Fiber data connections: LC vs SC (with pictures!)


Fiber patch panelIf you're working in data centers, you're going to need to know something about fiber optic connections. Fiber is the most common type of connection used for that last mile between backbone providers and your equipment. Just like any other cable, fiber cables have to connect to your equipment and this article is about two commonly used fiber connectiors: the SC and LC connectors.

A quick primer

First, some background. My exposure to data centers is generally a single cabinet in general population. My DC provider gives me rack, power, and a patch panel. The patch panel is the central part of this article.

The patch panel is generally mounted at the top of the rack because most data centers run their cables over head. The part of the patch panel that I use is referred to as the A-side and the other side is referred to as the Z-side.

When I deploy into a new DC, my backbone provider gives me a Letter of Authorization (LOA). I take that LOA and order a cross connect from the DC using that letter. The DC techs need that LOA because it is their authority to connect my backbone provider to my patch panel. The DC techs will run cable from the ports my backbone provider has specified on their patch panel in the bowels of the DC (the Z side) to the ports I dictate on the patch panel in my rack (the A side). I then show up and start plugging things into my patch panel (the A side).

Connectors matter (kinda)

The LOA will specify whether the DC techs should use LC or SC connectors on the Z side and I can usually instruct the DC what type of connectors I want on my end. It is important for me to know what type of connectors are being installed on my patch panel because I need to bring a compatible set of pig tails with me. More on that later.

There are really only two considerations when choosing between LC and SC connectors, in my opinion.

  1. SC connectors are bigger and will take up an entire port on my patch panel.
  2. LC connectors are exactly half the size of an SC connector and I can therefore fit twice as many connections on my patch panel.

If you are operating in a high density patch panel configuration, you may want to chose LC solely for the reason that you can plug more stuff in.

A third consideration which doesn't have a lot of bearing for me is the mating cycles of each type of connector. A mating cycle is the entirely too apropos name to describe removing and re-inserting the connector. Fibre is glass and the connectors are porcelain. Both of those materials aren't terribly resistant to abuse and each push/pull wears the porcelain ferules down just a little bit more until they finally will not be able to transmit the light well. An LC connector is rated for 500 mating cycles in its lifetime whereas an SC connector is rated for 1,000 mating cycles.

Initially LC connectors were much more expensive due to licensing fees. Now the costs are not a consideration unless you're buying millions of them.

In my situation, I install a data center once and may never go back to it, so I'll never come anywhere near 500 mating cycles. Consequently, that is not a concern for me. If you're in a situation where you're constantly shuffling fiber cables around, then you may care more.

Practical information

Here are pictures of each type of connector plugged into a patch panel.

LC connectors

The image above shows LC connectors and you can see that each port on the patch panel can take two LC connectors.

SC connectors

This image (above) shows SC connectors and illustrates how each SC connector requires its own port.

Finally, on to the pig tails pictured below. The most common use for fiber connections is to run the fiber channel from the patch panel into a switch. Many switches use MPO (Multi fiber Push On) connectors. There is also a newer type of connector called MTP which is the same form factor, but more advanced.

Pigtail with MPO

The loose blue ends are LC connectors and the green end is the MPO connector. the MPO goes into the adapter pictured on the right, and then into an optical port on my switch. The LC ends go into the patch panel which is why it is important for me to know what type of connectors the DC techs installed into my patch panel.

And that's the 25-cent tour of fiber connections in a data center.

#datacentre #networking #fiber

First image courtesy of Fiber Optical Networking