Guided Vs Unguided Transmission Media

Classes of transmission media

1. Twisted pair consists of two conductors each surrounded by an insulating material.

Technical Focus: Why Twisted-Pair Cable Reduces NoiseInterference from devices such as a motor can create

unequal noise over two parallel lines. The line that is closer to the device receives more interference than the one that is farther. If however, the wires are twisted around each other at regular intervals, each wire is closer to the noise source for half of thetime and farther for the other half. Both receive the same amount of noise. The noise from two wires cancel each other out at the receiver.

Technical Focus: Categories of Twisted-Pair CablesCategory 1: The basic cabling used for voice

Category 2: Suitable for voice and data up to 4 MbpsCategory 3: Suitable for data up to 10 MbpsCategory 4: Suitable for data up to 16 MbpsCategory 5: Suitable for data up to 100 Mbps

Business Focus: Use of UTP in Telephone NetworkUTP [Unshielded twisted pair ]In the early days of the

telephone network, the local loop, the two wires that connect a residence to the telephone office, was made of two parallel wires. The telephone companies later changed the local loop to a pair of twisted-pair wires, which is less prone to noise than the previous parallel wires.

Shielded twisted-pair cable

Technical Focus: Shielding and Lightning

We can be immune from lightning in a desert if we are surrounded by a metallic cover (inside a car, for example).

The reason is that electromagnetic energy (lightning) can penetrate insulators, but not metals. Lightning on a metallic shell creates a current on the outside surface, which can be dangerous; the inside is protected. Note that this phenomenon is opposite from what happens with electricity. The electrical current passes through a metal, but it is stopped by an insulator.

Electromagnetic energy passes through an insulator, but isstopped by a metal. The same idea is used in shielded twisted-pair cable. The shield is a metal that protects the cable from outside noise, electromagnetic interference.

2. Coaxial cable is the kind of copper cable used by cable TV companies between the community antenna and user homes and businesses. Coaxial cable is sometimes used by telephone companies from their central office to the telephone poles near users. It is also widely installed for use in business and corporation Ethernet and other types of local area network.

Figure 7-4

Coaxial cable

Figure 7-6

Propagation modes

Figure 7-5

Refraction and reflection

Advantages of Optical Communications

Fundamentals of Fiber Optic System’s

Radio waves are used for multicast communications, such as radio and television, and paging systems. They can penetrate through walls.

Highly regulated. Use Omni directional antennas

1. Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

2. Higher frequency ranges cannot penetrate walls.3. Use directional antennas - point to point line of sight communications - Unidirectional antennas

Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.

Unguided Transmission - Technical Focus:

Also called wireless communication. It transports electromagnetic Waves without using a physical conductor. Signals are broadcast through the air (in some cases water). There are available to anyone who can receive them.

Radio Frequency Allocation - the selection of the electromagnetic spectrum defined as radio communication is divided into 8 ranges, called bands, each regulated by government. These bands are rated from very low frequency (VLF) to very high frequency (EHF)

VLF and HF - this propagate very close to the surface. MF and HF - upper layers and then reflect back to the earth VHF and UHF - transmitted using line of sight - EHF and SHF - space propagation.