The evolution of DWDM systems stemmed from a need to increase the capacity of a single fiber, and thus the entire network, in an inexpensive way. While the idea behind using a single fiber to carry multiple channels seems simple, in reality it is a complex endeavour. By converting incoming optical signals into the precise ITU-standard wavelengths to be multiplexed, transponders are currently a key determinant of the openness of DWDM systems. This article is mainly to introduce the transponder based DWDM system.
Generally, a transponder based DWDM system includes terminal multiplexer, terminal demultiplexer, intermediate line repeater or Optical Add-Drop Multiplexer (OADM), and optical amplifier or optical supervisory channel.
Terminal multiplexer consists of transponders and optical multiplexers (DWDM MUX). For each signal in the fiber, there is a corresponding transponder. The transponder takes the signal and transmits it in the C-Band through the use of a laser. The optical multiplexer transmits these signals in the C-Band through one fiber. In the course of ten years, DWDM system capacity grew from 4 signals to 128 signals. (Here is a picture of a transponder used in DWDM systems.)
DWDM systems also employ terminal demultiplexers which consist of transponders and optical demultiplexers. In their first incarnations, terminal demultiplexers were passive systems. As the complexity of DWDM systems increased, the need for an active approach did, too. Terminal demultiplexers take the signal, which is composed of several wavelengths by this point, and breaks it down to its constituent signals. These signals are then sent through individual fibers to their destinations. The active terminal demultiplexers first go through an output transponder before they are transmitted, which can also go through an error correction procedure. These transponders can also be placed a longside the input transponders.
Note: For a bi-directional transponder based DWDM system, the terminals contain both multiplexers and demultiplexers.
Intermediate line repeaters are placed between 80 and 100 km apart along the path of the fiber. If the optical signal has travelled more than 140 km before arriving at its destination, an OADM integrated optical amplifiers (aka an intermediate optical terminal) is placed. It serves to not only amplify the signal, but also as a diagnostic point. If locations further down the path of the fiber are having issues with the signal, these sites can be used to determine if the fiber has been damaged or otherwise impaired.
To counteract the losses in curred to the signal, optical amplifiers are needed. For example, an Erbium-Doped Fiber Amplifier (EDFA) is used to amplify the optical signal in the intermediate line repeater. An EDFA can also be placed in the terminal multiplexer as a pre-amplifier to amplify the signal before it is transmitted. (An EDFA is shown in the figure below.)
When an EDFA cannot be used, an optical supervisory channel is. This occurs when the signal occurs outside of the C-Band. Here is a table that shows the wavelength ranges for each optical wavelength band.
|Optical Bands||Wavelength Range (nm)|
|O-Band||1260 to 1360|
|E-Band||1360 to 1460|
|S-Band||1460 to 1530|
|C-Band||1530 to 1565|
|L-Band||1565 to 1625|
|U-Band||1625 to 1675|
Signal regeneration was not initially implemented in transponders. At first, these transponders were only used to convert the wavelengths of incoming external signals into wavelengths that worked with the DWDM systems: namely, those in the C-Band. This conversion also serves to stabilize the frequencies and amplify the power of these signals into something compatible with the EDFA in the DWDM system. The sophistication of the signal regeneration components in transponders grew as they progressed from 1R to 3R:
- 1R is shorthand for Retransmission. It was used in the earliest transponders. As the name implies, 1R does not employ any methods to “clean up” the signal. Rather, it simply takes the incoming external optical signal and converts it to analog. This process happened regardless of signal integrity. As a result, if the incoming optical signal was “junk”, the analog version of it would be “junk” as well. Another consequence of 1R was that, because of the signal degradation inherent to long-range communications that would occur, the practical distance of DWDM systems was limited.Note: DWDM systems operate in the C-Band with an attenuation value of about 0.3 dB/km. Even though this value is much lower than the attenuation values of other methods of communication, the degradation over long distances adds up. This necessitated the development of systems that did more than just retransmit.
- 2R is shorthand for Retime and Retransmit. Before the incoming external signal is retransmitted, it first goes through a process to clean it up. The quality of the signals was monitored at this point.
- 3R is shorthand for Retime, Retransmit, and Reshape. This system is more advanced than both 1R and 2R systems. Signal quality can be monitored more closely and accurately, thanks to the inclusion of quality bits embedded in the signal. The quality bits inform the system of the level of health and degradation of the signal. 3R systems are capable of the monitoring of bi-directional communication.
Within the DWDM system a transponder converts the client optical signal from back to an electrical signal and performs the 3R functions (the figure below shows the 3R functions of transponders in the terminal). This electrical signal is then used to drive the WDM laser. Each transponder within the system converts its client’s signal to a slightly different wavelength. The wavelengths from all of the transponders in the system are then optically multiplexed. In the receive direction of the DWDM system, the reverse process takes place. Individual wavelengths are filtered from the multiplexed fiber and fed to individual transponders, which convert the signal to electrical and drive a standard interface to the client. (Future designs include passive interfaces, which accept the ITU-compliant light directly from an attached switch or router with an optical interface.)
The figure above shows the end-to-end operation of a unidirectional DWDM system using transponder. And the following steps describe the operation shown in the figure:
- The transponder of Terminal A accepts input in the form of standard single-mode or multimode laser. The input can come from different physical media and different protocols and traffic types.
- The wavelength of each input signal is mapped to a DWDM wavelength.
- DWDM wavelengths from the transponder are multiplexed into a single optical signal and launched into the fiber. The system might also include th e ability to accept direct optical signals to the multiplexer; such signals could come, for example, from a satellite node.
- A post-amplifier (booster amplifier) boosts the strength of the optical signal as it leaves the system (optional).
- Optical amplifiers (in-line amplifiers) are used along the fiber span as needed (optional).
- A pre-amplifier boosts the signal before it enters the end system (optional).
- The incoming signal is demultiplexed into individual DWDM lambdas (or wavelengths).
- The individual DWDM lambdas are mapped to the required output type and sent out through the transponder of Terminal B.
Transponders in DWDM systems facilitate a wide variety of applications, some of which include broadcasters and cable operations, data networks, and satellite and wireless communications. Transponder based DWDM systems can be implemented as a replacement for any existing WDM systems if the advantage of doing so justifies the cost. If a company has already invested in laying down fiber, that initial investment can be protected by using such a DWDM system. Using this system multiplies the capacity of the existing fiber by up to 10 or more times. This type of system is necessary for internet providers because of the rapid expansion of internet subscribers. If DWDM systems did not exist, the only way for these companies to meet the demand of internet users would be to lay new fiber. It is much more cost-effective for them to implement DWDM systems and thus alleviate the bandwidth concern. DWDMs also allow for more flexibility in the design of networks.
DWDM systems are continually being improved. Research is advancing the technology to the point where 800 wavelengths on a single fiber could be feasible. The amount of data that modern applications require continues to grow. Where bit rates of a few Gbps were once sufficient, modern consumer and corporate needs necessitate Tbps. This type of growth could not have been anticipated when the first WDM systems were introduced, but the tranponder based DWDM systems are capable of meeting modern demands.
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