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Class Note for COSC 6397 at UH


Class Note for COSC 6397 at UH

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This 38 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Houston taught by a professor in Fall. Since its upload, it has received 20 views.

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Date Created: 02/06/15
Location amp Context Awareness 117 39 I quot39 Potential Killer App s for Wireless Data Networks I Exploit location specific information il Find the nearest caf movie theater l l V reless dating matchmaker game How to play BotFighters 1 Set up your robot 2 Scan for enemies with your Robot Raclar39 i 3 Run after your opponent 4 Destroy your target or Flee in terror Ilinmmn39 mumnv 1 18 MobiCom 2002 Talk by Ed Knightly 39 I What is Context I Context any information that can be used to characterize the situation of an entity An entity is a person place or object that is considered relevant to the interaction between a user and an application including the user and applications themselves I Defined by enumerating the examples of contexts Computing context eg the network connectivity communication costs and communication bandwidth and nearby resources such as printers displays and workstations User context eg user s profile location people nearby even the current social situation Physical context eg lighting noise levels traffic conditions and temperature Time context eg time of a day season ofthe year etc 119 39 I How is Context Useful I Active context awareness iii Adaptation of applications to the context I Passive context awareness i i Acquisition of context information 120 39 I quot Example Applications I Call forwarding 2 Olivertti Research Ltd ORL 2 Active context user s location The location context is presented to the receptionist who forwards the phone calls to the destination user s nearest phone I Teleporting ORL 391 Active context user s location and location ofworkstation 391 follow me computing dynamically map the user interface onto the resources of the surrounding computer and communication facilities I Conference assistant Future computing environments at GIT Passive context current activity Active context attendee s location current time schedule of presentation Match conference schedule topics of presentations user s location and user s research interests to suggest the presentations to attend Extended to other reminder services Still lack of killer application 121 39 I Enable Technologies I Location sensing Cl Lowresolution means user logging to a system sliding hisher badge Ll GPS in Indoor localization systems I RFbased ultra sound l l Localization in multihop wireless networks I Clock synchronization I Process migration 122 Q Navigation in Old Times The Light by Hattie Vose Hall I am the light holding Nightly my torch on high Under me surge the waters Over me bends the sky Year after year I stand here Holding my steady light Sending its ray of comfort Into the darkest night 123 39 L Localization Methods I Time of arrival TOA I Angle of arrival AOA I Signal strength I Proximity based I Passive I Active 124 39 I 7 GPS The Global Positioning System GPS is a worldwide radionavigation system formed from a constellation of 24 satellites and their ground stations Allow an accuracy of 10 to 20 meters Doesn t work well in indoor environment Ii39Z Signal strength too low 1 i Multipath effects GPS receivers 50 1000 with maps 125 39 I Triangulation I Satellites follows deterministic trajectory 9 known locations I Distance measurements from three satellites in space can determine two gossible locations Required 4 in practice httpwwwtrimblecomgpshowhtml 126 quot Measuring Distance I Distance measurement using pseudo random code J39 L Leeel 39 m J W Reeeivee er 1 l Requires accurate clock synchronization 1 Satellites are equipped with atomic clocks but regular GPS receivers are not 127 39 1 U Mitigate Clock Errors Measured distance Real distance 128 39 1 U Mitigate Clock Errors Why this works 129 39 Enhanced911 ml I Require wireless carrier to provide the telephone number of a wireless 911 caller and the location of the antenna that received the call with location information within 50 to 100 meters i l Passive ii Use time of arrival TOA ortime difference of arrival TDOA a m 1 30 L Indoor Localization I Active Bat ATT Cambridge Lab Sound source 131 39 Indoor LocalizationiCont d I MIT Cricket locationsupport system 132 Localization Using APs in WLAN B VA U 133 39 U Areabased localization Use of SNR Map Markovian mobility model Practical Robust Localization over LargeScale 80211 Wireless Networks Andreas Haeberen Eliot Flannery Andrew Ladd Agis Rudys Dan Wallach Lydia 134 Kavraki Rice University USA A Comparison Hm rnal l39l39nn Z a 9 y 1 3 m 5SEWI IU a Nancy It may latency SCALE ROOM BUILDING COMPLEX CETY httpwwwubisensenet 135 39 I Localization in Multihop Wireless Networks l Methods like signal strength TOA TDOA AOA still apply l Use a few anchor points with known locations Ii39Z Problem accumulation of errors 136 I Use hop counts as rough distance measurements 137 39 I quot Time I Time is an important concept in distributed computing environment l Ecommerce need to timestamp events occurred at a merchant s computer and at a bank computer 1 Ordering of events wherewhen location time of mobile entities Sometimes to determine location time information is needed 1 Multimedia synchronization for realtime teleconferencing Fusion of data computer on 2144 2145 2146 2147 4 Time according which compiler l l to local clock l l l runs outputo created Computer on 2142 2143 2144 2145 9 Time according which editor 4 i i to local clock runs outputc created 138 39 I 111 Clock Synchronization I Clock synchronization is a nonproblem for a standalone computer I But clock skew happens across computers Clock time C UTC t 139 39 I Standard Clocks I Atomic clock 1 An atomic frequency standard as its count 109 seconds per day TAI Temps Atomique International International atomic time I Universe coordinated time UTC UTC is broadcast by NIST from Fort Collins Colorado over shortwave radio station WVW VVWV broadcasts a short pulses at the start of each UTC second Derived from TAI but synchronized with the passing of day and night based on astronomical observations 140 39 I Techniques for Time Synchronization We have learned I GPS I Time synchronization function in WLAN 1 Synchronization in WLAN V th AP TSF broadcast by AP in beacon messages I All mobile stations adopt the same value In ad hoc mode adopt the largest value Problems delay on the interface We also know that I Cell phones are loosely synchronized I Radio clocks get synchronized 141 39 I I NTP The Network Time Protocol I Format 1 32bit integer 32bit fraction in seconds I Accuracy 1 better than 1 ms in LAN and tens of ms in WAN Request 142 NTP Architecture 573539 Standard Time Stratum 1 Server 3 Stratum 2 Server Stratum I Hierarchical server client peer iii stratum levels Server E Primary stratum 1 servers synchronize to national time standards via radio satellite and modem r3 Subnets 143 39 I Network Delay and Clock Offset ml Stratum N T2 T3 T A Clock A d1 d2 Clock B St t N1 T m m T1 T4 B Suppose CIOCk B is to ahead of clock A T2 d1 T1 to T4 T3 d2 t0 t0 T2T1T3T42 d2 d1 2 11 If d2 d1 9 t0 T2T1T3T42 144 L Multiple Synchronization Path Fl Standard Time Standard Time Standard Time Stratum 1 Server Stratum 2 Server Stratum 3 Server 145 quot Combining Multiple Time Sources V H 7 I I39I fw39 I V k I HIIIII by W 5 Cilii39i if ii L V V I I I i ii I I i I I Timestamps E I NTP Messages 146 Critical Path in Ciock Synchronization Sender Receiver f Critical Path Time Read the clock eg gettime system call Construct a packet Transmit overthe network interface Propagation delay Received at the network interface Pass to the application Extract the time information 147 iii 39 1 Reference BroadcaetTSynchronization Sync 2 receivers with each other NOT sender with receiver Receive Time Propagation Time Physical Media 1 48 Jeremy Elson Lewis Girod and Deborah Estrin FineGrained Network Time Synchronization using Reference Broadcasts 39 I RBS cont d l RBS removes send and access time errors 1 Broadcast is used as a relative time reference n l Each receiver synchronizing to a reference packet Ref packet was injected into the channel at the same instant for all receivers uj Message doesn t contain timestamp Almost any broadcast packet can be used eg ARP RTSCTS route discovery packets etc iNI Sender Receiver 1 Receiver 2 lifl W J Critical Path gt 149 39 I quot RBS Procedure I Nodes periodically send beacon messages to their neighbors I Each receiver records the time that beacon was received according to its local clock I Receivers exchange observations to estimate clock screw t1 t1 at2 b t2 150 39 L Precision of R88 I Ignore propagation delay in 1m3e8 0003usec I Can achieve an accuracy on the order of usec tumulauve Error Probabiliw Synchronization Error with Light Network Load 10 RESKernel 39 09 RBSUsar spaca NTPUserspace 07 V V 0 5 01 00 001 01 1 10 100 1000 Error usec 151 39 Heavy Load Scenario L Eumulatiue Error Probability 10 09 08 0 05 05 04 03 02 01 00 Sgnohronization Error with Heavy Network Load RBS Userspace NTPUserspace 01 10000 Error usec 152 ID L Extend R88 to Multihop Wireless Network I Use relays 153 39 I quot Things to remember Know what context awareness means Understand how GPS mitigates inaccurate clocks Understand how different localization methods work How does NTP work What affects the accuracy of a synchronization method How does RBS take advantage of wireless broadcast medium 154


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