End-to-End Delay Distribution Analysis for Stochastic Admission Control in Multi-hop Wireless Networks

published_or_final_versio

Low-latency Networking: Where Latency Lurks and How to Tame It

While the current generation of mobile and fixed communication networks has been standardized for mobile broadband services, the next generation is driven by the vision of the Internet of Things and mission critical communication services requiring latency in the order of milliseconds or sub-milliseconds. However, these new stringent requirements have a large technical impact on the design of all layers of the communication protocol stack. The cross layer interactions are complex due to the multiple design principles and technologies that contribute to the layers' design and fundamental performance limitations. We will be able to develop low-latency networks only if we address the problem of these complex interactions from the new point of view of sub-milliseconds latency. In this article, we propose a holistic analysis and classification of the main design principles and enabling technologies that will make it possible to deploy low-latency wireless communication networks. We argue that these design principles and enabling technologies must be carefully orchestrated to meet the stringent requirements and to manage the inherent trade-offs between low latency and traditional performance metrics. We also review currently ongoing standardization activities in prominent standards associations, and discuss open problems for future research

Distributed Operation of Uncertain Dynamical Cyberphysical Systems

In this thesis we address challenging issues that are faced in the operation of important cyber-physical systems of great current interest. The two particular systems that we address are communication networks and the smart grid. Both systems feature distributed agents making decisions in dynamic uncertain environments. In communication networks, nodes need to decide which packets to transmit, while in the power grid individual generators and loads need to decide how much to pro-duce or consume in a dynamic uncertain environment. The goal in both systems, which also holds for other cyber-physical systems, is to develop distributed policies that perform efficiently in uncertain dynamically changing environments. This thesis proposes an approach of employing duality theory on dynamic stochastic systems in such a way as to develop such distributed operating policies for cyber-physical systems. In the first half of the thesis we examine communication networks. Many cyber-physical systems, e.g., sensor networks, mobile ad-hoc networks, or networked control systems, involve transmitting data over multiple-hops of a communication network. These networks can be unreliable, for example due to the unreliability of the wireless medium. However, real-time applications in cyber-physical systems often require that requisite amounts of data be delivered in a timely manner so that it can be utilized for safely controlling physical processes. Data packets may need to be delivered within their deadlines or at regular intervals without large gaps in packet deliveries when carrying sensor readings. How such packets with deadlines can be scheduled over networks is a major challenge for cyber-physical systems. We develop a framework for routing and scheduling such data packets in a multi-hop network. This framework employs duality theory in such a way that actions of nodes get decoupled, and results in efficient decentralized policies for routing and scheduling such multi-hop communication networks. A key feature of the scheduling policy derived in this work is that the scheduling decisions regarding packets can be made in a fully distributed fashion. A decision regarding the scheduling of an individual packet depend only on the age and location of the packet, and does not require sharing of the queue lengths at various nodes. We examine in more detail a network in which multiple clients stream video packets over shared wireless networks. We are able to derive simple policies of threshold type which maximize the combined QoE of the users. We turn to another important cyber-physical system of great current interest – the emerging smarter grid for electrical power. We address some fundamental problems that arise when attempting to increase the utilization of renewable energy sources. A major challenge is that renewable energy sources are unpredictable in their availability. Utilizing them requires adaptation of demand to their uncertain availability. We address the problem faced by the system operator of coordinating sources of power and loads to balance stochastically time varying supply and demand while maximizing the total utilities of all agents in the system. We develop policies for the system operator that is charged with coordinating such distributed entities through a notion of price. We analyze some models for such systems and employ a combination of duality theory and analysis of stochastic dynamic systems to develop policies that maximize the total utility function of all the agents. We also address the issue of how the size of energy storage facilities should scale with respect to the stochastic behavior of renewables in order to mitigate the unreliability of renewable energy sources

Dynamic control of wireless networks with confidential communications

Future wireless communication systems are rapidly transforming to satisfy everincreasing and varying mobile user demands. Cross-layer networking protocols have the potential to play a crucial role in this transformation by jointly addressing the requirements of user applications together with the time-varying nature of wireless networking. As wireless communications becoming an integral and crucial part of our daily lives with many of our personal data is being shared via wireless transmissions, the issue of keeping personal transactions confidential is at the forefront of any network design. Wireless communications is especially prone to attacks due to its broadcast nature. The conventional cryptographical methods can only guarantee secrecy with the assumption that it is computationally prohibitive for the eavesdroppers to decode the messages. On the other hand, information-theoretical secrecy as defined by Shannon in his seminal work has the potential to provide perfect secrecy regardless of the computational power of the eavesdropper. Recent studies has shown that information-theoretical secrecy is possible over noisy wireless channels. In this thesis, we aim to design simple yet provably optimal cross-layer algorithms taking into account information-theoretical secrecy as a Quality of Service (QoS) requirement. Our work has the potential to improve our understanding the interplay between the secrecy and networking protocols. In most of this thesis, we consider a wireless cellular architecture, where all nodes participate in communication with a base station. When a node is transmitting a confidential messages, other legitimate nodes are considered as eavesdroppers, i.e., all eavesdroppers are internal. We characterize the region of achievable open and confidential data rate pairs for a single and then a multi-node scenario. We define the notion of confidential opportunistic scheduler, which schedules a node that has the largest instantaneous confidential information rate, with respect to the best eavesdropper node, which has the largest mean cross-channel rate. Having defined the operational limits of the system, we then develop dynamic joint scheduling and flow control algorithms when perfect and imperfect channel state information (CSI) is available. The developed algorithms are simple index policies, in which scheduling and flow control decisions are given in each time instant independently. In real networks, instantaneous CSI is usually unavailable due to computational and communication overheads associated with obtaining this information. Hence, we generalize our model for the case where only the distributions of direct- and crosschannel CSI are available at the transmitter. In order to provide end-to-end reliability, Hybrid Automatic Retransmission reQuest (HARQ) is employed. The challenge of using HARQ is that the dynamic control policies proposed in the preceding chapter are no longer optimal, since the decisions at each time instant are no longer independent. This is mainly due to the potential of re-transmitting a variant of the same message successively until it is decoded at the base station. We solve this critical issue by proposing a novel queuing model, in which the messages transmitted the same number of times previously are stored in the same queue with scheduler selecting a head-of-line message from these queues. We prove that with this novel queuing model, the dynamic control algorithms can still be optimal. We then shift our attention to providing confidentiality in multi-hop wireless networks, where there are multiple source-destination pairs communicating confidential messages, to be kept confidential from the intermediate nodes. For this case, we propose a novel end-to-end encoding scheme, where the confidential information is encoded into one very long message. The encoded message is then divided into multiple packets, to be combined at the ultimate destination for recovery, and being sent over different paths so that each intermediate node only has partial view of the whole message. Based on the proposed end-to-end encoding scheme, we develop two different dynamic policies when the encoded message is finite and asymptotically large, respectively. When the encoded message has finite length, our proposed policy chooses the encoding rates for each message, based on the instantaneous channel state information, queue states and secrecy requirements. Also, the nodes keep account of the information leaked to intermediate nodes as well the information reaching the destination in order to provide confidentiality and reliability. We demonstrate via simulations that our policy has a performance asymptotically approaching that of the optimal policy with increasing length of the encoded message. All preceding work assumes that the nodes are altruistic and/or well-behaved, i.e., they cooperatively participate into the communication of the confidential messages. In the final chapter of the thesis, we investigate the case with non-altruistic nodes, where non-altruistic nodes provide a jamming service to nodes with confidential communication needs and receiving in turn the right to access to the channel. We develop optimal resource allocation and power control algorithms maximizing the aggregate utility of both nodes with confidential communication needs as well as the nodes providing jamming service

Delay Analysis for Multi-Hop Wireless Networks

Abstract—We analyze the delay performance of a multi-hop wireless network with a fixed route between each source-destination pair. There are arbitrary interference constraints on the set of links that can be served simultaneously at any given time. These interference constraints impose a fundamental lower bound on the delay performance of any scheduling policy for the system. We present a methodology to derive such lower bounds. For the tandem queue network, where the delay optimal policy is known, the expected delay of the optimal policy numerically coincides with the lower bound. We conduct extensive numerical studies to suggest that the average delay of the back-pressure scheduling policy can be made close to the lower bound by using appropriate functions of queue length. I