02-04-2011, 04:28 PM
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Transmission Media
Transmission media can be divided into two broad categories
• Guided Media
• Unguided Media
Guided Media
• Guided Media, Some thing physically , include twisted-pair cable, coaxial cable, and fiber-optic cable.
• Twisted-pair cable and coaxial cable use metallic (copper).
• Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light.
Twisted-Pair Cable
Unshielded Twisted-Pair (UTP) Cable:
• Unshielded Twisted-Pair (UTP) Cable is the most common type of telecommunication medium in use today.
• Frequency range is suitable for transmitting both data and voice.
• T.P consists of two conductors, each with its own colored plastic insulation.
UTP Connectors
• Connectors are either male (the plug) or female (the receptacle).
• Each wire in a cable is attached to one conductor (or pin) in the connector.
• RJ45 most frequently used plugs connector with eight conductors.
Shielded Twisted-Pair (STP) Cable
• Shielded Twisted-Pair (STP) Cable has a metal foil or braided-mesh covering that encases each pair of insulated conductors.
• Metal Casing prevents the penetration of electromagnetic noise.
• Eliminate Crosstalk, undesired effect of one channel to another channel.
Electronic Industries Association (EIA) Categories
• Category 1: The basic twisted-pair cabling used in telephone systems, good for voice but low speed data communication. (56 kbps)
• Category 2: The next higher grade, suitable for voice and for data transmission of up to 4 Mbps.
• Category 3: Required to have at least three twists per foot and can be used for data transmission of up to 10 Mbps.
• Category 4: At least three twists per foot, to bring the transmission rate to 16 Mbps.
• Category 5: Used for data transmission up to 100 Mbps.
Coaxial Cable
• Coaxial Cable has a single copper conductor a plastic layer provides insulation. Coaxial has a central core conductor of solid copper enclosed in an insulating sheath.
• Carries signals of higher frequency ranges than twisted-pair cable.
• Different coaxial cable designs are categorized by their radio government (RG) rating. Each RG number denotes a unique set of physical specifications, thickness, type of inner insulator, the construction of the shield, and the size and type of the outer casing.
• RG-8 is used in the thick Ethernet.
• RG-9 is used in the thick Ethernet.
• RG-11 is used in the thick Ethernet.
• RG-58 is used in the thin Ethernet.
• RG-59 is used for TV.
• Terminators are required for bus topologies where one main cable acts as a backbone with branches to several devices but does not itself terminate in a device.
Optical Fiber
• Optical Fiber is made of glass or plastic and transmits signals in the form of light.
• Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. The cost of fiber optic cabling is comparable to copper cabling; however, it is difficult to install.
• Light, a form of electromagnetic energy, travels at 300,000 kilometers/second. The speed decreases as the medium through which the light travels becomes denser.
• Advantages
– Noise resistance
– Less signal attenuation
– Higher bandwidth
• Disadvantages
– Cost
– Installation/maintenance
– Fragility
• Refraction
If a ray of light traveling through one substance suddenly enters another (more or less dense) substance, its speed changes abruptly, causing the ray to change direction. This change is called refraction.
• Angle of Incidence
When light travels into a more dense medium, the angle of incidence is greater than the angle of refraction; and when light travels into a less dense medium, the angle of incidence is less than the angle of refraction.
• Critical Angle
When the change in the incident angle results in a refracted angle of 90 degrees, with the refracted beam now lying along the horizontal. The incident angle at this point is known as the critical angle.
Angle of Reflection
When the angle of incidence becomes greater than the critical angle, a new phenomenon occurs called reflection. In this case the angle of incidence is always equal to the angle of reflection.
Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. Information is encoded onto a beam of light as a series of on-off flashes that represent 1 and 0 bits.
Multimode Step-Index
Multimode step-index uses multiple beams of light. The density of the core remains constant from the center to the edges. A beam of light moves through the constant density in a straight line until it reaches the interface of the core and the cladding. At the interface there is an abrupt change o a lower density that alters the angle of the beam's motion.
Multimode Graded-Index
Multimode graded-index uses fiber with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. Each density difference causes each beam to refract into a curve. Varying the refraction varies the distance each beam travels in a given period of time, resulting in different beams intersection at regular intervals. Careful placement of the receiver at one of these intersections allows the signal to be reconstructed with far greater precision.
Single Mode
Single mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. Propagation of different beams is almost identical and delays are negligible.
• A core is surrounded by cladding, forming the fiber. In most cases, the fiber is covered by a buffer layer that protects it from moisture. The entire cable is encased in an outer jacket.
• The light source can be either a light-emitting diode (LED) or an injection laser diode (ILD). LEDs are limited to short-distance use. Lasers can be focused to a very narrow range, allowing control over the angle of incidence. They preserve the character of the signal over considerable distances.
• Unguided Media or Wireless
• Communication transport electromagnetic waves without using a physical conductor.
• The section of the electromagnetic spectrum defined as radio communication is divided into eight ranges, called bands, each regulated by government authorities.
Propagation types of Radio Waves
• In surface propagation سطح الكرة الارضية radio waves travel through the lowest portion of the atmosphere, hugging the earth.
• Tropospheric propagation داخل الغلاف الجويcan work two ways. Either a signal can be directed in a straight line from antenna to antenna (line-of-sight), or it can be broadcast at an angle into the upper layers of the troposphere where it is reflected back to the earth's surface.
• In ionospheric propagation سطح الغلاف الجوي higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth. The density difference between the troposphere and the ionosphere causes each radio wave to speed up and change direction, heading back to earth. High frequency.
• In line-of-sight propagation, very high frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other, and either tall enough or close enough together.
• Space propagation utilizes satellite relays in place of atmospheric refraction
Type of Propagation Used in Radio Transmission
– The type of propagation used in radio transmission depends on the frequency (speed) of the signal.
• Very Low Frequency (VLF) 3-30KHz waves are propagated as surface waves, usually through air but sometimes through seawater. VLF waves are used mostly for long-range radio navigation.
• Low Frequency (LF) 30-300KHz waves are also propagated as surface waves. LF waves are used for long-range radio navigation.
• Middle frequency (MF) 300KHz,3MHz signals are propagated in the troposphere. These frequencies are absorbed by the ionosphere.
• High frequency (HF) 3MHz - 30 MHz signals use ionosphere propagation. Uses for HF signals include amateur radio (ham radio), citizen's band (CB) radio, international broadcasting, military communication, long-distance aircraft and ship communication, telephone, telegraph, and facsimile.
• Very high frequency (VHF) 30 MHZ - 300 MHZ waves use line of sight propagation. Uses include VHF television, FM radio, aircraft AM radio, and aircraft navigational aid.
• Ultrahigh frequency (UHF) 300 MHz - 3 GHz waves always use line-of-sight propagation. Uses include UHF television, mobile telephone, cellular radio, paging, and microwave links.
• Super high frequency (SHF) 3 GHz - 30 GHz waves are transmitted mostly line-of-sight and some space propagation. Uses include terrestrial and satellite microwave and radar communication.
Extremely high frequency (EHF) 30 GHz - 300 GHz waves use space propagation. Uses are predominantly scientific and include radar, satellite, and experimental communications.
