Until the wrong optical fiber connectors arrives on site and a patch panel that should have taken half an hour turns into a parts run, every optical connector on a…
Until the wrong optical fiber connectors arrives on site and a patch panel that should have taken half an hour turns into a parts run, every optical connector on a network seems to look like a small, interchangeable piece of hardware. Once an OEM is designing a fiber management system, or an NOC is specify-ing materials for a large-scale deployment, there is no point in worrying about optical fiber connectors capacity.
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To meet the performance requirements of the system or the scale of the future fiber rollout if they will not be physically compatible with existing infrastructure, will not provide the field performance (insertion loss) required by customers, or will require significantly more downstream field labour than alternatives. This guide explains the 4 connector types that are most commonly used in the telecom application industry (FC, SC, LC and the APC polish type) and what makes them different beyond the name on the housing.
FC Connector — Screw-Lock Stability
Ferrule Connector (FC) is a metal sleeve used for external reinforcement and a screw type locking mechanism instead of a push-pull mechanism. The defining feature of this connector is that the screw lock can withstand vibration and accidental unlocking much better than a push-pull design, so FC became the connector to use for test equipment, lab instruments and older telecom central office installations where optical fiber connectors could be left undisturbed — or accidentally bumped during rack work — for years.
The down side of the trade-off is time; screwing and torquing a screw-lock connector is more time consuming than seating a push-pull connector, which is important for the technician that needs to terminate dozens or hundreds of connections in one fiber management system. Although FC is still widely used in legacy exchange equipment and high precision test equipment, it is being continually phased out of new-build telecom equipment by SC and LC.
SC Connector — The Push-Pull Standard
The SC connector (Subscriber Connector, sometimes pronounced “Square Connector”, for its housing shape) features a 2.5mm ferrule, and requires no twist to seat or release. This push-pull action allowed SC to be even faster than a screw-lock connector to be disconnected and reconnected, which is why it was chosen as the standard connector for data communication networks, telecom infrastructure and gigabit Ethernet deployments into the 2000s and beyond.
The 2.5mm ferrule size is the physical benchmark against which other connector sizes are measured and the low insertion loss and high return loss make them a reliable connector type for demanding optical networking applications.
LC Connector — Built for Density
The LC connector features a smaller 1.25mm ferrule (half the size of SC) and a small housing with an RJ-style latching clip, which will be instantly recognizable to all those working with copper networking devices. That smaller footprint is the entire reason for the LC design, which enables about twice as much port density as SC in the same rack space, which is what modern high-density patch panels, data centers and space-constrained fiber management systems require.
As an example of why connector density is important at the enclosure level, CWIPL’s 1U FMS platforms have up to 24 fiber cores in a single rack unit, representing the physical size of the connector is the thing that ultimately drives the number of fiber cores that can fit within a rack unit.
APC — A Polish Type, Not a Connector Shape
APC is often thought of as a fourth connector shape that is used with SC and LC, but it is really an APC ferrule polish that may be used on SC or LC connector bodies (most commonly used as SC/APC, LC/APC). Standard PC (Physical Contact) or UPC (Ultra Physical Contact) ferrelled optical fiber connectors feature a flat or slightly convex end face, while APC — Angled Physical Contact — end faces the ferrule end-face at an 8-degree angle.
This angle helps to redirect the light out of the fiber core instead of back down the fiber line, resulting in substantially lower return loss and back reflection than with PC or UPC. This means that in GPON, passive optical LAN, FTTH and other passive optical networks, where reflected signal negatively impacts network performance, this is the required specification.
The two aren’t physically interchangeable: a UPC connector won’t fit into an APC port, and an APC connector can be plugged into a UPC port, but the angled connection causes a significant loss of signal, resulting in a poor connection. Singlemode optical fiber connectors are colour coded (UPC is blue, APC is green) so the difference is clearly visible with the human eye.
Optical Fiber Connectors Boots and Physical Durability
Here’s one aspect of a spec sheet that can slip under the radar but that can become apparent in the field: the connector boot, the strain-relief housing at the back of the connector that keeps the fiber from encountering bend stress and repeated flexing at the termination point. A push-pull sc connector or lc uniboot connector with a small or weak connector boot will start to bend small sections of the boot at the point where it meets the cable long before the connector fails.
This is particularly damaging in a dense patch panel where the cable is tightly bound and moved, added, and changed frequently. Even though the numbers are the primary consideration for comparing connector suppliers, it’s important to verify the boot material and bend-radius rating, because it’s the boot which can influence how many times a connector can be used in a working environment in an exchange or data center.
Specifying the Right Optical Fiber Connectors for Your Network
In practice the choices of optical fiber connectors will be based on three questions: What does the equipment either end require (which is normally fixed by the existing equipment), how much is the amount of port density required, and does the network’s performance allow for back reflection or does the network need an APC polish. FTTH/GPON deployments should use APC, high density data center/ exchange deployments favour LC for the amount of rack space saved, any equipment interface that specifically requires FC or SC compatibility should be matched exactly, not adapted, and connector mis-matching (especially mixing APC with PC/UPC) is a common and completely avoidable cause of field fault.
Why This Matters for OEMs Sourcing Fiber Management Hardware
Fiber management systems are a strategy that is being specified by OEMs at scale and cannot be chosen on the fly when termination time arrives. Because the right connector type and density for a network is based on network design, not a standard that is interchangeable like a commodity spec, CWIPL’s FMS platforms are designed to support the connector types and densities that a network requires, not a standard that will fit into a pre-specified niche.