Transmission media-Definition Transmission media can be broadly defined as anything that can carry information from source to a destination. For example, the transmission medium for two people having a dinner conversation is air. For a written message, the transmission medium might be a mail carrier, a truck, or an airplane. 7.2
7.3 Figure 7.1 Transmission medium and physical layer Transmission media usually located below the physical media and are directly controlled by physical layer.
7.5 7-1 GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-Pair Cable Coaxial Cable Fiber-Optic Cable Topics discussed in this section:
7.6 Twisted-pair cable A twisted pair cable consists of two conductors(normally copper), each with its own plastic insulation, twisted together. One of the wire is used to carry a signal to the receiver, and the other is used only as a ground reference. The receiver uses a 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.
7.10 Applications Twisted pair cables are used in telephone lines to provide voice and data channels. Local area networks, such as 10base-T and 100base-T, also use twisted-pair cables.
7.11 Coaxial cable Coaxial cable carries signals of higher frequency ranges than those in twisted –pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or standard wire enclosed in an insulating sheath, which is, in turn, enclosed in an outer conductor of metal foil. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.
7.15 Applications Coaxial cables were used in analog telephone networks. Later it was used in digital telephone networks. Cable TV networks also use co-axial cables. Ethernet LANs.
7.16 Fiber-optic cable A fiber optic cable is made up of glass or plastic and transmit signals in the form of light. To understand optical fiber, we must need to explore several aspects of the nature of light.
Propagation modes Multimode, Stepped Index the least expensive and used when performance is unimportant the boundary between the fiber and the cladding is abrupt, which causes light to reflect frequently dispersion is high Multimode, Graded Index fiber is slightly more expensive than the step index fiber it has the advantage of making the density of the fiber increase near the edge, which reduces reflection and lowers dispersion 7.21
Propagation modes Single Mode- In this laser light travels over single path with little reflection. fiber is the most expensive, and provides the least dispersion the fiber has a smaller diameter and other properties that help reduce reflection. Single mode is used for long distances and higher bit rates 7.22
7.25 Applications Fiber optic cable is often used in backbone networks. Local area networks such as 100Base-FX network and 1000Base-X.
7.26 7-2 UNGUIDED MEDIA: WIRELESS Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Radio Waves Microwaves Infrared Topics discussed in this section:
7.27 Figure 7.17 Electromagnetic spectrum for wireless communication
Radio waves Transmission Radio waves have frequencies between 10Khz to 1Ghz. Radio waves can broadcast signals omni-directionally or directionally. bandwidth capacity is 1-10Mbps. antennas are used to broadcast signals. power of RF(radio frequency) signal is determined by antenna and transceiver. can travel long distances. Easy to generate. low and medium frequencies can penetrate walls. So they are widely used for communication both indoors and outdoors. 7.31
Micro wave Transmission Microwaves are electromagnetic waves having frequencies between 1-300 GHz. Above 100 MHz, the wave travel in straight lines and can be narrowly focused concentrating all the energy into a small beam using parabolic antenna. produce better throughput and performance Unidirectional. 7.35
7.36 Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. Note
Infra-red wave Transmission Infra-red waves are the waves having frequencies ranges from 300-400 THz. transmit signals directionally. cannot penetrate through walls because of higher frequencies. can be used for short range communication. no interference is possible. cannot be used outside building because sun rays contain IR waves that may interfere with IR communication. Cheap and Easy to install. 7.37
7.38 Infrared signals can be used for short- range communication in a closed area using line-of-sight propagation. Note