Not All Light is Created Equal: Singlemode vs. Multimode

Not All Light is Created Equal: Singlemode vs. Multimode

Rick Dallmann | Data Center Architecture Director

They're Not on the Same Wavelength

Did you know that multimode and singlemode fiber enables transmission of signals on different wavelengths of light? The standard wavelengths for multimode are 850nm and 1300nm, and 1310 and 1550 for singlemode, respectively.  There’s also a new multimode fiber standard known as WBMMF (wideband multimode fiber) which uses the wavelengths between 850nm and 953nm.

The primary differences between singlemode and multimode fiber, however, lies in the following: fiber core diameter, the light source being used for transmission, signal transmission distance, and the bandwidth capacity.

Speaking of distance, relevant to your infrastructure planning is the transmission strength; singlemode fiber can transmit a 10-gigabit signal 33 times longer than multimode fiber! This is caused by a phenomenon known as modal dispersion, a fancy term for signal loss per distance of cable. In multimode, since the fiber core size is large, i.e. 50u, the signal “disperses” quickly whereas singlemode's fiber core size of approximately 8.3 microns is not as fast.

Bandwidth Differences Between Singlemode and Multimode

Bandwidth, which is defined as the information-carrying capacity, is very different between these two fiber types. The latest generation of multimode fiber bandwidth, OM5, is set at 28000MHz/km, while singlemode fiber bandwidth is unlimited!

Singlemode transmission is accomplished through a LASAR light source, (Light Amplification by Stimulated Emission of Radiation), (Whew, that’s a mouthful), while multimode light sources use a VCSEL (Vertical Cavity Surface-Emitting Lasers Light Source). VCSELs replaced LEDs a few decades ago. Just an FYI, these VSCEL’s cost pennies to produce.


The Major Difference Between Singlemode and Multimode

Unfortunately, these two different fiber types CANNOT be mixed and matched. As the data center design is being completed, this fact begs the question: Why would one install multimode fiber if singlemode has higher bandwidth and can carry this information over longer distances?

The answer is not as obvious or as cut-and-dry as it once was. Evaluating port for port singlemode vs multimode cabling infrastructure in the market today, singlemode will be approximately 30% less expensive than a multimode cabling infrastructure; this fact alone is consistent with the support of singlemode over fiber. So, the question still remains: Why is there a preference for multimode?

In one word? Transceivers.  In the majority of cases, multimode transceivers utilizing VSCELs are anywhere from three to ten times less expensive than LASER based singlemode transceivers (depending on the network required speed protocol). This fact drives most data centers to use multimode for cost savings.


Will Future Advancements Change Preferences?

What’s happening that could change this? The newly announced QSFP-100G-PSM4 specification, a.k.a. short-reach singlemode, defines singlemode 100 gigabit ethernet at 500 meters, uses less expensive LASERs in the transceivers and are now equal in cost to 100G multimode. If higher speeds define your data center it may be time to look at singlemode!

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