Lillykutty Jacob

Fairness Analysis of IEEE 802.11 Multirate Wireless LANs

This paper investigates the issue of fairness in IEEE 802.11-based multirate wireless local area networks (WLANs). Distributed coordination function, which is the medium-access control (MAC) protocol used in 802.11 WLANs, provides equal long-term channel access probability to competing stations, irrespective of the time required in sending a frame. When equal-sized frames are used and channel conditions are similar, each station, regardless of its data rate, achieves the same throughput. Furthermore, the aggregate throughput is reduced to a level much closer to what one gets when all stations are of lower rate. This anomaly in the performance is a result of unfair channel time allocation for stations when they use multiple data rates. We consider provisioning of time-based fairness in which each station receives an equal share of the wireless channel occupancy time. We demonstrate that under time-based fairness, the throughput that a tagged node achieves in a multirate WLAN with nodes is identical to what it would achieve in a single-rate WLAN with nodes all at the same data rate as the tagged node. Furthermore, we show that under time-based fairness scheme, the ratio of throughputs per station corresponding to two different bit rates is directly proportional to the ratio of their bit rates. We analyze different mechanisms in achieving time-based fairness by using an analytical model. Using Jains fairness index, optimal MAC parameters required in achieving maximum fairness between slow and fast stations are obtained. The impacts of these mechanisms on throughput of slow and fast stations are explored. We also consider the notion of proportional fairness in a multirate scenario and prove that it is equivalent to fair channel time allocation. Last, our investigation of an alternative fairness criterion also leads us to propose that the IEEE 802.11 MAC protocol should be redesigned with temporal fairness as a design objective in avoiding inefficiencies related to the performance anomaly.

TCP HACK: TCP header checksum option to improve performance over lossy links

Wireless networks have become increasingly common and an increasing number of devices are communicating with each other over lossy links. Unfortunately, TCP performs poorly over lossy links as it is unable to differentiate the loss due to packet corruption from that due to congestion. We present an extension to TCP which enables TCP to distinguish packet corruption from congestion in lossy environments resulting in improved performance. We refer to this extension as the HeAder ChecKsum option (HACK). We implemented our algorithm in the Linux kernel and performed various tests to determine its effectiveness. Our results have shown that HACK performs substantially better than both SACK and NewReno in cases where burst corruptions are frequent. We also found that HACK can co-exist very nicely with SACK and performs even better with SACK enabled.

Multicast zone routing protocol in mobile ad hoc wireless networks

In the past years, a variety of unicast and multicast routing protocols for mobile ad hoc wireless networks (MANETs) have been developed. Zone routing protocol (ZRP) is one of these unicast routing protocols, which is a hybrid protocol that proactively maintains routing information for a local neighborhood (routing zone), while reactively acquiring routes to destinations beyond the routing zone. In this paper, we adapt ZRP for multicast routing purpose, and call it multicast zone routing protocol (MZRP). MZRP is a shared tree multicast routing protocol, which proactively maintains the multicast tree membership for nodes local routing zone at each node while establishes multicast tree on-demand. It is scalable to large number of multicast senders and groups. The use of IP tunnel mechanism during data packets transmission improves the overall performance. The evaluation and comparison with ODMRP are performed on the ns-2 simulator.

TCP Vegas-A: Improving the Performance of TCP Vegas

While it has been shown that TCP Vegas provides better performance compared to TCP Reno, studies have identified various issues associated with the protocol. We propose modifications to the congestion avoidance mechanism of the TCP Vegas to overcome these limitations. Unlike the solutions proposed in the past, our solution, named TCP Vegas-A, is neither dependent on optimising any critical parameter values nor on the buffer management scheme implemented at the routers and hence can be implemented solely at the end host. Our simulation experiments over wired as well as over geosynchronous and lower earth orbit satellite links show that TCP Vegas-A is able to overcome several of the identified problems-it can obtain a fairer share of the network bandwidth in wired and satellite scenarios, tackle rerouting issues, rectify Vegass bias against higher bandwidth flows and prevail over fluctuating RTT conditions of a lower earth orbit satellite link. At the same time, Vegas-A is able to preserve the unique properties of Vegas that had made it a noteworthy protocol.

A QoS enabled MAC protocol for multi-hop ad hoc wireless networks

A medium access control (MAC) technique for multi-hop wireless networks with quality of service (QoS) assurance is introduced The proposed protocol deals with most of the hidden node problems in a multi-hop environment and also provides service differentiation for various types of traffic: real-time constant bit rate (CBR) traffic, real-time variable bit rate (VBR) traffic, and non-real-time datagram traffic, by guaranteeing bounded delay for real-time traffic, at the same time avoiding lockout of datagram traffic. It uses a distributed mechanism of scheduling and reserving the channel access for real-time sessions. Transmission of datagram or starting packet of a new realtime session is granted only after checking the reservation such that they do not affect the scheduled ongoing real-time sessions. The protocol has been implemented and tested using the VINT Network Simulator 2 (NS2). The performance evaluation and comparison confirm the improved QoS features and multimedia support compared to various existing QoS enabled MAC protocols.

Localization Using Ray Tracing for Underwater Acoustic Sensor Networks

Localization is an indispensable part for most Underwater Wireless Sensor Networks (UWSNs). In these networks, acoustic ranging signals are typically used. Because of the depth dependent sound speed, ray bends in water. This presents a unique problem making conventional localization algorithms assuming straight line distances between target and anchor nodes inaccurate. In this work, authors present a scheme that takes in to account bending of path, to provide an accurate location estimate in UWSNs. The approach makes use of standard ray equations to model the path followed by rays of sound in water. The results of the preliminary study using this scheme are also reported.

Distributed wireless sensor network localization using stochastic proximity embedding

In this paper, we propose a computationally efficient distributed Wireless Sensor Network (WSN) localization method based on Stochastic Proximity Embedding (SPE), which is a dimensionality reduction technique that finds a low dimensional embedding of a high dimensional data by preserving the pair-wise distance data information. Unlike the localization techniques based on classical Multidimensional Scaling (MDS), which is a popular dimensionality reduction technique, SPE method does not require a complete distance information matrix of the network and it scales linearly with the number of nodes in the network. Also the stochastic descent approach adopted in SPE provides an accurate position estimate in reasonable number of iterations. Through extensive simulation study of the proposed method, it is found to provide better results in both uniform and irregular shaped sensor networks.

SCTP vs TCP : performance comparison in MANETs

We provide a simulation-based performance comparison of SCTP (stream control transmission protocol) vs TCP (Transmission Control Protocol) in MANET (mobile ad hoc network) environments. We conclude that the behavior of SCTP and TCP in MANETs is similar, but TCP outperforms SCTP in most cases because of extra overheads present in SCTP.

TCP Vegas-A: solving the fairness and rerouting issues of TCP Vegas

In spite of the larger performance gain such as higher throughput and almost zero packet retransmissions compared to TCP Reno, TCP Vegas still has a few obstacles for it to be deployed in the Internet. Studies have shown unfair treatment to Vegas connections when they compete with Reno connections. Other issues identified with TCP Vegas are problems of rerouting, persistent congestion, and discrepancy in flow rate tied with starting times and link bandwidth. We reinvestigate these issues rind propose modifications to the congestion avoidance mechanism of the TCP Vegas, with the slow-start and congestion recovery algorithms of Vegas remaining untouched. Unlike the solutions proposed in the recent past to deal with some of these issues, our solution it neither dependent on any critical parameter values nor on the buffer management scheme at the routers (e.g., RED). Our experiments show that the modified TCP Vegas (Vegas-A) it able to obtain a fairer share of the network bandwidth when competing with other TCP flows. We also show that Vegas-A can tackle rerouting issues and rectify Vegass bias against higher bandwidth flows. At the same time, our experiments prove that Vegas-A preserves the properties of Vegas that have made it a noteworthy protocol.

Avoiding congestion collapse on the internet using TCP Tunnels

This paper discusses the application of TCP tunnels on the Internet and how Internet traffic can benefit from the congestion control mechanism of the tunnels. Primarily, we show the TCP tunnels offer TCP-friendly flows protection from TCP-unfriendly traffic. TCP tunnels also reduce the many flows situation on the Internet to that of a few flows. In addition, TCP tunnels eliminate unnecessary packet loss in the core routers of the congested backbones, which waste precious bandwidth leading to congestion collapse due to unresponsive UDP flows. We finally highlight that the use of TCP tunnels can, in principle, help prevent certain forms of congestion collapse described by Floyd and Fall [IEEE/ ACM Trans Networking 7 (4) (1999) 458].The deployment of TCP tunnels on the Internet and the issues involved are also discussed and we conclude that with the recent RFC2309 recommendation of using random early drop as the default packet-drop policy in Internet routers, coupled with the implementation of a pure tunnel environment on backbone networks makes the deployment of TCP tunnels a feasible endeavour worthy of further investigation.