Dense Wavelength Division Multiplexing (DWDM)

Optical Fiber DWDM

   

 DWDM

           Dense wavelength division multiplexing (DWDM) is an extension of optical networking. DWDM devices combine the output from several optical transmitters for transmission across a single optical fiber. At the receiving end, another DWDM device 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 bit-rate 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 co-exist 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 (1mm 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  

Optical transmitters/receivers

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. 
Example: 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 (for example; light = 1, little or no light =0) light into the core of an optical fiber. E-O conversion is nontraffic 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 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 (multiplexer filter). The output signal of an optical multiplexer is referred to as a composite signal.At the receiving end, an optical drop filter (demultiplexer) 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 block diagram shows in figure 2 is bidirectional DWDM operation. N light pulses of N different wavelengths carried by N different fibers are combined by a DWDM mux. The N signals are multiplexed onto a pair of optical fiber. A DWDM demultiplexer 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. Fig 2. Mux/demux (transmit/receive) components.     

Optical add/drop multiplexers (OADMs) 

This block diagram demonstrates the operation of a onechannel OADM. This OADM is designed to only add or drop optical signals with a particular wavelength (represented by the red light pulse above). 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 passthrough signals to form a new composite signal

Optical amplifiers 

Optical amplifiers (OAs) 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. OAs 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 fibre.    

Transponders (wavelength converters) 

Transponders convert optical signals from one incoming wavelength to another outgoing wavelength suitable for DWDM applications. Transponders are optical-electricaloptical (O-E-O) wavelength converters. A transponder performs an O-E-O operation to convert wavelengths of light. 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 block diagram shows bidirectional 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 bit stream and converts the signals back to the wavelength used by the client device.  

 

THE FUNCTION OF DWDM

DWDM systems generally perform the functions as follows:

1      Generating signals. Light source (LASER or LED) should provide a steady light with certain specifications, different bandwidth narrow that carry digital data, is modulated as an analog signal.

2       Combine the signals. Using DWDM multiplexer for combining the signals input. 

3       Transmits the signals. The effect of signal degradation or disruption can be minimised by controlling variables such as channel spacing, tolerance of the wavelength of light and LASER power levels.

4    Separating the signal received. At the receiver end, signals that had been dimultipleks, must be separated through a demultiplexer.

5       Receive the signal. The signal has been didemultipleks was received by a photodetector.

6      Strengthening of the signal and the regenator. The signal is passed through optical fiber must be experiencing reinforcement. Signal booster serves to strengthen the signals received to be forwarded again, regenerator while in addition to having the ability to strengthen signals can also improve the quality of the received signal, so the output signal has a good quality.

There are two types of optical amplifiers, namely:

Ø      Solid State Optical Amplifier optical amplifier form made of a semiconductor material.

Ø    Fiber Amplifiers in the form of reinforcement in an optical fiber which is divided into EDFA (Erbium Doped Fiber Amplifier) and Raman amplifiers.

While for the type of reinforcement signals in the optical fiber itself there are 3 kinds i.e. post ampifier, pre amplifiers and line amplifiers. In the image below in the show me illustration of the reinforcement. When the start signal is transmitted, then do the strengthening via post amplifier. At a time when the signal passes through an optical fiber, then back strengthening is done through the line amplifiers. In addition, line amplifier also serves to improve the received signals so that at a time when they will be sent back, the signal became nice as they are. At the end, when the signals will be received back, then done through strengthening pre amplifier.

As an additional function, the system should also come with a DWDM interfaces (interfaces) and client side to receive the input signal by a device called a transponder. Transponder function to change the signal input from the client side or another device that has different traffic types into types of signals that are known and can be transmitted by WDM system.

There is another function of the DWDM system devices namely ADM (Add/Drop Multiplexer) and OXC (Optical Cross Connect). ADM is required if between two connected terminals will be integrated to other terminals or inserted. ADM functions is also done by a multiplexer and a demultiplexer.

Optical cross connect or OXC is required if will do the integration or interconnection of several optical networks into one network. OXC is composed of mux/demux and optical switching as well. The following is an illustration of OXC optical networks.

THE WORKINGS OF DWDM

Working principles of DWDM technology in General has in common with other transmission media in sending information from one place to another. For using the DWDM technology in the form of fiber optic transmission medium, where all the source signal information (λ1-λn) of the transmitter will be dimultipleksikan into a single fiber, the signal information is transmitted to the device then enter demuktiplekser to redeployed in accordance with each of the signals will be received by the receiver.

On DWDM technology is the supporting components including filter type, fiber optic and optical amplifiers. The type of filter used in umumya include Dichroic interference Filters (DIF. ..), Fiber Bragg Gratings (FBG), Array Wavegiude Filters (AWG) and Hybrid Fiber Fused Cascade (FCF) and Mach-Zehnder (M-Z) interference. The next component is the fiber optic cables with low dispersion, while the optical amplifier that is widely used is the EDFA (Erbium Doped fibre Amplifiers (1530-1565 nm)) and many more types of amplifier msih other examples of raman amplifiers etc. The use of optical amplifiers are very important role in the development of DWDM technology such as optical signal amplifier and 3R (Reshaping, Retiming, Regenerating) to keep the maximum signal quality.

CHARACTERISTICS OF DWDM

DWDM has a characteristic in which discrete wavelengths from an orthogonal set of carrier wave can be separated, routed, and switches without any interference from each other. Wdm network needed to implement various passive and active components to the combination, distribution, isolation, and strengthening power optic on the length of grlombang. Passive components have no control for external operation, so it has limitations in flexibility of application. In contrast, the active components have a high degree of flexibility because it can be electronically controlled or optic

PARAMETERS OF DWDM

The system of processing and presentation of data in optical performance is still done manually, IE by mangamati raw data (data with extension .txt) on monitor terminal (NMS) which pretty much amount to choose certain parameters. So it is very tiring because it takes quite a long time. The choice of outcome data is then presented in the form of Excel. Such data can only be viewed on the screen of the monitor, terminal 

 

Optical port interface STM-16 (2.5 Gbps) operating wavelength, fiber type, power output, the SMSR (Side Mode Suppression ratio), extinction ratio, attenuasi, and sensitivity.

Optical interface port STM64 (10 Gbps) operating wavelength, fiber type, power output, the SMSR (Side Mode Suppression Ratio), extinction ratio, attenuasi, sensitivity. Parameters used to test the feasibility of the device against a system that is damping, dispersion, and receive power.

DEVELOPMENT OF DWDM TECHNOLOGY

Initially, the WDM technology (Wavelength Division Multiplexing), the forerunner of the inception of DWDM evolved from the existing limitations on the fiber-optic system, where the growth of traffic on a number of backbone network experience a high acceleration capacity of the network with the quickly filled. This has been the rationale for utilizing existing network compared to building a new network.

On the development of WDM, some systems have been successfully accommodate a number of wavelength in an optical fiber, each 2.5 Gbps with capacity up to 5 Gbps. But new problems pose a WDM, namely to-nonlinieran optical fiber dispersion effects and growing significant presence that causes the limited number of wavelength 2-8 fruit in at that time.

In further development, the number of long-wave that can be accommodated by a single optical fiber grow tens of fruit and a capacity for each wave-length increased in the range of 10 Gbps, this refers to the ability of the so-called DWDM.

But basically, DWDM is the solution of the problems found in the WDM, which in terms of its own infrastructure practically occur only the addition of the transmitter and receiver equipment for each wavelength used. Core improvements owned by DWDM technology lies in the type of filter, optical fiber and amplifier amplifier. A common type of filter used in DWDM systems include Dichroic Interference Filters (DIF. ..), Fiber Bragg Gratings (FBG), Array Waveguide Filters (AWG) and Fused Hybrid Cascaded Fiber (FCF) and Mach-Zehnder (M-Z) interferometers.

The next component is the optical fiber with low dispersion, where such indispensable characteristics considering the dispersion is directly related to the transmission capacity of a system. Types of optical fibers that are widely used for DWDM applications including LEAF which is a product of Corning, which by the carriers is believed to be the finest quality fibres. While optical amplifier that is widely used for such applications is the EDFA with flat characteristics for all long-wave spectrum in DWDM. Another technique that has been successfully tested is to shorten the distance between the channel, usually ranging from 1 nm be 0.3 nm. This is particularly useful on systems that the spectrum of the amplifier gain of optical properties less evenly.

Implementation Of DWDM

 

In the early development of DWDM backbone network is devoted to the provider with a large capacity. However with the development of the latest technologies can now be implemented in transimisi coverage of network subscribers. Provider Telkom using base Alcatel Lucent PSS (Western Digital My Service Switch) as a multiplexer. Latest technology DWDM 100 g-based Alcatel-Lucent 1830 Western Digital My Service Switch (PSS) offered to customers as a substitute technology Metro Ethertnet more their use so that growing bandwidth needs. Metro Ethernet has some limitations on traffic during peak hours and full data, often obtained packet loss on a customer's system. While DWDM has a number of advantages derived from either side of the operator or the user of the service, including the ease of adding capacity that is integrated with the latest technology than his predecessor Metro Ethernet-based networks.

Development Of Implementation

DWDM network used for BTS backhaul in GSM network. The system of processing and presentation of data in optical performance is still done manually, IE by mangamati raw data (data with extension .txt) on monitor terminal (NMS) which pretty much amount to choose certain parameters. So it is very tiring because it takes quite a long time. The choice of outcome data is then presented in the form of Excel. Such data can only be seen on the screen monitors the local terminal, while many of the parties who wanted this data accessible anywhere.

 

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