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Bandwidth Aggregation in Heterogeneous Wireless Networks

There are several issues that need to be addressed to enable simultaneous use of multiple interfaces. These span different layers of the protocol stack and need multi-disciplinary approaches to devise appropriate architecture and scheduling policies. We have looked in depth at the problem of bandwidth aggregation. Since TCP is the dominant transport protocol in use today, we have evaluated the performance of a file transfer using TCP over such bandwidth
aggregation. We have also looked at the performance of real-time interactive applications with strict QoS requirements.

Towards realizing the objective of simultaneous use of multiple interfaces, we have proposed a general framework in the form of an architecture based on an extension to Mobile-IP, a popular network layer mobility management protocol standardized by IETF. In contrast to transport/application layer solution, such a network layer approach introduces minimal changes to the infrastructure while handling mobility well.

While the use of multiple interfaces allows us to increase throughput, the varying characteristics of the different paths (corresponding to different interfaces) introduce problems in the form of packet reordering. With respect to TCP, this reordering can degrade application throughput significantly as TCP misinterprets reordering as indicative of packet loss and invokes congestion
control cutting down the sending rate. This could even counter any gains that can be had through bandwidth aggregation. Therefore, to improve overall performance of TCP, we took a twopronged
approach: (1) we proposed a scheduling algorithm that partitions traffic onto the different paths (corresponding to each interface) such that reordering is minimized; (2) a buffer management policy was introduced at the client to hide any residual reordering from TCP.
Simulation studies have shown that this approach can achieve good bandwidth aggregation under a variety of network conditions. The performance is comparable to "MTCP", an application layer solution that opens multiple TCP connections with one on each interface (the best possible).

For real-time interactive applications, packet reordering introduces excess delay. Because of stringent QoS requirements, this late arrival of packets is often equivalent to loss. Thus, to reduce the delay associated with reordering, we proposed a scheduling policy – Earliest Delivery First (EDF) that partitions the traffic onto different paths such that the QoS requirements of the application are met. We theoretically analyzed the performance of EDF and show that it performs close to an idealized Single Link (SL) discipline, where the multiple interfaces are replaced by a single interface with the aggregate bandwidth. We carried out simulations using video and delay traces and we observed that under a variety of network conditions, EDF mimics SL closely and
outperforms by a large margin other straight-forward scheduling policies like weighted roundrobin.

While our approach has been based on simulation and analysis so far, we plan to implement some of the ideas on an actual testbed. In addition, we will look into other aspects of bandwidth aggregation related to power consumption, security, etc. While use of multiple interfaces may be a drain on the battery, the power savings in time had through bandwidth aggregation need to be quantified. Security is another important issue in wireless networks. Use of multiple interfaces means securing a greater number of paths, but through proper encryption, connection can be made inherently more secure as it is difficult for an attacker to snoop on all the paths.

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