Testbed Measurements Research Articles

A modular framework for implementing joint wireless network coding and scheduling algorithms

Abstract Intersession network coding (NC) can provide significant performance benefits via mixing packets at wireless routers; these benefits are especially pronounced when NC is applied in conjunction with intelligent scheduling. NC, however, imposes certain processing operations, such as encoding, decoding, copying, and storage. When not utilized carefully, all these operations can induce tremendous processing overheads in practical settings. Our testbed measurements suggest that such processing operations can severely degrade the router throughput, especially at high bit rates. Motivated by this, we design network coding framework for rate adaptive wireless links (NCRAWL). The design of NCRAWL facilitates low overhead NC operations, thereby effectively approaching the theoretically expected throughput benefits of joint NC and scheduling. We implement and evaluate NCRAWL on a wireless testbed. Our experiments demonstrate that NCRAWL meets the theoretical predicted throughput gain while requiring much less CPU processing, compared to related frameworks.

RTC-GWLAN: A Real Time Characterization of Generic IEEE 802.11 for High Density Services

This research investigates and analysed generic Wireless Local Area Networks (IEEE 802.11a/b/g) in the context of their Quality of Service (QoS) in high density networks using some generic Key Performance Indicators (KPI) such as throughput (Bytes/Sec), Received Signal Strength Indicator (RSSI), and latency response (secs). The study focused on University of Nigeria Nsukka (UNN) which houses several hostels and faculties. Using Applied Research Methodology (ARM), this work characterized a real life generic hotspot testbed (UNN) for Received Signal Strength (RSS), mobile node distance, latency and throughput using Tranzio-Wavion Minipop Infrastructure in UNN as the network testbed. In this study, after sampling the selected KPIs, data were collected via our testbed measurements from the test nodes, Network Switching system/Network Operating Centres (NOCs) and their values were evaluated for an assessment study. The KPI results obtained showed that the traditional generic WLAN will scarcely scale in high density traffic environment in terms of scalability, speed performance (network convergence), cost, and other selected KPIs. This will lead to a proposal on a conceptual framework for Cognitive High Density Hotspot (CHDN) in our future work, which will address the limitations of generic WLANs for enterprise computing.

Comparing wireless flooding protocols using trace-based simulations

Most wireless multi-hop networks, such as ad hoc networks and wireless sensor networks, need network-wide broadcasting, which is best done with a flooding protocol. In this article, we use packet trace information from a real test-bed network to define a simulator for flooding protocol performance studies. Five protocols are compared using the simulator. Trace-based simulations promise to have the benefits of the simulator, such as reducing required work effort and repeatability but still produce results close to the real test-bed or deployment. We propose and evaluate different approaches on how to use collected trace data and how to tune the parameters to achieve the best possible accuracy in comparison with actual test-bed measurements. We study the resulting accuracy of the model so that performance studies know with what confidence a certain conclusion can be made. Using the new trace-based model and knowing its accuracy, we compare the five flooding protocols to gain additional insights into their performance. Finally, by modifying the trace data, we study how real-world effects, such as links with in-between qualities and asymmetric links, influence the different flooding protocols.

Data Timed Sending (DTS) Energy Efficient Protocol for Wireless Sensor Networks: Simulation and Testbed Verification

The Data Timed Sending (DTS) protocol contributes to the energy savings in Wireless Sensor Networks (WSNs) and prolongs the sensor nodes’ battery lifetime. DTS saves energy by transmitting short packets, without data payload, from the sensor nodes to the base station or the cluster head according to the Time Division Multiple Access (TDMA) scheduling. Placing the short packets into appropriate slots and subslots in the TDMA frames transfers the information about the measured values and node identity. This paper presents the proof of concept of the proposed DTS protocol and provides verification of the energy savings using the QualNet®; communication simulation platform (QualNet) and the SunTM Small Programmable Object Technology (Sun SPOT) testbed platform (for single hop and multi hop scenarios). The simulations and the testbed measurements confirm that the DTS protocol can provide energy savings up to 30% when compared with the IEEE 802.15.4 standard in unslotted Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) mode at 2.4 GHz frequency band.

Backpressure scheduling in IEEE 802.11 wireless mesh networks: Gap between theory and practice

Achieving efficient bandwidth utilization in wireless networks requires solving two important problems: (1) which packets to send (i.e., packet scheduling) and (2) which links to concurrently activate (i.e., link scheduling). To address these scheduling problems, many algorithms have been proposed and their throughput optimality and stability are proven in theory. One of the most well-known scheduling algorithms is backpressure scheduling which performs both link and packet scheduling assuming a TDMA (Time Division Multiple Access) MAC (Medium Access Control) layer. However, there has been limited work on realizing backpressure scheduling with a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) MAC layer (e.g., IEEE 802.11). In IEEE 802.11 networks, it is expected that the throughput optimality will not be achieved. In this paper, we investigate the extent of this throughput gap between theoretical TDMA-based backpressure scheduling and an approximation of it for IEEE 802.11 WMNs (Wireless Mesh Networks). Through extensive testbed measurements, we verify that there is indeed a non-negligible throughput gap. We present two main reasons behind this gap: Control inaccuracy that results from approximation of link scheduling and information inaccuracy that results from late or incorrect information, for instance, about queue lengths or network topology. Our results show that losses by MAC-layer collisions and backoff, which mainly occur due to control inaccuracy plays a major role for the throughput gap. On the other hand, while losses by queue drops, typically due to information inaccuracy, do occur, their effect can be tolerated. Nevertheless, both types of inaccuracies need to be mitigated in order to improve throughput.

Optimising the design of a piston-ring pack using DoE methods

This paper shows how design of experiments can be used with a ring-pack simulation program to optimise the design of a piston-ring assembly. Ten factors are varied—six describing the ring profile, three ring tensions, and the lubricant viscosity. Statistical analysis shows that there are some significant interactions between some of the factors—an issue that should be considered when performing test-bed measurements on engines. It is shown that an improved design can be achieved that reduces ring losses by 57% whilst reducing upward oil flow by 39%. This could lead to a 7% improvement in fuel economy provided there are no deleterious effects in other parts of the engine.

Comparison of average particle number emission factors for heavy and light duty vehicles derived from rolling chassis dynamometer and field studies

Fleet aggregate emission factors for particle number for light duty vehicles and heavy duty vehicles are available from a number of field studies by calculation from airborne concentrations. Data are also available from laboratory chassis dynamometer studies in which particle number emissions from engines of a range of capacity and technologies have been measured. In this study the data from the PARTICULATES project database of laboratory test bed measurements have been aggregated to estimate fleet average particle number emission factors for light duty and heavy duty vehicles which have then been compared with the results of field measurement studies. Measurements carried out by [Jones, A.M., Harrison, R.M., 2006. Estimation of the emission factors of particle number and mass fractions from traffic at a site where mean vehicle speeds vary over short distances. Atmospheric Environment 40, 7125–7137] at the Marylebone Road London, represented an ‘urban’ fleet and were estimated by the aggregated laboratory measured emission factors within one or two standard deviations of their values. For light and heavy duty vehicles travelling with an average speed of 39.5 km h −1 the NO x emission factors were estimated to be 0.62 and 5.88 g veh −1 km −1, respectively, and the PM 2.5 emission factors were estimated to be, respectively, 0.017 and 0.17 g veh −1 km −1. The total particle number emission factors (including volatiles) were estimated for light and heavy duty vehicles travelling according ‘urban’ Common ARTEMIS Driving Cycle (CADC) to be 6.31 × 10 13 and 70.6 × 10 13 num veh −1 km −1 and these were both within one standard deviation of the values measured using the field data. Similarly, the fractionated solid particle number emission factors (excluding volatiles), measured for the aerodynamic diameter ranges, 30–50 nm, 50–100 nm and 100–1000 nm, were estimated to within one standard deviation of the equivalent measured emission factors for the light duty vehicles and to within two standard deviations of the equivalent measured emission factors for the heavy duty vehicles. Extending this work to estimate the total number emission factors measured by other research groups, conditions representing both the ‘rural’ and ‘highway’ Common ARTEMIS Driving Cycles were considered. For the light duty vehicles, a general increase in the calculated value of the total number emission factor was observed with increased fleet speed and the observed scatter was accounted for by the split in the fleet according to the vehicle's fuel, whether gasoline or diesel. The calculated values of the total number emission factors for the heavy duty vehicles also show an increase with speed after 60 km h −1. High values are observed at 20 km h −1 suggesting an underlying inverted binomial trend similar to that observed for gaseous emission factors and arguably supported by the field measured values. The scatter of the calculated points about this trend was accounted for by the distribution of the fleet across the vehicle weight and emission legislation categories. With the exception of three of the field research studies considered – where the measured emission factors were significantly underestimated by the aggregated laboratory emission factors – there was a general agreement in the order of magnitude between the calculated and field emission factors, but not to the high degree shown for the Marylebone Road data. Furthermore, dilution temperature, fuel sulphur content and tunnel deposition effects did not individually account for the observed discrepancies in this work and point to the need for correction factors derived from these parameters together with additional information such as dilution ratio in giving a more complete description.

Service Middleware for Self-Managing Large-Scale Systems

Resource management poses particular challenges in large-scale systems, such as server clusters that simultaneously process requests from a large number of clients. A resource management scheme for such systems must scale both in the in the number of cluster nodes and the number of applications the cluster supports. Current solutions do not exhibit both of these properties at the same time. Many are centralized, which limits their scalability in terms of the number of nodes, or they are decentralized but rely on replicated directories, which also reduces their ability to scale. In this paper, we propose novel solutions to request routing and application placement- two key mechanisms in a scalable resource management scheme. Our solution to request routing is based on selective update propagation, which ensures that the control load on a cluster node is independent of the system size. Application placement is approached in a decentralized manner, by using a distributed algorithm that maximizes resource utilization and allows for service differentiation under overload. The paper demonstrates how the above solutions can be integrated into an overall design for a peer-to-peer management middleware that exhibits properties of self-organization. Through complexity analysis and simulation, we show to which extent the system design is scalable. We have built a prototype using accepted technologies and have evaluated it using a standard benchmark. The testbed measurements show that the implementation, within the parameter range tested, operates efficiently, quickly adapts to a changing environment and allows for effective service differentiation by a system administrator.

Performance effect in optical-communication systems caused by phase ripples of dispersive components.

We develop a simple theory that relates the performance of an optical component to the variance of the amplitude of its phase ripple that has been weighted by the incoming signal spectrum. Comparisons of model predictions for chirped fiber gratings with 10-Gbit/s test-bed simulations and measurements agree.

Design, Analysis, and Implementation of DVSR: A Fair High-Performance Protocol for Packet Rings

The Resilient Packet Ring (RPR) IEEE 802.17 standard is a new technology for high-speed backbone metropolitan area networks. A key performance objective of RPR is to simultaneously achieve high utilization, spatial reuse, and fairness, an objective not achieved by current technologies such as SONET and Gigabit Ethernet nor by legacy ring technologies such as FDDI. The core technical challenge for RPR is the design of a bandwidth allocation algorithm that dynamically achieves these three properties. The difficulty is in the distributed nature of the problem, that upstream ring nodes must inject traffic at a rate according to congestion and fairness criteria downstream. Unfortunately, we show that under unbalanced and constant-rate traffic inputs, the RPR fairness algorithm suffers from severe and permanent oscillations spanning nearly the entire range of the link capacity. Such oscillations hinder spatial reuse, decrease throughput, and increase delay jitter. In this paper, we introduce a new dynamic bandwidth allocation algorithm called Distributed Virtual-time Scheduling in Rings (DVSR). The key idea is for nodes to compute a simple lower bound of temporally and spatially aggregated virtual time using per-ingress counters of packet (byte) arrivals. We show that with this information propagated along the ring, each node can remotely approximate the ideal fair rate for its own traffic at each downstream link. Hence, DVSR flows rapidly converge to their ring-wide fair rates while maximizing spatial reuse. To evaluate DVSR, we develop an idealized fairness reference model and bound the deviation in service between DVSR and the reference model, thereby bounding the unfairness. With simulations, we find that compared to current techniques, DVSR's convergence times are an order of magnitude faster (e.g., 2 versus 50 ms), oscillations are mitigated (e.g., ranges of 0.1% versus up to 100%), and nearly complete spatial reuse is achieved (e.g., 0.1% throughput loss versus 33%). Finally, we provide a proof-of-concept implementation of DVSR on a 1 Gb/s network processor testbed and report the results of testbed measurements.

Phenomenological combustion modeling of a direct injection diesel engine with in-cylinder flow effects

A cycle simulation program is developed and its predictions are compared with the test bed measurements of a direct injection (DI) diesel engine. It is based on the mass and energy conservation equations with phenomenological models for diesel combustion. Two modeling approaches for combustion have been tested; a multi-zone model by Hiroyasu et al (1976) and the other one coupled with an in-cylinder flow model. The results of the two combustion models are compared with the measured imep, pressure trace and NOx and soot emissions over a range of the engine loads and speeds. A parametric study is performed for the fuel injection timing and pressure, the swirl ratio, and the squish area. The calculation results agree with the measured data, and with intuitive understanding of the general operating characteristics of a DI diesel engine.

Validation of an Integrated Modeling Methodology's Closed-Loop Performance Prediction Capability

This paper validates the integrated modeling methodology used for design and performance evaluation of complex optomechanicalsystems, particularly spaceborne interferometers. Themethodologyintegrates structural modeling, optical modeling, and control system modeling and design into a common environment, the Integrated Modeling of Optical Systems software package. The validation uses the Micro-Precision Interferometer testbed, a ground-based, full-scale hardware model of a spaceborne interferometer. Parallel development of the testbed and a corresponding integrated model enabled a unique opportunity to validate the modeling methodology with actual testbed measurements. This paper assesses the ability of the integrated modeling methodology to predict performance in a closed-loop conŽ guration, namely with high-bandwidth optical control loops operational. The assessment is a comparison of integrated model closed-loop predictions with testbed closed-loop measurements, indicating that the integrated modeling methodology is accurate to within a factor of two.

Influence of the armature-rail geometry on results of launch experiments of fibre armatures with railguns

To increase the transition velocity of solid fibre brush armatures, launch experiments have been performed with an increased armature height of about 15%. However, results of experiments under equal launch conditions did not show higher transition velocities up till now. In order to find an explanation for the disappointing results, a number of 3D-computer simulations has been performed to calculate the current density distribution and Lorentz force in the forward direction in monolithic solid armatures for different armature-rail height ratios. It appears the Lorentz force is influenced by relatively small changes in the armature-rail geometry. The results of the 3D-computer simulations of two armature-rail combinations of different geometry have been verified with (quasi) static experiments in the armature test bed and launch experiments with segmented copper fibre armatures in the 2.4 m long, 20 mm square bore rail accelerator. The results of computer simulations and armature test bed measurements are presented. Furthermore, results of the launch experiments for two different rail heights are discussed.

Design and Deployment of Optically Amplified Undersea Systems

Applying optical amplifier technology to undersea cable systems using repeaters has occurred in a remarkably short time. Exploratory development of this technology at AT&T-Submarine Systems, Inc. (AT&T-SSI) began in 1989, and the first two commercial systems were installed in the Caribbean Sea early in 1994. Among the steps taken to support this aggressive development and deployment schedule were construction and performance evaluation of optical system “testbeds.” Each testbed consists of an optical transmitter and receiver connected by thousands of kilometers of glass fiber and optical amplifiers configured as they would be in an installed system. This paper reports on the results of testbed measurements, and how they affected network system design. It also describes performance measures made during installation and commissioning of the first Caribbean system, to confirm design expectations, and what was learned, from special “first-system” testing, about the limits of this technology.