Screened and Shielded Cabling - Noise Immunity, Grounding, and the Antenna Myth
Introduction and History of Shielding
Screened and shielded twisted-pair copper cabling has been around for quite a while. A global standard in the 1980s, varieties of screened and shielded have remained a mainstay in some markets, while many others migrated largely to unshielded (UTP) cables.
Recently, however, the ratification of the 10GBASE-T standard for 10Gb/s Ethernet over copper cabling has reestablished the commercial viability of screened and shielded systems and fueled greater adoption of these systems in previously UTP centric markets.
In this competitive landscape, many confusing and often contradictory messages are finding their way to the marketplace, challenging both cabling experts and end-users alike. This whitepaper addresses the most common questions, issues and misconceptions regarding screened and shielded cabling.
Figure 1: IBM Type 1 cable
In the 1980's, LAN cabling emerged to support the first computer networks beginning to appear in the commercial building space. These first networks were typically supported by IBM Token Ring transmission, which was standardized as IEEE 802.5 in 1985. Cabling for the Token Ring network consisted of "IBM Type 1" cable mated to unique hermaphroditic connectors. IBM Type 1 cable consists of 2 loosely twisted, foil shielded, 150 ohm pairs surrounded by an overall braid as shown in figure 1.
This media was an optimum choice for the support of first generation LAN topologies for several reasons. Its design took advantage of the twisted-pair transmission protocol's ability to maximize distance (Token Ring served distances up to 100 meters) and data rates using cost effective transceivers. In addition, the foils and braid improved crosstalk and electromagnetic compatibility (EMC) performance to levels that could not yet be realized by early generation twisted-pair design and manufacturing capability. Not surprisingly, a handful of buildings are still supported by this robust cabling type today.
By 1990, LAN industry experts were beginning to recognize the performance and reliability that switched Ethernet provided over Token Ring. Concurrently, twisted-pair design and manufacturing capabilities had progressed to the point where individual foils were no longer required to provide internal crosstalk isolation and overall shields were not necessary to provide immunity against outside noise sources in the 10BASE-T and 100BASE-T bands of operation. The publication of both the 10BASE-T application in 1990 and the first edition ANSI/EIA/TIA-568 generic cabling standard in 1991, in conjunction with the lower cost associated with unshielded twisted-pair (UTP) cabling, firmly established UTP cabling as the media of choice for new LAN network designs at that time.
15 years later, as Ethernet application technology has evolved to 10Gbps transmit rates, a marked resurgence in the specification of screened and fully-shielded twisted-pair cabling systems has occurred. This guidebook addresses the practical benefits of screens and shields and how they can enhance the performance of traditional UTP cabling designs intended to support high bandwidth transmission. It also dispels common myths and misconceptions regarding the behavior of screens and shields.