Practical 13

Introduction of Transmission Media

Transmission media can be defined as physical path between transmitter and receiver in a data transmission system. Transmission media are of two types.

Guided

Unguided

Guided: Transmission capacity depends mainly on the medium, the length, and whether the medium is point-to-point or multipoint (e.g. LAN). Examples are co-axial cable, twisted pair, and optical fiber.

Twisted Pair Cable:

• The oldest, least expensive, and most commonly used media           
• The two wires are typically ``twisted'' together in a helix to reduce interference between the two conductors.
• Twisting decreases the interference between adjacent pairs in a cable (two straight    parallel wires tend to act as an antenna and pick up extraneous signals.)
• Typically, a number of pairs are bundled together into a cable by wrapping them in a tough protective sheath.
• Can carry both analog and digital signals..
• Quite highly susceptible to noise & interference.

Twisted Pair Cable

Typical characteristics:

• The data rates upto 1 Mb/s can be achieved for the maximum transmission distance of 1 km.
• For analog voice signals, amplifiers are required about every 6 Km and for digital signals, repeaters are needed for about 2 Km.
• To reduce interference, the twisted pair can be shielded with metallic braid. This type of wire is known as Shielded Twisted-Pair (STP) and the other form is known as Unshielded Twisted-Pair (UTP).
• Use: Telephone System, Point-to-point, LAN.

Coaxial Cable:

With ``coax'', the medium consists of a copper core surrounded by insulating material. The insulator is encased by a cylindrical conductor (copper mesh shield) which is again covered with the plastic protective sheath (outer insulation). 

  

                                                                                                                                                                                                                                                                                                               

Coaxial Cable

Co-axial cable has better shielding then twisted pair, so it can span longer distance at higher speeds. Two types of coaxial cable are widely used. One kind, 50 ohm cable commonly used for digital transmission and the other kind , 75 ohm cable commonly used for analog transmission.

Characteristic of Coaxial Cable

• Most versatile medium
• LANs, Cable TV, Long-distance telephones, VCR to TV connections
• Noise immunity is good
• Very high channel capacity
• Operates at distances up to 100 km (metropolitan area).
• Need repeater/amplifier every few kilometer 

Fiber Optics

In fiber optic technology, the medium consists of a hair-width strand of silicon or glass, and the signal consists of pulses of light. For instance, a pulse of light means “1'', lack of pulse means “0”. It has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket.

The core, innermost section consists of a single solid dielectric cylinder. The core is surrounded by a solid dielectric cladding of refractive index. As a consequence, the light is propagated through multiple total internal reflections. The core material is usually made of ultra pure fused silica or glass and the cladding is either made of glass or plastic. The cladding is surrounded by a jacket made of plastic. The jacket is used to protect against moisture, abrasion, crushing and other environmental hazards.

Three components are required:

1.      Fiber medium: Current technology carries light pulses for tremendous distances (e.g., 100s of kilometers) with virtually no signal loss.

2.      Light source: typically a Light Emitting Diode (LED) or laser diode. Running current through the material generates a pulse of light.

3.      A photo diode light detector, which converts light pulses into electrical signals.

Advantages:

•          Very high data rate, low error rate. 1000 Mbps (1 Gbps) over distances of kilometers common..

·         Difficult to tap, which makes it hard for unauthorized taps as well?

·         Much thinner (per logical phone line) than existing copper circuits. Because of its thinness, phone companies can replace thick copper wiring with fibers having much more capacity for same volume.

•          Not susceptible to electrical interference (lightning) or corrosion (rust).
•          Greater repeater distance than coax.
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