General description

Twisted pair is the cheapest and most common guided transmission media there is today. It's mostly used in LAN (Local Area Networks) and in telephone networking, as the medium that connects the host (the phone itself at your home or office) and the local telephone center (this part of the network is called the subscriber loop).
It is the most common media because it is very cheap to produce and it is easy to install and to work with (very flexible and thin).

Physical description

As its name suggests, the "Twisted Pair" is made of two copper wire twisted together. One wire transfers the signal voltage level and the other transfers the ground voltage level. The signal that is received at the receiver is the difference between the two wires.

There are two types of twisted pairs:

1. UTP – Unshielded Twisted Pair: a regular twisted pair as described above.
2. STP – Shielded Twisted Pair: Like UTP but each pair is shielded with metal shielding (invented by IBM).

Attenuation and bandwidth

As mentioned before, the attenuation of twisted pair is proportional in the thickness of the wire and in the square root of the frequency.
Twisted pair cables of different categories allow different bandwidths ranging from 1Mhz to 600Mhz.
The transfer rate on a twisted pair is different for different category cables, it varies from 10Mbps up to 10Gbps.
Because of its nature a twisted pair must have a repeater every few kilometers, if used to transfer analog data (like voice for phone network) it's enough to place a repeater every 5km, but if used to transfer digital data it requires a repeater every 2km. That is why twisted pair is mostly used for LAN, where it isn't used to cover great distance.

Distortion and noise

The twists in the twisted pair exists to cancel noise, the following lines describe how it works.
Whenever the pair is subjected to an interference, the signals on both wires are incremented/decremented by approximately the same amount of noise. The receiver measures the difference between the wires which is unaffected by the noise terms that end up canceling each other out. The twisting ensures the two wires are at the same distance from any noise source. That is why the more twists a pair has, the better its immunity to noise is.
Of course the shield in the STP also protects against noises therefore the STP is more immune to noise than the UDP.

Twisted pair categories

Over the years twisted pair evolved and changed to accommodate the requirements for faster data rate and and better noise immunity. Therefore several twisted pair categories were created over the years each one better than its predecessor. They were named Cat 1 – Cat 7. Cat 1 and Cat 2 are very old and aren't used in digital networks anymore. Cat 6 and Cat 7 are brand new and haven't been widely adopted yet.

The following table summarizes the different categories' attributes, these are typical numbers that may vary from one manufacture to another.

Category	Frequency	Data rate	Thickness	UTP/STP	Twists per meter
Cat 3	16Mhz	10Mbps	0.5106mm	UTP	12
Cat 4	20Mhz	16Mbps	0.5106mm	UTP
Cat 5	100Mhz	100Mbps	0.5106mm	UTP/STP	140
Cat 6	250Mhz	1Gbps	0.5106mm	UTP/STP
Cat 7	600Mhz	10Gbps	0.5733mm	STP

Applications

Telephone – Usually the cable that run between the user (your home or office for example) to the phone company closest station are twisted pair cables, because voice data requires very little bandwidth and the twisted pair cables are very cheap to install (you need to wire every house to the telephone exchange). Usually in office buildings and residential buildings, a network of twisted pair cables connects each of the houses/offices to the nearest phone station. The phone station is in turn connected to the next phone station by fiber optic cables.

LAN – Nearly every wired LAN in the world uses twisted pair media. Not just because it very easy to install and use but also because it's very cheap compared to any other media. These days most LANs consist of several hosts (PCs) connected to one router using a 1Gbps or 100 Mbps transfer rate.
Because LANs are short ranged networks, usually in a building or a complex, there is never a big distance between one network device to another. It's therefore possible to transfer a high frequency signal on top of the twisted pair with very little noise.
When twisted pair is used for LAN, multiple twisted pairs contained in a single cable are used to connect between network devices. This enables us to to achieve higher bit rate over inferior twisted pair categories, for example Cat 5 can go up to 1Gbps using 4 twisted pairs and Cat 3 can achieve up to 100Mbps with 3 twisted pairs.