复旦大学：《计算机网络 Computer Networking》课程教学资源（PPT课件讲稿）Wireless LANs（WLANs）

Wireless LANs(WLANs) Chapter 5 Updated January 2009 boXU Zhengchuan Fudan University

Chapter 5 Updated January 2009 XU Zhengchuan Fudan University Wireless LANs (WLANs)

Orientation LANs Are Governed by Layer 1 and 2 Standards So they are governed by osl Standards · Wired lan standards Chapter 3(UTP and optical fiber transmission) Chapter 4(Ethernet 802. 3 Layer 1 and 2 standards) Chapter 5 Wireless LAN (WLAN Standards Physical layer wireless transmission Wireless data link layer operation Management 5-2

5-2 Orientation • LANs Are Governed by Layer 1 and 2 Standards – So they are governed by OSI Standards • Wired LAN Standards – Chapter 3 (UTP and optical fiber transmission) – Chapter 4 (Ethernet 802.3 Layer 1 and 2 standards) • Chapter 5 – Wireless LAN (WLAN) Standards – Physical layer wireless transmission – Wireless data link layer operation – Management

Figure 5-1: Local wireless Technologies, Continued ·802.11 The dominant WLan technology today Standardized by the 802. 11 Working Group 802.11 5-3

5-3 Figure 5-1: Local Wireless Technologies, Continued • 802.11 – The dominant WLAN technology today – Standardized by the 802.11 Working Group 802.11

I Figure 5-2: Wireless LAN(WLAN) Access Point Large wired ethernet LaN Wireless Access Ethernet Switch Point UTP Radio Transmission Laptop Router Mobile Communication C lient Server Wireless access point (WAP)bridges wireless stations to resources on wired LAN-servers and Internet routers for Internet access

5-4 Figure 5-2: Wireless LAN (WLAN) Access Point Server Internet Router Ethernet Switch Laptop Mobile Client Wireless Access Point Large Wired Ethernet LAN UTP Radio Transmission Wireless access point (WAP) bridges wireless stations to resources on wired LAN—servers and routers for Internet access Communication

Figure 5-3: Access Router with Wireless Access Point and wireless Nics PC Card WNIC for a notebook Computer Access Router With Access point USB WNIC Internal WNIC For desktop p( 65

5-5 Figure 5-3: Access Router with Wireless Access Point and Wireless NICs PC Card WNIC for a Notebook Computer Internal WNIC For Desktop PC USB WNIC Access Router with Access Point

Figure 5-1: Local wireless Technologies, Continued ·802.11 Wireless lans oday, mostly speeds of tens of megabits per second with distances of 30 to 100 meters or more Can serve many users in a home or office Increasingly, 100 Mbps to 600 Mbps with 802. 11n Organizations can provide coverage throughout a building or a university campus by installing many access points 5-6

5-6 Figure 5-1: Local Wireless Technologies, Continued • 802.11 Wireless LANs – Today, mostly speeds of tens of megabits per second with distances of 30 to 100 meters or more • Can serve many users in a home or office – Increasingly, 100 Mbps to 600 Mbps with 802.11n – Organizations can provide coverage throughout a building or a university campus by installing many access points

Radio Propagation

Radio Propagation

Figure 5-5: Frequency Measurement Frequency Light waves are measured in wavelengths( Ch 3) Radio waves are measured in terms of frequency Measured in hertz(Hz)-the number of complete cycles per second 1 Second Two cycles in 1 second, so frequency is two Hertz(Hz) 5-8

5-8 Figure 5-5: Frequency Measurement • Frequency – Light waves are measured in wavelengths (Ch. 3) – Radio waves are measured in terms of frequency – Measured in hertz (Hz)—the number of complete cycles per second 1 Second Two cycles in 1 second, so frequency is two Hertz (Hz)

Figure 5-5: Frequency Measurement, Continued Measuring frequencies Frequency measures increases by factors of 1,000(not 1,024) Kilohertz( kHz) [Note the lower-case k MegahertZ(MHz) Gigahertz(GHz) 5-9

5-9 Figure 5-5: Frequency Measurement, Continued • Measuring Frequencies – Frequency measures increases by factors of 1,000 (not 1,024) – Kilohertz (kHz) [Note the lower-case k] – Megahertz (MHz) – Gigahertz (GHz)

Figure 5-6: Omnidirectional and dish Antennas Dish Antenna Om nidirectional Antenna Focuses signals in a narrow range spread signals in all directions Signals can be sent over long distances Rapid signal attenuation Must point at the sender No need to point at receiver Good for fixed subscribers Good for mobile subscribers 5-10

5-10 Figure 5-6: Omnidirectional and Dish Antennas Omnidirectional Antenna Spread signals in all directions Rapid signal attenuation ----- No need to point at receiver Good for mobile subscribers Dish Antenna Focuses signals in a narrow range Signals can be sent over long distances ----- Must point at the sender Good for fixed subscribers