Shared fiber delay line buffers in asynchronous optical packet switches

— Packet contention is a major issue in asynchronous optical packet and burst 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 shared optical buffering which can reduce the buffer size at a switch. Since no previous study is available to analyze the performance of asynchronous architectures with shared buffers, 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. I.

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.

Packet contention is a major issue in optical packet switching network. To resolve contention, optical buffering approach is used using Fiber Delay Lines (FDLs). Yet there is a heavy packet loss rate due to unavailability of output-port and free FDLs. This paper proposes a selective minimum delay value algorithm (SMDV) to resolve contention for shared FDL buffering architecture in order to ensure the availability of output-port for the contended packet after coming out from FDL. Simulation result shows that proposed scheme can achieve a packet loss rate of ~ even at heavy traffic load 0.9 for 32×32 switch using 32 variable length FDL.

Analytic and approximate traffic model is proposed for asynchronous optical packet switching (AOPS) networks to serve the network performance analysis. The study shows that AOPS network traffic could be modeled as Poisson arrival with Gaussian-distributed packet size, when the timeout-based optical packet assembly algorithm is adopted and the incoming IP traffic at AOPS edge routers is short-range dependent. These findings are helpful to accurately dimension the traffic model for AOPS networks.

In order to achieve service differentiation, especially loss differentiation, in optical burst switching (OBS) networks, we propose a dynamic fiber delay line (FDL) partitioning algorithm, which divides FDLs into several groups over a feed-forward output buffering architecture. In the proposed scheme, a plurality of traffic classes and FDL groups can be considered, and each FDL group is assigned to traffic classes, so that the target loss probabilities of classes are guaranteed. Also, the optimal number of FDLs for each FDL group by the proposed algorithm is decided in Poisson traffic environments. The extensive simulation results validate the effectiveness of the proposed dynamic FDL partitioning algorithm for the loss differentiation in OBS networks.

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 lack of optical buffer is still one of the main problems that hinder the development of all optical networks. Current works on this topic mainly focus on the emulation of optical buffers based on a combination of fiber delay line (FDL) and switch. These works have demonstrated the feasibility of FDL-based emulation for many kinds of optical buffers, like the First In First Out (FIFO) buffer, Priority buffer, etc. The Last In First Out (LIFO) buffer is another basic network component for congestion control and QoS guarantee. Recently, Huang et al. introduced a recursive construction for the LIFO buffer, which requires no less than 9 log2 B FDLs to build such a buffer of size B. In this paper, we first show that by a proper FDL grouping and a suitable FDL length assignment for each FDL-group, we need approximately 3 log2 B FDLs to emulate a LIFO buffer of size B. We then demonstrate that if a careful packet scheduling among FDL-groups is adopted, this number of FDLs can be further reduced to 2 log2 B.

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.

In optical burst switching and optical packet switching, contention of bursts (or packets) can be dealt with most effectively through a combination of wavelength conversion and optical buffering. While this is generally accepted in the optical networking community, and validated through simulation, analytic performance results for optical buffers were limited to the single-wavelength case, and the performance gain from wavelength conversion was never traced analytically for general assumptions. Quantifying this gain analytically is the scope of the current contribution. Relying on generating functions, we developed a fiber delay line (FDL) buffer model with wavelength conversion, which assumes the buffer located at the output of an optical switch, having access to multiple wavelengths. This document presents our model, validates its accuracy, and compares its output for different burst sizes (fixed or varying), scheduling policies and buffer sizes. Several numerical examples assess the applicability of our approximation, and show that our approach yields accurate results.

In this paper, we employ fiber delay line (FDL) as optical buffers to reduce the burst-loss probability. We studied an analytical model using a single forward FDL buffering for contention resolution, then built a new model for OBS architecture using both a forward FDL and a feedback FDL. Based on the two models, we designed algorithm for each models, and simulated them with C++ programming language.

In this work, we propose a new hybrid dynamic burst contention resolution scheme. The proposed scheme is based on the notion of burst segmentation and judiciously uses optical buffering (i.e., FDLs) and partial wavelength conversion, and at the same time dynamically selects contention resolution policies in response to the network traffic load change. We assume that the just-enough-time (JET) signaling protocol is used and that switches only have a limited number of full-range wavelength converters and some switches may not have FDLs or wavelength converters. The proposed hybrid scheme does not pre-reserve FDLs or wavelength converters. Instead, it uses them on an as needed basis to reduce the wavelength converters required. Extensive simulation was performed to evaluate the performance of the proposed scheme and to compare with other schemes. Results show that our hybrid scheme is effective.

Performance analysis of an improved burst-outputted scheme in a limited buffer equipped OBS core node

A 160 Gbit/s/port optical packet switch prototype with narrow-band optical code-label processing, optical switching, optical buffering, electrical scheduling, optical MUX/DEMUX, and variable-length packet BER and loss real-time measuring is proposed and experimentally demonstrated. (4 pages)

The lack of optical buffer is still one of the main problems that hinder the development of all optical network. The current works on this topic mainly focus on emulating optical buffers with fiber delay line (FDL). Recent advances have shown the feasibility of emulating many kinds of optical buffers, such as the First In First Out (FIFO) buffer, priority buffer, etc. The Last In First Out (LIFO) buffer is another important network components that can be used for providing QoS guarantee and congestion control. Huang et al. introduced a recursive construction of LIFO buffer with buffer size B by using no less than 9 log 2 B FDLs [1]. In this paper, we try to reduce the number of required delay lines there. We will show that by combining every 3 fiber delay lines into one group and applying exponential growth of FDLs among groups, one needs only 3 log 2 B delay lines to emulate a LIFO buffers of size B.

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.