DWDM Definition

In digital signal processing, DWDM is a technique for increasing the bandwidth of optical network communications. DWDM allows dozens of different data signals to be transmitted simultaneously over a single fiber. To keep the signals distinct, DWDM manipulates wavelengths of light to keep each signal within its own narrow band.

Wavelength Division Multiplexing (WDM) transport products are the scalable, optical, foundation for all transport services offerings, allowing for differentiation, capacity, predictability and security.

WDM is now a cost-effective, flexible and scalable technology for increasing capacity of a fiber network. WDM architecture is based on a simple concept – instead of transmitting a single signal on a single wavelength, transmit multiple signals, each with a different wavelength. Each remains a separate data signal, at any bit rate with any protocol, unaffected by other signals on the fiber.

Over the past ten years, WDM networks have evolved from simple point-to-point systems aimed at fiber exhaust, to single-ring networks to advanced, interconnected, mesh architectures. Reconfigurable Optical Add/Drop Multiplexers (ROADMs) are vital to these advanced networks, enabled by Wavelength Selective Switch (WSS) technology.

LightRiver’s portfolio of WDM transport products provides that scalable optical foundation, with the technical variety that allows you to differentiate your service offerings, and the best platform for you to deliver those custom services to your clients. From dynamic bandwidth allocation and compression for data center connectivity, to wire-speed encryption and uncompressed digital video transport, compete more effectively by offering unique SLAs and improved service quality.

As with most transport systems, there are requirements to add and drop traffic along ring and tapered networks. WDM systems support two types of add/drop structures – Fixed Optical Add/Drop Multiplexers (FOADM) and Reconfigurable Optical Add/Drop Multiplexers (ROADM).

FOADMs are based on simple static fibers that permit add/drop of predefined wavelengths. These systems are fully integrated and manageable and provide a fine balance of features and lower initial cost.

ROADMs add the ability to remotely switch traffic from a WDM system at the wavelength layer. While more expensive than FOADMs initially, ROADMs are used in applications where traffic patterns are not fully known or change frequently, and where this level of automation will save significant cost over time.

The key features and benefits of WDM include:

  • Protocol and Bit Rate Agnostic – wavelengths can accept virtually any services
  • Fiber Capacity Expansion – WDM adds up to 160X bandwidth to a single fiber
  • Hi Cap/Long Haul and Lo Cap/Short Haul Applications – CWDM and DWDM provide price performance for virtually any network
  • Remotely Provisionable – ROADMs provide the flexibility to change with changing network requirements

Primarily driven by Ethernet and packet-based services, the need for bandwidth has exploded. Even mid-sized network operators are demanding multiple 100 Gbps pipes to accommodate large increases for growing and diverse applications such as surveillance, ITV, and data-center connections, or simply to leverage the optical advantages of coherent optics in overcoming legacy fiber plant issues. This growth requires transport that is flexible and scalable.

ROADM Definition

A reconfigurable optical add-drop multiplexer (ROADM) is a form of optical add-drop multiplexer that adds the ability to remotely switch traffic from a wavelength-division multiplexing (WDM) system at the wavelength layer. This is achieved through the use of a wavelength selective switching module. This allows individual or multiple wavelengths carrying data channels to be added and/or dropped from a transport fiber without the need to convert the signals on all of the WDM channels to electronic signals and back again to optical signals.

Wave-Division Multiplexing (WDM) networks have evolved into advanced interconnected mesh architectures. Reconfigurable Optical Add/Drop Multiplexers (ROADMs) are vital to these advanced networks, enabled by Wavelength Selective Switch (WSS) technology.

ROADMs add the ability to remotely switch traffic from a WDM system at the wavelength layer. While more expensive than FOADMs on day one, ROADMs are used in applications where traffic patterns are not fully known or change frequently, and this flexibility and level of automation always support a lower Total Cost of Ownership for any dynamic network.

The keys to any ROADM are the optical switch fabric and optical switching technology. Advanced WSS modules are the optical switching “engine.” The WSS provides wavelength selection, switching, power monitoring, and auto-power balancing all within a single device. In addition, WSS ROADMS support advanced network architectures such as multi-degree hub nodes and mesh architectures.

Prior to ROADMs, DWDM systems were commonly implemented using “fixed filters” in what is known as a “banded” DWDM architecture. A typical banded DWDM system provided 32 channels in eight groups of four channels in each group.

ROADMS offer additional size, flexibility, and performance benefits, including:

  • The planning of the entire bandwidth assignment need not be carried out during initial deployment of a system. The configuration can be done as and when required without affecting traffic already passing the ROADM.
  • Improved network utilization due to single-channel granularity
  • Non-block access to any lambda – improved network efficiency
  • Flexibility & service velocity – drop any channel and any number of channels at any site
  • Integrated auto-power balancing, eliminating costly & timely manual tuning
  • Full band tunable transponders
  • Support for interconnected ring and mesh architectures
  • A-Z end-to-end wavelength provisioning

Coherent Definition

Coherent optics, which reduces the need for amplification of a signal and increases the distance it can be sent while reducing chromatic dispersion, has been combined with 100G technology to ease carrier network upgrades.

Although there’s no common definition of what DWDM coherent detection is, there is a consensus among vendors and service providers that it consists of four major elements: high order amplitude/phase modulation, polarization multiplexing, coherent detection using a local oscillator laser in the receiver, and high-speed ADCs and sophisticated digital signal processing in the receiver.

What has made coherent technology a good fit with 100G is that unlike initial DWDM systems that used the Intensity Modulation with Direct Detection (IMDD) modulation technique is it can overcome various fiber impairments, such as chromatic dispersion (CD) and polarization mode dispersion (PMD), as data rates increase beyond 10 Gbps.

The advent of 100G technology coupled with coherent optics means that network operators can more easily upgrade their backbone, core and edge networks with the right amount of bandwidth to ensure that the user gets a consistent experience, and is not constrained by capacity limitations.

Although 100G wavelength shipments have experienced over 75 percent CAGR over the last five years, future forecasts show that at least 60 percent of WDM shipments will be with 100 Gbps wavelengths, or higher, over the next 5 years.

But even as 100G gains momentum, the IEEE and the Optical Internetworking Forum are looking what lies ahead. The IEEE is developing a Terabit Ethernet standard, while the OIF’s Multi-Link Gearbox 2.0 project addresses how to process link technology with optical interfaces as bandwidth grows to 200G, 400G, 1T and perhaps beyond.

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