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INDICE PATRON DEL ÉXITO 4 LOS OCHO PASOS 5 PASO 1 DEFINA SU SUEÑO 6 PASO II HAGA ALGUNOS COMPROMI... more INDICE PATRON DEL ÉXITO 4 LOS OCHO PASOS 5 PASO 1 DEFINA SU SUEÑO 6 PASO II HAGA ALGUNOS COMPROMISOS 8 CICLO DEL ÉXITO 10 PASO III ESCRIBA SU LISTA DE CANDIDATOS 11 PASO IV INVITE A SUS CANDIDATOS 12 CUADRO PARA HACER CONTACTO E INVITAR 15 PASO V TENGA UNA REUNION EXITOSA 16 PASO VI SEGUIMIENTO 18 PASO VII REVISE SU PROGRESO 19 PASO VIII ENSEÑE EL PATRON DEL ÉXITO 21 EL SECRETO PARA DESARROLLAR UN NEGOCIO SOLIDO 22 PROGRAMADOR MENSUAL 23 3 AÑO 2009

General Relativity and Gravitation, 1998

The canonical quantization of diffeomorphism invariant theories of connections in terms of loop v... more The canonical quantization of diffeomorphism invariant theories of connections in terms of loop variables is revisited. Such theories include general relativity described in terms of Ashtekar-Barbero variables and extension to Yang-Mills fields (with or without fermions) coupled to gravity. It is argued that the operators induced by classical diffeomorphism invariant or covariant functions are respectively invariant or covariant under a suitable completion of the diffeomorphism group. The canonical quantization in terms of loop variables described here, yields a representation of the algebra of observables in a separable Hilbert space. Furthermore, the resulting quantum theory is equivalent to a model for diffeomorphism invariant gauge theories which replaces space with a manifestly combinatorial object.

Annals of The New York Academy of Sciences, 2001

The repeated, intermittent use of cocaine and other drugs of abuse produces profound and often lo... more The repeated, intermittent use of cocaine and other drugs of abuse produces profound and often long-lasting alterations in behavior and brain chemistry. It has been suggested that these consequences of drug use play a critical role in drug craving and relapse to addiction. This article reviews the effects of psychostimulant administration on dopaminergic and excitatory amino acid neurotransmission in brain regions comprising the brain's motive circuit and provides evidence that the activation of endogenous κ κ κ κ-opioid receptor systems in these regions opposes the behavioral and neurochemical consequences of repeated drug use. The role of this opioid system in mediating alterations in mood and affect that occur during abstinence from repeated psychostimulant use are also discussed.

This paper reports results of a performance comparison for four optical ring network architecture... more This paper reports results of a performance comparison for four optical ring network architectures envisaged for future metropolitan area networks (MANs), with particular emphasis on the design of and compatibility to possible 100-Gigabit-Metro-Ethernet (100GbME) standards. Both analytical and numerical modelling techniques were applied to quantify and compare network performance for all architectures in terms of achievable throughput, delay and the number of required wavelengths. Non-uniform traffic required additional resources and dynamic adaptation of the slotted ring architecture. The study also considered aspects of the physical transmission and interfaces (PHY) for consideration in 100GbME standards.

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Journal of Optical Networking, 2004

We describe results of the scalability analysis for dynamic wavelength-routed optical networks wi... more We describe results of the scalability analysis for dynamic wavelength-routed optical networks with end-to-end lightpath assignment and central network control with electronic scheduling and processing of lightpath requests. We investigate the effect of the algorithm complexity in both the scheduling and the dynamic routing and wavelength assignment (DRWA) of lightpath requests. Scheduling theory and static performance-prediction techniques were applied to define the bounds on the electronic processing time of requests, and hence the maximum number of nodes supported by a centralized dynamic optical network for given blocking probability, latency, and network diameter. Scalability analysis results show that medium-sized centralized networks (∼50 nodes) can be supported when these networks are reconfigured on a burst-by-burst basis. In addition, we found that real topologies showed a complex trade-off between the request processing time, blocking probability, and resource requirements. These findings can be used to determine the optimum combination of scheduling/DRWA algorithm, showing that the fastest DRWA algorithm does not necessarily lead to the minimum blocking probability and maximum scalability but that a careful consideration of both blocking and processing speed is required. The results are applicable both to dynamic network architectures with centralized request processing such as wavelength-routed optical networks and to the design of advanced optical switching matrices and routers. not disrupted, as the control of the network is transferred to the backup node. For an example of a real long-haul centralized optical network utilizing control node redundancy (see Ref.

This paper reports results of a performance comparison for four optical ring network architecture... more This paper reports results of a performance comparison for four optical ring network architectures envisaged for future metropolitan area networks (MANs), with particular emphasis on the design of and compatibility to possible 100-Gigabit-Metro-Ethernet (100GbME) standards. Both analytical and numerical modelling techniques were applied to quantify and compare network performance for all architectures in terms of achievable throughput, delay and the number of required wavelengths. Non-uniform traffic required additional resources and dynamic adaptation of the slotted ring architecture. The study also considered aspects of the physical transmission and interfaces (PHY) for consideration in 100GbME standards.

Bt Technology Journal, 2006

New dynamic optical switching technologies and adaptive transmission techniques will allow increa... more New dynamic optical switching technologies and adaptive transmission techniques will allow increased capacity and flexibility in future core and metro networks. The Optical Networks Group at UCL has carried out research focused on these areas, in collaboration with industry and other university research groups, for over a decade. In this paper, the technical challenges to be overcome in implementing dynamic, high-capacity networks are discussed, and recent results of the group’s work are presented. New network architectures based on wavelength-routing and optical burst switching have been proposed and analysed, and the experimental demonstration of burst switching using fast wavelength-tunable lasers is described. High-capacity dense wavelength division multiplexed transmission systems, using advanced regeneration and equalisation techniques, have been studied, both theoretically, and experimentally using a dynamically reconfigurable recirculating fibre loop. Research results on advanced optical signal formats, all-optical 3R regeneration at 40 Gbit/s channel rates, adaptive analogue and digital electronic processing techniques and optical finite impulse response equalisation are presented. Finally, the development of optical performance monitoring technology, a key component of future all-optical networks, is described. The paper concludes with a prediction of future trends in optical communications research.

Analytic equations are developed to quantify the maximum network size supported by a centralized ... more Analytic equations are developed to quantify the maximum network size supported by a centralized dynamic optical network architecture. Comparison with static networks establishes the maximum traffic load for which wavelength resources reduction is achievable.

It is widely believed that dynamic operation in wavelength-routed optical networks could overcome... more It is widely believed that dynamic operation in wavelength-routed optical networks could overcome the inefficiencies of static allocation in wavelength usage. In this paper this hypothesis is reviewed by quantifying the wavelength requirements in dynamic wavelength-routed optical networks and the comparison of these to those of the static approach. To do so, new analytical and heuristic lower bounds for the wavelength requirements in dynamic networks are proposed. They are used to evaluate the optimality of existing algorithms whose wavelength requirements are evaluated by means of simulation. Results show that dynamic wavelength-routed optical networks can save wavelengths only at low traffic loads (

The request scheduler carrying out the QoS provisioning with service differentiation was analysed... more The request scheduler carrying out the QoS provisioning with service differentiation was analysed for the wavelength-routed OBS network architecture. QoS performance for two traffic classes was analysed in terms of the burst blocking probability and end-to-end delays and as a function of the constraint of core wavelength over-provisioning. The results show that the WROBS is capable of a very efficient QoS performance, whilst significantly minimising the core wavelength requirement with regards to the statically-routed networks. It is demonstrated that the QoS provisioning for the premium traffic does not trade off the blocking probability of the best-effort traffic. Moreover, even with no core wavelength over-provisioning in WROBS, the desired blocking probability can be achieved for both premium and best-effort traffic whilst the delay bounds for both classes are not exceeded. The reported results are directly applicable for the provisioning of the guaranteed IP services and for RWA optimisation.

This work compares the maximum delay in five different burst assembly schemes. The mean packet de... more This work compares the maximum delay in five different burst assembly schemes. The mean packet delay is analytically derived and evaluated by simulation for Poisson and non-Poisson input traffic. The results allow to define an optimum burst assembly scheme in terms of the introduced delay.

IEEE/OSA Journal of Lightwave Technology, 2004

This paper investigates the challenges for developing the current local area network (LAN)-based ... more This paper investigates the challenges for developing the current local area network (LAN)-based Ethernet protocol into a technology for future network architectures that is capable of satisfying dynamic traffic demands with hard service guarantees using high-bit-rate channels (80. . .100 Gb/s). The objective is to combine high-speed optical transmission and physical interfaces (PHY) with a medium access control (MAC) protocol, designed to meet the service guarantees in future metropolitan-area networks (MANs). Ethernet is an ideal candidate for the extension into the MAN as it allows seamless compatibility with the majority of existing LANs. The proposed extension of the MAC protocol focuses on backward compatibility as well as on the exploitation of the wavelength domain for routing of variable traffic demands. The high bit rates envisaged will easily exhaust the capacity of a single optical fiber in the band and will require network algorithms optimizing the reuse of wavelength resources. To investigate this, four different static and dynamic optical architectures were studied that potentially offer advantages over current link-based designs. Both analytical and numerical modeling techniques were applied to quantify and compare the network performance for all architectures in terms of achievable throughput, delay, and the number of required wavelengths and to investigate the impact of nonuniform traffic demands. The results show that significant resource savings can be achieved by using end-to-end dynamic lightpath allocation, but at the expense of high delay.

INDICE PATRON DEL ÉXITO 4 LOS OCHO PASOS 5 PASO 1 DEFINA SU SUEÑO 6 PASO II HAGA ALGUNOS COMPROMI... more INDICE PATRON DEL ÉXITO 4 LOS OCHO PASOS 5 PASO 1 DEFINA SU SUEÑO 6 PASO II HAGA ALGUNOS COMPROMISOS 8 CICLO DEL ÉXITO 10 PASO III ESCRIBA SU LISTA DE CANDIDATOS 11 PASO IV INVITE A SUS CANDIDATOS 12 CUADRO PARA HACER CONTACTO E INVITAR 15 PASO V TENGA UNA REUNION EXITOSA 16 PASO VI SEGUIMIENTO 18 PASO VII REVISE SU PROGRESO 19 PASO VIII ENSEÑE EL PATRON DEL ÉXITO 21 EL SECRETO PARA DESARROLLAR UN NEGOCIO SOLIDO 22 PROGRAMADOR MENSUAL 23 3 AÑO 2009

General Relativity and Gravitation, 1998

The canonical quantization of diffeomorphism invariant theories of connections in terms of loop v... more The canonical quantization of diffeomorphism invariant theories of connections in terms of loop variables is revisited. Such theories include general relativity described in terms of Ashtekar-Barbero variables and extension to Yang-Mills fields (with or without fermions) coupled to gravity. It is argued that the operators induced by classical diffeomorphism invariant or covariant functions are respectively invariant or covariant under a suitable completion of the diffeomorphism group. The canonical quantization in terms of loop variables described here, yields a representation of the algebra of observables in a separable Hilbert space. Furthermore, the resulting quantum theory is equivalent to a model for diffeomorphism invariant gauge theories which replaces space with a manifestly combinatorial object.

Annals of The New York Academy of Sciences, 2001

The repeated, intermittent use of cocaine and other drugs of abuse produces profound and often lo... more The repeated, intermittent use of cocaine and other drugs of abuse produces profound and often long-lasting alterations in behavior and brain chemistry. It has been suggested that these consequences of drug use play a critical role in drug craving and relapse to addiction. This article reviews the effects of psychostimulant administration on dopaminergic and excitatory amino acid neurotransmission in brain regions comprising the brain's motive circuit and provides evidence that the activation of endogenous κ κ κ κ-opioid receptor systems in these regions opposes the behavioral and neurochemical consequences of repeated drug use. The role of this opioid system in mediating alterations in mood and affect that occur during abstinence from repeated psychostimulant use are also discussed.

This paper reports results of a performance comparison for four optical ring network architecture... more This paper reports results of a performance comparison for four optical ring network architectures envisaged for future metropolitan area networks (MANs), with particular emphasis on the design of and compatibility to possible 100-Gigabit-Metro-Ethernet (100GbME) standards. Both analytical and numerical modelling techniques were applied to quantify and compare network performance for all architectures in terms of achievable throughput, delay and the number of required wavelengths. Non-uniform traffic required additional resources and dynamic adaptation of the slotted ring architecture. The study also considered aspects of the physical transmission and interfaces (PHY) for consideration in 100GbME standards.

[

Journal of Optical Networking, 2004

We describe results of the scalability analysis for dynamic wavelength-routed optical networks wi... more We describe results of the scalability analysis for dynamic wavelength-routed optical networks with end-to-end lightpath assignment and central network control with electronic scheduling and processing of lightpath requests. We investigate the effect of the algorithm complexity in both the scheduling and the dynamic routing and wavelength assignment (DRWA) of lightpath requests. Scheduling theory and static performance-prediction techniques were applied to define the bounds on the electronic processing time of requests, and hence the maximum number of nodes supported by a centralized dynamic optical network for given blocking probability, latency, and network diameter. Scalability analysis results show that medium-sized centralized networks (∼50 nodes) can be supported when these networks are reconfigured on a burst-by-burst basis. In addition, we found that real topologies showed a complex trade-off between the request processing time, blocking probability, and resource requirements. These findings can be used to determine the optimum combination of scheduling/DRWA algorithm, showing that the fastest DRWA algorithm does not necessarily lead to the minimum blocking probability and maximum scalability but that a careful consideration of both blocking and processing speed is required. The results are applicable both to dynamic network architectures with centralized request processing such as wavelength-routed optical networks and to the design of advanced optical switching matrices and routers. not disrupted, as the control of the network is transferred to the backup node. For an example of a real long-haul centralized optical network utilizing control node redundancy (see Ref.

This paper reports results of a performance comparison for four optical ring network architecture... more This paper reports results of a performance comparison for four optical ring network architectures envisaged for future metropolitan area networks (MANs), with particular emphasis on the design of and compatibility to possible 100-Gigabit-Metro-Ethernet (100GbME) standards. Both analytical and numerical modelling techniques were applied to quantify and compare network performance for all architectures in terms of achievable throughput, delay and the number of required wavelengths. Non-uniform traffic required additional resources and dynamic adaptation of the slotted ring architecture. The study also considered aspects of the physical transmission and interfaces (PHY) for consideration in 100GbME standards.

Bt Technology Journal, 2006

New dynamic optical switching technologies and adaptive transmission techniques will allow increa... more New dynamic optical switching technologies and adaptive transmission techniques will allow increased capacity and flexibility in future core and metro networks. The Optical Networks Group at UCL has carried out research focused on these areas, in collaboration with industry and other university research groups, for over a decade. In this paper, the technical challenges to be overcome in implementing dynamic, high-capacity networks are discussed, and recent results of the group’s work are presented. New network architectures based on wavelength-routing and optical burst switching have been proposed and analysed, and the experimental demonstration of burst switching using fast wavelength-tunable lasers is described. High-capacity dense wavelength division multiplexed transmission systems, using advanced regeneration and equalisation techniques, have been studied, both theoretically, and experimentally using a dynamically reconfigurable recirculating fibre loop. Research results on advanced optical signal formats, all-optical 3R regeneration at 40 Gbit/s channel rates, adaptive analogue and digital electronic processing techniques and optical finite impulse response equalisation are presented. Finally, the development of optical performance monitoring technology, a key component of future all-optical networks, is described. The paper concludes with a prediction of future trends in optical communications research.

Analytic equations are developed to quantify the maximum network size supported by a centralized ... more Analytic equations are developed to quantify the maximum network size supported by a centralized dynamic optical network architecture. Comparison with static networks establishes the maximum traffic load for which wavelength resources reduction is achievable.

It is widely believed that dynamic operation in wavelength-routed optical networks could overcome... more It is widely believed that dynamic operation in wavelength-routed optical networks could overcome the inefficiencies of static allocation in wavelength usage. In this paper this hypothesis is reviewed by quantifying the wavelength requirements in dynamic wavelength-routed optical networks and the comparison of these to those of the static approach. To do so, new analytical and heuristic lower bounds for the wavelength requirements in dynamic networks are proposed. They are used to evaluate the optimality of existing algorithms whose wavelength requirements are evaluated by means of simulation. Results show that dynamic wavelength-routed optical networks can save wavelengths only at low traffic loads (

The request scheduler carrying out the QoS provisioning with service differentiation was analysed... more The request scheduler carrying out the QoS provisioning with service differentiation was analysed for the wavelength-routed OBS network architecture. QoS performance for two traffic classes was analysed in terms of the burst blocking probability and end-to-end delays and as a function of the constraint of core wavelength over-provisioning. The results show that the WROBS is capable of a very efficient QoS performance, whilst significantly minimising the core wavelength requirement with regards to the statically-routed networks. It is demonstrated that the QoS provisioning for the premium traffic does not trade off the blocking probability of the best-effort traffic. Moreover, even with no core wavelength over-provisioning in WROBS, the desired blocking probability can be achieved for both premium and best-effort traffic whilst the delay bounds for both classes are not exceeded. The reported results are directly applicable for the provisioning of the guaranteed IP services and for RWA optimisation.

This work compares the maximum delay in five different burst assembly schemes. The mean packet de... more This work compares the maximum delay in five different burst assembly schemes. The mean packet delay is analytically derived and evaluated by simulation for Poisson and non-Poisson input traffic. The results allow to define an optimum burst assembly scheme in terms of the introduced delay.

IEEE/OSA Journal of Lightwave Technology, 2004

This paper investigates the challenges for developing the current local area network (LAN)-based ... more This paper investigates the challenges for developing the current local area network (LAN)-based Ethernet protocol into a technology for future network architectures that is capable of satisfying dynamic traffic demands with hard service guarantees using high-bit-rate channels (80. . .100 Gb/s). The objective is to combine high-speed optical transmission and physical interfaces (PHY) with a medium access control (MAC) protocol, designed to meet the service guarantees in future metropolitan-area networks (MANs). Ethernet is an ideal candidate for the extension into the MAN as it allows seamless compatibility with the majority of existing LANs. The proposed extension of the MAC protocol focuses on backward compatibility as well as on the exploitation of the wavelength domain for routing of variable traffic demands. The high bit rates envisaged will easily exhaust the capacity of a single optical fiber in the band and will require network algorithms optimizing the reuse of wavelength resources. To investigate this, four different static and dynamic optical architectures were studied that potentially offer advantages over current link-based designs. Both analytical and numerical modeling techniques were applied to quantify and compare the network performance for all architectures in terms of achievable throughput, delay, and the number of required wavelengths and to investigate the impact of nonuniform traffic demands. The results show that significant resource savings can be achieved by using end-to-end dynamic lightpath allocation, but at the expense of high delay.