transmission media

Means by which a communication signal is carried from one system 

to another (for example, twisted-pair wire, co axial cable, fiber-

optics cable) and wireless links (for example, satellite, microwave 

and radio and infrared systems).

Transmission medium and physical layer

There are two categories of transmission media used in computer communications:

• BOUNDED/GUIDED MEDIA
• UNBOUNDED/UNGUIDED MEDIA

Classes of transmission media

 GUIDED MEDIA: 

Guided media, which are those that provide a conduit form one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. twisted-pair and coaxial cable use metallic conductors that accept and transport signals in the form of electric current. optical fiber is a glass or plastic cable that accepts and transports signals in the form of light.  

Types of guided media:

1. Twisted-pair cable
2. Coaxial cable
3. Fiber-optic cable       

    TWISTED-PAIR CABLE:

The least-expensive and most widely-used guided transmission medium is twisted pairs. A twister pair consists of two conductors (normally copper) .Each with its own plastic insulation,twisted together as shown in fig 
   

Types of twisted-pair cable:

1. Shielded twisted-pair cable (UTP)
2. Unshielded twisted-pair cable (STP)

They are mostley connected by RJ-45. Twisted Pair cables are used in telephone lines to provide voice and data channels. In LAN's, such are Base -T end 100 Base-T also use twisted pair cables.

COAXIAL CABLE:

Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. The inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material. The outer conductor is covered with a jacket or shield. A single coaxial cable has a diameter of from 0.4 to about 1 in. Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than is twisted pair. Coaxial cable can be used over longer distances and supports more stations on a shared line than twisted pair.

Categories of coaxial cables

connector used for it is BNC connector, BNC-T connector, BNC terminator. It is used in analog telephones and cable TV connection.

BNC connectors

OPTICAL FIBER:

Optical fiber is made of glass or plastic and transmit signals in the form of light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse of light through this cable to other device. These pulses translated into “1’s”and “0’s” at the other end.

Propagation modes of Optic Fiber

Modes

connector used in fiber optic cables is MT-RJ connector

Fiber-optic cable connectors

UNGUIDED MEDIA:

Unbounded / Unguided media or wireless media doesn't use any physical connectors between the two devices communicating. Usually the transmission is send through the atmosphere but sometime it can be just across the rule. Wireless media is used when a physical obstruction or distance blocks are used with normal cable media.

RADIO WAVES:

Radio waves frequency are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio waves also are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically.

Radio Frequency includes the following types-

          (i) Short wave used in AM radio.

          (ii) Very high frequency used in FM radio and TV.

          (iii) Ultra high frequency used is TV.

MICROWAVES:

Microwaves are unidirectional, when an antenna transmits microwaves they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas.

On the other hand microwaves.

Propagation is line-of-sight. Since the towers with the mounted antennas needs to be in direct sight of each other, towers that are for apart need to he very tall, the curvature of the earth as well as other blocking obstacles do not allow two short towers to communicate using microwaves, Repeaters are often needed for long distance communication very high frequency microwaves cannot penetrate walls.

INFRARED WAVES:

Unguided infrared and millimeter waves are widely used for short-range communication. The remote controls used on televisions, VCRs, and stereos all use infrared communication. They are relatively directional, cheap, and easy to build but have a major drawback: they do not pass through solid objects (try standing between your remote control and your television and see if it still works). On the other hand, the fact that infrared waves do not pass through solid walls well is also a plus. It means that an infrared system in one room of a building will not interfere with a similar system in adjacent rooms or buildings.