Wave Division Multiplexing: Why it’s Good for Fiber

A single optical fiber can carry a huge amount of bandwidth using Time-Division Multiplexing (TDM) and Coarse Wavelength Division Multiplexing (CWDM), which may be combined.

 

TDM was developed for digital telephony to send independent signals over a single fiber using synchronized switches at each end so that each signal appears on the line in short bursts, creating an alternating pattern. For audio/video, it is more efficient to convert any analog signals into digital and then combine them into one data stream using TDM.

 

CWDM was developed for the broadcast industry to combine signals from different bands onto a single fiber, using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm (technically, it was shifted by 1 nm to 1271 to 1611 nm). Channel spacing ensures that minor signal drifting will not contribute to crosstalk or otherwise affect different wavelengths negatively, as well as, permits the usage of less sophisticated transceiver designs, thus contributing to a reduction in cost.

 

To understand how CWDM works, we need to first understand how fiber signal transport makes use different wavelengths (colors) of laser light in the infrared zone, which is 700 nm to 1 mm (1,000,000 nm), to carry different signals.

 

The earliest fiber systems operated in the first band, 850 nm, which are shorter wavelengths best suited for multimode fiber. The bands or “optical windows” are regions within the optical fiber spectrum with low optical loss (“attenuation”). The second band is 1310 nm, which has a longer wavelength and is used by both multimode and singlemode fiber with zero dispersion, and the third band is 1550 nm, which is an even longer wavelength and is used exclusively by singlemode fiber. Optical loss or attenuation can vary depending on whether the fibers are plastic or glass, and which wavelengths are being used.

 

A CWDM system uses a multiplexer at the source to combine or “muxes” the signals, and a demultiplexer at the destination “demuxes” them to split them apart again. Some units can both multiplex and demultiplex simultaneously, which is called an “add-drop multiplexer,” combining the functionality into one.

 

Benefits of CWDM

The main advantage of CWDM is that it allows companies to expand their network capacity without laying more fiber. In a CWDM configuration, the capacity of a fiber link can be expanded simply by adding or upgrading the multiplexers and demultiplexers at both ends. With CWDM it is possible to carry the combined video/audio/data information from an entire equipment rack on just one fiber.

 

When this technology was originally developed in the 70s and 80s it was somewhat cost-prohibitive, but over time CWDM multiplexing has undergone considerable refinement even as costs have come down, so more companies can afford to use it. CWDM multiplexing is particularly popular in countries with limited infrastructures, where it is highly desirable to maximize usage of all installed fiber optic cabling.

 

One of the most significant benefits of CWDM is that you can use off-the-shelf Small Form-Factor Pluggables (SFPs). SFPs are optical transceivers for specific wavelengths and they are hot-swappable: if one should fail, you can easily substitute another one, and it will work as long as the data rate matches the same standard as the one being replaced.

 

Multimode vs. Singlemode Fiber

Multimode fiber is utilized between points that are a short distance apart, such as within the same building. The most common wavelengths used for multimode fiber are 850 nm and 1310 nm, with each wavelength going in different directions in the fiber, and also is ideally supported by CWDM multiplexing.

 

Telephony network designers were the first to take advantage of multimode fiber but by the early 1980s singlemode fiber, which can be run for much longer distances, began operating in the 1310 nm wavelength and later in the 1550 nm wavelength, so it became the more widely accepted standard.

 

Singlemode fiber continued to improve and now has a usable spectrum from about 1270 nm to 1610 nm. Since fiber can handle up to 8 channels of video per wavelength, and can have up to 18 CWDM wavelengths on one fiber, this means that more than 144 channels of video can be transported over one fiber! This makes fiber the unparalleled solution for high-bandwidth video transport. Other advantages of fiber include its light-weight cables as compared to copper, its immunity to lightning, EMI/RFI and crosstalk, and its increased security as it can’t be “tapped” like copper. Singlemode fiber is also less fragile than multimode fiber, allowing Installers to more easily handle it.

 

Optiva Fiber System

fiber-mart’s Optiva fiber system was designed to take advantage of TDM and CWDM technologies, to maximize the use of fiber lines and the signals handled per insert card. Many signals can be daisy-chained together, allowing additional signals to be added without adding additional fiber, or can be multiplexed onto a single fiber.

 

While most Optiva insert cards allow for CWDM multiplexing, it really depends how much bandwidth the signal being sent requires and what SFPs are being used in the transmitter/receiver cards, in order to determine the maximum distance capability, whether multimode or singlemode fiber is usable, and whether a single fiber (“simplex”) or 2 fibers (“duplex”) is needed to complete the system. As SFPs have evolved to handle increased bandwidth, the most commonly used ones are 2.97 Gbps (aka 3 Gbps), 4.25 Gbps, and 10 Gbps, the latter of which are called SFP+.

 

Optiva additionally has separate CWDM passive optical multiplexer/demultiplexer insert cards for 4-channels (MDM-7004), 8-channels (MDM-7008), or 16-channels (MDM-7016), designed to send or receive up to 4, 8, or 16 individual signals respectively, with bandwidths up to 3.125 Gbps per wavelength.