Higher data transmission speeds require superior performance to combat electromagnetic interference, writes Thorsten Punke.

Since the introduction of 10Gbps data transmission over copper wire, shielded systems have become more popular and the market share is still growing.

However, given the develpoment of 40Gbps by the Institute of Electrical and Electronics Engineers (IEEE), the next generation of copper cabling systems will be in shielded technology only.

This article provides background on different shielded cables and will help in  nding the right cable in practice.

Shielding has the same aim in a wide range of industries: appropriate performance to combat electromagnetic interference (EMI) for the desired application and environment.

This is fundamental in any electrical design but was often a controversial issue in the cabling sector, most likely driven by some North American manufacturers.

Shielded twisted-pair cables in several variations have been available for some time, and there is often discussion on which type is best or correct for individual applications.

In principle there are two ways of shielding twisted-pair cables – braiding and foiling. Both have a certain purpose and can be used alone or together.

Braid offers good EMI performance in the lower frequency range, and foil is better in the higher frequency range.

Another factor is mechanical behaviour. Braid allows cable to be  flexible, whereas foil makes it stiff and harder to handle in confined spaces (see Table 1).

With the introduction of screened foiled twisted pair (S/FTP) cables in Europe in 1996, the foil around the pairs has even more influence on transmission parameters, in this case near-end crosstalk (NEXT).

With the introduction of 10Gbps, shielding had a positive influence on NEXT and alien near-end crosstalk (ANEXT).

Cable choice depends on environment, performance class, application and budget.

The naming and coding of cables is standardised in the ISO/IEC 11801 Standard. Letters before the backslash describe the outer construction; subsequent letters describe the inner construction around the twisted pair.

So U/UTP denotes unshielded twisted pair. For S/FTP cable, S denotes foil braiding around the four pairs and F indicates individual foils.

A couple of considerations are necessary for transmission between data centres and of ces, and from harsh environments to areas with strong magnetic  elds.

Healthcare facilities offer a good example of environmental requirements, and an S/FTP installation provides for all areas.

In a normal of office environment, foil over unshielded twisted pair (F/UTP) offers suf cient performance.

As mentioned, coupling attenuation is crucial in making the right choice.

However, be careful – cable construction itself doesn’t say anything about quality. This is determined only in the cable specification sheet. In particular, the braiding of S/FTP cables is a key factor for pricing and overall shielding effectiveness.

Another invisible factor is the overall thickness of the shield. The thicker the shield, the better the EMI performance. Cheaper cables have a very thin foil and therefore much lower EMI performance.

Each shielded cable construction has a certain purpose and characteristic. From a technical prospective, S/FTP cables provide high performance in all settings and F/UTP cables fulfill all basic requirements. They reject the most common shielded cable types.

The F/FTP type is an interesting niche solution, as it is a compromise between S/ FTP and F/UTP.

To reiterate: cable performance cannot be determined by construction only. A 1,500MHz S/FTP cable is currently the only suitable one for 40Gbps. In future, other types may be optimised for this higher performance level.

Thorsten Punke is the Australia and New Zealand head of marketing for the enterprise division of TE Connectivity. He has 13 years’
experience and wide knowledge of copper, fibre, testing and active technology.