The explosion in bandwidth brought on by the popularity of the Internet has led to a paradigm shift in the telecommunications industry from a circuit optimized for voice - switched services to data-optimized packet-switched services. The rating of 'data directly to optical' support has been fueled by the promise that removing unnecessary network layers will result in significant reduction.the cost and complexity of the network.
In this view of reduced or reduced network layers, existing TDM systems such as synchronous digital hierarchy (SDH) play a decreasing role and the optical transport network emerges as the sub-transport infrastructure. underlying for the resulting "network of networks".
Optical Internet
Optical Internet operation, for example, as defined by the Optical Interworking Forum (OIF), is a data-optimized network infrastructure in which the Switches and routers have built-in optical interfaces and are directly connected by fiber or optical network elements, such as dense wavelength division multiplexers (dwdm).
At present, however, the notion of IP directly on WDM is little more than cleverly disguised marketing. Almost invariably, IP over WDM consistsof SDH-mapped IP packets, associated with dSDH-based point-to-point DWDM systems. SDH stand-alone elements, often referred to as time division multiplexers (TDM), are not required, but SDH remains an integral part of the data network equipment interface.
Ever increasing dependence on the presence of SDH in DWDM Systems Limi technological innovation. For example, it can inhibit packet-over-fiber applications such as hronone transfer mode (ATM), Gigabit Ethernet (GbE), and 10 GbE over DWDM. It also does not bring us any closer to realizing the ultimate vision of the optical transport network.
Compared to the current view of IP over WDM, there is a more balanced view of the evolution of the data transport network. This balanced view is based on two fundamental principles -
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Each data network is unique, in a market ruled by differentiation.
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The Optical Transport Network (OTN), as The underlying infrastructure, "network of networks ", should be able to carry a wide variety of client signals, regardless of their format.
Together, these fundamental principles form the basis of the concept of optical data networking.
Convergence Networks
Today's TDM-based transport networks have been designed to provide a guaranteed level of performance and reliability for core services voice and line-based. Proven technologies, such as SDH, have been widely deployed, providing high capacity transport, scalable at gigabit per second rates, for voice and leased line applications. SDH self-healing rings provide service level recovery within tens of milliseconds of network outages. All of these features are supported by well established global standards allowing a high degree of interoperability.multi-vendor tee.
Today 's network
Unlike today's TDM-based transport networks (and, to some extent, with ATM networks), IP networks “At best” generally lack the means to ensure high reliability and predictable performance. The best 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 (for example, number of users per unit of bandwidth) has been an important figure of merit for data networks, as links are typically carried on leased circuitsthrough the network. TDM transport.
Given the inherently bursty nature of data traffic, fixed bandwidth channels of TDM transport may not be an ideally efficient solution. However,this inefficiency has traditionally been viewed as less important than the network reliability and congestion isolation features of a TDM-based transport network provider.
The growing demand for high bandwidth and differentiated data services is now a challenge. Dual-architecture model of TDM-based transport and best-effort packet networks. Extending the utility of best effort networking by over-provisioning network bandwidth and keeping the network lightly loaded is not cost effective.
In addition, this approach cannot always be realized or guaranteed due to irregular growth in demand, and it is a particular problem for the network access domain, which is most sensitive to the economic constraintsof underused installations. As a result, in general, data service providers today do not havedoes not support the network infrastructure 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 use both transport network and enhanced service layers, working together in a complementary and interoperable manner. These next-generation networks will dramatically increase, and share as much as possible, the capacity of the main network infrastructure, and provide sophisticated service differentiation for emerging data applications.
The transport network allows service layers to operate more efficiently, freeing them from the constraintsof the physical topology to focus on the sufficiently large challenge of meeting service requirements. Therefore, in addition tomany service layer enhancements, the optical transport network will provide a unified and optimized layer of high capacity and high reliability bandwidth management, and create optical data network solutions for higher capacity data services with guaranteed quality.
Optical Transport Network: A Practical Vision
Optical network visions have captured the imaginations of researchers and network planners since the rapid and successful commercialization of WDM. In the original vision of the optical transport network, a flexible, scalable and robust transport network emerges, responding to an increasing variety of customer signals with equally varied service requirements(flexibility, scalability and survivability associated with bit rate and protocol independence).
The promise of a transport infrastructure capable of meeting the growing demand for bandwidthin this new century, where wavelengths replace time slotsas a means of providing reliable transfer of high bandwidth services over the network, is indeed enticing. But what is the optical network? The answer varies widely and has in fact evolved in recent years. Early attemptsat optical networking focused on optical transparency and the design of optically transparent networks on a global scale.
Convenient solution
In the absence of viable “all-optics” solutions More practical solutions for optical networks meet the need for optoelectronics to support regeneration of the optical signal and optical signal performance monitoring. In what is called an all-optical network, the signals pass through the network entirely in the optical domain, without any form of optoelectronic processing. This implies that all signal processing -including - signal regeneration, routing and wavelength exchange - takes place entirely in the optical domain.
Due to limitations of analog engineering (e.g., limiting factor in a properly designed digital system is a unique precision of the waveform conversion of the original analog message in digital form) and taking into account the current state of all-optical processing technology, the concept of global or even national optical networks is practically not feasible.
In particular, optoelectronic conversion may be necessary in opto network elementsto avoid the accumulation of transmission degradations - degradations resulting from such factors: chromatic dispersion and non-linearities of fiber fibers, cascading of non-ideal flat gain amplifiers, optical signal crosstalk, and narrowing transmission spectrum from non-flat filters in casecade. Optoelectronic conversion can also support wavelength exchange, which is currently a difficult feature to achieve in the all-optical field.
In short, in the absence of commercially available devices that perform signal regeneration to alleviate the accumulation of disturbances and Supporting wavelength conversion in the all-optical domain, some measure of optoelectronic conversion should be expected in practical optical network architectures in the short term. The resulting optical network architectures can be characterized by short-term optical network architectures. optically transparent (or fully optical) subnets, delimited by optoelectronics with enhanced functionality, as shown in the figure above.
Client signal transparency
Beyond analog network engineering, practical considerations will continue to govern the rfinal realization of the OTN. Among these considerations, the most important is the desire of the network operator for a high degree of customer signal transparency within the future transport infrastructure.
What is meant by "customer signal transparency"? Specifically, for the desired set of client signals targeted for transport over the OTN, individual mappings are defined to transport these signals as optical channel payloads. (OCh) signals from the server. Signals expected in OTN include old SDH and PDH signals and packet based traffic such as Internet Protocol (IP), ATM, GbE and Ssimple Ddata L (SDL). Once a client signal has been mapped into itsOCh server signal to the OTN input, an operator who deploys such a network does not need to have detailed knowledge of the signal. client (or access to), until it is unmapped when exiting the network.
The entry pointsand output of the optical network must delimit the transparency domain of the client OTN signal. Therefore, the most important factor in achieving customer signal transparency is to eliminate all customer specific equipment and processing between the OTN entry and exit points. Fortunately, it is easier to accept customer dependent equipment on entry / exit, as it is usually dedicated service by service.
Optical transport network via Digital Wrappers
The widespread use of DWDM technology has presented service providers with a new challenge: how to profitably manage the growing number of wavelengths to provide fast and reliable services to their end customers. To effectively manage wavelength or OChs, optical networks must support the operation, administration and maintenance (OAM) functions by wavelength ou at the level of OCh.
Rec. The G872 defines some functionality for OCh level OAM implemented as overhead without specifying how this overload is to be transported. Until now, the only possible way to support signal regeneration and to monitor, analyze and manage OChs (wavelengths) was to rely on SDH signals and equipment across the board. network. This requires that the signals on each of the wavelengths of the WDM system be formatted as SDH.
One optical channel (wavelength)
Taking advantage of the optoelectronic regeneration pointsexisting in DWDM systems, the concept of using digital packaging technology will provide functionality and reliability similar to SDH, but for any signal customer, bringing us closer to realizing the original vision of optical transport networking.
Digital wrapper technology provides the foNetwork management functions described in ITU-T Rec. ITU (T). G.872 to activate OTNs. These include optical layer performance monitoring, Fforward Eerror C (FEC) correction, ring protection, and wavelength network restoration, all regardless of the format of the network. input signal, as shown in the following figure.
The notion of using a digital wrapper (or TDM) by" around "the OCh client for Supporting the OCh overhead associated with the channel has recently been proposed and has in fact been adopted as the basis for the definition of OCh. This scheme will take advantage of the need for OCh regeneration to add additional capacity to the OCh client. Of course, Once we have a way to add overload to the OCh client signal digitally, it makes sense to use it to support all OCh level OAM requirements.
In particular, the digitally added overheadmakes it almost trivial to solve the major OTN performance monitoring problem of accessing Bbit Eerror Rrate (BER) in a client independent manner. By optionally using FEC, the digital wrapper method can dramatically improve the BER performance of the client signal, further minimizing the need for optoelectronic conversion.
One method to improve the performance of the transport network is to use FEC, which is currently provided in some equipment. Therefore, an additional benefit of the digital wrapper technique is the ability to optionally support FEC for system margin improvement.
OCh frame structure
In functional terms, the OCh and OAM payload should be separable from the FEC mechanism. This allows end-to-end payload and OAM to be transported over the network, while using different sFEC schemas on different links. An obvious example of where this could happen is between submarine and land links. In the first, new FEC codes are being explored for the next generation of systems.
The following figure below figure illustrates the proposed basic frame structure of the OCh, and the types of functions that can be carried in the OCh frame structure. While it can be argued that this proposal is incompatible with the long-term goals of the all-optical network, we should not expect the need for regeneration to go away.
The distance between regeneration pointswill continue to increase; however, the need for regeneration at signal transfer pointswill remain. Coupled with the use of the Ooptical Ssupervisory Cchannel (OSC) to manage OChs within optically transparent subnets, digital wrappers will support the management ofend-to-end OCh (wavelength) across national or global OTNs.
3R-regeneration (Remodeling, La (resynchronization and regeneration) are provided by means of optical-to-electrical conversion and vice versa, and the digital packaging proposal takes advantage of this. Would the picture change if all-optical 3R regeneration became available? full optical regeneration is able to add overload, argument is unchanged; only regenerator implementation would change.
If optical regenerators could not add overload, the need to OChs overload will not go away.; Optical regenerators would then simply increase the potential distance between optoelectronic regeneration points, and the digital envelope would flow through them seamlessly. The implications of using digital packaging for the evolution of networks optical transport canbe profound, especially in the context of trends in data networks.
Choice of protocol stack
The IP protocol is clearly the convergence in today's data communication networks, and it is predictable that it will extend this role to multiservice networks in the years to come. IP can be transported over a wide variety of data link layer protocols and underlying network infrastructures. The following figure below The figure shows some of the possible protocol stacks, or mappings, of IPs in a WDM network infrastructure.
What is IP on WDM?
The protocol stacks labeled a, b, and d in the figure below are the most commonly deployed today. They use the classic IP over ATM over SDH mapping as shown in figure (a);. packet over SDH (POS) as shown in figure (b); or the classic and well extended IP over Ethernet as shown ina figure (d). Cases (e) and (f) use Simple Data Link (SDL), a new data link layer recently proposed as an alternative to POS. The protocol stack labeled (c) is an alternative to case (a), where the intermediate SDH layer is removed and a direct mapping of ATM cells in WDM is performed.
These different protocol stacks offer different functionality, in terms of bandwidth overhead, rate scalability, traffic management, and quality of service. To claim that a particular mapping representsIP on WDM is extremely dishonest.
This Optical Data Networki ng
IP over WDM, as defined today, imposes a restrictive view of the capacities that data networks and optical networks can provide. The constraints, introduced by a single protocol stack and not by fully utilizing the networking capabilities at the optical layer, are very restrictive for some network applications.
The networking trends mentioned above require an optical network platform that can support a variety of protocol stacks, network architectures, and networking options. protection and restoration independently of the client signal. The point-to-point WDM choicepoint-to-point is best for some of the network applications of high-speed data networks, but certainly not for all. In addition, the optical platform chosen to implement and deploy these future data networks must ensure that unexpected new protocol stack mappings can be easily supported, and they can receive the same networking functionality from the network. optical layer network without requiring 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 instead exploitsheterogeneity to provide network solutions tailored to each application segment and network provider. Optical data networking combines networking capabilities at the service layers and transport.
Composmain ant of optical data networking
The This technology is cost effective and more flexible for upgrading channel capacity, adding / removing channels, routing and distributing traffic, supporting all types of topology of network and protection and synchronization systems. Here are the principlespaux components-
- TP (transponder)
- VOA (variable optical attenuator)
- MUX (multiplexer)
- DEMUX (demultiplexer)
- BA (booster amplifier)
- Line (OFC media)
- LA (line amplifier)
- PA (Preamplifier)
- OSC (Optical Supervisory Channel)
Transponder
This device is an interface between Optical signal STM-n wide pulse and MUX / DEMUX equipment. This optical signal can be co-located or come from different physical media, different protocols and types of traffic. It convertsthe wide pulse signal into a narrow wavelength (spot or colored frequency) of the order of nanometer (nm) with a spacing of 1.6 nm; send to MUX.
In the reverse direction, the colored output of the DEMUX is converted to a wide pulse optical signal. The output power level is between +1 and –3 dBm dans both ways. The conversion is optical to electrical and electrical to optical (O to E and E to O) in 2R or 3R method.
In 2R, regeneration and reshaping is done, while in 3R, regeneration, re-shaping and re-timing are done. TP can be wavelength color and bit rate dependent or tunable for both (expensive and not used). However, in 2R, any bit rate, PDH, STM-4 or STM-16 can be the channel rate. The unit has limitation with receiver sensitivity and overload point.
Although the intermediate electrical stage is inaccessible, the overload bytes of STN-n are used for supervision. This device also supportsOptical Safety Operation (ALS) on ITU-T recommendation G.957.
Variable Optical Attenuator (VOA)
This is a passive network as the pre-emphasis requiredarea to adjust for a uniform signal level distribution on the EDFA band so that the optical output power of each channel of the Mux unit remains the same regardless of the number of channels loaded into the system.
The optical attenuator is similar to a simple potentiometer or circuit used to reduce a signal level. The attenuator is used whenever a performance test needs to be run, for example, to see how the bit error is affected by the change in signal level in the link. One way is to have a precise mechanical configuration in which the optical signal passes through a glass plate with a different amount of darkness and then returns to the optical fiber as shown in the figure.
The glass plate is gray density ranging from 0% at one end to 100% at the other end. When the plate is moved through space, more or less light energy is allowed to passer. This type of attenuator is very precise, and can handle any light wavelength (since the plate attenuates all 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 multiple stations on a single fiber, they must include means to combine the incoming signals . This is done with the help of a multiplexer, which takes the optical wavelengths of several fibers and convertsthem into a bundle. On reception, the system must be able to separate the transmitted wavelengths of the light beam so that they can be detected discreetly.
Demultiplexers perform this function by separating the received beam into itswavelength componentsand coupling them into individual fibers.
TheMultiplexers and demultiplexers can be passive or active in their design. The passive design uses a prism, diffraction gratings, or filters, while the active design combines passive devices with tunable filters.
The main challenges with these devices are to minimize crosstalk and maximize channel separation (the difference in wavelength 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
Type of prism
A simple form of multiplexing or demultiplexing of wavelengths can be done by hand using a prism.
A parallel beam of polychromatic light strikes a prism surface and each component wavelength is refracted differently. This is the effand rainbow . 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 must enter a fiber. The componentscan be used in reverse to multiplex different wavelengths onto a fiber.
Type of diffraction grating
Another technology is based on the principle of diffraction and optical interference. When a polychromatic light source strikes a diffraction grating, each wavelength is diffracted at a different angle and therefore at a different point in space. By using a lens, these wavelengths can be focused on individual fibers, as shown in the following figure. The Bragg grating is a simple passive component, which can be used as a wavelength selective mirror and is widely used to add and remove channels in DWD systemsM.
Braggs gratings are fabricated using an ultraviolet laser beam to illuminate the core of a single mode fiber through a phase mask . The fiber is doped with phosphorus, germanium or boron to make it photo-sensitive. Once the light has passed through the mask, a pattern of fringes is produced, which is "imprinted" into the fiber. This creates a modulation Periodic permanent refractive index of the fiberglass core. The finite grating reflectslight at the Bragg wavelength (equal to twice the optical spacing between the high and low index regions) and transmitsall other wavelengths.
Tunable Bragg Grating
A Bragg fiber grating can be bonded to a piezoelectric element by applying a voltage to it. element, the element stretches so that the grating is stretched and the Bragg wavelength changes to a longer wavelength.can provide 2nm tuning range for 150v input.
Matrix waveguide network
Matrix waveguide networks (AWG) are also based on the principles of diffraction. An AWG device, sometimes referred to as an optical waveguide router or waveguide network router, consistsof a curved channel waveguide network with a fixed difference in path length between adjacent channels. The waveguides are connected to inlet and outlet cavities.
Optical multiplexer
When light enters the entrance cavity, it is diffracted and enters the waveguide array. Thus, the difference in optical length of each waveguide introduces phase delays in the output cavity, where an array of fibers is coupled. The process resultsin different wavelengths having maximum interference in different places, which corresponds to the output ports.
Multilayer interference filters
A different technology uses interference filters in devices called thin film filters or multilayer interference filters. By positioning the filters, made of thin films in the optical path, the wavelength can be demultiplexed. The property of EAC The filter h is such that it transmitsone wavelength, while reflecting the others. By cascading these devices, many wavelengths can be demultiplexed.
Filters provide good stability and isolation between channels at a moderate cost, but with high insertion loss (AWGs have a flat spectral response and low insertion loss ). The main disadvantage of the filter is that it is temperature sensitive and may not be used in virtually all environments. However, their big advantage is that they can be designed to perform omultiplexing and demultiplexing operations simultaneously.
OM coupling type
OM coupling is an interactive surface with two or more fibers welded together. Generally, it is used for OM, and itsworking principles are shown in the following figure.
Coupling OM can only perform multiplexing function with low manufacturing cost. Itsdisadvantage is high insertion loss. Currently, OM used in ZTWE DWDM equipment uses OM coupling. OD adoptsAWG components.
Amplifiers amplifiers (optical amplifiers)
Due to attenuation, there are limitsto the duration during which a fiber segment can propagate a signal with integrity, before regenerating. Before the arrival of optical amplifiers (OA), a repeater was needed for each signal transmitted. OA had made it possible to amplify all lengths dboth wave and without optical-electrical-optical (OEO) conversion. In addition to being used in optical links (as a repeater), optical amplifiers can also be used to increase signal power after multiplexing or before demultiplexing.
Types of optical amplifiers
In each optical channel, optical amplifiers were used as repeaters in simplex mode. One fiber was used in the outgoing path and the second fiber was used in the return path. The latest optical amplifiers will operate in t two directions at the same time. We can even use the same wavelength in two directions, as long as we use two different bit rates. Only one fiber can therefore be used for duplex operation.
Optical amplifiers must also have sufficient bandwidth to pass a range of signals operating at different longwave uours. For example, an SLA with a spectral bandwidth of 40 nm, for example, can handle ten optical signals.
In a 565 mb / s system, for an optical link of 500 km, five SLA optical amplifiers are needed, spaced at an interval of 83 km. Each amplifier provides a gain of about 12 dB, but also introduces noise into the system (BER of 10-9.)
SLA amplifiers have the following disadvantages -
- Sensitive to temperature changes
- Sensitive to changes in supply voltage
- Sensitive to mechanical vibrations
- Not very reliable
- Crosstalk-prone
Erbium-doped fiber amplifier (EDFA)
In DWDM systems, EDFAs are used . Erbium is a rare earth element which, when excited, emitslight around 1.54 micrometers, which is the low loss wavelength foroptical fibers used in DWDM. A weak signal enters the erbium-doped fiber, into which light at 980nm or 1480nm is injected using a pump laser.
This injected light stimulates the erbium atoms to release their stored energy in the form of additional light of 1550 nm. The signal is getting strong. Spontaneous emissions in EDFAs also add the noise figure of an EDFA. EDFAs have a typical bandwidth of 100nm and are required at an interval of 80 to 120 km along the optical route.
EDFAs also suffer from an effect called four-wave mixing due to the non-linear interaction between adjacent channels. Therefore, increasing the power of the amplifier to increase the distance between repeaters leads to more crosstalk.
Raman amplifier
The use of SLA and EDFA amplifiers in WDM is limited as already described and, the systemsmy modern WDMs turn to Raman amplification, which is about 300nm wide. Here, the pump laser is at the receiving end of the fiber. Crosstalk and noise are greatly reduced. However, Raman amplification requires the use of a high pump laser.
The dispersion in the fiber actually minimizes the "four-wave mixing" effect. Unfortunately, early optical links often used zero dispersion fibers in an attempt to minimize dispersion over long distances, when those same fibers are upgraded to carry WDM signals; they are not the ideal medium for broadband optical signals.
Special fibers in mono mode are under development for WDM use. These have alternating segmentsof positively and negatively dispersed fibers, therefore the total dispersion amountsto zero. However, the individual segmentsprovidedent a dispersion to prevent four-wave mixing.
Line amplifiers
This is a two-stage EDFA amplifier consisting of a preamplifier (PA) and a booster amplifier (BA). Without the two stages, it is not possible to amplify the signal up to 33 dB on the EDFA principle (to avoid the noise generated by spontaneous emission). The Line Amplifier (LA) compensates for line loss by 22dB or 33dB respectively for long and very long haul systems. This is entirely an optical stage device.
Online Media (OFC)
This is the fiber-optic media over which DWDM signals travel. Attenuation and dispersion are the main limiting factors determining transmission distance, bit rate capacity, etc. Normally 22dB and 33dB are considered line loss for the hop length of long haul and super long haul systems.ively.
The wavelength of the very long distance line can be 120 km without repeater (LA). However, with a number of cascade repeaters, the length can be up to 600 km, which can be further increased to 1200 km using the dispersion compensation module. After such a distance, it needs regeneration in the electrical stage instead of the repeater in the optical stage only.
Pre-amplifier (PA)
This amplifier alone is used at the terminal to interface the DEMUX and the line to receive the signal from the remote station. Therefore, the attenuated line signal is amplified to a level of +3 dBm to 10 dBm before entering the DEMUX unit.
Optical supervision channel
The function of transmitting additional data (2 mbps: EOW, user specific data, etc. via the interface) at a length separate wave (1480 nm according to the recommendation ITU-T G-692) lower optical level without any optical security provision, accompanied and independent of the main optical traffic STM-n signal, is performed by the OSC. EOW (0.3 to 3.4 KHz) for selective and omnibus channel is 64 kbps in 8-bit PCM code.
The Optical Supervision Channel (OSC) provides control and monitoring of optical line devices as well as fault location management, configuration, performance and security achieved using LCT.
Optical networks - Devices
In this chapter, we have the different componentsof optical devices.
Isolator
Isolator is a non-reciprocal device that allows light to pass along a fiber in one direction and provides very high attenuation in the opposite direction. Insulators are needed in the optical system to prevent unwanted reflections, from descending a fiber and disrupting the operation of a laser(producing noise). In the manufacture of insulators, " Faradays Effect " is used, which depends on the polarization.
Isolators are constructed using optical polarizers, analyzers and Faradays rotators. The optical signal passes through the polarizer, oriented parallel to the incoming polarization state. The Faradays rotator will rotate the polarization of the optical signal by 45 degrees.
The signal then passes through the analyzer, which is oriented 45 degrees to the input polarizer. The isolator passes an optical signal of left to right and changes itspolariz ation 45 degrees and produces a loss of about 2 dB.
Circulator
Circulators are micro-optical devices and can be used with Any number of ports, however, generally 3 port / 4 port circulators are used.It has relatively low loss of 0.5dB to 1.5dB port to port.
The basic function of a circulator is shown in the figure above. Light entering a particular port (say port 1) moves around the circulator and goes out to the next port (let's say port 2). Light entering port 2 goes to port 3, and so on. The device works symmetrically around a circle. Circulators are micro devices -optical and can be manufactured with any number of ports. However, 3- and 4-port circulators are very common. Circulators have very low losses. Typical port-to-port loss is approximately 0 , 5 to 1.5 dB.
Splitters and couplers
Couplers and splitters are used to combine optical signals and / or Three important characteristics are -
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Return Loss - The amount of reflected and lost energy .
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Insertion Loss - The amount of signal lost in total transit through a device.
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Excessive loss - Additional loss of a device above the 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 the trans path and the receiver from many final signals. The grids are threadsvery. Switches, modulators, AWG, multiplexers, etc. are considered to be types of filters.
Here are the types of filters -
- Fabry- Perot
- Adjustable filter
- Filter fiber Bragg grating
Filters are used in front of an LED to reduce the width of the line before transmission. Filters will be very useful in WDM networks for -
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A filter placed in front of an inconsistent receiver can be used to select a particular signal from among many incoming
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WDM networks are offered which use filters to control the path through a network that a signal will take.
Fiber Bragg networks are the most important optical filter in the communications world.
Modulators
Modulators are made of a material which changes itsoptical properties under the influencence of an electric or magnetic field. In general, three approaches are used -
- Electro-optical and magneto-optical effects
- Electro-absorption effects
- Acoustic modulators
By mechanical vibrations Ref. Index of significant changes. Acoustic modulators use very high frequency sound. By controlling the intensity of the sound, we can control the amount of deflected light and therefore build a modulator.
Here are some of itsadvantages -
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They can withstand quite high power.
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The amount of refracted light is linearly proportional to the intensity of sound waves.
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They can modulate different wavelengths at the same time.
Optical ADM
An optical filter is used to isolate or drop the wavelength of multiple wavelengths arriving ona fiber. Once a wavelength is removed, another channel using the same wavelength can be added or inserted on the fiber, as it leaves OADM.
A simple ADM has only 4 input and output channels, each with four wavelengths. In OADM, wavelengths can be amplified, equalized or further processed. OADM arranges the wavelengths from input fiber to output fiber using optical interconnect.
Optical Cross-Connect
An optical x-connect can take four input fibers, each carrying four lengths of wavelength, and rearrange the 16 wavelengths, on the four output fibers. A single transponder inside OXC will mix one of the wavelengths to an available channel.
Single and multi-hop networks
Telecommunications traffic continues to grow at a very rapid rate.Accelerated by the increase in the volume of data and mobile traffic, especially in India, thanks to the recent liberalization of the telecommunications market. A solution can be adopted to meet ever increasing traffic demands based on a combination of WDM, SDH and IP transport technologies.
Wavelength Division Multiplexing is used to multiplex multiple wavelength channels onto a single strand of fiber, thereby overcoming fiber congestion. SDH technology provides the granularity of capacity that customers demand today and the ability to protect these services from network failure. An IP transport network over WDM can provide high capacity Internet transit services to Internet service providers (ISPs).
Synchronous Digital Hierarchy
Synchronous Digital Hierarchy (SDH) networks have replaced PDH and have several key advantages.
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ITU recommendations G.707, G.708 and G.709 form the basis of a global network.
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Networks benefit from traffic resiliency to minimize traffic loss in the event of an equipment failure or failure.
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Integrated monitoring technology enables configuration remote and network troubleshooting.
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Flexible technology allows tributary access at all levels.
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One technology Future-proof allows for faster bit rates as technology advances.
European PDH networks could not interface with us networks, SDH networks can carry both types. The figure above shows how different PDH networks compare and which signals can be carried through the SDH network.
SDH - Network topologies
An online system is the topolo systemPDH network management. Traffic is added and removed only at network endpoints. Terminal nodes are used at the end of the network to add and remove traffic.
Line System
In any SDH network it is possible to use a node known as the name of regenerator . This node receives the high order SDH signal and retransmitsit. No access to lower order traffic 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 the traffic.
Ring System
A ring system is consistsof multiple add / remove mux (ADM) connected in a ring configuration. Traffic is accessible to any ADM around the ring and it is also possible for traffic to be dropped at multiple nodes for dissemination purposes.Ring network has the advantage of providing traffic resiliency, if there is a fiber break, the traffic is not lost. Network resilience is discussed in detail in a subsequent chapter.
SDH ation network synchronization
While PDH networks were not centrally synchronized, networks SDH are (hence the name Synchronous Digital Hierarchy). Somewhere on the operator's network will be a primary referral source. This source is distributed over the network either on the SDH network or on a separate synchronization network.
Each node can switch to backup sources, if the primary source becomes unavailable. Different quality levels are set and the node will switch to the next higher quality source it can find. In cases where the node uses inbound line sync, the S1 byte in the MS overhead is used in.ur indicate the quality of the source.
The lowest quality source available for a node is usually itsinternal oscillator. In the event that a node switches to itsown internal clock source, this should be corrected 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 carefully planned. If all the nodes in a network try to synchronize with itsneighbor on the same side, you will get an effect called synchronization loop , as shown in the figure above. This network will quickly start generating errors as each node tries to synchronize.
SDH Hierarchy
The following figure shows how the payload is constructed, and it 's not so scary at first.
Optical networks - Technology WDM
WDM is a technology which enablessend various optical signals through a single fiber. itsprinciple is essentially the same as that of frequency division multiplexing (fdm). in other words, several are transmitted using different carriers, occupying non-overlapping partsof a frequency spectrum. case wd m, the spectral band used is around 1300 or 1550 nm, which two wavelength windows at which the fibers have very low signal loss.
Initially, each window was used to transmit a single digital signal. With the advancement of optical components, such as distributed feedback lasers (DFBs), erbium doped fiber amplifiers (EDFAs), and photodetectors, it was quickly realized that every transmission window could in fact be used by multiple optical signals, each occupying a small pull of the total available wavelength window.
In fact, the number of optical signals multiplexed in a window doesn 'tis limited only by the accuracy of these components. With current technology, more than 100 optical channels can be multiplexed into a single fiber. The technology was later named dense WDM (DWDM).
WDM on the long-haul
In 1995, long-haul carriers in the United States began to deploy point-to-point WDM transmission systems to improve the capacity of their networks while leveraging their existing fiber infrastructure. Since then, WDM has also taken the long-haul market by storm. WDM technology helps meet ever increasing capacity requirementswhile deferring fiber depletion and increasing flexibility for capacity upgrades.
The most prevalent factor, however, is the cost advantage of the WDM solution over competing solutions, such as spatial division multiplexing (SDM) or time division multiplexing ( TDM) improvede to improve network capacity. The "open" WDM solution, shown in the following figure, uses transponders in WDM (TM) terminal multiplexers and in-line optical amplifiers shared by multiple wavelength channels.
The transponder is in essence a 3R opto-electro-optical (O / E / O) converter, which convertsan optical signal conforming to the G.957 standard into a channel of the appropriate wavelength (and vice versa) while re-energizing, reshaping and electrically resynchronizing the signal. The SDM solution uses mu Multiple pairs of fibers in parallel, each equipped with SDH regenerators instead of multiple wavelengths sharing the same in-line optical amplifier. Upgrading to higher TDM rates (e.g. 2.5 Gb / s STM-16 to 10 Gb / s STM-64) is only a short-lived solution as transmission degradations such that the dispersion does not adapt well to the increase.n TDM speeds, in particular single-mode fiber.
A case study has shown that long-distance point-to-point WDM systems are clearly a more cost effective than SDM, even for as little as three channels of STM-16. The figure above illustrates two link cost comparisons for the initial core of a transport network made up of 5,000 km of fiber with an average distance of 300 km between two access cities. Note that the 100% cost benchmark in the figure above is the cost of deploying an STM-16 channel, including the cost of fiber. Two conclusions can be drawn As shown in the following figure, if only the costsof transmission and regeneration equipment are taken into account (i.e. SDH regenerators in the case of SDM and WDMs TM with transponders with optical in-line amplifiers in the WDM case), the initial link cost of usen of WDM technology is more than double that of SDH. However, the WDM solution is more cost effective for the deployment of three and more channels in the network, due to the shared use of the optical amplifier in line.
As shown in the following figure, if it is in addition from above, the cost of fiber is also considered, the cost advantage of WDM box becomes even more evident and is amplified as the number of channels increases. WDM solution is more cost effective for deployment of three and more channels in the network.
WDM on the short haul
Regenerators are not necessary and optical degradations have less impact due to the limited distances in short haul networks, so 'where the advantages of WDM are less clear than those of enhanced SDM or TDM solutions. However, the depletion of fiber and low optical componentscost are now the driving force behind WDM in the metropolitan area.
The short-haul application is linked to the interconnection of several pointsof presence (POP) in the same city. Let's analyze an example. The following figure shows that the transport network has at least two POPs per city, where customers can interconnect. With two-node interconnection techniques, such as drop and continue, customer networks can be interconnected with the transport network through two different POPs.
The result is a very secure architecture that can even survive POP outages without any impact on traffic. Thus, the traffic flow between two POPs in a city consistsnot only of traffic that passes through the city, but also of traffic that ends in the city and is protected by Drop and Continue. These increased needs for intra-city capacity have led to the deployment of WDM in the short-haul section of a transport network.
The main reason why WDM is preferred over SDM is that the fibers in a city must be leased to a third party or a fiber optic network must be built. Leasing or building urban fiber is not only an expensive process, it is also a less flexible approach to improving capacity.In a dynamic environment, where distributions and traffic volumes change rapidly, the quantity of fiber to lease or build is difficult to predict in advance. Therefore, using g WDM technology has clear advantages in flexibility, as wavelength channels can be activated in very little time. time.
Although specific short haul WDM systems are available worldwide, it is advantageous to use the same type of WDM system for itslong haul network. While short haul WDM systems distance are cheaper than their homologatedThey are long-haul and because of their inexpensive optical componentsthey can be used, they lead to a heterogeneous network, which is not preferred for several reasons. First, using two different systems resultsin increased operating and management costs. For example, a heterogeneous network requires more spare partsof equipment than a homogeneous network. Second, interworking between two different systems could cause problems. For example, a bottleneck can occur because short-haul WDM systems typically support fewer wavelengths than long-haul WDM systems.
Optical Transport Network Architectures
Optical Transport Network (OTN), as illustrated in the following figure, representsa natural next step in the evolution of transport networks. From a high level architectural point of view, we do notOTN architectures do not have to differ significantly from those of SDH. However, the fact that SDH involves digital network engineering and OTN involves analog network engineering leads to important, albeit subtle, distinctions. Exploring these distinctions leads us to understand aspectsof OTN that may differ from their SDH counterparts.
Scalable OTN WDM architectures (including network topologies and survival patterns) will closely resemble - if not mirror - those of SDH TDM networks. This should be surprising, however, since SDH and OTN are both connection-oriented multiplexed networks. The main differences come from the form of multiplexing technology: digital TDM for SDH vs analog WDM for an OTN.
The digital vs analog distinction has a profound effect on the fundamental cost / performance tradeoffs in de many aspectsof the design of OTN networks and systems. In particular, the complexities associated with analog network engineering and maintenance implications represent the majority of challenges associated with OTN.
To meet the short-term need for increased capacity, WDM point-to-point line systems continue to be deployed on a large scale. As the number of wavelengths and the distance between terminals increase, there is a growing need to add and / or remove wavelengths at intermediate sites. Therefore, Flexible Reconfigurable Optical ADMs (OADMs) will become integral partsof WDM networks.
As more and more wavelengths are deployed in carrier networks, it will be increasingly necessary to manage capacity and transfer signals between networks at the network level. optical channel. Likewise, DXCs appeared to manage thecapacitance at the electrical layer, optical interconnects(OXC) will emerge to manage the capacitance at the optical layer.
Initially, the need for optical layer bandwidth management will be most acute in the core transport network environment. Here, logical mesh-based connectivity will be supported through physical topologies, including OADM-based shared rings of protection and OXC-based mesh restoration architectures. The choice will depend on the degree of bandwidth desired by the service provider "from the build" and the survival timescale requirements.
As similar bandwidth management requirementsemerge for inter-office and metro access environments, OADM ring-based solutions will also be optimized for these applications: rings shared optical protection for mail requestsDedicated optical protection ring and rings for hubbed applications. Therefore, just as OA has been the technological catalyst for the emergence of point-to-point WDM line systems, so will OADMs and OXCs be the catalystsfor EME. OTN emergency.
As the elementsof the optical network assume the transport layer functionality traditionally provided by SDH equipment, the optical transport layer will come to serve as a unifying transport layer capable of supporting the both the legacy packet core network and converged signal formats. Of course, the shift from service provider to OTN will be predicted on the transfer of functionality from the "SDH-type" transport layer to the optical layer, at the same time as the development of a maintenance philosophy. and associated network maintenance functionalities for the emerging optical transport layer.
Survival is at the heart of the role ofu optical network as a unifying transport infrastructure. As with many other architectural aspects, optical network survivability will bear a high level resemblance to SDH survivability, as network topologies and types of network elementsare so similar. Within the optical layer, survivors will continue to provide the fastest possible recovery from fiber cutsand other physical media faults, as well as provide efficient and flexible management of protection capacity. .
OTN is conceptually analogous to SDH, in that sublayers are defined that reflect client-server relationships. Since OTN and SDH are both connection-oriented multiplexed networks, it should come as no surprise that the restoration and protection schemes of the two are remarkably similar. The subtle but important difference is worth repeating: so qAs TDM networking is based on the manipulation of digital time slots, OTN / WDM networking is based on the manipulation of analog frequency slotsor optical (wavelength) channels. So while we can expect similar protection and recovery architectures to be possible with the two technologies, the types of network outages that may need to be accommodated in a particular survival pattern can be very different. .
Optical Layer Survivability
Telecommunication networks are necessary to ensure reliable uninterrupted quality service to their customers. The overall uptime requirementsare in the order of 99.999% or more, which would imply that the network cannot be down for more than 6 min / year on average. As a result, network survivability is a major factor affecting the way these networks are designed andexploited. Networks should be designed to handle link or fiber outages as well as equipment failures.
The network can be seen as made up of several layers interacting with each other, as shown in the figure above. Different operators choose different ways of realizing their networks using different combinations of stratification strategies The incumbent operators use their large installed base of SDH equipment and the extensive capabilities for preparing and monitoring digital interconnections.
In contrast, an operator offering services based on Internet Protocol (IP) seeks to simplify network infrastructure using IP as the base transport layer without using SDH. Carriers that differ in quality (and The optical layer provides light paths to the upper layers, which can be thought of as client layers that use the service provided by the optical layer. Lightpaths are circuit-switched pipes carrying traffic at fairly high bit rates (for example, 2.5 Gb / s or 10 Gb / s). These light paths are typically configured to interconnect client layer equipment, such as SDH ADMs, IP routers, or ATM switches. Once set up, they remain fairly static over time.
The optical layer consistsof optical line terminals (OLT), O ADM (OADM) and optical interconnects(OXC) as shown in the following figure. OLTs multiplex multiple channels into a single fiber or pair of fibers. OADMs remove and add a small number of channels from / to a flAggregate WDM ow. An OXC switches and manages a large number of channels in a high traffic node location.
We look at optical layer protection from a service perspective, in terms of the types of services to be provided by the optical layer to the upper layer. We then compare the different optical layer protection schemes that have been proposed in terms of cost and bandwidth efficiency depending on the mix of services to be supported. It is somewhat different, who tends to regard the protection of the optical layer as analogous to the protection of the optical layer. SDH layer.
Why optical layer protection?
The IP, ATM, and SDH layers shown above figure, all have built-in protection and restoration techniques. these layers have all been designed to work with other layers, they can also work directly on the fiber and nottherefore do not depend on other layers to manage the protection and recovery functions. As a result, each of these layers has itsown protection and restoration functions. So, the question arises, why do we need the optical layer to provide itsown set of protection and restoration mechanisms. Here are some of the reasons -
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Some of the layers operating above the optical layer may not be fully able to provide all the functions necessary protection in the network. For example, the SDH layer was designed to provide full protection and therefore would not depend on the protection of the optical layer. However, protection techniques in other layers (IP or ATM) alone may not be sufficient to ensure adequate network availability in the presence of faults.
There are currently manys proposals to make the IP layer work directly on the optical layer without using the SDH layer. Although IP incorporates fault tolerance at the routing level, this mechanism is cumbersome and not fast enough to provide adequate QoS. In this case, it becomes important for the optical layer to provide rapid protection to meet the overall availability requirementsof the transport layer.
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Most carriers have huge investmentsin existing equipment that do. do not provide any protection mechanism, but cannot be ignored. A seamless introduction of the optical layer between this equipment and the raw fiber provides a low cost upgrade of infrastructure over long fiber links with increased survivability.
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Protection and restoration of the optical layer can be used to provide an additional level of terminationnce in the network. For example, many transport networks are designed to handle a single failure at a time, but not multiple failures. Optical restoration can be used to provide resilience against multiple failures.
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Protection of the optical layer may be more effective in handling certain types of failures, such as fiber cuts. A single fiber carries multiple wavelengths of traffic (eg 16 to 32 SDH streams). A fiber cut therefore resultsin 16 to 32 of these SDH streams being restored independently by the SDH layer. The network management system is inundated with a large number of alarms generated by each of these independent entities. If the fiber cut is restored quickly enough by the optical layer, this operational inefficiency can be avoided.
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Significant savings can be achieved by using the protection and restoration of the optical layerue.
Limitations - Optical layer protection
Here are some of the limitations of optical layer protection.
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It cannot handle all types of failures in the network. For example, it cannot handle the failure of a laser in an IP router or SDH ADM connected to the optical network. This type of failure must be handled by the IP or SDH layer, respectively.
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It may not be able to detect all types of failures in the network. The light paths provided by the optical layer can be transparent so that they carry data at various bit rates. the optical layer in this case may in fact not know exactly who is being transported on these light paths. As a result, it would monitor traffic for degradations, such as increasing bit error rates, would normally invoke a protection switch.
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The couche optics protectstraffic in unitsof light paths. It cannot provide different levels of protection to different partsof the traffic carried on the light path (part of the traffic may be high priority, the other less priority). This function must be performed by a higher layer that handles traffic with this finer granularity.
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There may be link budget constraintswhich limit the protective capability of the optical layer. For example, the length of the protection road or the number of nodes crossed by the protection traffic can be limited.
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If the overall network is not carefully designed, there may be a race condition where the optical layer and the client layer both try to protect the traffic from a failure simultaneously.
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The technology and protection techniques have not yet been tested in the field, and theLarge-scale deployment of these new protection mechanisms will therefore take a few years.
Definitions of protected entities
Before going into the details of protection techniques and the compromises between them, it is interesting to define the entities that are protected by the optical layer and the client layer. These entities are shown in the following figure.
Client equipment port
Portson client equipment might fail. In this case, the optical layer cannot protect the client layer by itself.
Intrasite connections between client and optical equipment
Cables inside a site can become disconnected, mainly due to human error. This is considered a relatively probable event. Again, full protection against such occurrences can only be supported by combined protection of theclient layer and optical layer.
Transponder cards
Transponders are interface cards between client equipment and the optical layer. These cards convert the signal from the client equipment to a wavelength suitable for use within the optical network, using optical-to-electrical-to-optical conversion. Therefore, the failure rate of this card cannot be considered negligible. Given the large number of these cards in a system (one per wavelength), a special protective backing for them is in order.
External installations
This fiber installation between sites is considered to be the least reliable component of the system. Fiber cutsare quite common. This category also includes optical amplifiers which are deployed along the fiber.
Entire nodes
An entire node can go down due to 'Maintenance personnel errors (eg tripping circuit breakers) or any site failures. Site failures are relatively rare and usually occur due to natural disasters such as fires, floods, or earthquakes. Node failures have a significant impact on the network and therefore still need to be protected, despite their relatively low probability of occurrence.
Protection against recovery
Protection is defined as the main mechanism used to deal with a failure. It must be very fast (generally the traffic must not be interrupted for more than 60 ms in the event of failure of SDH networks). As a result, protection routes generally need to be pre-planned so that traffic can quickly switch from normal routes to protection routes.
Due to speed requirements, this function is usually performedin a distributed manner by the elementsof the network without depending on a centralized management entity to coordinate the protection actions. With the exception of recent (and not yet proven) fast mesh protection schemes, protection techniques tend to be fairly straightforward and are implemented in line or ring topologies. They all end up using 100% of the network access bandwidth.
In contrast, restore is not a primary mechanism used to handle failures. After the protection function is complete, recovery is used to provide efficient routes or additional resiliency against further failures before the first failure is corrected. As a result, it can afford to be quite slow (sometimes from a few seconds to a few minutes).
Dining routes do not need to be planned in advance and can be calculated on the flyby a centralized management system, without requiring a control function. More sophisticated algorithms can be used to reduce the excess bandwidth required, and more complex mesh topologies can be supported.
Sub-layers in the optical layer
The optical layer consistsof several sublayers. Protection and restoration can be performed at these different layers. We can have diagrams that protect individual light paths or optical channels. These diagrams deal with fiber cutsas well as terminal equipment failures, such as lasers or receivers.
We can have schemes that work at the aggregate signal level, which is the Optical Multiplex Section (OMS) layer. These diagrams do not distinguish between the different light paths which are multiplexed together, and restore them all simultaneously into them.switching as a group.
The term path layer protection is used to denote schemes that operate on individual channels or light paths and line layer protection is used to denote schemes that operate at the level of the light path. multiplex optical section layer. Refer to Table 1 for a comparison between the properties of path and line layer diagrams, and Table 2 and Table 3 for the different path and line diagrams.
Table 1: A comparison between line protection and path protection
| Criterion | Line protection | Path protection |
| Protectsagainst | Inter-office services Site / node outages | Cross-office services Site / node outages Equipment failures |
| Number of fibers | Four, if the Single level multiplexing is used | Two |
| Can handle failures / degradation of 'only one path | No | Yes |
| Support traffic that should not be protected | No | Yes |
| Equipment cost | Low | High |
| Bandwidth efficiency | Good forr protected traffic | Low for unprotected channels |
Table 2: A comparison between row layer schemes
| Schema | Protectsagainst | Topology | Constraints/ shortcomings | Customer benefits |
| 1 + 1 line | Line breaks | Point to point | Various routes needed to protect fibers | The easiest to implement and use |
| 1 + 1 line | Line cuts | Point to point | td = "vertical-align: middle; "> Various routes needed to protect fibers Support for low priority traffic Dreduction of losses (about 3 dB) |
| OULSR | Line breaks Node errors | Metropolitan ring | Degradation of the optical layer A loss additional power existsdue to line-level signal bridging | Simple to set up and use Can be done using passive elements(instead of optical switches) |
| OBLSR | Line breaks Node errors | Metropolitan ring | Degradation of the optical layer | Reuse of tape pass-through protection Support foru low priority traffic |
| Mesh line protection | Line breaks Node errors | < td class = "ts"> All Limited by degradations of the optical layer Based on an all-optical cross-connect Difficult to manage | Efficient Low cost |
Table 3: A comparison between path-layer schemes
| Schema | Protectsagainst | Topology | Constraints/ gaps | Client Benefits |
| Client layer protection | Customer equipment failures Intra-office installations Transponder faults Inter-office installations Node faults | All | Requires Most expensive | Most extensive protection |
| 1: N equipment protection | Defectstransponder | Linear or ring | | Very low cost Effective bandwidth |
| 1 + 1 path or OUPSR | Cross-desktop functions Node faults | All | Requires Bandwidth consumption | Similar to protection clients Easy to develop and use |
| OBPSR | Inter-office installations Node faults | Virtual ring | | Bandwidth reuse protection Supportslow priority traffic |
| Protection of mesh paths | Inter-office services Node faults | All | Requires an OXC Very complex to implement and operate | High performance |
The physical topology of the network can be any what mesh, passage of light paths between the nodes of client equipment. The virtual topology from the point of view of the client equipment is restricted according to the client layer (for example, rings for SDH). 2The physical topology is any mesh, while the virtual lightpath topology is a ring.
Consider, for example, the two protection schemes shown in the following figures. These two schemes can be thought of as 1 + 1 protection schemes, i.e. they
Line layer and path layer protection
There are significant differences in cost and complexity between the two approaches . Line protection requires an additional splitter and switching to an unprotected system. However, path protection requires one splitter and one switch per can.al. More importantly, trail protection typically requires twice as many transponders and twice the multiplex / demultiplexer resources of the line protection. Therefore, path protection is almost twice as expensive as line protection, if all channels are to be protected. The story changes, however, if not all channels need to be protected.
The basic protection schemes
A comparison of the protection schemes can be found in Tables -1, 2 and 3. Optical layer protection schemes can be classified in the same way as SDH protection schemes and can be implemented at the client layer, path layer or line layer.
Client protection
A simple option is to let the client layer take care of itsown protection and not let the optical layer do the protection. This may be the case in.on SDH client layers. Although this is simple from an optical layer point of view, significant cost advantages and bandwidth savings can be obtained by providing optical layer protection. Although the client protection method can support point-to-point, ring, or mesh client networks, it is important to note that from an optical network perspective, all of this translates into optical mesh support because even a point-to-point client link can cover an entire optical mesh network.
In client layer protection, the working client and protection paths are fully Path Layer Schemes
1 + 1 Path Protection
This scheme requires two wavelengths on the network, because as well as two setsof transponders to each end. When applied to a ring, this protection is also referred to as Optical Unidirectional Path Switching Ring (OUPSR) or Dedicated OCh Protection Ring (OCh / DP Ring).
Implementation Notes - Bridging is usually done via an optical coupler, while selection is via a 1 x 2 optical switch. The receiving end can decide to switch to the emergency path without coordination with the source.
Bidirectional path switched ring
This scheme is loosely based on the SDH 4-fiber bi-directional line switched ring (BLSR) and relies on a bandwidth of shared protection around the ring. When a working light path fails, the nodes coordinate with each other and try to send thee traffic through the designated protection bandwidth in the same direction around the ring (to overcome transponder faults). This is a range switch. If unsuccessful, nodes loop traffic around the alternate path around the ring to the other end of the failure. This action is a ring switch.
The scheme allows non-overlapping light paths to share the same protection bandwidth as long as they do not fail together. This scheme is also referred to as the OCh Shared Protection Ring (OCh / SPRing).
Implementation Notes - This scheme can be implemented in an OXC or, through much smaller switches in OADM. Switches are required for each protection channel l. It is similar to the SDH BLSR standard.
Mesh path protection
This scheme allows global mesh protection with very fast switching (en less than 100 ms) for each faulty light path separately from an emergency path, shared by several light paths potentially taking a different route per light path. If unsuccessful, it is signaled to all relevant nodes that set up backup paths.
Implementation Notes - These schemas are being implemented in OXCs. Due to time constraints, the predefined backup paths are stored in the nodes of the network and are activated according to the types of failure.
Mesh path restoration
Unlike the mesh path protection, this scheme does not work with strict time constraints. This device calculates alternative routes using itstopology and broadcastsnew configuration information to the nodes, which define these routes. Nodes don't need to keep n / w information.
Implementation Notes - The centralized nature of this scheme ensures more optimized protection routes and reduces the complexity of implementation and maintenance.
1: N Equipment protection
One of the most complex (and therefore prone to failure) modules of a typical WDM terminal is a transponder. The 1: N protection designates a spare transponder to take over in the event of failure of the normal transponder.
Implementation Notes - This scheme is more generally based on a designated protected wavelength. In the event of a failure, both ends must fail over using fast signaling protocols, not like APS in SDH.
Line layer schemes
1 + 1 linear protection
This scheme is based on bridging the entire WDM signal in bulk on a pair of Linear protection 1: 1
This scheme requires a configuration similar to the previous one (i.e. 1+ 1 linear), however the signal is switched to the work or protection path, but not on both. Although this increases the coordination load, it allows low priority traffic to run on the backup path (until needed to protect the working path). It also resultsin less optical power loss due to all signal energy being directed to one path instead of two.
Implementation Notes - Switching is usually done using a 1 optical × 2 switch. Coordination is achieved through a rapid signaling protocol.
Optical Unidirectional Line Switching Ring (OULSR)
The scheme is similar to the OUPSR scheme except that bridging and signal selection is done in.on the aggregated WDM signal. This allows for a more optimized design, lower cost, and very different implementations.
Implementation Notes - One implementation of this scheme is based on passive couplers which run the optical ring in a broadcast medium. Instead of using OADMs, this scheme is based on simple OLTs, each coupled in rings both clockwise and counterclockwise, so that each of the wavelengths are transmitted and received on both fibers. Under normal conditions, the link is artificially disconnected, resulting in a linear bus, when the fiber-cut link is reconnected.
Bi-directional line switched ring
This scheme is similar to the OBPSR scheme both in protocol aspectsand in the protection actions used (range and ring switching) . Like all line layer schemes, the aggregate WDM signal is switchede en masse to a dedicated shield fiber (requiring four fibers), or to a different WDM band in a single fiber (allowing only two fibers, but requiring a two-stage optical multiplexing scheme). This scheme is also referred to as the OMS Shared Protection Ring (OMS / SPRing).
Implementation Notes - As the back-up route loops optically around the entire ring, optical line amplifiers may be required along the way. relief to compensate for losses. The circumference of the ring is also limited by other optical degradations. Therefore, this option is best suited in metro politan applications.
Mesh Line Protection / Restore
This scheme is based on fully optical interconnectswhich divert the WDM signal from a failed installation to another route and back to the other end of the failed installation.
Implementation Notes - Like OBLSR, this scheme is limited by optical impairmentsthat can develop along alternate routes and requires careful optical design.
Taking into account the choice of protection scheme
The criteria that could be used by an operator to select the protection schemes to be used in the network. A simplified decision table for this is shown in the following figure assuming that equipment and line protection are required.
The cost of protection
Another criterion from the carrier's point of view is the cost of the system in at least two aspects-
- Cost of the equipment
- Bandwidth efficiency
Both depend on the combination of traffic services, ie the fraction of the traffic to be protected by the optical layer.
The following figureshows the cost of equipping path layer schemes and equivalent line layer schemes as a function of traffic composition. If all traffic needs to be protected, trail layer schemes require approximately twice the equipment of line layer schemes because there is less sharing of common equipment.
However, the cost of trail layer protection is proportional to the number of channels to be protected, as each channel requires an associated multiplexer / demultiplexer and terminating equipment. Thus, the cost of protecting the path layer decreases if fewer channels need to be protected. In the event that no channel needs to be protected, path layer schemes will cost roughly the same as line layer schemes, assuming no additional common equipment. is deployed.
The story is different from an effica point of viewquoted from the bandwidth, as shown in the following figure. In a line protected system, the protection bandwidth is used for light paths that require protection as well as those that do not require protection. In path protection systems, light paths that do not require protection can use up the bandwidth, allowing others to be unprotected. light paths to utilize bandwidth that would otherwise have been wasted on unwanted protection.
It follows that if a large portion of lightpaths could be left unprotected, the path layer protection recovers the cost by supporting more operational traffic on the same network as the line layer protection.
Optical networks - ROADM
Older optical networks deploy SDH / SONET technologies for r data transport over the optical network.These networks are relatively easy to plan and design. New network elementscan be easily added to the network. WDM static networks may require less investment in equipment, especially in metro networks. However, planning and maintaining these networks can be a nightmare as the engineering rules and scalability are often quite complex.
Bandwidth and wavelengths must be pre-allocated. Because wavelengths are grouped into groups and not all groups end at every node, accessing specific wavelengths may not be possible at some sites. Network expansions may require new amplifiers and optical-electrical-optical regeneration or at least power adjustmentsat existing sites. Operating a static WDM network is very labor intensive.
Network and bandwidth planning should be as simple as in SDH / SONET networks in t he past. In the given bandwidth of the ring, for example STM-16 or OC-48, each node could provide as much bandwidth as needed.
Full bandwidth access was possible at each ADM. Network expansion, for example, inserting a new node into an existing ring, was relatively easy and did not require any on-site visitsto existing nodes. The network diagram on the left illustrates this: Digital interconnect systems link with multiple SDH / SONET optical rings.
Reconfigurable optical networks act differently: bandwidth can be scheduled on demand and range is optimized as optical power is now managed per WDM channel. Scalability increases dramatically.
The key element to enable such a reconfigurable optical networkthe is the Reconfigurable Optical Multiplexer (ROADM) . It allows to redirect optical wavelengths to customer interfaces with just one click in the software. The other traffics remain unchanged. All this is achieved without the need to drive 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 each node).
- Bandwidth and wavelength pre-allocation
- Margin allocation for a fixed filter structure
- Insufficient power management
- Network expansion requires optical-electrical-optical (OEO) regeneration
SDH / SONET networks are easy to plan.
- Access to full bandwidth on each ADM
- Simple engineering rules (single united hopcement)
- Easy addition of new network elements
A reconfigurable optical layer enables the following.
- On-demand bandwidth planning
- Transparent reach extended through power management by WDM channel
- Safe Scalability
Static photonic layers are made up of separate optical rings. Consider a number of lo cated DWDM systems on each of these rings. Often times the information or data just stays on the same ring, so there is no problem. However, what happens in cases where the data needs to be transferred to a different optical ring?
In static systems, a large number of transponders are needed wherever a transition between rings is required. Indeed, each wavelength which passes from one ring to another needs two transponders: one on each side of the ring.bucket. This approach involves high costsand a lot of upfront planning, taking into account the allocation of bandwidth and channels.
Now imagine a dynamic reconfigurable photonic layer. Here there is only one DWDM system forming the interface between two optical rings. Therefore, transponder-based regeneration disappears and the number of DWDM systems decreases. The entire network design is simplified and wavelengths can now travel from one ring to another without 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 optical passage from the core to the access area, is the ROADM and the GMPLS control plane.
Simplifications via ROADM
ROADMs simplify the service provider's or carrier's network and processes. Thisinteraction summarizes some of these simplifications. After all, we have to keep in mind that all of these benefitstranslate into reduced time and costs. But what is more important is that they also lead to increased customer satisfaction and, therefore, customer loyalty.
Network planning is greatly simplified with ROADMs. Just consider the greatly reduced number of transponders, which must be stored in the warehouse.
Installation and commissioning - for example, when setting up a new wavelength on the network - requires much less effort and is much less complex . Service technicians should only visit the respective end sites to install transponders and ROADMs. Fixed Optical Multiplexers (FOADMs) required a visit to each staging site for installation and patch work.s can be performed.
Operations and maintenance are greatly simplified when a dynamic optical network is deployed. Optical diagnosis can be performed in minutes rather than hours as it used to be. Deficiencies can be dynamically detected and cleared instead of triggering truck runs to external locations.
With the deployment of tunable lasers and colorless ROADMs, maintenance of the fiber plant is made easier. With these features, providing services is now easier than ever. As with installation and commissioning work, it is also much easier to carry out network maintenance and possible upgrades.
ROADM Architecture
Many of the benefitsthat ROADMs bring to network design and operation have been discussed in the previous sections. Here are someothers -
- Power monitoring and leveling per channel to equalize the entire DWDM signal
- Full traffic control from the network remote operations center
One question, however, has remained unanswered so far: how does a ROADM work? Let's take a look at some basics.
A ROADM typically consistsof two major functional elements: a wavelength splitter and a selective length switch (WSS). Look at the block diagram above: A pair of optical fibers at network interface # 1 is connected to the ROADM module.
The fiber carrying the incoming data ( network) is routed to the wavelength splitter. Now all wavelengths are available on all output portsof the splitter, in this case 8. Local add / remove traffic (wavelengths) can be multiplexed / demultiplexed with a network waveguide filter (AWG). Using an AWG involves a fixed wavelength allocation and direction.
The Selective Wavelength Switch (WSS) selectively joins the different wavelengths and feeds them to the output of network interface # 1. The remaining splitter portsare connected to other network directions, for example, three other directions at a 4 degree junction node.
Note - One of the modules shown (completely gray boxed) is required per network direction at this node. Or to be more precise: in a trunk node serving four directions (4 degrees), four of these modules are needed.
The ROADM Heart - the WSS module
Let's start with the WDM signal coming from the left. It passes through the optical fiber from the top and is directed towards a mass diffraction grating. This mass diffraction grating actslike a spelle of prism. It separates the different wavelengths in different directions, although the angle variation is quite small. The separated wavelengths hit a spherical mirror, whichreflectsthe rays onto a set of micro-electromechanical systems (MEMS). Each microswitch is struck by a different wavelength, which is then reflected back to the spherical mirror.
From there the rays are sent back to the bulk diffraction grating and sent to the optical fiber. But now it is a different fiber than the one we started with. The single wavelength output signal indicates that this has occurred. This signal can then be combined with other single wavelength signals to fill another transmission fiber.
There are different versions available - the keywords here are colorless, directionless etc. .
ROADM - Degrees, colorless, without direction, and more
| Term | Explanation |
| Degree | The term Degree describes the number of supported DWDM line interfaces. A 2 degree ROADM node supportstwo DWDM line interfaces. It also allows two add / remove branches from all line interfaces. |
| Multi Degree | Multi-degree ROADMs support more than two interfaces of DWDM line. The number of possible add / remove branches is determined by the number of WSS ports. |
| Colorless | Colorless ROADM allows flexible assignment of any what wavelength or color at any port. Filter modules must be connected to implement this function. |
| Directionless | A directionless ROADM does not require physical reconnection of the transmission fibers. Restrictions on directions are eliminated. Directionless ROADMs are deployed for the purpose of restoration or temporary rerouting of services (e.g. due to network maintenance or on-demand bandwidth requirements). |
| Contentionless | Contentionless ROADMs eliminate the potential problem of two identical wavelengths colliding in the ROADM. |
| Gridless | Non-grid ROADMs support |
To understand this leveled ROADM approach, here are some key terms often used in relationship with ROADMs.
Incolore
Simple ROADMs include a WSS for each direction, also called “a degree”. Wavelengths are always assigned and added / removed fixed transceivers are used. Colorless ROADMs remove this limitation: with such ROADMs, any wavelength or color can be assigned to any port. No truck roll is required as the entire setup is software controlled. Filter modules must be implemented to achieve the colorless function.
No direction
This often appears in conjunction with the term "colorless". A directionless design removes another ROADM limitation. The need to physically reconnect transmission fibers is eliminated by using directionless ROADMs as there are no restrictions on the direction, for example, to the south or to the north.
Without contention
Although colorless and sIn management, ROADMs already offer great flexibility, two wavelengths using the same frequency could still collide in a ROADM. Contention-free ROADMs provide a dedicated internal structure to avoid such blockage.
Gridless
Gridless ROADMs support a very dense wavelength channel grid and can be adapted to future transmission speed requirements. This functionality is required for signal rates over 100 Gbit / s and different modulation formatswithin the same network.
When directionless
Directionless ROADMs are the most popular ROADM design as they allow adding / removing wavelength from the taken ITU grid supported on any line interface. In the case of a directionless variant only, the add / remove portsare specific to a defined wavelength. In utireading the colorless option, portscan also be wavelength nonspecific.
Directionless technology is primarily deployed for wavelength rerouting to other portsas needed for restoration purposes. Other applications are also possible, for example, in bandwidth on demand situations. ROADMs that do not support the directionless function are subject to certain limitations in terms of flexibility.
When they are colorless
Colorless ROADMs allow the wavelengths of a specific optical channel to be changed without any physical rewiring. A colorless ROADM can be reconfigured to add / remove any supported ITU grid wavelength on any add / remove port. The added / deleted wavelength can change (adjustable DWDM interface). This allows -
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Improved flexibility for wavelength provisioning and wavelength restoration
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Restore switching, directional switching, and color change
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The adv key Advantage of the colorless add / remove portsin combination with the tunable DWDM line interfaces is the improved flexibility for provisioning wavelength restoration and wavelength restoration. Automatic adjustment of the next free wavelength on a requested optical path.
One of the last bitsof full optical network automation is the deployment of colorless ROADMs. Using such ROADMs allows the add / remove of any supported ITU grid wavelength on any add / remove port. The wavelength on the port may change as tunable transceivers are used as optical front ends.
Supplying and restoring wavelength is even easier than before. When a wavelength is occupied, the system can automatically tune the transceiver to the next available free wavelength. ROADMs provide the ability to use fixed and colorless add / remove functionality within the same ROADM node.
If there is no conflict
Contentionless ROADMs can add / remove any wavelength to any add port / remove without any contention grid on any add / remove port. A dedicated wavelength color can be added / removed multiple times (from different DWDM line interfaces) on the same add / remove branch. If only 8 add / remove portsare equipped, it must be possible to drop the same wavelength from 8 directions ofdifferent lines on the 8 add / remove ports. As long as free add / remove portsare available, the ROADM node must be able to add / remove any wavelength from / to any line interface.
The combination of colorless, directionless and contention-free (CDC) functionality provides the ultimate level of flexibility.
When nodes without a network
ROADM nodes without a network support different ITU-T channel grids in the same DWDM signal. Network bandwidth can be provided per channel.
The gridless function is required for networks operating at data rates above 100 Gbit / s or for network operating with different modulation schemes. It is intended for next generation networks with consistent line interfaces. Different data rates require different wavelengths depending on the modulation schemeand data rate.
Transmission speeds are increasing and modulation schemes are becoming more and more complex. Several modulation technologies can now be mixed on a single optical fiber. All of this links back to ROADM technology and generates the requirementsfor networkless ROADMs. These ROADMs operate on a dense frequency grid and allow channel-based bandwidth provisioning. Data channels now require different wavelengths depending on their modulation scheme and data rate.
Typical applications are networks operating at data rates above 100 Gbit / s or performing different modulation schemes in parallel. The latter situation can, for example, easily exist when deploying coherent transmission technologies.
