A question often asked by people in the fiber optic business is why we have so many different connectors. The specific reference is usually to the 8-pin modular connector design used on 4-pair UTP cable for premises cabling, as there are thousands of different electrical connector designs available. So whenever I hear this question, I respond with a hearty laugh and a question in return. Do you really think all 8-pin modular connectors are the same? In actuality, there are dozens of 8-pin modular connector types, mostly incompatible.
We have RJ-45 connectors with different pinouts, for example, the 8-pin modular connector with USOC pinout for plain old telephone service, TIA-568A and TIA-568B for data. We also have a number of obsolete versions used before TIA began standardizing them around 1990. You can get 8-pin modular connectors, which are shielded or unshielded. There are different performance grades of 8-pin modular connectors, generally called Category 3, 5, 5e, 6 and 6A, which are compatible only with their corresponding grade of UTP cable. And, of course, since the standard UTP cabling system is undergoing constant upgrades, you have prestandard versions from different manufacturers of questionable compatibility.
So what’s wrong with fiber optics having three basic connector designs that are used for virtually all networks in the United States? The ST and SC hit the market about the same time in the mid-1980s. AT&T backs the ST and it costs less, so it became the market leader. When TIA standards first looked at fiber optics, licensing and patent issues led to standardizing the SC connector from Japan, but its higher cost and low market penetration at the time kept its popularity low for years. Today, the SC probably has more market share, as it is a better performing connector, but it still costs 10 percent more than an ST. ST and SC connectors use the same ferrule design, a 2.5-mm cylindrical ferrule usually made from a ceramic material, so they are intermateable with hybrid mating adapters, simplifying testing and network connections.
The third design, the LC, came along at the end of the 1990s as part of an industry trend toward smaller connectors to allow higher density for patch panels and transmission equipment. The LC uses a smaller cylindrical ferrule, only 1.25 mm diameter, which makes it easier to terminate. The LC has been adopted almost exclusively by the high-speed transceiver manufacturers, so it’s on practically every piece of communications equipment for gigabit speeds and above.
Fiber is much more standardized than copper cabling. Every glass optical fiber, multimode graded-index or single-mode, is the same size with a 125-micron outside diameter. Therefore, every connector has only one fiber size to accommodate. The larger core of multimode fiber allows connectors to have greater tolerances and, therefore, lower costs than single-mode connectors. However, the differences are more in selecting connectors with closer tolerances for single-mode.
Each of these connectors is described in complete mechanical detail in a Fiber Optic Connector Intermateability Standard (FOCIS) document from the TIA. FOCIS 2 covers the ST, FOCIS 3 covers the SC, and FOCIS 10 covers the LC.
What about all the other fiber optic connectors that have been on the market? When I was in the test equipment business, we had adapters for 84 different connectors. Most of those designs came from the first few years of the business when it seemed every company had a better idea of how to make fiber optic terminations. Many others were unique designs for specialized purposes, such as rugged military designs or cheap plastic fiber connectors. Once the ST and SC hit the market, new connector introductions declined until the small form factor connectors were introduced in the late 1990s, giving us the LC.
Most cabling systems today use ST or SC connectors. We now recommend using LCs on laser-optimized 50/125 fiber networks to prevent mixing 62.5/125 and 50/125 fibers with the resulting high fiber mismatch losses at connections. If users continue using the ST or SC on 50/125 fiber, then marking and color coding became very important. There is one more fiber optic connector you may see, the MTP/MTO, a 12-fiber connector used exclusively in preterminated cabling systems. It, too, has become a standard (FOCIS-5) for backbone cables, but the patch panels generally remain ST or SC.
Even with just these four connectors, you still can run into compatibility issues while testing. If your test equipment has a different connector from the cable plant (excepting the ST and SC, which are adaptable), you may have trouble setting the reference properly. International standards bodies have already recognized this and changed their test methods to use only the three-cable reference method (launch cable—“0” loss reference cable—receive cable), which accommodates any connector style. In the next revision of the TIA-568 standard (C revision), it will be an option to the one cable reference in previous standards, as long as the method is documented with test data. EC
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.