DWDM: shaping the future communications network

This paper describes the increase in the number of optical carriers per single fiber, a technology known as wavelength division multiplexing (WDM). This successful technology takes advantage of existing glass or synthetic fiber. WDM greatly simplifies the traditional signal regeneration since optical amplification is much simpler and more cost effective than single channel amplification. However, network bandwidth elasticity is best addressed with WDM. However photonic devices performing in the low-loss spectral band have enabled more than a single wavelength in the same fiber. Thus, DWDM technology exhibits an inherent flexibility.     

A Time Compression Multiplexing System for a Circuit Switched Digital Capability

The time compression multiplexing (TCM) technique offers a solution to the problem of achieving digital penetration of the two-wire loop plant. TCM is planned to provide the digital loop transmission in the new circuit switched digital capability of the Bell System, so that a full-duplex 56 khit/s digital connection is obtained between subscriber's premises and the serving central office. Herein, the concept of TCM and its parameters are described. Tradeoffs leading to determination of system parameters such as burst frequency, burst length, guard time, and delay as they affect the range of the system are discussed. Also, the performance of the digital loops in terms of eye diagrams, system range limitations due to crosstalk and impulse noise, and percentage of loops covered are summarized. In addition, the circuit implementation using both analog and digital technologies is described. Experiences with various digital architectures to implement TCM and tradeoffs leading to a low-power digital architecture suitable for VLSI are discussed.     

A primer on cryptography in communications

The secure transport of messages was the concern of many early civilizations. Since then, different methods have been developed to assure that only the sender and the addressee would be able to read a message, while it would be illegible or without significant meaning to a third party. Today, this practice continues with more fervor. Wireless, wired, and optical communication networks are able to transport unimaginable amounts of data and thus privacy of information and security of the network are of the utmost concern because a good part of the transported information may be very sensitive and/or confidential. Confidentiality of information has been particularly popularized with the explosive growth of the Internet, which has touched most people's lives. However, from the outset, the Internet was based on open network architecture with computer-based nodes and without network security, and thus was vulnerable to attackers and hackers. Wireless networks transmit in unguarded space and thus are also subject to malicious actors. Dense wavelength-division multiplexing (DWDM) optical networks transport a massive aggregate traffic that has exceeded terabits per second per single fiber; therefore, the issues of information privacy and network security are equally important. As a result, the development of unbreakable cipher keys, cipher-key distribution, identification of malicious actors, source authentication, physical-link signature identification, countermeasures, and so forth has been the major thrust of research efforts with regard to cyber-security. This article focuses on cryptography, and is the first of a series of three articles on cryptography and security in communications. Subsequent articles will cover wireless and IP network security, as well as optical network security, quantum cryptography, and quantum-key distribution processes specific to optical networks.     

Elastic bandwidth [optical-fiber communication]

Discusses dense wavelength division multiplexing (DWDM) photonic technology and the role it will play in shaping future communications networks. The flexibility of DWDM is in handling continually increasing bandwidth demands for future optical-fiber communication networks     

The reflected and transmitted shadow methods for the study of sharp V-notched plates under pure bending

An experimental study for investigating the constrained zones at the tip of edge V-notches in plexiglas plates and its dependence on the ratio depth of notch to the width of the plate, under static pure bending moment is developed.The optical reflected and transmitted shadow methods were used for various angles of the V-notch under different values of bending moment and for various ratios of depth of V-notch to width of the plate.The apparatus used is of the balance type to assure that the applied forces are distributed equally and that only pure moment forces are responsible for the stress field. The apparatus is suitable for both methods, the reflected shadow and the transmitted shadow, which are also compared.Experimental evidence shows that, the constrained zone exhibits a maximum size at the one extreme of angle of V-notch zero degrees, and a minimum at the other extreme of 180°, where it vanishes. Between the two extremes a second maximum is found at about 73°, and a second minimum at about 35°. At first approximation, it is deduced that the stress distribution, as the apex of a V-notch of angle up to about 90°, can be approximated by the stress distributions at the tip of a straight crack, under the same conditions.     

Next generation hierarchical CWDM/TDM-PON network with scalable bandwidth deliverability to the premises

Long haul optical networks have been on focus for more than two decades. With the advent of dense wavelength division multiplexing technology, optical long haul fiber networks have been so successful in delivering an unprecedented amount of bandwidth that they outperformed the traffic deliverability from/to the access network by orders of magnitude. The reason was a cost-efficiency mismatch; long haul ultra-high bandwidth networks can take advantage of state of the art and costly technology, which cost-sensitive access networks cannot. The result was an unbalanced traffic flow from/to access points to the network if one compares the aggregate flow of the long haul network with that of the access. Nevertheless, over the last decade technology at the access advanced and new standards have been developed so that in the access layer of the overall communications network the focus has shifted onto fiber optic access again. Thus, in the optical regime, two proposals have prevailed. One uses a time division multiplexing (TDM) scheme over a single wavelength and a comprehensive timing protocol, and the other uses coarse wavelength division multiplexing (CWDM) technology. Each technology has advantages and disadvantages, and the one attempts to address the disadvantages of the other. In this paper we describe a hierarchical CWDM/TDM passive optical network (PON). Our access network architecture is scalable, it is flexible to accommodate one of several topologies simultaneously, and it delivers any type of payload, synchronous and asynchronous that spans from DS0 to Gbps. We discuss the bandwidth flexibility, versatility, resiliency and cost efficiency of the access network. We also demonstrate that our network can deliver payload to more than 16,000 end-users using simple and existing optical technology. Thus, if one considers cost per bandwidth or per user, the cost-efficiency outperforms any previous PON access network. Moreover, we provide simulation results to support the viability of our network architecture.     

Interferometry for the evaluation of the dynamical optical and mechanical properties of transparent materials

A theory is developed which leads to an interferometric method for determining the dynamic ratio, Poisson's ratio to the modulus of elasticity, and the dynamic stress-optical coefficients of transparent plates, both during the same set of tests and without attaching mechanical or electrical sensors on the surface of the specimen.

The method requires a monochromatic light source, that is, a parallel beam is directed perpendicularly to the vicinity of a crack tip which exists in the plate specimen under investigation. The rays impinging on the crack tip are transmitted through the plate and reflected by the two parallel surfaces of the plate. At the same time, because of the variation of the optical path (index of refraction and thickness of the plate) in the said area, the rays are refracted in such a way as to form two caustic surfaces, one by the transmitted and one by the reflected rays. These two caustic surfaces, when intercepted with two opaque screens, provide two strongly illuminated curves of a general epicycloid shape called caustics.

The size of the caustics is directly related to the applied load, the material properties and the distance specimen-screen. Their ratio, however, as it is taken at equally distant screens, depends only on the optical and mechanical properties of the material in question.

In addition, an interferogram consisting of fringes, as formed from rays reflected by the two surfaces of the plate, is obtained. The fringe number depends on the wavelength of the light source and the optical path. When the plate specimen is loaded, the optical path changes. This results in a change of the fringe number and consequently to a displacement of fringes with respect to a reference point as they are observed on an opaque screen.

The ratio of the two caustics, taken during a static test, and the number of displaced fringes per applied load, taken during a dynamic test, provide enough information for the determination of the aformentioned mechanical and optical parameters.

Experiments conducted according to the method developed on acrylic plates and for a tensile impulse of total time duration of 600 μsec produced a reduced by 23.5% ratio of Poisson's ratio to modulus of elasticity from the static value and a reduced by 21% stress-optical coefficient from the static value.     

An associative RAM-based CAM and its application to broadband communications systems.

The content addressable memory (CAM) is a memory device that executes fast recognition and validation of binary patterns of a limited set, as opposed to the random access memory (RAM) that executes slow recognition of a large set of patterns. Here, we present a logic circuit which, combined with existing RAM, constructs an associative RAM-based content addressable memory (AR-CAM) that executes fast pattern recognition of very large pattern sets. In addition, we describe the applicability of the AR-CAM to broadband communications systems. Broadband communications systems with an aggregate bandwidth of tens or hundreds of Gbps require real-time recognition and translation of large pattern sets. Although CAM are a good candidate for such applications, nevertheless we demonstrate that AR-CAM can better and inexpensively accomplish this task.