7. Transmission Media

Transmission Media Transmission Medium and Physical Layer Classes of Transmission Media

Twisted-Pair Cable Consist of two conductors (normally copper), each with its own plastic insulation, twisted together One is used to carry signal to the receiver The other is used only as a ground reference Reason for Twisting Reduce noise caused by electromagnetic interference

Unshielded vs. Shielded TP Cables Unshielded TP (UTP) Most common TP cable used in communication Shielded TP (STP) Improve quality of cable by preventing noise or crosstalk Bulky and more expensive

Categories of UTP Cables Category Bandwidth Data Rate Digital/Analog Use 1 very low < 100 kbps Analog Telephone 2 < 2 MHz 2 Mbps Analog/digital T-1 lines 3 16 MHz 10 Mbps Digital LANs 4 20 MHz 20 Mbps 5 100 MHz 100 Mbps 6 (draft) 200 MHz 200 Mbps 7 (draft) 600 MHz 600 Mbps

RJ45 – Most Common UTP Connector

Coaxial Cable Better shielding than twisted pairs Coaxial Cable High bandwidth and excellent noise immunity Longer distances at higher speeds Coaxial Cable

Coaxial Cable (cont.) 75-Ohm Cable (Broadband) 50-Ohm cable (Baseband) Analog transmission on standard CATV “Broadband” means “cable using analog transmission” Anything wider than 4 kHz 300MHz (or 450MHz) over 100km Inferior to baseband for sending digital data Superior for huge amount of cable in place 50-Ohm cable (Baseband) Digital transmission 1 to 2 Gbps data rate for 1-km cable

Coaxial Cable Connectors BNC Connectors

Optical Fiber Optical Fiber Refraction/Reflection Made of glass or plastic and transmit signals in form of light Refraction/Reflection

Optical Fiber (cont.) Optical Fiber Use reflection to guide light through a channel

Propagation Modes

Propagation Modes (cont.)

Cable Composition Fiber Construction

Cable Composition (cont.) Light Sources for Optical Fiber Light-Emitting Diode (LED) Cheaper source Provide unfocused light Limited to short-distance use Laser Focused to a very narrow range Preserve character of signal over considerable distance Detector (Photodiode) Generate electrical pulse when light falls on it

Characteristics of Optical Fiber Advantage Noise resistance Less signal attenuation Higher bandwidth Disadvantage Cost Installation/maintenance Unidirectional Fragility

Unguided Media: Wireless Transport electromagnetic waves without using a physical conductor Signals are normally broadcast through air and thus are available to anyone who has a device capable of receiving them

Figure 7.17 Electromagnetic spectrum for wireless communication

Figure 7.18 Propagation methods

Radio Wave 3 KHz – 1 GHz Omnidirectional – signal propagates in all directions Sky transmission Used for multicast communications, such as radio and television

Figure 7.20 Omnidirectional antenna

Figure 7.21 Unidirectional antennas

Microwave 1 GHz – 300 GHz Unidirectional Line-of-sight propagation Used for unicast communication, such as cellular telephones, satellite networks, and wireless LANs

Infrared 300 GHz – 400 THz Line-of-sight propagation Used for short-range communication

Table 7.4 Bands

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