TRANSMISSION MEDIUMS IN COMPUTER NETWORKS
Data is represented by computers and other telecommunication devices using signals. Signals are transmitted in the form of electromagnetic energy from one device to another. Electromagnetic signals travel through vacuum, air or other transmission mediums to move from one point to another (from sender to receiver).
Electromagnetic energy (includes electrical and magnetic fields) consists of power, voice, visible light, radio waves, ultraviolet light, gamma rays etc.
Transmission medium is the means through which we send our data from one place to another. The first layer (physical layer) of Communication Networks OSI Seven-layer model is dedicated to the transmission media, we will study the OSI Model later.
Factors to be considered while selecting a Transmission Medium
1. Transmission Rate
2. Cost and Ease of Installation
3. Resistance to Environmental Conditions
4. Distances
BOUNDED OR GUIDED TRANSMISSION MEDIA
Guided media, which are those that provide a conduit from one device to another, include Twisted-Pair Cable, Coaxial Cable, and Fibre-Optic Cable. A signal travelling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fibre is a cable that accepts and transports signals in the form of light.
A. Twisted Pair Cable
This cable is the most commonly used and is cheaper than others. It is lightweight, cheap, can be installed
easily, and they support many different types of network. Some important points:
• Its frequency range is 0 to 3.5 kHz.
• Typical attenuation is 0.2 dB/Km @ 1kHz.
• Typical delay is 50 μs/km.
• Repeater spacing is 2km.
A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together. One of these wires is used to carry signals to the receiver, and the other is used only as ground reference. The receiver uses the difference between the two. In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources. This results in a difference at the receiver.
Twisted Pair is of two types:
• Unshielded Twisted Pair (UTP)
• Shielded Twisted Pair (STP)
Unshielded Twisted Pair Cable
It is the most common type of telecommunication when compared with Shielded Twisted Pair Cable which consists of two conductors usually copper, each with its own colour plastic insulator. Identification is the reason behind coloured plastic insulation.
Advantages of Unshielded Twisted Pair Cable
• Installation is easy
• Flexible
• Cheap
• It has high speed capacity,
• 100-meter limit
• Higher grades of UTP are used in LAN technologies like Ethernet.
It consists of two insulating copper wires (1mm thick). The wires are twisted together in a helical form to reduce electrical interference from similar pair.
Disadvantages of Unshielded Twisted Pair Cable
• Bandwidth is low when compared with Coaxial Cable Provides less protection from interference.
Shielded Twisted Pair Cable
This cable has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Electromagnetic noise penetration is prevented by metal casing. Shielding also eliminates crosstalk
(explained in KEY TERMS Chapter). It has same attenuation as unshielded twisted pair. It is faster the unshielded and coaxial cable. It is more expensive than coaxial and unshielded twisted pair.
Advantages of Shielded Twisted Pair Cable
• Easy to install
• Performance is adequate
• Can be used for Analog or Digital transmission
• Increases the signaling rate
• Higher capacity than unshielded twisted pair
• Eliminates crosstalk
Disadvantages of Shielded Twisted Pair Cable
• Difficult to manufacture
• Heavy
