When choosing what type of cable to choose for your network infrastructure a lot of factors have to be taken into consideration.  The budget is always an important factor, however one must also consider the performance expected from the network infrastructure, the electrical environment in which the network in being installed and the availability of adequate earthing points.  These are just a small number of considerations to be factored in when choosing the type of cabling you will opt for when installing a network.

 

Types of Cable Screening.

There are essentially two types of cable screening:

Overall Screen

The overall screen is screening that lies on the outside of all the pairs within a network cable.  This screening has two main aims; to prevent loss of signal from within the cable to the outside, and to protect the cable from external interference.  Normally this type of screening is either made from aluminium foil or tin plated copper.

Pair Screens

The pair screen is the foil screen around each of the individual pairs within the cables.  This type of screening stops interference between the signal in one pair with another pair in the same cable.  There is also a secondary function: it prevents noise from interfering with other nearby cables, what is termed alien crosstalk.

 

To screen or not to screen?

As a general rule of thumb the higher the frequency of the signal within the cable, the greater the screening needed.  As the frequency rises the signal tends to travel further away from the core of the wire, reaching a point where it is travelling as an electromagnetic field around the core of the cable.  This is essentially an aerial. At much higher frequencies this can create a problem with alien crosstalk, since the signal might seep out of the cable.

 

Unscreened cables are cheaper.  This is mainly due to the use of less materials utilised and lower manufacturing costs.  With low frequency cables in old standards such as Cat 5e, where frequencies reach 250Mhz, screening is not essential, since Alien Crosstalk is not normally expected.   However screening might be required even at these frequencies, if the network cabling is being installed in areas with high electromagnetic interference.

 

There are other options which might allow you not to install screened cables at these frequencies, such as separating the cables from electromagnetic interference sources,  and the utilisation of earthed metal conduit, which will provide screening for the cables passing through it.

 

Cat 6a cables can either be screened or unscreened.  If unscreened several steps need to be taken in order to avoid alien crosstalk.  This includes the use of special non circular extrusions that will assure that cables are not lying parallel to each other.  This normally results in more expensive installation since a lot of testing for alien crosstalk has to be carried out. Screened Cat6a cables tend to be smaller in diameter, thus can be installed in smaller areas.

Due to Cat 7A cables being able to carry frequencies of up to 1000 MHz, they are only available as screened cables.  This mainly due to high potential for alien crosstalk.

 

So do you go for an unscreened cable or a screened cable?  This hugely depends on various factors. When choosing your cabling it is important that all these factors are considered, since choosing the wrong type will cause issues, and might result in higher costs, due to replacements.