Friday, May 11, 2012

Shielded Twisted-Pair Connectors

In the United States, the most common connectors for cables that have individually shielded pairs in addition to an overall shield are based on a pre-1990 proprietary cabling system specified by IBM. Designed originally to support Token Ring applications using a two-pair cable (shielded twisted-pair, or STP), the connector is hermaphroditic. In other words, the plug looks just like the jack, but in mirror image. Each side of the connection has a connector and a receptacle to accommodate it. Two hermaphroditic connectors are shown in Figure 1. This connector is known by a number of other names, including the STP connector, the IBM data connector, and the universal data connector.

Figure 1: Hermaphroditic data connectors
The original Token Ring had a maximum throughput of 4Mbps (and later 16Mbps) and was designed to run over STP cabling. The 16Mbps Token Ring used a 16MHz spectrum to achieve its throughput. Cables and connectors rated to 20MHz were required to allow the system to operate reliably, and the original STP hermaphroditic connectors were limited to a 20MHz bandwidth. Enhancements to these connectors increased the bandwidth limit to 300MHz. These higher-rated connectors (and cable) are designated as STP-A.
STP connectors are the Jeeps of the connector world. They are large, rugged, and versatile. Both the cable and connector are enormous compared to four-pair UTP and RJ-type modular plugs. They also have to be assembled and have more pieces than an Erector set. Cabling contractors used to love the STP connectors because of the premium they could charge based on the labor required to assemble and terminate them.
Darwinian theory prevailed, however, and now the STP and STP-A connectors are all but extinct—they've been crowded out by the smaller, less expensive, and easier-to-use modular jack and plug.

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