计算机网络课件英文版Chapter6_Wireless and Mobile Networks.ppt

Chapter 6: Wireless and Mobile Networks Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access two important (but different) challenges wireless: communication over wireless link mobility: handling the mobile user who changes point of attachment to network Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9 Summary Elements of a wireless network Elements of a wireless network Elements of a wireless network Characteristics of selected wireless link standards Elements of a wireless network Elements of a wireless network Wireless network taxonomy Wireless Link Characteristics (1) Differences from wired link …. decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” Wireless Link Characteristics (2) SNR: signal-to-noise ratio larger SNR – easier to extract signal from noise (a “good thing”) SNR versus BER tradeoffs given physical layer: increase power - increase SNR-decrease BER given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) Wireless network characteristics Multiple wireless senders and receivers