An examination is made of fiber-optic code-division multiple-access (FO-CDMA), a technique in which low information data rates are mapped into very-high-rate address codes (signature sequences) for the purpose of achieving random, asynchronous communications free of network control, among many users. The need for a special class of signature sequences to achieve the multiple-access capability using fiber-optic signal processing techniques is discussed. A class of signature sequences called optical orthogonal codes (OOCs) that provide the auto- and cross-correlation properties required for FO-CDMA is introduced and used in an experiment to show the principles of FO-CDMA. The experiment demonstrates the auto- and cross-correlation properties of the codes. The concept of optical disk patterns, an equivalent way of representing the OOCs, is introduced. The patterns are used to derive the probability density functions associated with any two interfering OOCs. A detailed study of different interference patterns is presented, and the strongest and the weakest interference patterns are determined. >

Code division multiple-access techniques in optical fiber networks. I. Fundamental principles

The very broad bandwidth of low-loss optical transmission in a single-mode fiber and the recent improvements in single-frequency tunable lasers have stimulated significant advances in dense wavelength division multiplexed optical networks This technology, including wavelength-sensitive optical switching and routing elements and passive optical elements, has made it possible to consider the use of wavelength as another dimension, in addition to time and space, in network and switch design The independence of optical signals at different wavelengths makes this a natural choice for multiple-access networks, for applications which benefit from shared transmission media, and for networks in which very large throughputs are required Recent progress in multiwavelength networks are reviewed, some of the limitations which affect the performance of such networks are discussed, and examples of several network and switch proposals based on these ideas are presented Discussed also are critical technologies that are essential to progress in this field >

Dense wavelength division multiplexing networks: principles and applications

An increasing number of high performance internetworking protocol routers, LAN and asynchronous transfer mode (ATM) switches use a switched backplane based on a crossbar switch. Most often, these systems use input queues to hold packets waiting to traverse the switching fabric. It is well known that if simple first in first out (FIFO) input queues are used to hold packets then, even under benign conditions, head-of-line (HOL) blocking limits the achievable bandwidth to approximately 58.6% of the maximum. HOL blocking can be overcome by the use of virtual output queueing, which is described in this paper. A scheduling algorithm is used to configure the crossbar switch, deciding the order in which packets will be served. Previous results have shown that with a suitable scheduling algorithm, 100% throughput can be achieved. In this paper, we present a scheduling algorithm called iSLIP. An iterative, round-robin algorithm, iSLIP can achieve 100% throughput for uniform traffic, yet is simple to implement in hardware. Iterative and noniterative versions of the algorithms are presented, along with modified versions for prioritized traffic. Simulation results are presented to indicate the performance of iSLIP under benign and bursty traffic conditions. Prototype and commercial implementations of iSLIP exist in systems with aggregate bandwidths ranging from 50 to 500 Gb/s. When the traffic is nonuniform, iSLIP quickly adapts to a fair scheduling policy that is guaranteed never to starve an input queue. Finally, we describe the implementation complexity of iSLIP. Based on a two-dimensional (2-D) array of priority encoders, single-chip schedulers have been built supporting up to 32 ports, and making approximately 100 million scheduling decisions per second.

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The iSLIP scheduling algorithm for input-queued switches

An optical orthogonal code (OOC) is a family of (0,1) sequences with good auto- and cross-correlation properties, i.e the autocorrelation of each sequence exhibits the `thumbtack' shape and the cross correlation between any two sequences remains low throughout. The use of optical orthogonal codes enables a large number of asynchronous users to transmit information efficiently and reliably. The thumbtack-shaped autocorrelation facilitates the detection of the desired signal, and low-profile cross correlation reduces interference from unwanted signals. Theoretical upper and lower bounds on the maximum possible size of OOCs are derived. Methods for the design and analysis of OOCs, using tools from projective geometry, the greedy algorithm, iterative constructions, algebraic coding theory, block design, and various other combinational disciplines, are discussed

Optical orthogonal codes: design, analysis and applications

Optical orthogonal codes: design, analysis, and applications

This paper gives a broadband (total throughput approaching 1 terabit/s) self-routing packet switch design for providing flexible multiple bit-rate broadband services for an end-to-end fiber network. The switch fabric for the slotted broadband packet switch delivers exactly one packet to each output port from one of the input ports which request packet delivery to that output port. The denied requests would try again during the next slot. We discover an effective scheme, implemented by CMOS VLSI with manageable complexity, for performing this function. First, each input port sends a request for a port destination through a Batcher Sorting network, which sorts the request destinations in ascending order so that we may easily purge all but one request for the same destination. The winning request acknowledges its originating port from the output of the Batcher network, with the acknowledgment routed through a Batcher-banyan selfrouting switch. The acknowledged input port then sends the full packet through the same Batcher-banyan switch without any conflict. Unacknowledged ports buffer the blocked packet for reentry in the next cycle. We also give several variations for significantly improved performance. We then study switch performance based on some rudimentary protocols for traffic control. For the basic scheme, we analyze the throughput-delay characteristics for random traffic, modeled by random output port requests and a binomial distribution of packet arrival. We demonstrate with a buffer size of around 20 packets, we can achieve a 50 percent loading with almost no buffer overflow. Maximum throughput of switch is 58 percent. Next, we investigate the performance of the switch in the presence of periodic broadband traffic. We then apply circuit switching techniques and packet priority for high bit-rate services in our packet switch environment. We improve the throughput per port to close to 100 percent by means of parallel switch fabric, while maintaining the periodic nature of the traffic.

A Broadband Packet Switch for Integrated Transport

We consider the use of multiple high-capacity fibers for communications networking. Each user transmits, asynchronously, patterns of optical pulses distributed over the fibers and throughout a time frame. Each receiver has a distinct alphabet of patterns, which are detected by optical correlators. Optical correlation by fiber tapped delay lines provides speedy and easy-to-implement decoders. Thus the individual user obtains a transparent low-speed channel by code multiplexing. The reliability of this low-speed channel can be enhanced by redundantly coding the patterns sent by the user, for which the encoding and decoding processes can be performed electronically. This two-step encoding process is simple to implement, highly reliable at reasonable throughput, and provides asynchronous access with simple protocol. Various components and configurations of this access scheme are described. The information theoretic capacity and the error probability for these configurations are derived. We also demonstrate that hardlimiting and filtering at the receiver reduce error probability significantly.

J. Hui

Papers

A Broadband Packet Switch for Integrated Transport

Pattern Code Modulation and Optical Decoding - A Novel Code-Division Multiplexing Technique for Multifiber Networks