Optimising cabling infrastructure

Shielded versus unshielded — which cable is best for your application?

Since the early 1980s, shielded cables have been a popular choice for data networking applications. Most notably, IBM standardised an S/FTP cable for its Token Ring LAN systems, creating an industry norm. In many parts of the world, shielded cables continue to be the dominant ICT infrastructure choice, particularly across Europe. Conversely, other countries, including the United States, opt for unshielded varieties.

The preference for shielded versus unshielded cable has been the cause of passionate debate over the years. Ultimately, determining the appropriate choice comes down to a number of factors including the applications to be supported, the external environment and, of course, budget.

The primary reason for selecting a shielded cable is to protect the transmission signal within the cable from external electromagnetic interference (EMI). In addition, the shield also acts to stop these signals from exiting the cable and causing interference with nearby electrical and electronic systems.

Cable anatomy

Obviously, the construction of CAT6A cable will vary depending on its external nature — shielded or unshielded. However, even among shielded cables, there is variation in the shielding method used, which can impact on its suitability for a particular application.

To clarify, there are three main types of cable in broad use today:

1. U/UTP — unshielded twisted pairs with cross filler element/pair divider. (Figure 1.)
2. F/UTP — outer foil shield around all four pairs with cross filler element/pair divider. (Figure 2.)
3. S/FTP — foil shield around each pair, outer metal braid around all four pairs with no cross filler element/pair divider. (Figure 3.)

Figure 1. CAT6A U-UTP cable.

Figure 2. CAT6A F-TUP cable.

Figure 3. CAT6A S-FTP cable.

There is, however, another type of shielded cable construction that has also been around for many years and is about to change the status quo: U/FTP. This variant combines the noise immunity advantages of a fully shielded cable with the ease of installation of an unshielded one.

Benefits of U/FTP

The benefits of U/FTP are numerous, but relate mostly to performance, ease of installation and physical size.

Performance

When installed correctly, the U/FTP pair shielding provides superior crosstalk and EMI performance. The individually shielded pairs enable the use of a more relaxed twist rate than U/UTP or F/UTP.

For most installations, U/FTP also removes the need for dealing with the extra shielding found in an S/FTP cable construction. Additionally, permanent link tests have shown that U/FTP provides superior headroom for most measurement parameters when compared with U/UTP and F/UTP variants.

Ease of termination and installation

Perhaps the biggest surprise for installers is the increased speed and ease of termination of U/FTP in comparison to other available variants.

U/FTP has the advantage of not requiring a cross filler element to maintain pair geometry and crosstalk performance. In the case of F/UTP cable, there is a plastic layer which surrounds the pairs that needs to be removed for termination, and removal of that shield can often be problematic. The construction of U/FTP means that removal of the four shields is easy and quick, thanks to the looser twist rate. This facilitates rapid termination, saving time and money.

Figure 4. CAT6A U-FTP cable.

Figure 4a. CAT6A U-FTP cable with only outer sheath removed.

Physical characteristics

For applications up to CAT6A, U/FTP cable features a much smaller diameter than U/UTP and F/UTP varieties. This is because the cross filler element is not required, decreasing overall cable width.

The benefits are obvious — a smaller cable diameter reduces congestion in cable trays, conduits and in racks. The physical construction and smaller diameter also make U/FTP more flexible and therefore easier to route.

The upshot is that U/FTP cable is a viable alternative to the many other options and features excellent transmission performance, as well as physical characteristics that can save time and money in any installation.

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