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Best Practices for Cost Effective CATx Installations

Posted by admin on September 13, 2015 9:50 am

When setting up a CATx network (where x stands for 5, 5e, 6, 6a, 7, etc. – the rating of the copper cable), also known as twisted pair network, there are best practices you can use to make sure that your network operates at peak performance. Before beginning, let’s start with some technical terms you may not be familiar with.

  • Signal degradation – the loss of quality of a signal, especially over long distances or in areas of interference.
  • Electromagnetic interference (EMI) – where cables are able to influence each other or disrupt an electromagnetic signal just by proximity.
  • Radio Frequency interference (RFI) – similar to EMI but is a disturbance in radio waves by proximity.
  • Crosstalk – Undesirable transfer of energy from one cable to another.
  • Patch cables – patch cables are copper or optical (fiber) cables used to connect (“patch-in”) one device to another, e.g., a switch connected to a computer, or a switch to a router.
  • 8-bit 4:2:0 color – 8-bit graphics store images so that each pixel is represented by one 8-bit byte, and where each bit describes red, green, and blue values, typically 3 bits (8 levels) for red, 3 bits (8 levels) for green and 2 bits (4 levels) for blue, enabling 256 (8x8x4 or 28) different colors. The normal human eye is less sensitive to blue/yellow than red/green, so blue is assigned one bit less than the others. The eye is also more sensitive to brightness (luma or Y’) than color (chrominance or Cb and Cr), so we can scale back on bandwidth requirements (compress images) by devoting less bandwidth to the chrominance.
  • 12-bit 4:2:2 color – 12-bit graphics store images so that each pixel is represented by one 12-bit byte, typically 4 bits (16 levels) for red, 4 bits (16 levels) for green and 4 bits (16 levels) for blue, enabling 4096 (16x16x16 or 212) different colors. The 4:2:2 Y’CbCr scheme requires two-thirds the bandwidth of 4:4:4.

Buy the Best Cable You Can Afford

Cable performance is defined by the quality of the cable, the distance of the connection, the handling of the cable, and the video resolutions you plan to send. A rule of thumb is that the higher the video resolution, the higher rated the cable should be. Your choice of cable should also take into consideration future expansion needs, so buy the best cable you can afford.

As cable ratings increase so do the speeds, communication frequency (MHz), twisting of the wiring inside the sheaths and sheath thickness for greater insolation, to eliminate crosstalk. Cable manufacturers may also include a nylon spline that runs the length of the cable to help reduce crosstalk, although manufacturers may choose to leave out the spline for a smaller diameter cable. Crosstalk interference can occur when multiple cables are bundled, even over short distances, causing a dropped signal or degraded picture.

If cabling is intended for a permanent installation inside walls, you should choose solid cables, which uses a single, untwisted piece of copper. Solid cables are not as flexible but are more durable. If you plan to install to a desk or anywhere the cable will move around, you should instead choose stranded copper cables, which are twisted and more flexible.

Also, you should be aware of the plenum rating (or non-rating) of the cable’s sheath. Having a plenum rating means that it is approved for certain locations based on the cable’s ability to not spread fire. Because plenum cables are routed through air circulation spaces (“plenums”), which contains few fire barriers, they are required to not catch fire easily or generate a lot of toxic smoke if it does catch on fire. Non-plenum “riser cables” are intended for vertical applications, such as between floors of a building, whereas plenum cables are absolutely required in both ceilings and floors.

One more thing you should know about selecting the right CATx cable for the job is that some cables are unshielded and others shielded. Shielding is use of one or more conductive layers that surrounds the cables, inside the outer sheath, which helps to eliminate crosstalk and maintain needed temperatures. Shielding can be made from:

  • Braided strands of copper or aluminum, which are most effective for low frequencies to eliminate EMI.
  • Unbraided spiral winding of copper tape, aluminum tape and combinations of materials such as nickel-plated copper (collectively called “foil shields,” “spiral shields,” or “serve shields”), which are most effective for high frequency applications to eliminate RFI, but can add undesirable mass and may corrode.
  • Combined braided and foil shields to eliminate both EMI and RFI, as well as, resists corrosion, but are more costly because of multiple processing.
  • Conductive polymers (plastics) and fabrics that are combined with an electrically conducting metal, so that the metal particles come into contact with each other and form a continuous path in the materials for electrons to travel.

Most twisted pair networks in the world carry Ethernet data traffic, exclusively. This is now changing as AV and IT networks converge. Cat5e and Cat6 cables were not designed for resistance to crosstalk, and most Cat5e cables are unshielded making them more flexible, durable and inexpensive but also more susceptible to EMI. This can be a problem when transporting serialized, uncompressed, high bandwidth, video. Both shielded and unshielded cables should be kept at least 12 inches from power cables to prevent EMI. If using unshielded Cat5e cables, also keep them away from other cables, transformers and light fixtures. If you must have cables that share the same pathway, make sure they are at least Cat6a or Cat7.

Note that with Cat6a you can use any standard HDMI cable that is labeled “high speed,” which means it supports 4K/30Hz (3840 x 2160) and 4K/24Hz (4096 x 2160). However, keep in mind that HDMI cables are typically unshielded and prone to interference from electrical wires, etc. This is why you typically don’t have long runs of HDMI cable. Cat6a or Cat7 are best for longer runs and for preventing EMI.

Future Proofing

Cat6a is currently the most popular choice for new digital AV installations, but is limited to supporting 4K/30Hz and HDMI 1.4b with uncompressed 8-bit 4:4:4 color, and a maximum transmission rate of 4.46 Gbps. However, if you go that route, be ready to swap out cables at a later date.

Another option for sending 4K/30Hz and HDMI 1.4b is Opticomm-EMCORE’s AFC-HDMI2 fiber-copper hybrid cable. It can be used like any regular HDMI cable, but where the maximum distance achieved with CATx cabling is 100 meters (330 ft.) using HDBaseT, this plenum-rated hybrid cable allows for much longer runs of 100 meters (330 ft.) and easier installation through tight conduits, due to its 2.9mm width (Cat6a cable averages 9mm) and significantly tighter bender radius, up to 6mm. Since this is not a twisted-pair cable design, no CAT rating is applicable for the copper.

It may make more sense to future-proof your system and install Cat8 now to support coming HDMI 2.0 products, 4K/60Hz (3840 x 2160), and uncompressed 8-bit 4:2:0 color (note the color decrease from HDMI 1.4). Cat8 supports minimally a transfer rate of 8.91 Gbps and maximum of 40 Gbps, and bandwidth of 1.85 GHz – four times today’s maximum of 500 MHz.

If you already have high-speed HDMI cables, you likely will not need to buy new ones to support HDMI 2.0 unless the cable isn’t fully up to the high-speed spec, in which case you might get dropouts, sparkles (white pixels on the screen), or nothing at all, any of which indicates that the cable isn’t able to pass the signal.

HDMI 2.0 also can support 12-bit 4:2:2 color, but bandwidth requirements balloon to 2.97 GHz. As comparison, Cat8 can only support up to 1.85 GHz so at that point you will want to switch to fiber transmission, which can support up to 8K and beyond.

Termination

CATx cables are favored by installers because they are easily field terminated with cable strippers, electrician scissors and crimpers, and thus do not require a lot of pre-planning before the project can begin. When terminating a cable, make sure not to untwist more than half-an-inch (preferably quarter-of-an-inch) from the point of termination, and remove as little of the sheath as possible.

You should be aware that there are two different RJ45 wiring standards, T568B and an older standard, T568A. While they are similar in that they both provide wiring schemes for terminating twisted-pair CATx cables for 8-position RJ45 connectors, the pin positions for the green and orange pairs have been switched. “8 position” refers to the fact that RJ45s require 8 individual wires, one for each pin, which are provided by the 4 twisted-wire pairs (blue, orange, green and brown) in the cable. If you are building a new network from the ground up you will want to terminate with the newer standard, T568B. However, if you are expanding on an existing network, you will need to look at the build records or do cable testing to see which wiring schematic was originally used and continue with that standard. If you happen to need to connect T568A-wired devices with T568B-wired devices, you can use a patch cord that has an A-configured plug at one end and a B-configured plug at the other end.

Handle Cables Carefully

Neat, tight-knit cabling can result in signal degradation via EMI. Tie cables loosely with appropriate cable wraps and use trays, carriers or J-hooks to support them, and don’t overloaded cable supports as it can result in the cables at the bottom becoming crushed, which will harm the signal.

When pulling cable through installation areas, don’t pull too hard because you might loosen the twist in the cables. Don’t pull cables through electrical conduits because they have tight right-angle bends. Bend radius is an important consideration; you want to keep it to a minimum of at least at 4 times the diameter of the cable, e.g., Cat6A cable should have no less than a 36mm bend in order to not damage it. Over-bending may also loosen the twist in the cables, increase susceptibility to signal degradation.

Mind Your Distances

CATx cables are sold at a maximum of 100 meters (330 ft.) due to signal degradation issues. If you need to go past 100 meters, you can use repeaters for up to 8 hops or 800 meters (2624 ft.), or you can switch to fiber cables (supported by HDBaseT 2.0). If you are using HDBaseT, be sure to know which Valens chipset is in the equipment, as type B only goes up to 70 meters (230 ft.).

Limit the Number of Connected Devices

If possible, try to avoid patch cables because each connection introduces losses in the transmission. If you do need patch cables, try to use no more than 2 to 4 devices per cable run and keep each less than 5 meters (16 feet) from the end.

Test as You Go

Testing is critical to guarantee cable quality and signal integrity. Be sure to test every cable as it is installed and terminated to identify problems such as broken or mismatched wires as you go, and also label and document wires so future installations go more easily. If something isn’t working right, before you swap out the components, check the source and the display by connecting them with pre-tested cable to make sure they are operational under ideal conditions. Digital signal loss is much harder to diagnose than analog signals and diagnostic equipment can be expensive and inconclusive, therefore, take extra time when installing a digital system and employ the best practices described in this article.

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Topics: Video over CATx with HDBaseT

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