What is the purpose of outer sheath in FTP cable, i.e. individual pairs already have the shielding and thus both crosstalk between the pairs and alien crosstalk should be eliminated?
In addition, how is screening(for example SF/UTP or S/FTP cable types) different from foiling(for example F/FTP type)?
Best Answer
It appears to be the difference in the level of blocking EMI for the reason these different cables are made.
Copper braided shields are easiest to work with (crimping and connecting), but only cover about 70% of the area. Foil shields are more difficult to work with, but due to higher resistance, aluminum foil compared to copper braids aren't as good at blocking EMI. For greater EMI protection than either of them can provide alone, they're used together. If you're in a very noisy environment or you can't afford to have corrupted signals, you'll want to go the most expensive route. I can imagine vehicle connectivity being done with something like this as safety is a primary concern.
In summary, aluminum foil is the cheapest and worse protection from EMI. Copper braid comes next for usability and blocking ability. Lastly, both together offer the best protection.
For more details please see this reference.
Note that I assume foil screen is very similar to copper braid. It may be made out of aluminum though.