The core job of DSP is to process signals. In some cases, it’s adding or subtracting from the amplitude. In other cases, it’s enhancing or suppressing parts of the frequency response, and many DSPs are excellent for reducing and/or eliminating noise from the source signal. Unfortunately, the “Garbage In-Garbage Out” theory applies to AV signals: there’s only so much that DSP can do to clean up a noisy waveform. The cleaner the incoming signal, the cleaner it will be going out.
Ethernet cables, as well as many other types of cables used in AV, are in essence antennas (thin cross section + significant axial length = antenna). You don’t have to look any further than your garage door opener: the antenna is literally a 6” length of wire hanging down, and typically they work pretty well. When you combine the antenna-like behavior of an Ethernet cable with the increased transmission frequency of the signals it’s carrying, the effects and impact of alien crosstalk (AXT or ANEXT) become much more pronounced.
AXT is crosstalk caused by other cables (and connecting hardware) routed close to the cable of interest. While not a problem at the lower frequencies of Cat 5e and Cat 6 cabling, the higher frequency signals of Cat 6A makes AXT the limiting noise source for those systems. Active equipment can effectively reduce near-end and far-end crosstalk between pairs in a cable using cancellation technology that relies on crosstalk coming from a known source. However, the reason this type of crosstalk is called “alien” is because the source is unknown and cannot be cancelled out by the equipment.
The primary contributor of AXT is the in-wall cable runs. While you can physically mitigate it by separating the runs into separate joist bays, it’s not very practical. The same is true of electrical cables, especially in retrofit situations. 110 VAC 60 Hz is pretty noisy, and can greatly impact AXT, which is why the rule of thumb is to cross electrical cable with low voltage cables at a 90° angle to minimize cross sections.
Using a portable spectrum analyzer is a great way to get a good picture of EMI/RFI in a space, but again, a lot of times it’s not practical or realistic. In most cases, you won’t know what electrical equipment will be close by and whether the neighboring electric cable will have additional noise from motors, compressors, etc. Having said that, unshielded twisted pair (UTP) is perfectly fine for many applications. It all depends on what the customer considers acceptable for AXT, and most won’t care, so long as the system sounds good. The question to ask is how good will the system sound in the future, as technologies evolve and hardware reaches EOL and gets swapped out.
Shielded Cable Construction
There are three variants of a shielded cable: an STP construction that includes individual shields for each twisted pair, an S/UTP construction that has an overall cable shield, and an S/STP construction that has both an overall cable shield as well as individual shields for each twisted pair. Typically, the twisted pair shields are to reduce near-end crosstalk (NEXT), while the cable shield is to prevent AXT.
Cat 6a UTP Cables vs. STP Cables
While Cat 6a UTP cables support 10 Gigabit transmission, STP may have some advantages. The shield surrounding the pairs in the STP cable helps prevent EMI/RFI from coupling onto twisted pairs. This can help to eliminate the effects of noise from sources like machinery, generators, or medical imaging equipment, making STP systems an excellent choice for industrial environments and healthcare facilities.
While Cat 6 UTP may support 10 Gig in shorter runs of less than 55 meters, Cat 6 AXT performance will likely not enable it to support 10 Gig to the typical 100-meter test length. It is therefore not recommended for new commercial installations that are designed to support 10 Gig transmission either now or in the future.
Myths Surrounding STP
There are a few misconceptions in the industry surrounding STP cabling, including installation, bonding and grounding, and overall cost.
- Shielded cabling systems take longer to install. This notion has been around for many years and is based on older-style shielded systems that required extra steps during installation. Many of today’s shielded components include die-cast metal jacks that no longer require special bonding of the shield, making the installation time comparable for both UTP and STP. STP systems really only require a little extra time to separate the overall shield from the cable. In addition, STP systems do not typically need to be tested for AXT, which can be a fairly time-consuming process with UTP.
- Bonding and grounding is more complex and time consuming with STP. STP systems really require just the one additional step of bonding the shielded patch panel to the rack using a 6 AWG bonding conductor in exactly the same way that other equipment is bonded. The shield of the STP cable is bonded to the patch panel during termination, and the bonding conductor maintains the continuity of that shield to ground because the rack is bonded to the telecommunications room grounding busbar.
- Shielded Ethernet cable costs a lot more. Today, STP ranges from 20–40% more per foot versus UTP, which can be very significant for large scale installations. However, while shielded cable costs more than UTP, the increased performance and reduced testing time can ultimately lower the total cost of ownership.
As with many things in the AV industry, there’s no hard and fast rule when to use shielded vs. unshielded cable. Next generation cabling systems that perform beyond 10 GB/s transmission speeds will require shielded cabling and components, which makes STP systems more future-proofed than UTP. The result is similar lifetime costs for both STP and UTP systems.