Analytical model of quality of service scheduling for optical aggregation in data centers

Optical switching technologies represent a promising solution for data center interconnection networks to support the increasing bandwidth requirements of current cloud-based applications, while reducing interconnection complexity and energy consumption. Furthermore, the heterogeneity of intra- and inter-data center traffic characteristics requires some form of quality of service management. This paper describes modeling and design aspects of data center optical interconnections with particular emphasis on the aggregation level, where hybrid switching and packet scheduling are jointly applied to effectively implement service differentiation. Priority scheduling of three different service profiles is applied to maximize intra- and inter-data center traffic throughput, while guaranteeing time transparency for delay-sensitive services and zero loss/fixed delay for guaranteed connections. An analytical model is defined and validated to assess loss of real time and throughput of best effort traffic, in asynchronous packet context, when considering best effort traffic saturating the channels of the optical link. The model can also be used to dimension the optical output interface of the aggregation level switch.     

Scheduling Algorithms for a Slotted Packet Switch with either Fixed or Variable Length Packets

We address the problem of congestion resolution in optical packet switching (OPS). We consider a fairly generic all-optical packet switch architecture with a feedback optical buffer constituted of fiber delay lines (FDL). Two alternatives of switching granularity are addressed for a switch operating in a slotted transfer mode: switching at the slot level (i.e., fixed length packets of a single slot) or at the burst level (variable length packets that are integer multiples of the slot length). For both cases, we show that in spite of the limited queuing resources, acceptable performance in terms of packet loss can be achieved for reasonable hardware resources with an appropriate design of the time/wavelength scheduling algorithms. Depending on the switching units (slots or bursts), an adapted scheduling algorithm needs to be deployed to exploit the bandwidth and buffer resources most efficiently.     

Keeping the packet sequence in optical packet-switched networks

This paper deals with optical packet switches with limited buffer capabilities, subject to asynchronous, variable-length packets and connection-oriented operation. The focus is put on buffer scheduling policies and queuing performance evaluation. In particular a combined use of the wavelength and time domain is exploited in order to obtain contention resolution algorithms that guarantee the sequence preservation of packets belonging to the same connection. Four simple algorithms for strict and loose packet sequence preservation are proposed. Their performance is studied and compared with previously proposed algorithms. Simulation results suggest that by accepting some additional processing effort it is possible to guarantee very low packet loss probabilities while avoiding the out-of-sequence delivery.     

Immunoreactivity for bcl-2 protein in cutaneous lymphomas and lymphoid hyperplasias

BACKGROUND: In patients with unstable cervical spine injuries initially managed with in-line traction, there is some risk of loss of cervical alignment and of new neurologic deficit when the halo vest is applied. This report outlines a simple technique which facilitates halo application in these patients. METHODS: The back of the halo vest is "prepositioned" before traction is instituted. Once alignment is optimized, halo vest application is easily completed without lifting the patient or discontinuing cervical traction. RESULTS: The authors have used this technique in four patients without complication. CONCLUSIONS: In patients with unstable cervical spine injuries who require an initial period of in-line traction, halo vest application is made easier and safer by "prepositioning" the back of the vest.     

Survivable virtual topology mapping in IP-over-WDM networks using differential evolution optimization

In IP-over-wavelength division multiplexing networks, a virtual topology is placed over the physical topology of the optical network. Given that a simple link failure or a node failure on the physical topology can cause a significant loss of information, an important challenge is to make the routing of the virtual topology on to the physical topology survivable. This problem is known as survivable virtual topology mapping (SVTM) and is known to be an NP-complete problem. So far, this problem has been optimally solved for small instances by the application of integer linear programming and has been sub-optimally solved for more realistic instances by heuristic strategies such as ant colony optimization and genetic algorithms. In this paper, we introduce the application of differential evolution (DE) to solve the SVTM problem and enhancements based on DE are proposed as well. Three algorithms based on DE are developed. The enhanced variants have better convergence rate, get better quality of solutions and require few control parameters. We present the impact of these parameters on the system’s performance improvement. Algorithms are evaluated in different test bench optical networks, as NSFnet and USA, demonstrating that the enhanced DE algorithm overcomes the other two, for small instances. The three algorithms reach a 100  survivable mapping for small instances. The three algorithms also find positive survivable mappings and reduce the network wavelength links. Results show the effectiveness and efficiency of the proposed algorithms.     

Polymer-based symmetric electrochromic devices

The fact that conjugated polymers repeatedly undergo electrochemical doping/undoping processes, which are accompained by color changes, makes these materials very attractive, and much effort has been devoted to their use in advanced devices. There is renewed interest in electroactive polymers that reversibly undergo both p- and n-doping because of their potential application in symmetric electrochemical devices. We employed fused molecules, dithienothiophenes, as monomers to obtain polymers with a narrow band gap suitable for n- and p-doping. The performance results of two symmetric electrochromic devices having as electrodes both poly(dithieno[3,4-b:3',4'-d]thiophene) (pDTT1) and poly(dithieno[3,4-b:2',3'-d]thiophene) (pDTT3) are reported and discussed.     

The European IST project DAVID: a viable approach toward optical packet switching

In this paper, promising technologies and a network architecture are presented for future optical packet switched networks. The overall network concept is presented and the major choices are highlighted and compared with alternative solutions. Both long and shorter term approaches are considered, as well as both the wide-area network and multiple-area networks parts of the network. The results presented in this paper were developed in the frame of the research project DAVID (Data And Voice Integration over DWDM) project, funded by the European Commission through the IST-framework.     

Evaluation of Photopolymerization Kinetics by Means of Transmittance Measurements

Polymeric resins are widely used for dental reconstruction, and most resins use camphorquinone as activator of the polymerization reaction, through the absorption of light at a defined wavelength range (from 400 nm to 460 nm). During the photopolymerization curing, transparency of these resins changes and transmittance variation can be detected by photodiode and bolometer measurements. This change can be used as an index of the reaction rate, and the kinetic parameter k (reaction rate) can be evaluated from transmittance data by means of nonlinear regression. The relation between k and the light intensity impinging on the resin sample can thus be obtained. In the present work, tests were carried out using the resin Enamel Plus HFO GE2. Results reveal the presence of two different polymerization reactions at two different intensity ranges. The obtained k values were used to predict the most suited curing times for different light intensities. The proposed methodology can be applied to different dental reconstruction materials, provided that the material is partially transparent and that its transparency changes during the polymerization reaction.     

Optical switch emulation in programmable software router testbed

A programmable optical router is a key enabler for dynamic service provisioning in Future Internet scenarios. It is equipped with optical switching hardware to forward information at hundreds of Gigabits/s rates and above, controlled and managed through modular and flexible procedures according to emerging standards. The possibility to test such costly optical architectures in terms of logical and physical performance, without implementing complex and expensive testbeds, is crucial to speed-up the development process of high-performance routers. To this purpose, this paper introduces the software-based emulation testbed of a programmable optical router, which is here developed and applied to test optical switching fabrics. Accurate characterization of the optical devices and physical layer aspects is implemented with the Click software router environment. Power loss and optical signal-to-noise-ratio evaluation are provided through accurate software representation of the physical characteristics of the optical devices employed. The scalability of the proposed emulation testbed is also assessed on standard PC hardware. All the obtained results prove the effectiveness of the proposed tool to emulate an optical router at different levels of granularity.