In-service Performance Monitoring Research Articles

Damage Assessment in Composite Beam Using Infrared Thermography, Optical Sensors, and Terahertz Technique

Composite materials find wide range of applications due to their high strength-to-weight ratio. Due to this increasing dependence on composite materials, there is a need to study their mechanical behavior in case of damage. There are several extended nondestructive testing (ENDT) and structural health-monitoring (SHM) methods for the assessment of the mechanical properties each with their set of advantages and disadvantages. This paper presents a comparative study of three distinct damage detection methods (infrared thermography (IRT), neutral axis (NA) method based on optical strain sensor measurements, and terahertz spectroscopy) for the detection of delamination and temperature-induced damage in a simple glass fiber reinforced polymer (GFRP) beamlike structure. The terahertz spectroscopy is a specialized technique suitable for detecting deterioration inside the structure but has limited application for in-service performance monitoring. Similarly, the IRT technique in the active domain may be used for in situ monitoring but not in in-service assessment. Both methods allow the visualization of the internal structure and hence allow identification of the type and the extent of damage. Fiber optic sensors (especially fiber Bragg grating (FBG)) due to their small diameter and no need of calibration can be permanently integrated within the sample and applied for continuous dynamic strain measurements. The measured strain is treated as an input for neutral axis (NA) method, which as a damage-sensitive feature may be used for in-service monitoring but gives absolutely no information about the type and extent of damage. The results for damage detection based on proposed comparative studies give a complete description of the analyzed structure.

The INIM system: in-service non-intrusive monitoring for QoS-enabled transparent WDM

This paper presents the design and experimental implementation of an in-service performance monitoring system that uses a combination of non-intrusive spectral optical performance monitoring and Internet protocol (IP) metrics and is based on a low-complexity, distributed architecture. Apart from guaranteeing QoS in real time (less than 1 s) in an IP/wavelength-division-multiplexing (WDM) dynamic network, the system provides link-state information for impairment-aware routing and wavelength assignment. Another novel aspect is the nature of the service level agreements proposed, which are "all-optical" (transparent). Performance delays of the system are evaluated in a real test bed featuring dense WDM and transparent optical add-drop multiplexers. Strategies to estimate link-state parameters from real-time monitoring information are also proposed

Performance of a new bus assignment algorithm for an ATM switch fabric

The measurement of the quality of service in ATM* networks involves the activation of the ATM layer management OAM functions. Once an ATM connection is set up, in-service performance monitoring based on OAM cells can be used to measure and estimate the ATM layer QoS parameters. In-service measurement of ATM layer QoS parameters such as cell delay variation, cell transfer delay, cell loss ratio, etc., is necessary to verify on one hand, if the network is meeting the requested QoS and on the other hand, if the users are receiving their promised QoS. Congestion conditions in ATM switches may cause gradual deteriorations in the quality of virtual paths or virtual channel connections in an ATM network. QoS measurement using OAM cells can accurately measure the overall quality of monitored ATM layer QoS parameters in order to detect such deteriorations before the service quality has dropped bellow an acceptable level. Several factors such as network load, congestion, bottlenecks, and incremenTS in network load may affect the cell delay variation of video transmissions in an ATM network. In this work, several ATM layer QoS parameters have been measured on a VBR video source using OAM cells. The simulation resulTS obtained show that with low network loads, the contribution of each individual ATM switch to the local and to the end-to-end CDV is not significant. In the presence of a bottleneck switch, the CDV presented by this switch becomes the dominant one at both local and end-to-end sides. When the network load is between 0.5 and 0.6, small incremenTS in network load do not alter the buffer performance of the non-congested ATM switches, and low values of CDV can be obtained. In contrast, it was found that there is a critical load between 0.65 and 0.7 where small incremenTS in network load may force the CDV to grow abruptly.

In-service monitoring of quality of service in ATM networks using OAM cells

Congestion conditions in ATM switches may cause gradual deterioration in the QoS. QoS monitoring, using OAM, can accurately measure the overall quality of monitored parameters to detect such deterioration before the service quality has dropped below an acceptable level. Several factors call affect the accuracy of the in-service QoS performance monitoring technique, such as the burstiness of the background traffic, network load, monitoring frequency, and bit rate. However, acceptable levels of accuracy can be achieved using OAM cells. Because of the fluctuations presented by VBR video, OAM cells can measure their characteristics more accurately than CBR video. Also, for low bursty background traffic, the cell delay variation (CDV) of VBR is higher than CBR. As the burstiness of the background traffic increases, the CDV of CBR grows faster than that of VBR. At high burstiness, the CDV of CBR is higher than VBR. For all background data burstiness, the first multiplexer of the network absorbs most of the CDV.

Combined framing and error correction coding for the DS3 signal format

The paper proposes and analyzes a combined framing and control scheme for the DS3 format in which the primary framing bits are replaced with parity check bits of a rate 0.9912 extended and shortened Hamming code. A fast syndrome computation scheme is developed for the code to facilitate faster reframe times. The proposed scheme has better overall reframing performance than conventional framing, and reduces the output BER to approximately 1360P/sub e//sup 2/. This is effective against error-rate floors or dribble error phenomena in fiber optical transmission. Primary applications are in advanced in-service performance monitoring and for throughput enhancement in ATM networking applications using leased DS3 carrier signals. >

Simple method of multipath delay difference detection for π/4-shift QPSK

A simple method for detection of multipath delay difference in a digital mobile communication channel employing π/4-shift QPSK as the modulation scheme is proposed. The results show that this method is useful for inservice performance monitoring of such a communication channel.

Digital Data System: Testing and Maintenance

Reliability and maintainability are important aspects of the service objectives for the Digital Data System. Consequently, maintenance planning was an essential element in the DDS development. Maintenance features provided by the system include in-service performance monitoring, protection switching, comprehensive alarms, and the means for rapid fault isolation and repair.

Improving DCPSK Transmission by Means of Error Control

In this paper, the output of a differentially coherent detector using the phase of the k th previous bit as a reference is derived under noise free conditions for binary differential phase shift keyed signals, where k = 1, 2, 3 ... or ally integer. This is then related to the output of a conventional differentially coherent detector ( k = 1 ). In the presence of noise, this output relationship changes depending on which detector has made an error in detection. This property can be used to reduce the error rate for differentially coherent systems. A receiver consisting of a first-order (conventional) and second-order detectors, where the references are the phase of the previous and second previous bits, respectively, is analyzed theoretically. The result shows that the amount of improvement in error rate due to the addition of the second-order detector depends on the percentage of single errors in the conventional detector. If the error rate of a conventional differentially coherent system is close to the optimum, the theoretical improvement using the additional detector is small (≃ 0.2 dB). However, it is shown experimentally that if the original performance is poor due to intersymbol interference, then considerable improvement is possible with an additional detector (≃ 1.6 dB). In addition to error rate improvement, the circuit also provides error rate information that can be used for in-service performance monitoring and as a sensor for automatic equalizers.

A New Class of Selected Ternary Pulse Transmission Plans for Digital Transmission Lines

Digital transmission lines consisting of cable media and regularly spaced self-timed reconstructive repeaters are being considered for the transmission of binary digital information, such as arbitrary combinations of digitized speech, television, and data. For transmission over the line, the binary information is coded into a sequence of symbols. The transmission code is referred to as the pulse transmission plan. Among the significant practical problems which must be considered in choosing the pulse transmission plan are: 1) minimization of performance dependence on source statistics, 2) repeater timing jitter performance, 3) dc wander of the pulse stream, and 4) in-service performance monitoring. A new class of selected ternary pulse transmission plans has been devised to combat these problems. Emphasis in this paper is placed upon a member of this class, paired selected ternary (PST), in which the binary sequence to be transmitted is framed into pairs and translated into appropriately selected pairs of three level pulses.