Shivkumar Kalyanaraman

On the capacity improvement of ad hoc wireless networks using directional antennas

The capacity of ad hoc wireless networks is constrained by the interference between concurrent transmissions from neighboring nodes. Gupta and Kumar have shown that the capacity of an ad hoc network does not scale well with the increasing number of nodes in the system when using omnidirectional antennas [6]. We investigate the capacity of ad hoc wireless networks using directional antennas. In this work, we consider arbitrary networks and random networks where nodes are assumed to be static.In arbitrary networks, due to the reduction of the interference area, the capacity gain is proven to be √2π/α when using directional transmission and omni reception. Because of the reduced probability of two neighbors pointing to each other, the capacity gain is √2π/β when omni transmission and directional reception are used. Although these two expressions look similar, the proof technique is different. By taking advantage of the above two approaches, the capacity gain is 2π/√αβ when both transmission and reception are directional.For random networks, interfering neighbors are reduced due to the decrease of interference area when directional antennas are used for transmission and/or reception. The throughput improvement factor is 2π/α, 2π/β and 4π2/αβ for directional transmission/omni reception, omni transmission/direc-tional reception, and directional transmission/directional reception, respectively.We have also analyzed hybrid beamform patterns that are a mix of omnidirectional/directional and a better model of real directional antennas.

Rate-based flow controllers for communication networks in the presence of uncertain time-varying multiple time-delays

An H^~ based robust controller is designed for a rate-feedback flow-control problem in single-bottleneck communication networks. The controller guarantees stability robustness to uncertain time-varying multiple time-delays in different channels. It also brings the queue length at the bottleneck node to the desired steady-state value asymptotically and satisfies a weighted fairness condition. Lower bounds for stability margins for uncertainty in the time-delays and for the rate of change of the time-delays are derived. A number of simulations are included to demonstrate the time-domain performance of the controller. Trade offs between robustness and time-domain performance are also discussed.

Analytic models for the latency and steady-state throughput of TCP Tahoe, Reno, and SACK

Continuing the process of improvements made to TCP through the addition of new algorithms in Tahoe and Reno, TCP SACK aims to provide robustness to TCP in the presence of multiple losses from the same window. In this paper we present analytic models to estimate the latency and steady-state throughput of TCP Tahoe, Reno, and SACK and validate our models using both simulations and TCP traces collected from the Internet. In addition to being the first models for the latency of finite Tahoe and SACK flows, our model for the latency of TCP Reno gives a more accurate estimation of the transfer times than existing models. The improved accuracy is partly due to a more accurate modeling of the timeouts, evolution of cwnd during slow start and the delayed ACK timer. Our models also show that, under the losses introduced by the droptail queues which dominate most routers in the Internet, current implementations of SACK can fail to provide adequate protection against timeouts and a loss of roughly more than half the packets in a round will lead to timeouts. We also show that with independent losses SACK performs better than Tahoe and Reno and, as losses become correlated, Tahoe can outperform both Reno and SACK.

TCP rate control

This paper presents TCP rate control, a new technique for transparently augmenting end-to-end TCP performance by controlling the sending rate of a TCP source. The sending rate of a TCP source is determined by its window size, the round trip time and the rate of acknowledgments. TCP rate control affects these aspects by modifying the ack number and receiver window fields in acknowledgments and by modulating the acknowledgment rate. From a performance viewpoint a key benefit of TCP rate control is to avoid adverse performance effects due to packet losses such as reduced goodput and unfairness or large spread in per-user goodputs. Further, TCP rate control positively affects performance even if the bottleneck is non-local and the end-host TCP implementations are non-conforming. These aspects are demonstrated through a comparative study of TCP rate control, RED and TCP-ECN. The TCP rate control approach has been implemented and patented by Packeteer Inc.

Source behavior for ATM ABR traffic management: an explanation

The available bit rate (ABR) service has been developed to support data applications over asynchronous transfer mode (ATM) networks. The network continuously monitors its traffic and provides feedback to the source end systems. This article explains the rules that the sources have to follow to achieve a fair and efficient allocation of network resources.

An integrated model for the latency and steady-state throughput of TCP connections

Abstract Most TCP connections in today’s Internet transfer data on the order of only a few kilobytes. Such TCP transfers are very short and spend most of their time in the slow start phase. Thus the underlying assumptions made by steady-state models cease to hold, making them unsuitable for modeling finite flows. In this paper, we propose an accurate model for estimating the transfer times of TCP flows of arbitrary size. Our model gives a more accurate estimation of the transfer times than those predicted by Cardwell et al. [Proceedings of the IEEE INFOCOM, Tel Aviv, Israel, March 2000, pp. 1742–1751], which extends the steady-state analysis of Padhye et al. [IEEE/ACM Trans. Networking 8 (2) (2000) 133] to model finite flows. The main features of our work are the modeling of timeouts and slow start phases which occur anywhere during the transfer and a more accurate model for the evolution of the cwnd in the slow start phase. Additionally, the proposed model can also model the steady-state throughput of TCP connections. The model is verified using web based measurements of real life TCP connections. We also introduce an empirical model which allows a better “feel” of TCP latency and the nature of its dependence on loss probabilities and window limitation. Finally, the paper investigates the effect on window limitation and packet size on TCP latency.

Weak state routing for large scale dynamic networks

Forwarding decisions in routing protocols rely on information about the destination nodes provided by routing table states. When paths to a destination change, corresponding states become invalid and need to be refreshed with control messages for resilient routing. In large and highly dynamic networks, this overhead can crowd out the capacity for data traffic. For such networks, we propose the concept of weak state, which is interpreted as a probabilistic hint, not as absolute truth. Weak state can remain valid without explicit messages by systematically reducing the confidence in its accuracy. Weak State Routing (WSR) is a novel routing protocol that uses weak state along with random directional walks for forwarding packets. When a packet reaches a node that contains a weak state about the destination with higher confidence than that held by the packet, the walk direction is biased. The packet reaches the destination via a sequence of directional walks, punctuated by biasing decisions. WSR also uses random directional walks for disseminating routing state and provides mechanisms for aggregating weak state. Our simulation results show that WSR offers a very high packet delivery ratio ( ≥ 98%). Control traffic overhead scales as O(N), and the state complexity is Θ(N3/2), where N is the number of nodes. Packets follow longer paths compared to prior protocols (OLSR , GLS-GPSR , ), but the average path length is asymptotically efficient and scales as O(√N). Despite longer paths, WSRs end-to-end packet delivery delay is much smaller due to the dramatic reduction in protocol overhead.

LT-TCP: end-to-end framework to improve TCP performance over networks with lossy channels

TCP performance over wireless links suffers substantially when packet error rates increase beyond about 1% - 5%. This paper proposes end-end mechanisms to improve TCP performance over lossy networks with potentially much higher packet loss rates. Our proposed scheme separates congestion indications from wireless packet erasures by exploiting ECN. Timeout effects due to packet erasures are combated using a dynamic and adaptive Forward Error Correction (FEC) scheme that includes adaptation of TCP’s Maximum Segment Size. Proactive and reactive FEC overhead enhance TCP SACK to protect original segments and retransmissions respectively. Dynamically changing the MSS tailors the number of segments in the window for optimal performance. SACK and timeout mechanisms are used as a last resort. ns-2 simulations show that our scheme substantially improves TCP performance even for packet loss rates up to 30%, thus extending the dynamic range and performance of TCP over networks with lossy (e.g., wireless) links.

UBR+: improving performance of TCP over ATM-UBR service

ATM-UBR service responds to congestion by dropping cells when switch buffers become full. TCP connections running over UBR experience low throughput and high unfairness. For 100% TCP throughput, each switch needs buffers equal to the sum of the window sizes of all the TCP connections. Intelligent drop policies can improve the performance of TCP over UBR with limited buffers. The UBR+ service proposes enhancements to UBR for intelligent drop. The early packet discard scheme improves throughput but does not attempt to improve fairness. The selective packet drop scheme based on per-connection buffer occupancy improves fairness. The fair buffer allocation scheme further improves both throughput and fairness.

Error analysis of multi-hop free-space optical communication

In this paper we analyze the error performance of free-space optical (FSO) communication over multiple hops. We first develop an error model for a single hop based on visibility, atmospheric attenuation, and geometric spread of the light beam. We model atmospheric visibility by Gaussian distributions with mean and variance values to reflect clear and adverse weather conditions. Based on this, we find the end-to-end bit error distribution of the FSO link for single hop and multi-hop scenarios. We present simulation results for decoded relaying, where each hop decodes the signal before retransmitting. We demonstrate that multi-hop FSO communication achieves a significant reduction in the mean bit error rate and also reduces the variance of the bit error rate. We argue that by lowering mean error and error variance, multi-hop operation facilitates an efficient system design and improves the reliability of the FSO link by application of specific coding schemes (such as forward error correction techniques).