One of the oldest and still most common transmission media is twisted pair.
Overview of twisted pair:
- Twisted pair consists of two insulated copper wires, typically about 1 mm thick.
- The wires are twisted together in a helical form, just like a DNA molecule.
- Twisting is done because two parallel wires constitute a fine antenna.
- When the wires are twisted, the waves from different twists cancel out, so the wire radiates less effectively.
- The most common application of the twisted pair is the telephone system.
- Nearly all telephones are connected to the telephone company (Telco) office by a twisted pair.
- Twisted pairs can run several kilometers without amplification, but for longer distances, repeaters are needed.
- When many twisted pairs run in parallel for a substantial distance, such as all the wires coming from an apartment building to the telephone company office, they are bundled together and encased in a protective sheath.
- The pairs in these bundles would interfere with one another if it were not for the twisting.
- In parts of the world where telephone lines run on poles above ground, it is common to see bundles several centimeters in diameter.
- Twisted pairs can be used for transmitting either analog or digital signals.
- The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits/sec can be achieved for a few kilometers in many cases.
- Due to their adequate performance and low cost, twisted pairs are widely used and are likely to remain so for years to come.
Varieties of twisted pair cables:
- Twisted pair cabling comes in several varieties, two of which are important for computer networks.
- Category 3 twisted pairs consist of two insulated wires gently twisted together.
- Four such pairs are typically grouped in a plastic sheath to protect the wires and keep them together.
- Prior to about 1988, most office buildings had one category 3 cable running from a central wiring closet on each floor into each office.
- This scheme allowed up to four regular telephones or two multiline telephones in each office to connect to the telephone company equipment in the wiring closet.
- Starting around 1988, the more advanced category 5 twisted pairs were introduced.
- They are similar to category 3 pairs, but with more twists per centimeter, which results in less crosstalk and a better-quality signal over longer distances, making them more suitable for high-speed computer communication.
- Up-and-coming categories are 6 and 7, which are capable of handling signals with bandwidths of 250 MHz and 600 MHz, respectively (versus a mere 16 MHz and 100 MHz for categories 3 and 5, respectively).
- All of these wiring types are often referred to as UTP (Unshielded Twisted Pair), to contrast them with the bulky, expensive, shielded twisted pair cables IBM introduced in the early 1980s, but which have not proven popular outside of IBM installations.
- Twisted pair cabling is illustrated in Fig. 1.1
Figure 1.1 (a) Category 3 UTP. (b) Category 5 UTP.