By Rachel Tang, Fiber-Optic Communication Researcher, Cozlink

Rachel Tang, Fiber-Optic Communication Researcher, CozlinkAs an option for high-speed data centers, wideband multimode fiber (WBMMF) is far superior to traditional OM4 multimode fiber in terms of effective mode bandwidth (EMB). This means wideband is more capable of providing a margin space for higher-speed Ethernet. To further understand wideband multimode fiber, data center operators need to look at the development process of single-mode fiber and multimode fiber.

Single mode vs. Multimode Fiber

Single-mode fiber is mainly used in multi-frequency data transmission applications and its transmission mode is primarily wavelength division multiplexing (WDM). Single-mode fiber uses a multiplexed optical signal that can transmit data by only a single-mode fiber. In 2010, 100GBase-LR4 adopted dual core, single-mode fiber (one for receiving and one for transmission), which can reuse four wavelengths at one-core fiber at a time (with each wavelength transmission at 25Gbps).

Wideband Multimode Fiber - The Choice of Future Data Centers imageThe transmission rate of single-mode fiber is higher than that of multimode fiber, and offers up to 50 times more distance. The price is consequently higher. Compared with multimode fiber, single-mode fiber has a much smaller core diameter.  The small size and single-mode transmission characteristics make the optical signal transmitted in single-mode free of distortion due to the light pulse overlap. In all fiber types, single-mode fiber has the lowest signal attenuation rate and maximum transmission rate.

However, single-mode fiber requires a high-cost laser (LD) light source transceiver – at least three times more expensive than that of the multimode fiber optic transceivers – and the power consumption is at least double that of multimode fiber optic transceivers.

Another method to improve the network transmission rate for multimode fiber is to use parallel transmission mode, which increases the number of optical fiber deployed, to improve the transmission rate. 100GBase-SR10, which in 2010 adopted 10Gbps/channel transmission, requires 10-channel reception/transmission, thereby incorporating 20 core fibers.

The most common determinant between single-mode and multimode has been distance. Multi-mode is the preferred for distances under five miles because the LED transmitter/receiver is less expensive than the single-mode laser. Single mode fiber is used for distances over five miles. Bandwidth is another consideration. If future applications require transmission of large bandwidth data signals, then single mode will be the best choice.

Future data center option: Wideband multimode fiber (WBMMF)

With the introduction of 100G-NG, along with 200G/400G Ethernet and 1T Ethernet, the fiber count and distance of traditional multimode fiber will become a bottleneck for the development of faster Ethernet networks. However, the emergence of wideband multi-mode fiber (WBMMF) challenges such technical bottlenecks. WBMMF uses wavelength division multiplexing (WDM) technology of single-mode fiber, extends the available wavelength range of network transmission, can support four wavelengths on a core multimode fiber, and thereby reduces the required fiber core requirements. Further, WBMMF improves effective modal bandwidth (EMB), by extending transmission distance of 40/100G by approximately 300 meters.

Conclusion

Today in 96 percent of the world’s data centers, the distance between the network core area spine switch and the server cabinet branch switch is no more than 300 meters.  Consequently, shortwave wavelength division multiplexing technology (SWDM) and wideband multimode fiber (WBMMF) will continue to leverage multimode fiber as mainstream transmission media in data centers utilizing 40/100/400G Ethernet. In the future, by combining short wavelength division multiplexing (SWDM) with parallel transmission technology, 8-core wideband multimode fiber (WBMMF) will be able to support higher speed applications, including 200/400G Ethernet.

About the Author:
Rachel Tang is a researcher with over five years in the fiber-optic communication industry. Since graduating from the Electronics and Communication Engineering Institute of University of Technology in 2011, Tang joined Cozlink, where she has focused on optical communication technology and leads a team focused on data center ecosystems.

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