PHYSICAL TRANSMISSION MEDIA
Physical transmission media used in communications include twisted-pair cable, coaxial cable, and fiber-optic cable. These cables typically are used within or underground between buildings. Ethernet and token ring LANs often use physical transmission media. One of the more commonly used transmission media for network cabling and telephone systems is twisted-pair cable. Twisted-pair cable consists of one or more twisted-pair wires bundled together (Figure 8-24). Each twisted-pair wire consists of two separate insulated copper wires that are twisted together. The wires are twisted together to reduce noise. Noise is an electri- cal disturbance that can degrade communications.
Coaxial cable, often referred to as coax (pronounced KO-ax), consists of a single copper wire surrounded by at least three layers: (1) an insulating material, (2) a woven or braided metal, and (3) a plastic outer coating (Figure 8-25).
Cable television (CATV) network wiring often uses coaxial cable because it can be cabled over longer distances than twisted-pair cable. Most of today’s computer networks, however, do not use coaxial cable because other transmission media such as fiber-optic cable transmit signals at faster rates.
Fiber-Optic Cable The core of a fiber-optic cable consists of dozens or hundreds of thin strands of glass or plastic that use light to transmit signals. Each strand, called an optical fiber, is as thin as a human hair. Inside the fiber-optic cable, an insu- lating glass cladding and a protective coating surround each optical fiber (Figure 8-26). Fiber-optic cables have the following advantages over cables that use wire, such as twisted-pair and coaxial cables:
Disadvantages of fiber-optic cable are it costs more than twisted-pair or coaxial cable and can be difficult to install and modify. Despite these limitations, many local and long- distance telephone companies are replacing existing telephone lines with fiber-optic cables, enabling them to offer fiber Internet access to home and business users. |
This article is intended to give you a brief overview of the transmission media of computer networks. Let’s see what’s on this topic, A transmission medium is a communication channel that transmits information from the sender to the receiver using electromagnetic signals. The main purpose of this medium is to gradually retrieve data over a local area network (LAN). These electromagnetic signals travel from one place to another through a vacuum, air, water, copper wire, fiber optics, or another medium of transmission. Electromagnetic energy includes power, sound, visible light, radio waves, ultraviolet light, and gamma rays. Factors to consider when choosing a transmission medium, 1. Transmission Rate
2. Cost and Ease of Installation
3. Resistance to Environmental Conditions
4. Distances
There are two main types of network transmission media as guided and unguided.
1. Guided media
The signal strength is driven and directed in a fixed path in the guided media. Used for point-to-point connectors or shared connectors with multiple connections.
ex: Twisted-pair cables, coaxial cables, fiber optic cable
2. Unguided media
The signal energy is transmitted over a wireless medium in unguided media. Used for radio and long-distance broadcasting with the use of microwave connectors.
ex: Radio waves, microwaves, Infrared
Guided media
The twisted-pair cable consists of two insulated copper wires arranged in a regular spiral pattern. Both wire pairs act as a single communication connector. Adjacent pairs are twisted to eliminate crosstalk.
This is the most commonly used guide transmission medium and is the least expensive. There are two types of twisted wire pairs,
Unshielded Twisted Pair — Usually consists of wire and insulators. Have the ability to block interference. These twisted pair cables are used for telephonic applications.
Shielded Twisted Pair — Each pair of insulating wires has a braided wire mesh covered. Gives a better performance with a higher data rate.
Unshielded twisted pair and shield twisted pairCoaxial cables
The center of a coaxial cable is wrapped in a concentric solid or stranded copper wire enclosed in an insulating sheath, which is encased in an external conductor of metal strips, braids, or both. The external metal winding serves as a shield for sound and as a second conductor to complete the circuit. This outer conductor is also encased in an insulating sheath and the entire cable is protected by a plastic sheath.
The coaxial cable carries a higher frequency range than twisted-pair cables.
Coaxial cableFiber optic cable
A fiber optic cable is made of glass or plastic and transmits signals in the form of light with the use of reflection to guide light through a channel. The difference in density of the two materials is so great that a beam of light passing through a core must be reflected from the mantle instead of being refracted into it.
There are two main types of fiber optic cables,
Single-mode cable— A single strand of glass fiber that has one mode of transmission. Have a higher bandwidth, but require a light source with a narrow spectral width.
Multi-mode cable — Gives a high bandwidth at high speeds over medium distances. There are two types of multi-mode cables as step-index and graded-index.
Fiber optic cableUnguided media
Radio Waves
Radio waves use omnidirectional antennas to transmit signals in all directions. This feature of radio waves is useful for multiple transmissions, where there is one sender but many receivers. There are several types of antennas depending on the wavelength, power, and purpose of transmission.
Radio waves are used for AM and FM radio, maritime radio, cordless phones, and televisions. Radio frequencies can be used for direct voice communication within 10 miles.
Radio WavesMicrowaves
Electromagnetic waves with a frequency of 1–300 GHz are called microwaves. Microwave waves require directional antennas that can transmit signals in one direction and they are propagation is line-of-sight.
Microwaves are used for unitary communication such as mobile phones, satellite networks, and wireless LANs. There are two types of antennas used for microwave communication as parabolic dish and horn antenna.
MicrowavesInfrared
Infrared waves with frequencies from 300 GHz to 400 THz can be used for short-distance communication in a closed area using line-of-sight propagation. They cannot penetrate through the barriers that intersect between systems. These are considered as one of the most secure transmission modes due to the short-range.
Infrared is used in TV remotes, wireless speakers, automatic doors, printers, remote controls, wireless mouse, keyboards, and other handheld devices, etc.
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