When planning a copper network some questions need to be answered like “Do I need a shielded cable or an unshielded will be enough?”.
To deal with this question we need to know the different kinds of shields available in a Twisted pair cable and the structure to name these several shields available and the different types of shields available in a Twisted pair cable
Structure of shields names
- The first X is the general shield of the cable. It involves all the pairs in the cable.
- The second X is the individual shield of each pair.
- TP is twisted pair
Types of twisted pair cables
- U/UTP (Unshielded Twisted Pairs) – Usually called UTP, this is the most common cable used and it has no shield. The only resistance to Electromagnetic interference (EMI) is the twist in each pair that balance the transmission.
The different twist in each pair helps to reduce the crosstalk between pairs.
- F/UTP (Foiled with Unshielded Twisted Pairs) – Also known as FTP, it has a foil of aluminum wrapped around the pairs and a drain wire to redirect the noise to the ground acting as a protection against EMI and RFI.
- S/UTP (Shielded with Unshielded Twisted pairs) – Or STP, has a braid screen around the pairs. This braid provides better protection against EMI or RFI than the foiled. It also gives better mechanical protection to the cable.
- SF/UTP (Shielded cable and foiled with Unshielded Twisted pairs) – This cable has a braid screen and a foil of aluminium wrapped around the pairs. It provides a better shield and ground than S/UTP.
- U/FTP (Unshielded with foiled Twisted Pairs) It has an aluminium foil wrapped around each pair. It provides same protection against EMI and RFI, and great protection against crosstalk from other pairs in the same cable.
- F/FTP (Foiled with Foiled Twisted Pairs) – similar to F/UTP but with an additional foil wrapped around each pair to increase the protection against crosstalk between pairs in the same cable.
- S/FTP (Shielded cable with Foiled Twisted Pairs) – Similar to F/FTP with a braid screen instead of the foil. It gives a better protection to EMI and RFI and some mechanical protection to the cable.
- SF/FTP (Shielded cable and foiled with Foiled Twisted Pairs) – The best protection against EMI and RFI, Crosstalk and alien-crosstalk. It as a braid screen and a foil around all pairs and a foil tape in each pair.
Why use a shielded cable?
There are 3 types of noise in the cable:
- Noise induced from other pairs in the same cable. (x/Xtp)
- Noise induced from the environment (other cables in the same pathway), or Aliencrosstalk (X/xtp), and the radio frequencies in the environment.
- Noise induced by the difference in the ground potential in each end of the cable.
These noises will interfere in the communication if they get close to the strength of the original signal.
If it is expected to have a big environmental noise (industrial environment) a F/UTP or even an SF/UTP should be the choice. To prevent the interference of a pair in the others in the same cable (NEXT or FEXT), the choice should be a U/FTP.
If the installation has a specific ground connection connected to the cabinet, and the electric power network, connected to the workstation, as a different ground connection. If these two ground connections have different voltage potentials, it will induce a noise in the ground loop.
This noise can only affect the shield and/or foil but, in many cases, appears also in the twisted pairs.
When this problem appears, one easy solution is to use an Unshielded RJ 45 connector and patchcords, at the workstation, isolating the entire system from the electrical Ground but keeping the telecommunication ground in all circuit.
The Antenna Effect
A common disadvantage appointed to the shielded cables is that they act as antennas and attract the signals in the surrounding environment, or even radiate the signal traveling in the cable pairs.
In reality, both shield and twisted pairs act like an antenna but the foil shield is super effective to frequencies higher than 30MHz because of the thickness of this foil and the Skin effect, and the balanced twist of the pairs is effective to lower than 30MHz frequencies.
So, the UTP cable works more like an antenna than a shielded cable.
Advantages of a Shielded infrastructure
- Lower crosstalk between pairs in the same cable
- Lower Alien-crosstalk
- Great increase in noise reduction especially in frequencies over 30MHz
- Increase SNR (signal to noise ratio)
- Decrease BER (Bit error ratio)