Effective-capacity-based stochastic delay guarantees for systems with time-varying servers, with an application to IEEE 802.11 WLANs

Many network applications rely on stochastic QoS guarantees. With respect to loss-related performance, the effective bandwidth/capacity theory has proved useful for calculating loss probabilities in queues with complex input and server processes and for formulating simple admission control tests to ensure associated QoS guarantees. This success has motivated the application of the theory for delay-related QoS too. However, up until now this application has been justified only heuristically for queues with variable service rate. The paper fills this gap by establishing rigorously that the effective bandwidth/capacity theory may be used for the asymptotically correct calculation and enforcement of delay tail-probabilities in systems with variable rate servers too. Subsequently, the paper applies the general results to IEEE 802.11 WLANs, by representing each IEEE 802.11 station as an On/Off server and employing the effective capacity function for this model. Comparison of analytical results with simulation validates the effectiveness of the On/Off IEEE 802.11 model for delay-related QoS, complementing earlier results on loss-related performance.     

Towards efficient decision rules for admission control based on the many sources asymptotics

This paper introduces new admission criteria that enable the use of algorithms based on the many sources asymptotics in real-life applications. This is achieved by a significant reduction in the computational requirements and by moving the computationally intensive tasks away from the timing-sensitive decision instant. It is shown that the traditional overflow probability type admission control method can be reformulated into a bandwidth requirement type and into a buffer requirement type method and that these methods are equivalent when used for admission control. The original and the two proposed methods are compared through the example of fractional Brownian motion (fBm) traffic.     

A novel Effective Capacity-based framework for providing statistical QoS guarantees in IEEE 802.11 WLANs

This article proposes a performance model of the IEEE 802.11 MAC layer that employs the notion of Effective Capacity. In particular, the paper establishes that an IEEE 802.11 mobile station can be regarded as a Semi-Markovian bursty server of the On/Off type, with known distributions for the On and Off periods, and subsequently applies known results for Semi-Markovian models to derive the Effective Capacity function of this On/Off server. The general Effective Bandwidth/Capacity theory can then be used for computing buffer overflow probabilities and for employing simple traffic control policies to enforce related QoS guarantees. The policies guarantee a soft bound on the buffer overflow probability and are suitable for real-time traffic control over WLANs. The Effective Capacity model of IEEE 802.11 stations is originally developed by assuming that the other competing stations are saturated. This is a conservative assumption that becomes very accurate in a highly loaded network. Subsequently, the model is adapted to encompass lightly loaded networks as well. In the adapted model, each mobile station directly measures a few model parameters, instead of calculating them on the basis of the saturation assumption, and uses these measurements in the computation of its Effective Capacity function. The theoretical results are checked against simulations, validating the appropriateness of the model.     

Extended EDCA for delay guarantees in wireless local area networks

Despite their very broad diffusion, IEEE 802.11 Wireless Local Area Networks (WLANs) are not able to provide service differentiation and to support real-time multimedia applications, due to their channel access methods. To overcome these limitations, the 802.11e working group has proposed the Enhanced Distributed Coordination Access (EDCA) scheme, which achieves service differentiation on a statistical basis by properly mapping user Quality of Service (QoS) requirements to channel contention parameters. Such a scheme will be included in the emerging 802.11n standard and in the revision of the 802.11 standard. However, it has been widely demonstrated that, especially at high network loads, EDCA does not provide an effective usage of the channel capacity. In particular, it is unable to provide a bounded delay service to all kinds of multimedia flow because flows with lower channel access priorities are starved to advantage only those with the highest priority. To fix this undesired behavior and improve wireless LAN performance, this paper proposes a new Extended EDCA (E²DCA) scheme, that is compliant with 802.11e specifications. By exploiting a closed-loop control algorithm, E²DCA performs a distributed dynamic bandwidth allocation, providing guarantees on average/absolute delays to real-time media flows, regardless of their priorities. Moreover, an innovative Call Admission Control (CAC) procedure has been developed. Using the ns-2 simulator, the effectiveness of the algorithm has been investigated in realistic network scenarios, involving a mix of audio, video, and FTP flows, at several network loads and with random losses. Results have shown that the proposed scheme is able to provide a bounded delay service to multimedia flows in a wide range of network loads and frame loss ratios.     

Optimal control of LTI systems over unreliable communication links

In this paper, optimal control of linear time-invariant (LTI) systems over unreliable communication links is studied. The motivation of the problem comes from growing applications that demand remote control of objects over Internet-type or wireless networks where links are prone to failure. Depending on the availability of acknowledgment (ACK) signals, two different types of networking protocols are considered. Under a TCP structure, existence of ACK signals is assumed, unlike the UDP structure where no ACK packets are present. The objective here is to mean-square (m.s.) stabilize the system while minimizing a quadratic performance criterion when the information flow between the controller and the plant is disrupted due to link failures, or packet losses. Sufficient conditions for the existence of stabilizing optimal controllers are derived.     

Adaptive explicit congestion control based on bandwidth estimation for high bandwidth-delay product networks

Existing congestion control protocols for high bandwidth-delay product (HBDP) networks either suffer from efficiency degradation caused by insufficient feedback of the network state, or are difficult to deploy in real networks because they require modification in IP header to provide enough feedback information. Based on VCP, this paper proposes the VCP-BE protocol that can provide more accurate network state information by estimating available bandwidth for sender with an adaptive bandwidth estimator. The estimator can dynamically adjust its parameters (e.g., observing interval) to achieve different accuracy and responsiveness to adapt to the network state indicated by two ECN bits, without no more modification in IP header than VCP. Furthermore, when the network load is high, VCP-BE calculates the ratio of current throughput to the available bandwidth to judge if network is close to the overload state. With these fine-grained network state estimation, VCP-BE can adjust the congestion window more precisely than VCP, thus greatly improves its convergence speed of achieving high bandwidth utilization and fairness. Simulation results show that VCP-BE also outperforms MLCP, which uses seven bits for explicit feedback while VCP-BE only uses two ECN bits.     

适合于移动多媒体网络的呼叫接纳控制算法

移动多媒体网络应该能够同时支持传统的数据业务和实时交互式多媒体业务,并能够为用户提供QoS保证.在无线移动网中提供QoS保证,呼叫接纳控制扮演着重要的角色.通过对呼叫接纳算法中资源预留方案进行了分析总结,提出了一种适合于移动多媒体网络的自适应呼叫接纳控制算法.     

Dynamic control of service rates in queuing networks

A method for dynamic control of service rates in closed exponential queuing networks is proposed. The performance of queuing networks with the service-rate control is analyzed, and the main steady-state network characteristics are computed using an analytic approximation. A simple example of a queuing network with controlled service rates is considered as an illustration. The efficiency of the service-rate control is confirmed by Monte Carlo simulations, which, as a by-product, also show acceptable accuracy of our analytical approximations.     

Robust stability analysis of simple systems controlled over communication networks

There is increasing interest in controlling systems over communication networks. Using a simple method called dual-locus diagram, this communique proposes complete stability criteria for a mass-spring-damper system controlled over the network. The stability region is divided into a delay-dependent stability region and a delay-independent stability region, which offers a nice graphical view on the conservativeness of the delay-independent stability criteria.     

A neural network model of a communication network with information servers

This paper models information flow in a communication network. The network consists of nodes that communicate with each other, and information servers that have a predominantly one-way communication to their customers. A neural network is used as a model for the communication network. The existence of multiple equilibria in the communication network is established. The network operator observes only one equilibrium, but if he knows the other equilibria, he can influence the free parameters, for example by providing extra bandwidth, so that the network settles in another equilibrium that is more profitable for the operator. The influence of several network parameters on the dynamics is studied both by simulation and by theoretical methods.The author was with the Intelligent Systems Unit, BT Laboratories, Martlesham Heath, Ipswich IP5 7RE, UK.     

Design issues encountered in the development of a mobile multimedia augmentative communication service

Augmentative and alternative communication (AAC) systems can be mounted on a range of different hardware platforms, from custom-designed units to desktop or laptop personal computers and hand-held and palmtop systems. Palmtop devices such as personal data assistants (PDAs) offer great advantages of portability. The small display size and limited storage and processing capacity of a PDA compared to larger systems are likely to impose some limitations on the range of AAC applications which can be supported, however, particularly when multimedia-based applications are considered. This paper addresses issues involved in migrating a multimedia AAC application onto a palm-top PDA and discusses the user involvement in the re-engineering of the system for that environment. Outcomes from an initial practical trial with a person who uses AAC are reported.     

Dynamics and stability in optical communication networks: a system theory framework

This paper addresses the problem of dynamics analysis in optical networks from a system control perspective. A general framework for finding the transfer matrix representation of an optical network is developed, based on linear fractional transformations. Under the natural assumption of equal time-delay for all channels in a link, the network transfer matrix is simplified such that channel cross-coupling is evidenced. The optical network stability problem is then reformulated as a robust stability problem and stability conditions are developed by applying μ-analysis.     

通信网络在智能建筑中的应用研究

对智能建筑通信网络(IBN)进行了研究，重点分析了：智能建筑信息系统(IBIS)网络的集成以及将不同的建筑系统集成于一个框架内的标准协议的应用．而ISDN与ADSL被认为是实现IBN的合适平台。     

On the large deviations approximation for the stationary distribution of skip-free regulated queueing networks

Large deviations papers like that of Ignatyuk et al. [Russ. Math. Surv. 49 (1994) 41–99] have shown that asymptotically, the stationary distribution of homogeneous regulated networks is of the form

with the coefficient being different in various “boundary influence domains” and also depending on some of these domains on n. In this paper, we focus on the case of constant exponents and on a subclass of networks we call “strongly skip-free” (which includes all Jackson and all two-dimensional skip-free networks). We conjecture that an asymptotic exponent is constant iff it corresponds to a large deviations escape path which progresses gradually (from the origin to the interior) through boundary facets whose dimension always increases by one. Solving the corresponding large deviations problem for our subclass of networks leads to a family of “local large deviation systems” (LLDSs) (for the constant exponents), which are expressed entirely in terms of the cumulant generating function of the network. In this paper, we show that at least for “strongly skip-free” Markovian networks with independent transition processes, the LLDS is closely related to some “local boundary equilibrium systems” (LESs) obtained by retaining from the equilibrium equations only those valid in neighborhoods of the boundary.

Since asymptotic results require typically only that the cumulant generating function is well-defined over an appropriate domain, it is natural to conjecture that these LLDSs will provide the asymptotic constant exponents regardless of any distributional assumptions made on the network.

Finally, we outline a practical recipe for combining the local approximations to produce a global large deviations approximation , with the coefficients K_j determined numerically.     

On the complexity of bandwidth allocation in radio networks

We define and study an optimization problem that is motivated by bandwidth allocation in radio networks. Because radio transmissions are subject to interference constraints in radio networks, physical space is a common resource that the nodes have to share in such a way, that concurrent transmissions do not interfere. The bandwidth allocation problem we study under these constraints is the following. Given bandwidth (traffic) demands between the nodes of the network, the objective is to schedule the radio transmissions in such a way that the traffic demands are satisfied. The problem is similar to a multicommodity flow problem, where the capacity constraints are replaced by the more complex notion of non-interfering transmissions. We provide a formal specification of the problem that we call round weighting  . By modeling non-interfering radio transmissions as independent sets, we relate the complexity of round weighting to the complexity of various independent set problems (e.g. maximum weight independent set, vertex coloring, fractional coloring). From this relation, we deduce that in general, round weighting is hard to approximate within n^1−ε$n^{1-\epsilon }$ (n$n$ being the size of the radio network). We also provide polynomial (exact or approximation) algorithms e.g. in the following two cases: (a) when the interference constraints are specific (for instance for a network whose vertices belong to the Euclidean space), or (b) when the traffic demands are directed towards a unique node in the network (also called gathering, analogous to single commodity flow).     

High performance power control and opportunistic fair scheduling in TH-PPM UWB ad-hoc multimedia networks

Ultra wideband (UWB) systems are currently an important wireless infrastructure for high performance short-range communications and mobile applications. Indeed, forming ad-hoc networks among various UWB enabled devices is considered as an important mobile data exchange operating environment. In our study, we explore the problem of jointly optimizing the power level and data rate used in the devices in such a UWB based ad-hoc network. We propose a practical optimization algorithm based on judicious power control for real-time applications and opportunistic scheduling for non-real-time applications. Simulation results indicate that our proposed techniques are effective under various practical scenarios.

A preemptive channel allocation scheme for multimedia traffic in mobile wireless networks

This paper presents a channel allocation model with both partial and full preemptive capabilities for multimedia traffic in mobile wireless networks. The different types of multimedia traffic in transmitting through next-generation networks possess different characteristics and demand an adequate channel allocation scheme to fulfill individual Quality of Service (QoS) requirements. In the proposed model, multimedia traffic is classified into three classes corresponding to different priority levels in a decreasing order. To effectively reduce the dropping probability, a higher-priority handoff call is allowed to fully or partially preempt any lower-priority ongoing calls when the channel capacity becomes insufficient. In addition, to prevent from possible starvation of the lowest-priority traffic, a portion of system channels are purposely reserved. Performance measures, including the dropping probability of handoff calls, the call-interruption probability that an ongoing call is forced to termination due to full preemption, and the bandwidth reduction ratio due to partial preemption, are investigated through an analytical model built with 4-D Markov chains. The numerical results demonstrate the effectiveness of our model.     

Radio resource management in emerging heterogeneous wireless networks

Deployment of heterogeneous wireless networks is spreading throughout the world as users want to be connected anytime, anywhere, and anyhow. Meanwhile, these users are increasingly interested in multimedia applications such as video streaming and Voice over IP (VoIP), which require strict Quality of Service (QoS) support. Provisioning network resources with such constraints is a challenging task. In fact, considering the availability of various access technologies (WiFi, WiMAX, or cellular networks), it is difficult for a network operator to find reliable criteria to select the best network that ensures user satisfaction while maximizing network utilization. Designing an efficient Radio Resource Management (RRM), in this type of environment, is mandatory for solving such problems. In order to provide a better understanding of RRM’s design, this paper presents a detailed investigation of key challenges that constitute an efficient RRM framework. More importantly, an overview with a classification of recent solutions, in terms of decision making, is provided along with the discussion.     

Part 2: Traffic control in Multimedia Networks An efficient algorithm for multimedia call-admission control

This paper presents a call admission control (CAC) algorithm for multimedia communication systems with diverse bit-rate media sources. The proposed algorithm for making bandwidth allocation decisions is based on bounding the probability of demand for excess bandwidth and the loss (or blocking) probability. Recursion formulas for calculating these probabilities are derived. An algorithm based on these probabilities executes quickly enough so that the response time to a bandwidth request is acceptable.