What are some of the various switches used in DWDM network and explain how one can switch based on frequency of light.
DWDM(dense wavelength division multiplexing)
There are three categories of wavelength division multiplexing:
WDM (wavelength division multiplexing) Two to four wavelengths per fiber. The original WDM systems were dual-channel 1310/1550 nm systems.
CWDM (coarse wavelength division multiplexing) From four to 8 wavelengths per fiber, sometimes more. Designed for short to medium-haul networks (regional and metropolitan area).
DWDM (dense wavelength division multiplexing) A typical DWDM system supports eight or more wavelengths. Emerging systems support hundreds of wavelengths.
The spacing between wavelengths in CWDM is about 10 to 20 nm, while the spacing in DWDM is about 1 to 2 nm. Due to the tight spacing and number of lasers, DWDM systems require elaborate cooling systems. Also, precision light sources and complex optical multiplexers are required to ensure that channels do not interfere with one another. In contrast, CWDM systems are simple and easy to manufacture, and cost much less than DWDM systems. They are also smaller. A CWDM device can be held in your hand, while a DWDM device is a large box that requires rack mounting.
The development of EDFAs (erbium-doped fiber amplifiers) provided a boost in cable distance and capacity for fiber-optic networks. EDFAs can amplify optical signals directly by injecting light into the cable via a light pump. Weak signals enter the amplifier and stimulate excited erbium atoms in the erbium-doped fiber to emit more light, thus preserving the original signal and boosting its output signal. Best of all, EDFAs can simultaneously boost the signals of multiple wavelengths in the same cable. EDFAs work in the 1,500- to 1,600-nm range, so a typical DWDM system has a range of lambda circuits operating in this range.
Prior to the development of EDFAs, optoelectronic amplifiers were used to boost optical signals. The process is often called “3R” regeneration, referring to reamplify, regenerate, and retime. Weak incoming light is converted to a voltage signal, amplified, and then converted back to light. This is impractical in high-speed core networks.
With the potential of hundreds of lambdas per fiber, it is practical for carriers to lease entire optical circuits to businesses. For example, a television network could lease lambda circuits to transmit video signals among media centers and stations. Recently, MPLS (Multiprotocol Label Switching) has been considered an ideal protocol for controlling optical switches in DWDM networks. It already controls LSPs (label switched paths) across routed networks and can also be used to control optical paths.
Switches used in DWDM:
- Optical Switch is a switch that enables signals in optical fibers or integrated optical circuits (IOCs) to be selectively switched from one circuit to another in telecommunication. Away from telecom, an optical switch is the unit that actually switches light between fibers, and a photonic switch is one that does this by exploiting nonlinear material properties to steer light (i.e., to switch wavelengths or signals within a given fiber).
An optical switch may operate by mechanical means, such as physically shifting an optical fiber to drive one or more alternative fibers, or by electro-optic effects, magneto-optic effects, or other methods. Slow optical switches, such as those using moving fibers, may be used for alternate routing of an optical switch transmission path, such as routing around a fault. Fast optical switches, such as those using electro-optic or magneto-optic effects, may be used to perform logic operations; also included in this category are semiconductor optical amplifiers, which are optoelectronic devices that can be used as optical switches and be integrated with discrete or integrated microelectronic circuits.
Optical Switching Technology
Optical switching technology as an important foundation for all-optical communication network technology, its development and application will greatly affect the development direction of future optical communication networks. So, how does it work?
Optical signals are multiplexed in three ways, space division, time division, and WDM. The corresponding optical switching methods space division switching, time division switching and wave division switching to complete the three multiplexed channels.
Space Division SwitchingIt is the domain swap space on the optical signal, the basic functional components of the spatial light switch. Spatial light switch is the principle of optical switching components gate array switch can be in any of the multiple input multiple output fiber established path. It can constitute an empty spectroscopic switching unit, and other types of switches can also together constitute a time-division switching unit or wave stars. Empty spectral switches generally have both fiber-based and space-based space division switching is a division of swap space.
Time Division SwitchingThis multiplexed signal multiplexing method is a communication network, a channel is divided into a number of different time slots, each optical path signal distribution occupy different time slots, a baseband channel to fit the high-speed optical data stream transmission. Need to use time division switching time slot interchange. The time slot interchanger of the input signal is sequentially written to the optical buffer, and then read out in accordance with established order, thus achieving a one frame at any one time slot exchange to another time slot and outputs completed the timing exchange program. Usually bistable lasers can be used as an optical buffer, but it is only the bit output, and can not meet the demand of high-speed switching and large capacity. While the optical fiber delay line is a more time-division switching device, the time-division-multiplexed signal light input to the optical splitter, so that each of its output channels are only a light signal of the same timeslot, then these signals combined through different optical delay line, after a signal of the type of delay line to obtain a different time delay, the final combination fits before the signals are multiplexed with the original signal, thereby completing a time-division switching.
Wave Division SwitchingShips in WDM systems, the source and destination are required to transmit signals using the same wavelength, such as non-multiplexed so multiplexed in wavelength division multiplexing technology is widely used in the optical transmission system, each multiplex terminal using additional multiplexers, thus increasing system cost and complexity. In the WDM system, wave spectral exchange in the intermediate transmission nodes, to meet no additional devices to achieve wavelength division multiplexing system source and destination communicate with each other, and you can save system resources, improve resource utilization rate. Wave spectroscopic switching system first lightwave signal demultiplexer is divided into plural wave splitting is required to exchange the wavelength channels in each channel wavelength switching the last signal obtained after multiplexing composed of a dense wave division multiplexing signal from an optical output, which take advantage of the characteristics of the fiber-optic broadband, low-loss band multiplexing multiple optical signals, greatly improving the utilization of the Fiber Channel, to improve the communication system capacity.
There are also hybrid switching technologies which are used in large-scale communication network in a variety of the optical path switching technology a mixture of multi-level link connection. In large-scale networks need to be multi-channel signal splitter and then access different link, making the advantages of wavelength division multiplexing can not play, so using wavelength division multiplexing technology levels connecting link, and then space division switching technology used in all levels of link exchange to complete the interface between the link, finally destination and then wave of the exchange of technical output corresponding optical signals, signal combined final sub output. Mixed-use switching technology time mixed, air separation – after midnight – wavelength division mixed several minutes – hours of mixing, air separation – wavelength division.
All-Optical Network Switching Technology
To realize the all optical network switching, the first is to use the circuit switch based optical add-drop multiplexing (OADM) and OXC (optical cross connect) technology to achieve wavelength switching, and then further realization of optical packed switching.
Wavelength switching is based on wavelength in units of optical circuit switched domain, wavelength switching optical signals to provide end-to-end routing and wavelength assignment channel. Wavelength switching key is to use the corresponding network node equipment, optical add-drop multiplexing optical cross-connect. Optical add-drop multiplexing the working principle is based on all-optical network nodes drop and insert the required wavelength path. Its main constituent elements of the multiplexer reconciliation multiplexer, as well as optical switches and tunable harmonic, etc.. Optical add-drop multiplexing of the working principle and the synchronous digital hierarchy (SDH) multiplexer separate interpolation function is similar, but in the time domain, while the other is acting in the optical domain. The optical cross-connect and the synchronous digital system digital cross-connect (DXC) similar effect, but to achieve the cross-connection to the passage in the wavelength at which the optical network node.
Optical wavelength to exchange essentially took office contingent is not efficient optical switching, connection-oriented attribute it established wavelength channel re-distribution to achieve maximum utilization efficiency can not be achieved, even if the communication is idle. Optical packet switching can be implemented with a minimum switching granularity multiplexing of bandwidth resources, improve the communication efficiency of the optical network. Optical packet switching is generally light and transparent packet-switched (OTPS), optical burst switching (OBS) and optical label switching (OMPLS). The optical the transparent packet switching characteristics is the packet length is fixed, the use of synchronous switching manner, the need for all input packets are synchronized in time, thus increasing the technical difficulty and increase the use of cost. The transmission optical burst the use of a variable-length packet data transfer header control information and separated in time and space, to overcome the shortcomings of the synchronization time, but it is possible to generate the packet loss problem. Optical label switching is carried out to add a tag in the IP packet in the core network access re-packet, and the routing method according to the tag inside the core network.
Although optical switching communication occasion require a higher (generally more than 10Gbps) is more suitable for lower transmission costs and greater system capacity can be achieved; via digital transmission rate when the system requirements require a lower transmission rate (2.5Gbps or less), the connection configuration more flexible access may be more appropriate to use the old-fashioned way of photoelectric conversion. Therefore, the practical application of the current should be selected according to the application scenarios appropriate system deployment.
With the future communication network technology development and all-optical network, optical switching technology will be more innovative and more efficient ways for communication network photochemical contribute to become an important part of social development and people’s lives.
Types of Optical Switches
- Optical switches can be divided into mechanical and non-mechanical ones according to the driving methods.
- Mechanical optical switch relies on the movement of optical fiber or optical elements to convert the optical path, such as a mobile optical fiber type, moving the sleeve to move the lens (including mirrors, prisms and self-focusing lens) types. The biggest advantage of this kind of optical switch is a low insertion loss and low crosstalk. Its disadvantage is slow and easy to wear, easy to vibration, impact shocks.
- Non-mechanical optical switch relies electro-optic, magneto-optic, thermo-optic and other effects to change the refractive index of the optical waveguide, the optical path changes, such as electro-optic switch, magneto-optic switch, and thermo-optic switch. This kind of optical switch has good repeatability, fast switching speed, high reliability, long life and other advantages, and small size, can be monolithically integrated. The disadvantage is that the insertion loss and crosstalk performance is not ideal, which should be improved.Here are three common optical switches.
- Opto-Mechanical Switch
Opto-mechanical switch is the oldest type of optical switch and the most widely deployed at the time. These devices achieve switching by moving fiber or other bulk optic elements by means of stepper motors or relay arms. This causes them to be relatively slow with switching times in the 10-100 ms range. They can achieve excellent reliability, insertion loss, and crosstalk. Usually, opto-mechanical optical switches collimate the optical beam from each input and output fiber and move these collimated beams around inside the device. This allows for low optical loss, and allows distance between the input and output fiber without deleterious effects. These devices have more bulk compared to other alternatives, although new micro-mechanical devices overcome this.
- Thermo-Optic Switch
Thermo-optic switches are normally based on waveguides made in polymers or silica. For operation, they rely on the change of refractive index with temperature created by a resistive heater placed above the waveguide. Their slowness does not limit them in current applications.
- Electro-Optic Switch
These are typically semiconductor-based, and their operation depends on the change of refractive index with electric field. This characteristic makes them intrinsically high-speed devices with low power consumption. However, neither the electro-optic nor thermo-optic optical switches can yet match the insertion loss, backreflection, and long-term stability of opto-mechanical optical switches. The latest technology incorporates all-optical switches that can cross-connect fibers without translating the signal into the electrical domain. This greatly increases switching speed, allowing today’s telcos and networks to increase data rates. However, this technology is only now in development, and deployed systems cost much more than systems that use traditional opto-mechanical switches.
Optical Switch Protection System for DWDM Network Security
Optical switch protection system for the security of communication network provides a set of economic, practical solutions, the formation of a non-blocking, high reliability, flexible, anti-disaster ability of the optical communication network. Optical switch protection system by the automatic switching and network management stations, you can achieve light switch protection, monitoring and the optical path of the optical power emergency dispatch three main functions.
DWDM system in the trunk and local fiber optic transmission network has a large number of applications. Due to the amount of traffic carried by focus on the importance of safety more and more attention in the event of full resistance will affect all business network hosted. The DWDM network security has always been the most important in the transmission maintenance work. However, DWDM protection technology by its own limitations, has problems such as not flexible, large investment, and the effect is not ideal. Then the optical switch protection technology comes to play a very important role in the DWDM network security.
The optical switch protection system switching control module is a set of optical switches, optical power monitoring, stable light source monitoring in one of the high level of integration modules. Optical power monitoring module and optical switch control module coordination, selection of splitting ratio of 97:3 is more appropriate on the trunk, the equivalent of approximately 0.2dB attenuation on the transmission line; optical switching module contains 1×2 or 2×2 optical switch, controlled by the switch between the main and backup light routing operation.
Real-time monitoring of the optical power monitoring module communication optical fiber optical power value reported to the main control module; analysis and comparison of the main control module, found that the change in value of the optical power exceeds a preset threshold switching immediately issued instructions to the optical switch module; optical switch module by the Directive instantly switching action has occurred. In order to achieve a switching operation.
The optical path automatically switch protective equipment involved in trunk transmission system did not affect the transmission characteristics. In fact, switching equipment involved in the optical switch and splitter only two passive optical devices.
One end of the switching unit is connected to the transceiver of the transmission system, the main fiber optic cable and the spare cable, respectively connected to two output terminals of the 2×2 optical switch. When the optical path occurs when the optical power is abnormal, the optical switch is automatically switched to the alternate route.
It is understood that the optical switch protection system has the following advantages. Fast switching speed, the optical switch switching speed ships 5ms, plus system analysis, the response time of a single-ended switching time of less than 20ms, the switching time of less than 50ms for the entire system, the basic switching operation can be done without interrupting the communication, to achieve business grade level of protection.
Switching, high reliability, implemented through the optical power monitoring, to avoid false alarm of the optical frame, ensure switched judgment is correct. The spare fiber routing monitoring, to ensure the validity of the switch, and continue to be monitored after switching optical path.
Emergency dispatch function, simply switching command issued from the program, you can deploy routing to facilitate the realization of the non-blocking cutover and line maintenance work. The switch device for a transmission system is transparent, i.e. the switching device does not require the type of transmission system can use either SDH or DWDM.
The optical switch protection DWDM is an economical and safe a line protection method, but the the light automatic protection system intervention to DWDM systems, there are many issues to consider. Splitter 97:3 spectral, optical switching device insertion loss is about 2 dB intervention light switching device, the system has an additional two-fiber jumper whose fiber insertion loss is estimated as 1 dB, so the whole switching device Interventional theoretically maximum will bring 3dB attenuation, and many cases of practical use only in 1.5-2.5dB.
Optical automatic switching system for the DWDM line protection is both safe and economical means of protection. The future, as the size of the network continues to expand, optical switch protection systems will play a more important role to meet the requirements of the assessment indicators, to improve the safety of operation of the transmission network.
Telecommunications makes wide use of optical techniques in which the carrier wave belongs to the classical optical domain. The wave modulation allows transmission of analog or digital signals up to a few gigahertz (GHz) or gigabits per second (Gbps) on a carrier of very high frequency, typically 186 to 196 THz. In fact, the bitrate can be increased further, using several carrier waves that are propagating without significant interaction on a single fiber. It is obvious that each frequency corresponds to a different wavelength. Dense Wavelength Division Multiplexing (DWDM) is reserved for very close frequency spacing. This blog covers an introduction to DWDM technology and DWDM system components. The operation of each component is discussed individually and the whole structure of a fundamental DWDM system is shown at the end of this blog.
- lambda switching (photonic switching, or wavelength switching)
Lambda switching (sometimes called photonic switching, or wavelength switching) is the technology used in optical networking to switch individual wavelengths of light onto separate paths for specific routing of information. In conjunction with technologies such as dense wavelength division multiplexing (DWDM) – which enables 80 or more separate light wavelengths to be transmitted on a single optical fiber – lambda switching enables a light path to behave like a virtual circuit.ad this free guide
Although the ability to redirect specific wavelengths intelligently is, in itself, a technological breakthrough, lambda switching works in much the same way as traditional routing and switching. Lambda routers – which are also called wavelength routers, or optical cross-connects (OXC) – are positioned at network junction points. The lambda router takes in a single wavelength of light from a specific fiber optic strand and recombines it into another strand that is set on a different path. Lambda routers are being manufactured by a number of companies, including Ciena, Lucent, and Nortel.
Multiprotocol Lambda Switching is a variation of Multiprotocol Label Switching (MPLS, confusingly, the abbreviation for both variants) in which specific wavelengths serve in place of labels as unique identifiers. The specified wavelengths, like the labels, make it possible for routers and switches to perform necessary functions automatically, without having to extract instructions regarding those functions from IP addresses or other packet information.
Lambda switching gets its name from lambda, the 11th letter of the Greek alphabet, which has been adopted as the symbol for wavelength. In networking, the word is used to refer to an individual optical wavelength.
Introduction to DWDM Technology
DWDM technology is an extension of optical networking. DWDM devices (multiplexer, or Mux for short) combine the output from several optical transmitters for transmission across a single optical fiber. At the receiving end, another DWDM device (demultiplexer, or DeMux for short) separates the combined optical signals and passes each channel to an optical receiver. Only one optical fiber is used between DWDM devices (per transmission direction). Instead of requiring one optical fiber per transmitter and receiver pair, DWDM allows several optical channels to occupy a single fiber optic cable.
A key advantage to DWDM is that it’s protocol and bitrate independent. DWDM-based networks can transmit data in IP, ATM, SONET, SDH and Ethernet. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel. Voice transmission, email, video and multimedia data are just some examples of services which can be simultaneously transmitted in DWDM systems. DWDM systems have channels at wavelengths spaced with 0.4 nm spacing.
DWDM is a type of Frequency Division Multiplexing (FDM). A fundamental property of light states that individual light waves of different wavelengths may coexist independently within a medium. Lasers are capable of creating pulses of light with a very precise wavelength. Each individual wavelength of light can represent a different channel of information. By combining light pulses of different wavelengths, many channels can be transmitted across a single fiber simultaneously. Fiber optic systems use light signals within the infrared band (1 mm to 400 nm wavelength) of the electromagnetic spectrum. Frequencies of light in the optical range of the electromagnetic spectrum are usually identified by their wavelength, although frequency (distance between lambdas) provides a more specific identification.
DWDM System Components
A DWDM system generally consists of five components: Optical Transmitters/Receivers, DWDM Mux/DeMux Filters, Optical Add/Drop Multiplexers (OADMs), Optical Amplifiers, Transponders (Wavelength Converters).
Transmitters are described as DWDM components since they provide the source signals which are then multiplexed. The characteristics of optical transmitters used in DWDM systems is highly important to system design. Multiple optical transmitters are used as the light sources in a DWDM system. Incoming electrical data bits (0 or 1) trigger the modulation of a light stream (e.g., a flash of light = 1, the absence of light = 0). Lasers create pulses of light. Each light pulse has an exact wavelength (lambda) expressed in nanometers (nm). In an optical-carrier-based system, a stream of digital information is sent to a physical layer device, whose output is a light source (an LED or a laser) that interfaces a fiber optic cable. This device converts the incoming digital signal from electrical (electrons) to optical (photons) form (electrical to optical conversion, E-O). Electrical ones and zeroes trigger a light source that flashes (e.g., light = 1, little or no light =0) light into the core of an optical fiber. E-O conversion is non-traffic affecting. The format of the underlying digital signal is unchanged. Pulses of light propagate across the optical fiber by way of total internal reflection. At the receiving end, another optical sensor (photodiode) detects light pulses and converts the incoming optical signal back to electrical form. A pair of fibers usually connects any two devices (one transmit fiber, one receive fiber).
DWDM systems require very precise wavelengths of light to operate without interchannel distortion or crosstalk. Several individual lasers are typically used to create the individual channels of a DWDM system. Each laser operates at a slightly different wavelength. Modern systems operate with 200, 100, and 50-GHz spacing. Newer systems support 25-GHz spacing and 12.5-GHz spacing is being investigated. Generally, DWDM transceivers (DWDM SFP, DWDM SFP+, DWDM XFP, etc.) operating at 100 and 50 GHz can be found on the market nowadays.
DWDM Mux/DeMux Filters
Multiple wavelengths (all within the 1550 nm band) created by multiple transmitters and operating on different fibers are combined onto one fiber by way of an optical filter (Mux filter). The output signal of an optical multiplexer is referred to as a composite signal. At the receiving end, an optical drop filter (DeMux filter) separates all of the individual wavelengths of the composite signal out to individual fibers. The individual fibers pass the demultiplexed wavelengths to as many optical receivers. Typically, Mux and DeMux (transmit and receive) components are contained in a single enclosure. Optical Mux/DeMux devices can be passive. Component signals are multiplexed and demultiplexed optically, not electronically, therefore no external power source is required. The figure below is bidirectional DWDM operation. N light pulses of N different wavelengths carried by N different fibers are combined by a DWDM Mux. The Nsignals are multiplexed onto a pair of optical fiber. A DWDM DeMux receives the composite signal and separates each of the N component signals and passes each to a fiber. The transmitted and receive signal arrows represent client-side equipment. This requires the use of a pair of optical fibers; one for transmit, one for receive.
Optical Add/Drop Multiplexers
Optical add/drop multiplexers (i.e. OADMs) have a different function of “Add/Drop”, compared with Mux/DeMuxfilters. Here is a figure that shows the operation of a 1-channel OADM. This OADM is designed to only add or drop optical signals with a particular wavelength. From left to right, an incoming composite signal is broken into two components, drop and pass-through. The OADM drops only the red optical signal stream. The dropped signal stream is passed to the receiver of a client device. The remaining optical signals that pass through the OADM are multiplexed with a new add signal stream. The OADM adds a new red optical signal stream, which operates at the same wavelength as the dropped signal. The new optical signal stream is combined with the pass-through signals to form a new composite signal.
OADM designed for operating at DWDM wavelengths are called DWDM OADM, while operating at CWDM wavelengths are called CWDM OADM. Both of them can be found on the market now.
Optical amplifiers boost the amplitude or add gain to optical signals passing on a fiber by directly stimulating the photons of the signal with extra energy. They are “in-fiber” devices. Optical amplifiers amplify optical signals across a broad range of wavelengths. This is very important for DWDM system application. Erbium-Doped Fiber Amplifiers (EDFAs) are the most commonly used type of in-fiber optical amplifiers. EDFAs used in DWDM systems are sometimes called DWDM EDFA, compared to those used in CATV or SDH systems. To extend the transmission distance of your DWDM system, you can get all the types of Optical Amplifiers in Fiberstore, including DWDM EDFA, CATV EDFA, SDH EDFA, EYDFA, and Raman Amplifier etc. (Here is a figure that shows the operation of a DWDM EDFA.)
Transponders (Wavelengths Converters)
Transponders convert optical signals from one incoming wavelength to another outgoing wavelength suitable for DWDM applications. Transponders are Optical-Electrical-Optical (O-E-O) wavelength converters. A transponder performs an O-E-O operation to convert wavelengths of light, thus some people called them “OEO” for short. Within the DWDM system a transponder converts the client optical signal back to an electrical signal (O-E) and then performs either 2R (Reamplify, Reshape) or 3R (Reamplify, Reshape, and Retime) functions. The figure below shows bi-directional transponder operation. A transponder is located between a client device and a DWDM system. From left to right, the transponder receives an optical bit stream operating at one particular wavelength (1310 nm). The transponder converts the operating wavelength of the incoming bitstream to an ITU-compliant wavelength. It transmits its output into a DWDM system. On the receive side (right to left), the process is reversed. The transponder receives an ITU-compliant bitstream and converts the signals back to the wavelength used by the client device.
Transponders are generally used in WDM systems (2.5 to 40 Gbps), including not only DWDM systems, but aslo CWDM systems. Fiberstore provides various WDM transponders (OEO converters) with different module ports (SFP to SFP, SFP+ to SFP+, XFP to XFP, etc.).
How DWDM System Components Work Together with DWDM Technology
As DWDM system is composed of these five components, how do they work together? The following steps give out the answer (also you can see the whole structure of a fundamental DWDM system in the figure below):
1. The transponder accepts input in the form of a standard single-mode or multimode laser pulse. The input can come from different physical media and different protocols and traffic types.
2. The wavelength of the transponder input signal is mapped to a DWDM wavelength.
3. DWDM wavelengths from the transponder are multiplexed with signals from the direct interface to form a composite optical signal which is launched into the fiber.
4. A post-amplifier (booster amplifier) boosts the strength of the optical signal as it leaves the multiplexer.
5. An OADM is used at a remote location to drop and add bitstreams of a specific wavelength.
6. Additional optical amplifiers can be used along the fiber span (in-line amplifier) as needed.
7. A pre-amplifier boosts the signal before it enters the d e muliplexer.
8. The incoming signal is demultiplexed into individual DWDM wavelengths.
9. The individual DWDM lambdas are either mapped to the required output type through the transponder or they are passed directly to client-side equipment.
Using DWDM technology, DWDM systems provide the bandwidth for large amounts of data. In fact, the capacity of DWDM systems is growing as technologies advance that allow closer spacing, and therefore higher numbers, of wavelengths. But DWDM is also moving beyond transport to become the basis of all-optical networking with wavelength provisioning and mesh-based protection. Switching at the photonic layer will enable this evolution, as will the routing protocols that allow light paths to traverse the network in much the same way as virtual circuits do today. With the development of technologies, DWDM systems may need more advanced components to exert greater advantages.