Fiber Patch Connectors, LC and SC and ST and More
Written by Don Schultz, Fluke Networks Copper/Fiber CCTT, BICSI INST1, INSTC, INSTF Certified
In the world of copper Ethernet Category cable, very little has changed in regards to how you terminate it in the last 20 years. Whether back in the late 1990s or today, you will see 8P8C RJ45 type connectors at the end of Ethernet patch cords and keystone jacks mounted in walls running back to patch panels. The T568A and T568B color code has remained the same too, dictating the wiring color code sequence to make proper connections. One key thing about copper Ethernet is that it is nearly impossible to directly splice it if you need to extend it. You have to terminate it somehow (connector, patch panel, etc.) in order to get from A to B and be mindful of the rather strict length limitations.
Extending and finally terminating fiber optic cable is very unlike copper Ethernet cable. To start with, if an installer needs to extend a fiber run that does not need to be connected or disconnected then the fibers are spliced together (either fusion or mechanical splices). During the installation at the point where the fiber cable has to be plugged and/or unplugged you need to switch up your strategy and put a connector of some kind on. Fiber optic cable has gone through quite the evolution of connectors, and none of these connector styles are compatible with each other. Some connector types are actually being phased out. What is interesting about fiber optic connectors is how you choose which one to use. The choice is largely driven by the equipment requirements and to a lesser extent the installation environment.
In this blog we will cover:
Simplex vs Duplex
In order to make some sense of the various types of connectors out there, first we should talk about assembly styles. Connectors come in two primary different assembly styles--simplex and duplex. This sounds terribly complicated but it is not. Simplex = single and duplex = double. A simplex connector is simply a single connector terminated onto a single fiber. A duplex connector is essentially two single connectors side by side, often in a plastic assembly. Duplex style fiber optic cord is associated with the term “zip cord” and that literally means two fiber patch cords that are joined together at the jacket and can be separated…or unzipped, I guess? That is the term and it stuck, so we will go with it. What defines whether you use a simplex or duplex style connector is your end point equipment and how you are hooking things up. It should be noted that simplex and duplex have nothing to do with single mode or multi-mode or the actual connector type like LC or SC. The key take-away is that no matter what assembly we are talking about, the individual connector terminates a single strand of fiber.
Here are a couple of pictures to illustrate the idea:
Duplex connector assembly on left. Simplex on the right. Both are LC.
FC connector (seen attached to single mode fiber in duplex configuration)
The FC connector has been around for quite some time and is the earliest form of fiber optic connector. The acronym FC means “Ferrule Connector” but is often used as an acronym for “Fiber Channel” as well. This is a screw-on connector that is made of metal and is extremely secure against vibration and accidental disconnections. The closest analog in the copper cable world would be the F-Connector seen on coaxial cable. FC connectors make use of a connector key and must be inserted with care to avoid scratching the end face of the fiber. This connector is falling out of favor due to improvements in connector technology and the costs associated with precision made metal connectors. In the field, it is still seen in robotic and industrial applications where reliability is paramount. FC connectors make use of a 2.50mm ferrule.
SC connectors are probably the most common fiber connector in use today, but they are quickly being overtaken by the much smaller LC connector as SC connectors do not lend themselves to ultra high density applications. SC is an acronym for “Subscriber Connector” and literally means that. It is often the connector terminated onto fiber coming into a home or business (the end subscriber). It has also been called a “Standard Connector”. Man, everything has multiple names around here!
SC connector shown in simplex format terminated onto single mode fiber
SC connectors are plastic push-pull type, and are keyed with a tab. There is no fancy screw-in mechanism here, and there does not need to be. SC connectors are wiggle proof and moderately pull proof (but not vibration proof) while being easy to plug or unplug. There is less risk of ruining the end face of the fiber during connection/disconnection compared to FC. The biggest downside to these connectors is size. Like FC connectors, SC connectors make use of a 2.50mm ferrule.
SFF (Small Form Factor) connectors are becoming all the rage these days, and for good reason. Their significantly smaller size allows for very high density installations. Enter the LC connector. The LC acronym means “Lucent Connector” but of course everyone just says LC. Like the SC connector, this connector type is keyed, wiggle proof, and constructed of plastic. Unlike SC connectors, the LC connector is fully pull proof due to the positive latch and about half the size. LC connectors use a 1.25mm ferrule.
SC connector on left. LC connector on the right. Big difference!
LC connectors are often seen used with fiber optic transceivers that are hot pluggable devices that can be plugged into an Ethernet switch that has SFP/SFP+ ports. The job of the transceiver is to translate electrical signals into optical pulses and this strategy offers huge versatility.
LC Transceivers. These happen to be 10 Gigabit Multi-Mode.
The manufacturer of the Ethernet switch will often not include the transceivers since what transceiver you need will depend upon the fiber optic cable you intend to install and this is often driven by the length or speeds involved. This has the effect of lower prices for the switch, initially. In all previous types of installations, the end point equipment has the optical transmitter built in, which drives up costs and the need to have many different versions of the same piece of equipment, supporting the various types of fiber technology. Transceivers and LC connectors bring modularity and flexibility, because switching over from MMF at 1G and 1,500 feet to SMF at 10G and 4,000 feet may be as simple as switching out to low(er) cost transceivers and using a different cable as opposed to replacing the whole switch!
The end result is the LC connector and transceiver strategy is transforming the fiber optic market in many unprecedented ways, such as bringing the technology right to the home market and small-time DIY installer.
The last type of connector we will talk about that is meant for a single strand of fiber is the ST connector. ST is an acronym for “Straight Tip”. This type of connector is metal and uses a bayonet style plug and socket. Essentially, this means the connector is twist and lock (but NOT actual screw on) like the common BNC connector seen used in A/V applications (often seen used on coaxial cable).
ST connectors on the left. SC duplex assembly on right for comparison.
ST connectors are not pull or wiggle proof and are arguably the least reliable way of terminating a strand of fiber. They are not often used anymore, having been supplanted by far superior connectors such as SC and LC. ST connectors use a 2.50mm ferrule like FC and SC.
Up until now, we have been talking about connectors that terminate onto a single fiber strand. Are there connectors that allow for plugging in large numbers of strands at once? Oh yes, yes there are. One of the common strategies for running fiber optic cable is to haul your 12 or 24 fiber distribution cable into a telecommunications room (TR), and then you can do a number of things to terminate that distribution cable so you can patch it into your equipment:
- Haul in a high fiber count cable that has nothing but unconnectorized fibers hanging out the end and then fusion or mechanically splice each fiber individually to a fan out “pigtail” that has already been connectorized with SC or LC connectors on the other side. This is a laborious process, increasing the possibility of mistakes.
- Haul in a pre-connectorized high fiber count cable (MPO/MTP connector already terminated to it) and then simply plug it into a fan out cable that has a MPO/MTP connector on one side and anywhere from 12 to 24 SC or LC connectors on the other side. It is factory made, and ready to go! Hooray for plug and pray (...oops, I meant PLAY)
MPO connectors stand for Multi Fiber Push On, and is an industry recognized and defined connector style. MPO connectors come in male and female styles. MTP connectors fall under the MPO umbrella but are in fact a brand name. All MTPs are MPOs, but not all MPOs are MTPs since “MTP” is a registered trademark of US Conec. MTP stands for “Multi Fiber Termination Push On”. MTP connectors are essentially the same idea but have improvements such as a more secure latching mechanism and more accurate fiber alignment. Both work the same way and allow two rows of 12 fibers (up to 24) to plug in at the same time. Of course not all MPO patch cords are the same. There are MPO > MPO as well. It all depends on the installation and equipment. Here are a couple of examples:
MPO > MPO cable on the left. MPO > LC connector fan out on the right.
So, there you have it. This is a quick overview of the various connector styles you may or may not be familiar with. The vast majority of the time you will encounter SC, LC, or MPO connectors as the others are being used less and less. We did not talk about end face polishes (PC, UPC, APC) nor the standard color coding being to help differentiate these cables. That will be a future blog! For now I will say…
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