The integration of Wake-Up Radios (WURs) into Wi-Fi presents a powerful paradigm for enabling multi-year battery life in power-constrained devices. However, existing approaches, such as the IEEE 802.11ba standard, rely on dedicated, spectrally-inefficient transmissions that consume valuable channel resources. This paper introduces AuraWake, a novel technique that creates a spectrally-efficient control plane by superimposing control signals directly onto ongoing Wi-Fi data packets, leveraging the primary link’s existing signal-to-noise ratio margin. We first present a detailed simulation model and a low-complexity architecture for the AuraWake receiver. Through comprehensive link-level simulation, we conduct a rigorous performance evaluation of its reliability and quantify its minimal, controllable impact on the primary data communication. We then demonstrate the utility of this superimposed control plane by applying it to solve critical, practical challenges in modern Wi-Fi. Our system-level analysis and simulation studies confirm that using AuraWake to mitigate clock drift for long-sleeping Target Wake Time (TWT) stations, enforce quiet periods to protect Restricted TWT (R-TWT) transmissions, and dynamically adapt TWT schedules significantly enhances the reliability and efficiency of these next-generation energy-saving features.

Next-generation IEEE 802.11 wireless local area network (WLAN) amendments have been proposed to support Wi-Fi stations (STAs) and access points (APs). IEEE 802.11be (Wi-Fi 7) features multi-link operation (MLO) with multi-link device (MLD), where the enhanced multi-link single-radio (EMLSR) operation is promising. Also, IEEE 802.11bf launches a sensing capability, paving the way for integrated sensing and communications (ISAC). Pioneering an innovative combination of EMLSR operation and ISAC functionality in this paper, we propose WiHound, a novel method for target tracking with ISAC using EMLSR in IEEE 802.11 WLANs. Specifically, we adopt the Kalman filter for target tracking and develop a score-based ISAC decision approach for the AP MLD to decide between sensing and communications within each transmit opportunity (TXOP). For a sensing TXOP, we solve a discrete convex optimization problem based on Cramér-Rao lower bound (CRLB) to select three STA MLDs required in trilateration. Conversely, for a communications TXOP, we develop an efficient fairness-aware STA MLD selection heuristic approach toward weighted proportional fairness. Simulation results confirm the superiority of WiHound on striking a balance between sensing and communications. Moreover, we investigate the effect of number of STA MLDs on the sensing performance of WiHound.

Commercialized as Wi-Fi 7, the IEEE 802.11be extremely high throughput (EHT) amendment is aimed to further boost the network performance. As a key feature in the IEEE 802.11be EHT amendment, multi-link operation (MLO) allows an operation over multiple links. Having a modest hardware demand while providing sufficient benefits, the enhanced multi-link single-radio (EMLSR) operation is the most promising MLO option, but it confronts a link selection issue which is less explored in previous works. Therefore, in this paper, we propose Equitas, a fairness-aware dynamic link selection heuristic method for EMLSR operation, taking link quality into account while considering both throughput and fairness. Through the link quality assessment and fairness-aware probabilistic link selection processes, Equitas is an efficient scheme to a unique multiple knapsack problem which aspires to achieve weighted proportional fairness with a balance between throughput and fairness under the constraints of EMLSR operation. Simulation results confirm the superior performance of Equitas in terms of throughput and fairness. Besides, we explore the effect of different weight update choices on the fairness performance of Equitas.

The IEEE 802.11bf amendment is aimed to provide Wi-Fi networks with essential support for their potential sensing capability, in addition to their renowned communications paradigm. Taking advantage of both sensing and communications, integrated sensing and communications (ISAC) is a promising direction for Wi-Fi that has been less investigated in the existing literature. Therefore, in this paper, we propose a novel method for target tracking with ISAC in IEEE 802.11bf. Particularly, the Kalman filter is adopted for tracking the state of the target and the Cramér-Rao lower bound (CRLB) is employed to develop a proper performance metric for trilateration, where the access point (AP) needs a selection of three stations (STAs). By solving a discrete convex optimization problem, the AP decides between sensing and communications within each TXOP and selects the three STAs for trilateration if sensing is conducted. Simulation results confirm that the proposed method strikes a good balance between the sensing and communications performance. Moreover, the sensing performance of the proposed method improves as the number of STAs increases.

A low-complexity direction of arrival (DoA) estimation approach based on transmissive intelligent surfaces (TISs) is proposed for single-target scenarios. The proposed DoA estimator is composed of one TIS with pre-designed phase shifts and two receive antennas. The signal from the target transmits through the TIS before being captured by the two antennas, and the DoA of the target is estimated solely based on the ratio of power received at the two antennas. An optimization method is proposed to design the phase shifts of the TIS and the relative positions of the two antennas with two primary objectives: (1) enhancing DoA estimation accuracy, and (2) guaranteeing an analytical expression of the estimated DoA derived from the power ratio. To be specific, the power ratio is approximated using a limited number of Fourier series coefficients, so that the optimization problem is formulated as a small set of quadratic programming problems aimed at optimizing these Fourier series coefficients. Simulation results validate the effectiveness of the proposed TIS-based DoA estimator and the optimization method. The method demonstrates comparable or even lower root mean squared error (RMSE) of DoA estimation in comparison to classic approaches. Unlike classic approaches that rely on complex-valued received signals, the proposed method offers reduced hardware complexity, relying solely on power measurements. Additionally, it involves reduced computational complexity compared to classic approaches including the multiple signal classification (MUSIC) algorithm and the discrete Fourier transform (DFT)-based DoA estimation method.

Target Wake Time (TWT) is a new scheduling capability in Wi-Fi 6 that can improve network performance by reducing contention and increasing energy efficiency. In this paper, we revisit the classic Adaptive Tree Walk Protocol (ATWP) and show its relevance to the TWT-based scheduling problem in Wi-Fi. Inspired by ATWP, we propose a new algorithm, Accordion, which dynamically switches between contention-based and collision-free channel access strategies using TWT. Preliminary simulations show that Accordion can significantly enhance Wi-Fi network performance in terms of average throughput and latency with varying network loads.

Brain-computer interfaces (BCI) have surfaced as a powerful and attractive modality in human-machine interaction. BCIs establish a firsthand communication channel between a human and a computer, using the human subject's neural activity signals as input. Depending on the application, BCIs use distinct brain signals, which are tailored to specific tasks. Systems designed for BCI suffer from two major problems. Firstly, the signal recorded by off-the-shelf devices is extremely noisy and suffers from a very poor signal-to-noise ratio along with low resolution and the presence of signal artifacts. This impedes achieving high accuracy in models even designed for single-user BCI systems. One method to overcome this is to utilize multiple users' observations and average their signals to get an aggregate signal. However, the second problem arises as BCI signals generalize poorly to other subjects which makes user-specific training mandatory for the model to perform well across the board or generalize well to unseen data. This places a lot of overhead on new users who want to use a BCI device in the form of calibration and training. These problems have been the subject of BCI research and various solutions have been proposed throughout the literature. In this paper, we investigate these problems and propose using multiplicity as a viable dimension along which we can find a solution. We augment our method using transfer learning (few-shot learning specifically) to address the problem of poor generalization of the BCI signal across users. We conduct experiments where multiple test subjects observe an AI agent navigate a maze in an Atari-based game environment, where the agent takes occasional incorrect actions. This generates a BCI signal called the error potential (ErrP). ErrPs have been used throughout scientific literature in cyber-physical systems to detect anomalous situations, correct robot mistakes, train machine learning algorithms, etc. However, systems designed for ErrP detection suffer from poor generalization accuracy (≈60%). In order to test the viability of using multiplicity in a single-user BCI system, we compare our performance with the baseline of the traditional single-user BCI systems and achieve a much-improved generalization accuracy$(\approx 90\%)$.

In this paper, we investigate the optimal line-of-sight (LoS) coverage problem for multiple access point (multi-AP) mmWave wireless LANs in indoor scenarios. Due to the weak diffraction ability of mmWave signals at 60 GHz, maintaining LoS communications between APs and client devices is critical to achieve ultra-high data rates with mmWave communications. We focus on the use of multiple APs deployed to maximize LoS coverage in a target area, and we develop multi-AP placements that maximize LoS coverage by means of both analytical and algorithmic methods. We consider two main scenarios, which differ in their assumptions about knowledge of obstacles and clients. In a random-obstacle, random-client scenario, we derive the LoS-optimal positions of APs by solving a thinnest covering problem. For a fixed-obstacle, random-client scenario, we propose an efficient algorithm that produces a multi-AP placement, which is shown through simulation to provide near-optimal LoS coverage. Finally, through extensive ns-3 simulations based on the IEEE 802.11ad protocol and mmWave-specific channel models, we show that our multi-AP placements are significantly better than existing placement approaches, both in terms of LoS coverage and aggregate throughput.

Netflix is the most popular video streaming site contributing to nearly a quarter of global video traffic. Given the dominance of Netflix on Internet traffic, understanding how individual users consume content on Netflix is of interest to not only the research community, but to network operators, content creators and providers, users and advertisers. In this context, we collect Netflix viewing activity from 1060 users spanning a 1 year period, and consisting of over 1.7 million episodes and movies. We group the users based on their activity level, and provide key insights pertaining to the user’s watch patterns, watch-session length, user preferences, predictability and watch-behavior continuation tendencies. We also implement and evaluate classifiers which are used to predict the user’s engagement in a series based on their past behavioral patterns.

Commercially available EEG-based Brain-Computer Interface (BCI) wearable headsets are always-on and are thus power hungry, requiring users to charge the headsets multiple times a day. In this paper, we tackle the problem of wake-up command design and detection for BCI headsets, and explore how battery life can be made to last for approximately a whole day. The key challenge that we address is enabling the headset to operate in a near-sleep mode but still reliably detect and interpret an EEG-based wake-up command from the user. Towards addressing the challenge, we present a solution that is built upon eye-blinks. Our core contribution is Trance, a user-friendly and robust wake-up command for BCI headsets that is computationally lightweight. We show using experimental results coupled with multiple data sets collected through user-studies that Trance can extend battery life by approximately 2.7x or to approximately 10 hours for a typical wearable battery, while remaining user-friendly.

Biometric authentication is becoming popular in a varied range of applications because of its unique specificity for an individual user. In this context, electro-encephalogram (EEG) signals from a user is an interesting candidate for authentication. In this work, we specifically focus on the EEG signal corresponding to the human eye-blink to create an authentication system that could be used to distinguish between multiple users accurately and efficiently while also being burden-less and convenient to the users. We use a dataset of eye-blink related EEG signals, collected from 20 users, to study our solution. Our results show that blink signals can be used for accurately distinguishing between different users and hence can be used for authentication.

mmWave is emerging as an essential technology for next-generation wireless networks due to its capability of delivering multi-gigabit throughput performance. To achieve such a promising performance in mmWave communications, Line-of-sight (LOS) connectivity is a critical requirement. In this work, we explore the strategy of infrastructure mobility to alter the location of an access point (AP) in order to provide LOS connectivity to stations (STAs) in indoor mmWave WiFi networks. Through both simulation-based and theoretical analyses, we make a detailed case for infrastructure mobility by identifying the impact of AP mobile platforms configurations on network performance and propose a ceiling-mounted mobile (CMM) AP model. Then, we compare the performance of a CMM AP with multiple static APs, and we identify that the throughput and fairness performance of a CMM AP is better than as many as 5 ceiling-mounted static APs.

YouTube is the most popular video sharing platform with more than 2 billion active users and 1 billion hours of video content watched daily. The dominance of YouTube has had a big impact on the performance of Internet protocols, algorithms, and systems. Understanding the interaction of users with YouTube is thus of much interest to the research community. In this context, we collect YouTube watch history data from 243 users spanning a 1.5 year period. The dataset comprises of a total of 1.8 million videos. We use the dataset to analyze and present key insights about user-level usage behavior. We also show that our analysis can be used by researchers to tackle a myriad of problems in the general domains of networking and communication. We present baseline characteristics and also substantiated directions to solve a few representative problems related to local caching techniques, prefetching strategies, the performance of YouTube's recommendation engine, the variability of user's video preferences and application specific load provisioning.

Millimeter-wave communication is a highly promising technology to deliver multi-gigabit-per-second transmission rates for next-generation wireless LANs (WLANs). To achieve such ultra-high throughput performance in indoor scenarios, line-of-sight (LoS) connectivity becomes a critical requirement. Prior work has proposed access point (AP) mobility as an approach to improve LoS conditions and, thereby, approach optimum mmWave WLAN performance. In this work, we present a comprehensive simulation study of linear AP mobility that investigates various dimensions, including the number of mobile APs, the placement of the mobile AP platforms, and the length of the platforms. The results show how WLAN performance varies across these dimensions and also compares the results against a varying number of static APs to quantity the performance gains achievable from mobility. The results show that even 2 or 3 mobile APs can significantly outperform a much larger number of static APs and that deploying up to 3 mobile APs in a room brings substantial performance gains.

The load on wireless cellular networks is not uniformly distributed through the day, and is significantly higher during peak periods. In this context, we present MANTIS, a time-shifted prefetching solution that prefetches content during off-peak periods of network connectivity. We specifically focus on YouTube given that it represents a significant portion of overall wireless data-usage. We make the following contributions: first, we collect and analyze a real-life dataset of YouTube watch history from 206 users comprised of over 1.8 million videos spanning over a 1-year period and present insights on a typical user's viewing behavior; second, we develop an accurate prediction algorithm using a K-nearest neighbor classifier approach; third, we evaluate the prefetching algorithm on two different datasets and show that MANTIS is able to reduce the traffic during peak periods by 34%; and finally, we develop a proof-of-concept prototype for MANTIS and perform a user study.

Line-of-sight (LOS) is a critical requirement for mmWave communication. In this work, we make the case for a ceilingmounted mobile (CMM) AP by comparing its performance with other types of AP mobility and single static AP. We then present Hawkeye to solve the optimal location discovery problem for a CMM AP using a machine learning (ML) algorithm. Hawkeye relies purely on the connectivity matrix between STAs and the AP to decide if and where the AP should move to for maximizing LOS connectivity. Using a prototype implementation, we show that the throughput of Hawkeye is 219% and 129% compared with single static AP and other approaches for AP mobility, respectively.

The load on wireless cellular networks is not uniformly distributed through the day, and is significantly higher during peak-times. In this context, we present a time-shifted prefetching solution that prefetches content during off-peak periods of network connectivity. We specifically focus on YouTube as it represents a significant portion of overall cellular data-usage. We make the following contributions: first, we establish that a significant portion of a user's YouTube watch behavior is indeed predictable by analyzing a real-life dataset of YouTube watch history spanning a 1-year period, from 206 users comprised of over 1.8 million videos; second, we develop an accurate prediction algorithm using a K-nearest neighbor classifier approach; and finally, we evaluate the prefetching algorithm on two different datasets and show that MANTIS is able to reduce the traffic during peak periods by 34% for a typical user.

mmWave communication in 60GHz band has been recognized as an emerging technology to support various bandwidth-hungry applications in indoor scenarios. To maintain ultra-high throughputs while addressing potential blockage problems for mmWave signals, maintaining line-of-sight (LoS) communications between client devices and access points (APs) is critical. To maximize LoS communications, one approach is to deploy multiple APs in the same room. In this paper, we investigate the optimal placement of multiple APs using both analytical methods and simulations. Considering the uncertainty of obstacles and clients, we focus on two typical indoor settings: random-obstacle-random-client (RORC) scenarios and fixed-obstacle-random-client (FORC) scenarios. In the first case, we analytically derive the optimal positions of APs by solving a thinnest covering problem. This analytical result is used to show that deploying up to 5 APs in a specific room brings substantial performance gains. For the FORC scenario, we propose the shadowing-elimination search (SES) algorithm based on an analytic model to efficiently determine the placement of APs. We show, through simulations, that with only a few APs, the network can achieve blockage-free operation in the presence of multiple obstacles and also demonstrate that the algorithm produces near-optimal deployments. Finally, we perform ns-3 simulations based on the IEEE 802.11ad protocol at mmWave frequency to validate our analytical results. The ns-3 results show that proposed multi-AP deployments produce significantly higher aggregate performance as compared to other common AP placements in indoor scenarios.

Mobilizing enterprise applications enable employees to perform their job functions on their smartphones, even when away from a Desktop. Among the different strategies used to mobilize web enterprise applications, application refactoring based services are gaining popularity due to their ease of deployment. Refactoring involves hosting the application as-is on the enterprise cloud and allowing a user to interact with it through a smartphone-optimized UI. These services require APIs provided by the applications to accurately map the smartphone UI to the web application UI.

Lightweight control planes are techniques that create a control plane in WiFi networks without any additional spectrum requirements. Flash signals are an example of such control signals that exploit the link margin that typically exists in WiFi communication. In this paper we consider the problem of allowing transmitters and receivers of a transmission to exploit such control channels while the communication is ongoing. We present a mechanism called switch that facilitates switches in communication modes (Tx to Rx and vice-versa). We then use switch as the core building block to solve problems in WiFi networks such as starvation due to hidden terminals, early collision termination, and frequency backoffs. We rely on WARP radios to experimentally verify that switch is indeed possible, and use ns-3 simulations to study the impact of using switch to solve the aforementioned WiFi problems.

Brain-computer interfaces (BCIs) allow users to communicate to a nearby computing device (computer, smartphone, etc.) using thoughts or other covert actions that result in a detectable change in brain-waves. Consider a BCI command to be a word consisting of a sequence of characters. Each character is a thought or action that can be reliably detected through brain waves. For this work, we specifically consider eye-blinks as the user action of interest. Eye-blinks are an interesting modality for BCI commands because of their easy detectability and naturalness (and hence covertness). It turns out that there is an interesting trade-off between the complexity of characters and the length of words. In this work, we perform a user-study to answer a simple, but important, question pertaining to eye-blinks based BCI command design: do users prefer shorter characters (and hence longer words) or shorter words (and hence longer characters) when performing commands?. We present a simple eye-blink language consisting of words and characters and use real user-experiments to study the aforementioned trade-off.

The position of a wireless access point (AP) in wireless networks has been found to have a considerable impact on the overall network performance in indoor scenarios. Recent work has investigated the impact of the AP's location on the received signal strength of clients [1], and has shown that a 1:7x throughput improvement can be achieved by simply moving the AP in a 2ft. x 2ft. region. The benefits of small scale AP mobility is chiefly caused by mitigating multipath effects. In fact, the multipath effect has a significant impact on network performance and can be dramatically altered even with mere centimeter level movement of a Tx or a Rx. In this work, we investigate how network throughput performance can be improved if the AP is able to adapt its orientation. We consider two types of orientation changes - that of the AP's base platform (base orientation), and that of its antennas (antenna orientation). We show using experimental analysis that network throughput performance can be improved 1:8x by simply adapting AP's orientation.

Application Mobilization, or the ability of an enterprise employee to rely on mobile devices such as smartphones and tablets, to continue to perform business workflows even when mobile, is seen as a game changer to improve productivity. However, the practical adoption of enterprise mobility is very much in its in fancy, and seemingly has barriers. We posit that these barriers include heavy user-burden in accomplishing tasks (e.g. number of actions required to execute a workflow), high cost of mobile access (e.g. latency for content fetching), and irrelevance of available mobile functions (e.g. mobile app defeaturization done inappropriately).The novelty of our research is in a unified observe-patternize-mimic paradigm we explore to address these barriers, based on a simple question: could patterns in user-behavior be learned, and leveraged for reducing user-burden? If patterns are discovered, then we show that intelligent mimicking of these patterns at appropriate junctures can considerably relieve the mobile user burden. We motivate this paradigm through three application scenarios representing read, write, and act usage modalities.

More email is opened on mobile devices today than on other platforms [1]. At the same time, enterprises are constantly investing in approaches to improve employee productivity. In this paper, we consider the problem of automated information suggestions to assist in reply construction. The basic premise of the work is that a significant portion of the information content of a reply is likely to be present in prior emails. We first show that the premise is valid by analyzing both public and private email datasets. We then present a simple algorithm that relies on inverse document frequency (IDF) and keyword matching to provide relevant suggestions during reply construction. Through prototype evaluations done using the Email datasets, we show that the proposed algorithm has attractive benefits.

We ask the following question in this paper: Can the goals of centralized WiFi scheduling be achieved using purely distributed operations? We present a solution called Look Who's Talking (LWT) that allows for arbitrary schedules to be distributed to nodes in a WiFi network. The nodes in the network then use purely local and distributed operations to achieve the prescribed schedule. The scope of LWT in this paper is restricted to a single collision domain (single or multiple cells), but we discuss how LWT can be extended to multiple collision domains. We use both experimental evaluations (using a WARP-based testbed) and simulation-based analysis (using ns3) to evaluate LWT.

The ubiquitous adoption of WiFi implicitly introduces large diversity in types of application requirements and topological characteristics. Consequently, considerable attention is being devoted to making WiFi networks controllable without compromising their scalability. Within this broad paradigm, we propose Rhythm, a MAC protocol that achieves scheduled WiFi efficiently and that is subject to the following constraints: (i) It does not need fine-grained time synchronization, (ii) it adds no "active listening" time, (iii) it does not need to gather the queue status from clients, and (iv) it requires no additional hardware. It also has the following properties: (i) low overhead, (ii) work conservation, (iii) robustness to partial connectivity, and (iv) backward compatibility.

In this work we consider nano-scale communication using bacterial populations as transceivers. We demonstrate using a microfluidic test-bed and a population of genetically engineered Escherichia coli bacteria serving as the communication receiver that a simple modulation like on-off keying (OOK) is indeed achievable, but suffers from very poor data-rates. We explore an alternative communication strategy called time elapse communication (TEC) that uses the time period between signals to encode information. We identify the severe limitations of TEC under practical non-zero error conditions in the target environment, and propose an advanced communication strategy called smart time elapse communication (TEC-SMART) that achieves over a 10× improvement in data-rate over OOK.

An attractive approach to overcome capacity limitations in a densely deployed WLAN environment is to enable transmission concurrency. In this paper, we propose a co-channel concurrent transmission scheme, referred to as C2SMA/CA, which determines whether to allow multiple concurrent transmissions based on interference estimation using positional information. A distributed multi-link concurrency scheduling algorithm is implemented, and its performance is evaluated through extensive simulations.

Bacterial populations housed in microfluidic environments can serve as transceivers for molecular communication, but the data-rates are extremely low (e.g., 10-5bits per second.). In this work, genetically engineered Escherichia coli bacteria were maintained in a microfluidic device where their response to a chemical stimulus was examined over time. The bacteria serve as a communication receiver where a simple modulation such as on-off keying (OOK) is achievable, although it suffers from very poor data-rates. We explore an alternative communication strategy called time-elapse communication (TEC) that uses the time period between signals to encode information. We identify the limitations of TEC under practical non-zero error conditions and propose an advanced communication strategy called smart time-elapse communication (TEC-SMART) that achieves over a 10x improvement in data-rate over OOK. We derive the capacity of TEC and provide a theoretical maximum data-rate that can be achieved.

In this paper, we consider the use of cooperative transmissions in multihop wireless networks to achieve Virtual Multiple Input Single Output (VMISO) links. Specifically, we investigate how the physical layer VMISO benefits translate into network level performance improvements. We show that the improvements are nontrivial (15 to 300 percent depending on the node density) but rely on two crucial algorithmic decisions: the number of cooperating transmitters for each link; and the cooperation strategy used by the transmitters. We explore the tradeoffs in making routing decisions using analytical models and derive the key routing considerations. Finally, we present Proteus, an adaptive diversity routing protocol that includes algorithmic solutions to the above two decision problems and leverages VMISO links in multihop wireless network to achieve performance improvements. We evaluate Proteus using NS2-based simulations with an enhanced physical layer model that accurately captures the effect of VMISO transmissions.

Cochannel links in a Wireless LAN are separated across orthogonal time slots to avoid interference. With increasing density of links, time sharing the channel leads to severe capacity problems. In this paper, we identify a specific class of interference scenarios called asymmetric interference scenarios, where the nature of interference is different at the receivers of the concurrent signals. We show that, with appropriate handling, asymmetric interference allows each receiver to decode its intended reception successfully. We represent the signal combination at the receiver as a function fc and propose a solution called Symbiotic Coding (SC) such that fc(E1(d1;d2);E2(d1; d2)) is equal to E1(d1;d2), where d1and d2are the intended and interfering data symbol sequences and E1is the encoder at sender 1 and E2at sender 2, respectively. SC, thus, enables successful simultaneous cochannel transmissions even if they result in a collision. The performance of SC scales with the number of interfering links achieving median throughput improvements of 30 and 86 percent over time sharing with two and three interfering links, respectively. We address fundamental challenges in realizing SC including synchronization, coding algorithms, extensions to different modulations. We also implement SC on software defined radios and demonstrate its practical feasibility.

Network deduplication (dedup) is an attractive approach to improve network performance for mobile devices. With traditional deduplication, the dedup~source uses only the portion of the cache at the dedup~destination that it is aware of. We argue in this work that in a mobile environment, the dedup~destination (say the mobile) could have accumulated a much larger cache than what the current dedup~source is aware of. This can occur because of several reasons ranging from the mobile consuming content through heterogeneous wireless technologies, to the mobile moving across different wireless networks.

Sensors of various modalities and capabilities, especially cameras, have become ubiquitous in our environment. Their intended use is wide ranging and encompasses surveillance, transportation, entertainment, education, healthcare, emergency response, disaster recovery, and the like. Technological advances and the low cost of such sensors enable deployment of large-scale camera networks in large metropolises such as London and New York. Multimedia algorithms for analyzing and drawing inferences from video and audio have also matured tremendously in recent times. Despite all these advances, large-scale reliable systems for media-rich sensor-based applications, often classified as situation-awareness applications, are yet to become commonplace. Why is that? There are several forces at work here. First, the system abstractions are just not at the right level for quickly prototyping such applications on a large scale. Second, while Moore's law has held true for predicting the growth of processing power, the volume of data that applications are called upon to handle is growing similarly, if not faster. Enormous amount of sensing data is continually generated for real-time analysis in such applications. Further, due to the very nature of the application domain, there are dynamic and demanding resource requirements for such analyses. The lack of right set of abstractions for programing such applications coupled with their data-intensive nature have hitherto made realizing reliable large-scale situation-awareness applications difficult. Incidentally, situation awareness is a very popular but ill-defined research area that has attracted researchers from many different fields. In this paper, we adopt a strong systems perspective and consider the components that are essential in realizing a fully functional situation-awareness system.

The IEEE 802.11n standard is gaining popularity to achieve high throughput in Wireless LANs. In this paper, we explore link adaptation in practical 802.11n systems using experiments with off-the-shelf hardware. Our experiments reveal several non-trivial insights. Specifically, (1) trivial extensions of algorithms developed for 802.11g provide minimal benefits in 802.11n systems; (2) in contrast to theoretical expectation, multiple antenna transmission does not always lead to higher throughput in practice; (3) both stream and antenna selection are essential to reap the full benefits of MIMO technologies. We use insights developed from experiments to develop a new metric for stream selection called the Median Multiplexing Factor (MMF). The proposed metric can be used to develop intelligent rate selection algorithms that can achieve high throughput with purely software changes.

The focus of this work is to study the efficacy of TCP's flow control algorithm on mobile phones. Specifically, we identify the design limitations of the algorithm when operating in environments, such as mobile phones, where flow control assumes greater importance because of device resource limitations. We then propose an adaptive flow control (AFC) algorithm for TCP that relies not just on the available buffer space but also on the application read-rate at the receiver. We show, using NS2 simulations, that AFC can provide considerable performance benefits over classical TCP flow control.

While a remote computing solution such as VNC is an effective technology for PC users to access a remote computer, it is not as effective while being used from smartphones. In this paper, we propose techniques to improve remote computing from smartphones that help deliver near-PC level experience to users. We introduce a key building block called smart-macros that have the robustness of application macros but at the same time possess the generality of raw macros. Using smart-macros we design and prototype SmartVNC, a remote computing solution for smartphones. We show using experimental studies and a trace based analysis of real user activity, that SmartVNC can improve user experience considerably.

We explore the design of a high capacity multi-radio wireless network using commercial 802.11n hardware. We first use extensive real-life experiments to evaluate the performance of closely located 802.11n radios. We discover that even when tuned to orthogonal channels, co-located 802.11n radios interfere with each other and achieve significantly less throughput than expected. Our analysis reveals that the throughput degradation is caused by three link-layer effects: (i) triggering of carrier sensing, (ii) out of band collisions and (iii) unintended frequency adaptation. Using physical layer statistics, we observe that these effects are caused by fundamental limitations of co-located radios in achieving signal isolation. We then consider the use of beamforming antennas, shielding and antenna separation distance to achieve better signal isolation and to mitigate these problems. Our work profiles the gains of different physical isolation approaches and provides insights to network designers to realize high-performance wireless networks without requiring synchronization or protocol modifications.

Studies have shown the presence of considerable amounts of redundancy in Internet traffic content. Recent works are exploring possibilities for exploiting network traffic redundancy, but these works invariably focus on fixed wireline networks. Unlike wireline networks, wireless and mobile environments exhibit unique challenges and opportunities in the context of redundancy elimination. In this work, we explore leveraging network traffic redundancy, but exclusively focus on wireless and mobile environments. We first analyze real Wi-Fi traces, and based on insights obtained from the analysis, we propose Wireless Memory (WM), a two-ended AP-client solution to effectively exploit traffic redundancy for such environments. Trace-driven evaluation results show that WM can help deliver significant throughput improvement.

The use of smart antennas in multihop wireless networks has garnered significant attention over the last few years. Given the unique capabilities of smart antennas, and how they can improve performance in a typically constrained multihop wireless network (MWN) environment, the attention is with merit. However, not much light has been shed on MWNs that have nodes with varying antenna capabilities. While homogeneous MWNs with all nodes having the same antenna capabilities will have certain applications, we argue that MWNs with nodes having heterogeneous antenna capabilities are more likely to be the norm due to a variety of motivating factors. In the context of such heterogeneous smart antenna networks (HSANs), we investigate and motivate the need for a simple form of node cooperation called retransmit diversity. We show that while such a simple form of node cooperation cannot bring significant benefits to homogeneous omni-directional antenna networks, they can bring great gains (several folds improvement) to heterogeneous smart antenna networks. We then present several key properties pertaining to node cooperation in HSANs along with analytical reasoning. In the process, we identify a fundamental trade-off between exploiting smart antenna gain and cooperation gain, that undermines the ability of HSANs to leverage node cooperation to their maximum potential. To address this trade-off, we then present an adaptive cooperation mechanism and incorporate this mechanism through the design of a simple but efficient MAC protocol. The performance of the MAC protocol is evaluated through ns2 simulations along with a realistic physical layer channel model for smart antenna environments.

In this paper, we focus on securing communication over wireless data networks from malicious eavesdroppers by using smart antennas. While conventional cryptography-based approaches focus on hiding the meaning of the information being communicated from the eavesdropper, we consider a complimentary class of strategies that limit knowledge of the existence of the information from the eavesdropper. We profile the performance achievable with simple beamforming strategies using a newly defined metric called exposure region. We then present three strategies within the context of an approach calledAegis, which uses virtual arrays of physical arrays to significantly improve the exposure region performance of a wireless LAN environment. Using simulations, analysis, and field trials, we validate and evaluate the proposed strategies.

Most mobile devices today are equipped with multiple and heterogeneous wireless interfaces. In this paper we ask the following question: what is the best approach to leverage the multiple interfaces available at a mobile device in terms of the performance delivered to the user? In answering the question we argue that simple "bandwidth aggregation" approaches do not provide any meaningful benefits when the multiple interfaces used have highly disparate bandwidths as is true in many practical environments. We then present super-aggregation, a set of mechanisms that in tandem use the multiple interfaces intelligently and in the process is able to achieve a performance that is "better than the sum of throughputs" achievable through each of the interfaces individually. We prototype super-aggregation on both a laptop and the Google Android mobile phone and demonstrate the significant (up to 3x throughput) performance improvements it provides in real-world experiments.

WLANs have become an important last-mile technology for providing internet access within homes and enterprises. In such indoor deployments, the wireless channel suffers from significant multipath scattering and fading that degrades performance. Beamforming is a smart antenna technology that adjusts the transmissions at the transmitter to reenforce the signals received through multiple paths at the receiver.

Wi-fi standards have provisions for multiple orthogonal channels where the orthogonality allows them to be used simultaneously both in time and frequency without interference concerns. In this paper, we pose the following question: Can devices use the multiple orthogonal channels in wi-fi networks simultaneously to realize a high data-rate wireless link and hence cater to applications requiring high bandwidths? In other words, given that there are 3 orthogonal wi-fi channels in the 2.4GHz band and 12 orthogonal wi-fi channels in the 5GHz band, can a pair of devices each equipped with 15 wi-fi radios use all the available orthogonal channels to achieve a high data-rate link operating at 600Mbps? Surprisingly, we find through experimental evaluation that the actual observed performance when using all fifteen orthogonal channels between two devices is a mere 91Mbps. We identify the reasons behind the low performance and present Glia, a software only solution that effectively exercises all available radios. We prototype Glia and show using experimental evaluations that Glia helps achieve close to 600Mbps data-rate when using all possible wi-fi channels.

In this paper, we consider the use of cooperative transmissions in multi-hop wireless networks to achieve virtual MISO (multiple input single output) links. Specifically, we investigate how the physical layer VMISO benefits translate into network level performance improvements. We show that the improvements are non-trivial (15% to 300% depending on the node density) but rely on two crucial algorithmic decisions: the number of co-operating transmitters for each link; and the cooperation strategy used by the transmitters. Finally, we present Proteus, an adaptive diversity routing protocol that includes algorithmic solutions to the above two decision problems and leverages VMISO links in multi-hop wireless network to achieve performance improvements. We evaluate Proteus using NS2 based simulations with an enhanced physical layer model that accurately captures the effect of VMISO transmissions.

A tremendous amount of research has been done toward improving transport-layer performance over wireless data networks. The improved transport layer protocols are typically application-unaware. In this paper, we argue that the behavior of applications can and does dominate the actual performance experienced. More importantly, we show that for practical applications, application behavior all but completely negates any improvement achievable through better transport layer protocols. In this context, we motivate an application-aware, but application transparent, solution suite called A^3 (application-aware acceleration) that uses a set of design principles realized in an application-specific fashion to overcome the typical behavioral problems of applications. We demonstrate the performance of A^3 through both emulations using realistic application traffic traces and implementations using the NetFilter utility.

In this paper, we focus on securing communication over wireless data networks from malicious eavesdroppers, using smart antennas. While conventional cryptography based approaches focus on hiding the meaning of the information being communicated from the eavesdropper, we consider a complimentary class of strategies that limit knowledge of the existence of the information from the eavesdropper. We profile the performance achievable using simple beamforming strategies using a newly defined metric called exposure region. We then present three strategies within the context of an approach called virtual arrays of physical arrays to significantly improve the exposure region performance of a wireless LAN environment. Using simulations and analysis, we validate and evaluate the proposed strategies.

Peer-to-peer (P2P) data networks dominate Internet traffic. In this work, we study the problems that arise when mobile hosts participate in P2P networks. We primarily focus on the performance issues as experienced by the mobile host, but also study the impact on other fixed peers. Using Bit Torrent as a key example, we identify several unique problems that arise due to the design aspects of P2P networks being incompatible with typical characteristics of wireless and mobile environments. We then present a wireless P2P (wP2P) client application that is backward compatible with existing fixed peer client applications, but when used on mobile hosts can significant improve performance.

In this paper we present a new approach called cue-based networking that uses hints or cues about the physical environment to optimize networked application behavior. We define the notion of cues and describe how cues can be obtained using a wireless sensor network as the underlying platform. We identify both the research and system challenges that need to be addressed to realize benefits of the approach under a target application of video delivery over IP networks. In the process, we identify a key challenge of wireless sensor networks, namely the timeliness-robustness tradeoff. We design an adaptive algorithm that balances this tradeoff satisfying both timeliness and robustness requirements. Through an implementation of the video delivery application using the proposed algorithm in a real home environment, we highlight the practical benefits of the proposed approach.

There exist several applications of sensor networks where the reliability of data delivery can be critical. Although the redundancy inherent in a sensor network might increase the degree of reliability, it by no means can provide any guaranteed reliability semantics. In this paper, we consider the problem of reliable sink-to-sensors data delivery. We first identify several fundamental challenges that need to be addressed and are unique to the environment of wireless sensor networks. We then propose a scalable framework for reliable downstream data delivery that is specifically designed to both address and leverage the characteristics of the wireless sensor networks while achieving the reliability in an efficient manner. Through ns2-based simulations, we evaluate the proposed framework.

Current popular web-browsers simply fetch the entire web-page from the server in a greedy fashion. This simple web fetching mechanism employed by browsers is inappropriate for use in low-bandwidth networks, since they cause large response times for users unneccesarily. In this paper, we first analyze the reasons that cause large response times by considering several factors including the properties of typical web-pages and browsers, the interaction of the HTTP and TCP protocols, and the impact of server-side optimization techniques. We then propose three easy-to-deploy browser-side optimization mechanisms to reduce the user response time. Through simulations, we compare the performance of our solution with that of current browsers and show that the proposed scheme brings significant performance benefits in terms of user-perceived response times.

The expected VoIP call capacity in a one hop IEEE 802.11b network with G.711 voice codec is about 85 simultaneous calls, but the actual observed capacity is only 5 calls even at the highest data rate and under zero loss conditions. In this paper we analyze the reasons behind this inferior performance of VoIP traffic. We also present algorithms at the medium access control layer to improve the observed call capacity. Finally, using ns-2 based simulations, we evaluate the algorithms and show that performance improvements of up to 300% can be achieved.

The ability of smart antennas to improve performance in a typically constrained ad-hoc network environment, has helped them garner significant attention over the last few years. However, not much light has been shed on wireless ad-hoc networks that have nodes with varying antenna capabilities. While homogeneous ad-hoc networks with all nodes having the same antenna capabilities will have certain applications, we argue that ad-hoc networks with nodes having heterogeneous antenna capabilities are more likely to be the norm due to a variety of motivating factors. In the context of such heterogeneous smart antenna networks (HSANs), we investigate and motivate the need for a simple form of node cooperation called retransmit diversity. We show that while such a simple form of node cooperation cannot bring significant benefits to homogeneous omni-directional and smart antenna networks, they can bring several folds improvement to heterogeneous smart antenna networks. We then present several key properties pertaining to node cooperation in HSANs. In the process, we identify a fundamental trade-off between exploiting smart antenna gain and cooperation gain, that undermines the ability of HSANs to leverage node cooperation to their maximum potential. To address this tradeoff, we then present an adaptive cooperation mechanism and incorporate this mechanism through the design of a simple but efficient MAC protocol. The performance of the MAC protocol is evaluated throughns2 simulations.

Smart antennas include a broad range of antenna technologies ranging from the simple switched beam to the more sophisticated adaptive arrays and multiple input multiple output (MIMO) links. Their ability to exploit multiple degrees of freedom helps them operate in different strategies to achieve different objectives ranging from rate increase, range increase, transmission power reduction, and higher link reliability. While these antennas have been significantly researched at the PHY layer leading to results that are easily translatable to single-hop wireless networks, very little is understood about their use in multi-hop wireless ad-hoc networks. Specific unanswered questions in this context include: (i) what kind of performance improvements can the different technologies and their strategies provide? (ii) for each technology, which of the different possible strategies is the optimal strategy to employ for a given network condition? and (iii) for a given network setting, which of the different smart antenna technologies will deliver the best performance? In this paper, we systematically answer these questions by comprehensively evaluating the relative benefits of the different smart antenna technologies.

Web-based information access suffers tremendously in low-bandwidth wireless data networks due to the non-correlation between the content transferred across the wireless links and the actual data that is used to serve the user requests. As a result, the current web-access mechanisms face such problems as unnecessary bandwidth consumption, large response times, no service for partial disconnections, and low system utilization in wireless networks. In order to solve these problems with web-transfer in wireless networks, we present a new middleware for wireless web-access called Cut-Load, which performs application unaware content-partitioning in the graphical domain residing at both the mobile client and the proxy server that the mobile client communicates with. Cut-Load uses dynamic mode selection, opportunistic hoarding, transparent mode transfer, and display caching for efficient wireless web-access. Through simulations, we compare the performance of Cut-Load with that of the current web-access mechanisms and show that the proposed middleware brings significant performance benefits both in terms of bandwidth consumption and user-perceived response times.

In this paper, we consider the channel assignment problem in single radio multi-channel mobile ad-hoc networks. Specifically, we investigate the granularity of channel assignment decisions that gives the best trade-off in terms of performance and complexity. We present a new granularity for channel assignment that we refer to as component level channel assignment. The strategy is relatively simple, and is characterized by several impressive practical advantages. We also show that the theoretical performance of the component based channel assignment strategy does not lag significantly behind the optimal possible performance, and perhaps more importantly we show that when coupled with its several practical advantages, it significantly outperforms other strategies under most network conditions.

A tremendous amount of research has been done toward improving transport layer performance over wireless data networks. The improved transport layer protocols are typically application-unaware. In this paper, we argue that the behavior of applications can and do dominate the actual performance experienced. More importantly, we show that for practical applications, application behavior all but completely negates any improvements achievable through better transport layer protocols. In this context, we motivate an application-aware, but application transparent, solution suite called A 3 (application-aware acceleration) that uses a set of design principles realized in an application specific fashion to overcome the typical behavioral problems of applications. We demonstrate the performance of A3 through emulations using realistic application traffic traces.

A typical wireless sensor network (WSN) performs only one action: sensing the environment. The need for smart interaction with the environment has led to the emergence of wireless sensor and actor networks (WSANs). The evolution from WSNs, which can be thought of to perform only read operations, to WSANs, which can perform both read and write operations, introduces unique and new challenges that need to be addressed. In this context, we identify the problem of mutual exclusion, which is the requirement to act only to the desired level for any particular location and command. We define the different types of mutual exclusion and the associated challenges in the context of WSANs, and show the undesirable consequences of not providing mutual exclusion with example applications. To address this problem efficiently, we propose a greedy centralized approach, and a distributed and fully localized approach based on the centralized approach. Through simulations, we study the performance of the proposed solution with the centralized approach and a baseline strategy, and show that the proposed solution is efficient for a variety of network conditions

Abstract The use of smart antennas in wireless ad hoc networks has garnered attention over the last few years. Given the unique capabilities of smart antennas, and how they can improve performance in a typically constrained ad hoc network environment, the attention is with merit. However, not much light has been shed on wireless ad hoc networks that have nodes with varying antenna capabilities. While homogeneous ad hoc networks with all nodes having the same antenna capabilities will have certain applications, we argue that ad hoc networks with nodes having heterogeneous antenna capabilities are more likely to be the norm due to a variety of motivating factors. In this context, we answer two fundamental questions in this paper: (i)what is the theoretical capacity of an ad hoc network when its nodes have varying antenna capabilities? We consider nodes equipped with only omnidirectional antennas to those that are equipped with more sophisticated adaptive‐array and MIMO capable antennas. Perhaps more interestingly, we provide insights into what percentage of nodes in an ad hoc network have to be ‘smart’ to get a certain overall network capacity. (ii)What medium access control (MAC) and routing protocols should nodes use in such a heterogeneous network? We present MAC and routing protocols for ad hoc networks with heterogeneous antenna capabilities that allow nodes to interact with each othereffectivelydespite their heterogeneity, and in the process deliver considerably improved performance over simplistic strategies. We substantiate our theoretical results, and evaluate the proposed protocols using ns2‐based network simulations. Copyright © 2006 John Wiley & Sons, Ltd.

A typical wireless sensor network performs only one action: sensing the environment. The requirement for intelligent interaction with the environment has led to the emergence of wireless sensor and actor networks (WSANs). In WSANs, the sensors monitor the environment based on which the sink issues commands to the actors to act on the environment. In order to provide tight coupling between sensing and acting, an effective coordination mechanism is required among sensors and actors. In this context, we identify the problem of "hazards", which is the out-of-order execution of queries and commands due to a lack of coordination between sensors and actors. We identify three types of hazards and show with an example application, the undesirable consequences of these hazards. We also identify and enumerate the associated challenges in addressing hazards. In this context, we discuss the basic design needed to address this problem efficiently. We propose a distributed and fully localized hazard-free approach that addresses the problem and the associated challenges based on the design. Through simulations, we study the performance of the proposed solution and two basic strategies, and show that the proposed solution is efficient for a variety of network conditions.

The problem of congestion in sensor networks is significantly different from conventional ad-hoc networks and has not been studied to any great extent thus far. In this paper, we focus on providing congestion control from the sink to the sensors in a sensor field. We identify the different reasons for congestion from the sink to the sensors and show the uniqueness of the problem in sensor network environments. We propose a scalable, distributed approach that addresses congestion from the sink to the sensors in a sensor network. Through ns2 based simulations, we evaluate the proposed framework, and show that it performs significantly better over a basic approach which does not provide any congestion control.

Smart antennas include a broad variety of antenna technologies ranging from the simple switched beams to the sophisticated digital adaptive arrays. While beam-forming antennas are good candidates for use in strong line of sight (LOS) environments, it is the multiple input multiple output (MIMO) technology that is best suited for multipath environments. In fact, the MIMO links exploit the multipath induced rich scattering to provide high spectral efficiencies. The focus of this work is to identify the various characteristics and tradeoffs of MIMO links that can be leveraged by routing layer protocols in rich multipath environments to improve their performance. To this end, we propose a routing protocol called MIR for ad-hoc networks with MIMO links, that leverages the various characteristics of MIMO links in its mechanisms to improve the network performance. We show the effectiveness of the proposed protocol by evaluating its performance through ns2 simulations for a variety of network conditions.

The communication model typically assumed in wireless ad-hoc networks is based on a traditional "pipelined relay" (PR} strategy. In PR, an end-to-end flow has multiple outstanding packets (or data units) along the path from the source to the destination. In this paper, we argue that due to several unique properties of wireless ad-hoc networks, PR can be fundamentally improved upon. We present a new non-pipelined relay (nPR) strategy, where end-to-end flows have exactly one outstanding packet (or data unit) along the end-to-end path. We show that nPR has the following properties: (i) under idealized network conditions, it provides performance improvement, in terms of end-to-end throughput capacity and network transport capacity over PR, and achieves proportional fairness; and (ii) under practical network conditions, it further increases the above performance improvements, both in terms of the throughput achieved, and in terms of the fairness between flows. Finally, we present a forwarding protocol that practically realizes nPR. Through analysis and ns2 based packet level simulations, we evaluate the performance of the proposed strategy, and that of the forwarding protocol.

Wireless sensor networks (WSNs) are typically characterized by a limited energy supply at sensor nodes. Hence, energy efficiency is an important issue in the system design and operation of WSNs. In this paper, we introduce a novel communication paradigm that enables energy-efficient information delivery in wireless sensor networks. Compared with traditional communication strategies, the proposed scheme explores a new dimension - time, to deliver information efficiently. We refer to the strategy as Communication through Silence (CtS). We identify a key drawback of CtS - energy - throughput trade-off, and explore optimization mechanisms that can alleviate the trade-off. We then present several challenges that need to be overcome, primarily at the medium access control layer of the network protocol stack, in order to realize CtS effectively.

Abstract — Correlation of data sent by different sensors in a wireless sensor network can be exploited during the data gathering process to improve energy efficiency. In this paper, we study the energy efficiency of correlation aware data aggregation trees under various sensor network conditions and the tradeoffs involved in using them. The following two related questions are specifically investigated in the study: (i) Is there any practical limit on the achievable improvement in energy efficiency in adopting a correlation aware aggregation structure as opposed to a correlation unaware structure? (ii) Is there a practical maximum useable delay bound that can deliver the maximum achievable improvement? In answering the above questions, we present comprehensive simulation results and draw inferences based on the results. We also conclude two rather surprising results that the energy improvement in using correlation aware aggregation is not significant under many network scenarios, and the maximum useable delay bound is not large compared with the delay along the maximum length shortest-path in the default shortest path tree. I.

Existing works have approached the problem of reliable transport in ad-hoc networks by proposing mechanisms to improve TCP's performance over such networks. In this paper we show through detailed arguments and simulations that several of the design elements in TCP are fundamentally inappropriate for the unique characteristics of ad-hoc networks. Given that ad-hoc networks are typically stand-alone, we approach the problem of reliable transport from the perspective that it is justifiable to develop an entirely new transport protocol that is not a variant of TCP. Toward this end, we present a new reliable transport layer protocol for ad-hoc networks called ATP (ad-hoc transport protocol). We show through ns2 based simulations that ATP outperforms both default TCP and TCP-ELFN.

We present a new medium access control (MAC) protocol for ad-hoc networks with multiple input multiple output (MIMO) links. Links that use multiple element arrays (MEAs) at both ends are referred to as MIMO links. MIMO links are known to provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad-hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm that has the optimization considerations incorporated in its design. Finally, we present a distributed protocol that approximates the centralized algorithm, and compare its performance against that of baseline protocols that are variants of the CSMA/CA protocol.

There exist several applications of sensor networks where reliability of data delivery can be critical. While the redundancy inherent in a sensor network might increase the degree of reliability, it by no means can provide any guaranteed reliability semantics. In this paper, we consider the problem of reliable sink-to-sensors data delivery. We first identify several fundamental challenges that need to be addressed, and are unique to a wireless sensor network environment. We then propose a scalable framework for reliable downstream data delivery that is specifically designed to both address and leverage the characteristics of a wireless sensor network, while achieving the reliability in an efficient manner. Through ns2 based simulations, we evaluate the proposed framework.

Smart antennas represent a broad variety of antennas that differ in their performance and transceiver complexity. The superior capabilities of smart antennas, however, can be leveraged only through appropriately designed higher layer network protocols, including at the medium access control (MAC) layer. Although several related works have considered such tailored protocols, they do so in the context of specific antenna technologies. In this paper, we explore the possibility for a unified approach to medium access control in ad-hoc networks with smart antennas. We first present a unified representation of the PHY layer capabilities of the different types of smart antennas, and their relevance to MAC layer design. We then define a unified MAC problem formulation, and derive unified MAC algorithms from the formulation. Finally, using the algorithms developed, we investigate the relative performance trade-offs of the different technologies under varying network conditions.

In this paper, we consider the problem of file synchronization when a mobile host shares files with a backbone file server in a network file system. Several diff schemes have been proposed to improve upon the transfer overheads of conventional file synchronization approaches which use full file transfer. These schemes compute the binary diff of the new file with respect to the old copy at the server and transfer the computed diff to the server for file-synchronization. However, Lee et al. have shown that the performance of diff can be significantly improved upon by shipping user operations as opposed to the data itself. Using this as motivation, we present a purely application-unaware approach called Mimic that relies on transferring raw user activity to the server for file synchronization. Through a simple prototype of the proposed approach, we show that Mimic can outperform diff under many common conditions. We also identify conditions under which diff based approaches do perform better than the proposed approach, but show that detection of such conditions is straightforward, thus enabling both schemes to be used in tandem with a mobile file system for bandwidth-efficient file synchronization.

A recent class of approaches for enhancing the performance of cellular wireless data networks has focused on improving the underlying network model. It has been shown that using the peer-to-peer network model, a mode of communication typically seen in ad hoc wireless networks, can result in performance improvements such as increased data rate, reduced transmission power, better load balancing, and enhanced network coverage. However, the true impact of adopting the peer-to-peer network model in such an environment is yet to be fully understood. In this paper, we investigate the performance benefits and drawbacks of using the peer-to-peer network model for Internet access in cellular wireless data networks. We find that, although the peer-to-peer network model has significantly better spatial reuse characteristics, the improved spatial reuse does not translate into better throughput performance. Instead, we observe that using the peer-to-peer network model as-is might actually degrade the throughput performance of the network. We identify and discuss the reasons behind these observations. Using the insights gained through the performance evaluations, we then propose two categories of approaches to improve the performance of the peer-to-peer network model: approaches that leverage assistance from the base station and approaches that leverage the relaying capability of multihomed hosts. Through simulation results, we show that using the peer-to-peer network model in cellular wireless data networks is a promising approach when the network model is complemented with appropriate mechanisms.

we present a medium access control (MAC) protocol for ad hoc networks with multiple input multiple output (MIMO) links. MIMO links provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links coupled with several key optimization considerations, necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm called stream-controlled medium access (SCMA) that has the key optimization considerations incorporated in its design. Finally, we present a distributed SCMA protocol that approximates the centralized algorithm and compare its performance against that of baseline protocols that are CSMA/CA variants.

Abstract In this paper, we study the performance of the transmission control protocol (TCP) over mobile ad‐hoc networks. We present a comprehensive set of simulation results and identify the key factors that impact TCP's performance over ad‐hoc networks. We use a variety of parameters including link failure detection latency, route computation latency, packet level route unavailability index, and flow level route unavailability index to capture the impact of mobility. We relate the impact of mobility on the different parameters to TCP's performance by studying the throughput, loss‐rate and retransmission timeout values at the TCP layer. We conclude from our results that existing approaches to improve TCP performance over mobile ad‐hoc networks have identified and hence focused only on a subset of the affecting factors. In the process, we identify a comprehensive set of factors influencing TCP performance. Finally, using the insights gained through the performance evaluations, we propose a framework calledAtraconsisting of three simple and easily implementable mechanisms at the MAC and routing layers to improve TCP's performance over ad‐hoc networks. We demonstrate thatAtraimproves on the throughput performance of a default protocol stack by 50%–100%. Copyright © 2004 John Wiley & Sons, Ltd.

We propose a deployable approach to improving QoS by using the familiar overlay architecture approach. The goals of this work are: (1) to create an overlay architecture which allows us to sample specific path quality metrics among different paths; (2) to utilize the proposed overlay architecture in order to implement our proposed QoS-based routing scheme, application layer switching (ALSW). We show that we are able to achieve better than best-effort QoS without modifying intermediate nodes (i.e., routers), thus encouraging immediate deployment. Additionally, this research is performed on an actual wide area network testbed, comprised of universities across the nation. Also, we assemble this architecture as a peer-to-peer framework, encouraging collaborating individuals with average workstations to improve the QoS of their traffic.

In this paper we consider TCP based applications that use bandwidth guarantees, but can also benefit from any additional best-effort service offered by the network. We show that default TCP cannot offer such applications the aggregate throughput offered by the two services. We propose an adaptation of TCP called GTCP that uses changes to TCP's congestion control mechanisms to provide the optimal aggregate throughput. GTCP does not require any additional implementation overheads, and does not change the TCP receiver. Through simulations we show that GTCP achieves significantly better performance than default TCP in the target environment.

In this work, we propose a priority buffer management scheme for MPEG-2 video traffic called MPFD. The MPFD algorithm is based on constructing a virtual buffer that represents the future buffer occupancy using information about video frames within a future lookahead period. The virtual buffer is then used to determine whether incoming low priority frames need to be dropped in order to protect future high priority frames. MPFD is compared to existing buffer management schemes that has been used for video traffic. MPFD shows a significant frame loss reduction as compared to other buffer management schemes.

Numerous transport protocols have been proposed in related work for use by mobile hosts over wireless environments. A common theme among the design of such protocols is that they specifically address the distinct characteristics of the last-hop wireless link, such as random wireless errors, round-trip time variations, blackouts, handoffs, etc. In this paper, we argue that due to the defining role played by the wireless link on a connection's performance, locating the intelligence of a transport protocol at the mobile host that is adjacent to the wireless link can result in distinct performance advantages. To this end, we present a receiver-centric transport protocol called RCP (Reception Control Protocol) that is a TCP clone in its general behavior, but allows for better congestion control, loss recovery, and power management mechanisms compared to sender-centric approaches. More importantly, in the context of recent trends where mobile hosts are increasingly being equipped with multiple interfaces providing access to heterogeneous wireless networks, we show that a receiver-centric protocol such as RCP can enable a powerful and comprehensive transport layer solution for such multi-homed hosts. Specifically, we describe how RCP can be used to provide: (i) a scalable solution to support interface specific congestion control for a single active connection; (ii) seamless server migration capability during handoffs; and (iii) effective bandwidth aggregation when receiving data through multiple interfaces, either from one server, or from multiple replicated servers. We use both packet level simulations, and real Internet experiments to evaluate the proposed protocol.

Existing works have approached the problem of reliable transport in ad-hoc networks by proposing mechanisms to improve TCP's performance over such networks. In this paper we show through detailed arguments and simulations that several of the design elements in TCP are fundamentally inappropriate for the unique characteristics of ad-hoc networks. Given that ad hoc networks are typically stand-alone, we approach the problem of reliable transport from the perspective that it is justifiable to develop an entirely new transport protocol that is not a variant of TCP. Towards this end, we present a new reliable transport layer protocol for ad-hoc networks called ATP (ad-hoc transport protocol). We show through ns2 based simulations that ATP outperforms both default TCP and TCP-ELFN.

In this work, we present an algorithm to analytically estimate the queueing performance of MPEG-2 video using goodput as the metric; where we define goodput as the ratio between the number of cells in uncorrupted and correctly displayable frames to the total number of cells that arrive at the queue. The estimation algorithm is used to evaluate three buffer management schemes. The effect of congestion at the output link is also investigated. The algorithm produces a good approximation of the frame goodput metric and closely agrees with the corresponding simulation results.

We propose a hybrid network model called Sphinx for cellular wireless packet data networks. Sphinx uses a peer-to-peer network model in tandem with the cellular network model to achieve higher throughput and lower-power consumption. At the same time, Sphinx avoids the typical pitfalls of the pure peer-to-peer network model including unfair resource allocation, and throughput degradation due to mobility and traffic locality. We present simulation results showing that Sphinx outperforms the cellular network model in terms of throughput and power consumption, and achieves better fairness and resilience to mobility than the peer-to-peer network model.

Transmission power control in ad-hoc networks has hitherto been used only for achieving connectivity of networks. It has been implicitly assumed that the optimal throughput performance in ad-hoc networks can be achieved when using the minimum transmission power required to keep the network connected. However, in this paper we argue that such an assumption remains valid only under high node densities, which is not the characteristic of typical ad-hoc networks. Using both throughput and throughput per unit energy as the optimization criteria, we demonstrate that the optimal transmission power depends on several network characteristics such as the number of stations, the network grid area, and the traffic load. In particular, we show that the optimal power is a function of the network load for typical network scenarios. Finally, we propose two transmission power control algorithms called common power control (CPC), and independent power control (IPC) that adjust the transmission power adaptively, based on the network conditions to optimize throughput performance. Simulation results show that both adaptive power control algorithms achieve better throughput per unit energy than constant power control.

The TCP transport layer protocol is designed for connections that traverse a single path between the sender and receiver. However there are several environments in which multiple paths can be used by a connection simultaneously. We consider the problem of supporting striped connections that operate over multiple paths. We propose an end-to-end transport layer protocol called pTCP (parallel TCP) that allows connections to enjoy the aggregate bandwidths offered by the multiple paths, irrespective of the individual characteristics of the paths. We show that pTCP can have a varied range of applications through instantiations in three different environments: (a) bandwidth aggregation on multi-homed mobile hosts; (b) service differentiation using purely end-to-end mechanisms; (c) end-systems based network striping. In each of the applications, we demonstrate the applicability of pTCP and how its efficacy compares with existing approaches through simulation results.

We present an algorithm to estimate analytically the frame goodput of a correlated MPEG-2 video source that traverses a first network node's buffer; where frame goodput is defined as the ratio of the number of cells in completely and correctly transmitted frames through the buffer to the total number of cells arriving at the buffer. The effect on the frame goodput of changing the output-to-input line speeds is also investigated. By using this algorithm, a buffer that is three times the maximum frame size is seen to yield loss probability of 10/sup -6/ at a normalized output line rate of 0.28 for a load of 20%. A maximum simulation error margin of /spl plusmn/30% for values of the order of 10/sup -5/ with a 98% confidence was achieved.

We study the performance trade-offs between conventional cellular and ad-hoc peer-to-peer wireless networks. We compare, through simulations, the performance of the two network models in terms of throughput, delay, power consumption, per-flow fairness, impact of mobility, impact of traffic locality, and impact of node distribution on the network performance. The simulation results show that while peer-to-peer networks perform better in terms of throughput, delay, and power, they suffer from unfairness, and poor performance in the event of mobility and low traffic locality. We discuss the trade-offs involved in the performance of the two network models and contend that the trade-offs preclude the adoption of either of the network models as a clear solution for future wireless packet data networks. Thus, we present a simple hybrid wireless network model that uses peer-to-peer network and conventional cellular network models in tandem. It supports a dual mode of operation and has the combined advantages of cellular and peer-to-peer wireless networks without suffering from the disadvantages of either. We present simulation results showing that the hybrid network model outperforms the conventional cellular network model in terms of throughput, delay, and power consumption, and achieves better fairness and resilience to mobility than the peer-to-peer network model.

Due to the availability of a wide variety of wireless access technologies, a mobile host can potentially have subscriptions and access to more than one wireless network at a given time. In this paper, we consider such a multi-homed mobile host, and address the problem of achieving bandwidth aggregation by striping data across the multiple interfaces of the mobile host. We show that both link layer striping approaches and application layer techniques that stripe data across multiple TCP sockets do not achieve the optimal bandwidth aggregation due to a variety of factors specific to wireless networks. We propose an end-to-end transport layer approach called pTCP that effectively performs bandwidth aggregation on multi homed mobile hosts. We show through simulations that pTCP achieves the desired goals under a variety of network conditions.

While several approaches have been proposed in literature for improving the performance of wireless packet data networks, a recent class of approaches has focused on improving the underlying wireless network model itself. Several of such approaches have shown that using peer-to-peer communication, a mode of communication used typically in ad-hoc wireless networks, can result in performance improvement in terms of both throughput and energy consumption. However, the true impact of using the ad-hoc network model in wireless packet data networks has neither been comprehensively studied, nor characterized. In this paper, we investigate the benefits of using an ad-hoc network model in cellular wireless packet data networks. We find that while the ad-hoc network model has significantly better spatial reuse characteristics, the improved spatial reuse does not translate into better throughput performance. Furthermore, although considerable improvement is seen in energy consumption performance, we observe that using the ad-hoc network model as-is might actually degrade the throughput performance of the network. We identify and discuss the reasons behind these observations. Finally, using the insights gained through our performance evaluations, we discuss strawman versions of three techniques which when used in tandem with the ad-hoc network model result in better throughput, energy consumption, fairness, and mobility-resilience characteristics. Through our simulation results, we motivate that using the ad-hoc network model in conventional wireless packet data networks is a promising approach when the network model is complemented with appropriate mechanisms.

Ad-hoc networks are multi-hop wireless networks that can operate without the services of an established backbone infrastructure. While such networks have obvious applications in the military and disaster relief environments, more recent works that have motivated their use even in regular wireless packet data networks have increased their significance. The focus of this paper is to study the performance of the TCP transport layer protocol over ad-hoc networks.Recent works in transport protocols for ad-hoc networks have investigated the impact of ad-hoc network characteristics on TCP's performance, and proposed schemes that help TCP overcome the negative impact of such characteristics as random wireless loss and mobility. The primary mechanism proposed involves sending an explicit link failure notification (ELFN) to the source from the point of link failure. The source, upon receiving the ELFN freezes TCP's timers and state, re-computes a new route to the destination, and either releases the timers and state or re-starts them from their respective initial values. While the goal of ELFN based approaches is to prevent the route disruption time from adversely impacting TCP's performance, in this paper we contend that there are several other factors that influence TCP's performance degradation. We briefly outline the different factors below:• TCP Losses: Every route failure induces upto a TCP-window worth of packet losses. While the losses have an absolute impact on the performance degradation, the TCP source also reacts to the losses by reducing the size of its window. Note that ELFN will prevent this negative impact on TCP's performance by appropriately freezing TCP's state.• MAC Failure Detection Time: Since the MAC layer (802.11) has to go through multiple retransmissions before concluding link failure, there is a distinct component associated with the time taken to actually detect link failure since the occurrence of the failure. Importantly, the detection time increases with increasing load in the network. While an external mechanism to detect link failures (e.g. through periodic beacones at the routing layer) would solve this problem, it comes at the cost of beacon overheads and associated trade-offs.• MAC Packet Arrival: When a failure is detected as described above, the link failure indication is sent only to the source of the packet that triggered the detection. If another source is using the same link in the path to its destination, the node upstream of the link failure will wait until it receives a packet from that source before informing it of the link failure. This also contributes to the magnitude of the delay after which a source realizes that a path is broken.• Route Computation Time: Once a source is informed of a path failure, the time taken to recompute the route also increases with increasing load. With ELFN, for a load of 25 connections, the per-flow average of the aggregate time spent in route computation during a 100 second simulation was as high as 15 seconds. In addition to the absolute impact of the idle periods, TCP is also likely to experience timeouts, especially in the heavily loaded scenarios where the route computation time can be high.In the next section, we present a framework of mechanisms called Atra targeted toward addressing each of the above components. We show through representative simulation results that the proposed mechanisms outperform both the default protocol stack and an ELFN-enabled protocol stack substantially. We assume the default protocol stack to comprise of the IEEE 802.11 MAC protocol, the Dynamic Source Routing (DSR) routing protocol, and TCP-NewReno as the transport layer protocol. For a more detailed analysis of TCP performance in mobile ad-hoc networks, and description of the Atra framework, please see [1].

Several router mechanisms have been proposed to achieve fair rate allocations. However, most of them require the proposed mechanisms to be deployed at all routers within an autonomous domain in order to achieve the desired, or any appreciably improved, quality of service. We present an incrementally deployable QoS architecture called iQ. iQ consists of a set of core- and edge-router mechanisms that allow for changing one router at a time, improving the rate fairness provided by the network gracefully for each additional iQ router deployed. We use simulations to compare the incremental deployability of the iQ architecture with that of existing approaches.

We introduce nanoProtean, a new router operating system and execution environment that reduces system overhead, making it possible to process the packets produced by gigabit networks. The ovearhead decreases as the offered load per packet increases due to the following features in nanoProtean (i) a completely preemptable operating system, (ii) efficient management of the system's job queue, and (iii) system support for fine-grain sharing of processing time amongst packets. These features are a result of a novel integration of efficient thread scheduling for multiprocessors and interrupt handling. Experimental analysis used to test our systems scalability uses a technique to emulate processing requests generated in real-time at 802.3z (gigabit) line speeds and greater. Our results demonstrate 2 Gbps routing; with MAE-East tables on two processors, and system overheads decreasing from 3.6 /spl mu/s per packet to 1.64 /spl mu/s per packet on one processor. By reducing the system overhead, we also demonstrate that nanoProtean enables active networking in a router supporting gigabit connections.

Several routing algorithms for mobile ad hoc networks (MANETs) have been proposed previously. With the exception of a few, these protocols (i) involve all nodes in the route management process, (ii) rely on the use of broadcast relays for route computation, and (iii) are primarily reactive in nature. Related work has shown that the capacity utilization in ad hoc networks decreases significantly when broadcast relays or "broadcast storms" are performed frequently. This effect is compounded when all nodes in the network take part in the route computation. We propose and study an approach based on overlaying a virtual infrastructure adaptation of the core, proposed by Sivakumar, Sinha and Bharghavan (see IEEE Journal on Selected Areas in Communications, vol.17, no.8, p.1454-65, 1999) on an ad hoc network and operating routing protocols over the infrastructure. The core enables routing protocols to use only a subset of nodes in the network for route management and avoid the use of broadcast relays. Using the ns-2 simulator we evaluate the performance of two ad hoc routing protocols, dynamic source routing (DSR) and ad hoc on demand distance vector (AODV), when they are operated over the core and compare their performance against those of their basic versions.

Corelite is a quality of service architecture that provides weighted max-min fairness for rate among flows in a network without maintaining any per-flow state in the core routers. There are three key mechanisms that work in concert to achieve the service model of Corelite: the introduction of markers in a packet flow by the edge routers to reflect the normalized rate of the flow; weighted fair marker feedback at the core routers upon incipient congestion detection; and linear increase/multiplicative decrease based rate adaptation of packet flows at the edge routers in response to marker feedback.

We present a core-stateless quality of service architecture for achieving delay differentiation between flows. There are two key components in our approach: (1) per-class per-hop relative average delay-the average queueing delay perceived by the packets in a delay class at a link is inversely proportional to the delay weight of the class; (2) per-flow end-to-end delay class adaptation-the delay class of a flow is dynamically adjusted based on its perceived end-to-end delay in order to maintain the desired end-to-end average delay requirement of the flow. We show through simulations and analysis that these two components can, in concert, support end-to-end delay differentiation between flows using only simple mechanisms at the core routers in the network.

CEDAR is an algorithm for QoS routing in ad hoc network environments. It has three key components: (a) the establishment and maintenance of a self-organizing routing infrastructure called the core for performing route computations, (b) the propagation of the link-state of stable high-bandwidth links in the core through increase/decrease waves, and (c) a QoS route computation algorithm that is executed at the core nodes using only locally available state. But preliminary performance evaluation shows that CEDAR is a robust and adaptive QoS routing algorithm that reacts effectively to the dynamics of the network while still approximating link-state performance for stable networks.

Most current wireless wide area packet data networks (WWANs) are characterized by low and variable bandwidths, high and variable delays, significant non-congestion related loss, asymmetric uplink and downlink channels, and occasional blackouts.Additionally, the majority of the latency in a WWAN connection is incurred over the wireless link.Under such operating conditions, most contemporary wireless TCP algorithms do not perform very well.In this paper, we present WTCP, a reliable transport protocol that is designed to operate efficiently over commercial WWAN networks such as CDPD.WTCP is rate-based, uses only end-to-end mechanisms, performs rate control at the receiver, and uses the ratio of sending rate to receiving rate as the primary metric for rate control.We have implemented and evaluated WTCP over the CDPD network, and also simulated it in the ns-2 simulator.Our initial results indicate the WTCP can improve on the performance of comparable algorithms such as TCP-NewReno, TCP-Vegas, and Snoop-TCP by between 20% to 200% for typical operating conditions.'Most of the current deployment uses the proxy model for three reasons: (a) the connection quality of the WWAN network is too poor to sustain typical client-server applications, (b) the amount of data transfered to the mobile host must be filtered because of the orders of magnitude difference in bandwidth between the wired and WWAN connections, and (c) many portable computing devices have display and processing limitations that must be addressed by the proxy before sending the processed/filtered response to the mobile user.Permission to make digital or hard copies of all or part of this work tw personal or classroom use is gxnted without fee provided that coPi= are oat ma& or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first Page.To COPY othenvise, to rqublish, to post on servers or to redistribute to lists, reqllires prior specific permissioll and/or a fee.

In this paper, we present the MCEDAR (multicast core extraction distributed ad hoc routing) multicast routing algorithm for ad hoc networks. MCEDAR is an extension to the CEDAR architecture and provides the robustness of mesh based routing protocols and the approximates the efficiency of tree based forwarding protocols. It decouples the control infrastructure from the actual data forwarding infrastructure. The decoupling allows for a very minimalistic and low overhead control infrastructure while still enabling very efficient data forwarding.

Many current wireless wide area packet data networks are characterized by very low and variable bandwidths, very high and variable delays, significant non-congestion related loss, asymmetric uplink and downlink channels, and occasional blackouts. Additionally, the majority of the latency in a WWAN connection is incurred over the wireless link. Under such operating conditions, most contemporary wireless TCP algorithms do not perform very well. Sinha, Venkitaraman, Sivakumar and Bharghavan (see Proc. of ACM Mobicom '99, 1999) we presented WTCP, a reliable transport protocol that is designed to operate efficiently and fairly over wireless wide area networks. WTCP is rate-based, uses only end-to-end mechanisms, performs rate control at the receiver, and uses the ratio of sending rate to receiving rate as as the primary metric for rate control. In this paper, we evaluate the performance of WTCP over CDPD networks. We have implemented and evaluated WTCP over the CDPD network, and also simulated it in the ns-2 simulator. Our performance results indicate that WTCP can improve on the performance of comparable algorithms such as TCP-NewReno and TCP-Vegas by between 20% to 200% for typical operating conditions.

We present CEDAR, a core-extraction distributed ad hoc routing algorithm for quality-of-service (QoS) routing in ad hoc network environments, CEDAR has three key components: (a) the establishment and maintenance of a self-organizing routing infrastructure called the core for performing route computations; (b) the propagation of the link-state of high bandwidth and stable links in the core through increase/decrease waves; and (c) a QoS-route computation algorithm that is executed at the core nodes using only locally available state. The performance evaluations show that CEDAR is a robust and adaptive QoS routing algorithm that reacts quickly and effectively to the dynamics of the network while still approximating the performance of link-state routing for stable networks.

This work proposes a self-organizing, dynamic infrastructure called a spine for efficient routing in ad hoc networks. We present a scalable framework for routing that encompasses a range of knowledge at each spine node, and identify the trade-offs involved for routing at different points in this range. Our routing algorithm requires only partial topology information at each spine node, consisting of the spine structure, dependants of each spine node, propagation of long-lived links, and snooped routing information from ongoing flows. Through worst-case theoretical bounds and simulation of typical scenarios, we show that the spine-based routing with only partial topology information provides good routes at low overhead.

We present a two-level hierarchical routing architecture for ad hoc networks. Within each lower level cluster, we describe a self-organizing, dynamic spine structure to (a) propagate topology changes, (b) compute updated routes in the background, and (c) provide backup routes in case of transient failures of the primary routes. We analyze and bound the worst case of movements between upper level clusters to show that this hierarchical architecture scales well with network size.