What fiber connector should I use?

Over the past couple of weeks we talked about the tools needed to troubleshoot fiber connections on the LAN along with methods of troubleshooting those connections. This week I want to talk about the fiber connectors.
At the end of every fiber there is a connector. This connector is very important. The connector needs to match your optical module or your patch panel. The wrong connector will not work well, or may not work at all. Here is a list of the most common connectors that I have experienced in my carrier.

ST – These are very common on older equipment and patch panels, they are the ones you push in and twist into place.
SC – These are common today. Many optical modules still use these connectors.
LC – These connectors seem to be today’s standard for use on optical modules and patch panels. They are the smallest and therefore more can fit on a line card.

The end of the connector where the light is omitted is polished. This polish has different levels of back reflection (This is where the light would bounce off the end of the fiber and go back to the source. Measurement of light off of the connector).

In General, you only need to be concerned with the polish on Single-Mode connectors. The connectors that I have used have always been SC. The three polishes are Physical Contact (PC), Ultra Physical Contact (UPC) and Angled Physical Contact (APC). The proper term for the connector would be an SC/UPC or SC/APC. UPC connectors are usually blue and usually have blue barrels in the patch panel. APC connectors are usually green and usually have green barrels in the patch panel. If you look at the tip of the APC connector you can see that it is angled instead of flat on the UPC. The color of the patch panel does not matter; it simply helps you identify the connector type.

I have seen both UPC and APC be used in large fiber plants. Some locations would use SC/APC, others would use SC/UPC. As long as the connectors match up, it usually doesn’t matter what is used. If there is ever a choice, you should use the APC connector because it produces less back reflection. Due to a lack of correct fiber jumpers, I have made a good connection with an SC/APC and an SC/UPC before. The connection degraded the signal, but the link came up. Unless you are in a bind, I don’t recommend doing this.

I have only had one instance that truly required the APC connector. I was using a single strand Single-mode optic. The transmitter and receiver were on the same fiber and when the laser hit the end of the UPC connector it would bounce back. The receiver would see it’s own light and it would tell the switch that the link was up. As you probably imagine, this was a problem. Once identified, APC connectors were used and the reduction in back reflection resolved this issue.

When you connect a fiber into a piece of network equipment you always use a PC or UPC connector. I have never seen a requirement for an APC connector by a hardware manufacturer.

Instead of pictures for you, I found a nice video from L-Com that does a good job of showing the connectors. Please click here to see the video.

What experiences have you had with APC or UPC?
Why did you use one over the other?

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About Scape

Over 10 Years in the networking field. Have worked in the Service provider and Enterprise environments. I have worked with Cisco, Foundry/Brocade, F5, Riverbed, Scientific Atlanta, Routers, Switches, Firewalls, Load Balancers, WAN Accelerators, DWDM, SONET, Multicast etc...

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