Ranveer Chandra, Jitendra Padhye, Lenin Ravindranath, Alec Wolman.
Transcript of Ranveer Chandra, Jitendra Padhye, Lenin Ravindranath, Alec Wolman.
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Ranveer Chandra, Jitendra Padhye, Lenin Ravindranath, Alec Wolman
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Which AP is serving client C? At any given point in time, there should be only
one answer Responsibility for relaying downstream traffic Secondary purposes: PSM, capability
negotiation
“broadcast” applications: certain information is Inherently relevant to a particular location Largely independent of the receiving client▪ e.g. not confidential
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Wi-Fi clients perform scans to discover nearby APs (disconnected and connected) Passive Scans▪ Stations switch channels, listening for
beacons Active Scans▪ Stations send probe requests, listen for probe
responses Networks Found
Connected
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Low bandwidth communication protocol for IEEE 802.11 (Wi-Fi) networks
Construct a transport protocol by overloading beacons
Clients receive information from nearby APs Even when they are disconnected When connected to another AP
Primarily for “broadcast” applications Two way communication is also possible
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Network SelectionLocalized Advertisements
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Beacons overloaded with Pricing Information Number of active users Other network utilization Information (e.g. 802.11k )
Help end users or client software in making connection decisions
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Handheld devices with Wi-Fi are proliferating Embed advertisements in beacons Clients receive ads regardless of their
connection status Implicitly location-sensitive “Push” model of ad delivery – improves privacy
model Can include dynamic information
e.g. 4 tables free, or 15 minute wait
Delivery of advertisements over the Internet has become a huge market Ads are targeted – Relevant to the user
Location-sensitive advertisements already becoming an important market
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AdCenter APat CoffeeDay
WiFi Beacon
Within 250 m of CoffeeDay
CoffeeDay close to me..Hmm!
I love Hot Choc! Go to CoffeeDay
Coffee DayDrink of the day is Hot Chocolate
Client running our software
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AP fragments each message and sends in successive beacons
Client application performs reassembly, and presents the information to users
Each fragment is of the form: <UniqueID>
- This identifies the specific message <SeqNo>
- Fragment number <More Flag>
- Boolean indicating whether there are more fragments in this message
<Content>- Arbitrary byte stream (usually text)
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Three encoding strategies Overload the SSID Overload the BSSID Create a new Information Element
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SSID is network name with 32 byte limit
Pros User level client software is enough, no driver mods
Cons Low Bandwidth Spams the user-interface (e.g. zero-config)
Networks Found
Connected
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BSSID length: only 6 bytes per beacon All messages can use the same SSID, say
“BeaconTransport” Client assembles message from multiple beacons
Pros User level client software Avoids the spam problem
Cons Very low bandwidth
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Each beacon carries a set of Information Elements Other management frames too (Probe Request, Probe
Response) 802.11 is extensible (sort-of) – Element IDs 32-255
“reserved” Maximum length of an Information Element: 253 bytes
Pros Moderate bandwidth Also avoids spam Feels less like an “ugly hack”
Cons Requires driver modifications on most clients
(except Vista)
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When a fragment is lost, so is entire message FEC, or probes to request retransmissions
Throughput 100 ms is typical beacon interval, easy to
adjust Airtime utilization
Not necessary to send at lowest data rate Power management
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We have built access point & client implementations on XP
Recently finished version of client for Windows Mobile
Future Test deployment (at MS? Or a local mall?) Implement bi-directional communication
(e.g. pub/sub for ads)
Questions?
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Clients receive beacons (hence ads) even when they are not connected to any network
Clients receive beacons from other networks even when they are connected to a particular network
Client need not send any information to the Access Point
Can update Ad text to include dynamic information
Number of tickets left Daily specials in a restaurant Stock quotes
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Starbucks: Get 10% off a Hot Chocolate
Step 1: Coupon Distribution Step 2: Coupon Validation
Present coupon
Discounted Coffee
User does not have to connect to the Internet!
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Competitor’s AP sends fake coupons e.g. Tully’s AP sends invalid coupons for Starbucks
Solution: ASP generates public-private key pair for all registered stores Coupon is encrypted with store’s private key
▪ E[coupon + time of validity]store_private_key
▪ Time of validity prevents replay of outdated coupons
Store/ASP sends encrypted coupon to APs APs broadcasts encrypted coupons Clients validate coupon by decrypting with store’s public
key
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User awareness: Require user input
AP tracking: Track which AP distributed the coupon
Limited coupons: Limit number of distributed coupons
AP not connected to Internet