Continued… DWDM Part III

In my first posting on DWDM (Part I), I briefly discussed how DWDM works and perhaps today I’ll discuss further the way it functions from various angles and examples for clarity purposes.

DWDM System Functions

At its core, DWDM involves a small number of physical-layer functions. These are depicted in figure below, which is similar to figure given in Part I. Figure below shows a DWDM schematic for four channels. Each optical channel occupies its own wavelength (wavelength is expressed (usually in nanometers) as an absolute point on the electromagnetic spectrum. The effective light at a given wavelength is confined narrowly around its central wavelength).

DWDM Functional Schematic

DWDM Functional Schematic

From the above figure (view it from left to right), the DWDM system performs the following main functions:

  • Generating the signal—The source, a solid-state laser, must provide stable light within a specific, narrow bandwidth that carries the digital data, modulated as an analog signal.
  • Combining the signals—Modern DWDM systems employ multiplexers to combine the signals. There is some inherent loss associated with multiplexing and demultiplexing. This loss is dependent upon the number of channels but can be mitigated with optical amplifiers, which boost all the wavelengths at once without electrical conversion.
  • Transmitting the signals—The effects of crosstalk and optical signal degradation or loss must be reckoned with in fiber optic transmission. These effects can be minimized by controlling variables such as channel spacings, wavelength tolerance, and laser power levels. Over a transmission link, the signal may need to be optically amplified.
  • Separating the received signals—At the receiving end, the multiplexed signals must be separated out. Although this task would appear to be simply the opposite of combining the signals, it is actually more technically difficult.
  • Receiving the signals—The demultiplexed signal is received by a photodetector.

In addition to these functions, a DWDM system must also be equipped with client-side interfaces to receive the input signal. This function is performed by transponders. On the DWDM side are interfaces to the optical fiber that links DWDM systems.

As mentioned earlier above and in Part I, optical networks use Dense Wavelength Multiplexing as the underlying carrier. The most important components of any DWDM system are transmitters, receivers, Erbium-doped fiber Amplifiers (EDFA), DWDM multiplexors (aka Mux) and DWDM demultiplexors (aka Demux). The block diagram below gives the structure of a typical DWDM system with these components (view it from right to left).

Block Diagram of a DWDM System

Block Diagram of a DWDM System

Optical Transmission Principles

Optical fiber transmission plays a major role in deciding the throughput of the DWDM network. The DWDM system has an important photonic layer, which is responsible for transmission of the optical data through the network. The following basic principles are necessary for the proper operation of the system.

Channel Spacing
The minimum frequency separation between two different signals multiplexed is known as the Channel spacing. Since the wavelength of operation is inversely proportional to the frequency, a corresponding difference is introduced in the wavelength of each signal. The factors controlling channel spacing are the optical amplifier’s bandwidth and the capability of the receiver in identifying two close wavelengths sets the lower bound on the channel spacing. Both factors ultimately restrict the number of unique wavelengths passing through the amplifier.

Signal Direction
An optical fiber helps transmit signal in both directions. Based on this feature, a DWDM system can be implemented in two
ways:

  • Unidirectional: All wavelengths travel in the same direction within the fiber. It is similar to a simplex case. This calls in for laying one another parallel fiber for supporting transmission on the other side.
  • Bi-directional: The channels in the DWDM fiber are split into two separate bands, one for each direction. This removes the need for the second fiber, but, in turn reduces the capacity or transmission bandwidth.

Signal Trace
Signal Trace is the procedure of detecting if a signal reaches the correct destination at the other end. This helps follow the light signal through the whole network. It can be achieved by plugging in extra information on a wavelength, using an electrical receiver to extract if from the network and inspecting for errors. The receiver then reports the signal trace to the transmitter.

Taking into consideration the above two factors, the international bodies have established a spacing of 100GHz to be the worldwide standard for DWDM. This means that the frequency of each signal is less than the rest by atleast 0.1THz.

Sources:

Introduction to DWDM for Metropolitan Networks

Optical Networking And Dense Wavelength Division Multiplexing (DWDM) by Muralikrishna Gandluru

To be continued… DWDM Part IV

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