With high bandwidth applications emerging in and out of the data center, the need for shielded cable has increased. Many of these new installations are in harsh environments. Robotic equipment, motors, generators, air conditioners, fluorescent lights, printers, and they often generate a large amount of EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference). You may need to install shielded cables in these harsh environments.
EMI and RFI can cause crosstalk between circuits and interfere with data transmission on a copper cable. The interference will cause increased errors resulting in mire network traffic due to packet retransmissions, and downtime.
Shielded cables reduce interfernece
EMI is an unwanted signal that is induced into the cable. EMI typically comes from a source that is external to the cable, such as an electrical cable or device. Cables can be both a source and receiver of EMI. As a source, the cable can either conduct noise to other equipment or act as an antenna radiating noise. As a receiver, the cable can pick up EMI radiated from other sources.
RFI is a disturbance that affects an electrical circuit due to either electromagnetic conduction or radiation emitted from an external source. Conducted RFI is unwanted high frequencies that ride on the AC waveform. Radiated RFI is emitted through the air.
Ethernet cables can be shielded to deal with EMI and RFI. The shield surrounds the inner signal carrying conductors. Shielded Ethernet cables can deal with interference in two ways. It can reflect the energy, or it can pick up the interference and conduct it to ground. Both methods use shielding to cut the EMI and RFI reaching the twisted pairs located under the shielding. Whether the EMI/RFI is reflected off the shield or “rides” the shield to ground, some energy can still pass through the shielding, but since it is so highly attenuated it will not cause interference.
Types of shielded cables
Two basic types of shielded cables are available. The first are cables with an overall shield known generically as STP. STP cable may be made either with a foil or a braid for the shield. STP cables with an overall foil are often known as FTP. In practice, FTP and STP cables may be interchanged with no apparent difference in performance according to BlackBox
The second type of shielded cable is S/STP. S/STP cables have a shield around each individual pair and an overall outer shield. BlackBox says the purpose of the inner shields is to cut the Alien Cross Talk parameter in CAT7 and CAT6a systems.On both STP and S/STP the primary task of the outer shield is to resist external RF noise such as electrical spikes. S/STP cables may also have an extra braid for strength and to simplify connection to the metal shields around the connectors.
To reduce EMI/RFI interference, shielded Ethernet cables must use shielded connectors to maintain the benefits of STP cabling. High-quality shielded cable includes a drain wire to provide grounding that cancels the effects of EMI and can ease termination of the cable shield for crimping or soldering.
Proper Ground connections
The shielded cable system must have proper ground connections for the shields. Incorrect grounding opens the possibility for ground loop currents and associated interference to the Ethernet signal. In the worst cases with no proper grounding, the shields can actually act as antenna broadcasting high-frequency signals out into the environment interfering with electronic equipment and allowing external detection of the Ethernet data.
Standards bodies have requirements for shielded Ethernet systems. The American National Standards Institute (ANSI) specifics how shielded communications cables are properly grounded. Normal practice says the cable shields should be grounded in the telecommunications closet (TC). Typically, the cable shield is grounded through the connector to the patch panel. Then the panel is grounded to the rack, which is grounded to the telecommunications grounding busbar in the TC. Do not ground the cable shield at the work-area outlet. Fortunately, many of today’s shielded outlets automatically connect to the patch panel’s ground so there’s no need to set up ground paths for each cable.
ISO/IEC requires that shielding must be complete for an entire channel, shielded and shielded cables, connectors or network controllers should not be mixed.
Cable Types
Category cable types comparison.| Max Data Rate | Max TX Rate | Max Length | Sheilding | Year Introduced | Status | |
|---|---|---|---|---|---|---|
| Cat 3 | 10Mbps | 16MHz | 100m | No | 1983 | Obsolete |
| Cat 5 | 1,000Mbps | 100MHz | 100m | Optional | 1995 | Obsolete |
| Cat 5e | 1,000Mbps | 250MHz | 100m | Optional | 2001 | Obsolete |
| Cat 6 | 10,000Mbps | 500MHz | 100m | Optional | 2002 | |
| Cat 6a | 10,000Mbps | 500MHz | 100m | Optional | 2008 | |
| Cat 7 | 10,000Mbps | 600MHz | 100m | Required | 2002 | Not recognized by TIA |
| Cat 7a | 10,000Mbps | 1GHz | 100m | Required | 2010 | Not recognized by TIA |
| Cat 8.1 | 40,000Mbps | 2GHz | 30m | Required | 2016 | Backward compatible with Cat 6A |
| Cat 8.2 | 40,000Mbps | 2GHz | 30m | Required | 2016 | Not recognized by TIA |
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interference is not a new problem. It dates back to the earliest communication systems. In 1881 Alexander Graham described the interaction between many twisted pairs in US Patent 244,426.
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Ralph Bach has been in IT long enough to know better and has blogged from his Bach Seat about IT, careers, and anything else that catches his attention since 2005. You can follow him on LinkedIn, Facebook, and Twitter. Email the Bach Seat here.
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