Ranveer Chandra, Jitendra Padhye, Lenin Ravindranath, Alec Wolman

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

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

Low bandwidth communication protocol for IEEE (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

Network Selection Localized Advertisements

Beacons overloaded with Pricing Information Number of active users Other network utilization Information (e.g k ) Help end users or client software in making connection decisions

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

AdCenter AP at 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

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: -This identifies the specific message -Fragment number -Boolean indicating whether there are more fragments in this message -Arbitrary byte stream (usually text)

Three encoding strategies Overload the SSID Overload the BSSID Create a new Information Element

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

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

Each beacon carries a set of Information Elements Other management frames too (Probe Request, Probe Response) is extensible (sort-of) – Element IDs 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)

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

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?

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

Starbucks: Get 10% off a Hot Chocolate Step 1: Coupon DistributionStep 2: Coupon Validation Present coupon Discounted Coffee User does not have to connect to the Internet!

Competitors AP sends fake coupons e.g. Tullys AP sends invalid coupons for Starbucks Solution: ASP generates public-private key pair for all registered stores Coupon is encrypted with stores 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 stores public key

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