CSCI 4550/8556 Computer Networks Comer, Chapter 4: Transmission Media

Data transmission – basic ideas At the origin, encode data as energy. Then transmit the energy to the destination. At the destination, capture the transmitted energy and decode it back into data. The form of energy used for transmission can be electrical, light, electromagnetic (radio waves), sound,.... Each form of energy has different properties and requirements for transmission.

Transmission media Transmitted energy is carried through some sort of medium. The transmitter encodes data as energy and transmits it through the medium. Special hardware is needed for data encoding. A hardware connection to the transmission medium is required. Media can be copper, glass, air,...

Copper wires Twisted pair uses two wires: Coaxial cable includes shield for improved performance

Glass fibers (“fiber optics”) A thin glass fiber carries light with encoded data. A plastic jacket allows limited bending of the fiber without breaking. Fiber is very clear and is designed to reflect light internally for efficient transmission. A light emitting diode (LED) or laser injects light into fiber. Light sensitive receiver (phototransistor) at other end translates light back into data.

Electromagnetic (radio) Data can be transmitted using radio waves. Energy travels through the air rather than copper or glass Conceptually similar to radio, TV, cellular telephones. Can travel through walls and through an entire building. Can be long distance or short distance Long distance with satellite relay Short distance - wireless computer network

Wireless examples Wireless bridge and antenna (right) Wireless transceiver, antenna, and RS-232 cable (below)

Wireless example Remote station (laptop) interface

Microwaves Microwaves are high frequency radio waves. They are unidirectional (focused by dish antennas), for point-to-point communication. Antennas mounted on towers relay transmitted data.

Infrared light Infrared light transmits data through the air. It is similar to the technology used in TV remote controls. IR can propagate throughout a room (bouncing off surfaces), but it will not penetrate walls. It is becoming common in personal digital assistants.

Laser (coherent light) Laser light is unidirectional, like microwaves. It can transmit data faster than microwaves. It uses a laser transmitter and photo-sensitive receiver at each end. It is used for point-to-point communication, typically between buildings. Laser beams can be adversely affected by weather.

Choosing a medium Copper wire is a mature technology, rugged and inexpensive; maximum transmission speed is limited. Glass fiber (compared to copper): higher speed more resistant to electro-magnetic interference spans longer distances requires only single fiber (for one-way communication) more expensive and less rugged Radio and microwave don't require physical connections. Radio and infrared can be used for mobile connections. Laser also does not need physical connection and supports higher speeds than copper.

Media in use at UNO Copper/fiber for long-distance connection to Internet Fiber between buildings Copper, fiber, and wireless within buildings