Chapter 4. Transmission Media

Transmission Media

• Transmission medium defined as physical path that carry information between sender and receiver
• it is key elements of data communication
• Because choosing the appropriate media, making cost-efficient of data communications

Design Factors Determining Data Rate and Distance

• Bandwidth : Higher bandwidth gives higher data rate
• Transmission impairments : Impairments, such as attenuation, limit the distance
• Interference : Overlapping frequency bands can distort or wipe out a signal
• For guided media, interference can be caused by emanations coupling from nearby cables or adjacent conductors under the same cable sheath
• For example, twisted pairs are often bundled together
• Proper shielding of a guided medium can minimize this problem
• Also be caused by electromagnetic coupling from unguided transmissions
• Number of receivers : More receivers introduces more attenuation

Twisted Pair

• Twisted pair is the least expensive and most widely used guided transmission medium
• It consists of two insulated copper wires arranged in a regular spiral pattern
• A wire pair acts as a single communication link
• Pairs a re bundled together into a cable
• Most commonly used in the telephone network and for communications within buildings

• Most common transmission wires because of good performance at low costs
• But, two or more wires lying together can interfere with each other's signals
  

PROS	CONS
Inexpensive	High interference
Easy to get	Signal weakens over time
Flexible and lightweight	Relatively low bandwidth
Easy to work with

Unshielded and Shielded Twisted Pair

Unshielded Twisted Pair (UTP)

• It consits of one or more twisted-pair cables, typically enclosed within an overall thermoplastic jacket which provides no electromagnetic shielding
• It is used in telephone wire and local area networks (LANs)
• It is subject to external electromagnetic interference
• The tighter the twisting, the higher the supported transmission rate and the greater the cost per meter

Shielded Twisted Pair (STP)

• It is enclosed in a shield that functions as a grounding mechanism
• This is done to provide greater protection from electromagnetic interference and radio frequency interference compared with UTP
• It provides better performance at higher data rates
• it is difficult to install and more expensive

Unshielded and Shielded Twisted Pair

UTP	STP
Cheap	More expensive
Easy to install	Harder to handle
Suffers from interference	Sheathing reduces interference

Coaxial Cable

• Coaxial cable is a type of copper cable specially built with a shield and other components engineered to block signal interference
• It is a transmission medium used in a wide variety of applications

• Coaxial cable can be used over longer distances than twisted pair
• Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies
  
• A single coaxial cable has a diameter of from 1 to 2.5 cm

• Coaxial cable is a versatile transmission medium, used in a wide variety of applications. The most important of these are
• Television distribution
• Long-distance telephone transmission
• Short-run computer system links
• Local are networks
• Coaxial cable is widely used as a means of distributing TV signals to individual home
  

Optical Fiber

• Optical fiber is a thin flexible medium capable of guiding an optical ray
• Optical fiber uses light to transmit data signals
• Various glasses and plastics can be used to make optical fibers
• It has a cylindrical shape with three sections - core, cladding, coating
• It is widely used in long distance telecommunications
• Performance, price and advantages have made it popular to use

• Core
• The core is the innermost section and consists of thin strands made of glass or plastic
• The core has a diameter in the range of 8 to 62.5㎛
• Cladding
• The core is surrounded by a cladding, which is a glass or plastic coating that has optical properties different from those of the core and a diameter of 125㎛
• Jacket
• The outermost layer is the coating, which is as hard plastic coating that protects the glass from physical damage

• Optical fiber already enjoys considerable use in long-distance telecommunications, and its use in military applications is growing
• The continuing improvements in performance and decline in prices, together with the inherent advantages of optical fiber, have made it increasingly attractive for local are networking

Optical Fiber - Benefits

• Greater capacity
• Data rates of hundreds of Gbps over tens of kilometers have been demonstrated
• Smaller size and lighter weight
• Considerably thinner than coaxial or twisted pair cable
• Reduces structural support requirements
• Lower attenuation
• Electromagnetic isolation
• Not vulnerable to interference, impulse noise, or crosstalk
• High degree of security from eavesdropping
• Greater repeater spacing
• Lower cost and fewer sources of error