Dense Wavelength Division Multiplexing (DWDM)

Dense Wavelength Division Multiplexing (DWDM)

  1. Dense wavelength division multiplexing (DWDM)—

It is a technology that puts data from different sources together on a optical fiber, with each signal carried at the same time on its own separate light wavelength.

The DWDM is analogous to FDMA (frequency division multiple access) as the wavelength is related to the frequency by the equation

Using DWDM, up to 80 (theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. A particular standard has to be followed for the channel spacing in the DWDM. For 1500 nm DWDM wavelength the channel spacing is 100 GHz (0.8nm) i.e40 ch/λ or 50 GHz i.e 80 ch/λ. In the case of Coarse DWDM the channel spacing is 20 nm. (G.652 attenuation fiber)

Since each channel is demultiplexed at the end of the transmission back in to the original source, different data formats being transmitted at different data rates can be transmitted together. Specifically internet protocol (IP) data, synchronous optical network data (SONET), and asynchronous transfer mode (ATM) data can all be travelling at the same time within the optical fiber. Apart from this the other clients are SDH, Ethernet in the NG-DWDM.

  1. DWDM components
  • Optical Transmission Unit (OTU)
  • Optical multiplexers (mux) and demultiplexers (Demux).
  • Optical Amplifiers

Transmitters and transponders – transmitters in DWDM are high resolution, precision narrow-band lasers. These lasers allow close channel spacing, which increases the number of wavelengths that can be used in the 1500 nm band while minimizing the effects of signal impairment such as dispersion. These lasers can also use optical amplifiers.

Some DWDMs use transponders, which converts broadband optical signals to specific wavelengths using optical-to-electrical-to-optical (OEO) conversion.

Optical amplifiers – optical amplifiers boost optical signals to minimize the effects of power loss and attenuation that result from sending light pulses over optical fiber.

Example – erbium doped fiber amplifier (EDFA)

Optical multiplexers and demultiplexers–multiplexers combine the transmit signals from different wavelengths onto a single optical fiber. Optical demux separate the combined signals into their component wavelengths at the receiving end.

Types of mux and demux---thin film dielectric and optical gratings. Dwdmmux are passive optical devices and donot need electrical input. High precision Prisms are used as passive mux.

Optical receivers – detect incoming light wave signals and convert them to an appropriate signal for processing by the receiving device.

OADMs – allow you to add or drop channels from a fiber that is dense wave division multiplexed, while enabling other wavelength to pass through.

Simple configuration

Transmitlasert

OTU multiplexeramplifier demultiplexerOTU

EDFA

Optical Transport Network (OTN):

OTN is an industry standard optical transport protocol proposed by ITU-T (INTERNATIONAL TELECOMMUNICATION UNION).

It is a open system which provides the standard frame structure for the DWDM.

An Optical Transport Network (OTN) is composed of a set of Optical Network Elements connected by optical fibre links.

OTN supports the operation and management aspects of optical networks of various architectures.

OTN is intended to promote network evolution beyond SONET/SDH.

OTN is divided into several network layers and interface structure but the most important are OTUk and ODUk

The Och is the wavelength port/cross connecting.

The OTUk (k=1/2/2e/3/3e2/4) is an information structure into which another information structure called ODUk (k=1/2/2e/3/3e2/4) is mapped. Also the OTU structure which includes the FEC provides supervisory functions and conditions the signal for transport between optical channel termination points where 3R functionality (retiming, reshaping, and regeneration) takes place. The ODU provides end-to-end path supervision and supports tandem connection monitoring.

  • OTU1has a line rate of approximately 2.66 Gbit/s and was designed to transport aSONETOC-48or synchronous digital hierarchy (SDH)STM-16signal.
  • OTU2has a line rate of approximately 10.70 Gbit/s and was designed to transport anOC-192,STM-64orWAN PHY (10GBASE-W).
  • OTU2ehas a line rate of approximately 11.09 Gbit/s and was designed to transport an10 gigabit Ethernet LAN PHYcoming from IP/Ethernet switches and routers at full line rate (10.3 Gbit/s). This is specified in G.Sup43.
  • OTU3has a line rate of approximately 43.01 Gbit/s and was designed to transport anOC-768orSTM-256signal or a40 Gigabit Ethernetsignal.[2]
  • OTU3e2has a line rate of approximately 44.58 Gbit/s and was designed to transport up to four OTU2e signals.
  • OTU4has a line rate of approximately 112 Gbit/s and was designed to transport a100 Gigabit Ethernetsignal

Examining the ODUk header:

TrailTraceIdentifier(TTI): The TTI is a 64-Byte signal that occupies one byte of the frame and is aligned with the OTUkmultiframe. It is transmitted four times per multiframe. The definition of what the fields’ mean is inG.709/Section 15.2.

BIP-8: This byte provides a bit interleaved parity-8 (BIP-8) code.

BackwardDefectIndication(BDI): This is defined to convey the “Signal Fail” Status detected at the Path Terminating Sink Function, tothe upstream node.

BackwardErrorIndicationandBackwardIncomingAlignment Error(BEI/BIAE):This signal is used to convey in the upstream direction the count of interleaved-bit blocks that have been detected in errorby the corresponding ODUk path monitoring sink using the BIP-8 code.This count has nine legal values, namely 0-8 errors. The remaining seven possible values represented by these four bits can only result fromsome unrelated condition and are interpreted as zero errors

Features of OTN

Compared with SDH and SONET:

  • Ultra capacity with high accuracy, terabit/second per fiber via DWDM lines.
  • Services transparency for client signals
  • Asynchronous mapping, powerful FEC function, predigest network design and reduce the cost.

Compared with traditional WDM:

  • Enhanced OAM and networking functionality for all services
  • Dynamically electrical/optical layer grooming.

OTN system:

  • Management
  • Jitter and wander
  • Network protection
  • Equipment function and features
  • Structure and mapping
  • Physic layer features
  • Architecture

FOADM: fixed optical add and drop mux.

Two types—serial and parallel OADM

Serial OADM –

  • MR2+MR2/MR4+MR4
  • Adopts the thin film filter
  • Add / drop 2 wavelength in eastward/westward direction
  • When more than two then cascade

Parallel OADM –

  • Using MUX/DMUX connected back-to-back to add/drop any wavelength
  • Optical Amplifiers are needed for insertion loss compensation

Problems of FOADM:

  • Wavelength distribution in FOADM is fixed physically. It is not flexible.
  • The wavelength services can’t be activated in time
  • Maintenance personnel need to perform the on-site commissioning for capacity expansion at each node
  • Incase of wavelength adjustment, the maintenance need to perform on-site interference.
  • The power budget will be changed node expansion and wavelength adjustment.

ROADM – reconfiguration OADM

Both the pass through and the added/drop wavelengths are reconfigurable. Basically means that any wavelength can be added/dropped to/from any ports. ROADM can solve problems with physical budget of power, noise and dispersion.

Core components of ROADM are Planar Light Circuit (PLC) and Wavelength Selective Switch (WSS)

PLC ROADM:

  • It is composed by MUX/DEMUX, 2*1 optical switches and VOA array
  • 2 AWG are integrated in the PLC ROAM used to perform MUX and DMUX function. Between the MUX and DMUX, 2*1 optical switch array and one dimension VOA array are integrated.
  • Optical switch is operated through electrical signals to determine whether the optical signals passed through/added to main path. The VOA array is used to control the power of optical signals of different wavelength.
  • After entering the PLC board, the chromatic optical signal is DMUXed to monochromatic optical signals travelling in multiple channels. At the same time, multiple monochromatic optical signals are added at the local station. By selecting between the monochromatic optical signals sent from the upstream through the 2*1 optical switch, and optical signal added at the local station we can choose whether to pass through the wavelength or add it to main path
  • The PLC ROADM integrated the O/E detection function. It can monitor and control the optical power of each wavelength channel through the O/E diode and it can also adjust the power through the VOA and supports the optical power equilibrium function.

WSS ROADM :

The switching or reconfiguration functions of a ROADM can be achieved using a variety of switching technologies including mirror- MEMS (micro-electrical-mechanical) or LCOs (LCD which is non-mechanical).

  • In the NG DWDM system, there are two types of WSS: WSD (demultiplex) and WSM(multiplex). Considering the optical channel revertive principle, the WSM and WSD are the same in the interior structure and the mechanism.

In the figure is the WSD where multiple optical ports are avaliable

WSD drops wavelength from any port. It implements colorless features on any port, also de-multiplex the input optical signal into any wavelength grouping then output to any port, thereby grooming function among the multiple optical multiplex section is implemented.

  • Chromatic signal is demuxed to monochromatic optical signal in MUX channels. The in-built VOAs adjust the power of each monochromatic optical signal and then guide each of the monochromatic signals to different optical multiplexer by controlling the 1*N (i.e N=9) optical switch array. Finally the signals are mixed and output. We can achieve transmitting of any monochromatic optical signal to any output.
  • The mirror or LCD mechanism allows some wavelengths to pass through and while drops the wavelength from other ports.

WSM has the same interior structure.

Advantages of WSS over PLC:

  • It can choose a wavelength to be switched in multiple directions.
  • The WSS can be directly used as DMUX, which can add/drop any wavelength at any port. Besides it can also groom optical signals in multiple directions.

IP over DWDM

The DWDMs are now integrated with IP routing (Ethernet)

Earlier DWDM lacked the protection and management scheme of SDH/SONET. It requires additional equipments such as amplifiers, multiplexers, demultiplexers. These equipments require continuous monitoring to ensure reliablitiy. For example if the transponder link fails the data cannot be transported.

Converged IP and DWDM networks provide significant benefits to service providers, as router-integrated transponders eliminate transponder shelves.

Hence, now OTN has found its way into IP router interfaces.

This is basically the Ethernet based technology.

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