The implementation of optical packet-switched networks requires that the problems of resource contention, signalling and local and global synchronization be resolved. A possible optical solution to resource contention is based on the use of switching matrices suitably connected with optical delay lines. Signalling could be dealt with using subcarrier multiplexing of packet headers. Synchronization could take advantage of clock tone multiplexing techniques, digital processing for ultra-fast clock recovery, and new distributed techniques for global packet-slot alignment. To explore the practical feasibility and effectiveness of these key techniques, a consortium was formed among the University of Massachusetts, Stanford University, and GTE Laboratories. The consortium, funded by ARPA, has three main goals: investigating networking issues involved in optical contention resolution (University of Massachusetts), constructing an experimental contention-resolution optical (CRO) device (GTE Laboratories), and building a packet-switched optical network prototype employing a CRO and novel signaling/synchronization techniques (Stanford University). This paper describes the details of the project and provides an overview of the main results obtained so far.

CORD: contention resolution by delay lines

A detailed analytical traffic model for a photonic wavelength division multiplexing (WDM) packet switch block is presented and the requirements to the buffer size is analyzed. Three different switch architectures are considered, each of them representing different complexities in terms of component count and requirements to the components, it is shown that the number of fiber delay-lines, that form the optical buffer, can be substantially reduced by the use of tunable optical wavelength converters, thereby exploiting the wavelength domain to solve contention of optical packets. For a 16/spl times/16 switch with four wavelength channels per inlet, all at a load of 0.8, the number of delay-lines is reduced from 47 to 12 by use of tuneable optical wavelength converters. Apart from the number of delay-lines the physical buffer structure is analyzed with special attention to the possibilities offered by optics, i.e., the possibility of several outlets sharing the same physical buffer. For the three architectures presented here, a tradeoff in the buffer architectures is addressed: a buffer physically shared among an outlets requires many wavelengths internally in the switch block, whereas, architectures with buffers dedicated to each outlet require a smaller number of wavelengths.

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WDM packet switch architectures and analysis of the influence of tunable wavelength converters on the performance

For realistic traffic, i.e., bursty traffic patterns, the use of tuneable wavelength converters is recognized as essential for reducing the complexity of photonic wavelength division multiplexing (WDM) packet switches. Results are obtained from an analytical traffic model that includes buffering in the wavelength domain and accounts for bursty traffic. The theoretical model is verified by simulations and from the model we find that higher traffic loads as well as burstiness can be accepted when tuneable wavelength converters are used. Consequently, a larger throughput of the photonic packet switches is obtained and very importantly, this is achieved while keeping the number of gates needed to realize the space switches nearly constant.

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Analysis of a WDM packet switch with improved performance under bursty traffic conditions due to tuneable wavelength converters

Multicasting in a network is needed for a number of applications where a session requires connectivity between more than two participants. In this paper, we consider a wavelength-division multiplexing (WDM)-based all-optical network. We derive asymptotic upper bounds on the number of wavelengths needed to support multicasting capability in an all-optical network. The number of wavelengths in a WDM network continues to be a major constraint. The results derived here should be seen as the limit on scalability of all-optical networks that support multicasting. The derivation of the upper bound uses some properties of expander graphs.

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Wavelength requirements for multicasting in all-optical networks

We demonstrate a rapidly tunable narrowband wavelength filter using unbalanced Mach-Zehnder interferometers in lithium niobate. It is designed to switch among eight channels spaced 50 GHz apart in optical frequency (approximately 0.4 nm in wavelength) in less than 50 ns. The insertion loss is 19 dB and crosstalk is less than -22 dB. The filter consists of a cascade of three electrooptically tunable unbalanced Mach-Zehnders and a feedback circuit to keep the filter centered on the desired wavelength.

Rapidly tunable narrowband wavelength filter using LiNbO/sub 3/ unbalanced Mach-Zehnder interferometers

A proposal for developing a precompetitive, commercially viable all-optical network architecture using dense optical wavelength division is described. The objective was to demonstrate a packet-switched high-capacity all-optical LAN/MAN network based on wavelength-division multiplexing (WDM) and capable of supporting several hundred to a thousand nodes, each requiring gigabit-per-second throughputs. The discussion covers the rationale and objectives of the program; tunable components, optical amplifiers, network protocol, and application development; network interconnection studies; and testbed construction. >

P.E. Green

Papers

All-optical packet-switched metropolitan-area network proposal