19 - Wireless LANsSlide 2Chapter 6: Wireless and Mobile NetworksElements of a wireless networkSlide 5Slide 6Characteristics of selected wireless link standardsSlide 8Slide 9Wireless Link CharacteristicsWireless network characteristicsCode Division Multiple Access (CDMA)CDMA Encode/DecodeCDMA: two-sender interference6: Wireless and Mobile Networks 6-119 - Wireless LANs6: Wireless and Mobile Networks 6-2Chapter 6Wireless and Mobile NetworksComputer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004. A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.Thanks and enjoy! JFK/KWRAll material copyright 1996-2004J.F Kurose and K.W. Ross, All Rights Reserved6: Wireless and Mobile Networks 6-3Chapter 6: Wireless and Mobile NetworksBackground: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challengescommunication over wireless linkhandling mobile user who changes point of attachment to network6: Wireless and Mobile Networks 6-4Elements of a wireless networknetwork infrastructurewireless hostslaptop, PDA, IP phonerun applicationsmay be stationary (non-mobile) or mobilewireless does not always mean mobility6: Wireless and Mobile Networks 6-5Elements of a wireless networknetwork infrastructure base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its “area”e.g., cell towers 802.11 access points6: Wireless and Mobile Networks 6-6Elements of a wireless networknetwork infrastructure wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates, transmission distance6: Wireless and Mobile Networks 6-7Characteristics of selected wireless link standards384 Kbps384 Kbps56 Kbps56 Kbps54 Mbps54 Mbps5-11 Mbps5-11 Mbps1 Mbps1 Mbps802.15802.11b802.11{a,g}IS-95 CDMA, GSMUMTS/WCDMA, CDMA2000 .11 p-to-p link2G3GIndoor10 – 30mOutdoor50 – 200mMid rangeoutdoor200m – 4KmLong rangeoutdoor5Km – 20Km6: Wireless and Mobile Networks 6-8Elements of a wireless networknetwork infrastructure infrastructure modebase station connects mobiles into wired networkhandoff: mobile changes base station providing connection into wired network6: Wireless and Mobile Networks 6-9Elements of a wireless networkAd hoc modeno base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network: route among themselves6: Wireless and Mobile Networks 6-10Wireless Link CharacteristicsDifferences 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 wellmultipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times…. make communication across (even a point to point) wireless link much more “difficult”6: Wireless and Mobile Networks 6-11Wireless network characteristicsMultiple wireless senders and receivers create additional problems (beyond multiple access):ABCHidden terminal problemB, A hear each otherB, C hear each otherA, C can not hear each othermeans A, C unaware of their interference at BABCA’s signalstrengthspaceC’s signalstrengthSignal fading:B, A hear each otherB, C hear each otherA, C can not hear each other interfering at B6: Wireless and Mobile Networks 6-12Code Division Multiple Access (CDMA)fourth type of medium access used in several wireless broadcast channels (cellular, satellite, etc) standardsunique “code” assigned to each user; i.e., code set partitioningall users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode dataencoded signal = (original data) X (chipping sequence)decoding: inner-product of encoded signal and chipping sequenceallows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)6: Wireless and Mobile Networks 6-13CDMA Encode/Decodeslot 1slot 0d1 = -11 1 111-1-1- 1-Zi,m= di.cmd0 = 11 1 111-1-1- 1-1 1 111-1-1- 1-1 1 111-1-1- 1-slot 0channeloutputslot 1channeloutputchannel output Zi,msendercodedatabitsslot 1slot 0d1 = -1d0 = 11 1 111-1-1- 1-1 1 111-1-1- 1-1 1 111-1-1- 1-1 1 111-1-1- 1-slot 0channeloutputslot 1channeloutputreceivercodereceivedinputDi = Zi,m.cmm=1MM6: Wireless and Mobile Networks 6-14CDMA: two-sender
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