Transmission Media
Cable
Twisted Pair Cable
Twisted pair cable consists of pairs of insulated comer wire twisted together (to reduce interference) and covered with an outer jacket. The twisted pair cable used in networks is called Unshielded Twisted Pair (UTP). It is call unshielded because there is no shielding between the insulated pairs and the outer jacket. Its main advantage is that it is cheap to purchase and install
A current standard for UTP is 100BASE-T. The number 100 is the maximum data speed of 100 Mbs, the BASE refers to the baseband cable and the T refers to the twisted pair. 100BASE-T is light in weight and very flexible.
Links using low cost UTP cables are ideal for star networks because each cable only needs to carry data between a single node (such as a workstation) and the central node. Using broadband for this task would be an unnecessary extra expense for most systems.
Coaxial Cable
Coaxial cable is made of a stiff wire surrounded by insulating material then encased by a cylindrical conductor like a mesh. The mesh helps block out electrical interference. It is the encased in a plastic cover.
The mesh shielding makes it a more reliable carrier of data than UTP but it is heavier, less flexible and takes up more space than UTP.
Coaxial is more expensive than twisted pair but it is capable of higher bandwidth, as much as 150 times the capacity of twisted pair.
Fibre Optic Cable
Optical fibre cable allows data to be sent as pulses of light. One light pulse can signal a 1 bit while no light pulses signals a 0 bit. The optical fibre cable consists of an extremely thin fibre of clear glass or plastic surrounded by a layer of a different type of glass or plastic. The boundary between the two different materials acts like a mirror and traps the light pulses inside the inner cable.
A laser diode at one end of the cable emits light pulses into the cable. A photodiode (light detecting diode) at the other end of the cable detects light pulses.
The advantages of optical fibre cable are:
· They are not affected by electrical interference such as lighting and power surges.
· They are thinner and lighter than coaxial cables
· They provide greater security because they are difficult to tap into
· They have a high transmission speed and a low error rate
· The can carry thousands of separate signals.
Disadvantages of optical fibre cable are:
· They are expensive to make and repair
· They need terminators at each end of the link to convert the data between light pulses and electrical signals.
Optical fibre cable is becoming the standard media for connecting LANs together or for linking the terminals in a high speed WAN
A WAN (wide area network) covers a much larger area than a LAN and is more likely to be use externally owned and controlled communications links, such as the public telephone system.
As fibre optic cable is more secure from human interference, it is often used for long distance connections, such as between Sydney and Melbourne.
Characteristic
/ Unshielded Twisted Pair / Coaxial Cable / Optical FibreCost
/ Cheap / Moderate / ExpensiveMaximum cable length without repeaters / 100 meters / 185 meters / 2000 meters
Materials / Pair of copper wires / Copper wire with a surrounding mesh / Glass and plastic
Common uses / LANs / LANs / Joining LANs
LAN topologies / Star, ring / Bus / Star, ring
Speeds / 10 Mbps,
100 Mbps /
10 Mbps
/ 10 Mbps to1 Gbps
International standards / 10BASE-T
100BASE-T / 10BASE-2 / 10BASE-f
100BASE-F