Transmission Media

Definition of Terms

• Transmission media lie below the physical layer.
• A guided medium provides a physical conduit from one device to another.  
• Twisted-pair cable, coaxial cable, and optical fiber are the most popular types of guided media. 
• Twisted-pair cable consists of two insulated copper wires twisted together. Twisting allows each wire to have approximately the same noise environment. Twisting ensures that both wires are equally, but inversely, affected by external influences such as noise.
• Twistedpair cable is used for voice and data communications.
• Coaxial cable consists of a central conductor and a shield. Coaxial cable can carry signals of higher frequency ranges than twisted-pair cable. Coaxial cable is used in cable TV networks and traditional Ethernet LANs.
• Coaxial cable has the following layers (starting from the center): a metallic rod-shaped inner conductor, an insulator covering the rod, a metallic outer conductor (shield), an insulator covering the shield, and a plastic cover.  
• The inner core of an optical fiber is surrounded by cladding. The core is denser than the cladding, so a light beam traveling through the core is reflected at the boundary between the core and the cladding if the incident angle is more than the critical angle.
• Fiber-optic cables are composed of a glass or plastic inner core surrounded by cladding, all encased in an outside jacket. 
• Fiber-optic cables carry data signals in the form of light. The signal is propagated along the inner core by reflection. 
• Fiber optic transmission is becoming increasingly popular due to its noise resistance, low attenuation, and high-bandwidth capabilities. 
• Fiber-optic cable is used in backbone networks, cable TV networks, and Fast Ethernet networks.
• Signal propagation in optical fibers can be multimode (multiple beams from a light source) or single-mode (essentially one beam from a light source).
• In multimode step-index propagation, the core density is constant and the light beam changes direction suddenly at the interface between the core and the cladding. 
• In multimode graded-index propagation, the core density decreases with distance from the center. This causes a curving of the light beams. 
• Unguided media (free space) transport electromagnetic waves without the use of a physical conductor.
• Wireless data are transmitted through ground propagation, sky propagation, and line-of- sight propagation.
• In sky propagation radio waves radiate upward into the ionosphere and are then reflected back to earth. In line-of-sight propagation signals are transmitted in a straight line from antenna to antenna.
• Wireless waves can be classified as radio waves, microwaves, or infrared waves. Radio waves are omnidirectional; microwaves are unidirectional. 
• Radio waves are omnidirectional. The radio wave band is under government regulation. 
• Microwaves are unidirectional; the propagation is line of sight. Microwaves are used for cellular phone, satellite, and wireless LAN communications.
• Microwaves are used for cellular phone, satellite, and wireless LAN communications.
• The parabolic dish antenna and the horn antenna are used for transmission and reception of microwaves. 
• Infrared waves are used for short-range communications such as those between a
  PC and a peripheral device. It can also be used for indoor LANs.

Categories of Transmission Media

• Guided media - have physical boundaries
• Unguided media - are unbounded.

Three Major classes of Guided Media

• Twisted-pair cable
•  Coaxial cable
• Optical fiber

Propagation modes

• 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.
• Multimode -  multiple beams from a light source move through the core in different paths. It can be implemented in two forms: step-index or graded-index

Modes

Fiber Types

Categories of unshielded twisted-pair cables

Categories of coaxial cables

Electromagnetic spectrum for wireless communication

Propagation methods

• Ground Propagation
• Sky Propagation
• Line-of-sight Propagation

Frequency Bands

Wireless Transmission

• Radio Wave - used for multicast communications, such as radio and television, and paging systems.
• Micro Wave - used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.
• Infrared - used for short-range communication in a closed area using line-of-sight propagation.

Note: You can proceed to take the multiple choice exam regarding this topic. Transmission Media - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com

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