Thyagarajan Nandagopal

Achieving MAC layer fairness in wireless packet networks

Link-layer fairness models that have been proposed for wireline and packet cellular networks cannot be generalized for shared channel wireless networks because of the unique characteristics of the wireless channel, such as location-dependent contention, inherent conflict between optimizing channel utilization and achieving fairness, and the absence of any centralized control.nIn this paper, we propose a general analytical framework that captures the unique characteristics of shared wireless channels and allows the modeling of a large class of system-wide fairness models via the specification of per-flow utility functions. We show that system-wide fairness can be achieved without explicit global coordination so long as each node executes a contention resolution algorithm that is designed to optimize its local utility function.nWe present a general mechanism for translating a given fairness model in our framework into a corresponding contention resolution algorithm. Using this translation, we derive the backoff algorithm for achieving proportional fairness in wireless shared channels, and compare the fairness properties of this algorithm with both the ideal proportional fairness objective, and state-of-the-art backoff-based contention resolution algorithms.nWe believe that the two aspects of the proposed framework, i.e. the ability to specify arbitrary fairness models via local utility functions, and the ability to automatically generate local contention resolution mechanisms in response to a given utility function, together provide the path for achieving flexible service differentiation in future shared channel wireless networks.

Fair queuing in wireless networks: issues and approaches

Fair queuing has long been a popular paradigm for providing bounded delay channel access and separation between flows in wireline networks. However, adapting fair queuing to the wireless domain is not a trivial task because of the unique problems in wireless channels such as location-dependent and bursty channel error. In this article we identify the key issues in wireless fair queuing, define a wireless fair service model, present a generic framework for designing wireless fair queuing algorithms, and explore solutions within this framework. Using simple examples, we show that some of the wireless fair queuing algorithms proposed in literature can achieve wireless fair service.

A wireless fair service algorithm for packet cellular networks

Wireless Fair Service Algorithm For Packet Cellular Networks Sonwu Lu Thyagarajan Nandagopal Vaduvur Bharghavan Coordinated Science Laboratory University of Illinois email: {sIu, thyagu, bharghav}~timely. crhc .uiuc. edu b order to support diverse communication-intensive redtirne and non red-time data flows over a scarce, varying and shared wireless channel with location-dependent and bursty errors, we defie a service model that has the following charxteristi~ short-tern fairness among flows which perceive a clean channel, worst-case delay bounds for packets, short-term throughput bounds for flows with clean channels and Iong-tem throughput bounds for dl flows with bounded channel error, optimal schedulable region, and support for both delay sensitive and error sensitive data flows. We present a wireless fair service algorithm, and show that it achieves the requirements of the service model through both analysis and simulation. The key aspects of the dg~ rithm are the following (a) an enhanced fair queueing based service scheme that supports decoupling of delay and bandwidth, (b) graceful service compensation for lagging flows and graceful service degradation for leading flows, (c) support for red-time delay sensitive flows as well as non realtime error sensitive flows, and (d) implementation of the wireless fair service rdgorithm within the fiarnework of the simple and robust CSMA/CA wireless medium access pro tocol.

Delay differentiation and adaptation in core stateless networks

We present a core-stateless quality of service architecture for achieving delay differentiation between flows. There are two key components in our approach: (1) per-class per-hop relative average delay-the average queueing delay perceived by the packets in a delay class at a link is inversely proportional to the delay weight of the class; (2) per-flow end-to-end delay class adaptation-the delay class of a flow is dynamically adjusted based on its perceived end-to-end delay in order to maintain the desired end-to-end average delay requirement of the flow. We show through simulations and analysis that these two components can, in concert, support end-to-end delay differentiation between flows using only simple mechanisms at the core routers in the network.

WTCP: a reliable transport protocol for wireless wide-area networks

Wireless wide-area networks (WWANs) are characterized by very low and variable bandwidths, very high and variable delays, significant non-congestion related losses, asymmetric uplink and downlink channels, and occasional blackouts. Additionally, the majority of the latency in a WWAN connection is incurred over the wireless link. Under such operating conditions, most contemporary wireless TCP algorithms do not perform very well. In this paper, we present WTCP, a reliable transport protocol that addresses rate control and reliability over commercial WWAN networks such as CDPD. WTCP is rate-based, uses only end-to-end mechanisms, performs rate control at the receiver, and uses inter-packet delays as the primary metric for rate control. We have implemented and evaluated WTCP over the CDPD network, and also simulated it in the ns-2 simulator. Our results indicate that WTCP can improve on the performance of comparable algorithms such as TCP-NewReno, TCP-Vegas, and Snoop-TCP by between 20% to 200% for typical operating conditions.

Design and analysis of an algorithm for fair service in error-prone wireless channels

In order to support diverse communication‐intensive real‐time and non‐real‐time data flows over a scarce, varying and shared wireless channel with location‐dependent and bursty errors, we define a service model that has the following characteristics: short‐term fairness among flows which perceive a clean channel, long‐term fairness for flows with bounded channel error, worst‐case delay bounds for packets, short‐term throughput bounds for flows with clean channels and long‐term throughput bounds for all flows with bounded channel error, expanded schedulable region, and support for both delay sensitive and error sensitive data flows. We present the wireless fair service algorithm, and show through both analysis and simulation that it achieves the requirements of the service model in typical wireless network environments. The key aspects of the algorithm are the following: (a) an enhanced fair queueing based service scheme that supports decoupling of delay and bandwidth, (b) graceful service compensation for lagging flows and graceful service degradation for leading flows, (c) support for real‐time delay sensitive flows as well as non‐real‐time error sensitive flows, and (d) an implementation within the framework of the simple and robust CSMA/CA wireless medium access protocol.

Coping with link failures in centralized control plane architectures

Recently proposed SoftRouter and 4D network architectures recommend having the least amount of intelligence in the switches. They advocate transferring control plane functionalities to general-purpose servers that could govern these switches remotely. A link failure in such architectures could result into switches losing contact with the controller or even generating routing loops. These scenarios could generate a large amount of unnecessary traffic in the network. We study the implications of a failed link in such architectures. We develop an algorithm that would inform only the relevant switches to refrain from sending messages in the direction of the failed link, and yet have the minimum amount of intelligence on the switches. We implement our algorithm on a network formed by OpenFlow switches and evaluate its performance. Our experiments verify that the performance of our algorithm is dependent on the total number of flow-table entries in a switch. We also verify that by implementing our algorithm all the necessary switches are informed of the failed link significantly sooner than the controller identifies the failed link and sends out an update.

A unified architecture for the design and evaluation of wireless fair queueing algorithms

Fair queueing in the wireless domain poses significant challenges due to unique issues in the wireless channel such as location-dependent and bursty channel errors. In this paper, we present a wireless fair service model that captures the scheduling requirements of wireless scheduling algorithms, and present a unified wireless fair queueing architecture in which scheduling algorithms can be designed to achieve wireless fair service. We map seven recently proposed wireless fair scheduling algorithms to the unified architecture, and compare their properties through simulation and analysis. We conclude that some of these algorithms achieve the properties of wireless fair service including short-term and long-term fairness, short-term and long-term throughput bounds, and tight delay bounds for channel access.

Service differentiation through end-to-end rate control in low bandwidth wireless packet networks

With the increased commercial deployment of wireless networks, the issue of service differentiation among flows in a wireless/wireline network is becoming important. While many wireline solutions for service differentiation depend on end to-end rate control upon packet loss, this approach does not work well for wireless networks because packet losses do not necessarily indicate congestion in the network in the wireless domain. In a related work, we presented WTCP, a reliable transport protocol that is designed to operate efficiently and fairly over wireless wide area networks. The rate control algorithm in WTCP is based on the use of the ratio of the inter-packet separation at the receiver and the sender as the metric for detecting and reacting to congestion. In this paper, we generalize the rate control mechanisms of WTCP to support both reliable and unreliable flows, and to provide service differentiation among flows. We present preliminary results using the ns2 simulator to show that our rate control algorithm provides service differentiation for low bandwidth wireless networks and scales well with the variations in the link loss characteristics.

Scalable service differentiation using purely end-to-end mechanisms: features and limitations

We investigate schemes for achieving service differentiation via weighted end-to-end congestion control mechanisms within the framework of the additive increase/multiplicative decrease (AIMD) principle, and study their performance as instantiations of the TCP protocol. Our first approach considers a class of weighted AIMD algorithms. This approach does not scale well in practice because it leads to excessive loss for flows with large weights, thereby causing early timeouts and a reduction in throughput. Our second approach considers a class of loss adaptive weighted AIMD algorithms. This approach scales by an order of magnitude compared to the previous approach, but is more susceptible to short-term unfairness and is sensitive to the accuracy of loss estimates. We conclude that adapting the congestion control parameters to the loss characteristics is critical to scalable service differentiation; on the other hand, estimating loss characteristics using purely end-to-end mechanisms is an inherently difficult problem.