Modeling the performance of FDL buffers with wavelength conversion

We present a performance model for fiber delay line (FDL) buffers having access to multiple wavelengths on an output fiber. In optical burst switching and optical packet switching, contending bursts (or packets) need to be dealt with in an effective way, and both wavelength conversion and optical buffering are viable solutions. The buffer studied here includes both solutions, with full wavelength conversion. It is situated at the output, and handles independent arrivals. We apply an analytic discrete-time queueing model to evaluate performance in terms of loss. We mainly consider the impact of burst size (fixed or varying), scheduling policy, and buffer size. Several numerical examples assess the accuracy of our approximation, and show that our approach is applicable when burst sizes are fixed, and, when a round-robin scheduling policy is adopted, also when burst sizes vary.

The prospects of Optical Burst Switching (OBS) as a cost-effective switching paradigm for future all-optical transport networks could greatly benefit from avoiding the use of complex and expensive all-optical wavelength converters at the network nodes. However, contention resolution is essential to achieve reasonable bandwidth utilization efficiency in OBS networks and replacing the wavelength converters by Fiber Delay Line (FDL) buffers requires huge numbers of FDLs and large space switches, thus also rendering complex network nodes. This paper proposes combining the use of a priority-based wavelength assignment and burst scheduling strategy at the ingress nodes, which minimizes the probability of contention, with the use of shared FDL buffers at the core nodes to resolve the unavoidable contentions. The objective is to design OBS networks employing only moderate numbers of simple FDL buffers while matching the performance of a network using full-range wavelength converters. Simulation results using a reference network topology show that this goal is feasible. Moreover, the FDL buffer requirements are shown to depend on the offered traffic load, the number of wavelengths per link, and the maximum burst delay at the ingress node.

Optical Circuit Switching (OCS) provides a good support for the services requiring high bandwidth and Quality of Service (QoS). When the character of network traffic changes from voice-oriented to data-oriented, OCS must cooperate with new switching mechanisms such as Optical Burst Switching (OBS) to improve performance. So a Hybrid Optical Switching (HOS) system is proposed. In HOS, different traffics are classified and both OBS and OCS can be processed. At the same time, delay buffer mechanisms, which are Fiber Delay Line (FDL) and electronic buffering, are introduced in HOS to solve the contention of OBS bursts. And a wavelengths classification mechanism, which assigns some wavelengths dedicated to OCS traffics, is provided to guarantee the QoS of OCS. To analyze the performance in theory, models based on Markovian theory and iterative method are provided. The results got by theory are accurate compared with simulations. The use of delay buffer can decrease the blocking probability of OBS and increase resource utilization. Wavelength classification mechanism benefits to improve OCS's performance.

Optical packet switching (OPS) is being considered as one of the switching technologies for a future optical internet. For contention resolution in an optical packet switching (OPS) system, the wavelength dimension is generally used in combination with a fibre delay line (FDL) buffer. In this study, the authors propose to reduce the number of tunable wavelength converters (TWCs) by sharing TWCs for cost-effective design of an asynchronous OPS system with a shared or an output FDL buffer. Asynchronous and variable-length packets are considered in the OPS system design. To investigate the number of TWCs needed for the OPS system, an algorithm is proposed, which searches for an available TWC and an unused internal wavelength, as well as an outgoing channel. This algorithm is applied to an OPS system with a shared or an output FDL buffer. Also, the number of internal wavelengths (i.e. the conversion range of the TWC) needed for an asynchronous OPS system is presented for cost reduction of the OPS system.

At a switching node in an optical packet switching (OPS) network, packet contention occurs whenever two or more packets try to reach the same output port of the node at the same wavelength, consequently it degrades network performance. Previous works on optical packet switching flourishingly adopted optical buffering with optical fibre delay line (FDL), wavelength conversion, or deflection routing to resolve packet contention. In this paper, we focus on the optical fibre delay line approach, especially the selection of proper length of FDL under asynchronous variable length packet traffic. As the length of fibre coils equipped in a FDL optical buffering system is impossible to be reconfigured frequently, a proper length of FDL should be determined during manufacture. The motivation is to observe the impact factors on FDL optical system performance in a WDM network scenario, and provide the empirical data as supports to select the proper length of FDL in an OPS node system.

In an effort to reduce switch cost, we, for the first time, present the optimum numbers of tunable wavelength converters (TWCs) and internal wavelengths required for contention resolution of asynchronous and variable length packets, in the optical packet switch (OPS) with the shared fiber delay line (FDL) buffer. To optimize TWCs and internal wavelengths related to an OPS design cost, we proposed a scheduling algorithm for the limited TWCs and internal wavelengths. For three TWC alternatives (not shared, partially shared, and fully shared cases), the optimum numbers of TWCs and internal wavelengths to guarantee minimum packet loss are evaluated to prevent resource waste. Under a given load, TWCs and internal wavelengths could be significantly reduced, guaranteeing the same packet loss as the performance of an OPS with full TWCs and internal wavelengths

In an effort to reduce switch cost, we present the optimum numbers of tunable wavelength converters (TWCs) and internal wavelengths required for contention resolution of asynchronous and variable length packets, in the optical packet switch (OPS) with the shared fiber delay line (FDL) buffer. To optimize TWCs and internal wavelengths related to OPS design cost, we proposed a scheduling algorithm for the limited TWCs and internal wavelengths. For three TWC alternatives (not shared, partially shared, and fully shared cases), the optimum numbers of TWCs and internal wavelengths to guarantee minimum packet loss are evaluated to prevent resource waste. Under a given load, TWCs and internal wavelengths could be significantly reduced, guaranteeing the same packet loss as the performance of an OPS with full TWCs and internal wavelengths.

Optical burst switching (OBS) is proposed as a high-speed, flexible, and transparent technology. It is thought to be the best way to adapt the bursty IP traffic over optical wavelength division multiplexing (WDM) networks. OBS technology facilitates the efficient integration of both IP and WDM. It provides statistical multiplexing gains and avoids long end to end setup time of traditional virtual circuit configuration. However, there are still a lot of challenges, one of which is burst contention. Owing to the fact that random access memory like buffering is not available in the optical domain at present, there exists a real possibility that bursts may contend with one another at a switching node. Many contention resolutions are proposed. The major contention resolutions in literature are wavelength conversion, fiber delay lines, and deflecting routing. In this paper, a new data burst scheduling scheme, called group scheduling based on control-packet batch processing (GSCBP) was proposed to reduce burst contention. Like transmission control protocol, GSCBP has a batch processing window. Control packets which located in the batch processing window are batch processed. A heuristic scheduling algorithm arranges the relevant bursts' route based on the processing result and the network resource. A new node architecture supporting group scheduling was presented. The GSCBP algorithm is combined with wavelength converter and/or fiber delay lines which is shared by some data channels. Meanwhile, an extended open shortest path first (E-OSPF) routing strategy was proposed for OBS. Both GSCBP and E-OSPF are introduced into 14-node national science foundation network by means of simulations. The ETE delay, burst blocking probability, as well as burst dropping probability were attained. Results show that the GSBCP lead to the higher-priority traffic drop rate decrease one order of magnitude, if drop rate and ETE delay of lower priority traffic is sacrificed.

Optical Burst Switching (OBS) is a new paradigm for future all-optical networks. It has been noted that performance of an OBS node depends on the wavelength assignment algorithm that is used. In this paper we present a new class of wavelength allocation algorithms called cost-based algorithms. We note that bursts compete for more types of resources than wavelengths alone. For example if a given burst is to be allocated successfully, a Fiber Delay Line (FDL) or a wavelength converter may have to be used. It can be expected, however, that the set of available resources will be limited. If at a given time all the converters are used, then any arriving burst will have to be allocated on the same wavelength - if it is available. Similarly, the unavailability of FDLs will decrease the probability of a burst being accepted. In a cost-based algorithm, each resource is assigned a metric (or price). Channels are priced according to their suitability for a particular burst. When a control packet arrives at a core node, all the possible ways of handling the corresponding burst are found (the outgoing channel, with or without a FDL or wavelength converter), and the one with a lowest metric is chosen. To show how the performance of a cost-based algorithm compares to other algorithms we present the results of our simulations for a node with full conversion capability and a shared FDL.

Optical Burst Switching (OBS) is a promising switching technology for the next generation all-optical networks. An OBS network without wavelength converters and fiber delay lines can be implemented simply and cost-effectively using the existing technology. However, this kind of networks suffers from a relatively high burst loss probability at the OBS core nodes. To overcome this issue and consolidate OBS networks with QoS provisioning capabilities, we propose an absolute QoS differentiation scheme, called Absolute Fair Quality of service Differentiation (AFQD), which is based on a wavelength partitioning scheme, called Optimization Topology-aware Wavelength Partitioning scheme (OTWP). AFQD is the first absolute QoS provisioning scheme that guarantees loss-free transmission for high priority traffic inside the OBS network. Simulation results show that AFQD not only guarantees loss-free transmission for high priority traffic but also substantially decreases the loss probability of best effort traffic to a remarkable level compared to the existing schemes.

As the bit rate of one wavelength channel and the number of channels keep increasing in the telecommunication networks thanks to the advancement of optical transmission technologies, switching is experiencing the transition from the electrical domain to the optical domain. All-optical signal processing, including wavelength conversion, optical logic gates and signal regeneration, etc, is one of the most important enabling technolgies to realize optical switching, including optical circuit switching, optical burst switching and optical packet switching. Semiconductor optical ampli??ers (SOAs) are very promising in all-optical signal processing because they are compact, easy to manufacture and power e??cient. It is therefore very important to develop numerical models for the SOAs to understand their behaviour in di??erent system con??gurations, especially when the interacting pulse duration becomes shorter and shorter with increasing bit rate, where several e??ects that are neglected in previous models have to be accounted for. To investigate high-speed SOA-based all-optical signal processing systems, in this thesis we develop a comprehensive model, which includes both inter- and intra-band carrier dynamics, gain dispersion and group velocity dispersion, in this thesis. Polarization dependent e??ects can also be taken into account through introducing an imbalance factor f. Finite-di??erence beam propagation method is employed to solve the numerical model. Mode-locking lasers o??er a lot of applications in all-optical signal processing systems. In this thesis we investigate a novel mode-locked laser based on nonlinear polarization rotation in an SOA. The pulse narrowing process is demonstrated numerically, achieving good agreement with our experimental results. The pulse performance is largely determined by the ultrafast SOA gain dynamics and the cavity dispersion. The laser can produce a pulse train of sub-picosecond pulse width at a repetition rate of 28 GHz, which is limited by the carrier lifetime, for a moderate SOA current level. For higher currents instabilities occur in the laser. vi One of the drawbacks of the SOA-based devices is the relatively long gain recovery time which results in strong pattern e??ects for high bit rate operaion. In this thesis we extensively investigate a very high bit rate wavelength converter based on a single SOA and an optical bandpass ??lter. The enhancement in operation speed is based on ??ltering an amplitude- and phase-modulated signal. We study the underlying working principle and perform detailed analysis of the high-speed wavelength converter, which leads to optimization rules for high-speed SOA-based wavelength conversion. Moreover, both inverted and non-inverted wavelength conversion at much higher bit rate(1 T b/s), is also predicted. Furthermore, genetic algorithm is introduced in the optimization of the transfer function of the OBF following the SOA. Through optimization, eye opening of more than 33 dB is shown for non-inverted wavelength conversion. The optimized ??lter can be experimentally implemented through a combination of asymmetric Mach-Zehnder interferometer and a Gaussian ??lter. Enlightened by the working principle of the wavelength converter, we proposed and demonstrated experimentally a novel optical logic gate with a very simple structure: an SOA followed by an OBF. This logic gate can realize AND, OR and XOR gate functions based on the same setup but with di??erent operation conditions. This novel device can be integrated.

Packet contention is a major issue in asynchronous optical packet switching networks. Optical buffering, which is implemented by fiber delay lines (FDLs), is fundamental to many optical switch implementations for resolving contention. Most existing optical buffering implementations are output-based and require a huge amount of FDLs as well as larger switch sizes, which impose extra cost on the overall system. In this paper, we consider a shared optical buffering architecture which can reduce the buffer size at a switch. We propose an analytical model to evaluate the packet loss probability and the average delay For shared buffers at a single switch. We then compare the performance of output buffers to shared buffers under different granularities of FDLs. We observe that, by choosing an appropriate granularity, the shared buffering scheme can significantly reduce packet loss with much smaller switch sizes and fewer FDLs than the output buffering architecture. The accuracy of the analytical model is also confirmed by extensive simulation

Topology-aware wavelength partitioning for DWDM OBS networks: A novel approach for absolute QoS provisioning

Optical burst switching (OBS) is a switching technology that can efficiently operate in the optical core network using WDM technology and can also develop the future optical internet. The OBS switching technology presents a trade-off between the two switching technologies: optical circuit switching (OCS) and optical packet switching (OPS). This switching approach increases resource utilization compared with OCS technology and avoids the technological barriers of OPS networks. In OBS networks, many packets are assembled into one data burst at the edge node and a burst header packet (BHP) is sent before the burst transmission by an offset time in order to reserve the required resources in core nodes. This can cause security issues in the network and more specifically denial of service attacks that can occur if a source node is compromised. In this paper, we study a specific denial of service attack which we refer to as "BHP flooding attack", which prevents legitimate traffics from reserving the required resources at intermediate core nodes. We also propose the design of a reconfigurable BHP flooding countermeasure module that allows to counter against this type of attacks in an OBS network.

In optical switching paradigm, there are three approaches; optical circuit switching, optical packet switching, and optical burst switching (OBS). OBS is a technology that takes the best features of packet switching and circuit switching by providing a flexible infrastructure for carrying IP traffic and at the same time ensuring efficient utilization of the fiber link. In this paper we propose an effective contention resolution scheme to achieve minimum burst loss at core nodes. Algorithm of zero burst loss for bimodal burst switching is extended from single node star network topology to two node star topology. Scheduling at the core node, typically an optical cross connect (OXC), is performed in two modes i.e., Mode 0 & Mode 1. Mode 0 is for burst transmission between all directly connected edge nodes whereas Mode 1 is for burst transmission through remote core node to a particular destination edge node. Synchronization of the reservation request between the core nodes is a critical task in such a topology. In order to achieve minimum burst loss, using a small length of fiber delay line in recirculation mode results in an effective strategy.