In order to support emerging wireless applications, it is critical to provide users with ubiquitous high data rate connections. The emerging 6G standards are considering utilizing part of the terahertz spectrum for this purpose. However, this part of the spectrum suffers from significant distance based attenuation as well as blockage by most materials including foliage, paper, walls, etc. The challenge therefore is to ensure coverage in complex indoor environments. A second factor that needs to be considered is that for line of sight connections, MIMO capacity is lower than in a rich multi-path environment. Unfortunately, due to propagation limitations, multi-path propagation is mostly absent at this frequency.This paper proposes a novel design for a reflector that satisfies the need for coverage planning as well as providing a multi-path environment which can support high data rate MIMO links. The reflectors consist of multiple passive elements that add a random phase to the reflected signal. We build such reflectors and characterize their performance in a terahertz testbed for frequencies in the range 325 - 500 GHz. We use the channel impulse response to estimate the maximum possible capacity. We show that our reflectors do indeed provide a channel for high capacity MIMO links as compared to polished reflectors that are commonly used for coverage planning.

Line of Sight (LoS) Multiple Input Multiple Output (MIMO) terahertz communication channels suffer from a decrease in capacity in the far field due to the channel matrix only having single rank. Prior work in millimeter wave (mmWave) systems has considered the design of Intelligent Reflective Surfaces (IRSs) to mitigate this problem by introducing phase variation in reflected signals. Given the high cost of IRSs, we propose inexpensive Random Reflective Surfaces (RRSs) to create a rich multi-path environment for terahertz.In this work, we build three RRSs using high-density polyethylene (HDPE) sheets embedded with tiny copper flakes at different densities. The reflection behavior of these systems is studied in a terahertz testbed where we focus on the frequency band centered at 410GHz. Using 2x2 MIMO measurements in this testbed, we show that our RRS designs indeed do improve capacity as compared to reflections from polished aluminium (which behaves like a LoS channel). In addition, we examine the range of reflected phase produced and show that, indeed, the high variation in phase leads to improved channel capacity for the case when the channel state information is only available at the receiver (CSIR) for 2x2 MIMO.

Communication at the terahertz band is a challenging yet exciting technology. It promises to deliver terabit/sec data rates for short distances but, due to its unique propagation characteristics, it suffers from significant impairments. To enable these high data rates, we need to use MIMO (Multiple Input Multiple Output) systems. However, given that natural reflected paths display enormous attenuation, we need to rely either on LoS (Line of Sight) MIMO or carefully placed reflectors to overcome environmental obstacles. We have observed that when using polished reflectors (e.g., metal plates), the channel behaves similarly to a LoS MIMO channel in that there is a decrease in capacity with distance and a periodicity. In this paper, we describe the construction of a novel reflector that maintains a steady capacity with distance. The reflector creates multiple independent paths between the transmit and receive antennas such that the eigenvalues of the channel matrix are non-trivial. This provides us with the capacity improvement. Our work is experimental and we focus on the frequency band around 410 GHz. We constructed a variety of reflectors and identified a design that improves capacity. We show that our reflector maintains a high capacity with increasing distance while a LoS path and a path constructed with a polished reflector show a decrease in capacity. We also used the same reflector at other frequencies and observed poorer behavior illustrating a dependence between wavelength and reflector design. Our results show that we can maintain high-capacity indoor channels by carefully designed reflectors.

Communication at the terahertz band is increasingly seen as vital for future short-range very high datarate channels. However, these channels suffer from significant environmental impairments and, as a result, providing coverage in indoor settings requires the use of directional line of sight (LoS) paths to visible users and reflected paths, using smooth metal reflectors, for users in the shadow of an obstruction. Previous work has shown that these types of reflected paths display similar characteristics to LoS paths and we call them R-LoS (reflected LoS). MIMO for LoS and R-LoS channels is feasible at terahertz frequencies and delivers very high capacity at some distances. Unfortunately, channel capacity varies greatly with small changes in distance (the channel matrix fluctuates between full rank and rank 1) which is undesirable for communication systems. In this paper, we utilize diffusive reflectors to create multiple reflections such that the MIMO channel capacity for R-LoS is better behaved. We conduct experiments at 410 GHz for reflections from different artificially created diffuse surfaces. We use measurements to estimate channel capacity for 2×2 MIMO when the only path is the diffuse reflected one. We show that by creating multiple controlled reflections, it is possible to achieve relatively stable capacity up to 13 - 16 bits/sec/Hz at varying distances. We also analyze the phase of the received signals and the beam profile in detail. Overall, our results indicate that by utilizing artificially created reflections, we can maintain a stable MIMO channel at high capacity.

Terahertz frequencies are an untapped resource for providing high-speed short-range communications. As a result, it is of interest to study the propagation characteristics of terahertz waves and to develop channel models. In previous work we used a measurement-based approach to develop an accurate channel model for line of sight (LoS) links. In this paper we extend that work by developing channel models for non-line of sight (NLoS) links where the signal suffers one reflection. We study reflections that occur off a metal plate as well as a piece of wood.Our model for received magnitude includes the effects of standing waves that develop between the transmitter and receiver. Measurements show an excellent agreement between empirical data and the model. In addition, we have analyzed the received phase of the reflected signal at frequencies in the range 320-480 GHz. We observed a linear error between the predicted and actual phase and developed a model to accommodate that discrepancy. The final model we have developed for predicting received phase is very accurate for the entire range 320 - 480 GHz and for both materials.

In recent years, the task of detecting mosquito presence through acoustic data has drawn the attention of many researchers. However, just like in any other detection task, these researchers are often confronted with the distribution shift problem, which alludes to the situation where the training and test datasets do not share the same distribution. A detection system is almost always guaranteed to fail during testing when this situation arises. Solutions to this issue have been proposed over the years, but they are often computationally expensive and complex to implement. In this paper, we propose a simple solution that consists in (1) identifying and getting rid of the noise present in the input data, (2) performing a dimensionality reduction, and (3) classifying the data. We tested our technique on a large and publicly available dataset of mosquito recordings (HumBugDB) and the results showed a maximum improvement of nearly 28% when compared to a baseline classification system.

This paper examines the performance of a 2×2 Line of Sight (LoS) Multiple Input Multiple Output (MIMO) channel at three terahertz frequencies-340 GHz, 410 Ghz, and 460 GHz. While theoretical models predict very high channel capacities, we observe lower capacity which is explained by asymmetric transmit-to-receive signal strengths as well as due to signal attenuation over longer distances. Overall, however, we note that at 460 Ghz, channel capacity of higher than 12 bps/hz is possible even at sub-optimal inter-antenna spacings (for different distances). An important observation is also that we need to maintain appropriate receive signal levels at receive antennas in order to improve capacity.

This paper examines a 2x2 LoS (Line of Sight) MIMO (Multiple Input Multiple Output) channel for terahertz communications. Utilizing measurements, we extract channel models which are then used in a simulation to produce bit error rate curves for uncoded QPSK (Quadrature Phase Shift Keying). The specific focus of this work is on studying the impact of changing transmitter receiver distances and relative orientation. Since the terahertz band has a small wavelength, such small movements can have a large impact on the channel. The results support this intuition and show that horizontal displacements of one antenna relative to another by distances as small as 1-2 cm can cause the bit error rate to increase by an order of magnitude. Similarly, utilizing sub- optimal antenna spacing at the transmitter and receiver can also result in similar types of performance degradation. This study illustrates that while LoS MIMO is an effective approach for utilizing the large terahertz bandwidth, we need to consider the effects of antenna deployments.

Present-day datacenter networks (DCNs) are designed to provide full-bisection bandwidth in order to support high network throughput and server agility. However, many studies have shown average utilization of the DCN is very low even at high server loads. As a result, there is a significant waste of energy resources in modern datacenters. In this paper we describe the construction and evaluation of a hardware device that consolidates traffic going between servers and the top- of-rack (ToR) switch. The consolidator operates entirely in the optical domain and has very low power consumption. We evaluate the performance of the consolidator in a small network. Subsequently, using simulations, we show that for larger systems, the consolidator allows energy savings of almost 70% at light loads and scales with increasing loads.

The terahertz band (0.1-10 THz) has a great deal of unused spectrum that could satisfy the ever-increasing demand for wireless communications. However, as a first step in realizing this goal, we need to understand how terahertz signals behave in indoor environments. In this study, we present broadband channel measurements of the 0.1 to 2 THz band using convex lenses at the transmitter and collimating lenses at the receiver. The convex lens provides coverage over an indoor region while a collimating lens at the receiver maximizes signal strength. The measurements are based on Line-Of-Sight (LOS) since reflected signals in this frequency band are absent. Applying geometric analysis, we propose a path loss model which matches our measurements well.

The large available bandwidth of terahertz (THz) channels makes THz technology a promising approach to achieve very high data rates for future applications. In this paper, we characterize the THz channel using detailed measurement and analysis. Based on the results, we choose three THz bands in which atmospheric attenuation is smaller compared to other bands. We propose two rate adaptive downlink THz algorithms in which the modulation order is changed based on the channel properties of individual receivers. We demonstrate data rates of several Gbps per user while maintaining fairness.

Many data center network topologies are designed to provide full bisection bandwidth for tens of thousands of servers in order to achieve high network throughput and server agility. However, the utilization rate of DCNs on average is below 10%, which results in a significant waste of network resources and energy. Many researchers propose consolidating network traffic flows to maximize the set of idle network equipment and switching them to low power mode to save energy. In this paper, we propose using skinnier network topologies to meet performance requirements of realistic loads thus saving not only energy but capital cost as well. We undertake a comprehensive study of the sub-graphs of fat-trees for different traffic characteristics and present an analytical model for the number of switches required in each layer of the topology. Furthermore, we examine the potential energy savings of utilizing higher-port-count switches at the edge layer and evaluate the reduced energy consumption using the power data of Cisco modular switches.

We consider the problem of building a communication channel based on modulation of terahertz pulses. Given the constrained propagation properties of this spectrum, one of our goals is to determine the potential data rates achievable for different environmental conditions. To do this, we first present a detailed channel model based on measurements carried out on a terahertz time-domain system. We next simulate frequency-domain differential quadrature phase shift keying (FD-DQPSK) and Adaptive FD-DQPSK pulse modulation schemes. Finally, we simulate communication for different cities in winter and summer seasons where the primary channel impairment includes humidity. We show that data rates of well over 1 Tbit/s are achievable for all cases for distances of up to three meters. We also examine the error behavior of this transmission channel as a function of humidity and frequency band.

Fat-tree is a popular topology for data center networks (DCNs). Many recent papers propose new energy-efficient techniques that reduce the power consumption of fat-tree DCNs by dynamically powering off idle network switches and links. However, fat-tree DCNs adopting those techniques still consume a significant amount of energy even when lightly loaded. In this paper, we examine the approach of merging traffic in a fat-tree DCN in order to consolidate the traffic into fewer switches. We first derive an analytical model for the lower-bound of active switches required in a fat-tree DCN as a function of load, assuming traffic merging. Then we formulate a power optimization model and propose a scalable heuristic algorithm to find the minimum subset of network switches and links that satisfies a variety of traffic patterns and loads in actual DCNs. Simulation results show that our approach can substantially reduce the number of active switches and lower the energy consumption of a fat-tree DCN when the load is light. We show that our solution achieves near energy proportionality.

There are numerous scenarios where embedding static information into objects would prove to be very useful. For example, by embedding navigation information into the road surface we could enable driverless cars, similarly embedding information into major infrastructure such as building walls would enable easier classification of debris, and so forth. The key requirement is that the embedded information should survive as long as the infrastructure itself. We discuss the concept of wireless barcodes that allow us to achieve exactly this goal of tagging infrastructure. The barcodes are designed to be read at terahertz frequencies that allows good packing of information (bits/m). We use a TDoA (Time Difference of Arrival) technique to read the barcodes which provides robustness to wear and tear of the surface. This paper reports on the results of an experimental study that examines the challenges in building such barcodes and demonstrates the idea by constructing such a barcode.

We study the problem of wireless communication over indoor pulsed terahertz (THz) channels. First, we characterize the THz channel using measurements and derive a channel model. This channel model is then implemented in a Matlab simulator that we use to study different modulation schemes. In this paper, our focus is on examining frequency-domain differential phase shift keying (FD-DPSK) modulation schemes. We show that absorption due to water vapor leads to high bit error rate (BER). To combat this, we propose adaptive modulation schemes in which, depending on the humidity level of the channel and the distance between transmitter (Tx) and receiver (Rx), the system excludes some bands of the pulsed signal from the modulation scheme. Using FD-D64PSK modulation scheme and multiple transmitters in a time division multiplexing (TDM) channel, data rates as high as 5.994 Tbit/s can be achieved when the relative humidity (RH) is 0% over a distance of 50 cm. When the RH increases to 40% and 80% data rates decrease to 5.938 Tbit/s and 5.817 Tbit/s, respectively.

Reducing energy consumption in the Internet has become an increasingly important goal recently. Previous work on reducing energy consumption has primarily looked at either changing link rates or putting interfaces to sleep. Due to the unpredictable nature of traffic, the energy savings achieved have been modest, do not scale, and incur losses and delay. This paper proposes a different approach to the problem which involves a new switch architecture combined with a simple sleep/awake algorithm. The main results we obtain are that energy consumption, measured as the fraction of interfaces that are awake, scales linearly with load for all loads with no delay and very low packet loss. We demonstrate the behavior of the algorithm via detailed simulations using synthetic traffic and traffic traces collected at a LAN switch in our university. We obtain over 90% energy saving in all real traffic traces and zero packet loss. For the synthetic traces, we see linear scaling of energy use with load of up to 85%.

60GHz is well-suited for device-to-device communication and for general in-home applications due to its huge available bandwidth. However, links at this frequency are easily degraded by environmental conditions. Indeed, the poor multipath makes it necessary to maintain good LoS (Line of Sight) paths between communicating pairs of nodes. In this paper we analyze the problem of link breakage and degradation in point-to-point 60GHz networks. We propose using repeaters to provide alternate paths between communicating nodes when the direct path degrades. The decision on which path to use is driven by the goal of maximizing data rate per connection as well as overall throughput in the network. Using extensive simulations we show that by carefully using repeaters, we can maintain Gbps rates for each pair of communicating nodes in indoor spaces. We develop an efficient distributed algorithm for allocating repeaters to links as needed and explicitly deal with the problem of interference caused by the repeaters themselves. Finally, we study the scalability of the solution with increasing numbers of communicating pairs and show that Gbps rates can be maintained even when there are as many as eight communicating pairs in a small 10m×10m room.

With the opening of the 60 GHz spectrum for WLANs, there has been a great deal of interest in academia and industry on how best to exploit the more than 5 GHz of available bandwidth. One goal in the wireless community has been delivering gigabit data rates to end users. For this, a variety of approaches have been studied in the past. In this paper, we present two SDMA (spatial division multiple access) algorithms that exploit the peculiar propagation properties of this part of the frequency. We show that in typical indoor environments, one access point can deliver over 8 Gbps total throughput while using only 640 MHz of the bandwidth. We generalize the algorithms to the case when multiple channels are available and show that with seven channels, we get aggregate throughput of over 31 Gbps.

In the design of wireless networking protocols and systems, simulation has become the primary form of initial validation and performance evaluation. Hence, ensuring the realism of simulators and simulation methods is fundamental for simulated results to be interpretable. In this paper, we provide a simulation framework for infrastructure wireless network traffic that allows researchers to use publicly available captured traces as a primary or background traffic source. We investigate the question of trace classification as a necessary task for these traces to be useful and apply our framework to a well-known power-saving application, showing that the use of real traffic provides substantially different results as compared to traffic generated from an application-specific fitted model or contrived source. Additionally, we show how trace classification provides unique insights into application behavior in both typical and extreme scenarios.

The 60 GHz spectrum gives us the opportunity to deliver gigabit rates to users in a WLAN (wireless local area network) setting. The constrained propagation of signals at this frequency band ensures limited coverage which in turn enables the construction of very efficient STDMA (spatial time division multiple access) schedules. In this paper we study the achievable aggregate capacity in a room when using two types of smart antenna arrays - linear and circular. Using detailed Matlab simulations, we show that with just 400 MHz of the spectrum, aggregate data rates of 9 Gbps (4.5 Gbps) can be achieved with linear (circular) arrays. We also study the energy efficiency of the communication and show that the energy/bit is as low as 0.2 times 10-10(0.2 times 10-9)Joules/Bit by using variable transmit powers at different parts of the room. Finally, we study the problem of coverage due to blocking of the LoS (line of sight) path. To mitigate this problem we study the use of static reflectors and show that coverage in the entire room can indeed be maintained.

Understanding user behavior in wireless environments is useful for a variety of reasons ranging from the design of better sleep algorithms for components of mobile devices to appropriately provisioning the wireless network itself to better serve the user. Our work goes in a different direction from prior work on WLAN modeling and attempts to undersand the protocol independent behavior of users by developing packet-level models for user activity using diverse training data. Additionally we validate the derived model using a stochastic similarity metric adapted from human control strategy modeling and present a novel way to compare traces using this metric.

In this paper we use a large dataset of wireless user activity traces to test the various dynamic power management schemes. We also present and test our own empirically-driven dynamic power-saving algorithms, which are based on prior observations of user activity patterns. We believe that this sort of analysis can guide adoption of a user-behavior driven approach to radio and communications power management, and, in networking-centric devices, power management for the entire device. Additionally, understanding the characteristics of user-activity and efficient mechanisms to predict this activity can help inform the design of power-saving schemes for future networking protocols.

A WANET is a wireless network where the wireless nodes can be located anywhere over the globe. However, the underlying design is such that the nodes believe they are part of a single-hop or multi-hop wireless network at the PHY and MAC layers. This is accomplished by using Software Defined Access Points (SoDA) that are based on the idea of Software Defined Radio (SDR). For the uplink, each SoDA samples the down-converted channel using an ADC (analog-to-digital converter). The sampled data is then multicast to the other SoDAs via the Internet. At each end-point, the received digital signals from the other SoDAs are summed and sent through the DAC (digital-to-analog converter) and transmitted on a designated channel after upconversion. In effect what we have done is to mix the RF environment at geographically separate locations (albeit with a time shift). This simple technique leads to a whole new model for designing and using wireless networks. Indeed, we can think of these networks as following the end-to-end model where only the end nodes understand the PHY/MAC layers and the access points and Internet serve simply as dumb relays. This paper describes a series of network models that are enabled and outlines the open research challenges.

This paper considers the problem of providing gbps/user data-rate in indoor environments. The technology that we study uses the 60 GHz spectrum whose special propagation properties make it ideal when combined with antenna array technology. We present two algorithms. The first algorithm is SINR threshold based which uses dynamic spectrum allocation and adaptive modulation. We show data rates of 2 Gbps per user even when 10 users are present in a small room. The second algorithm dynamically assigns users to channels without using a fixed SINR threshold. The results show that it can achieve up to 4 Gbps/user for the 10 users case. We obtain a bandwidth efficiency of up to 12 b/s/hz.

Recent studies of network traffic utilization on campus networks have shown that data networks remain heavily under-utilized. Yet currently there is little attempt to save energy on network interfaces by using the low power modes available in Ethernet transceivers during periods of inactivity or low utilization. In this paper we design and evaluate a dynamic ethernet link shutdown (DELS) algorithm that utilizes current technology leading to significant benefits in energy savings with little noticeable impact on packet loss or delay. The algorithm uses buffer occupancy, the behavior of previous packet arrival times and a configurable maximum bounded delay to make sleeping decisions. The scheme was evaluated using simulations with inputs generated using a synthetic traffic generator for smooth and bursty traffic. The results show that the percentage of total time that a link can be shut down can be anywhere from 80% to 60% for traffic loads up to 5%.

Most Ethernet interfaces available for deployment in switches and hosts today can operate in a variety of different low power modes. However, currently these modes have very limited usage models. They do not take advantage of periods of inactivity, when the links remain idle or under-utilized. In this study, we propose methods that allow for detection of such periods to obtain energy savings with little impact on loss or delay. We evaluate our methods on a wide range of real-time traffic traces collected at a high-speed backbone switch within our campus LAN. Our results show that Ethernet interfaces at both ends can be put in extremely low power modes anywhere from 40%-98% of the time observed. In addition, we found that approximately 37% of interfaces studied (on the same switch) can be put in low power modes simultaneously which opens the potential for further energy savings in the switching fabric within the switch.

The question that we consider here is the following: "How can a source verify the quality of service (QoS) experienced by its packet(s) at each hop to the destination in a multihop wireless network?" For example, if Bob needs to forward packets within some maximum delay of deltaB, how can the source verify that Bob in fact forwarded the packets within this bound? Answering this question will enable innovations in multihop wireless network deployments, where nodes may receive payment not only for forwarding packets but also for meeting some QoS guarantees. In this paper, we present protocols that enable verification of delivered QoS for individual packets, as well as verification of statistical QoS for groups of packets. The protocols are proven to be cheat proof. We also provide expressions for the minimum verifiable delay.

The presence of heterogeneous nodes (i.e., nodes with an enhanced energy capacity or communication capability) in a sensor network is known to increase network reliability and lifetime. However, questions of where how many, and what types of heterogeneous resources to deploy remain largely unexplored. We focus on energy and link heterogeneity in ad hoc sensor networks and consider resource-aware MAC and routing protocols to utilize those resources. Using analysis, simulation, and real testbed measurements, we evaluate the impact of number and placement of heterogeneous resources on performance in networks of different sizes and densities. While we prove that optimal deployment is very hard in general, we also show that only a modest number of reliable, long-range backhaul links and line-powered nodes are required to have a significant impact. Properly deployed, heterogeneity can triple the average delivery rate and provide a 5-fold increase in the lifetime (respectively) of a large batten-powered network of simple sensors.

This paper discusses various approaches to save power in relay-based cellular networks. We introduce an optimization method based on the Hungarian assignment problem to globally minimize the power-based allocation of relays. We compare this approach with the greedy approach of choosing a relay. We introduce another approach based on the concept of intelligent relays. Here, based on the expected power values of mobiles at the relay's location, the relay chooses to carry call for a mobile. We show that this approach introduces less intra-cell interference

Smart antennas enable a receiver to determine the direction of arrival (DOA) of multiple transmissions as well as to form nulls in some number of directions to maximize SINR (Signal to Interference and Noise Ratio) of the received signal. We utilize the benefits of these capabilities to develop a simple modified version of the popular 802.11b protocol. This protocol exhibits high throughput under a variety of network conditions and is fair. The performance of the protocol is examined exhaustively using joint simulation in OPNET and Matlab.

We examine the feasibility of introducing power management schemes in network devices in the LAN. Specifically, we investigate the possibility of putting various components on LAN switches to sleep during periods of low traffic activity. Traffic collected in our LAN indicates that there are significant periods of inactivity on specific switch interfaces. Using an abstract sleep model devised for LAN switches, we examine the potential energy savings possible for different times of day and different interfaces (e.g., interfaces connecting to hosts to switches, or interfaces connecting switches, or interfaces connecting switches and routers). Algorithms developed for sleeping, based on periodic protocol behavior as well as traffic estimation are shown to be capable of conserving significant amounts of energy. Our results show that sleeping is indeed feasible in the LAN and in some cases, with very little impact on other protocols. However, we note that in order to maximize energy savings while minimizing sleep-related losses, we need hardware that supports sleeping.

We present results from a detailed energy measurement study of TCP. We focus on the node-level cost of the TCP protocol and obtain a breakdown of the energy cost of different TCP functions. We analyze the energy consumption of TCP on two platforms (laptop and iPAQ) and three operating systems (FreeBSD 4.2, 5 and Linux 2.4.7). Our results show that 60-70% of the energy cost (for transmission or reception) is accounted for by the kernel NIC (network interface card) copy operation. Of the remainder, /spl sim/15% is accounted for in the copy operation from user space to kernel space with the remaining 15% being accounted for by TCP processing costs. We then further analyze the 15% TCP processing cost and show that the cost of computing checksums accounts for 20-30% of TCP processing cost. Finally, we determine the processing costs of two primary TCP functions - timeouts and triple duplicate ACKs. Pulling all these costs together, we present techniques whereby energy savings of between 20%-30% in the computational cost of TCP can be achieved.

This work presents a tone-based 802.11b protocol for use in wireless networks where nodes are equipped with adaptive array antennas. The protocol relies on the ability of the antenna and angle of arrival (AoA) algorithms to identify the direction of transmitters and then beamform appropriately to maximize SINR (signal to interference and noise ratio) at the receiver. The performance of the protocol is evaluated using detailed simulation in OPNET and Matlab where we demonstrate the benefits of using adaptive antenna arrays. The impact of using different number of antenna elements is also studied for this environment.

It has been shown that gaps exist in the useful coverage service area of a cell and the reliability of coverage drops due to buildings as well as interference in CDMA cells. We have used the generalized poisson process to model the areas where signal level fall below threshold and formulated the mathematical definition of useful service area in the presence of relays. We used approaches "with replacement" and "without replacement" of idle mobiles to serve as relays or intermediaries. We also use a constraint to make sure that the intermediary doesn't fall in a below threshold area. Monte Carlo simulations show that 2-hop relaying can increase the useful service coverage area by 4-14% in sparse and heavy building density areas.

This paper presents a novel slotted MAC (medium access control) protocol for nodes equipped with adaptive antenna array in ad hoc network. The protocol relies on the ability of antenna to uses DOA (direction-of-arrival) information to beamform by placing nulls in the direction of interferers thus maximize SINR (signal to interference and noise ratio) at the receiver. We studied the performance of the protocol using joint simulation in OPNET and Matlab. We studied the impact of variable number of antenna elements, DOA algorithm, and nulling. The performance of our new protocol is compared against one of the recent directional MAC protocols [R.R.N.H.V. Romit Roy Choudhury, Xue Yang, 2002]. We observe that despite the simplicity of our protocol it achieves high throughput.

In this paper we examine the somewhat controversial subject of energy consumption of networking devices in the Internet, motivated by data collected by the U.S. Department of Commerce. We discuss the impact on network protocols of saving energy by putting network interfaces and other router & switch components to sleep. Using sample packet traces, we first show that it is indeed reasonable to do this and then we discuss the changes that may need to be made to current Internet protocols to support a more aggressive strategy for sleeping. Since this is a position paper, we do not present results but rather suggest interesting directions for core networking research. The impact of saving energy is huge, particularly in the developing world where energy is a precious resource whose scarcity hinders widespread Internet deployment.

In this paper we present results from a measurement study of TCP (Transmission Control Protocol) running over a wireless link. Our primary goal was on obtaining a breakdown of the computational energy cost of TCP at the sender and receiver (excluding radio energy costs) as a first step in developing techniques to reduce this cost in actual systems. We analyzed the energy consumption of TCP in FreeBSD 5 running on a wireless laptop. Our initial results showed that 60 - 70% of the energy cost (for transmission or reception) is accounted for by the Kernel -- NIC (Network Interface Card) copy operation. Of the remainder, 15% is accounted for in the copy operation from user space to kernel space with the remaining 15% being accounted for by TCP processing costs. We then further analyzed the TCP processing cost and determined the cost of computing checksums accounts for 20 -- 30% of TCP processing cost.

In this paper we study the energy cost (protocol processing and communication cost) and goodput of two different flavors of TCP in ad hoc networks. We implemented a testbed and measured the actual energy cost as well as goodput of running TCP-SACK in ad hoc network scenarios. We also implemented Explicit Link Failure Notification (ELFN)[1] and Explicit Congestion Notification (ECN)[2] in Newreno and measured its performance. We see that the use of ECN & ELFN does yield higher goodput in most cases with a corresponding lower total energy cost. We see an energy savings of between 20% and 500% depending on the network conditions.

In this paper we compare the energy consumption behavior of three versions of TCP --- Reno, Newreno, and SACK. The experiments were performed on a wireless testbed where we measured the energy consumed at the sender node. Our results indicate that, in most cases, using total energy consumed as the metric, SACK outperforms Newreno and Reno while Newreno performs better than Reno. The experiments emulated a large set of network conditions including variable round trip times, random loss, bursty loss, and packet reordering. We also estimated the idealized energy for each of the three implementations (i.e., we subtract out the energy consumed when the sender is idle) and here, surprisingly, we find that in many instances SACK performs poorly compared to the other two implementations. We conclude that if the mobile device has a very low idle power consumption then SACK is not the best implementation to use for bursty or random loss. On the other hand, if the idle power consumption is significant, then SACK is the best choice since it has the lowest overall energy consumption.

Transport connections set up in wireless ad hoc networks are plagued by problems such as high bit error rates, frequent route changes, and partitions. If we run the transmission control protocol (TCP) over such connections, the throughput of the connection is observed to be extremely poor because TCP treats lost or delayed acknowledgments as congestion. We present an approach where we implement a thin layer between Internet protocol and standard TCP that corrects these problems and maintains high end-to-end TCP throughput. We have implemented our protocol in FreeBSD, and we present results from extensive experimentation done in an ad hoc network. We show that our solution improves the TCP's throughput by a factor of 2-3.

This paper presents a radically new multicast model for ad hoc wireless networks. In this model, the content of the multicast data determines the receiver set for the data and the receiver set changes dynamically as the content of the multicast changes. Delivering the multicast to intended receivers while minimizing the message overhead and reducing battery consumption is a challenge made harder because of node mobility. We develop a novel multicast model that distributes this form of threat information in a message efficient manner. We present results from detailed simulations that demonstrate the efficiency of our protocol and discus the scalability of this model to larger networks.

Node mobility in ad hoc networks makes the problem of efficiently routing packets quite difficult. Several authors have developed routing protocols for these types of networks but the performance of these protocols suffers if the nodes in the network move at high speeds. We present a protocol that explicitly takes into account the mobility patterns of nodes to construct routes that last for long periods of time. We show that this protocol has the same routing overhead as other on-demand protocols while ensuring that the number of packets lost due to old or improper routes is minimized. The LLRP protocol is shown to be suited for small to medium sized networks (50-100 nodes) where the nodes move at high speeds and the time to link failure is small. We validate our protocol via numerous simulations and compare it against one other popular on-demand routing protocol.

We present a protocol for managing mobile wireless ad hoc networks. The protocol uses hierarchical clustering of nodes to reduce the number of messages exchanged between the manager and the agents (mobiles). Clustering also enables the network to keep track of mobiles as they roam. The ad hoc network management protocol (ANMP) is fully compatible with simple management protocol, version 3 (SNMPv3) and uses the same protocol data units (PDUs) for data collection. The protocol also implements sophisticated security mechanisms that can be fine-tuned to meet specific requirements. Finally, we have implemented the protocol along with a graphical user interface that allows a manager to change the view or specify management parameters on the fly.

In this paper we present a case for using new power-aware metrics for determining routes in wireless ad hoc networks. We present five different metrics based on battery power consumption at nodes. We show that using these metrics in a shortest-cost routing algorithm reduces the cost/packet of routing packets by 5-30% over shortest-hop routing (this cost reduction is on top of a 40-70% reduction in energy consumption obtained by using PAMAS, our MAClayer protocol) . Furthermore, using these new metrics ensures that the mean time to node failure is increased significantly. An interesting property of using shortest-cost routing is that packet delays do not increase. Finally,we note that our new metrics can be used in most traditional routing protocols for ad hoc networks. 1 Introduction AdHoc networks are multi-hop wireless networks where all nodes cooperatively maintain network connectivity. These types of networks are useful in any situation where temporary network connectivityisneede...

In this paper we develop a new multi-access protocol for multi-hop radio networks. The unique feature of our protocol is that it is energy conserving. Radios that are not actively transmitting or receiving a packet power themselves off. The manner in which nodes power themselves off does not influence the delay or throughput characteristics of our protocol. Simulation results indicate that power savings of between 10% and 70% are attainable in most systems.

We consider the problem of transport state handoff in mobile networks. The problem is motivated by the observation that an efficient way to implement transport connections, where one end is mobile, is to split these connections in two at a node close to the mobile. In such an environment we need to ensure that transport state can be handed off to a new intermediate node if the mobile roams to a location far away from the location where the connection(s) were opened. Our handoff protocols are efficient and we show that they work correctly despite any sequence of moves made by the mobile.

We report on an ongoing project to design and build the network and transport layers for mobile networking. The network architecture used is unique in that it separates the mobile network(s) from fixed networks and provides connectivity between the two via special gateways. These gateways provide QOS guarantees to mobile users for all their open connections. We provide summaries of all the protocols we are implementing (or have implemented) and discuss possible improvements.

In a distributed system an algorithm used to select a distinguished node or leader in the system is known as a leader election algorithm. Leader election algorithms are examined that attempt to locate the leader at a good node (from a performance standpoint) in the system. In the preference-based approaches examined, each node in the system uses locally available information to vote for the various candidates (potential leaders) on the basis of the performance level it would realize under each of them. The preference-based leader election algorithms proposed and examined are simple, and are shown to perform almost as well as a traditional optimization-based approach to leader election.>