In a communication channel, data is transmitted in the form of signals (analog signal). The data transmission is measured in bandwidth. The bandwidth will be higher if more signals can be transmitted. Actually, the bandwidth measures the amount of information that can be transmitted through the media within the given period of time. For analog signals, bandwidth is represented in hertz (Hz). It means number of signals transmitted per second. For digital signals, it is represented in bits per second (bps). Different transmission media have different bandwidths. The higher the bandwidth of the transmission media, the more information can be transmitted.
Types of Communication Channel:
The communication channel or media is divided into two types.
1. Guided Media.
2. Unguided Media.
1. Guided Media:
In guided communication media, communication devices are directly linked with each other via cables or physical media for transmission of data. The data signals are bounded to a cabling media. Therefore, guided media is also called bounded media. The guided media are usually used in LAN. The examples of guided or bounded media are:
Twisted pair wire.
Fiber optic cable.
Twisted Pair Cable: Twisted pair cable is one of the most commonly used communication media. It is used in local area network (LAN) for data communication between different computers. It is also used in telephone lines to carry voice and data signals.
A twisted pair cable consists of a pair of thin diameter copper wires. These wires are covered by insulating material (such as plastic). These pair of wires are twisted together to form a cable. The wires are twisted around each other to minimize (or reduce) interference from other twisted pairs in the cable.
The data transmission speed through twisted pair cable is about 9600 bits per second in a distance of 100 meters. However, this transmission speed is less than coaxial cable or optical fiber.
The twisted pair cable has been the standard communication channel for voice and data communication. But now its use is reducing because today more reliable communication media are available such as coaxial cable, fiber optic cable microwave and satellite.
Coaxial Cable: Coaxial cable is also referred to as Coax. It carries signals of higher frequency ranges than twisted-pair cable. Coaxial cable consists of a single solid copper wire, which is called the inner conductor.
Coaxial cable can be used for telephone lines for voice and data transmission with very high frequency. The bandwidth of coaxial cable is 80 times greater than that of twisted pair media. Coaxial cable is also widely used in local area network (LAN). It is more expensive than twisted-pair wire.
Fiber-Optic Cable: In twisted-pair cable and coaxial cable, data is transmitted in the form of electric frequencies. The fiber optic cable uses light to transmit data. The data transmission speed is very high (because fiber-optic cable uses light to transmit data). The data transmission speed is up to billions bits per second. Today, most of the telephone companies and cable TV operators are using fiber optic cables in their networks.
A fiber optic cable consists of tubes of glass (or thin glass fibers) through which data is transmitted as pulses of light. A typical optical fiber consists of a very narrow strand or fiber of glass called the core. It is thinner than a human hair. The core is surrounded by a concentric layer of glass called Cladding. The diameter of a core is 62.5 microns (I micron = 10-6 meters). The diameter of cladding is about 125 microns. The cladding is coated with insulating material such as plastic, which is called the Jacket.
2. Unguided Media:
In unguided communication media, data is communicated between communication devices in the form of wave. Unguided media provides means to transmit data signals but does not guide them along a specific path. The data signals are not bounded to a cabling media. Therefore,. unguided media is also called unbounded media.
This transmission medium is used when it is impossible to install the cables. The data can be transmitted all over the world through this medium. The examples of unguided or unbounded media are:
Microwaves: In microwave transmission, data is transmitted through air or space, instead of through cables or wires. Microwaves are high frequency radio waves. These waves can only travel in straight lines.
The data is transmitted and received through a microwave station. A microwave station is also called relay station or booster. A microwave station contains an antenna, transmitter, receiver, and other equipments that are required for microwave transmission. Microwave antennas are placed on the high towers or buildings and these are placed within 20 to 30 miles of each other. There may be many microwave stations between the sender and receiver. Data is transmitted from one microwave station to another. Each microwave station receives signals from previous microwave station and transmits to next station. In this way, data is transmitted over larger distances.
The data transmission speed of microwave transmission is up to 150 Mbps. Microwave transmission is used in environments where installing physical transmission media is impossible and where line-of-sight transmission is available, it is used in wide-open areas. Today, it is used by telephone companies, cable television providers, universities etc.
Satellite Communication: A communication satellite is a space station. It receives microwave signals (or messages) from earth stations. Satellite transmission station that can send and receive messages is known as earth station. The earth based stations often are microwave stations. Other devices, such as PDAs and GPS receivers, also functions as earth based stations.
Satellites rotate approximately 22,300 miles above the earth in precise locations. The communication satellite consists of solar powered, transceiver that receives and sends signals. The signals are transmitted from one earth station to the satellite. The satellite receives and amplifies the signals and sends them to another earth station. This entire process takes only a few seconds. In this way, data or messages are transferred from one location to another. Transmitting a signal from ground or earth station to a satellite station in space is called up-linking and the reverse is called the down-linking. The data transmission speed of communication satellite is very high such as upto 1 Gbps.
Different communication satellites are used to carry different kinds of information such as telephone calls, television transmissions, military communication, weather data, and even radio stations use them for broadcasting. The global positioning systems and Internet also use the communication satellites.
Radio Broadcast: It is a wireless transmission medium that is used to communicate information through radio signals in air, over long distance such as between cities and countries.
In this medium, a transmitter is required to send messages (signals) and receiver is required to receive them. To receive the radio signal, the receiver has an antenna that is located in the range of signal. Some networks use a special device called transceiver used to send and to receive messages in the form of radio signals. The data transmission speed of radio broadcast is up to 54 Mbps.
Cellular Radio: Cellular radio is a form of radio broadcast that is used for mobile communications such as cellular telephones and wireless modems. A cellular, telephone is a telephone device that uses high frequency radio waves to transmit voice and digital messages. Some mobile users connect their laptop computer or other mobile devices to a cellular telephone to access the Web, send and receive e-mail etc. while away from a standard telephone line.