A wide range of wireless channel models have been developed to model variations in received signal strength. In contrast to prior work, which has focused primarily on channel modeling on a short, per- packet timescale (millisecond), we develop and validate a finite-state Markov chain model that captures variations due to shadowing, which occur at coarser time scales. The Markov chain is constructed by partitioning the entire range of shadowing into a finite number of intervals. We determine the Markov chain transition matrix in two ways: (i) via an abstract modeling approach in which shadowing effects are modeled as a log-normally distributed random variable affecting the received power, and the transition probabilities are derived as functions of the variance and autocorrelation function of shadowing; (ii) via an empirical approach, in which the transition matrix is calculated by directly measuring the changes in signal strengths collected in a 802.16e (WiMAX) network. We validate the abstract model by comparing its steady state and transient performance predictions with those computed using the empirically derived transition matrix and those observed in the actual traces themselves.

Virtual network (VN) mapping is a useful tool for mapping VNs to physical mesh networks. This study extends the idea of mapping VNs from the wired world to the wireless domain by showing its potential applications. Since the generic VN mapping problem is NP-Hard, this study shows how the wireless VN mapping problem can be simplified and be used instead as a mechanism for provisioning wireless points of presences (POPs) as additions to conventional cellular voice and data services. Two heuristic algorithms GSA and GDR are proposed for producing a 2-phase solution to the mapping problem, which corresponds to conventional network deployment process. The results obtained from the VN mapping algorithms proposed here can be used for comparison of overall performance achieved by deploying a particular type of physical network. Further, using this setup, the network operator can determine the costs and benefits associated with setting wired or wireless links on the physical network. Performance is determined based on perceived revenue, and substrate utilization.

This paper presents an adaptive interference control method to mitigate undesirable interference from femtocells to macrocell users in hierarchical cellular networks. Such mechanisms usually require over-the-air signalling for estimation of interference resulting significant bandwidth overhead. The proposed `Adaptive Interference Scaling' (AIS) method uses geolocation information for femtocell power control for interference avoidance. In this approach, each femtocell calculates interference contributed to nearby macrocell users and adjusts the power to meet specific target signal-to-interference-plus-noise (SINR) level. Results from simulations show that AIS is able to increase the number of macrocell users achieving target data rates by up to 158% relative to baseline without adaptive control, while resulting in only 12.2% femtocell users receiving rates below the target. AIS achieves improved performance by using location information to calculate and limit the interference power contributed by femtocells to macrocell users, while allowing the network operator to set any desired target rates.

Providing air-time guarantees across a group of clients forms a fundamental building block in sharing an access point (AP) across different virtual network providers. Though this problem has a relatively simple solution for downlink group scheduling through traffic engineering at the AP, solving this problem for uplink (UL) traffic presents a challenge for fair sharing of wireless hotspots. Among other issues, the mechanism for uplink traffic control has to scale across a large user base, and provide flexible operation irrespective of the client channel conditions and network loads. In this study, we propose the SplitAP architecture that address the problem of sharing uplink airtime across groups of users by extending the idea of network virtualization. Our architecture allows us to deploy different algorithms for enforcing UL airtime fairness across client groups. In this study, we will highlight the design features of the SplitAP architecture, and present results from evaluation on a prototype deployed with: (1) LPFC and (2) LPFC+, two algorithms for controlling UL group fairness. Performance comparisons on the ORBIT testbed show that the proposed algorithms are capable of providing group air-time fairness across wireless clients irrespective of the network volume, and traffic type. The algorithms show up to 40% improvement with a modified Jain fairness index.

The 802.16e standard for broadband wireless access mandates the presence of QoS classes, but does not specify guidelines for the scheduler implementation or mechanisms to ensure air time fairness. Our study demonstrates the feasibility of controlling downlink airtime fairness for slices while running above a proprietary WiMAX basestation (BS) scheduler. We design and implement a virtualized infrastructure that allows users to obtain at least an allocated percentage of BS resources in the presence of saturation and link degradation. Using Kernel virtual machines for creating slices and Click modular router for implementing the virtual network traffic shaping engine we show that it is possible to adaptively control slice usage for downlink traffic on a WiMAX Basestation. The fairness index and coupling coefficient show an improvement of up to 42%, and 73% with preliminary indoor walking mobility experiments. Outdoor vehicular measurements show an improvement of up to 27%, and 70% with the fairness index and coupling coefficient respectively

Frame aggregation is a wireless link optimization mechanism that aims to reduce transmission overheads by sending multiple frames as the payload of a single MAC frame. Static assignment of frame aggregation parameters can result in delay penalties due to variations in traffic type or load levels. Another possible side effect is an increase in packet error rate in noisy environments due to large aggregated frame size. Adaptive aggregation methods have previously been proposed to deal with the above problems independently, but there is still a need for a unified adaptation algorithm that addresses both aspects in an integrated manner. In this study, a backlogged queue (BQ) aggregation approach is proposed that considers both these aspects, and also ensures inter-operability with other WLAN devices that are not capable of frame aggregation. Performance evaluation of the proposed algorithm on the ORBIT testbed shows throughput improvements of up to 56% in the presence of channel noise and 25% in scenarios with high contention over using a simple txop. An experimental case study shows improvement in FTP file transfer times of up to 11% while preserving performance for real time traffic.

Wireless network emulator testbeds have become increasingly important for realistic, at-scale experimental evaluation of new network architectures and protocols. Typically, wireless network performance measurements are made at multiple layers of the wireless protocol stack, i.e. link layer, MAC layer and network layer. This study highlights the impact of layer 2 frame aggregation that is enabled by default in the software drivers for commodity wireless 802.11 devices while it is still not a part of the core 802.11 standard. Using experimental measurements, it is shown that this feature has an impact across a diverse set of wireless experiments and should be considered while comparing results. Measurements on the ORBIT testbed show that throughput measurements can vary up to a startling 25% for certain packet sizes and the variance in receiver side interframe delays can almost double if MAC aggregation and preset transmission opportunities are not taken into consideration. Further results for VoIP traffic show a deterioration in jitter of up to 8 times when coupled with MAC layer aggregation in 802.11.

A promising application for RFID tags is to trace valuable assets in an inventory. In such systems, the key challenge is to achieve reliable and energy-efficient tag reads. This paper proposes a novel tag reading protocol, Relay-MAC, which aims at reducing the information sent over the network and the energy spent in collision detection and handling by introducing deliberate sequencing at runtime. This paper provides an in-depth study of the design issues one may face in implementing such a protocol on RFID tags, and validates its feasibility using simulation studies. These studies clearly demonstrate that Relay- MAC can yield much better throughput and energy conservation when compared to a conventional select-and-read protocol.