SC- and ST-compatible connectors coexist in premises networks

The standard SC connector and the non-standard ST-compatible connector are acceptable for use in existing networks, but the SC is the choice for new deployments

Jetta pyatt

siecor corp.

SC- and ST-compatible connectors have emerged as the popular choices for premises cabling systems. The ST-compatible connector has a larger installed base and has been accepted as a de facto standard for several years. The SC connector, which can be duplexed, has been recommended by the Telecommunications Industry Association/Electronic Industries Association-568A Commercial Building Telecommunications Cabling Standard.

Because network planners have two connector options that provide essentially the same performance characteristics, they must evaluate such connectivity issues as polarity, connector specifications and installation practices for the cabling infrastructure. In most installations, users can implement either the simplex ST-compatible connector or the duplexed SC (568SC) connector for premises networks in conjunction with field-installable connectors, connecting hardware, cable assemblies and recommended practices. Users who are satisfied with their installed base of ST-compatible connectors do not have to change connectors. New users of optical fiber or users who are considering the deployment of fiber-to-the-desk connections should install 568SC-type connectors.

The overriding requirement by the TIA/EIA-568A standards group was to select a connector that would be suitable for the outlet to the fiber-to-the-desk connection. To allow users to plug in a computer at the desk, the group decided the interface should be a duplex interface that prohibits the reversal of transmit and receive fibers.

Contenders eliminated

Consequently, this requirement eliminated a number of contenders. The traditional bayonet ST-compatible connector was dismissed because it could not be duplexed. The fiber distributed data interface-media interface connector was dismissed because of its size, cost and difficulty in field termination. The field was narrowed to the SC and push-pull ST types, which have a 2.5-millimeter ferrule and similar optical and mechanical performance.

The duplexed SC connector, referred to as 568SC, was ultimately selected because two major optical fiber application standards--Fibre Channel and low-cost FDDI--adopted it over the ST-compatible connector. In addition, more manufacturers hold build-rights to the SC rather than to the push-pull ST connector. Furthermore, the international equivalent of TIA-568A, known as DIS 11801, also adopted the SC connector.

The SC connector offers several advantages over the ST-compatible connector. Because of its push-pull latching mechanism, the SC connector can be duplexed and more easily mated than the one-quarter-turn bayonet latching style of the ST-compatible connector. The SC connector should more accurately be called "duplexible" because it can be used in a simplex or duplex configuration for duplex and non-duplex applications.

An installer can terminate the optical fibers with simplex SC connectors on the cabling side of the connecting hardware or outlet. This setup is favored because an installer can plug the simplex SC connectors into a duplex SC adapter, effectively duplexing the connectors.

Another advantage is the ease of making a patch cord. Fibers are terminated singularly by installing simplex SC connectors. An elementary clip or yoke is then installed to hold the two connectors together in a duplex configuration. This structure eases cable strain-relief and allows for remakes of only one connector. Moreover, reversing the position of the two connectors within the yoke is possible if they had not been correctly installed initially.

ST-compatible connector

From a standards viewpoint, the ST-compatible connector appears out of contention. The TIA-568A, DIS 11801, Fiber Channel, low-cost FDDI, broadband integrated services digital network (T1E1) and asynchronous transfer mode standards groups have adopted the SC. The ST-compatible connector has been adopted only by the Ethernet 10Base-F standards group. From a commercial viewpoint, however, the industry de facto standard connector for cabling and most electronics is the ST-compatible connector.

Despite growth in SC connector sales, industry analysts claim it is difficult to predict whether the SC connector will become a market standard, given that many standards have never been adopted by the marketplace. However, they agree that the advantages of the duplexed SC connector and the widespread adoption by numerous standards groups are expected to make the SC connector the primary choice in the marketplace.

This forecast, however, does not imply the demise of the ST-compatible connector, particularly because other than the duplex issue, there are no significant technical benefits of the SC connector over the ST-compatible connector. A quality ST-compatible connector can support the same-speed networks as a quality SC connector. For example, the benefits of the ST-compatible connector over the biconic and SMA connector types have not fully eliminated them from usage.

Customer decides

As always, the choice of connector is still a customer preference. In a large backbone network that needs additional connections to that backbone, the simplex ST-compatible connector gets the call. Customers planning a fiber-to-the-desk network should evaluate the 568SC interface because of duplex connection benefits at the workstation. The TIA/EIA-568A and DIS 11801 standards contain a grandfather clause and suggested migration path options for users who have an installed base of ST-compatible connectors.

Duplex ST-compatible connectors and hybrid 568SC-to-ST-compatible adapters are available, but the advantages do not outweigh the disadvantages. The duplex ST-compatible and push-pull ST-compatible connectors have not been adopted by any application or cabling standards; also, only a limited number of vendors support this connector. Hybrid adapters do not help connectivity because they prove troublesome in testing and maintenance. These options are not recommended to be used as intermediate steps.

A short-term drawback of the SC connector is its price. Customers are paying a slight premium for the SC connector vs. the ST-compatible connector and adapters. Furthermore, many users will not experience the full benefits of the 568SC because most electronics are still shipped with ST-compatible interfaces. Although the installation of hybrid patch cords with the 568SC on one end and the ST-compatible connector on the other does not create problems, the patch cords do not completely address the duplex issue. In regard to test equipment, however, the SC connector comes with most equipment or is easily accommodated with a hybrid jumper.

The simplex SC connector and adapter are keyed, ensuring the same orientation of the connector and consistent losses upon mating. Installation of the duplex clip establishes the orientation of one fiber to the other (polarity) because the 568SC connector can be inserted into the adapter or receptacle in one direction only. A duplex connector alone does not ensure polarity and correct connectivity. Therefore, connectivity practices were standardized to ensure uniformity between manufacturers and installers.

All known specifications for a multimode duplex SC are "intermateable" and include the low-cost FDDI, Fibre Channel, ATM, broadband ISDN and TIA/EIA-568A. The only specification dissimilarity occurs for the singlemode Fibre Channel duplex SC connector. To prevent the possibility of inserting a multimode fiber into a singlemode receptacle, Fibre Channel has specified a narrower key on the connector and keyway on the receptacle associated with the transmitter. This dimensioning is done for laser safety at the electronic interface.

However, this dimension is not a concern for the cabling infrastructure. If a singlemode fiber is plugged into an the electronic port, acceptable power levels are still maintained. This connection means that a Fibre Channel singlemode connector will plug into a 568SC adapter; however, a 568SC singlemode connector will not plug into a Fibre Channel receptacle. u

Jetta Pyatt is connector product specialist at Siecor Corp. in Hickory, NC.