Comparison Of Cat 5e/6/6a Cabling Testing
When comparing Cat5e to Cat6 and Cat6a cable, both Cat5e and Cat6 fail to deliver the level of reliable performance needed, as shown in Figures 12, 13 and 14, from an industry-standard field-test instrument. All measurements should be completely below the red limit line, which represents power sum alien near-end crosstalk (PSANEXT).
The test results for Cat5e cable (Figure 12) reveals -20dB of AXT margin: fail. This results in a dropped signal for complete video loss. Measurements of Cat6 cable (Figure 13) are also above the red limit line, indicating the presence of significant alien crosstalk that results in signal degradation or complete video loss. In Figure 13, the Cat6a cable measurement is well below the red limit line, indicating significant reduction of alien crosstalk and greatly improved signal reliability.
(A detailed look at the test results can be found in “AV Cable Recommendations for HDBaseT” at www.leviton.com/ns/whitepapers.)
Shielded Cabling & HDBaseT
Shielded cable is ideal for high electromagnetic interference (EMI) environments, applications near high-voltage wiring and infrastructures that require increased levels of security, such as casinos or government facilities. Shielded cable works to prevent alien crosstalk in two ways:
- creates a barrier between pairs in adjacent cable where the inherently “open” structure of the twisted pairs allows magnetic fields to escape. The magnetic field sets up eddy currents in the shield barrier, which then creates a magnetic counter field, preventing magnetic coupling from one pair to the other
- creates an electrostatic barrier over multiple pair twists in order to create a “zero-average” voltage potential without peaks that can also couple to an adjacent pair in an adjacent cable.
At high frequencies, the important function of the shield is to perform the two aforementioned barrier tasks. When low (100s) MHz to GHz frequencies are required, such as those needed for an HDBaseT signal, the actual grounding of the shield is not relevant. However, failing to do so creates the potential for setting up resonances that can cause alien crosstalk or other signal degradation.
Due to the point-to-point nature of typical HDBaseT applications, it is not feasible to follow TIA-568 or TIA-607 bonding and grounding standards. Most HDBaseT products use a power supply or Power over Ethernet (PoE) with no ground connection. Using shielded channels on a point-to-point application with no connection in a telecommunications room (TR) will result in a floating ground that can add EMI and should be avoided. (See Figure 15.)
Additionally, there are added safety risks when using shielded channels in a point-to-point application with no ground connection. Should an energized conductor come into contact with the shield foil, the entire cable pathway may become live and dangerous. National Electrical Code (NEC) 250 sets the minimum requirements for grounding and bonding. Compliance with the NEC and local codes is essential to the proper application of the ANSI/TIA-607 standard.
UTP Cable With Isolation Wrap
UTP cable with special isolation wrap takes advantage of the phenomenon that the shield can be segmented and the length of the shield can be adjusted to cover only a few twists, establishing eddy currents and the magnetic counter field, and also creating a capacitive effect sufficient to prevent alien crosstalk. This technology tunes the length and shape of the shielded sections to perform best in the desired frequency range for category-rated cables, and it installs just as easily as standard UTP cable. Because UTP cable with isolation wrap does not require grounding or bonding, you get the benefits of a shielded cable without any of the installation headaches or costs. (See Figure 16.)
As seen in Figure 17, Cat5e and Cat6 UTP cable can’t provide the PSANEXT margin necessary for IT/AV signals in bundles or over long distances, and in most cases it does not support the signal at all. Standard Cat6a UTP cable comes close to meeting signal requirements and delivers 1dB of margin for modest performance. Across the board, Cat6a cable delivers the level of signal required for higher resolution over long distances. A Cat6a FTP or UTP with alien crosstalk prevention solution is capable of mitigating alien crosstalk to provide top-tier performance. However, the shielded solution is difficult to implement and requires additional labor and cost where a UTP cable with alien crosstalk prevention technology provides the desired performance and lower cost for an optimal HDBaseT system.
PoH powers the HDBaseT link over category cable. This allows for integrated transmitters in devices such as matrix switches to power remote receivers. Similar to PoE standards, HDBaseT transmitter and receiver pairs draw only 10 to 15 watts (typically <300mA @ 48VDC on the four pairs) with PoH. Only half of that wattage is sent to the opposite end.
If implemented as a bidirectional solution, the transmitter or receiver can be powered at either end, which is ideal for environments where power outlet locations are limited. It can also be used to power multiple auxiliary devices, such as HDBaseT control panels and autoswitching wallplates.
The best way to ensure dependable signal performance over an IT/AV system, and avoid video signal loss, is to use Cat6a cabling installed in line with BICSI practices and tested and certified with the manufacturer. However, when encountering performance issues, there are a few steps you can take to fix video signal loss. This starts by dividing and isolating the power, HDBaseT link, HDCP at TX (transmitter) and RX (receiver), category cable link and extender hardware.
HDBaseT extender transmitters and receivers typically include status indicators that can help identify any issues. See Figure 19:
- lights up red or green when powered (depending on the manufacturer)
- self-check blinks green when the device is working properly
- lights up green when the TX and RX are correctly connected
- HDCP compliance indication lights up solid green when the connected device supports HDCP; blinks when the device does not support HDCP.
Field-testing HDMI signals is generally impractical, but there are a number of reliable testers available that provide a reference HDMI source and a reference sink. They are useful for monitoring HDCP key sets, EDID transactions and hot-plug events. However, most testers are not suitable for field testing an end-to-end link, and the relative expense of the tester often outweighs the benefits. (See Figure 20.)
A standardized approach can often provide better testing results at a lower cost. Standard testing of the category cable infrastructure relies on extensive TIA, ISO and IEC standards, and there are a number of field-proven test instruments readily available. Plus, industry-standard field testers often allow integrators to certify and warrant the cable link, providing customers added peace of mind.
A standardized field tester provides the capability to identify the following in an HDBaseT link:
- the actual link distance for comparison with the distance capability of the extender (Figure 21)
- the presence of an incorrectly terminated connector (Figures 22 and 23)
- any opens or shorts in the channel (Figure 24)
- cabling and connectivity capability aligns with TIA, ISO and IEC standards (Figure 25).
Should the link test check out and signal loss is still occurring (no video), the next step is to verify the source (disc player, PC, laptop, etc.) and sink (TV, projector, flatscreen display, etc.).
- First, check that power is applied at the display or projector.
- Connect the HDMI source directly to the display or projector using a known, good three- to 10-foot-long high-quality HDMI cable. Verify that the display is set to the connected HDMI input. If the video is correctly displayed, then proceed to the next step. If video is not displayed, ensure that the source is connected to the selected HDMI input on the display device. If video is still not displayed, substitute an alternate HDMI source or display to isolate the failing component.
- Once video is correctly displayed when using only the HDMI cable, add the HDBaseT transmitter and receiver to the link, substituting a Cat6a patch cord in place of the entire category cable link. Plug the patch cord directly into the output of the HDBaseT transmitter and into the input of the HDBaseT receiver. Add the HDMI source to the transmitter using a known, quality short HDMI cable and then connect the receiver to the display device using the same short HDMI cable as before.If video is correctly displayed, proceed to the next step. If video is not displayed, then one at a time, substitute a known good transmitter and receiver into the channel to isolate the failing component.
- Now that video is correctly displayed, using the short patch cable HDBaseT link, remove the Cat6a patch cord and connect to the full category cable link. You should have video correctly displayed; if not, you have isolated the issue back to the category cable link and should retest using a standard field test set to verify connection, distance and Cat6a signal capability. Verify that the link distance does not exceed the capability of the HDBaseT extender set.
- If you are not able to identify the source of the issue, reach out to the manufacturer’s technical support team for help with identifying potential problems.
HDBaseT technology is fast becoming commonplace throughout enterprise environments, especially classrooms and conference rooms. This places datacom professionals in a unique position to become the AV experts for their customers. However, to establish complete customer confidence, it’s critical to understand the variations in HMDI, HDBaseT and video resolutions; know the practical applications for PoH; and be prepared to apply optimal troubleshooting strategies should performance issues occur. Familiarizing yourself with HDBaseT best practices now ensures that your customers benefit from a reliable, high-quality IT/AV system for years to come.