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A path through which information are transmitted fl-nm one place to another is called communication channel. It is also referred to as communication medium or link. In network communication, the communication media’ are wires, cables arid other means through which information travels from its source to its destination. The examples of communication media are twisted-pair cable, coaxial cable, fiber-optic cable and wireless links. In communication channel, data is transmitted inthe 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 bandwidth. The higher the bandwidth of the transmission media, the more information can be transmitted.
Transmission media are divided into:
- Physical Transmission Media
- Wireless Transmission Media
Physical Transmission Media
In Physical Transmission Media, communication devices are directly linked with each other via cables or other physical materials for data communication. These are also referred to as wired channels. The physical transmission media are usually used in LAN.
Physical transmission media used in transmission are:
- Twisted-Pair Cable
- Coaxial Cable
- Fiber-Optic Cable
I. Twisted-Pair Cable
Twisted-pair cable is one of the most commonly used communication media for network cabling. It consists of two separate copper wires that are covered by insulating material such as plastic and are twisted together to form a cable. One of the wires is used to carry signals to the receiver, and other is used only as ground reference.
The wires are twisted together to reduce the noise. The noise is created due to high-voltage equipment or due to light energy of sun. The noise creates disturbance in the signal when it is received. It means that noise creates problem in communication.
The telephone cable wiring at wire is often called the telephone wire. 10 Mbps (Mega bits per second) to 100 home uses twisted-pair wire, so this type of The data transmission speed of twisted pair is Mbps or up to 1Gbps (Giga bits per second).
Twisted Pair
II. 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. Usually, the inner conductor is surrounded by three layers:
- An insulting material, which covers the inner conductor.
- An outer conductor of solid metal foil (shield). The outer conductor (shield) serves both as a shield against noise and as a second conductor, which completes the circuit.
- An insulting material, which covers the outer conductor.
The whole cable is protected by an external plastic cover. The structure of coaxial cable is shown in figure below.
Coaxial Cable
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 twisted pair media, Coaxial cable is also widely used in local area network (LAN). It is more expensive than twisted-pair wire. The main advantages of coaxial cable are:
- It creates less noise and protects the cable from interference of external electromagnetic waves. Therefore coaxial cable has low transmission error rates.
- These cables can transmit data over longer distances than that of twisted pair.
The data transmission speed of coax is from 10 Mbps to 200 Mbps. Now-a-days, this transmission medium is not used in computer network because other transmission media such as fiber-optic cables have higher transmission speed.
III. Fiber-Optic Cable
In twisted-pair cable and coaxial cable, data is transmitted in the form of electric frequencies. So copper wire has to be protected from water and electromagnetic waves. In fiber optic cable, these types of problems are not occurred. A fiber optic cable is made of glass or plastic and transmits signals in the form of light. 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 as thin as a human hair). The core is surrounded by a concentric layer of glass called Cladding. The diameter of a core is 62.5 microns. (1 micron l0-6 meters). The diameter of cladding is about 125 microns. Optic-fiber cable uses reflection phenomena to guide light through a channel. The data transmission in fiber optic cable is shown below.
Fiber Optic
However, the fiber-optic cable is becoming more popular. Now-a-days, telephone and television companies are replacing their existing telephone and coaxial cables with fiber-optic cables.
Advantages
The major advantages of fiber-optical media are:
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.
- It has high level of security.
- It is not affected by electromagnetic waves.
- It is more reliable and has lower data transmission errors.
Disadvantages
The disadvantages of fiber optic cable are:
- It is more costly.
- It is difficult to install and modify.