improved by around two-fold. A detailed analysis of simulation results shows that parallel transmission opportunities are not always utilized because of the collision of either RTS or CTS messages with other packets. This is hard to avoid when RTS/CTS signaling is mixed with heavy data traffic in the same channel.
Some researchers found similar problem and called that as masked node problem or jammed node problem. Due to the same reason, the MACA-P scheme also behaves underperformed. Those observations suggest that the MAC schemes featuring minor modifications upon on CSMA/CA MAC only achieve a limited portion of throughput gains. The performance are not comparable to that of optimal link scheduling schemes yet.

Further Reading

[MACA-P] Arup Acharya, Archan Misra, and Sorav Bansal. Design and analysis of a cooperative medium access scheme for high-performance wireless mesh networks. In Proceedings of 1st IEEE International Conference on Broadband Networks, 2004.

[DCMA] Arup Acharya, Archan Misra, and Sorav Bansal. A label-switching packet forwarding architecture for multi-hop wireless lans. In WOWMOM ’02: Proceedings of the 5th ACM international workshop on Wireless mobile multimedia, pages 33–40, New York, NY, USA, 2002. ACM Press.

[MaskNode] S. Ray, J. Carruthers, and D. Starobinski. Evaluation of the masked node problem in ad hoc wireless lans. IEEE Transaction on Mobile Computing, 4(5), 2005.

[JamNode] Mathilde Durvy and Patrick Thiran. Understanding the gap between the IEEE 802.11 protocol performance and the theoretical limits. In Proceedings of IEEE SECON Conference, 2006.