Chapter 7 Transmission Media

Figure 7.1 Transmission medium and physical layer Transmission media are located below the physical layer and are directly controlled by the physical layer.

signals: electromagnetic Figure 7.2 Classes of transmission media signals: electromagnetic

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. Uses a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver. Transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint A signal traveling 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 fiber is a cable that accepts and transports signals in the form of light.

Figure 7.3 Twisted-pair cable Telephone line in your home; Low frequency transmission medium Consists of two conductors (normally copper), each with its own plastic insulation, twisted together. One is used to carry signals to the receiver, the other is used only for ground reference. The receiver uses the difference between the two. Interference (noise) and crosstalk may affect both wires and create unwanted signals. What if the two wires are parallel? Why twisted? To minimize the electromagnetic interference between adjacent pairs. Twising the pairs, a balance is maintained. One twist one is closer, then next twist, the other is closer. So both wires are equally affected by external influences. The unwanted signals are mostly canceled out.

Figure 7.4 UTP and STP cables

Table 7.1 Categories of unshielded twisted-pair cables

Figure 7.5 UTP connector

Figure 7.6 UTP performance

Twisted Pair Advantages Inexpensive and readily available Flexible and light weight Easy to work with and install Disadvantages Susceptibility to interference and noise Attenuation problem For analog, amps needed every 5-6km For digital, repeaters needed every 2-3km Relatively low bandwidth

Figure 7.7 Coaxial cable Used for cable television, LANs, etc Conductors share a common center axial, hence the term “co-axial”

Table 7.2 Categories of coaxial cables

Figure 7.9 Coaxial cable performance

Coax Advantages Higher bandwidth Can be tapped easily Much less susceptible to interference than twisted pair Disadvantages High attenuation rate makes it expensive over long distance Bulky

Figure 7.10 Bending of light ray Relatively new transmission medium used by telephone companies in place of long-distance trunk lines Also used by private companies in implementing local data communications networks

Figure 7.11 Optical fiber

Figure 7.12 Propagation modes

Figure 7.13 Modes

Table 7.3 Fiber types

Figure 7.16 Optical fiber performance

Fiber Optic Advantages greater capacity smaller size and lighter weight lower attenuation immunity to environmental interference highly secure due to tap difficulty and lack of signal radiation Disadvantages expensive over short distance requires highly skilled installers adding additional nodes is difficult

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.

Figure 7.17 Electromagnetic spectrum for wireless communication

Figure 7.18 Propagation methods

Table 7.4 Bands

Figure 7.19 Wireless transmission waves

Figure 7.20 Omnidirectional antenna Radio waves are used for multicast communications, such as AM, FM, maritime radio, cordless phones, television, and paging systems. Ranged from 3kHz – 1GHz

Figure 7.21 Unidirectional antennas Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. Ranged from 1-300GHz

Infrared Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation. Ranged from 300 GHz to 400 THz. Cannot penetrate walls: prevents interference between two systems. Useless for long-range communication Cannot be used outside of a building Applications?