Introduction of Passive DWDM and Active DWDM

DWDW is short for dense wavelength division multiplexing. It is an optical multiplexing technology used to increase bandwidth over existing fiber networks. DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. It has revolutionized the transmission of information over long distances. DWDM can be divided into passive DWDM and active DWDM. This article will detail these two DWDM systems.

 

Passive DWDM

Passive DWDM systems have no active components. The line functions only due to the optical budget of transceivers used. No optical signal amplifiers and dispersion compensators are used. Passive DWDM systems have a high channel capacity and potential for expansion, but the transmission distance is limited to the optical budget of transceivers used. The main application of passive DWDM system is metro networks and high speed communication lines with a high channel capacity.

 

Active DWDM

Active DWDM systems commonly refer to as a transponder-based system. They offer a way to transport large amounts of data between sites in a data center interconnect setting. The transponder takes the outputs of the SAN or IP switch format, usually in a short wave 850nm or long wave 1310nm format, and converts them through an optical-electrical-optical (OEO) DWDM conversion. When creating long-haul DWDM networks, several EDFA amplifiers are installed sequentially in the line. The number of amplifiers in one section is limited and depends on the optical cable type, channel count, data transmission rate of each channel, and permissible OSNR value.

active DWDM

The possible length of lines when using active DWDM system is determined not only with installed optical amplifiers and the OSNR value, but also with the influence of chromatic dispersion—the distortion of transmitted signal impulses, on transmitted signals. At the design stage of the DWDM network project, permissible values of chromatic dispersion for the transceivers are taken into account, and, if necessary, chromatic dispersion compensation modules (DCM) are included in the line. DCM introduces additional attenuation into the line, which leads to a reduction of the amplified section length.

 

Passive DWDM vs. Active DWDM

Both passive DWDM and active DWDM have their own pros and cons.

 

Pros and Cons of Passive DWDM

 

Cost savings: unlike active backbone networks with amplifiers and dispersion compensators, the passive DWDM allows to arrange a high speed system having high channel capacity with substantial cost savings.

Less complex: passive DWDM isn’t complex at all. It’s really plug and play, and there is nothing to provision.

 

Even though passive DWDM has the two main benefits, it still has the drawback.

 

Scalability: you are limited to colored optics, and less wavelengths on the transport fiber. As you grow, you would be required to have more passive devices. Furthermore, with the more passive devices, you have more difficulty to manage. And you will have to start managing the same wavelength on multiple passive devices and they could be serving different purposes on each depending on your setup.

 

Control: if you need to change a wavelength or connection for whatever reason, your option is limited to taking it out of service and disconnecting the physical cabling as the wavelength is tied to the optic.

 

Pros and Cons of Active DWDM

Active DWDM can fit more wavelengths onto a single fiber pair. The composite signal that is sent over a single fiber pair can carry more bandwidth than a passive of the same size could. In turn, you don’t need as much physical fiber between your two sites. This is advantageous when distance is a problem because it allows you to get more out of a single fiber pair as opposed to passive.

 

Active setups grant you more control over your optical network. You can dynamically re-tune wavelengths without dropping connections. It’s transparent to whatever is riding on that wavelength. Moreover, active DWDM can be easier to scale as your network grows. You can fit more wavelengths on the fiber.

 

Expensive: active DWDM setups are extremely expensive compared to passive DWDM. If you don’t need that long distance requirements, not choose active DWDM.

 

Configuration: depending on your vendor, configuration can be a serious undertaking, and require a solid understanding of optical networks. There are many more components in active builds. DWDM necessarily require transponders, further, after muxing the signals, they typically need active amplification to have any interesting reach. Without this, you’re only going a relatively short distance.