In Telecommunication, transmission can be divided into two broad categories: Guided and Unguided. Guided Media includes twisted pair, Coaxial cables, and fiber-optic cable. Unguided media is free space.

(i) Coaxial Cables:

Coaxial cable carries signals of higher frequency ranges than those in twisted pair cable, in part because two media are constructed differently. Instead of having two wires, coax has a central core conductor of solid or standard wire enclosed in insulating sheath which in turn, encased in outer conductor of metal foil, braid, or a combination of two. The outer metallic wrapping serves both as a shield against noise and as a second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by plastic cover.

The wire mesh protects the wire from Electromagnetic Interface(EMI). It is often called a shield. A tough plastic jacket forms the cover of the cable providing insulation and protection.

• The important application is cable modem termination system.
• One important application is Ethernet LAN using coaxial cable. The co-axial cable is used for its large bandwidth and high noise immunity.

Characteristics:

• It has a large bandwidth and low losses.
• This cable is suitable for point to point or multipoint applications. In fact this is the most widely used medium for local area network.
• Due to shield provided, this cable has excellent noise immunity.

Applications:

• Analog telephone network
• Cable TV
• Digital transmission
• Traditional Ethernet connection

(ii) Twisted Pair Cables:

A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together as shown in above figure. One of the wires is used to carry signals to the receiver and other is used only as a ground reference. The receiver uses the difference between the two. In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are in different locations relative to the noise or crosstalk sources (eg.one is closer other is farther). This results in difference at the receiver. By twisting the pairs, a balance is maintained. For example, suppose in one twist one wire is closer to the noise source and other is farther, in the nest twist the reverse is true. Twisting makes it possible that both wires are equally affected by the external influences. This means that the receiver calculates the difference between the two, receives no unwanted signals. The unwanted signals are mostly cancelled out. It has two types: UTP (unshielded twisted pair) and STP (shielded twisted pair).

Applications:

• In the telephone loop.
• In the DSL line (ADSL).
• Local area networks such as 10-Base-T and 100 Base-T.
• In telephone lines to carry voice and data channels.

(iii) Optical Fiber cables:

A fiber optic cable is made up of glass or plastic and transmits signals in the form of light. To understand optical fiber, we need to explore several aspects of light. Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light travelling through a substance suddenly enters another substance, the ray changes direction. The construction of optical fiber cable is comprising of an inner glass core surrounded by a glass cladding which has a lower refractive index. Digital signals are transmitted in the form of intensity- modulated light signal which is trapped in the glass core.

Advantages:

• Small size and light weight: The size of the optical fibres is very small. Therefore a large number of optical fibres can fit into a cable of small diameters.
• Easy availability and low cost: The material used for the manufacturing of optical fibres is the “Silica glass”. This material is easily available.
• No electric or electromagnetic interference: Since the transmission takes place in the form of light rays the signal is not affected due to any electrical or electromagnetic interference.
• Large Bandwidth: As the light rays have a very high frequency in the GHz range, the bandwidth of the optical fibre is extremely large.