Guided Media
Guided Media
In networks that use wire line media, the transmission of information takes place on a wire or cable. The three types of wire line media are twisted-pair wire, coaxial cable and fibre-optic cable. (Try and find examples of each of these media, and their relative speeds). While twisted-pair and coaxial cable are more commonly used today, fibre optic cables are becoming increasingly popular.
Twisted Pair
• A twisted pair cable consists of four pairs of copper wires coated with an insulating material like plastic or Teflon, twisted together. The twisting of wires reduces electromagnetic interference from external sources.
• Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to optical fiber and coaxial cable.
• Twisted pair is of two kinds—Shielded Twisted Pair (STP), and Unshielded Twisted Pair (UTP).
• STP cable has an extra layer of metal foil between the twisted pair of copper wires and the outer covering. The metal foil covering provides additional protection from external disturbances. However, the covering increases the resistance to the signal and thus decreases the length of the cable. STP is costly and is generally used in networks where cables pass closer to devices that cause external disturbances.
• UTP is the most commonly used medium for transmission over short distances up to 100m. Out of the four pairs of wires in a UTP cable, only two pairs are used for communication.
• UTP cables are defined in different categories. The commonly used UTP cable is the Cat-5 cable which is used with fast Ethernet.
Coaxial Cable
• A coaxial cable has a single inner conductor that transmits electric signals; the outer conductor acts as a ground. The two conductors are separated by insulation. The inner conductor, insulator, and the outer conductor are wrapped in a sheath of Teflon or PVC.
• The copper wire is used for both inner and outer conductor. The signal is transmitted over the surface of the inner conductor.
• In an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors. This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other transmission lines, and provides protection of the signal from external electromagnetic interference.
• A thicker coaxial cable can transmit more data than a thinner one.
• The commonly used coaxial cable is 10 base 2 that transmits over a distance of 185 m, and 10 base 5 that transmits over a distance of 500 m.
Optical Fiber
• Optical fibers are being used for transmission of information over large distances more cost-effectively than the copper wire connection. Communication systems are now unthinkable without fiber optics.
• Optical fiber transmits data as light signals instead of electric signals.
• Modern optical fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodate large demands for bandwidth on a point to-point basis.
• Optical fibers come in two types: (a) Single-mode fibers, and (b) Multi-mode fibers damage.