Something that one getting into fiber will run into, and that is very different than in the world of copper-based networking is APC versus UPC. Depending on the type of fiber you are using, you may see either, and putting them together incorrectly can mean damaging cables. So this is an ultra-important concept, but we are also going to give some easy guidelines to help you spot both.
APC and UPC What Does This Even Mean?
First, let us get a few terms out of the way.
- APC stands for “Angled Physical Contact”
- UPC stands for “Ultra Physical Contact”
These are the two we are going to focus on today. If you are running modern OM3, OM4, OM5 multi-mode cable or OS1/ OS2 single-mode fiber optic cable, you are likely going to be using either UPC or APC (more on that in a bit.) There is another standard called just PC for “Physical Contact” that was used in older cables in the OM1 and OM2 generations. These days, you are going to see UPC or APC on new fiber connectors that you install, but it is very important to understand the difference.
The Big UPC versus APC Difference
Here is a quick diagram in terms of the difference between the two polish types.
We have to remember here that the goal is to mate two strands of fiber and continue to transmit light (data) over that mating.
UPC as one can see has a slight curvature for better alignment. They may look flat, but they are actually slightly curved for that. The key challenge is that light can reflect off of the end of the end face and will travel back through the fiber optic cable towards the light source.
APC on the other hand is polished at an 8-degree angle. That may not seem like a lot, but that angled edge changes how light is reflected back. With the 8-degree angle, light that reflects off of the end face is directed toward the cladding around the fiber optic cable instead of more directly back to the light source as we would see in APC. This means we usually get less return loss with APC.
Given the different connector angles, one should not try to connect APC and UPC cable together. The goal should be mating a flat-ish UPC to another flat-ish UPC or angled APC to APC. As one may imagine, not only is there a potential for loss, but damage can also occur if one attempts an incorrect mating.
Caution, Potential for Damage!
If you see the above, you may have realized the challenge this can pose in the field, or simply when trying to build a fiber infrastructure. The pointed end of the APC face plate is very small. A good mental model is to think of a multi-mode fiber core as roughly the same diameter as a human hair. A single-mode fiber optic core is 9 micrometers or significantly smaller in diameter than a human hair. Still, with the core, cladding, and so forth, we generally just use the concept of thinking about a fiber strand as being about the size of a human hair. As a result, an 8-degree angle for the APC polish leaves a leading glass edge that is relatively thin.
There is basically the rule:
- UPC to UPC is good
- UPC to APC is bad
- APC to UPC is bad
- APC to APC is good
- Aligning APC to APC incorrectly is bad
Here is the conceptual diagram of these rules:
It is important to mate these ends correctly so that one does not damage cable and connectors. If there is a chance for damage, one may wonder why even have APC. That comes down to loss.
Key Loss Stats for APC, UPC, and PC
Although we are mostly focusing on APC and UPC here, we are going to include PC as well.
- UPC return loss should be -50db or greater
- APC return loss should be -60db or greater
- PC, the older standard, has a return loss of -40db or greater
Return loss is a measurement of reflected light, or in simpler terms the amount of light that does not make it through the connection and is instead reflected back into the cable. This return loss figure is expressed as a negative dB value (-XXdb.) Perhaps the easy way to remember this is the “-” is loss, so the higher number after the negative sign is a smaller loss.
A Trick for Telling APC versus UPC Polish
The easy trick for being able to tell if a connector is using APC or UPC is to look at its color. The industry realized that with potential incompatibility and potential for damage, there needed to be an easy way to tell the two polishes apart. Typically, one will see green connectors for APC and blue/ teal connectors for UPC. Here are examples of MTP connectors:
Looking here we can see the yellow OS2 single-mode MTP cable having green latches signaling that we have an APC face. We also have a pink OM4 cable (OM4 is usually either teal like OM3 or pink) with a teal latch telling us we have a UPC face.
Since we are focusing on MTP cables for this series as that is the higher-end and less understood option, we are going to discuss with that lens. Generally, one will also see OS1 and OS2 single-mode fiber using APC, and multi-mode OM3, OM4, OM5 fiber using UPC, but it is always best to double-check cable specs and the connector as well. With lower-density simplex/ duplex cables, there is more variation.
If you are installing fiber, we are generally suggesting folks skip OM1, OM2, and OM3 for multi-mode and just install OM4 (OM5 that is lime green is still pricey at the time of this writing.) Installing OM4, we also suggest UPC since that ecosystem is largely UPC. If you are installing single-mode fiber then you will often see APC on the LC and SC connectors but there are certainly UPC there as well. For single-mode MTP, you are likely to see APC being used. Especially in long runs and with service providers bringing service to a building it is common to see bundles terminated to MTP using APC and then connected into patch panels. Lower loss makes less of a difference on a 10m cable, but on a 10km run, it can have a huge impact.
The key here is to ensure you are mating UPC to UPC, APC to APC, and when mating APC, ensure you have the proper orientation. We have seen some folks online get “creative” trying to mate female-to-female MTP cables and only to find they damaged cables trying to incorrectly mate two APC ends.
We hope this at least helps understand what it means when you see APC and UPC listed for fiber optic cables, and what to look for.