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“ROADM TECHNOLOGY: An optimal solution for economic and reliable data transfer”
TABLE OF CONTENTS PAGE NO
- INTRODUCTION……………………………………….. PAGE || 1
ARCHITECTURE……………………………………….. PAGE || 2
- WORKING OF WSS…………………………… PAGE || 3
- CHANNEL MONITOR…………………………. PAGE || 4
- AMPLIFIER……………………………………. PAGE || 4
- PERFORMANCE PARAMETERS……………………. PAGE || 4
- BENEFITS……………………………………………….. PAGE || 4
- FUTURE SCOPE & CURRENT ISSUES……………… PAGE || 4
- SUMMARY……………………………………………… PAGE || 5
- REFERENCE……………………………………………. PAGE || 6
In today’s technology-oriented world, communication plays a vital role in all aspects of our day to day activities. Continuous efforts are being made by technologist to achieve optimum data transfer through minimum bandwidth. Fiber optic communication is a technology used for transmitting information from one place to another by sending light waves through optical fibers. The light waves act as an electromagnetic carrier that can be modulated to carry information. 
In fiber optic communication Wavelength Division Multiplexing (WDM) is a technology which mixes numerous optical carrier signals onto a single optical fiber using different wavelengths of laser light. WDM systems allow telecommunication companies to expand the capacity of their network without installing new fiber cables. The capacity of a fiber optic link can be expanded by upgrading multiplexers and demultiplexers at each end. 
DWDM – Dense Wavelength Division Multiplexing is a technology in optical fiber communication that enhances the bandwidth capacity of existing fiber networks. By using this technology multiple signals of different wavelengths can be transmitted simultaneously through the same fiber. 
Optical multiplexing is the main function of WDM networks for reliable data transfer. Configurations in WDM network like OADM (Optical Add-Drop Multiplexer) provide flexibility and simplicity for data transmission. One of the most popular technologies evolved from OADM is ROADM (Reconfigurable Optical Add-Drop Multiplexer). 
Fig. 1: SONET Network
OADM is a device used in WDM systems for adding or dropping one or more wavelengths to the existing signals. It’s simple to install and requires no power to operate and maintain but the disadvantage is that it can’t be easily changed without disturbing the entire network. ROADM can be connected at several locations and can be easily modified as network requirements changed. However, it requires additional space and power to operate. 
Present growth in DWDM systems has made an achievable transmission with hundreds of channels presenting total capacity in terabits per second.
A simple synchronous optical network requires O-E-O (Optical-Electrical-Optical) translation which makes the network configuration complex, costly as well as less effective. ROADM permits single or many wavelengths to be added or dropped without O-E-O translation and offers huge bandwidth for data transfer at a minimum expense. 
Since the commercial introduction of ROADM in the early 2000s, they have evolved through three main generations: 
Wavelength Blocker (WB),
- The first-generation ROADM were a demux-switch-mux approach to add/drop/pass through. They were complex, large and expensive. The signal quality was poor due to polarization dependent loss.
- In the wavelength blocker technology, a splitter and a filter tray are used to add/drop any number of selected wavelengths. WB ROADM can equalize, attenuate, add or block all wavelengths with low dispersion and low power dissipation without regeneration. They are primarily used in the long haul.
Planar Lightwave Circuit (PLC),
- The PLC ROADM allowed the service providers to dynamically insert/terminate (add/drop) or route any particular wavelength through wavelength directors. This approach provides a reduced networking cost over WB-based ROADMs.
Wavelength selective switches.
- The wavelength selective switches allow adding dropping and express routing traffic through network nodes which help in efficient bandwidth utilization. 
Fig. 2: ROADM Architecture
ROADM architecture consists of three main components which make up a ROADM blade.
- Wavelength selective switch,
- Channel monitor
Fig. 3: Wavelength Selective Switch
The WSS consist of single common input port against multiple wavelength output ports, each wavelength input from the common port can be switched to any one multi-wavelength output ports regardless of all other wavelength routes. Each wavelength can be individually attenuated for channel power control and utilization.
2.1 Working of WSS:
1XN wavelength selective switch
Fig. 4: Wavelength Selective Switch 1x N
The light from the common input fiber collimates by a microlens at point A, The following lens creates the image of the collimated beam on the diffraction grating point at C, The wavelength dispersed beam falls on to the plane D where the beams are reflected with certain tilt depending on the micromirror setting all the reflected beams are focused on point B where angle space conversion will direct the beam towards the output fiber. The WSS is similar to fiber Bragg grating where multiple layers are used to reflect particular wavelengths.
2.2 Channel Monitor:
The Channel Monitor in a ROADM attenuates the output wavelength according to the channel requirements. The wavelength selective switch just redirects the light so WSS along with channel monitor checks that all frequencies are properly tuned, and no particular frequency occupies the entire channel volume. The attenuation is performed by using MEMS-Micro-Electro-Mechanical-System or Liquid Crystal Switching principle.
The amplifier amplifies the waves approaching towards the output fiber.
3. Performance Parameters:
ROADM allows building dynamic and flexible network which can efficiently transmit data at the rate of 100Gb/s. The service operators must pay attention to details like using efficient dual polarization Quadrate Phase Shift Keying modulation technique to avoid channel interference, optimizing OSNR (Optical Signal to Noise Ratio) for the same BER bit error rate, Polarization mode dispersion value of fiber is also an important parameter. ROADM enables add drop channels on every site so channel power balance is very important. 
Heavy Density 1 x 20 WSS switches have been devised by component providers at a very economic rate compared to the amount of data they can transfer.
Advanced modulation schemes enabled by coherent detection are leading to flexing spectrum ROADM hardware
Touchless Automated provision and activation of network bandwidth on intermediate sites of the network allow the service provider to respond to customer demands quickly and efficiently with a minimal budget. ROADM provides flexible reconfigurability at intermediate as well as at endpoints of the network
Increased fiber utilization capacity eliminates the need for excess equipment installation to meet the rising customer demands.
Avoids O-E-O translations thus eliminates the need for high-speed switching of electronics.
5. Future Scope and Current issues
N degree ROADM can be built using (N) (N x 1) WSS with each serving a given degree by changing access structure additional (N + 1) degree wavelength can realized using (N + 1)(1 x N) cross-connects but this does not allow all possible interlink connections.
Today the challenge is to develop a fully functional CDC-ROADM i.e. colorless directionless contention less ROADM using additional WSSs Such a module would allow any wavelength or combination of wavelengths to be switched between any interlink connections, CDC ROADM is conceptually simple, but it requires a high level of photonic integration.
Fig. 4: CDC ROADM
WSS is the important component of ROADM, and better-improved ROADMs can be fabricated by improving modules with additional WSSs.
ROADM have efficiently made 100G data transmission reliable and economic by omitting the optical electronic translations, Today the technologist are looking for enhancements in the ROADM architecture by optimizing the component performance and making Directionless, Colorless and Contention less ROADM possible to achieve transmission up to 400G
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