Optical Networks – Introduction The current thinking about IP over WDM by outlining a path to optical data networking, that includes multiple data networking protocol coupled with a protocol-neutral optical networking infrastructure is challenged. This tutorial discusses the diversity of data networking protocols and network architectures for optical data networking. The bandwidth explosion ushered in by the popularity of the Internet has led to a paradigm shift in the telecommunication industry from voice-optimized circuit-switched services to data-optimized packet-switched services. The notation of supporting “data directly over optics” has been fueled by the promise that elimination of unnecessary network layers will lead to a vast reduction in the cost and complexity of the network. In this view of reduced or collapsed network layers, existing TDM systems such as Synchronous Digital Hierarchy (SDH) plays a diminishing role, and optical transport networking emerges as the underlying transport infrastructure for the resultant “network of networks”. Optical Internet Optical internet working, for example, as defined by the Optical Interworking Forum (OIF), is a data-optimized network infrastructure in which switches and routers have integrated optical interfaces and are directly connected by fiber or optical network elements, such as Dense Wavelength-Division Multiplexers (DWDMs). At present, however, the notion of IP directly over WDM is little more than cleverly disguised marketing. Almost invariably, IP over WDM is IP packets mapped into SDH, coupled with SDH based point-to-point DWDM systems. SDH standalone elements, often referred to as Time-Division Multiplexer (TDMs), are not required, but SDH remains an integral element of the data networking equipment interface. Ever-increasing reliance on the presence of SDH in DWDM systems limits technological innovation. For example, it may inhibit packet over fiber applications such as Asynchronous Transfer Mode (ATM), Gigabit Ethernet (GbE) and 10 GbE over DWDM. Nor does it bring us any closer to realizing the ultimate vision of optical transport networking. As compared to the present view of IP over WDM, there is a more balanced view of data/transport network evolution. This balanced view is based on two fundamental principles − Every data network is unique, in a marketplace governed by differentiation. The Optical Transport Network (OTN), as the underlying infrastructure “network of networks” should be capable of transporting a wide variety of client signals, independent of their format. Together, these fundamental principles form the basis for the notion of optical data networking. Learning working make money
Category: optical Networks
Optical Networks Tutorial Job Search Optical networks are telecommunications network of high capacity. They are based on optical technologies and components, and are used to route, groom, and restore wavelength levels and wavelength-based services. This tutorial is divided into distinct chapters, which explains the structural features of optical fibers and their connections in networks. The nature of optical networks along with the recent developments in the Optical and Networking systems using optical sources and devices is also dealt with. Audience This tutorial is designed for learners who have interest in learning the networking concepts using optical sources. It will be useful for those having some idea regarding networking and optical sources. Prerequisites Readers will benefit from this tutorial if they are aware of basic networking concepts. A fair idea on digital networking and digital communication systems will be a plus. Learning working make money
Single and Multi-hop Networks Telecommunications traffic continues to grow at a very fast pace. This is accelerated through the increasing volume of data and mobile traffic, especially in India, through the recent liberalization of the telecommunications market. A solution can be adopted to meet the ever-increasing traffic requirements based on a combination of WDM, SDH, and IP transport technologies. Wavelength-division multiplexing is used to multiplex several wavelength channels on a single strand of fiber, thus overcoming fiber congestion. SDH technology offers the capacity granularity, which the customers demand today and offers the possibility to protect these services against network outages. An IP-over-WDM transport network can offer high capacity Internet transit services to Internet Services Providers (ISPs). Synchronous Digital Hierarchy Synchronous Digital Hierarchy (SDH) networks replaced PDH and have several key advantages. G.707, G.708, and G.709 ITU recommendations provide the basis for global networking. Networks benefit from traffic resilience to minimize traffic loss in the event of fiber break or equipment failure. Built-in monitoring technology allows remote configuration and troubleshooting of network. Flexible technology allows tributary access at any level. Future proof technology allows for faster bit rates as technology advances. European PDH networks could not interface with US networks, SDH networks can carry both types. The above figure shows how the different PDH networks compare and which signals can be carried across the SDH network. SDH – Network Topologies A line system is the system to the PDH network topology. Traffic is added and dropped only at the endpoints of the network. Terminal nodes are used at the end of the network for adding and dropping the traffic. Line System Within any SDH network, it is possible to use a node known as a regenerator. This node receives the high order SDH signal and retransmits it. No lower order traffic access is possible from a regenerator and they are only used to cover long distances between sites, where the distance means that the received power would be too low to carry traffic. Ring System A ring system consists of several add/drop muxes (ADMs) connected in a ring configuration. Traffic can be accessed at any ADM around the ring and it also possible for traffic to be dropped at several nodes for broadcast purposes. The ring network has the benefit of offering traffic resilience, if there is a fiber break traffic is not lost. Network resilience is discussed in detail in a subsequent chapter. SDH Network Synchronization While PDH networks were not centrally synchronized, SDH networks are (hence, the name synchronous digital hierarchy). Somewhere on the operator’s network will be a primary reference source. This source is distributed around the network either over the SDH network or over a separate synchronization network. Each node can switch to backup sources, if the main source becomes unavailable. Various quality levels are defined and the node will switch the next best quality source it can find. In cases where the node uses the incoming line timing, the S1 byte in the MS overhead is used to denote the quality of the source. The lowest quality source available to a node is generally its internal oscillator. In a case where a node switches to its own internal clock source, this should be remedied as soon as possible, as the node may start to generate errors over time. It is important that the synchronization strategy for a network is planned carefully. If all the nodes in a network try to synchronize off its neighbor on the same side, you will get an effect called a timing loop, as shown in the above figure. This network will quickly start to generate errors as each node tries to synchronize off each other. SDH Hierarchy The following figure shows how the payload is constructed, and it isn’t as scary as it looks at first. Learning working make money
Optical Networks – Devices In this chapter, we will discuss the various components of optical devices. Isolator Isolator is a non-reciprocal device that allows light to pass along a fiber in one direction and offers very high attenuation in the opposite direction. Isolators are needed in the optical system to prevent unwanted reflections, coming back down a fiber and disrupting the operation of a laser (producing noise). In manufacturing isolators “Faradays Effect” is used, which is polarization dependent. Isolators are constructed using optical polarizers, analyzers and Faradays rotator. The optical signal passes through the polarizer, oriented parallel to the incoming state of polarization. Faradays rotator will rotate the polarization of optical signal by 45 degrees. The signal then passes through the analyzer, which is oriented at 45 degrees with respect to the input polarizer. The isolator passes an optical signal from left to right and changes its polarization by 45 degrees and produces about 2 dB loss. Circulator Circulators are micro-optic devices and can be used with any number of ports, however, commonly 3 ports/4 ports circulators are used. It has a relatively low loss 0.5 dB to 1.5 dB port-to-port. The basic function of a circulator is shown in the above figure. Light entering any particular port (say port 1) travels around the circulator and exits at the next port (say port 2). Light entering at port 2 leaves at port 3, and so on. The device is symmetric in operation around a circle. Circulators are micro-optic devices and can be made with any number of ports. However, 3 and 4 port circulators are very common. Circulators have very low loss. Typical port-to-port loss is around .5 to 1.5 db. Splitters and Couplers Couplers and splitters are used to combine optical signals and/or split the optical signals. The vast majority of single mode optical couplers employ the principle of resonant coupling. Two SM fiber cores are placed parallel and close to one another. Optical power transfers from one core to another and back by electromagnetic wave induction. Power coupling depends on the length of the coupling section. Three important characteristics are − Return Loss − The amount of power reflected and lost. Insertion Loss − The amount of signal lost in total transit through a device. Excess Loss − Additional loss of a device above theoretical loss. Types of Couplers Y couplers Star couplers Fused fiber Mixing plate Planar (free space) 3 dB coupler Beam splitter Filters Filters are used to select the signal in trans path and receiver from many signals. The gratings are filters. Switches, modulators, AWGs, multiplexers, etc. are considered as types of filters. Following are the types of filters − Fabry-Perot Tunable filter In-fiber Bragg grating filter Filters are used in front of an LED to narrow the line width before transmission. Filters will be very useful in WDM networks for − A filter placed in front of an incoherent receiver can be used to select a particular signal from many arriving signals. WDM networks are proposed which use filters to control which path through a network a signal will take. Fiber Bragg Gratings are the most important optical filter in the communications world. Modulators Modulators consist of a material that changes its optical properties under the influence of an electric or magnetic field. In general, three approaches are used − Electro-optic and Magneto-optic effects Electro-absorption effects Acoustic modulators Due to mechanical vibrations Ref. Index of material changes. Acoustic modulators use very high frequency sound. By controlling the intensity of sound, we can control the amount of light deflected and hence, construct a modulator. Following are some of its advantages − They can handle quite high power. Amount of light refracted is linearly proportional to the intensity of sound waves. They can modulate different wavelengths at the same time. Optical ADM An optical filter is used to isolate or drop the desired wavelength from multiple wavelengths arriving on a fiber. Once a wavelength is dropped, another channel employing the same wavelength can be added or inserted on to the fiber, as it leaves OADM. A simple ADM has only 4 input and output channels, each with four wavelengths. In OADM, wavelengths might be amplified, equalized or further processed. OADM arranges the wavelengths from input fiber to output fiber using optical cross-connect. Optical Cross-Connect An optical x-connect can take four input fibers, each carrying four wavelengths, and rearrange the 16 wavelengths, on to the four output fibers. A simple transponder inside OXC will shuffle one of the wavelengths to an available channel. Learning working make money
Convergence Networks Today”s TDM-based transport networks have been designed to provide an assured level of performance and reliability for the predominant voice and based-line services. Proven technologies, such as SDH, have been widely deployed, providing high-capacity transport, scalable to gigabit per second rates, for voice and leased-line applications. SDH self- healing rings enable service-level recovery within tens of milliseconds following network failures. All of these features are supported by well- established global standards enabling a high degree of multivendor interoperability. Today’s Network In contrast to today”s TDM-based transport networks (and, to some extent, with ATM networks), “best-effort” IP networks generally lack the means to guarantee high reliability and predictable performance. The best-effort service provided by most legacy IP networks, with unpredictable delay, jitter, and packet loss, is the price paid to achieve maximum link utilization through statistical multiplexing. Link utilization (e.g. the number of users per unit of bandwidth) has been an important figure of merit for data networks, since the links are usually carried on leased circuits through the TDM transport network. Given the inherently bursty nature of data traffic, the fixed-bandwidth pipes of TDM transport may not be an ideally efficient solution. However, this inefficiency has traditionally been considered of less importance important than the network reliability and congestion isolation features of a TDM-based transport network provider. The surging demand for high bandwidth and differentiated data services is now challenging this dual architecture model of TDM-based transport and best effort packet networks. It is not cost- effective to extend the usefulness of best-effort networking by over provisioning network bandwidth and keeping the network lightly loaded. Furthermore, this approach cannot always be achieved or guaranteed due to spotty demand growth, and is a particular issue for the network access domain, which is most sensitive to the economic constraints of underutilized facilities. As a result, in general, data service providers today do not have the network infrastructure support to provide customer- specific differentiated service guarantees and corresponding service-level agreements. Next Generation Network Next generation network architectures for cost-effective, reliable, and scalable evolution will employ both transport networking and enhanced service layers, working together in a complementary and interoperable fashion. These next-generation networks will dramatically increase, and maximally share, backbone network infrastructure capacity, and provide sophisticated service differentiation for emerging data applications. Transport networking enables the service layers to operate more effectively, freeing them from constraints of physical topology to focus on the sufficiently large challenge of meeting service requirements. Hence, complementing the many service-layer enhancements, optical transport networking will provide a unified, optimized layer of high-capacity, high reliability bandwidth management, and create so-called optical data networking solutions for higher capacity data services with guaranteed quality. Optical Transport Networking : A Practical View Visions of optical networking have captured the imagination of researchers and network planners alike, since the rapid and successful commercialization of WDM. In the original vision of optical transport networking, a flexible, scalable, and robust transport network emerges, catering to an expanding variety of client signals with equally varied service requirements (flexibility, scalability, and survivability coupled with bit rate and protocol independence). The promise of a transport infrastructure capable of meeting the burgeoning bandwidth demands well into this new century, wherein wavelengths replace timeslots as the medium for providing reliable transfer of high-bandwidth services across the network, is indeed tantalizing. But what is optical networking ? The answer varies widely, and in fact has evolved over recent years. Early attempts at optical networking focussed on an optical transparency and the design of optically transparent networks on a global scale. Practical Solution In the absence of viable “all-optical” solutions more practical solutions for optical networking accommodate the need for opto-electronics to support optical signal regeneration, and optical signal performance monitoring. In what is termed all-optical networking, signals traverse the network entirely in the optical domain, with no form of opto-electronic processing. This implies that the all signal processing- including – signal regeneration, routing, and wavelength interchange – takes place entirely in the optical domain. Due to limitations of analog engineering (e.g. limiting factor in a properly designed digital system is an one accuracy of the conversion of the original analog message waveform into digital form) and considering the current state- of- the- art in all-optical processing technology, the notion of global or even national all optical networks is not practically attainable. In particular, opto-electronic conversion may be required in opto network elements to prevent the accumulation of transmission impairments – impairments that result from such factors areas fiber fibre chromatic dispersion and nonlinearities, cascading of non-ideal flat-gain amplifiers, optical signal crosstalk, and transmission spectrum narrowing from cascaded non-flat filters. Opto-electronic conversion can also support wavelength interchange, which is currently a challenging feature to realize in to the all optical domain. In short, in the absence of commercially available devices that perform signal regeneration to mitigate impairment accumulation and support wavelength conversion in the all-optical domain, some measure of opto-electronic conversion should be expected in near-term practical optical networking architectures. The resulting optical network architectures can be characterized by optically transparent (or all-optical) subnetworks, bounded by feature-enhanced opto-electronics, as shown in the above figure. Client Signal Transparency Beyond analog network engineering, practical considerations will continue to govern the ultimate realization of the OTN. Paramount among these considerations is the network operator”s desire for a high degree of client signal transparency within the future transport infrastructure. What is meant by “Client signal transparency”? Specifically, for the desired set of client signals targeted for transport on the OTN, individual mappings are defined for carrying these signals as payloads of optical channel (OCh) server signals. Signals expected in the OTN include legacy SDH and PDH signals, and packet-based traffic such as Internet Protocol (IP), ATM, GbE and Ssimple Ddata Llink (SDL). Once a client signal has been mapped into its OCh server signal at the ingress of the OTN, an operator deploying such a network need not have detailed knowledge of (or access to) the client signal, until it is demapped at
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Optical Data Networking IP over WDM, as defined today, imposes a restrictive view of the capabilities that data networks and optical networks can provide. The constraints, introduced by a single protocol stack and not by fully using the networking capabilities at the optical layer are very restrictive for some network applications. The networking trends mentioned above require an optical networking platform that can support a variety of protocol stacks, network architectures, and protection and restoration options in a client-signal independent way. The POS over point-to-point WDM choice is best for some of the network applications in high-speed data networks, but certainly not for all. Also, the optical platform selected to implement and deploy these future data networks must ensure that new, unexpected protocol stack mappings can easily be accommodated, and they can receive the same networking features from the optical layer network without the need for an intermediate protocol conversion. Optical data networking is an alternative approach that does not try to reduce the heterogeneity of protocol stacks and network architectures, but rather exploits the heterogeneity to provide tailored network solutions to each particular application and network provider segment. Optical data networking combines networking features at both the service and transport layers. Main Component of Optical Data Networking The diversity of protocol stacks, reflected in the multiplicity of client signal types to be supported in the OTN, is accommodated by the use of digital wrappers. The use of true optical networking features offer additional flexibility and robustness via OCh routing, fault and performance monitoring, protection, and restoration, all performed on a selective per OCh basis. All these elements combined together render a powerful and flexible networking solution that is future-proof and open to any particular vision of data service providers. This technology is cost-effective and more flexible for the upgradation of channel capacity, adding/dropping of channels, re-routing and traffic distribution, supporting all types of network topology and protection systems and synchronization. Following are the main components − TP (Transponder) VOA (Variable Optical Attenuator) MUX (Multiplexer) DEMUX (De-multiplexer) BA (Booster Amplifier) Line (OFC media) LA (Line Amplifier) PA (Pre Amplifier) OSC (Optical Supervisory Channel) Transponder This unit is an interface between STM-n wide pulse optical signal and MUX/DEMUX equipments. This optical signal may be co-located or coming from different physical mediums, different protocols, and traffic types. It converts the wide pulse signal into a narrow wavelength (spot or colored frequency) of the order of nano-meter (nm) with spacing of 1.6 nm; sending to MUX. In the reverse direction, colored output from the DEMUX is converted to a wide pulse optical signal. The output power level is +1 to –3 dBm in both directions. The conversion is Optical to Electrical and Electrical to Optical (O to E & E to O) in 2R or 3R method. In 2R, regeneration and re-shaping are done, while in 3R, regeneration, re-shaping, and re-timing are performed. TP may be the wavelength color and bit rate dependent or tunable for both (costly and not used). However, in 2R, any bit rate, PDH, STM-4 or STM-16 may be the channel rate. The unit has a limitation with the receiver sensitivity and overload point. Though the intermediate electrical stage is inaccessible, overhead bytes of STN-n are utilized for supervisory purpose. This unit also supports optical safety operation (ALS) over ITU-T Recommendation G.957. Variable Optical Attenuator (VOA) This is a passive network like pre-emphasis required to adjust for uniform distribution of signal level over EDFA band so that individual channel optical output power of Mux unit remains the same irrespective of the number of channels being loaded in the system. The optical attenuator is similar to a simple potentiometer or circuit used to reduce a signal level. The attenuator is used whenever performance test must be run, for example, to see how the bit error is affected by varying the signal level in the link. One way is to have a precise mechanical setup in which the optical signal passes through a glass plate with differing amount of darkness and then back to the optical fiber, as shown in the figure. The glass plate has grey density ranging from 0% at one end to 100% at the other end. As the plate is moved across the gap, more or less light energy is allowed to pass. This type of attenuator is very precise, and can handle any light wavelength (since the plate attenuates any light energy by the same amount, regardless of the wavelength), but it is mechanically expensive. Multiplexer (MUX) and Demultiplexer (De-MUX) As DWDM systems send signals from several stations over a single fiber, they must include some means to combine the incoming signals. This is done with the help of a Multiplexer, which takes optical wavelengths from multiple fibers and converges them into a beam. At the receiving end, the system must be able to separate out the transmitted wavelengths of the light beam so that they can be discreetly detected. Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them into individual fibers. Multiplexers and Demultiplexers can be either passive or active in design. Passive design uses prism, diffraction gratings, or filters while active design combines passive devices with tunable filters. The primary challenges in these devices are to minimize crosstalk and maximize channel separation (the wavelength difference between two adjacent channels). Crosstalk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength. Types of Multiplexer/ Demultiplexer Prism Type A simple form of multiplexing or demultiplexing of wavelengths can be done using a prism. A parallel beam of polychromatic light impinges on a prism surface and each component wavelength is refracted differently. This is the rainbow effect. In the output light, each wavelength is separated from the next by an angle. A lens then focuses each wavelength to the point where it needs to enter a fiber. The components can be used in reverse to multiplex different wavelengths
Optical Networks – ROADM Legacy optical networks deploy SDH/SONET technologies for transporting data across the optical network. These networks are relatively easy to plan and to engineer. New network elements can be easily added to the network. Static WDM networks may require less investment in equipment, especially in metro networks. However, the planning and maintenance of those networks can be a nightmare as engineering rules and scalability are often quite complex. Bandwidth and wavelengths must be pre-allocated. As wavelengths are bundled in groups and not all groups are terminated at every node, access to specific wavelengths might be impossible at certain sites. Network extensions might require new Optical-Electrical-Optical regeneration and amplifiers or at least power adjustments in the existing sites. Operating static WDM network is manpower intensive. Network and bandwidth planning should be as easy as in SDH/SONET networks in the past. Within the given ring bandwidth, for example STM-16 or OC-48 each node could provide as much bandwidth as needed. Access to the entire bandwidth was possible at every ADM. Network extension, for example, introduction of a new node in an existing ring, was relatively easy and did not require any on-site visits of the existing nodes. The network diagram on the left illustrates this: Digital cross-connect systems link up with multiple optical SDH/SONET rings. Reconfigurable optical networks act differently: Bandwidth can be planned on-demand and the reach is optimized as the optical power is now managed per WDM channel. The scalability goes up significantly. The key element for enabling such a reconfigurable optical network is Reconfigurable Optical Add-drop Multiplexer (ROADM). It enables optical wavelengths to be redirected to client interfaces on just a click in the software. Other traffic remains unaffected by this. All this is achieved without needing any truck rolls to the respective sites to install filters or other equipment. Reconfigurable WDM Network with ROADMs Static WDM engineering rules and scalability can be quite complex (OADM in every node). Bandwidth and wavelength pre-allocation Margin allocation for fixed filter structure Insufficient power management Network extension requires Optical-Electrical-Optical (OEO) regeneration SDH/SONET networks are easy to plan. Access to entire bandwidth at every ADM Easy engineering rules (single hop only) Easy addition of new network elements A reconfigurable optical layer enables the following. On-demand bandwidth planning Extended transparent reach due to power management per WDM channel Hitless scalability Static photonic layers consist of separate optical rings. Consider a number of DWDM systems located on each of these rings. Frequently information or data simply remains on the same ring, hence there is no issue. However, what happens in cases where data needs to be handed over to a different optical ring? In static systems, a large number of transponders is required wherever a transition between rings is needed. Actually, each wavelength which passes from one ring to another needs two transponders: one on each side of the network. This approach incurs high costs and a lot of initial planning, considering the allocation of bandwidth and channels. Let us now imagine a dynamic reconfigurable photonic layer. Here, there is only one single DWDM system forming the interface between two optical rings. Consequently, transponder-based regeneration disappears and the number of DWDM system drops. The whole network design is simplified and wavelengths can now travel from one ring to another without any further obstruction. Any wavelength can propagate to any ring and to any port. The key to such a fully flexible and scalable network design, with an optical pass-through from the core right to the access area, is the ROADM and the GMPLS control plane. Simplifications Through ROADMs ROADMs provide simplifications in the network and in the service provider’s or carrier’s processes. This interaction summarizes some of these simplifications. After all, we need to bear in mind that all these advantages result in reduced time effort and cost. But what is more important is that they also lead to increased customer satisfaction and, in turn, customer loyalty. Network planning is vastly simplified using ROADMs. Just consider the significantly reduced number of transponders, which need to be stocked in the warehouse. Installation and commissioning − for example, when setting up a new wavelength to the network − require significantly less effort and are much less complex. Service technicians only need to visit the respective end sites to install the transponders and ROADM. Fixed Optical Add/Drop Multiplexers (FOADMs) used to require a visit to each intermediate site so that installation work and patches could be carried out. Operations and maintenance are greatly simplified when a dynamic optical network is deployed. Optical diagnostics can be carried out in a few minutes rather than hours, as was previously the case. Impairments can be detected and dynamically cleared instead of triggering truck rolls to external sites. With the deployment of tunable lasers and colorless ROADMs, the maintenance of the fiber plant is easier. Using these features, service provisioning is now easier than ever before. As with the installation and commissioning work, it is also significantly easier to perform network maintenance and any potential upgrades. ROADM Architecture Many advantages ROADMs bring to network design and operation were covered in the previous sections. Here are a few more − Per-channel power monitoring and leveling to equalize the entire DWDM signal Full traffic control from the remote network operation center One question, however, has so far been left unanswered: How does a ROADM work? Let’s take a look at some fundamentals. A ROADM generally consists of two major functional elements: A wavelength splitter and a wavelength selective switch (WSS). Take a look at the block diagram above: An optical fiber pair at network interface No. 1 is connected with the ROADM module. The fiber carrying the incoming data (from the network) is fed to the wavelength splitter. Now, all the wavelengths are available at all output ports of the splitter, in this case 8. Local add/drop traffic (wavelengths) can be multiplexed/de-multiplexed with an Arrayed Waveguide Filter (AWG). Using an AWG implies a fixed wavelength allocation and direction. The Wavelength
Optical Networks – Quick Guide Optical Networks – Introduction The current thinking about IP over WDM by outlining a path to optical data networking, that includes multiple data networking protocol coupled with a protocol-neutral optical networking infrastructure is challenged. This tutorial discusses the diversity of data networking protocols and network architectures for optical data networking. The bandwidth explosion ushered in by the popularity of the Internet has led to a paradigm shift in the telecommunication industry from voice-optimized circuit-switched services to data-optimized packet-switched services. The notation of supporting “data directly over optics” has been fueled by the promise that elimination of unnecessary network layers will lead to a vast reduction in the cost and complexity of the network. In this view of reduced or collapsed network layers, existing TDM systems such as Synchronous Digital Hierarchy (SDH) plays a diminishing role, and optical transport networking emerges as the underlying transport infrastructure for the resultant “network of networks”. Optical Internet Optical internet working, for example, as defined by the Optical Interworking Forum (OIF), is a data-optimized network infrastructure in which switches and routers have integrated optical interfaces and are directly connected by fiber or optical network elements, such as Dense Wavelength-Division Multiplexers (DWDMs). At present, however, the notion of IP directly over WDM is little more than cleverly disguised marketing. Almost invariably, IP over WDM is IP packets mapped into SDH, coupled with SDH based point-to-point DWDM systems. SDH standalone elements, often referred to as Time-Division Multiplexer (TDMs), are not required, but SDH remains an integral element of the data networking equipment interface. Ever-increasing reliance on the presence of SDH in DWDM systems limits technological innovation. For example, it may inhibit packet over fiber applications such as Asynchronous Transfer Mode (ATM), Gigabit Ethernet (GbE) and 10 GbE over DWDM. Nor does it bring us any closer to realizing the ultimate vision of optical transport networking. As compared to the present view of IP over WDM, there is a more balanced view of data/transport network evolution. This balanced view is based on two fundamental principles − Every data network is unique, in a marketplace governed by differentiation. The Optical Transport Network (OTN), as the underlying infrastructure “network of networks” should be capable of transporting a wide variety of client signals, independent of their format. Together, these fundamental principles form the basis for the notion of optical data networking. Convergence Networks Today”s TDM-based transport networks have been designed to provide an assured level of performance and reliability for the predominant voice and based-line services. Proven technologies, such as SDH, have been widely deployed, providing high-capacity transport, scalable to gigabit per second rates, for voice and leased-line applications. SDH self- healing rings enable service-level recovery within tens of milliseconds following network failures. All of these features are supported by well- established global standards enabling a high degree of multivendor interoperability. Today’s Network In contrast to today”s TDM-based transport networks (and, to some extent, with ATM networks), “best-effort” IP networks generally lack the means to guarantee high reliability and predictable performance. The best-effort service provided by most legacy IP networks, with unpredictable delay, jitter, and packet loss, is the price paid to achieve maximum link utilization through statistical multiplexing. Link utilization (e.g. the number of users per unit of bandwidth) has been an important figure of merit for data networks, since the links are usually carried on leased circuits through the TDM transport network. Given the inherently bursty nature of data traffic, the fixed-bandwidth pipes of TDM transport may not be an ideally efficient solution. However, this inefficiency has traditionally been considered of less importance important than the network reliability and congestion isolation features of a TDM-based transport network provider. The surging demand for high bandwidth and differentiated data services is now challenging this dual architecture model of TDM-based transport and best effort packet networks. It is not cost- effective to extend the usefulness of best-effort networking by over provisioning network bandwidth and keeping the network lightly loaded. Furthermore, this approach cannot always be achieved or guaranteed due to spotty demand growth, and is a particular issue for the network access domain, which is most sensitive to the economic constraints of underutilized facilities. As a result, in general, data service providers today do not have the network infrastructure support to provide customer- specific differentiated service guarantees and corresponding service-level agreements. Next Generation Network Next generation network architectures for cost-effective, reliable, and scalable evolution will employ both transport networking and enhanced service layers, working together in a complementary and interoperable fashion. These next-generation networks will dramatically increase, and maximally share, backbone network infrastructure capacity, and provide sophisticated service differentiation for emerging data applications. Transport networking enables the service layers to operate more effectively, freeing them from constraints of physical topology to focus on the sufficiently large challenge of meeting service requirements. Hence, complementing the many service-layer enhancements, optical transport networking will provide a unified, optimized layer of high-capacity, high reliability bandwidth management, and create so-called optical data networking solutions for higher capacity data services with guaranteed quality. Optical Transport Networking : A Practical View Visions of optical networking have captured the imagination of researchers and network planners alike, since the rapid and successful commercialization of WDM. In the original vision of optical transport networking, a flexible, scalable, and robust transport network emerges, catering to an expanding variety of client signals with equally varied service requirements (flexibility, scalability, and survivability coupled with bit rate and protocol independence). The promise of a transport infrastructure capable of meeting the burgeoning bandwidth demands well into this new century, wherein wavelengths replace timeslots as the medium for providing reliable transfer of high-bandwidth services across the network, is indeed tantalizing. But what is optical networking ? The answer varies widely, and in fact has evolved over recent years. Early attempts at optical networking focussed on an optical transparency and the design of optically transparent networks on a global scale. Practical Solution In the absence of viable “all-optical” solutions more practical solutions for
Optical Networks – WDM Technology WDM is a technology that enables various optical signals to be transmitted by a single fiber. Its principle is essentially the same as Frequency Division Multiplexing (FDM). That is, several signals are transmitted using different carriers, occupying non-overlapping parts of a frequency spectrum. In case of WDM, the spectrum band used is in the region of 1300 or 1550 nm, which are two wavelength windows at which optical fibers have very low signal loss. Initially, each window was used to transmit a single digital signal. With the advance of optical components, such as Distributed Feedback (DFB) lasers, Erbium-doped Fiber Amplifiers (EDFAs), and photo-detectors, it was soon realized that each transmitting window could in fact be used by several optical signals, each occupying a small traction of the total wavelength window available. In fact, the number of optical signals multiplexed within a window is limited only by the precision of these components. With the current technology, over 100 optical channels can be multiplexed into a single fiber. The technology was then named dense WDM (DWDM). WDM in the Long Haul In 1995, long-haul carriers in the United States started deploying point-to-point WDM transmission systems to upgrade the capacity of their networks while leveraging their existing fiber infrastructures. Since then, WDM has also taken the long-haul market by storm. WDM technology allows to cope with ever-increasing capacity requirements while postponing the exhaustion of fiber and increasing the flexibility for capacity upgrade. The most prevailing driver, however, is the cost advantage of the WDM solution compared to competing solutions, such as Space Division Multiplexing (SDM) or enhanced Time Division Multiplexing (TDM) to upgrade the network capacity. The “open” WDM solution, illustrated in the following figure makes use of transponders in WDM terminal multiplexers (TMs) and inline optical amplifiers that are shared by multiple wavelength channels. The transponder is in essence a 3R opto-electro-optic (O/E/O) converter, that converts a G.957 standard compliant optical signal into an appropriate wavelength channel (and vice versa) while repowering, reshaping and retiming the signal electrically. The SDM solution uses multiple fiber pairs in parallel, each equipped with SDH regenerators instead of multiple wavelengths sharing the same inline optical amplifier. Upgrading to higher TDM rates (e.g., from 2.5 Gb/s STM-16 to 10 Gb/s STM-64) is only a short-lived solution since transmission impairments such as dispersion do not scale well with increasing TDM rates, especially on standard single-mode fiber. A case study has demonstrated that long haul point-to-point WDM systems are clearly a more cost-effective solution than SDM, even for as low as three channels of STM-16. The above figure illustrates two link cost comparisons for the initial core of a transport network consisting of 5000 fiber km with an average distance of 300 kms between two access cities. Note that the 100 percent cost reference point in the above figure corresponds to the cost of deploying one STM-16 channel, including fiber cost. Two conclusions can be derived from the above figure. As shown in the following figure, if only transmission and regeneration equipment costs are considered (i.e., SDH regenerators in the SDM case and WDM TMs with transponders with inline optical amplifiers in the WDM case), the initial link cost of using WDM technology is more than double that of SDH. However, WDM solution is more cost-effective for the deployment of three channels and more in the network, because of the shared use of the inline optical amplifier. As shown in the following figure, if in addition to the above consideration, the fiber cost is also considered, the cost advantage of WDM case becomes even more evident and is amplified as the number of channels increase. WDM solution is more cost-effective for the deployment of three channels and more in the network. WDM in the Short Haul Regenerators are not necessary and optical impairments have less impact because of the limited distances in the short haul networks, hence the benefits of WDM are less clear than those of SDM or enhanced TDM solutions. However, fiber exhaustion and low-cost optical components are now driving WDM in the metropolitan area. The short-haul application is related to the inter-connection of multiple Points of Presence (POPs) within the same city. Let us consider an example. The following figure shows that the transport network has at least two POPs per city, where the customers can interconnect. With dual node interconnection techniques, such as drop and continue, customer networks can be interconnected with the transport network via two different POPs. This results in a very secure architecture that can even survive POP failures without any traffic impact. Thus, the traffic flow between two POPs in a city consists of not only traffic that passes through the city, but also of traffic that is terminated in the city and protected using Drop and Continue. These increased intra-city capacity requirements have led to the deployment of WDM in the short-haul section of a transport network. The main reason WDM is preferred over SDM is because fibers in a city have to be leased from a third party or a fiber optic network has to be built. Leasing or building city fiber is not only an expensive process, it is also a less flexible approach to upgrade capacity. In a dynamic environment, where traffic distributions and volumes evolve rapidly, the amount of fiber to be leased or built is hard to predict in advance. Therefore, using WDM technology has clear flexibility advantages because the wavelength channels can be activated in a very short time. Although specific short-haul WDM systems are available in the world, it is advantageous to use the same type of WDM system for its long-haul network. While short-haul WDM systems are less expensive than their long-haul counterparts and due to their low-cost optical components can be used, they lead to a heterogeneous network, which is not preferred for several reasons. First, using two different systems leads to an increased operational and management cost. For instance, a heterogeneous network requires more