The physical layer of an optical network is vulnerable to a variety of attacks, including jamming, physical infrastructure attacks, eavesdropping, and interception. As the demand for network capacity grows dramatically, the issue of securing the physical layer of optical network cannot be overlooked. In this survey paper, we discuss the security threats in an optical network as well as present several existing optical techniques to improve the security. In the first part of this paper, we discuss various types of security threats that could appear in the optical layer of an optical network, including jamming, physical infrastructure attacks, eavesdropping, and interception. Intensive research has focused on improving optical network security, in the above specific areas. Real-time processing of the optical signal is essential in order to integrate security functionality at the physical layer while not undermining the true value of optical communications, which is its speed. Optical layer security benefits from the unique properties of optical processing-instantaneous response, broadband operation, electromagnetic immunity, compactness, and low latency. In the second part of this paper, various defenses against the security threats outlined in this paper are discussed, including optical encryption, optical code-division multiple access (CDMA) confidentiality, self-healing survivable optical rings, anti-jamming, and optical steganography.

The 31 papers in this special issue focus on using the physical layer for securing the next generation of communication systems.

We propose and demonstrate the use of real-time optical signal processing, implemented with integrated and packaged off-the-shelf commercial technology, to enhance the security of data networks. Optical steganography, based on spread spectrum, is used to inhibit the observation of data traffic and the analysis of traffic patterns. Using optical CDMA to modulate the data, service interruption is prevented in presence of infrastructure attacks. Unlike conventional wavelength and time multiplexing approaches, the soft blocking capability of optical CDMA provides protection paths without permanently allocating bandwidth. The large size of the optical CDMA code set also reduces the probability that the secure signal can be intercepted by an eavesdropper. Finally, high-speed all-optical data encryption is demonstrated, which prevents side-channel attacks and enables real-time encryption with no delay.

Given K simultaneously active users, an Optical CDMA (OCDMA) receiver needs to discriminate among K codes. Since the number of available codes C is always C Gt K at any given time for any truly asynchronous OCDMA system that employs quasi-orthogonal codes, there is always a set of unused codes. We here propose to exploit these unused codes to increase the spectral efficiency of the system. This can be accomplished by exclusively assigning to each user a set of M codes which represent a log2(M)-tuple of bits so that each user effectively uses a multi-dimensional modulation (multiple information bits per code are conveyed). In this paper, we analyze the performance of such a system and find in closed form an upper bound on the average probability of error. Moreover, we report for the first time that, under lightly loaded, non interference-limited conditions, the multi-dimensional OCDMA scheme here proposed allows us to obtain both higher spectral efficiency and lower bit- error probability (BEP) than a conventional OCDMA system, which employs one code only per user. The analysis here presented allow us to formulate a useful system design criterion that yields truly asynchronous OCDMA systems with very high spectral efficiency.

To support multimedia services with different discrete bit-rate requirements, families of multilength optical codes, such as the carrier-hopping prime code (CHPC), extended CHPC, and multiwavelength optical orthogonal code, were recently constructed for wavelength-time optical code-division multiple-access (O-CDMA). In this paper, the performance of these multilength optical codes in a multimedia O-CDMA system with a variable-weight operation are analyzed. Our study shows that short-length codes generate stronger interference than long-length codes. This supports services prioritization in O-CDMA. Our study also shows that code weight is a more important factor than code length in determining code performance (i.e., quality of service).

To support multimedia services with different discrete bit-rate requirements, families of multilength optical codes, such as the carrier-hopping prime code (CHPC), extended CHPC, and multiwavelength optical orthogonal code, were recently constructed for wavelength-time optical code-division multiple-access (O-CDMA). In this paper, the performances of these multilength optical codes in a multimedia O-CDMA system with a variable-weight operation are analyzed. Our study shows that short-length codes generate stronger interference than long-length codes. This supports services prioritization in O-CDMA. Our study also shows that code weight is a more important factor than code length in determining code performance.

Puncturing is studied as a physical layer mechanism for efficiently transmitting datastreams containing bits of unequal priority via wavelength-time optical code-division multiple access. Puncturing increases system capacity, while ensuring that important bits are received with low bit-error rate

The capacity of an optical code-division multi-access (CDMA) [OCDMA] network has traditionally been defined as the number of continuously transmitting circuits supported by the network. In this paper, we use teletraffic models to determine the teletraffic capacity of a circuit-switched OCDMA network where circuits carry bursty traffic. Our analysis is independent of the OCDMA implementation or spreading code. In conventional networks, e.g., a wavelength-routed network (WRN), new circuits are blocked when all wavelengths are occupied. In OCDMA when the number of codewords exceeds number of network subscribers, new circuits need not be blocked. Instead, capacity is limited by multiple access interference: when the number of actively transmitting circuits becomes excessive, the bit-error rate (BER) of all circuits on the network degrades, causing an outage. We find that through statistical multiplexing, the capacity of OCDMA exceeds that of a WRN except when circuit activity is very high while the constraints on outages are more stringent than those on blocking. In such cases, we show how OCDMA with call admission control can be used to match or exceed the capacity of a WRN. Overall, our analysis shows that OCDMA is well suited to applications when conventional blocking is undesirable, and/or circuits carry bursty traffic.

Given K users simultaneously active, an optical CDMA (OCDMA) receiver needs to discriminate among K codes. Since the number of available codes C is always CGtK at any given time for an asynchronous OCDMA system employing quasi-orthogonal codes, there is always a set of unused codes and these codes can be exploited to increase the spectral efficiency of the system. This can be accomplished by exclusively assigning to each user a set of M codes which represent a log2(M)-tuple of bits so that each user effectively uses a multidimensional modulation (multiple information bits per code are conveyed). In this paper, we analyze the performance of such a system, and find that an upper bound on the probability of error can be expressed in terms of the performances of the conventional case where one code only per user is employed. Moreover, we also find that, under certain conditions, the multidimensional modulation here proposed allows us to obtain higher spectral efficiency, higher power efficiency, and lower bit-error rate (BER) than conventional OCDMA, which employs one code per user

As microprocessors become increasingly interconnected, the power consumed by the interconnection network can no longer be ignored. Moreover, with demand for link bandwidth increasing, optical links are replacing electrical links in inter-chassis and inter-board environments. As a result, the power dissipation of optical links is becoming as critical as their speed. In this paper, we first explore options for building high speed optoelectronic links and discuss the power characteristics of different link components. Then, we propose circuit and network mechanisms that can realize power-aware optical links -links whose power consumption can be tuned dynamically in response to changes in network traffic. Finally, we incorporate power control policies along with the power characterization of link circuitry into a detailed network simulator to evaluate the performance cost and power savings of building power aware optoelectronic networked systems. Simulation results show that more than 75% savings in power consumption can be achieved with the proposed power aware optoelectronic network.

A new family of two-dimensional (2-D) wavelength-hopping time-spreading codes, which employs wavelength hopping algebraically under prime-sequence permutations on top of time-spreading optical orthogonal codes, is studied and analyzed. Different from other 2-D codes, our new codes allow the number of wavelengths and code length to be chosen independently and, at the same time, the code cardinality is a quadratic function of the number of wavelengths without sacrificing the maximum cross-correlation value (i.e., still at most one). They are particularly suitable for high bit-rate optical code-division multiple-access systems with broadband mode-locked lasers, in which the number of time slots is very limited, and system capacity can only be grown by increasing the number of wavelengths, rather than code length. Finally, a novel wavelength-aware detector for wavelength-hopping time-spreading codes is discussed and shown to provide improved code performance.

In this paper, we study the use of shifted carrier-hopping prime codes (CHPC) for multicode keying in wavelength-time optical code-division multiple-access (O-CDMA) systems. By using the code's good properties of zero autocorrelation sidelobes and cross-correlation functions of at most one, each user is assigned M=2/sup m/ shifted copies of its own code matrix to represent m data bits per symbol with minimal interference. The advantages of our scheme are that: 1) a lower baud rate O-CDMA system can now support a higher bit rate; 2) no network synchronization is required; 3) no M-fold increase in code cardinality is needed, as compared with standard M-code keying; 4) the numbers of optical encoders and decoders can be reduced from M to one per user, a major hardware cost savings; and 5) user code confidentiality is enhanced, as compared with the transmission of one code matrix for each data-bit one in conventional "on-off keying" O-CDMA schemes. Finally, our performance analysis shows that there is a tradeoff between the error probability and the choice of M.

Most research efforts to date on optical networks have concentrated on wavelength-division multiplexing (WDM) techniques where the information from different channels is routed via separate optical wavelengths. The data corresponding to a particular channel is selected at the destination node by a frequency filter. Optical time-division multiplexing (OTDM) has been considered as an alternative to WDM for future networks operating in excess of 10 Gb/s. Systems based on TDM techniques rely upon a synchronized clock frequency and timing to separate the multiplexed channels. Advances in device technologies have opened new opportunities for implementing OTDM in very high-speed long-haul transmission as well as networking. The multiterahertz bandwidth made available with the advent of optical fibers has spurred investigation and development of transparent all-optical networks that may overcome the bandwidth bottlenecks caused by electro-optic conversion. This paper presents an overview of current OTDM networks and their supporting technologies. A novel network architecture is introduced, aimed at offering both ultra-high speed (up to 100 Gb/s) and maximum parallelism for future terabit data communications. Our network architecture is based on several key state-of-the-art optical technologies that we have demonstrated.

Multihop networks with wavelength-division multiplexing (WDM) are one possible way to conduct high data-rate communication. We provide an in-depth study of the generalization of the well-known shuffle network for ultrafast multihop lightwave communication. In the classical definition of a shuffle network, i.e., N=kp/sup k/ where N is the number of nodes and k is the number of stages with nodes of degree p, the realizable values of N are very sparse and many of the intermediate values of N are not realizable. We use a new definition of the shuffle network, N=nk, where n is the number of nodes per stage, which was originally proposed by Krishna and B. Hajek (1990) as the shuffle-ring network. Based on this definition, we divide the shuffle networks into two classes: extra-stage and reduced-stage. We derive an exact model and an approximate model of the expected number of hops for various network topologies. The results can be used to determine an optimal network topology when given a value of N.

This paper presents an extension of a known analytical model for the performance evaluation of nonpriority deflection routing networks in uniform traffic. The extension allows the analysis of improved access techniques. The key features of the analytical technique are described by casting it in a very simple setting: nonpriority hot-potato in a two-connected slotted shufflenet (SN) network. Results are presented for three access techniques: transmit-no-hold (TXNH), transmit-hold (TXH), and bypass queuing (BQ).

Crossbar switches are rarely considered for large, scalable multiprocessor interconnect systems because they require O(n2) switching elements, are difficult to control efficiently and are hard to implement once their size becomes too large to fit on one integrated circuit. However these problems are technology dependent and a recent innovation in fiber optic devices has led to a new implementation of crossbar switches that does not share these problems while retaining the full advantages of a crossbar switch: low latency, high throughput, complete connectivity and multi-cast capability. Moreover, this new technology has several characteristics that allow a distributed control system which scales linearly in the number of attached nodes.The innovation that led to this research is an optical and-gate that can be used to demultiplex multiple high speed data streams that are carried on one common optical medium. Optical time domain multiplexing can combine the data from many nodes and broadcast the result back to all nodes. This paper discusses OTDM technology only to the extent necessary to understand its characteristics and capabilities. The main contribution lies in the description and analysis of interconnect architectures that utilize OTDM to achieve a level performance that is beyond electronic means. It is expected that cost-reduced OTDM systems will become competitive with the next generation of interconnect systems.

The steady state behavior of regular two-connected multihop networks in uniform traffic under a hot-potato and a simple single-buffer deflection routing technique is analyzed for very high bit rate optical applications. Manhattan Street Network and ShuffleNet are compared in terms of throughput, delay, deflection probability, and hop distribution both analytically and by simulation. It is analytically verified that this single-buffer deflection routing technique recovers in both networks more than 60% of the throughput loss of hot-potato with respect to store-and-forward when packets are generated with independent destinations. This gain, however, decreases to below 40% when the average message length exceeds 20 packets.>

New single-receiver/single-transmitter/single node structures for two-connected multihop transparent optical packet-switching networks with deflection routing are introduced. A ShuffleNet topology in uniform traffic is adopted to compare various shared optical memory schemes and their control algorithms. These simple structures minimize the number of crossbar switches needed or the node and have moderate control complexity, while still yielding high throughput and low delay. Analytical results are obtained by an extension of the existing theory and verified by simulation.>

Photonic packet switches offer high speed, data rate and format transparency, and flexibility required by future computer communications and cell-based telecommunications networks. In this paper, we review experimental progress in state-of-the-art photonic packet switches with an emphasis on all-optical guided-wave systems. The term all-optical implies that the data portion of a packet remains in optical format from the source to the destination. While the data remain all-optical, both optical and optoelectronic techniques have been used to process packet routing functions based on extremely simple routing protocols. An overview of the design issues for all-optical photonic packet switching is given and contrasted with electronic packet switch implementations. Low-level functions that have been experimentally implemented include routing, contention resolution, synchronization, and header regeneration. System level demonstrations, including centralized photonic switching and distributed all-optical multihop networks, will be reviewed.>

A phase-noise-cancelled coherent subcarrier fiberoptic communication system that uses integrated-optic Mach-Zehnder waveguide modulators for frequency-pair encoding is introduced. Optical double-sideband suppressed-carrier signals with the same random phase noise and information encoded in the frequency separation of the two sidebands are generated by properly biasing the modulator. Phase noise is eliminated at the receiver by a nonlinear operation on the two sidebands. System performance analysis with numerical examples, taking into account the modulation index and laser normalized linewidth, is presented.

The steady-state behavior of regular two-connected multihop networks in homogeneous load under hot-potato and single-buffer deflection routing is analyzed for ultra-fast optical applications. The Manhattan street network and ShuffleNet are compared in terms of throughput, delay, and deflection probability both analytically and by simulation. It is analytically verified that single-buffer deflection routing recovers in both networks more than 60% of the throughput loss of hot-potato with respect to store-and-forward when packets are generated with independent destinations. This gain, however, decreases to below 40% when the average message length exceeds 20 packets.>

A space-division photonic switch which has the potential to achieve high dimensionality is presented. The proposed switch, which resembles a collapsed network, does not use optical crosspoints; rather a dedicated path is provided for all input/output port connections on a common high-bandwidth transmission medium. This eliminates the restrictions imposed by 2*2 switching elements in classical space-division switching fabrics. The demonstration of a fully connected 120*120 space-division time-multiplexed photonic switch is reported. The dimensionality and blocking performance of a shared-medium photonic switch that uses time-multiplexing is analyzed.>

Synchronous code-division multiple access (S/CDMA) is investigated for fiber-optic local area networks. It is shown that the large bandwidth expansion required by spread-spectrum techniques, such as CDMA, can be accommodated by using a fiber-optic channel for transmission and incoherent optical signal processing for code generation and correlation. Prime sequence codes, previously developed for a fiber-optic network using (asynchronous) CDMA, are modified to fit a synchronous transmission format. A performance comparison of CDMA and S/CDMA systems reveals that S/CDMA can accommodate a larger number of subscribers and more simultaneous users than CDMA. An environment for which S/CDMA would be suited is discussed.>

A reliable and economical new transmission code is presented with the following properties: zero dc content, baseband bandwidth conservation, self-clocking capability, and data-rate-transparent decoding and synchronization. Simple encoder/decoder and clock extractor circuits are given. The code is demonstrated in a wavelength-multiplexed fiber-optic communication system.

A new self-clocking channel code is demonstrated and compared to other codes used in digital communication systems. The new code requires two channels and is best suited for wavelength-multiplexed optical fiber transmission systems. An experimental demonstration of the proposed code in a fiber-optic communication link is reported and the code performance is analyzed.

An experimental fiber optic local area network utilizing code division multiple access (CDMA) and fiber optic delay-line signal processing is reported. The throughput-delay characteristics are compared to ALOHA and CSMA/CD. Since CDMA allows many users to access the network simultaneously, it can achieve higher throughput with no delay.

Two synchronous multiple access schemes, TDMA and CDMA, are proposed for fiber optic networks using optical signal processing. Network synchronization is achieved by using a central modelocked laser which also serves as the source for each station. The data are converted into a high-bandwidth optical signal using electrooptic modulators. The accessing schemes use optical fiber delay lines. The feasibility of these schemes is discussed.

Theoretical results predict that by using orthogonal modulation in direct detection fiber optic systems, the minimum optical power requirements can be reduced by up to 6 dB, compared to nonorthogonal systems. This letter reports on the results of an experiment comparing orthogonal and nonorthogonal modulation in fiber optic systems.

The effect of nonparalyzable dead time on Poisson point processes with random integrated rates is studied. The case of exponentially decreasing rate, plus background (pedestal), with a uniformly uncertain starting time is explicitly presented. The decay time is considered to be slow compared to the refractory time. No constraints on the sampling time are imposed for calculating the mean and variance, though for the counting distribution, the sampling time must be short compared to the decay time. The results are expected to be useful in neurobiology, neural counting, psychophysics, photon counting, nuclear counting, and radiochemistry.

The conditions under which processing reduces to a single threshold comparison is derived for the binary orthogonal likelihoodratio, detection of a signal embedded in additive, independent noise. The range of the signal and noise is assumed to be discrete. Optimal processing of the observation vector, as Well as two commonly used ad hoc processing schemes, are discussed. It is shown, in all three cases, that single-threshold processing is optimal, provided a weak condition on the noise is satisfied, independent of the signal. The results are presented in the context of a PCM/FSK photocounting fiber-optic communication system in which the noise is considered to be of the Poisson or avalanche types. Although single-threshold processing is shown to be optimal in most cases, examples are presented where single-threshold processing is not optimal.

A single-threshold detector is derived for a wide class of classical binary decision problems involving the likelihood-ratio detection of a signal embedded in noise. The class of problems considered encompasses the case of multiple independent (but not necessarily identically distributed) observations of a nonnegative (or nonpositive) signal embedded in additive and independent noise, where the range of the signal and noise is continuous. It is shown that a comparison of the sum of the observations with a unique threshold comprises an optimum detector if a weak condition on the noise is satisfied independent of the signal. Examples of noise densities that satisfy and that violate this condition are presented. A sufficient condition on the likelihood ratio which implies that the sum of the observations is also a sufficient statistic is considered.