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What's the Difference in Singlemode vs. Multimode Fiber Cables?

by John Ciarlone on September 24, 2014

fiber optic cables

Which is correct for your network, Single-Mode or Multi-Mode fiber cables?

This is a question we sometimes hear from clients looking into high-end fiber upgrades, but like so many technical questions in networking, there isn't a clear-cut answer. Both kinds of fiber have different applications and different areas in which they excel. The difference is largely down to individual usage, not one being "better" than the other.

So, we wanted to talk a little about types of fiber, and when each is appropriate.

 

Short on time? Dowloand our guide- How to Avoid the Expense of Re-wiring your  Office 

ethernet cableSingle-Mode vs Multi-Mode: The Physical Differences

The differences in fiber types start with their construction.

Every fiber optic cable has the same basic structure. On the outside are two layers of cloth and/or plastic, used for insulation and protection. These are roughly the same for every cable. The difference lies inside. At the heart of the wire lies its core, made of pure glass, surrounded by another layer of reflective glass, called cladding, which keeps the light beams focused inside the core.

Single-Mode fiber uses a very narrow core, only 9 microns across, with a comparatively thick cladding. This focuses the light into a single coherent one-way beam.

Multi-Mode fiber has a larger core, typically 50 or 62.5 microns, and a thinner cladding. This allows the light waves to "bounce around" within the core as they reflect off the cladding. Certain frequencies and amplitudes create different reflection patterns, which are known as modes. Different modes have different transmission properties affecting the speed and bandwidth.  Hence, Multi-Mode.

As a comparison, by the way, a typical human hair is roughly 75-100 microns across.  We're talking about incredibly tiny spaces here.

Different Uses For Different Fiber Cable Types

Boiled down, the difference is this: Because Multi-Mode cable can utilize those reflections to pack more light into the fiber core, it can carry more data. However, those reflections also increase internal interference and reduce the overall effectiveness of the system at long ranges.

So, Single-Mode fiber is generally used only over long distances, to reliably pump a lot of bandwidth from Point A to Point B with an absolute minimum of interference or data corruption. One Single-Mode cable can transmit for many kilometers without degrading. On the other hand, Multi-Mode is best used in local deployments and LANs which want maximum possible speed and don't have to worry about long-distance signal corruption.

There are also two subtypes of Multi-Mode cable, called Step Index and Graded Index. Step Index is the older technology, and rarely used any more. It created zig-zag patterns within the fiber, which boosted speeds but introduced a lot of interference. Graded Index fiber creates smooth waveforms which are more efficient and less prone to signal degradation. There's rarely a reason to use Step Index rather than Graded Index these days.

You Can't Mix And Match Fiberfiber cables

Finally, we wanted to include a small warning. It's extremely difficult to mix and match fiber types without incurring huge signal loss when moving from one type to another. Any translation back and forth will require hardware to handle the transition and also to boost the signal.

Therefore, it's best to plan ahead and lay the one fiber type that best suits your needs. Otherwise you're going to create new problems for yourself getting the two types to link up.

Is It Time For A Move To Fiber?

If you're looking at moving into faster network backbone speeds, especially 10Gbe Ethernet switches, you're probably looking at laying some fiber. For most local deployments, Multi-Mode is the better option, but not if you're looking at connecting geographically distant offices. For that, look to Single-Mode fiber.  Make sure you purchase the correct Cisco SFP for the application if you are using a Cisco switch.

If you're uncertain how to proceed, give Hummingbird Networks a call. We have extensive experience deploying networks large and small, and can ensure you install the right cable for your needs today AND your needs tomorrow.

Contact Hummingbird Networks today for a free consultation!

cisco compatible optical transceivers

Topics: Switches