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Increasing efficiency in existing fiber networks

January 6, 2020  

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Digital communications are an important part of the day-to-day lives of many. As they evolve, the infrastructure that supports them has remained stagnant, resulting in a need for more efficient networks.

In a recent whitepaper from Cailabs, Is your network cabling infrastructure limiting your productivity? Everything you need to know before migrating to 10 Gb/s, the French deep-tech company explains that the demand for bandwidth in local area networks (LANs) is growing. More and more, people are transferring large data, audio and video files, while traffic in general has grown due to the increased daily use of real-time apps such as VoIP, video conferencing and collaborative online applications, as well as other assorted programs that use a great deal of data. The number of devices online has also grown significantly. More devices attempting to transfer data means there is more demand on the bandwidth.

These LANs require a higher transfer speed, but according to the whitepaper, the large majority of deployed multimode optical fibers (MMF) do not support speeds as high as 10 Gb/s or even 1 Gb/s over long distances, as these fibers were designed for lower transfer speeds (or “bit rates” to avoid repetition of “transfer speed”).

Optical fibers are widely used to carry data due to their small size, low linear losses and insensitivity to electromagnetic disturbances, to name a few reasons, however these cables have some limitations that are not easily overcome due to their physical properties.

To get more specific, for all MMFs, the fiber’s ability to support high speed transmission is limited due to modal dispersion. This is a distortion mechanism that occurs in MMFs. As modes travel at different speeds, the signal is spread out and can often become deformed in time during the transmission through the fiber. This happens because as light from the transceiver enters the MMF, it “splits” into different rays, or “modes,” which enter at different angles and travel down different paths: some take a direct path, while others reflect themselves. This results in the time it takes to arrive at the end of the fiber to differ. This is known as differential mode delay (DMD).

Principle of multimode propogation. Image: Cailabs

Effects of modal dispersion during propagation in a multimode fiber. Image: Cailabs

 As a result of DMD, inter-symbol interference can result in the inability to retrieve data that has been weakly transmitted. As the transmitted throughput gets higher, so does the distortion. Therefore, DMD degrades the width of the bandwidth capacity of the multimode optical fiber. For a given throughput, it reduces the distance that can be reliably travelled between the transmitter and the receiver.

Multimode isn’t the only fiber available: single-mode fibers (SMFs) are technologically advanced and allow only one mode of transmission. As such, they are not bandwidth limited due to the modal dispersion and used for high broadband transmissions over long distances. MMFs were first on the market and mainly deployed over campus for LAN.  To solve the problem of throughput and distance limitations in currently operational MMFs, the option exists to replace them with new generation MMFs or SMFs, as both options do not experience modal dispersion. However, this can turn out to be a difficult task. It depends on the distance involved and difficulties in deploying the fibers.

Cailabs says an audit of the cabling infrastructure must be performed to verify the availability and condition of the cable ducts. Depending on the case, civil engineering may be required to deploy new sheaths for new optical cables. As a result, the costs of installing a new cable can be prohibitive, in that it would be an expensive undertaking and require a long and complex installation process, which can also negatively impact the business.

However, another Cailabs whitepaper, Enhancing bandwidth in legacy multimode fiber links, addresses two potential solutions that may allow a user to conserve their existing cabling infrastructure while also meeting the growing demand for increased bandwidths in local area networks. These solutions allow the increase of bandwidth in MMFs by changing the light launching condition. They are: mode conditioning patch cables that slightly reduced the impact of modal dispersion combined with electronic dispersion compensation signal processing module within long reach multimode (LRM) transceivers, and Cailabs’ recently developed beam-shaping passive technology known as multi-plane light conversion technology (MPLC), which enables single-mode transmission capacity over MMFs by overcoming the modal dispersion effect.

Theoretical MMF bandwidth gain according to launching conditions. Image: Cailabs

According to Cailabs, both techniques allow customers to upgrade their existing LAN bandwidth capacity without having to undergo a complete overhaul of their fiber infrastructure. In the end, it’s up to each user to decide which technology best suits their needs, now and in the future.

To find out more about how each technique works, download both whitepapers from Cailabs.