1Wireless Networks1Wireless NetworksBackground: # 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 networkWireless Networks2Outline1. Introduction 2. Wireless link characteristics and PHY Layer design.3. Multiple access and interference management:o IEEE 802.11 wireless LANs (“wi-fi”)o cellular standards (eg. GSM, CDMA)4. Mobility management2Wireless Networks3Elements of a wireless networknetwork infrastructurewireless hosts laptop, PDA, IP phone run applications may be stationary (non-mobile) or mobile wireless does notalways mean mobilityWireless Networks4Elements of a wireless networknetwork infrastructurebase station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its “area” e.g., cell towers 802.11 access points3Wireless Networks5Elements of a wireless networknetwork infrastructurewireless link typically used to connect mobile(s) to base station also used as backbone link multiple access protocol coordinates link access various data rates, transmission distanceWireless Networks6Elements of a wireless networknetwork infrastructureinfrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network4Wireless Networks7Elements of a wireless networkAd hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselvesWireless Networks8History 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.5Wireless Networks9Wireless 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 Networks10Characteristics of selected wireless standards384 Kbps56 Kbps54 Mbps5-11 Mbps1 Mbps802.15802.11b802.11{a,g}IS-95 CDMA, GSMUMTS/WCDMA, CDMA2000.2G3GIndoor10 – 30mOutdoor50 – 200mMid rangeoutdoor200m – 4KmLong rangeoutdoor5Km – 20Km6Wireless Networks11Cellular 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 mobilityWireless Networks12Wireless 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)7Wireless Networks13Wireless ChannelChannel varies at two spatial scales:large scale small scale (mulipath fading)Wireless Networks14Large-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 reuse8Wireless Networks15Small-scale multipath fading•Multipath fading due to constructive and destructiveinterference of the transmitted waves at very high carrier frequency.16dBWireless Networks16Example Difference in phases of direct and reflected waves: where λ is the wavelength of the signal. Movement of λ/4 goes from a peak to a valley. (this is 0.3m at frequency 900 MHz)WallTransmit antennaReceive
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