Showing papers on "Optical Transport Network published in 1995"

A heuristic wavelength assignment algorithm for multihop WDM networks with wavelength routing and wavelength re-use

Abstract: Presents a heuristic algorithm for effectively assigning a limited number of wavelengths among the access stations of a multihop network wherein the physical medium consists of optical fiber segments which interconnect wavelength-selective optical switches. Such a physical medium permits the limited number of wavelengths to be re-used among the various fiber links, thereby offering very high aggregate capacity. Although the optical connectivity among the access station can be altered by changing the states of the various optical switches, the resulting optical connectivity pattern is constrained by the limitation imposed at the physical level. The authors also study two routing schemes, used to route requests for virtual connections. The heuristic is tested on a realistic traffic model, and the call blocking performance of new requests for virtual connections is studied through extensive simulations and compared against the blocking performance of an ideal infinite capacity centralized switch (lowest possible call blocking caused exclusively by congestion on the finite capacity user input/output links, never by the switch fabric itself). Surprisingly, the authors find that, for a wide range of parameters, the blocking performance of the lightwave network is almost the same as that of the ideal centralized switch. From these results, they conclude that the heuristic algorithm is effective and the routing scheme is efficient. >

Method and system for detecting optical faults within the optical domain of a fiber communication network

Abstract: A method and system for detecting optical signal degradation or loss within the optical domain of a fiber network. Optical cross-connect switches (OCCS) are provided at network nodes. Optical data traffic is conducted between nodes via a fiber link. The fiber link extends between respective ports of the optical cross-connect switches at each network node. Dedicated signals for detecting faults are introduced and removed within the optical domain of the fiber communication network. In one embodiment, transmitter/receiver units are coupled directly to the fiber link for broadcasting optical signals for fault detection. In another embodiment, transmitter/receiver units are coupled to the fiber link through one or more dedicated ports within optical cross-connect switches (OCCS) at network nodes. Optical signals for fault detection are then broadcast or switched through working and/or spare fibers. Distinctive optical fault detection signals are used when the signals are broadcast or switched over working fibers. Optical signal impairment or loss for a particular link or span is detected simply and quickly in the network optical domain. Rapid, accurate optical fault capability can be added to communication networks without requiring changes to existing light terminal elements and processing circuitry in the electrical domain.

Emerging technologies for fiber network survivability

Abstract: Reducing network protection costs, while maintaining an acceptable level of survivability, has become an important challenge for network planners and engineers. This article will review technology and architectures that may be used to implement cost effective survivable fiber networks for each transport layer, and discuss the interworking system between survivability mechanisms across different layers and associated open issues. Standards development, product availability and the current status of deployment will also be reviewed. The first section reviews a class of survivable fiber network architectures that has been deployed or is scheduled to be deployed. Next is a review of emerging technologies for these survivable architecture implementations. These emerging technologies include SONET, ATM, and passive optical technology. Finally the issue of multiple layer interworking on SONET/ATM networks is discussed. >

The design of a European optical network

Abstract: This paper presents a number of design issues under consideration in the development of an all-optical network linking major centers in Europe. The proposed network is characterized by spanning a large geographical area, the diameter is in excess of 3000 km, but with a relatively small number of nodes (20). The study combines aspects of two optical network projects funded by the European Commission: COST 239-ultra-high capacity optical transmission networks and RACE 2028: multiwavelength transport network. The network uses multiwavelength technology and combines wavelength selection and space switching to achieve signal routing. One of the objectives of the study is to understand the extent to which transparency can be achieved in such a large network. For example the effect of using amplifiers to achieve transparency is to provide extremely long fiber paths in which non-linear effects may become significant and effectively limit the number of wavelengths that can be employed; this limit may be at conflict with the number of wavelengths required from a traffic viewpoint. The paper therefore considers capacity requirements, transmission limitations and node and network architecture issues. It is concluded that partitioning of the network may be necessary to maximize transparency and suggestions are made as to how this might be achieved. >

Dynamically reconfigurable WDM optical communication systems with optical routing systems

Abstract: The present invention provides reconfigurable wavelength division multiplexed systems which include configurable optical routing systems. Through the use of the configurable optical routing systems, optical channel traffic patterns may be flexibly and dynamically determined based on overall system conditions at any particular time. In one embodiment, a dynamically reconfigurable wavelength selector system for a wavelength division multiplexed optical communication system is provided. An optical transfer system includes an input port for receiving at least one of N optical channels from an optical transmission medium. An optical switch is coupled to a first output port of the optical transfer system and includes an optical filter comprised of at least one Bragg grating configured to reflect at least one optical channel toward the optical transfer system on a first switch path and a transmission medium for transmitting the N optical channels on a second switch path. The reflected optical channel exits the optical transfer system through the second output port. In another aspect, the present invention provides a wavelength division multiplexed communication system with optical cross-connects for routing selected optical traffic between a pair of optical paths. Plural optical transfer systems are provided which optically communicate with at least two optical transmission lines. Optical filters are provided such that the optical channels which are not reflected by the filter cross over to the opposite optical transmission line. The optical cross-connects are used as wavelength-selective crosspoints to create blocking or non-blocking switching networks.

Tunable add/drop optical filter providing arbitrary channel arrangements

Abstract: A novel add/drop optical filter providing arbitrary channel arrangements between two input wavelength division multiplexing (WDM) signals and two output WDM signals is presented. This device can have useful applications in WDM optical communication systems. >

Wavelength division multiplexing passive optical network including broadcast overlay

Abstract: A passive optical network is provided that spectrally slices optical signals transmitted in both upstream and downstream directions utilizing wavelength division multiplexing routing. The passive optical network preferably includes an incoherent signal source at both ends to provide signals that are spectrally sliced according to optical frequency. The downstream information may be transmitted in a conventional data format. The upstream transmissions may be segregated by subcarrier multiplexing, time scheduling or wavelength division multiplexing. A broadcast signal can be overlayed with point-to-point information for transmission to plural network units.

Optical access communications network with a multiplexer/demultiplexer in each optical network terminating equipment

Abstract: For subscriber access, via optical fibre, to communications networks that offer various types of service by means of an optical access node (AN). The connections between the optical network terminating equipments (ONT), to which the subscribers are connected, and the access node (AN) can be made with point-to-point or point-to-multipoint fibre optic links. The links between the access node (AN) and the switching centre (CE) are made via optical fibres through which pass the lightwave signals corresponding to a number of subscribers, multiplexed together by means of wavelength division multiplexing. This network permits access to all types of communications services in which each type of service employs a reserved wavelength for its connection between the access node (AN) and the optical network terminating equipment (ONT). This reserved wavelength is fixed and depends only on the type of service.

Coherent Optical Communications Systems

Abstract: Theory of Optical Communications. Fiber Devices for Optical Communication Systems. Integrated Devices for Optical Communication Systems. Direct Detection Optical Communication Systems. Coherent Systems: Structure and Ideal Performance. Performance Degradation Sources in Coherent Optical Systems. Multilevel Coherent Optical Systems. Multichannel Optical Systems. Present--Day Systems Implementations. Appendices. Index.

Optical data communications network with a plurality of optical transmitters and a common optical receiver connected via a passive optical network

Abstract: An optical data communications network, for example a TPON (telecommunications over passive optical networks) network, has a common optical receiver and a plurality of optical transmitters connected to the common optical receiver by a passive optical network which consists of optical splitters/combiners linked by lengths of an optical fibre. The optical signals produced respectively by the different optical transmitters are interleaved together in a predetermined time-division multiple access format and are modulated using a return-to-zero modulation format. The common optical receiver can operate satisfactorily at desirably-high bit rates despite wide variations in the power levels of the optical signals reaching the receiver from the different transmitters, which variations arise due to the different attenuations experienced by those signals as they propagate along optical fibre paths of different lengths containing different numbers of splitters/combiners. As a result, power levelling of the transmitters is not required.

An optical infrastructure for future telecommunications networks

Abstract: For the transport of the increasing traffic volume caused by existing and new narrowband services and evolving broadband services, the enhancement of the existing public telecommunication transport network is necessary. For this purpose an optical network layer with cross-connect and add/drop functionalities will be added to the existing transport network. A comparative analysis of space, time, and optical frequency division multiplexing has shown that for the time being optical frequency multiplexing is best suited for the realization of that new network layer. This multiplexing scheme offers the greatest advantages such as very high bandwidth utilization in the fiber and simple and efficient cross-connecting of high bitrate streams. In the near future, technology will be mature enough for the realization of a demonstrator network based on optical frequency division multiplexing. The functionalities of the optical network are evaluated and the results clearly show that optical frequency conversion and regeneration should be provided by the optical network. The article also deals with the realization aspects (cross-connecting, supervision, and operation and maintenance) of an optical node. >

Method and system for detecting optical faults in a network fiber link

Abstract: A method and system for detecting optical faults by tapping a portion of a data signal from along a fiber network link at an optical switching site or node. The tapped data signal is evaluated to determine whether an optical fault exists along the fiber link. In this way, optical faults such as a fiber failure, cable cut, or nodal LTE failure occurring within a network path, are isolated to a specific fiber span or link. Network switch controllers can then take full advantage of optical switches provided between spans in an all-optical core network to restore end-to-end path communication. In one embodiment, a presence detector simply evaluates the magnitude of the tapped signal to determine an optical fault. In a second embodiment, a protocol analyzer evaluates selected bits or fields of the protocol used in the tapped data signal to determine an optical fault. Finally, in a third embodiment, a sub-carrier modulation signal is added to the high rate data signal prior to transport over the link. The subcarrier signal is significantly lower in both frequency and amplitude than the main data signal so as to not impact reliable reception of the main data signal. A low-pass filter tuned to the sub-carrier modulation signal filters the tapped data signal. By detecting the sub-carrier signal, fault can be determined inexpensively and reliably in the presence of optical noise.

Electronic wavelength translation in optical networks

Abstract: We study the benefits of electronic (regenerative) wavelength translation in optical networks providing wavelength channel circuit-switching among users. The electronic translation means that an optical signal on one wavelength is converted to electronics and then converted again into an optical signal on another wavelength. A previous study has demonstrated that wavelength translation can significantly improve the performance of a large mesh network. We consider an optical network architecture based on a mesh topology where each node is supplied with an array of W transmitters and receivers (where W is the number of wavelengths). For this architecture we study effectiveness of electronic wavelength translation as a low cost alternative to all-optical wavelength translation. We propose wavelength assignment algorithms over a given routing path which minimize the number of wavelength changes. The results of our performance study for a static routing mesh network show that electronic translation with such algorithms can be almost as effective as all-optical wavelength translation. In fact, the performance of electronic translation converge to those of all-optical translation as the size of a large mesh increases.

Multiwavelength self-healing ring transparent networks

Abstract: This paper describes the use of optical cross-connect nodes (as used in RACE project MWTN) for implementing multiwavelength unidirectional and bi-directional self-healing ring networks. A comparison of these two ring structures is presented in terms of the number of wavelength required to satisfy a uniform random traffic demand. Two protection strategies based on the electronic self-healing ring principles are applied to the wavelength division multiplexed (WDM) networks. It is shown how the bi-directional WDM rings can be protected against failure by means of spare wavelengths as well as spare fibres. The proposed network protection schemes are accomplished within the optical network with failure restoration transparent to the layers below. The ring networks described are also flexible and adaptable to changes in traffic patterns.

Increasing the transmission capacity of coherence multiplexed communication systems by using differential detection

Abstract: We experimentally verify the impact of optical beat noise on a coherence-multiplexed optical communication system. We then show that optical beat noise can be significantly reduced, and transmission capacity increased, by using differential detection. We demonstrate transmission at low bit error rate of four coherence multiplexed channels, each having a capacity of 1 Gb/s, over 8 km of dispersion-shifted fiber. This is the highest capacity demonstration to date for any code-division multiplexing scheme.

The Case for Opaque Multiwavelength Optical Networks

Abstract: Wavelength-division multiplexing (WDM) now seems certain to play a leading role in public telecommunications networks of the future. What is unclear, however, is whether these WDM networks will be "transparent," i.e., whether their signals will travel from source to destination through a sequence of intervening nodes with little or no optoelectronic conversion along the way. In this work we consider "opaque" WDM networks, in which network elements include optoelectronic conversion and signal regeneration along the path, thus preventing accumulation of impairments. There are two chief arguments for building transparent WDM networks. The first is the argument from "format-transparency"the ability to transport signals in analog or digital or unspecified modulation formats on separate wavelength channels. This is a remarkable feature of transparent WDM networks. However, given the current momentum in public networks towards digital formats in general, and towards SONET and ATM in particular, it seems unlikely that the case for network-transparency will rest on formabtransparency alone. The second argument for transparency turns on the ability of a transparent WDM network to provide high-speed nodebypass and to reconfigure in response to failures and changing traffic conditions. Here we argue that opaque WDM networks also offer these same advantages, but without the complexity and cost associated with transparency. Moreover, we show that in opaque VJDM networks, wavelength-translation can be added nearly for free. The argument for transparency is often made by referring to some variant of the node architecture in Fig. 1, which contains a wavelength add-drop multiplexer (W-ADM) with fixed optical routing. We assume that the W-ADM resides in an arbitrary ring or bus network, and that each wavelength transports a highspeed signal at rates on the order of OC-48 or above. The transparent node of Fig. l a has a weighty virtue: for wavelengths 1-4, which simply transit the node, no high-speed tra.nsmitters or receivers are needed. This offers potential cost savings over the opaque node of Fig lb. However, the transparency of Fig. la also carries costs. The optical amplifiers will require gain-equalization, which they will not require in opaque implementations. Fiber dispersion and nonlinearity will often make it difEcult or impossible to use existing, installed fiber. Component-crosstalk requirements will be stringent. The network's wavelength-multiplexers must have broad, flat passbands to avoid the transmissionspectrum-narrowing that results from cascaded non-flat filters. Moreover, passband-narrowing, passbandmisalignment, and fiber dispersion will impose tight requirements on the frequency-registration and spectral purity of the laser transmitters. Finally, transparency introduces a host of new operations and management challenges, including the tasks of network monitoring and partitioning transmission impairments. Trans parent networks will thus require new approaches to creating operations and management systems. To summarize, transparency does offer a substantial benefithigh-speed nodebypassbut it offers this at substantial cost. Node-bypass does not require transparency. The opaque node of Fig. l b provides the benefits of node bypass without incurring any of the casts listed above. This is chiefly because the digital receiver/transmitter pairs in channels 1-4 of Fig. l b prevent signal impairments from acm"mting through the network, thus alleviating the tight performance requirements that must otherwise be placed on the network's optical components. Of course, opacity also carries a cost, namely, the cost of an added digital receiver/transmitter pair for each wavelength transiting the node. It is open to argument whether the cost of these optoelectronic converters will be more or less than the costs of transparency enumerated above. However, when wavelength-translation is added, the advantage turns clearly in favor of opacity. One implementation of a transparent W-ADM with wavelength-translation is shown in Fig. 2a. It requires both optical switches and optical wavelength-converters. Practical wavebengthconverters, however, have yet to be demonstrated. An opaque W-ADM with wavelength reconfiguration and translation is shown in Fig. 2b. It merely requires an electrical crassbar switch operating at the line rate per wavelength (e.g. OC-48) to implement wavelength-translation. The cost of such electrical switches is expected to be small compared with the cost of optical wavelength-converter. Moreover, opaque networks avoid the host of transparency difficulties listed above. We conclude that when wavelength-translabtion is needed in WDM networks, the advantage tips strongly toward opacity. For larger reconfigurable, wavelength-converting WDM crossconntcts used in arbitrary mesh networks the situation is much the same. Figure 3 represents an opaque reconfigura.ble wavelength-converting crossconnect with optical input and output ports. The opaque crossconnect terminates 4 input fibers and 4 output fibers. Each fiber transports 8 wavelengths. The network element of Fig. 3 then performs the following function: it crossconnects the 8 . 4 = 32 input WDM channels with the 32 output channels. The sole restriction is the usual one: the mapping must be one-bone. Note that wavelength-translation again presents a substantial practical challenge for transparency, but comes naturally with opacity. This is clear from Fig. 3, which shows an opaque WDM crossconnect that provides reconfigurable routing with full wavelength-translation, all of which is purchased with a modest amount of optoelectronic conversion and electronic crosspoint switching. Once again, a transparent implementation requires both optical switches and optical wavelength-converters.

A wavelength division multiplexing passive optical network with bi-dir ectional optical spectral slicing

Abstract: A passive optical network is provided that spectrally slices optical signals transmitted in both upstream and downstream directions utilizing wavelength division multiplexing routing. The passive optical network preferably includes an incoherent signal source at both ends to provide signals that are spectrally sliced according to optical frequency. The downstream information may be transmitted in a conventional data format. The upstream transmissions may be segregated by subcarrier multiplexing, time scheduling or wavelength division multiplexing.

Optical multiplexing in fiber networks: progress in WDM and OTDM

Abstract: WDM systems are maturing and are in use in real point-to-point applications. The technology has also become of interest in the development of all-optical networks, and in this context the performance of WDM can be more critical. In some networks, multiple-access demands that a large number of channels be deployed over long distances, and in these cases the performance of WDM may be degraded by crosstalk arising from fiber nonlinearity. Other problems arise from non-ideal components such as optical amplifiers, switches, and multiplexers/demultiplexers, Much activity in WDM has been understanding its limitations and improving component performance, Some of the limitations imposed may be overcome by the use of OTDM, and research in this area continues to be of interest and importance. However, OTDM is less mature, and much activity is concerned with the realization of robust subsystems such as sources and demultiplexers. It is likely that in the longer term networks will employ a combination of WDM and OTDM.

Tree-type photonic switching networks

Abstract: The enormous bandwidth offered by optical systems makes photonic switching a very attractive solution for broadband communications. Tree-type architectures play an important role in the design of photonic switching networks. The authors present and discuss all known tree-type space-division photonic switching networks architectures in a unified framework, and propose a number of new solutions. The discussed networks can be implemented with guided-wave-based switching elements, or laser diodes and passive splitters and combiners. The following network types are considered: conventional, simplified, and two-active stage networks. Techniques for improving SNR as well as waveguide crossover minimization are presented and discussed. >

Optical communications network.

Abstract: For subscriber access, via optical fibre, to communications networks that offer various types of service by means of an optical access node (AN), connections between the optical network terminating units (ONT) unit to which the subscribers are connected, and the access node (AN) can be made with point-to-point or point-to-multipoint fibre optic links. The links between the access node (AN) and the switching centre (CE) are made via optical fibres through which pass the lightwave signals corresponding to a number of subscribers, multiplexed together by means of wavelength division multiplexing. This network permits access to all types of communications services in which each type of service employs a reserved wavelength for its connection between the access node (AN) and the optical network terminating equipment (ONT) unit. This reserved wavelength is fixed and depends only on the type of service.

Polyhedral integrated and free space optical interconnection

Abstract: An optical communication system uses holographic optical elements to provide guided wave and non-guided communication, resulting in high bandwidth, high connectivity optical communications. Holograms within holographic optical elements route optical signals between elements and between nodes connected to elements. Angular and wavelength multiplexing allow the elements to provide high connectivity. The combination of guided and non-guided communication allows compact polyhedral system geometries. Guided wave communications provided by multiplexed substrate-mode holographic optical elements eases system alignment.

Photonic transport networks based on optical paths

Abstract: This paper explores the technologies that will enable a further leap forward in transport network evolution. First, requirements of the future transport network are elucidated. Existing network cost is evaluated and the viable means to reduce transport network cost are identified. It is demonstrated that optical path technologies such as WP (wavelength path) and VWP (virtual wavelength path) will play a key role in this. They enhance not only transmission capacity but also cross-connect node throughput cost-effectively by capitalizing on the wavelength routeing scheme. Optical path realization technologies focusing on optical path cross-connect systems are examined. Requirements for the optical path cross-connect system are then elucidated, and a new cross-connect system architecture is evaluated. The architecture exploits PLC (planar lightwave circuit) and other commercially available technologies suitable for large-scale production. The evaluations include cross-connect node upgradability, modular growth capability, total node cost, optical loss and switching power consumption. It is proved that the architecture has significant advantages over existing switch architectures. The optical path technologies provided in this paper will pave the way for a new transport network paradigm, a ubiquitous, bandwidth-abundant and affordable broadband ISDN.

Optical network with subcarrier multiplexing

Abstract: Disclosed is an optical network which has a plurality of subordinate optical networks, each of which being connected with a plurality of optical terminals which function as a transmitter and a receiver of a signal light, wherein multiplexing of transmission signals and demultiplexing of reception signals at the optical terminals are carried out in the subcarrier multiplexing manner; and a main optical network which is connected with the subordinate optical networks, wherein multiplexing of signal lights transmitted from the subordinate optical networks is carried out in the wavelength division multiplexing manner.

Number of wavelengths required for constructing optical path network considering restoration

Abstract: Optical networks that support high-speed, low-cost communication services are required. an optical path is proposed that utilizes wavelength multiplexing technology at the path layer of transport network layers. Optical path schemes, wavelength path (WP)/virtual wavelength path (VWP) that can realize optical path networks are also proposed. Although wavelengths are a resource in optical networks, the number of wavelengths required to construct an optical path network has not been evaluated. This paper describes a method of network protection using the optical path technique that enhances network reliability and proposes a method for establishing a restoration path that restores failed active paths in both WP and VWP schemes. Proposed herein is an optical path accommodation design to establish WPs or VWPs in a physical network considering the proposed restoration method. Furthermore, optical path networks are constructed by establishing WPs or VWPs on a physical network that has a regular structure and the number of wavelengths required in the network are evaluated. This evaluation makes it clear that the VWP scheme requires fewer wavelengths than the WP scheme.

Optical cross-connect architecture using free-space optical switches based on PI-LOSS topology

Abstract: An optical cross-connect (OXC) system is an essential ingredient needed to create an optical transport network for B-ISDN. We propose an OXC architecture, incorporating failure restoration based on reserved wavelengths, which consists of a routing block and bypass block. We compare the number of required optical devices in our structure with a conventional structure. We also propose a polarization controlled free-space switch based on path-independent insertion loss (PI-LOSS) topology, adopt it to the routing block, and demonstrate a 4/spl times/4 prototype free-space switch.

Optical CDMA based on spectral encoding with integrated optical devices

Abstract: In this paper we show the application of optical CDMA based on spectral encoding with integrated optical devices and discuss the use of cyclic shifted m-sequences to realize full orthogonal transmission. Degradations of the system performance due to non-ideal source spectra, crosstalk and losses in the optical coder devices are examined. Some bounds for different kinds of realization are shown. Possible sources are broadband semiconductor diodes like SLD, optical semiconductor amplifiers or multiwavelength lasers. For optical coding solutions with acoustical tunable optical filters, arrayed waveguide structures and fiber grating filters are investigated. The total network capacity will be analyzed under the above mentioned conditions and it is shown that such a network can accommodate some hundred users with a network throughput of some Gb/s.

Optical path cross-connect systems transport experiment with simulated five-node network

Abstract: The authors describe an optical path cross-connect systems transport experiment performed by simulating a five-node network. Eight wavelength-division multiplexing signals, directly modulated at 2.5 Gbit/s, are confirmed to be transported over 330 km from the origination to the termination node via a cross-connection node. Optical path cross-connection is successfully performed with a crosstalk level of better than –35 dB.

Interferometric noise reduction in crosstalk corrupted optical WDM and TDM switching fabrics

Abstract: Interferometric noise due to crosspoint crosstalk in optical WDM and TDM switching fabrics is substantially reduced by manipulation in the time and/or wavelength domains. Furthermore, providing sufficient discrete wavelengths are available, or sufficient time compression can be realized, almost all noise may be eliminated at the expense of additional equipment. Practical realizations of the proposed scheme are discussed. >

Highly efficient interconnection for use with a multistage optical switching network with orthogonally polarized data and address information

Abstract: A novel optical interconnection is introduced for a multistage optical switching network that uses orthogonally polarized data and address information. The network is unique in that the data information is never regenerated and remains in optical form throughout (i.e., it is never converted into electrical information). This has two main consequences: (1) the bandwidth of the data is not restricted by electrical circuit considerations, and (2) the optical interconnections from one stage of the network to the next must be highly efficient. The interconnection meets several goals: high efficiency, preservation of cross polarization of data and address, low cross talk between polarizations, good manufacturability, resistance to misalignment caused by thermal expansion, and absence of significant aberrations. In addition, sychronization of the signals is maintained, as the optical path lengths for all routes through the system are equal.