Thierry E. Klein

Assignment strategies for mobile data users in hierarchical overlay networks: performance of optimal and adaptive strategies

Hierarchical wireless overlay networks have been proposed as an attractive alternative and extension of cellular network architectures to provide the necessary cell capacities to effectively support next-generation wireless data applications. In addition, they allow for flexible mobility management strategies and quality-of-service differentiation. One of the crucial problems in hierarchical overlay networks is the assignment of wireless data users to the different layers of the overlay architecture. In this paper, we present a framework and several analytical results pertaining to the performance of two assignment strategies based on the users velocity and the amount of data to be transmitted. The main contribution is to prove that the minimum average number of users in the system, as well as the minimum expected system load for an incoming user, are the same under both assignment strategies. We provide explicit analytical expressions as well as unique characterizations of the optimal thresholds on the velocity and amount of data to be transmitted. These results are very general and hold for any distribution of user profiles and any call arrival rates. We also show that intelligent assignment strategies yield significant gains over strategies that are oblivious to the user profiles. Adaptive and on-line strategies are derived that do not require any a priori knowledge of the user population and the network parameters. Extensive simulations are conducted to support the theoretical results presented and conclude that the on-line strategies achieve near-optimal performance when compared with off-line strategies.

911-NOW: A network on wheels for emergency response and disaster recovery operations

Public safety organizations increasingly rely on wireless communication technology to provide effective command, control, and communication during emergencies and disaster response operations. Since emergencies can vary in scale from day-to-day operations to large-scale and widespread catastrophic events, any previously deployed network infrastructure may not be able to handle the traffic load. Worse, the wireless infrastructure may be damaged or destroyed, as occurred during the events of 9/11 and Hurricane Katrina. The 911-network on wheels (911-NOW) solution is a novel portable cellular system based on base station routers (BSRs) that does not require any pre-existing wireless infrastructure and provides capacity and coverage on demand. It is an auto-configurable system with a fully integrated service architecture that can be deployed as a single-cell solution for local communication or be configured to operate as an ad hoc network of cells. This paper describes the 911-NOW vision and discusses some of the differentiating features such as auto-configuration, network management, wireless mesh networking, and interoperability with existing public safety systems. We also highlight some of the research challenges associated with mobile and rapidly deployable wireless networks. In particular we provide an overview of issues centered upon dynamic assignment and management of Internet Protocol (IP) addresses, online and real-time calculation and maintenance of routing information, mobility management, and dynamic configuration and optimization of radio parameters.

Improved TCP performance in wireless IP networks through enhanced opportunistic scheduling algorithms

Current and next-generation wireless networks rely on multiuser diversity and scheduling techniques, such as the commonly used proportional fair (PF) algorithm, to achieve greater system throughput and higher efficiencies for wireless data applications over a time-varying wireless channel. We show that the variability of the inter-scheduling intervals, as introduced by the PF scheduling algorithm, can have adverse effects on TCP and its congestion control mechanism and lead to spurious timeouts and unnecessarily low throughput. We propose an enhanced scheduling algorithm that is explicitly tuned towards throughput performance at the TCP layer. However, this algorithm does not use any explicit information from the TCP layer and solely relies on information readily available at the link layer at which the scheduler resides. The performance of this improved algorithm is assessed through extensive simulations to show an average TCP throughput improvement of 12% compared to PF. In addition, the TCP-level fairness across all users is increased, as is the individual user throughput.

Power control for the additive white Gaussian noise channel under channel estimation errors

We investigate the time-varying additive white Gaussian noise channel with imperfect side-information. In practical systems, the channel gain may be estimated from a probing signal and estimation errors cannot be avoided. The goal of this paper is to determine a power allocation that a priori incorporates statistical knowledge of the estimation error. This is in contrast to prior work which calculates the effect of the estimation error a posteriori.

Avoiding spurious TCP timeouts in wireless networks by delay injection

The Transmission Control Protocol (TCP) has been designed to provide reliable transport of packets by adjusting the transmission rate to the network congestion level. While TCP can adapt to small fluctuations in the delay between the sender and the receiver, adverse affects (most importantly, spurious timeouts) have been observed under large delay variability. We exhibit the presence of such delay spikes in wireless networks and discuss their possible origins. We then investigate a new methodology for avoiding spurious TCP timeouts by appropriately injecting additional random delay along the communication path. Different algorithms for the delay injection are presented and we assess their relative performances and merits through simulations. In particular, we show, by numerical examples, that the delay injection methodology can significantly decrease the number of timeouts and increase the achieved TCP throughput by about 8% in the network scenario considered. One of the attractive features of the new methodology is that it does not require any changes to the TCP and can be applied independently of the TCP version used.

Methods to improve TCP throughput in wireless networks with high delay variability [3G network example]

Highly variable round-trip times (RTTs) in wireless networks can induce spurious timeouts, thus unnecessarily degrading throughput for the transmission control protocol (TCP). In this paper, we propose and study two effective ways to improve TCP throughput in wireless networks. The first technique is to select a retransmission timeout (RTO) threshold higher than that in the de facto standard. Simulation reveals that the proposed method reduces timeouts and provides a relative throughput gain up to 13.7%, based on RTT measurements in a commercial 3G network and in a simulated network environment. The second technique is an appropriate use of selective repeat (SR) and go-back-N (GBN) as retransmission policies upon packet timeout. We find that when RTTs have reasonable temporal correlation and packets can arrive out-of-order at the receiver, GBN can improve throughput over the SR policy. Specifically, based on the RTT measurements in the 3G network, our results show that GBN provides a 12% throughput gain over the SR policy.

System aspects and handover management for IEEE 802.16e

The IEEE 802.16 family of standards that are currently being developed target wide area, cellular, high data rate transmission for non-line-of-sight terminals. The initial phase of the standard (802.16d) supports fixed terminal locations and the subsequent phase of the standard (802.16e) provides support for mobile terminals. The standards specify physical layer (PHY) and media access control (MAC) layer functionalities with several flexible options for frequency reuse and handover. We describe various system architectures for handling handovers in an 802.16e system based on the capabilities of the current draft standard (802.16e D8). The system architecture description combines the functionalities provided in the standard at the PHY and MAC layers with network layer signaling to provide a complete solution. Specifically, we propose hard handover with MAC context reset for a low mobility scenario and a layered migration procedure using fast base station switching (FBSS) with MAC context transfer for a high-mobility scenario and Internet Protocol (IP) packet forwarding for seamless mobility. Our handover solution is applicable to both flat and centralized network architectures. We discuss evolution paths through these various options, keeping in mind that the initial deployments are likely to be for limited mobility and low capacity, while future deployments will target high mobility and increased capacity.

Base station assignment and power control algorithms for data users in a wireless multiaccess framework

This paper considers the problem of assigning mobile data users to base stations and modulating their transmit powers according to their respective channel gains in order to maximize the total system throughput. We consider two scenarios of time-invariant and time-varying channel behaviors. We show that the base station assignment problem is NP complete and propose sub-optimal polynomial time algorithms and bound their performance. When the channel gains are time-varying, we present an iterative algorithm to compute the assignment and power control functions according to the probability distribution of the channel gains. This algorithm is shown to converge to the optimal allocation in the special cases of a single user or a single base station. Simulation results demonstrate the performance of our algorithms, which are especially significant under asymmetric loading of the network

Seamless TCP mobility using lightweight MPTCP proxy

We present a TCP mobility solution for the mobile Internet which enables seamless session end-point migration across multi-provider network environments. The solution can be applied by mobile devices to conduct energy-efficient network selection and dynamic spectrum sharing or by data centers to facilitate service migration across IP domains. We leverage MPTCPs in-band signaling protocol to provide the necessary robustness against firewall- and middle-box policies encountered in these environments. The mobility solution builds on a lightweight MPTCP proxy function which is inserted into the data path and performs header rewriting without packet buffering or stream assembly. The proxy is therefore easily integrated into mobile devices, network routers or data center nodes hence facilitating fast deployment. We discuss the use cases, deployment scenarios and algorithmic challenges of the proxy design. We also present a Linux implementation, demonstrate its functionality and prove the proxys interoperability with native MPTCP transport-layer implementations.

Distributed paging and registration in wireless networks

The success of current and future wireless networks depends on their ability to provide connections to mobile terminals anywhere and at any time. It is therefore of crucial importance that wireless networks are able to quickly and efficiently locate mobile users at the time of an incoming call, which is achieved in current networks through a combination of paging and registration. In this article we present novel distributed paging and registration procedures that are naturally suited to future distributed wireless network architectures. In addition, the distributed nature of these strategies allows us to effectively balance the paging and registration traffic and the required signal processing throughout the network and alleviate any potential overloads of individual base stations.