Cellular networks enable high reliability of vehicle-to-vehicle (V2V) communications thanks to centralized, efficient coordination of radio resources. Collision-free transmissions are possible, where base stations could allocate orthogonal resources to the vehicles. However, in case of limited resources in relation to the data traffic load, the resource allocation task becomes a challenge. Current solutions propose heuristic algorithms that focus on resource reuse, often based on the location of the vehicles. Such schedulers are mainly designed assuming ideal network coverage conditions and are prone to performance degradation in case of coverage loss. Further, they typically rely on frequent scheduling updates, which increases the dependency on coverage. In this paper, we propose a reinforcement learning-based approach to scheduling V2V communications. Our solution, called iVRLS, delivers higher reliability than an enhanced version of a state-of-the-art benchmark algorithm in case of intermittent coverage conditions, while requiring less frequent scheduling. Following this approach, we enable a unified scheduler deployment irrespective of coverage, which offers graceful performance behavior across varying coverage conditions, thus making iVRLS a robust alternative to existing schedulers.

WiFi networks suffer from severe network utility degradation due to the usage of diverse modulation and coding schemes. The proportional-fair allocation, that has been shown to be a good remedy, can be enforced through the proper selection of contention window values. This has been achieved so far for centralized systems by an explicit solution of an optimization problem or, as proposed recently, by following a learning-based approach. In this paper, we present the first fully distributed solution in which each of the WiFi nodes independently tunes its contention window to achieve proportional fairness. Our solution is therefore applicable also for a set of collocated, unconnected WiFi networks. We compare the throughput and air-time allocation that this algorithm achieves to the values achieved by standard WiFi binary exponential back-off and values achieved by known centralized algorithms.

Despite exhibiting very high theoretical data rates, in practice, the performance of LTE-U/LAA and WiFi networks is severely limited under cross-technology coexistence scenarios in the unlicensed 5GHz band. As a remedy, recent research shows the need for collaboration and coordination among colocated networks. However, enabling such collaboration requires an information exchange that is hard to realize due to completely incompatible network protocol stacks. We propose OfdmFi, the first cross-technology communication scheme that enables direct bidirectional over-the-air communication between LTE-U/LAA and WiFi with minimal overhead to their legacy transmissions. Requiring neither hardware nor firmware changes in commodity technologies, OfdmFi leverages the standard-compliant possibility of generating message-bearing power patterns, similar to punched cards from the early days of computers, in the time-frequency resource grid of an OFDM transmitter which can be cross-observed and decoded by a heterogeneous OFDM receiver. As a proof-of-concept, we have designed and implemented a prototype using commodity devices and SDR platforms. Our comprehensive evaluation reveals that OfdmFi achieves robust bidirectional CTC between both systems with a data rate of up to 84kbps, which is more than 125× faster than state-of-the-art.

Widespread adoption of indoor positioning systems based on WiFi fingerprinting is at present hindered by the large efforts required for measurements collection during the offline phase. Two approaches were recently proposed to address such an issue: crowdsourcing and RSS radiomap prediction, based on either interpolation or propagation channel model fitting from a small set of measurements. RSS prediction promises better positioning accuracy when compared to crowdsourcing, but no systematic analysis of the impact of system parameters on positioning accuracy is available. This paper fills this gap by introducing ViFi, an indoor positioning system that relies on RSS prediction based on Multi-Wall Multi-Floor (MWMF) propagation model to generate a discrete RSS radiomap (virtual fingerprints). Extensive experimental results, obtained in multiple independent testbeds, show that ViFi outperforms virtual fingerprinting systems adopting simpler propagation models in terms of accuracy, and allows a seven-fold reduction in the number of measurements to be collected, while achieving the same accuracy of a traditional fingerprinting system deployed in the same environment. Finally, a set of guidelines for the implementation of ViFi in a generic environment, that saves the effort of collecting additional measurements for system testing and fine tuning, is proposed.

Future cities are to be not only an intelligent and green living environment but also provide human-centered public services at a lower cost. The Internet of Things (IoT) and artificial intelligence (AI) are two cornerstone technologies enabling the smart city concept, which are fusing into an organic whole in recent years. Some particular joint points where IoT meets AI are intelligent IoT devices, smart sensing boosted by AI, and IoT big data mining with AI. Such “IoT meets AI” trend is already casting significant impact to enable a smart city. Some examples are: smartphones are able to learn the touching pattern of users; home Wi-Fi router can intelligently detect and locate an intruder; vehicles locations and movement information can be exploited for traffic control; video cameras installed in the street can perform face recognition locally.

LTE in Unlicensed (LTE-U) constitutes a new source of interference in the 5 GHz ISM band with a potentially strong impact on WiFi performance. Cross-technology interference and radio resource management are the best ways to assure efficient coexistence but require proper signaling channels. We present LtFi, a system which enables to set-up a cross-technology communication between nodes of co-located LTE-U and WiFi networks. LtFi follows a two-step approach: using an innovative side channel on their air-interface LTE-U BSs are broadcasting connection and identification information to adjacent WiFi nodes, which is used in a subsequent step to create a bi-directional control channel over the wired backhaul. The simple LtFi is fully compliant with LTE-U and works with COTS WiFi hardware. The achievable data rate on the air-interface based broadcast side channel (up to 665 bps) is sufficient for this and multiple other purposes. Experimental evaluation of a fully operational prototype has demonstrated reliable data transmission even in crowded wireless environments for LTE-U receive power levels down to -92 dBm. Moreover, system-level simulations demonstrate accurate recognition of the complete set of interfering LTE-U BSs in a typical LTE-U multi-cell environment.

The IEEE 802.11 and the IEEE 802.15.4e TSCH were designed with different goals in mind and both play important roles for respective applications. However, they suffer from mutual interference and experience degraded performance while operating in the same space and spectrum band. To improve this situation we propose an approach to enable a cooperative control for sharing time and frequency. The Time-Slotted Channel Hopping (TSCH) based sensor network is expected to occupy an only small share of time. We develop mechanism enabling over-the-air synchronization of the Wi-Fi network with the tightly synchronized TSCH based sensor network. We demonstrate that Wi-Fi network can avoid transmitting in the "collision periods". We provide the full design for which we have developed and tested a prototype implementation based on the Commercial off-the-shelf (COTS) devices.

To avoid the foreseeable spectrum crunch, LTE operators have started to explore the option to directly use the underutilized unlicensed spectrum in 5 GHz UNII bands being mainly used by IEEE 802.11 (WiFi). However, as LTE is not designed with shared spectrum access in mind, it has a potential to seriously harm Wi-Fi, Currently suggested solutions focus on forcing LTE-U to introduce coexistence gaps in either frequency, time, or space domain, and are addressing the coexistence only indirectly due to the lack of coordination among the coexisting WiFi and LTE-U networks. Contrary to these schemes, our proposal introduces explicit cooperation between neighboring LTE-U and WiFi networks. We suggest that LTE-U BSs equipped with multiple antennas can create coexistence gaps in space domain in addition to the time domain gaps by means of cross-technology interference nulling towards WiFi nodes in the interference range. In return, LTE-U can increase its own airtime utilization while trading off slightly its antenna diversity. We demonstrate that such cooperation offers benefits to both WiFi and LTE-U in terms of improved throughput and decreased channel access delay. More specifically, system-level simulations reveal a throughput gain up to 221% for LTE-U network and up to 44% for WiFi network depending on the setting, e.g., distance between the two cells, number of LTE antennas, and WiFi nodes in the LTE-U BS neighborhood. Our approach provides significant benefits especially for moderate separation distances between LTE-U/WiFi cells where interference from a neighboring network might be severe due to the hidden network problem.

Classical control and management plane for computer networks is addressing individual parameters of protocol layers within an individual wireless network device. We argue that this is not sufficient in phase of increasing deployment of highly re-configurable systems, as well as heterogeneous wireless devices co-existing in the same radio spectrum. They demand harmonized, frequently even coordinated adaptation of multiple parameters in different protocol layers (cross-layer) in multiple network devices (cross-node). We propose UniFlex, a framework enabling unified and flexible radio and network control. It provides an API enabling coordinated cross-layer control and management operation over multiple wireless network nodes. The controller logic may be implemented either in a centralized or distributed manner. This allows to place time-sensitive control functions close to the controlled device (i.e., local control application), off-load more resource hungry control application to compute servers and make them work together to control entire network. The UniFlex framework was prototypically implemented and provided to the research community as open-source. We evaluated the framework in a number of use-cases, what proved its usability.

Aggregation of location estimates from multiple services for provisioning of location information enhances the accuracy and robustness of the final location information. Recent Internet of Things (IoT)-based localization service architectures therefore envision a "manager" for selecting and invoking provisioning services and, in the later step, aggregating the received information and providing it to location-based applications. The selection of provisioning services should take into account the accuracy and latency requirements from the applications and accuracy, latency, and power consumption characteristics of provisioning services. However, it is yet unclear how such selection should be made. We propose two algorithms for the selection of provisioning services aiming at meeting latency and subsequently accuracy requirements from the applications, one subject to minimizing per-request power consumption, while the other subject to a per-time bucket power minimization. In the considered examples, we show that the per-time bucket optimization achieves around 25% better performance in terms of power consumption, while trading-off accuracy satisfaction.

With the abundance of mobile connected devices and coexisting networks, localization solutions are inevitably subject to interference. In this paper, effects of interference on the performance of Fingerprinting Localization Algorithms (FPS) are studied both theoretically and through experimentation. The previously introduced theoretical framework based on Hypothesis Testing (HT) problem is employed to characterize the performance of FPS and provide guidelines for combating negative impact of interference. In particular, it is shown that background interference interestingly can improve the performance of FPS, while the interference in the measurement phase incurs an error by pushing the reported location closer to the anchors. Moreover, the anchors provide the highest interference robustness for locations in their proximity. These results are further verified through simulations and experimentation in realistic setups.

Location information of mobile devices is a foundational input to location-based services and a valuable source of context information in wireless networks. To maximize the value, we need location information that is accurate, robust, and promptly and seamlessly available. Unfortunately, individual localization services seldom satisfy all these requirements. For achieving that vision, a set of challenges has to be addressed, pertaining to handover, fusion, and integration of different sources of location information. Current approaches for integration of individual localization services are either not specific enough or are limited in scope and lack flexibility. In the following, we provide a detailed design and a prototypical implementation of the Standardized Localization Service (SLSR), a middleware architecture for achieving those goals. We instantiate the service in an office environment and perform exhaustive performance benchmarking in a testbed specifically designed for supporting such experimentation. Our results characterize the effects of different functional components envisioned in the SLSR on its performance. Our results also quantify the accuracy benefits of fusion of representative sources of location information.

As computationally powerful mobile devices with high-resolution screens have become ubiquitous, network operators are facing the problem of coping with the huge traffic load caused by various video streaming applications. One approach to mitigate the problem is to dynamically offload parts of the traffic to Wi-Fi networks that offer higher data rates at lower costs. Offloading can be performed in the most seamless way by deploying multi-path transport protocols that incorporate mechanisms to distribute the load over the available network interfaces. In the present work, we develop such mechanisms that allow to efficiently transmit a low-delay live video stream over multiple paths. We propose a cross-layer solution that operates on the transport and application layers. It consists of two components. The Sender-Side Path Scheduling component allows to use multiple network interfaces for concurrent transmission of video data in order to achieve bandwidth aggregation, and packets loss reduction. The Sender-Side Video Adaptation component dynamically adapts the video bit rate to the network conditions. The joint operation of these two components allows to increase the Quality of Experience of the user by providing a smooth playback at the best possible video quality level with a minimum of playback interruptions and corrupted video frames. In order to efficiently adapt the video quality to the network conditions, our proposed solution leverages Scalable Video Coding. We prototyped the proposed solution using the Multipath Real-Time Transport Protocol, and evaluated it w.r.t. the achieved Quality of Experience. We observed that it allows to reduce the amount of playback interruptions by up to 50%, as compared to two baseline approaches.

We present a quality-selection policy for Quality of Experience (QoE) demanding video streaming in wireless networks. The proposed policy predicts the TCP throughput and adapts video segment requests in order to assure high QoE by taking into account the client buffer level. We introduce the concept of Affordable delivery Time (AT) and we design a buffer-based algorithm, hereafter referred to as Buffer-based lolyPOP (BPOP). The AT accounts for the ideal chunk download time which equals the chunk playout time, as well as for the client buffer status. In a nutshell, the buffer-based AT favors the download of higher quality video chunks when the client has more buffered data than a pre-established target buffer occupancy. Conversely, AT inhibits higher quality video chunks download when the buffer is below the targeted occupancy.

Ubiquitous Wi-Fi access points (APs) constantly send out beacon and probe response frames to inform potential stations (STAs) about their existence. Beacon and probe responses can be extended by adding additional information (so-called beacon stuffing) making it possible to deliver this information to mobile devices (e.g. smart-phones equipped with 802.11 interfaces) without the need for association with the local infrastructure. This feature can be used to support location-based information services (LBS) related for e.g. advertising local opportunities, temporary obstacles and traffic disturbances or emergency notifications. In this paper we introduce a demonstrator for Location-based Wi-Fi Services (LoWS) based on beacon stuffing. The LoWS system can be easily installed within an existent 802.11 infrastructure while the receiver application can be installed on nearly all up-to-date commercial off-the-shelf smartphones, e.g. based on Android or iOS.

Mobile app testing and evaluation requires exposing the app to a wide array of real world context conditions viz. location, sensor values, network conditions etc. Such comprehensive context conditions are difficult to create in a development environment on a real device, therefore, simulating them in a mobile emulator is a promising alternative. We present ContextMonkey, a framework for context generation in a mobile emulator. ContextMonkey can generate realistic context by leveraging traces in correlated and interdependent way from heterogenous sources: remote trace databases, models, and trace files. It eases the burden on developers to collect correlated traces, convert them to a common format, and feed them to the emulator in an orchestrated manner. We present examples demonstrating the simplicity and potential of ContextMonkey in app testing scenarios.

Individual localization services seldom satisfy the requirements for accurate, robust, punctual, and seamless location information. To enhance the provisioning, a set of challenges has to be addressed, pertaining to handover, fusion, and integration of individual localization services. In the following, we advertise the Standardized Localization Service (SLSR), a middleware architecture for achieving those goals. We instantiate the SLSR in a office-like indoor environment and evaluate its performance in a specifically designed testbed for evaluation of indoor localization approaches. Our results typify the contributions of different functional components envisioned in the SLSR on its overall performance.

Despite the popularity of Fingerprinting Localization Algorithms (FPS), general theoretical frameworks for their performance studies have rarely been discussed in the literature. In this work, after setting up an abstract model for the FPS, we show that a fingerprinting-based localization problem can be cast as a Hypothesis Testing (HT) problem and therefore various results from the HT literature can be used to provide insights, guidelines, and performance bounds for the FPS. This includes the scaling limits of error probability in terms of the number of measurements and the precise characterization of localization error. The provided results hold for the general FPS. Additionally, Received Signal Strength (RSS)-based fingerprinting algorithms are particularly considered from the theoretical viewpoint due to their widespread practical usage. Simulations and experimental results characterize numerically the findings of the theoretical framework and demonstrate its consistency with realistic localization scenarios.

Device-to-Device (D2D) communication has a high potential in reducing the amount of network traffic and improving the latency and energy efficiency of communication. Currently, D2D link establishment decisions are based on active probing between devices that wish to establish a D2D link. The main drawback of such approaches is a large overhead as during active probing no data communication can take place. We leverage physical locations of the devices that wish to establish a D2D link in order to estimate the probability of success in establishing the link before making an attempt to communicate. The probability of success is given as a closed form equation that takes into account the imperfections of location information of the devices and intrinsic randomness of wireless environments. We experimentally evaluate the proposed location-based decision- making mechanism for D2D link establishment in a complex office-like indoor environment. We show that setting the Signal-to-Noise Ratio (SNR) threshold of the proposed mechanism to a value that is 5 dB higher than the nominal SNR required for communication results in reliable link establishment with false positive rate of less than 2%. Furthermore, we show a relatively small loss of link establishment potential due to an increase in the inaccuracies of location information.

The increased popularity of IEEE 802.11 LANs (WLANs) in residential (home) environments leads to dense, unplanned and thus chaotic deployments. Access Points (APs) are frequently deployed unfavorably in terms of radio coverage and use interfering frequency/power settings. In fact, in some parts of a one's apartment the usage of the neighbor's AP might be preferred as it might provide better signal quality than the own AP. Moreover, the network performance could be dramatically improved by balancing the network load over spatially co-located neighbor APs operating on different radio channels or having asymmetric workloads. We address these problems by presenting NxWLAN (Neighborhood extensible WLAN) which enables the secure virtual extension of user's home WLANs in residential environments through enabling the usage of neighboring APs operating on the same or on different radio channels. NxWLAN requires no additional software to be installed on the client STAs while providing the same level of security as the home AP, e.g. WPA2, without revealing any kind of security credentials to untrusted neighboring APs. In this paper we prove, by prototyping and simulating the NxWLAN solution, that it is operational, performant and easy to deploy using off-the-shelf hardware. Moreover, we provide the full source code of our prototype to the community as open-source.

The increasing number of mobile devices with high processing power and high-resolution screens had led to an enormous growth of mobile video traffic. Mobile network operators face the requirement to efficiently support large numbers of concurrent unicast streaming sessions. In the present work, the long-term quality of experience perceived by the user, the fairness, and the overall system efficiency are addressed simultaneously from the cross-layer perspective by jointly optimizing the video adaptation and the wireless resource allocation. One fundamental challenge of the cross layer design is that the time scale of video adaptation - seconds - differs by several orders of magnitude from the one of resource allocation - milliseconds. We focus on the low-delay live streaming, which is particularly sensitive to the throughput fluctuation. We consider the streaming both in the downlink and in the uplink, explicitly taking into account the imperfect synchronization in the uplink. Our proposed solution consists of two components. First, we formulate the problem of video adaptation as a quality of experience based max-min optimization problem that leverages the link rate estimate in the lower network layers. Second, we propose a dynamic resource allocation scheme that takes into account the demands of the streaming clients. These two components together aim at a fair and efficient cross-layer streaming system. An accurate estimation of link rate on the time scale of seconds, required for this problem, is particularly difficult in mobile networks. As a separate contribution, several link rate estimation approaches are evaluated. The prediction algorithms assuming the static resource allocation, although computationally less complex, may lead to inaccurate prediction results in comparison to the one when dynamic resource allocation scheme is used. In this work, the spectral efficiency gain by dynamic resource allocation can be approximated and used to improve throughput predictions. We evaluate the proposed approach against state-of-the-art baselines. The results reveal significant improvements of quality of experience in all studied use cases.

Nowadays, the usage of multiple virtual wireless networks on top of one physical AP is very common in home Wi-Fi networks. Be it Hotspots of Internet Service Providers (ISPs) or Mobile Network Operators (MNOs) used for offloading their traffic, community networks or plain so-called "Guest-Networks". Whereas the home user (AP owner) in the best case should not even be aware that his network connection is shared. Most of the ISPs and MNOs are now trying to convince their customers to install an additional virtual wireless hotspot network on their home AP and offer them in return the free usage of all other available hotspots. But, currently most costumers are skeptical as the providers cannot guarantee a downlink slice of air-time or real separation in time on the wireless access network. In this paper we demonstrate by using a novel downlink slicing scheme applied on commercial off-the-shelf hardware, how slicing on MAC level can be applied to truly guarantee a fixed amount of air-time for the home user and provide complete traffic separation in time between the home and the hotspot network. Moreover, our demonstrator shows the benefits of the approach by comparing the quality of a high definition video stream with and without our MAC slicing approach.

The efficient management of radio resources in today's home or residential Wi-Fi networks is still an open research question. Due to the chaotic and unplanned deployment of Wi-Fi Access Points (APs) and the fact that all APs are managed individually by their owners, home Wi-Fi networks suffer from performance degradation due to contention and interference. In this paper we present and showcase a distributed radio channel assignment scheme, implemented using the ResFi platform, in which neighboring home Wi-Fi APs cooperate with each other in order to negotiate radio channel selection by taking into account the instantaneous network load of neighboring APs (two hop neighborhood). While the information about the network load can be directly exchanged between ResFi-enabled APs using the ResFi out-of-band Internet control channel, the information about the network load of co-located non-ResFi APs first needs to be estimated passively by monitoring the data traffic on the radio channel. The presented demonstrator of the approach is implemented using commodity hardware and enables the audience to observe the distributed channel adaptation in real-time.

The interest for RF-based indoor localization, and in particular for WiFi RSSI-based fingerprinting, is growing at a rapid pace. This is despite the existence of a trade-off between the accuracy of location estimation and the density of a laborious and time consuming survey for collecting training fingerprints. A generally accepted concept of increasing the density of a training dataset, without an increase in the amount of physical labor and time needed for surveying an environment for additional fingerprints, is to leverage a propagation model for the generation of virtual training fingerprints. This process, however, burdens the user with an overhead in terms of implementing a propagation model, defining locations of virtual training fingerprints, generating virtual fingerprints, and storing the generated fingerprints in a training database. To address this issue, we propose the Enriched Training Database (ETD), a web-service that enables storage and management of training fingerprints, with an additional “enriching” functionality. The user can leverage the enriching functionality to automatically generate virtual training fingerprints based on propagation modeling in the virtual training points. We further propose a novel method for defining locations of virtual training fingerprints based on modified Voronoi diagrams, which removes the burden of defining virtual training points manually and which automatically “covers” the regions without sufficient density of training fingerprints. The evaluation in our testbed shows that the use of automated generation of virtual training fingerprints in ETD results in more than 25% increase in point accuracy and 15% in room-level accuracy of fingerprinting.

Localization services available on today's mobile devices are proprietary and leverage a limited set of sources of location information. Integration of new location estimation methods is therefore cumbersome, requiring adaptation to the specific interfaces of the proprietary location service. In addition, location-based applications are tightly interwoven with the location service that is typically provided by the operating system, hence these applications require significant restructuring to be able to run with another location service. To address these problems, we propose a modular localization service architecture that consists of location-based applications, an integrated location service enabling a fusion of so-called elementary location services, and resources for generating location information. A unified style of interaction among these components is enabled by a set of well-defined Application Programming Interfaces (APIs). The practicability and advantages of the proposed design is demonstrated by outlining how the APIs can be realized using modern types of component interactions.

This demonstration presents a user-friendly web-based platform that supports intuitive remote benchmarking of different indoor localization solutions. It reduces the barriers for experimental evaluation and fair comparison of their performance across a set of testing environments. The platform is aimed at addressing the limitations in the current praxis of publishing indoor localization research evaluated only in local, potentially biased environments. To this end, it provides a holistic support for the benchmarking process offering: (i) multiple pre-collected raw data-traces from different RF technologies, (ii) high-level interface to control remote wireless testbed facilities, and (iii) a set of tools for creating, storing, comparing and visualizing the performance results of multiple indoor localization solutions.

mmWave (millimeter-Wave) is a very promising technology for the future wireless communication. To mitigate its high attenuation characteristics, mmWave communication frequently employs directional beamforming for both transmission and reception. Localization commonly takes advantage of directionality in RF frequencies in urban and indoor environments. In this paper, we use lessons learned from classical RF-based localization for discussing a set of feasible localization approaches in the context of mmWave bands. We further map the requirements of each discussed localization approach to design requirements for future mmWave devices and assess the expected accuracy of such approaches for a set of realistic scenarios. Our results show that mmWave-based localization is promising in both its availability and accuracy, even in the presence of a limited number of localization anchor nodes.

We demonstrate BIGAP, a novel architecture providing both high network performance as well as seamless handover in Enterprise IEEE 802.11 networks. The former is achieved by assigning different channels to co-located APs to fully utilize the available radio spectrum. The latter is achieved by providing a mechanism for below MAC-layer handover through exploiting the Dynamic Frequency Selection (DFS) capability in IEEE 802.11. In essence BigAP forces clients to change AP whilst they `believe' they are simply changing channel. BIGAP is fully compatible with 802.11 and requires no modifications to the wireless clients.

Enterprise IEEE 802.11 networks need to provide high network performance to operate a large number of diverse clients like laptops, smartphones and tablets as well as capacity hungry and delay sensitive novel applications like mobile HD video & cloud storage efficiently. Moreover, such devices and applications require much better mobility support and higher QoS/QoE. Existing solutions can either provide high network performance or seamless mobility but not both. We present BIGAP, a novel architecture achieving both of the above goals. The former is achieved by assigning different channels to co-located APs in order to fully utilize the available radio spectrum. The latter is achieved by providing a mechanism for below MAC-layer handover through exploiting the Dynamic Frequency Selection capability in 802.11. In essence BIGAP forces clients to change AP whilst they `believe' they are simply changing channel. BIGAP is fully compatible with 802.11 and requires no modifications to the wireless clients. Testbed results demonstrate a significant improvement in terms of network outage duration (which is 32 x smaller as compared to state-of-the-art solutions) and negligible throughput degradation during handover operation. In this way frequent and seamless handover operations can take place thus supporting both seamless mobility and efficient load balancing.

This paper describes a major effort to collect smartphone data useful for indoor localization. Data has been collected with four Android phones according to numerous scenarios in each of four large buildings. The data includes time-stamped traces of various environmental, position, and motion sensors available on a smartphone, Wi-Fi and cellular signal strengths, and GPS fixes, whenever available. Quantitative evidence is presented to validate the collected data and attest to its quality. This unique, extensive data repository is made available to the R&D community through the PerfLoc web portal to facilitate development of smartphone indoor localization apps and to enable performance evaluation of such apps according to the ISO/IEC 18305 international standard in the near future.

In dense deployments of residential WiFi networks individual users suffer performance degradation due to both contention and interference. While Radio Resource Management (RRM) is known to mitigate this effects its application in residential WiFi networks being by nature unplanned and individually managed creates a big challenge. We propose ResFi - a framework supporting creation of RRM functionality in legacy deployments. The radio interfaces are used for efficient discovery of adjacent APs and as a side-channel to establish a secure communication among the individual Access Point Management Applications within a neighborhood over the wired Internet backbone. We have implemented a prototype of ResFi and studied its performance in our testbed. As a showcase we have implemented various RRM applications among others a distributed channel assignment algorithm using ResFi. ResFi is provided to the community as open source.

Industrial control systems have traditionally been built around dedicated wired solutions. The requirements of flexibility, mobility, and cost have created a strong push toward wireless solutions, preferably solutions requiring low power. Simultaneously, the increased need for interoperability and integration with the wider Internet made a transition to IP-based communication unavoidable. Following these trends, we survey 6TiSCH, the emerging family of standards for IP-based industrial communication over low-power and lossy networks. We describe the state of the standardization work, the major issues being discussed, and open questions recently identified. Based on extensive first-hand experience, we discuss challenges in implementation of this new wave of standards. Lessons learned are highlighted from four popular open-source implementations of these standards: OpenWSN, Contiki, RIOT, and TinyOS. We outline major requirements, present insights from early interoperability testing and performance evaluations, and provide guidelines for chip manufacturers and implementers.

Recently, Hypertext Transfer Protocol (HTTP)-based adaptive streaming has become the de facto standard for video streaming over the Internet. It allows clients to dynamically adapt media characteristics to the varying network conditions to ensure a high quality of experience (QoE)—that is, minimize playback interruptions while maximizing video quality at a reasonable level of quality changes. In the case of live streaming, this task becomes particularly challenging due to the latency constraints. The challenge further increases if a client uses a wireless access network, where the throughput is subject to considerable fluctuations. Consequently, live streams often exhibit latencies of up to 20 to 30 seconds. In the present work, we introduce an adaptation algorithm for HTTP-based live streaming called LOLYPOP (short for low-latency prediction-based adaptation), which is designed to operate with a transport latency of a few seconds. To reach this goal, LOLYPOP leverages Transmission Control Protocol throughput predictions on multiple time scales, from 1 to 10 seconds, along with estimations of the relative prediction error distributions. In addition to satisfying the latency constraint, the algorithm heuristically maximizes the QoE by maximizing the average video quality as a function of the number of skipped segments and quality transitions. To select an efficient prediction method, we studied the performance of several time series prediction methods in IEEE 802.11 wireless access networks. We evaluated LOLYPOP under a large set of experimental conditions, limiting the transport latency to 3 seconds, against a state-of-the-art adaptation algorithm called FESTIVE . We observed that the average selected video representation index is by up to a factor of 3 higher than with the baseline approach. We also observed that LOLYPOP is able to reach points from a broader region in the QoE space, and thus it is better adjustable to the user profile or service provider requirements.

Range-based IEEE 802.15.4 localization systems currently require relatively high anchor density for indoor deployments. It can therefore be beneficial to use external sources of transmission as additional anchors. We present methods for using WiFi beacons to improve localization accuracy of a range-based IEEE 802.15.4 localization system in cases where only two IEEE 802.15.4 anchor nodes are available. We do this by identifying WiFi beacons from RSSI traces that we dynamically sample online, and applying fingerprinting and range-based methods using the RSSI values of the identified beacons. However, because the data of the WiFi traffic is not decodable by the IEEE 802.15.4 devices, these RSSI measurements lack identifiers that can associate them to specific WiFi Access Points (APs). Therefore, novel methods are required for both fingerprinting and range-based approaches to allow for these additional WiFi APs to be used as anchors. We show by using real-world measurements that our beacon identification method gives a false-positive rate of only 3%, and that if the range measurements to the IEEE 802.15.4 anchors are relatively accurate, with a standard deviation of 1 and 3 m, a localization accuracy improvement of 47% and 24% can be gained, respectively.

Real-time Ethernet is expected to become the core technology of future in-car communication networks. Following its current adoption in subsystems for info- and entertainment, broadband Ethernet promises new features in the core of upcoming car series. Its full potential will enfold when deploying Ethernet-based backbones that consolidate all automotive domains on a single physical layer at increased bandwidth but reduced complexity and cost. In such a backbone, traffic with a variety of real-time requirements and best-effort characteristics will share the same physical infrastructure. However, certain applications like online diagnosis, data- or firmware updates, and access to off-board backends will introduce bursty high traffic loads to the sensitive core of the cars communication network. In this work, we analyze the robustness against cross-traffic of real-time Ethernet protocols. Based on a realistic in-car scenario, we demonstrate that background cross-traffic can have significant impact on in-car backbone networks—even for real-time protocols with strict prioritization. By comparing the real-time approaches Ethernet AVBs asynchronous credit based shaping with the time-triggered and rate-constrained traffic classes of Time-triggered Ethernet (AS6802) we quantify how different media access policies suffer from low priority bursts of applications such as diagnosis, online updates or backend-based services. Our simulation study of a realistic in-car backbone design and traffic model reveals that in a realistic in-car network design, cross-traffic may increase end-to-end latency by more than 500% while the jitter can become 14 times higher than for a network without background tasks. We discuss ways to mitigate these degrading effects.

Accurate performance testing of mobile apps require comprehensive simulation of real world context in a mobile emulator viz. location, sensor values, network conditions etc. Existing mobile emulators support such simulation, however they lack the capability for automated generation of realistic, correlated, and dependent context traces. As a result, the burden of their generation and injection is left to the developers who have to create their own traces. This is not straightforward: there are heterogenous remote databases of traces, mathematical models and trace files can be used as well, but the trace values should be correlated, and traces have to be converted to a common format. We are developing ContextMonkey - a framework that addresses these concerns by leveraging context traces from databases such as FourSquare [1], OpenSignal [2], Google Street View [3]. It acts as a harness to mobile emulators, and aims at improving the efficiency of mobile app performance tests.

In this paper, we consider the problem of improving Quality of Experience (QoE) of multiple video streams in OFDMA uplink. The proposed system leverages both video adaptation and adaptation of resource allocation (RA). Three different time scales arises in this scenario, namely (1) long time scale of QoE, evaluated over the sequence of Groups of Pictures (GOPs), (2) medium time scale of video adaptation for each GOP, and (3) short time scale of RA inside each GOP. We deal with the time scale differences through a two step process. We first formulate, for each GOP, an optimization problem of video quality which takes into account the long time scale QoE. Then we convert this problem to a sequence of quality-driven RA optimization. At each RA step inside a GOP, resources are allocated to users according to their utility determined by QoE constraints and quality fairness. The performance of proposed quality-driven RA is evaluated under realistic uplink scenario. It is shown that the proposed system improves significantly users' QoE compared to other solutions.

We study the effect of interference on localization algorithms through the study of the interference effect on signal features that are used for localization. Particularly, the effect of interference on packet-based Received Signal Strength Indicator (RSSI), reported by IEEE 802.11 and IEEE 802.15.4 technologies, and on Time of Flight (ToF), reported by IEEE 802.15.4 technology, is studied using both theoretical discussions and experimental verifications. As for the RSSI values, using an information theoretic formulation, we distinguish three operational regimes and we show that the RSSI values, in dBm, remain unchanged in the noise-limited regime, increase almost linearly with interference power in dBm in the interference- limited regime and cannot be obtained due to packet loss in the collision regime. The maximum observable RSSI variation is dependent on the transmission rate and Signal to Noise Ratio (SNR). We also show that ToF is, interestingly, decreased under interference which is caused in the symbol synchronization procedure at the receiver. After providing the experimental results, we discuss how the localization algorithms are affected by interference.

Cognitive Radio (CR) will enhance the efficiency in spectrum usage by re-using temporally unused licensed spectrum. A promising transmission scheme to utilize even very fragmented and fast changing spectrum is Non-Contiguous OFDM (NCOFDM). Unfortunately, NC-OFDM requires tight synchronization between sender and receiver, i.e. the receiver needs perfect up-to-date knowledge about the set of subcarriers being used by the NC-OFDM sender. Therefore, a reliable and always available Control Channel (CC) is crucial for signaling the spectrum allocation information. Although, underlay Impulse-Radio Ultra-WideBand (IR-UWB) meets the theoretical requirements for such a CC in CR networks, i.e. communication range of up to 1km on a sufficient high data rate, there is a lack of practical studies of IR-UWB in real world environments, especially with respect to co-existence with NC-OFDM as in the envisioned CR multi-technology station. In this paper we present results of measurements in our state-of-the-art IR-UWB testbed. We show that IR-UWB can reach the required communication range of a few hundreds of meters in unobstructed as well as slightly obstructed Lineof- Sight propagation only. Although, IR-UWB is a wideband technology we show that it is severely affected by narrowband interference from close proximity sources which is typically the case in the envisioned multi-technology stations. Such a mutual disturbance can be mitigated by increasing the spatial separation between both air interfaces, orthogonalization in time or using only those parts of the radio spectrum for NC-OFDM which are outside the main IR-UWB transmission mask.

Recently, the way people consume video content has been undergoing a dramatic change. Plain TV sets, that have been the center of home entertainment for a long time, are losing ground to hybrid TVs, PCs, game consoles, and, more recently, mobile devices such as tablets and smartphones. The new predominant paradigm is: watch what I want, when I want, and where I want. The challenges of this shift are manifold. On the one hand, broadcast technologies such as DVB-T/C/S need to be extended or replaced by mechanisms supporting asynchronous viewing, such as IPTV and video streaming over best-effort networks, while remaining scalable to millions of users. On the other hand, the dramatic increase of wireless data traffic begins to stretch the capabilities of the existing wireless infrastructure to its limits. Finally, there is a challenge to video streaming technologies to cope with a high heterogeneity of end-user devices and dynamically changing network conditions, in particular in wireless and mobile networks. In the present work, our goal is to design an efficient system that supports a high number of unicast streaming sessions in a dense wireless access network. We address this goal by jointly considering the two problems of wireless transmission scheduling and video quality adaptation, using techniques inspired by the robustness and simplicity of proportional-integral-derivative (PID) controllers. We show that the control-theoretic approach allows to efficiently utilize available wireless resources, providing high quality of experience (QoE) to a large number of users.

Many indoor positioning applications focus on determining the location of a device in a particular area, like a room of a building, rather than its metric coordinates. It is common that most buildings are partitioned in more or less a regular way and discovering the relative positions of the nodes can give us an accurate estimate of the space, e.g. a room, they are located in. In this study, we have developed a probabilistic methodology to detect the sequence of wireless sensor nodes placed in an indoor environment. We assume no preliminary training, preconfiguration or infrastructure other than the identity of one reference node at the beginning of the sequence. We trigger the nodes under consideration to transmit a series of packets, and use received signal strength information as observed by the neighbors to infer about the sequence of the nodes. In this paper, a centralized processing of the information is used. The proposed methodology is verified both by simulation and by experiments in a building with regular rooms, high shadowing and multipath distortions. Rarely observed classification errors in the experiments have motivated some extension to the basic methodology.

Despite their popularity, the current praxis of comparative experimental evaluation of fingerprinting-based localization algorithms is lacking rigor, with studies typically following an ad-hoc evaluation process and focusing on black-box comparison of complete algorithms. In this paper we present a systematic benchmarking methodology that is focused on gaining finegrained insight about the relative contributions of the individual phases of the fingerprinting-based localization algorithms to their overall performance. To this end, we decompose the localization algorithms in common phases (collection of raw measurements, creation of fingerprints, pattern matching and post-processing) and systematically asses the performance of different procedures that can be applied in each of these phases. We illustrate the application of the proposed methodology using a comprehensive experimental case-study of 3 WiFi fingerprinting algorithms with 4 raw RSSI collection procedures, 3 fingerprint creation and pattern matching procedures, 4 different post-processing procedures in 3 testbeds and 4 evaluation scenarios, resulting in 36 individual experiments. The results demonstrate that in the evaluated scenarios, a lower number of WiFi APs and rather simple fingerprint creation and pattern matching can achieve better performance in terms of location accuracy than more sophisticated alternatives. The results also show that postprocessing steps like k-Nearest Neighbours (kNN) procedure are indeed effective in reducing the localization error variability and extremes, thus increasing the stability of the location estimation.

In OFDMA uplink, Multiple Access Interference (MAI) caused by synchronization offsets in user signals can considerably degrade user throughputs. In this paper, we investigate the problem of applying Dynamic Resource Allocation (DRA) to mitigate MAI and simultaneously improve user throughputs. Firstly, we introduce a novel solvable optimization problem (OP) following DRA to maximize the minimum user throughput. Using this proposed OP, we show the optimal resource allocation schemes can increase minimum user throughput up to 300% compared to existing DRA heuristics, and to 25% compared to the estimation and correction based approach. Secondly, by investigating the frequency assignment, we show an insight about the advantage of DRA in uplink, where better exploitation of frequency and multiuser diversities leads to efficient MAI mitigation and significant throughput gain. Finally, we introduce suboptimal OPs to reduce the complexity, among that the one based on chunking can significantly reduce the complexity and still provide comparable throughput gain.

In many application areas for wireless sensor networks, having some location knowledge about the sensor nodes is important, but this information is often not readily available. In the past, plethora of different solutions have been proposed that focused on recovering the locations of the sensor nodes, often at the cost of high complexity or limited accuracy. We claim that, in many scenarios the precise location in the x, y, z coordinates is not really needed. Frequently, due to the scenario of usage, the set of potential locations of the nodes is limited, and assigning the nodes to proper potential position is sufficient. In this study, we consider a single dimensional case and we introduce a novel methodology to detect the geographic sequence of wireless sensor nodes indoors, using only a limited number of message exchanges among the involved nodes and statistical reasoning applied to the set of received signal strength measurements. The proposed approach is verified experimentally in heavy non-line-of-sight settings with high shadowing and multipath distortions.

IEEE 802.11 WLANs are currently one of the most popular wireless technologies, but their immediate success results in dense deployments and high demand of user traffic. This in turn leads to decrease in throughput and poor spectrum utilization. Especially in the 2.4 GHz ISM band, where the spectrum is a very scarce resource, all available WLAN channels should be exploited in the best possible way to achieve higher utilization. One way to reach this goal is the usage of partially overlapping channels (POC). Most of the previous work related to POC is based on two major studies addressing 802.11 b, but none of them evaluates the POC behavior in the 802.11 g networks. Moreover, most of the previous results are based on simulations. The main contribution of this work is an experimental evaluation of POC in 802.11g networks. In this paper we confirm quantitatively that 802.11b reacts as expected from the previous studies, while 802.11 g reacts entirely different to the presence of adjacent channel interference. That leads to the conclusion that the usage of POC for 802.11g is not recommended.

Data rate adaptation (RA) schemes are the key means by which WLAN adapters adjust their operation to the variable quality of wireless channels. The IEEE 802.11 standard does not specify any RA preferences allowing for a competition in performance among vendors, thus numerous proprietary solutions coexist. While the RA schemes implemented in individual user terminals are unknown to the AP of a hotspot, it is well known that the way how individual stations adapt their rates strongly influences the performance of the whole WLAN cell. Therefore, the knowledge of the scheme applied by each station may be useful for the radio resource management in complex networks (e.g., HetNets or dense WLAN deployments in enterprise networks). In this paper, we present a novel approach to estimate the features of the RA schemes implemented in individual stations and demonstrate its efficiency using both simulated WLAN configurations as well as measurements.

In the uplink of OFDMA systems, Multiple Access Interference (MAI) can cause considerable reduction in the throughputs of the User Terminals (UTs). It is well known that applying optimal resource allocation is an efficient means to mitigate MAI, thereby improving UTs' throughputs. In this paper, we show the importance of the consideration of MAI in the resource allocation optimization. To do that we develop a new optimization model based on a solvable minimization of maximum of normalized MAI, which considers MAI and offers suboptimal resource allocation subject to the desired throughput goal. We show that the proposed optimization model improves significantly the minimum UT throughput by approximately 32%. Further, we numerically evaluate and analyze the spread of the resource allocation. As a result of the analysis, we show that, counterintuitively, spreading resource allocation can mitigate MAI and improve user throughputs.

Due to the requirements to provision a proper Quality of Service level in enterprise WLANs supporting both voice and data services the typical densities in the deployment of access points (APs) may exceed 4000 APs per square kilometer. While such density is necessary under heavy traffic conditions, it is obviously superfluous during the time of lower load- and dramatically excessive at night periods, with only marginal traffic intensity. We present a novel, aggressive approach for adjusting the AP density to the actual traffic conditions. In the limiting case of a very low traffic, we postulate keeping operational only a skeleton deployment, sufficient just to recognize that there is a station attempting an association. In this case additional APs can be powered up, in order to assure the requested connectivity, locally in this area. Using data from commercially available APs we estimate the potential of power saving in such an operation mode and relate it to the best approaches proposed so far.

Network discovery is a fundamental task in different scenarios of IEEE 802.15.4-based wireless personal area networks. Scenario examples are body sensor networks requiring health- and wellness-related patient monitoring or situations requiring opportunistic message propagation. In this paper, we investigate optimized discovery of IEEE 802.15.4 static and mobile networks operating in multiple frequency bands and with different beacon intervals. We present a linear programming model that allows finding two optimized strategies, named OPT and SWOPT, to deal with the asynchronous and multichannel discovery problem. We also propose a simplified discovery solution, named SUBOPT, featuring a low-complexity algorithm requiring less memory usage. A cross validation between analytical, simulation, and experimental evaluation methods is performed. Finally, a more detailed simulation-based evaluation is presented, when considering varying sets of parameters (i.e., number of channels, network density, beacon intervals, etc.) and using static and mobile scenarios. The performance studies confirm improvements achieved by our solutions in terms of first, average, and last discovery time as well as discovery ratio, when compared to IEEE 802.15.4 standard approach and the SWEEP approach known from the literature.

We present the design and prototyping of an OpenFlow-enabled Gigabit Ethernet switch based on an ATCA industrial standard platform. Our work consists of an architectural design for introducing OpenFlow switch functionalities to the ATCA switching platform, design and development of an enhanced library for OpenFlow protocol stack, and porting the data-path functionalities and protocol end-point to the selected platform. We also carry out several performance evaluation tests on the developed switching platform and present selected results. The results demonstrate the successful introduction of OpenFlow protocol to the ATCA platform and contribute, among other things, to the identification of performance bottlenecks and crucial design issues in OpenFlow switches.

The idea of using multiple paths to transport TCP traffic seems very attractive due to the potential benefits it may offer. Despite a solid theoretical background and quite substantial implementation attempts, none of the provided proposals was successful enough to fully establish its mark. Therefore, this paper presents and develops a novel approach to carry TCP traffic over multiple paths. The main features of the proposed architecture include: (1) end-to-end transparency, (2) autonomy of the operation (protocol stack transparency), and (3) interoperability with different operating systems (OSs). To demonstrate the potential of the presented architecture, experimental results are provided in one of possible application scenarios, a high capacity local mesh backbone network with bottleneck gateway links.

Offloading data traffic to IEEE 802.11 hotspots is one of the key techniques with which mobile operators aim at dealing with the increasing traffic demand of their users in cellular networks. Usually, these offloads are simply conducted once an end-device has WLAN connectivity. This paper identifies gains if the decisions for accommodating certain traffic flows in WLAN consider the characteristics of this technology, i.e., the suitability of traffic being offloaded in terms of occupation of the channel and MAC overhead as result of contention, interference, and fluctuating channels. The contribution of this work is twofold: A performance evaluation compares the novel scheme with common received-signal-strength-based decisions as well as with simpler randomized choices for offloading particular traffic flows. Our scheme shows substantial improvements over a broad range of factors. Lastly, the results identify operational points of the WLAN cell allowing operators to dynamically choose the mix of the offloaded traffic according to their requirements.

Future wireless mobile devices will have to support a variety of heterogeneous access technologies while having a limited number of transceiver chains. One of the most challenging tasks remains how to support wireless access in one of the technologies, while preparing a “smooth transition” to another, especially to technologies with a lengthy network entry such as WiMAX. This work proposes an efficient way to organize a WiMAX network entry process while continuing a communication session with stringent QoS requirements via WLAN. We advocate the prioritization of the strict WiMAX timing over the contention-based WLAN channel access. Our results show the applicability of this approach over a wide range of mobile WiMAX parameters and identify bounds on the WiMAX downlink load for which the WiFi communication does not suffer quality distortions.

Ethernet-based in-car communication is currently a hot topic in the automotive industry. Soon Ethernet will start to oust MOST bus in its domain of info- and entertainment applications. However, the full benefit of a technologically integrated in-car network will only become rewarding with the deployment of an Ethernet-based backbone that integrates all automotive domains on a single layer at increased bandwidth, reduced complexity and cost, while opening car intelligence for future innovations. Such backbone must transport critical control data in real-time. Standard Ethernet requires extensions to comply with the strict timing requirements of driver assistance and safety applications while simultaneously supporting broadband multimedia traffic. In this paper, we compare IEEE 802.1 AVB and Time-triggered Ethernet, two competing real-time approaches. While the first fosters over- provisioning and prioritisation, the second is based on a coordinated time-division-multiple-access (TDMA) policy for media access. By simulating a realistic in-car backbone design and traffic model, we reveal the strengths and weaknesses of both protocols and point to the diverging characteristics of event- and time-triggered policies. Our results show that in this in-car network scenario both protocols are able to meet the rigid timing requirements, while each has its unique benefits and disadvantages.

We consider the problem of neighbor discovery in wireless networks with nodes operating in multiple frequency bands and with asymmetric beacon intervals. This is a challenging task when considering such heterogenous operation conditions and when performed without any external assistance. We present linear programming (LP) optimization and two strategies, named OPT and SWOPT, allowing nodes performing fast, asynchronous, and passive discovery. Our optimization is slotted based and determines a listening schedule describing when to listen, for how long, and on which channel. We compare our strategies with the passive discovery of the IEEE 802.15.4 standard. The results confirm that our optimization improves the performance in terms of first, average, and last discovery time.

In this paper, we present a Genetic Algorithm approach to solving the joint Internet gateway allocation, routing and scheduling problem in wireless ad hoc networks with the goal of minimizing the average packet delay in the network. We analyze the performance of the proposed Genetic Algorithm by means of simulations and compare the solution that it provides to a hop count based routing and gateway selection solution. We also investigate the benefits that can be achieved by optimizing only the allocation of Internet gateways or the routing. It is shown that the Genetic Algorithm provides significantly better performance in terms of delay and packet delivery ratio.

The peer-to-peer networking concept has revolutionized the cost structure of Internet data dissemination by making large scale content delivery with low server cost feasible. In a peer-to-peer network, the total upload capacity increases with the number of down loaders instead of staying constant as in a client-server architecture, making it highly scalable. Despite of its importance, the problem of efficient data transport in a peer-to-peer network is still an open issue, mainly due to its complex combinatorial structure. In the presented work, we formulate the problem of optimizing a peer-to-peer download with respect to its make span (time until all peers finish downloading)as a mixed integer linear program. Other than previous studies, we consider the case of arbitrary heterogeneous uplink and downlink capacities of the peers. Moreover, we do not consider the fluid limit case but allow the file to be subdivided in finitely many chunks. On the one hand, our results allow to infer the capacity of a peer-to-peer network, providing a benchmark for performance analysis of existing peer-to-peer protocols. On the other hand, we believe that our results build a step towards the development of efficient algorithms serving as a base for the design of data transport protocols leveraging the peer-to-peer concept.

The major requirement for Cognitive Radio (CR) based opportunistic spectrum re-usage is reliable protection of the primary communication. This calls for a reliable detection of the presence of the Primary Users (PUs) as well as for an immediate reconfiguration of the secondary communication: Each time the PU has been detected, the Secondary Users (SUs) have to vacate the respective part of the spectrum and continue their communication elsewhere. Unfortunately enough, also false positives in the sensing process trigger the same type of reconfiguration, leading to more reconfigurations than actually necessary. Therefore, the efficiency of this reconfiguration process is of high interest. For a frequently postulated OFDM based spectrum pooling SU system two basic questions are considered: (1) How should the parameters of the secondary communication link be selected in order to achieve a stable Quality of Service (QoS) in spite of reconfigurations, and (2) how strongly is the QoS of the SU influenced by reconfigurations caused by a non ideal sensing process, i. e. by an excessive number of false positives.

The handshake gesture is an important part of the social etiquette in many cultures. It lies at the core of many human interactions, either in formal or informal settings: exchanging greetings, offering congratulations, and finalizing a deal are all activities that typically either start or finish with a handshake. The automated detection of a handshake can enable wide range of pervasive computing scanarios; in particular, different types of information can be exchanged and processed among the handshaking persons, depending on the physical/logical contexts where they are located and on their mutual acquaintance. This paper proposes a novel handshake detection system based on body sensor networks consisting of a resource-constrained wrist-wearable sensor node and a more capable base station. The system uses an effective collaboration technique among body sensor networks of the handshaking persons which minimizes errors associated with the application of classification algorithms and improves the overall accuracy in terms of the number of false positives and false negatives.

Recently, ad hoc networks have been proposed as a means of providing Internet access to passengers on board aircraft flying in oceanic or remote regions. The vast majority of data in such networks is either destined for or originated at an Internet gateway, which connects the ad hoc network to the Internet. Therefore, the allocation of aircraft to gateways plays an important role for the overall network performance. In this paper, we propose a new scheme for gateway selection based on estimation of the path delay, and demonstrate that this approach is indeed superior to common solutions based on hop count or gateway utilization in terms of packet loss ratio and mean delay.

In this paper we consider the problem of how several observer nodes can cooperatively make reliable judgements about one mobile node. These judgments shall not only cover the mere presence or absence of the mobile node, but also include coarse indications about the distance of the mobile node to the observers. To this end, we present and investigate in this paper a scheme for distance classification. The results show that our scheme greatly reduces the number of times where the mobile node is classified as "absent" and furthermore provides reliable classifications into one of a set of pre-defined distances, provided this pre-defined set is small enough.

This paper proposes a framework for interference mitigation in multi-BSS infrastructure 802.11 WLANs. Our interference mitigation approach is based on access point (AP) coordination. With this approach, interfering BSSs negotiate and switch from the 802.11 CSMA/CA to a time slotted mechanism if users' QoS is observed to be degraded, diagnoses conclude that the cause is high interference, and the switch to the time slotted modus is expected to be useful and feasible. The proposed algorithms within the framework are driven by measurements. We utilize the wireless bandwidth and improve the fairness among WLAN users. We present results of detailed simulation experiments as well as real implementation.

The IEEE 802.11 working group currently integrates the option of mesh networking into its standard for WLANs. In addition to WLAN meshes, future wireless networks will be expanded by multiple heterogeneous access technologies such as UMTS and WiMax. These technologies differ greatly regarding medium access scheme, network capacity, QoS support, choice of data rates, and other various parameters such as power consumption and AAA aspects. Even though current research focuses on different network access selection and handover strategies maximizing the utilization for such a heterogeneous network, it is still an open issue how to select ldquoinefficientrdquo mobiles as vertical handover candidates. This work presents a novel scheme for the selection of handover candidates which is exemplarily discussed for an IEEE 802.11 WLAN mesh. After exploring the design rationale of the decision metric, simulation studies show the impact of single and multiple handovers on remaining users in the cell.

Dynamic mechanisms such as adaptive modulation and coding or scheduling have been shown to significantly improve the performance of cellular systems. This gain comes at the cost of additional signaling overhead that needs to be delivered from the transmitter to the receiver. In this paper, we explore a resource- constraint control channel that delivers resource assignments in a multi-cell environment with a frequency reuse of 1. We show that despite the presence of co-channel interference signaling data can reliably be delivered. We also present means to increase the channel's reliability by dynamic power control.

In distributed virtual environments, hosts typically have to react to events within a time span which is less than the network latency. As a consequence, hosts do routinely take actions although the system is in an inconsistent state. This has a noticeable influence on the perceived quality of these actions and their effect on the application. We argue that the level of this influence depends on the degree of inconsistency. In this paper, we tackle two fundamental questions: How does the degree of inconsistency influence the perceived quality of the users' actions? How can the degree of inconsistency be quantified? We propose a benchmark test for comparing different consistency algorithms with each other which consists of two measures of inconsistency and a sample scenario. For two different consistency algorithms, we compare the results of our benchmark test with the results of a user evaluation test and a simple yield measure.

Despite the fact that the Request-To-Send/Clear-To-Send (RTS/CTS) protocol significantly reduces collisions and retransmissions due to the hidden node problem, it is well known that it adds considerable overhead specially with small payload packets. The IEEE 802.11 standard defined a manageable parameter, RTS threshold, above which a data packet should be preceded with RTS/CTS handshake. In this paper, we propose new dynamic criteria for setting the RTS/CTS mechanism. We believe that RTS/CTS settings should consider the characteristics of userspsila traffic, data rates, activities and locations. While most of the algorithms proposed for controlling RTS/CTS have been investigated under single transmission rate for all users, we evaluate our criteria in a multi-rate scenario. We validate our ideas using both synthetic and real traces as well as real experiments.

Observing the environment is the raison dpsilaetre of sensor networks, but the precise reconstruction of the measured process requires too many messages for a low power sensor network. The number of messages can be reduced by focussing on events relevant for the application. This paper examines three event definitions for a home automation example and compares the message reduction. In the extreme case, the number of transmitted messages drops from more than 3000 messages per node and hour to a maximum of about two messages per node and hour.

This paper proposes a new decentralized Access Point Selection Policy for 802.11 Wireless Local Area Networks (WLANs). We derive a decision metric towards AP selection improvement. The derived metric comprises both Inter-BSS and Intra-BSS interference. Our proposed policy is decentralized in the sense that the decision is performed by each station. The core of the policy is based on measurement reports standarized in the IEEE 802.11 k standard. The main question we would like to address is: To what extent can the impact of interference be reduced at the selection phase? The performance of the proposed policy is evaluated through detailed simulation experiments. We identify the scenarios where the proposed policy is likely to provide great gain, and scenarios where the gain is minimal.

One of the major challenges in designing cellular cognitive radio (CR) networks is the avoidance of secondary user (SU) interference to so called primary users (PUs) operating in the licensed bands. Usually, SU operation has to be interrupted periodically in order to detect PU activity and avoid the respective frequencies. Recently, dynamic frequency hopping (DFH) mechanisms have been suggested to enable reliable PU detection and continuous SU operation at the same time. Applying DFH in a multi-cell environment adds the challenge of mitigating co-channel interference (CCI). In this paper, we introduce a new DFH approach for cellular CR networks to allow reliable PU detection and continuous SU operation while avoiding CCI: double hopping (DH). We present a distributed frequency assignment heuristic for DH and compare it to the optimal assignment. We show that the performance of the sub-optimal distributed assignment is only slightly worse than the optimal performance, and, thus, outperforms existing distributed approaches by far.

Dynamic OFDMA systems provide a significant performance gain compared to static OFDM approaches. In reality this gain is reduced by an overhead due to signaling. Previous work has shown that this loss is particularly high if the number of sub-carriers is large. In this paper we present two approaches to reduce this overhead by exploiting the correlation in frequency. These two schemes are shown to provide a significant reduction of the signaling overhead.

The localization of a mobile device is of essential need in order to provide location based services to the users, e.g. in an emergency situation. Several mechanisms have been proposed so far to determine the positions, requiring rather expensive enhancements of existing access network equipment or mobile devices. In this paper, we present a new approach which uses relatively coarse, cell ID based location determination mechanisms. If the location information precision provided by a carrier is not sufficient for a service, it can be refined with data from other location servers. In contrast to other approaches for the enhancement of location information precision, our approach relies on existing location determination mechanisms inside the networks and requires only updates for the servers providing the information.

In this paper we consider optical burst/packet switching networks with core nodes in which contentions are resolved by means of a hybrid scheme incorporating FDL buffering and wavelength conversion. For such a network we investigate the influence of traffic shaping at the edge nodes, as introduced by optical burst generation algorithms, on the packet drop rate at the core nodes. In particular we focus our attention on the impact of the algorithms that generate fixed-size optical bursts, i.e., volume-based burst assembly. The simulative investigation reveals that the total number of burst flows multiplexed at the core node under consideration as well as the assembly threshold are the two most influential factors, and that the influence of these factors on the drop rate could be as high as several orders of magnitude. Additionally, it is discussed that the assembly process can negatively influence the performance of the so-called void-filling channel scheduling algorithms.

Possessing knowledge about access point coverage areas and neighbors is of essential need for the provisioning of, e.g., location based handovers or push services. A convenient method for their discovery is the usage of information obtained by the mobile devices which are currently associated with the access points. The time span being required for the discovery of coverage areas and neighbors is fundamental in order to assess the suitability of such a method in a dynamic environment. In this paper, exemplary numerical results of the required bootstrapping phase for the information determination are presented. Additionally, a relatively simple enhancement of the algorithm computing the coverage areas is introduced, reducing the number of vertices for the description of the areas. Besides, the paper shows how the upcoming IEEE 802.11k standard enables APs to query mobile devices for location and neighbor information and hence presents for the first time an entirely standard compliant acquisition scheme.

In wireless local area networks (WLANs) often a station can potentially associate with more than one Access Point(AP). In IEEE 802.11, the station simply associates to the AP from which it has received the strongest signal during the scanning process. However, this may result in a significant load imbalance between several APs since some of them might be highly loaded while others are lightly loaded or even idle. Moreover, the multi-rate exibility provided by several IEEE 802.11 variants can cause low bit rate stations to negatively affect high bit rate ones and consequently degrade the overall network throughput. Therefore, a relevant question is how to optimally distribute stations among APs so as to maximize the overall network performance. This paper presents a centralized optimal association policy for IEEE 802.11 WLANs. We first derive the optimal solution for stations association. Then, we evaluate the effectiveness of the solution through the results obtained from Lingo optimization and NCTUns simulation packages.

Earlier paper have demonstrated that the achievable throughput of OFDM systems can benefit significantly from individual modulation/transmit power selection on a per sub-carrier basis according to the actual gain of individual sub-carriers (so called dynamic OFDM scheme). Usage of such approach requires, however, providing support for additional functionality like: acquisition of the subcarrier gains, signaling of the used modulation types between the sender and receiver, etc. Therefore dynamic OFDM is actively pursued for future radio interfaces, rather then considered as extension of existing OFDM based standards. In this paper we present for the first time a proposal how the widely accepted IEEE 802.11a/g systems might be extended to support the dynamic OFDM in a singleuser (point-to-point) setting while assuring backward compatibility. We address these issues by a) presenting a set of protocol modifications; and b) a performance evaluation of the suggested extension (referred further on to as single-user 802.11 DYN mode) demonstrating the potential of performance improvement.

Energy management is a critical concern in wireless sensornets. Despite its importance, sensor network operating systems today provide minimal energy management support, requiring applications to explicitly manage system power states. To address this problem, we present ICEM, a device driver architecture that enables simple, energy efficient wireless sensornet applications. The key insight behind ICEM is that the most valuable information an application can give the OS for energy management is its concurrency. Using ICEM, a low-rate sensing application requires only a single line of energy management code and has an efficiency within 1.6 % of a hand-tuned implementation. ICEM’s effectiveness questions the assumption that sensornet applications must be responsible for all power management and sensornets cannot have a standardized OS with a simple API.

Channel-adaptive techniques increase the efficiency of wireless communications, as they are efficient in coping with the quality variation of the wireless channels. However, channel-adaptive mechanisms require a prediction of the future behaviour of the channel. Heuristics are an important alternative to more complex predictors in WLAN scenarios. In this paper we use WLAN measurement traces for the simulative evaluation of the influence of channel prediction errors in the performance of a threshold-based adaptive modulation scheme, considering also the case of delayed channel feedback. The results show that, as long as the necessary prediction horizon does not exceed 2 ms, assuming the channel to stay constant leads to less than 15% capacity loss compared to the case when perfect channel prediction is used. When farther prediction horizons are needed, the moving average of the received signal should be used for channel prediction.

By geocast protocols, information can be directly disseminated to users of mobile devices who currently reside in selected geographical regions. Time-stable geocast protocols enhance these mechanisms by providing the information to the users in the regions for a specified duration. This becomes a challenging task under the harsh conditions in mobile ad-hoc networks with sparse user densities, especially if the information should be provided to a new user immediately when he enters a region. This paper introduces a new approach which increases the probability of a user to immediately receive respective information when he enters a region. It is based on an enlarged message forwarding region, being dynamically adapted depending on the currently perceived spatial user density. Besides, the paper presents the realization of the time-stable geocast mechanisms based on IEEE 802.11 technology.

Abstract To solve the IP mobility problem, the use of multicast has been proposed in a number of different approaches, applying multicast in different characteristic ways. We provide a systematic discussion of fundamental options for multicast‐based mobility support and the definition and experimental performance evaluation of selected schemes. The discussion is based on an analysis of the architectural, performance‐related, and functional requirements. By using these requirements and selecting options regarding network architecture and multicast protocols, we identify promising combinations and derive four case studies for multicast‐based mobility in IP‐based cellular networks. These case studies include both the standard any‐source IP multicast model as well as non‐standard multicast models, which optimally utilize the underlying multicast. We describe network architecture and protocols as well as a flexible software environment that allows to easily implement these and other classes of mobility‐supporting multicast protocols. Multicast schemes enable a high degree of flexibility for mobility mechanisms in order to meet the service quality required by the applications with minimal protocol overhead. We evaluate this overhead using our software environment by implementing prototypes and quantifying handoff‐specific metrics, namely, handoff and paging latency, packet loss and duplication rates, as well as TCP goodput. The measurement results show that these multicast‐based schemes improve handoff performance for high mobility in comparison to the reference cases: basic and hierarchical Mobile IP. Comparing the multicast‐schemes among each other the performance for the evaluated metrics is very similar. As a result of the conceptual framework classification and our performance evaluations, we justify specific protocol mechanisms that utilize specific features of the multicast. Based on this justification, we advocate the usage of a source‐specific multicast service model for multicast‐based mobility support that adverts the weaknesses of the classical Internet any‐source multicast service model. Copyright © 2006 John Wiley & Sons, Ltd.

Summary form only given. Controlled experiments, with larger sensor networks configurations (100 + nodes), as well as discovering inefficiencies in their operation are rather complex. In this paper we present a concept of a testbed for WSNs supporting easy change of configuration, multi tier operation and precise observation of network behaviour using a wired backbone connectivity. The design considerations are accompanied by early usage experience

Impairments in wireless data communication due to time and location dependent errors can be overcome by using channel-adaptive techniques, like channel-aware scheduling or adaptive modulation. These techniques require information about channel behaviour obtained from channel predictors. Unfortunately enough the accuracy of predictors usually suggested in the literature has not been confirmed and comparative studies do not exist (with one exception). In addition, existing predictors are frequently difficult to compute and sensitive to numerous parameters, e.g. noise and channel variability. In this paper, we investigate and compare the performance accuracy of three simple heuristics for channel prediction. Our investigation includes a comparison with a verified reference predictor using Rayleigh channels and measured WLAN traces. Our study indicates the usefulness of the investigated heuristics for fixed systems and pedestrian speed mobility.

In performance evaluation of optical burst-switched (OBS) networks using simulation a significant part of the simulation time, particularly in case of a network with a large number of ingress nodes, is used by the implementation of the burst assembly algorithms. This is due to the fact that each data burst is result of aggregating several short-length packets which - in a straightforward approach - have to be generated individually and afterwards "melted" into the burst. This paper presents a novel approach to fast generation of bursts based on an analytical model for both burst length and interarrival time distributions as well as an efficient generation technique (composition) supporting generation of these distributions. It is shown that the emulators developed in this work can speed up the burst generation process up to an order of magnitude while assuring a high accuracy.

In wireless local area networks often a station can potentially associate with more than one access point. Therefore, a relevant question is which access point to select "best" from a list of candidate ones. In IEEE 802.11, the user simply associates to the access point with the strongest received signal strength. However, this may result in a significant load imbalance between several access points, as some accommodate a large number of stations while others are lightly loaded or even idle. Moreover, the multi-rate flexibility provided by several IEEE 802.11 variants can cause low bit rate stations to negatively affect high bit rate ones and consequently degrade the overall network throughput. This paper investigates the various aspects of "best" access point selection for IEEE 802.11 systems. In detail, we first derive a decision metric the selection can be based on. Using this metric we propose two new selection mechanisms which are decentralized in the sense that the decision is performed by each station, given appropriate status information of each access point. In fact, only few bytes of status information have to be added to the beacon and probe response frames which does not impose significant overhead. In addition, we show that our mechanism improves station quality of service and better utilizes network resources compared to the conventional one implemented today in IEEE 802.11 devices

Following widespread availability of wireless Internet, a wide range of applications with differing QoS requirements must share the wireless access. The objective of this work is to provide QoS support to different applications delivered over a wireless link shared by different users. We propose a link-layer packet scheduler following the cross layer concept: the channel-aware scheduling approach is extended by information about the importance of individual packets for the application. For this purpose we use utility curves, which map network service delivered to a given data flow to application-perceived quality. We define a single scheduling metric which expresses, for each packet, the balance between the quality increase of the flow served and the quality loss of the flows that are not. We evaluated our concept through extensive simulations with mixes of VoIP and file download flows. The results show that our maxsum utility-based scheduler clearly improves the quality seen by the VoIP flows when compared to a channel-aware round robin (caRR)/proportional fair scheduler (PFS), respectively. Further, this is achieved while delivering similar amount of data to file download users. We also show how different forms of the utility curves can be used to achieve differentiated QoS provisioning to users/applications with different priorities/ requirements

Area based triggers denote notifications being generated if a mobile client changes its spatial relation to a defined area, e.g. if it enters or leaves it. By these triggers, new valuable services can be provided for mobile users. One of the main challenges is to design a suited approach for the computation of the triggers based on the defined areas and the current whereabouts of the mobile clients. With a rising number of areas and participants of these services, the need for a scalable solution to process the triggers emerges. This paper examines a new approach for the distributed, hierarchical processing of area based triggers based on the aggregation and segregation of both triggers and the setup messages requesting the services in a carrier access network. We investigate the signaling effort and computational complexity of the approach based on the required effort for an emerging position update of a mobile client in the network. The results are compared analytically with two other approaches, a centralized and a hierarchical one. Our analysis shows that our approach leads to a superior performance in terms of the effort for signaling and computation

Detection of nearby nodes in wireless ad hoc networks requires periodic neighborhood scanning which is, in terms of energy, a rather expensive procedure: The nodes transmit the discovery packets and, furthermore, their reception circuits have to be active most of the time. Since distributed moving group detection algorithms rely on those nearby node-detection procedures, this kind of overhead seems unavoidable for them. However, in most of the scenarios some fixed infrastructure co-exist which may support the detection process. In this paper we propose a method that uses the infrastructure to gain hints about moving groups and triggers the distributed process only if there is some degree of confidence that such a group exists. This reduces the overhead of the neighborhood scans by approximately 90% translating e.g. to longer lifetime for the battery-driven nodes

This paper presents an end-to-end reservation protocol for quality-of-service (QoS) support in the medium access control layer of wireless multihop mesh networks. It reserves periodically repeating time slots for QoS-demanding applications, while retaining the distributed coordination function (DCF) for best effort applications. The key features of the new protocol, called "distributed end-to-end allocation of time slots for real-time traffic (DARE), are distributed setup, interference protection, and scheduling of real-time data packets, as well as the repair of broken reservations and the release of unused reservations. A simulation-based performance study compares the delay and throughput of DARE with those of DCF and the priority-based enhanced distributed channel access (EDCA) used in IEEE 802.11e. In contrast to DCF and EDCA, DARE has a low, nonvarying delay and a constant throughput for each reserved flow

Within the last decade, the orthogonal frequency- division multiplexing (OFDM) transmission scheme has become part of several standards for wireless systems. Today, OFDM is even a candidate for fourth-generation wireless systems. It is well known that dynamic OFDMA systems potentially increase the spectral efficiency. They exploit diversity effects in time, space, and frequency by assigning system resources periodically to terminals. Informing the terminals about new assignments creates a signaling overhead. Up to now, this overhead has not been taken into account in studies on dynamic orthogonal frequency-division multiplexing access (OFDMA) systems. Yet this is crucial for a realistic notion of the performance achieved by dynamic approaches. In this paper, we close this gap. We introduce two forms of representing the signaling information and discuss how these affect system performance. The study of the signaling impact on the performance is conducted for an exemplary dynamic approach. We find that the throughput behavior of dynamic OFDMA systems is significantly influenced by the signaling overhead. In many situations, neglecting the overhead leads to wrong performance conclusions. Also, the performance difference between dynamic and static schemes is now much more sensible to the specific parameter set of the transmission scenario (e.g., frame length, subcarrier number, etc.). This leads to the proposal of access points which should adapt certain system parameters in order to provide optimal performance.

OFDM systems are known to overcome the impairment of the wireless channel by splitting the given system bandwidth into parallel sub-carriers, on which data symbols can be transmitted simultaneously. This enables the possibility of enhancing the system's performance by deploying adaptive (dynamic) mechanisms, namely power and modulation adaption and dynamic sub-carrier assignments. In multi-user communication systems (OFDM-FDMA), these mechanisms can be used to achieve a level of system fairness ensuring that each terminal receives at least an environment-specific minimum amount of data per down-link phase. However, it has been doubted by multiple previous investigations that dynamic power adaption provides enough performance gain in order to be applied in such systems, as it increases the computational load significantly. In this study we discuss the performance gain due to the different approaches and show that in specific communication scenarios enabling a dynamic power distribution provides a significant performance increase compared to dynamic schemes without power adaption

Power efficiency is one of the most critical factors for wireless sensor networks. In this paper, we present a family of receiver-initiated pseudo-asynchronous rendezvous schemes designed specifically to reduce sensor nodes' power consumption. We analyze their performances in terms of power efficiency and latency, under different static and dynamic system parameters. In addition, we verify our analysis by extensive network simulations. Based on the results, our proposed scheme demonstrates superior performance compared to previous proposed schemes

In this paper, a detailed study of the performance of geographic routing protocols in the presence of localization errors is carried out. Both analytical and simulation results illustrate the major impact or localization errors on the protocol goodput and route discovery energy. The performance metrics observed were the packet delivery ratio and the power consumed at a node for routing. It is shown that significant performance deterioration occurs with location errors as low as 20% of a node's radio range with no other obstacles in the network. To counteract this degradation, an enhancement is proposed that increases the error tolerance to about 40% radio range and in addition improves the performance consistently for any location error. Furthermore, the effect of obstacles in conjunction with location errors on the routing performance is also investigated.

Recently, centralized wireless network architecture has been proposed for ef cient system resource management and cost-effectiveness. Radio over ber (or cable) networks and centralized wireless LAN systems are representative instances. However, this architecture requires us to reconsider system level control schemes since most of research efforts in this eld has been focused on distributed network architecture. In this paper, we propose a load balancing scheme for centralized wireless networks using the capability that users in the overlapping area between cells can ef ciently and dynamically be reconnected to one of the accessible base stations by a central control station. A simulation study is done to compare the proposed scheme with conventional load balancing ones, showing the proposed scheme is more ef cient in system resource utilization.

The common belief is that a multi-hop configuration with rather small per-hop distance is the only viable energy-efficient option for wireless sensor networks. We discuss a single hop configuration, utilizing the asymmetry between lightweight sensor nodes and a more powerful "base station" and demonstrate that such a single hop configuration can actually have lower overall power consumption than a multi-hop counterpart.

Opportunistic routing protocols have been proposed as efficient methods to exploit the high node densities in sensor networks to mitigate the effect of varying channel conditions and non-availability of nodes that power down periodically. They work by integrating the network and data link layers so that they can take a joint decision as to the next hop forwarding node based on its availability and suitability as a forwarder. This cross-layer integration makes it harder to optimize the protocol due to the dependencies among the different components of the protocol stack. In this paper, we provide a framework to model opportunistic routing that breaks up the functionality into three separate components and simplifies analysis. The framework is used to model two variants of opportunistic routing and is shown to match well with simulation results. In addition, using the model for performance analysis yields important guidelines for the future design of such protocols.

With the success of wireless technologies in consumer electronics, standard wireless technologies are envisioned for the deployment in industrial environments as well. Industrial applications involving mobile subsystems or just the desire to save cabling make wireless technologies attractive. Nevertheless, these applications often have stringent requirements on reliability and timing. In wired environments, timing and reliability are well catered for by fieldbus systems (which are a mature technology designed to enable communication between digital controllers and the sensors and actuators interfacing to a physical process). When wireless links are included, reliability and timing requirements are significantly more difficult to meet, due to the adverse properties of the radio channels. In this paper, we thus discuss some key issues coming up in wireless fieldbus and wireless industrial communication systems: 1) fundamental problems like achieving timely and reliable transmission despite channel errors; 2) the usage of existing wireless technologies for this specific field of applications; and 3) the creation of hybrid systems in which wireless stations are incorporated into existing wired systems.

The multicast-based mobility architecture, MOMBASA, has proven to be an efficient and elegant approach for achieving low latency handover with minimum packet loss in mobile Internet communications (Festag, A. et al., Proc. Performance Tools, p.212-19, 2002). The original MOMBASA specification, however, did not include any precautions against malicious attacks on its protocol operation. We present the principal results of a security analysis of the MOMBASA architecture and describe our security concept to counter the identified threats. A main focus is put on attacks against the MOMBASA protocol operation coming into the access network from two main sources: the public Internet and the wireless link. The design of our security concept is specifically suited to ensuring a seamless handover by augmenting the predictive handover functionality of MOMBASA with an accompanying predictive distribution of authentication keys. Furthermore, the security concept includes a rate control mechanism for traffic destined for idle mobile nodes in order to limit the risks of potential denial of service (DoS) attacks against the paging mechanism. While our security measures effectively counter the identified threats from the wireless link and the Internet, first measurements with our prototype implementation show only negligible degradation of handover performance compared to unsecured MOMBASA operation.

We propose two generic rendez-vous schemes for dense wireless sensor networks, including the transmitter- and receiver-initiated cycled receiver schemes. Our studies include the analyses and comparisons of their power efficiencies, especially under a fading channel as a realistic physical layer. We believe our modeling strategies as well as the results are applicable to any rendez-vous scheme of the cycled- receiver nature. The paper further proposes precise guidelines and optimization strategies for synchronization in wireless sensor networks.

In this paper we integrate the energy models of all the components of the data link layer of a wireless sensor network into a single framework, which interacts with the abstracted models of the network layer, the physical layer, and the channel. Such a framework enables intra layer and cross layer optimization. By identifying the important design parameters, it provides the venues and guidelines for energy' reduction and design improvement, as demonstrated in the paper. The validity of the models is verified using OMNET++ network simulations.

When optimizing the operations of large wireless ad hoc networks, neither global nor local information-based approaches fits well: they require either information about the entire network structure, which is in most cases impossible to get, or are not capable of optimizing beyond a very narrow horizon. We propose a novel optimization scheme based on regional information to compute the network-wide optimizations, taking the peculiarities of large ad hoc networks into account, and obtain an "emergent algorithm" out of a global optimization algorithm. Our solution uses a clustering algorithm to define regions but needs neither cluster maintenance nor inter-cluster communication protocols, thus is expected to be very robust. The problem of distributed frequency assignment is used as a case study to demonstrate the performance of our method as compared to algorithms based on local- or global information.

In multihop ad hoc networks the IEEE 802.11 MAC protocol, in its distributed version, dramatically degrades the performance in terms of throughput and delay. This is due to a protocol property, which tries to provide equal probability of channel access to all nodes. But in multihop ad hoc network a more frequent channel access might be necessary at certain nodes that are intermediate hops and forward other nodes' data. This paper proposes a modification to the protocol that leads to a higher throughput and lower delay by combining the ACK for one packet with the channel access procedure for another, queued-to-be-sent packet. Hence, nodes receiving many packets, probably forwarder, have a higher chance of accessing the channel. Additionally, these changes provide a scheme of packet forwarding more robust to different layouts than original IEEE 802.11, where performance can heavily deteriorate in certain configurations.

Wireless networks using radio over fiber (RoF) technology operating in millimeter-wave (mm-wave) bands have been suggested as promising solutions to meet increasing user bandwidth and mobility demands. Due to the high penetration loss of mm-wave band signal, a wireless LAN (WLAN) based on this technology has properties quite different from those of conventional WLAN systems. That is, every room in a building should have at least one base station (BS). Thus, a challenging problem lies in the medium access control (MAC) protocol design so that it can support QoS requirements as well as a fast and easy handover. A MAC protocol (chess board protocol) based on frequency switching (FS) codes has been proposed by the authors considering the situation (H. B. Kim et al., April 2003). In this paper performance evaluation results for simple six variants of it are described and discussed.

IEEE 802.11b compliant WLAN technology is increasingly used for cordless telephone services. Often, a WLAN phone is moved during a call. In the present paper we explore to what extent slow user motion influences the wireless link quality. We conducted extensive measurements with speech over commercial WLAN equipment using an experimental environment enforcing controlled motion. Our experiments show that - in contrast to the common assumption - an increase of motion speed can result in a better link quality: The packet loss rate and variance, measured after link-layer retransmissions, decreases. In addition, our measurements demonstrate that the modulation type, the maximal number of retransmissions, the experimental setting, and even the quality of power supply are the dominant influences on link quality.

In a wireless, multi-hop sensor network, choosing transmission power levels has an important impact on energy efficiency and network lifetime. Two algorithms for dynamically adjusting transmission power level on a per-node basis are proposed here. Network lifetime, convergence speed as well as resulting network connectivity are used as figures of merit for these two algorithms. They have been evaluated in an indoor sensor environment. The network lifetime metrics of these two local algorithms are also benchmarked against power control algorithms using global information. We show that these local algorithms outperform fixed power level assignment and that the lifetime achieved by them is usually within a factor of two of globally computed solution while being scalable.

Metropolitan WDM networks play an important role in the emerging Internet hierarchy; they interconnect the backbone WDM networks and the local access networks. The current SONET/SDH-over-WDM-ring metropolitan networks are expected to become a serious bottleneck - the so-called metropolitan gap-as they are faced with an increasing amount of bursty data traffic and quickly increasing bandwidths in the backbone networks and access networks. Innovative metropolitan WDM networks that are highly efficient and able to handle variable-size packets are needed to alleviate the metropolitan gap. In this paper we study an AWG-based single-hop WDM metropolitan network. We analyze the photonic switching of variable-size packets with spatial wavelength reuse. We derive computationally efficient and accurate expressions for the network throughput and delay. Our extensive numerical investigations - based on our analytical results and simulations - reveal that spatial wavelength reuse is crucial for efficient photonic packet switching. In typical scenarios, spatial wavelength reuse increases the throughput by 60% while reducing the delay by 40%.

Modern telecommunication networks promise new perspective in information access at any time, from any place. On the other hand recent spectrum auctions made it clear, that the transmission capacity (at least for wireless access!) becomes really an economic issue. In this talk we advocate the thesis, that both flat rate and usage based charging of the end-users are structurally wrong, and discuss an alternative dual charging model.

This study investigates the potential of using transmission power control in wireless packet networks with differing number of hops between source and destination. Here we exploit the benefits of power control in the context of multi-hop wireless ad hoc type networks with a distributed media access control. For our investigations we choose several general ad hoc network topologies and studied the effect of power control with respect to energy consumption and network capacity. We show that power control largely improves the network capacity and energy savings in all investigated scenarios, and that utilizing a greater number of intermediate hops between base source and destination improves the energy savings, but often causes a tradeoff in capacity, depending on the network topology scenarios.

"Best effort" packet-switched networks, like the Internet, do not offer a reliable transmission of packets to applications with real-time constraints such as voice. Thus, the loss of packets impairs the application-level utility. For voice this utility impairment is twofold: on one hand, even short bursts of lost packets may decrease significantly the ability of the receiver to conceal the packet loss and the speech signal playout is interrupted. On the other hand, some packets may be particular sensitive to loss as they carry more important information in terms of user perception than other packets.We first develop an end-to-end model based on loss run-lengths with which we can describe the loss distribution within a flow. These packet-level metrics are then linked to user-level objective speech quality metrics. Using this framework, we find that for low-compressing sample-based codecs (PCM) with loss concealment isolated packet losses can be concealed well, whereas burst losses have a higher perceptual impact. For high-compressing frame-based codecs (G.729) on one hand the impact of loss is amplified through error propagation caused by the decoder filter memories, though on the other hand such coding schemes help to perform loss concealment by extrapolation of decoder state. Contrary to sample-based codecs we show that the concealment performance may "break" at transitions within the speech signal however.We then propose mechanisms which differentiate between packets within a voice data flow to minimize the impact of packet loss. We designate these methods as "intra-flow" loss recovery and control. At the end-to-end level, identification of packets sensitive to loss (sender) as well as loss concealment (receiver) takes place. Hop-by-hop support schemes then allow to (statistically) trade the loss of one packet, which is considered more important, against another one of the same flow which is of lower importance. As both packets require the same cost in terms of network transmission, a gain in user perception is obtainable. We show that significant speech quality improvements can bem achieved and additional data and delay overhead can be avoided while still maintaining a network service which is virtually identical to best effort in the long term.

For multimedia senders to benefit the most of dynamically changing availability of network resources they need to adjust their transmission behavior in accordance with the network congestion state. In this paper we investigate various mechanisms for realizing an additive increase and multiplicative decrease adaptation scheme based on the real-time transport protocol (RTP). Through analysis and simulations we identify optimal approaches for increasing and decreasing the transmission rate. Here, we will mainly concentrate on the aspects of TCP-friendliness, dynamical setting of the adaptation parameters, fair bandwidth distribution and stability in addition to the obligatory requirements of high bandwidth utilization and low losses in the network. Note that the main challenge here is in achieving these goals using the rather infrequent RTP feedback messages.

This paper has two parts: Firstly, we investigate today's ATM network, especially the most load NFS applications. Secondly, we propose a fast simulation based on fractional ARIMA processes and an importance sampling scheme. In our collected data, we have found the traffic characteristics of today's ATM network present both long range dependence (LRD) and short range dependence (SRD) structures. Because of the strong SRD and LRD of the empirical traces, models which can present both LRD and SRD are necessary to simulate the traffic of today's networks. However, traditional Monte Carlo simulations with long range dependent structure make the cost of the computation very large and for CLP (cell loss probability)<10/sup -9/ traditional Monte Carlo simulations are almost impossible to achieve it. The proposed fast ARIMA simulation based on importance sampling efficient to simulate the queuing performance with self-similar inputs which exhibit both SRD and LRD properties.

We start with a general discussion clarifying the different meanings of the frequently misused term 'mobility'. Following we consider how different types of mobility could be introduced into ISDN, focusing especially on a constraint mobility approach where we provide a wireless counterpart to the usual ISDN access interfaces. Both the technical issues of such approach and possible application scenarios are discussed.

The first systematic investigation is presented of the consequences of using out-of-date load information in a server selection process. The authors suggest a selection policy allowing for the use of significantly outdated information and demonstrate the possible consequences of client autonomy, i.e. of clients using other server selection rules, such as the one suggested in the common trading concept.>