3.2 Guided Transmission Media

Waves are guided along solid medium

Twisted Pair

• Pair of twisted conductors

• Twisting reduces interference (two parallel wires constitute a simple antenna; a twisted pair does not.)
• Cheap medium
• Commonly used for communications within buildings and in telephone networks
• Produced in unshielded (UTP) and shielded (STP) forms, and in different performance categories.
• Cables may hold hundreds of pairs. Neighbor pairs typically have different twist lengths to reduce crosstalk.

Coaxial Cable

Pair of conductors separated by insulation

• Offers longer distances and better speeds than twisted pair, due to better shielding.
• Used for cable TV and local-area networks. Had been widely used in telephone systems, but optical fibre is now assuming this task.

Baseband Coaxial Cable 50-ohm cable, commonly used for digital transmission.

Broadband Coaxial Cable 75-ohm cable, commonly used for analog transmission.

Optical Fibre

• Three components: light source, transmission system, and a detector

• The detector generates an electric pulse when hit by light

• 1-a pulse of light; 0-missing pulse of light.

• optical rays travel in glass or plastic core

• When light move from one medium to another it bend at the boundary. The amount of bending depends on the properties of the media.

• Light at shallow angles propagate along the fibre, and those that are less than critical angle are absorbed in the jacket
• The cladding is a glass or plastic with properties that differ from those of the core
• Used in long distance communication, in locations having small amount of space, and with reduction in price is starting to get also to LANs.
• Not affected by external electromagnetic fields, and do not radiate energy. Hence, providing high degree of security from eavesdropping.
• Provide for multimode of propagation at different angles of reflections. Cause signal elements to spread out in time, which limits the rate in which data can be accurately received.
• Reduction of the radius of the core implies less reflected angles. Single mode is achieved with sufficient small radius.
• A multimode graded index transmission is obtained by varying the index of reflection of the core to improve on the multi mode option without resolving to the cost of single mode. (index of reflection=speed in vacuum / speed in medium.)
• 1 Gbps is the current limitation, with the bottle neck in the conversion from electrical to optical signals. Large improvements are expected.

Comparison

[next] [prev] [prev-tail] [front] [up]