Wireless 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 communication over wireless link handling mobile user who changes point of attachment to network Wireless Networks 1 Outline 1 Introduction 2 Wireless link characteristics and PHY Layer design 3 Multiple access and interference management o o IEEE 802 11 wireless LANs wi fi cellular standards eg GSM CDMA 4 Mobility management Wireless Networks 2 1 Elements of a wireless network network infrastructure wireless hosts laptop PDA IP phone run applications may be stationary non mobile or mobile wireless does not always mean mobility Wireless Networks 3 Elements of a wireless network base station typically connected to wired network relay responsible network infrastructure for sending packets between wired network and wireless host s in its area e g cell towers 802 11 access points Wireless Networks 4 2 Elements of a wireless network wireless link typically used to network infrastructure connect mobile s to base station also used as backbone link multiple access protocol coordinates link access various data rates transmission distance Wireless Networks 5 Elements of a wireless network infrastructure mode base station connects network infrastructure mobiles into wired network handoff mobile changes base station providing connection into wired network Wireless Networks 6 3 Elements of a wireless network Ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network route among themselves Wireless Networks 7 History Cellular concept Bell Labs early 70 s AMPS analog early 80 s GSM digital narrowband late 80 s IS 95 digital wideband early 90 s 3G 4G systems for wireless data UMTS CDMA 2000 Explosive growth of 802 11 WiFi Lan in past 5 years Wireless Networks 8 4 Wireless Link Key Parameters Carrier frequency f c 900 Mhz or 1 9 GHz for cellular 2 4 Ghz or 5 3 Ghz for 802 11 Transmission bandwidth W 200 kHz for GSM 1 25 MHz for CDMA 83 5 MHz for 802 11b divided into 3 channels Determines the symbol rate Data rate R bps eg 11 Mbps for 802 11b Range Wireless Networks 9 Characteristics of selected wireless standards 54 Mbps 5 11 Mbps 802 11 a g 802 11b 1 Mbps 802 15 3G UMTS WCDMA CDMA2000 384 Kbps 2G IS 95 CDMA GSM 56 Kbps Indoor Outdoor Mid range outdoor Long range outdoor 10 30m 50 200m 200m 4Km 5Km 20Km Wireless Networks 10 5 Cellular vs Wi Fi Cellular wide area coverage proprietary networks more coordination between BSs Wi Fi local area coverage last link for the Internet little coordination between APs Cellular licensed and expensive spectrum around 900 MHz and 1 9 GHz Wi Fi unlicensed free spectrum 2 4 Ghz and 5 3 GHz Cellular high mobility Wi Fi low or no mobility Wireless Networks 11 Wireless Link Characteristics decreased signal strength radio signal attenuates as it propagates through matter path loss multipath fading constructive and destructive interference of multiple signal paths from transmitter to receiver interference Between different users in a cell Between users across different cells From external sources in unlicensed bands eg your microwave oven Wireless Networks 12 6 Wireless Channel Channel varies at two spatial scales large scale small scale mulipath fading Wireless Networks 13 Large scale channel variations In free space received power attenuates like 1 r2 Can also experience shadowing by objects Important for cell site access point planning Coverage Frequency reuse Wireless Networks 14 7 Small scale multipath fading 16dB Multipath fading due to constructive and destructive interference of the transmitted waves at very high carrier frequency Wireless Networks 15 Example Transmit antenna d Wall r Receive antenna Difference in phases of direct and reflected waves where is the wavelength of the signal Movement of 4 goes from a peak to a valley this is 0 3m at frequency 900 MHz Wireless Networks 16 8
View Full Document
Unlocking...