香港浸会大学：《Data Communications and Networking》课程教学资源（PPT讲稿）Chapter 4 Transmission Media

Data communications and Networking Chapter 4 Transmission media Reading Book Chapter 4 Data and Computer Communications, 8th edition By wIlliam Stallings

1 Data Communications and Networking Chapter 4 Transmission Media Reading: Book Chapter 4 Data and Computer Communications, 8th edition By William Stallings

Transmission media In a data transmission system, the transmission medium is the physical path between transmitter and receiver · Guided mediun Electromagnetic waves are guided along a solid medium Unguided media Wireless transmission occurs through the atmosphere, outer space, or water The characteristics and quality of a data transmission are determined bot by the medium and the signal For guided media, the medium itself is more important in determining the limitations of transmission For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important. One key property of signals transmitted by antenna is directionality Key concerns are data rate and distance: the greater the data rate and distance. the better

2 Transmission Media • In a data transmission system, the transmission medium is the physical path between transmitter and receiver. • Guided medium — Electromagnetic waves are guided along a solid medium. • Unguided media — Wireless transmission occurs through the atmosphere, outer space, or water. • The characteristics and quality of a data transmission are determined both by the medium and the signal. — For guided media, the medium itself is more important in determining the limitations of transmission. — For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important. One key property of signals transmitted by antenna is directionality. • Key concerns are data rate and distance: the greater the data rate and distance, the better

Electromagnetic Spectrum Frequency (Hertz)102 104105 10 10 10 11 12 1011101210 1310 1410 ELF VF VLF LF MF HF VHF UHF SHFEHF Power and telephone Radio Microwave Infrared Visible Rotating generators Radios and televisions Radar asers light Musical instruments Electronic tubes Microwave antennas Guided missiles Voice microphones Integrated circuits Magnetrons Rangefinders Cellular Telephony Twisted Pair Optical Fiber axial cable AM Radio FM Radio Terrestrial and TVand Satellite Transmission Wavelength 10 10 10010-110 3 10 10 In space (meters) ELF EXtremely low frequency MF Medium frequency UHF Ultrahigh frequency VF Voice frequency HF = High frequency SHF = Superhigh frequency VLF Very low frequency VHF Very high frequency EHF Extremely high frequency

3 Electromagnetic Spectrum

Guided transmission media · Twisted pair · Coaxial cable Optical fiber Table 4.1 Point-to-Point Transmission Characteristics of Guided Media [GLov98] Frequency Range Typical Attenuation Typical Delay Re epeater spacing Twisted pair(with 0 to 3.5 kHz 0.2 dB/km a l khz Hs/km 2 km loading) Twisted pairs(multi-pair 0 to I MHz 0.7 dB/km a 1 kHz 5 us/km 2 km axial c 0 to 500 MHz 7 dB/km (a 10 MHz 4 us/km I to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 us/km 40 km THz= TeraHerz= 102H

4 Guided Transmission Media • Twisted Pair • Coaxial cable • Optical fiber

Twisted Pair length Separately insulated Twisted together -Often bundled" into cables -Usually installed in building during construction (a)Twisted pair a wire pair acts as a single communication link typically a number of these pairs are bundled together into a cable The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable, which usually contain hundreds of pairs 5

5 Twisted Pair A wire pair acts as a single communication link. Typically, a number of these pairs are bundled together into a cable. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable, which usually contain hundreds of pairs

Twisted Pair- Applications Most common medium · Telephone network Individual residential telephone sets are connected to the local telephone exchange(or end office )by twisted-pair wire. These are referred to as subscriber loop Within an office building Each telephone is connected to a twisted pair, which goes to the in-house private branch exchange(PBX) system These twisted-pair installations were designed to support voice traffic using analog signaling. However, by means of a modem, these facilities can handle digital data traffic at modest data rates For digital signaling: connections to a digital data switch or a digital PBX For local area networkS (LAN Data rates can be around 1ombps, 100Mbps, or even 1 Gbps 6

6 Twisted Pair - Applications • Most common medium • Telephone network — Individual residential telephone sets are connected to the local telephone exchange (or “end office”) by twisted-pair wire. These are referred to as subscriber loop. • Within an office building — Each telephone is connected to a twisted pair, which goes to the in-house private branch exchange (PBX) system. • These twisted-pair installations were designed to support voice traffic using analog signaling. However, by means of a modem, these facilities can handle digital data traffic at modest data rates. • For digital signaling: connections to a digital data switch or a digital PBX • For local area networks (LAN) — Data rates can be around 10Mbps, 100Mbps, or even 1Gbps

Twisted Pair- Pros and cons ros Twisted pair is much less expensive than other commonly used guided transmission media Twisted pair is easier to work with Cons: limited in distance. bandwidth and data rate -Short range not good for long-distance -E. g, the data rate of Adsl depends on the distance 1. 5Mbps for 18.000ft 2 mBps for 16,000ft 6. mBps for 12,000ft ·9.0 Mbps for9000ft

7 Twisted Pair - Pros and Cons • Pros —Twisted pair is much less expensive than other commonly used guided transmission media. —Twisted pair is easier to work with. • Cons: limited in distance, bandwidth, and data rate —Short range: not good for long-distance —E.g., the data rate of ADSL depends on the distance • 1.5Mbps for 18,000ft • 2.0Mbps for 16,000ft • 6.0Mbps for 12,000ft • 9.0Mbps for 9000ft

Unshielded and shielded Tp Unshielded Twisted Pair (UTP) Ord inary telephone wire Easiest to install Suffers from external electromagnetic interference UTP categories EIA-568-A: Commercial Building Telecommunications Cabling Standard Category 3 The transmiss ion characteristics are specified up to 16MHz Eg, the attenuation at 16MHz is about 13db per 100m Voice grade found in most ofices Twist length: 7.5 cm to 10 cm Category 5 The transmiss ion characteristics are specified up to 100MHz Eg, the attenuation at 16MHz is about 8db per 100m, the attenuation at 100MHz is about 22db per 100m Commonly pre-installed in new office buildings Twist length: 0.6 cm to 0.85 cm Remark: Category 3 Category 5 cables are widely used in LAN Shielded Twisted Pair(STP) Metal braid or sheathing that reduces interference 二 More expensive Harder to handle(thick, heavy) 8

8 Unshielded and Shielded TP • Unshielded Twisted Pair (UTP) — Ordinary telephone wire — Cheapest — Easiest to install — Suffers from external electromagnetic interference • UTP categories — EIA-568-A: Commercial Building Telecommunications Cabling Standard — Category 3 • The transmission characteristics are specified up to 16MHz – E.g., the attenuation at 16MHz is about 13db per 100m • Voice grade found in most offices • Twist length: 7.5 cm to 10 cm — Category 5 • The transmission characteristics are specified up to 100MHz – E.g., the attenuation at 16MHz is about 8db per 100m, the attenuation at 100MHz is about 22db per 100m • Commonly pre-installed in new office buildings • Twist length: 0.6 cm to 0.85 cm — Remark: Category 3 & Category 5 cables are widely used in LAN. • Shielded Twisted Pair (STP) — Metal braid or sheathing that reduces interference — More expensive — Harder to handle (thick, heavy)

Coaxial cable Outer conductor Outer sheath Insulation. Inner conductor Outer conductor is braided shield Inner conductor is solid metal Separated by insulating material Covered by padding Two conductors outer conductor inner conductor 9

9 Coaxial Cable Two conductors: outer conductor & inner conductor

Coaxial cable- transmission Characteristics Transmission Characteristics Analog signal Amplifiers every few km Closer if higher frequency Up to 500MHz(about 4MHz for each tv channel) Digital signal Repeater every l km or so Closer for higher data rates A pplications -Television distribution · Cable t Long distance telephone transmission Can carry 10,000 voice calls simultaneously Being replaced by fiber optic Local area networks Short-range connections between devices such as high-speed 1/O channels on computer systems 10

10 Coaxial Cable - Transmission Characteristics • Transmission Characteristics — Analog signal • Amplifiers every few km • Closer if higher frequency • Up to 500MHz (about 4MHz for each TV channel) — Digital signal • Repeater every 1km or so • Closer for higher data rates • Applications — Television distribution • Cable TV — Long distance telephone transmission • Can carry 10,000 voice calls simultaneously • Being replaced by fiber optic — Local area networks — Short-range connections between devices such as high-speed I/O channels on computer systems