Prerequisite : Types of Transmission Media
Transmission media can be divided into two parts Guided Media and Unguided Media. In guided media, the signal is contained within the physical limits of the transmission medium and is directed along with it.
Twisted Pair Cables :
These are a type of guided media. It was invented by Alexander Graham Bell. Twisted pair cables have two conductors that are generally made up of copper and each conductor has insulation. These two conductors are twisted together, thus giving the name twisted pair cables.
One of the conductors is used to carry the signal and the other is used as a ground reference only. The receiver uses the difference of signals between these two conductors. The noise or crosstalk in the two parallel conductors is high but this is greatly reduced in twisted pair cables due to the twisting characteristic. In the first twist, one conductor is near to noise source and the other is far from the source but in the next twist the reverse happens and the resultant noise is very less and hence the balance in signal quality is maintained and the receiver receives very less or no noise. The quality of signal in twisted pair cables greatly depends upon the number of twists per unit length of the cable.
Twisted Pair Cables are further of two types :
1. Unshielded Twisted Pair Cables (UTP) :
These are a pair of two insulated copper wires twisted together without any other insulation or shielding and hence are called unshielded twisted pair cables. They reduce the external interference due to the presence of insulation. Unshielded twisted pair cables are arranged in pairs so that we can add a new connection whenever required. The DSL or telephone lines in our houses have one extra pair in them. When UTP are arranged in pairs, each pair is coded with a different color as defined by the 25-pair color code developed by AT&T Corporation. The Electronic Industries Association divides UTP into 7 categories based on some standards. Categories are based upon cable quality where 1 is the highest quality and 7 is the lowest quality. Each cable in a category is put to a different use as needed.
- These cables are cost-effective and easy to install owing to their compact size.
- They are generally used for short-distance transmission of both voice and data.
- It is less costly as compared to other types of cables.
- The connection established using UTP is not secure.
- They are efficient only for a distance up to 100 meters and have to be installed in pieces of up to 100 meters.
- These cables have limited bandwidth.
2. Shielded Twisted Pair Cables (STP) :
These types of cables have extra insulation or protective covering over the conductors in the form of a copper braid covering. This covering provides strength to the overall structure of the cable. It also reduces noise and signal interference in the cable. The shielding ensures that the induced signal can be returned to the source via ground and only circulate around the shield without affecting the main propagating signal. The STP cables are also color-coded like the UTP cables as different color pairs are required for analog and digital transmission. These cables are costly and difficult to install.
- They are generally used for long-distance communication and transmission and are installed underground.
- The protective shield prevents external electromagnetic noise penetration into the cable.
- They have a higher bandwidth as compared to UTP.
- These cables are very expensive.
- They require a lot of maintenance which increases the cost more.
- These can be installed underground only.
- The length of the segment is similar to UTP for these cables.
Applications of Twisted pair cables :
- Twisted Pair cables are used in telephone lines to provide data and voice channels.
- The DSL lines make use of these cables.
- Local Area Networks (LAN) also make use of twisted pair cables.
- They can be used for both analog and digital transmission.
- RJ-45 is a very common application of twisted pair cables.