Location Based Services

Lenin Ravindranath, Microsoft Research India

Motivation Location is a key element of user context Mobile devices a natural conduit for location

based services They are ubiquitous Have many sensors sensors

Location -> GPS GPS based devices are not common GPS is power hungry

Many apps care about relative location relative to landmarks relative to people

Scenario

Demo!

Aspects Location Determination

need relative location Neighborhood Discovery

spatial and temporal Communication

push vs. pull with or without Internet access

Service model stateful vs. stateless

Overview Beacon-Stuffing

WiFiAds Neighborcast

People Near Me Neighborhood GPS

Landmark based Services Post-its in the air Ads, comments and reviews Location and Neighborhood based reminders,

profiles Social networking

Beacon-Stuffing

Access Point

Beacon Packets

Network NameSSID: NYCWireless

Network NameSSID: T-Mobile

Networks Found

Connected

Connected

WiFi Beaconing Protocol

Beacon-Stuffing Stuff beacon packets with information

Unused fields in beacons (Beacon Information Element)

Stripe across multiple beacons, if needed Clients receive beacons even when they are

not connected to any network Clients receive beacons from other networks

even when they are connected to a particular network

Send dynamic information

WiFiAds – Location sensitive Ads

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

Beacon-Stuffing Summary No absolute location required

Clients near the broadcasting AP gets the information Push model of information delivery No connectivity required Stateless

Published in HotMobile 2007 Joint work with Ranveer Chandra, Jitu Padhye, Alec Wolman

Techfest 2007 Demo

Overview Beacon-Stuffing

WiFiAds Neighborcast

Buddies Near Me Neighborhood GPS

Landmark based Services Post-its in the air Ads, comments and reviews Location and Neighborhood based reminders,

profiles Social networking

Neighborcast Goal: communication among nearby nodes

2 nodes are nearby if they hear a common AP Nearby nodes can be on different frequency

channelsLAN/Internet

Neighborcast Solution: Leverage multicast techniques

Mapping from AP BSSID to multicast group address BSSIDs correspond to AP’s MAC ID, and is globally unique

Client joins group of every AP around it APs it hears as part of its scanning process

Clients frequently publish info on group of associated AP Infrequently publishes info on other groups it is

subscribed to

NeighborcastLAN/

Internet

00:0B:80:12:34:5600:0A:80:12:34:55 00:0C:80:12:34:57

Multicast group for BSSID 00:0B:80:12:24:56

Implementing Neighborcast 3 approaches:

IP Multicast Application Level Multicast (ALM) Pub-sub using RSS feeds

Tradeoffs: overhead, scalability, ease of deployment

Neighborcast using IP Multicast Map AP’s BSSID to IP multicast group

address BSSID (6 bytes): <Manufacturer’s

prefix>:XX:XX:XX:XX IP multicast address (4 bytes): <224 –

239>:XX:XX:XX

Our mapping approach: 228.XX.XX.XX, where XX:XX:XX is last 3 bytes of

BSSID Example: 00:17:95:81:CA:30 will be

228.129.202.48 Since 0x81 = 129, 0xCA = 202, 0x30 = 48

Might have collisions! Probably not in same subnet…

Neighborcast using IP multicast: Every client maps AP’s BSSID to IP multicast

address Joins multicast group of all BSSIDs it hears Publishes info of multicast group of AP it is

associated with Pros:

No need for special infrastructure/server Approach is scalable since all traffic is local

Cons: Limited deployment of IP multicast, only in edge

networks

Neighborcast using IP Multicast

Buddies Near Me Each client joins Neighborcast group of all

APs around it It announces its presence (along with signal

strengths) to its Neighborcast group Learns of nearby buddies Can also estimate “nearness”

Signal strength

Neighborhood GPS Dynamically help neighbors leverage GPS

data from other phones

GPS

computed-location

Cloud/Server

Neighborcast Summary No absolute location required

Two clients are nearby if they hear a same AP Neighborcast forms IP overlay among nearby

nodes Client needs to be connected Stateless

Published in HotMobile 2008 Joint work with Ranveer Chandra, Jitu Padhye

Techfest 2008 Demo

Overview Beacon-Stuffing

WiFiAds Neighborcast

People Near Me Neighborhood GPS

Landmark based Services Post-its in the air Ads, comments and reviews Location and Neighborhood based reminders,

profiles Social networking

Location as Landmarks For many apps

absolute location (GPS) is not required

Location as perceived by a user

Landmarks Access Points BSSID GSM Tower ID

GSM Tower Access Point

Information tagged with Landmarks Users publish information

Landmark Tagged Information Automatically tag information published with landmarks Pictures, Videos, Blogs, Ads

Users search for information Landmark based Search

Query information based on landmarks

Landmark based services

Publish (info, AP BSSID, GSM ID)

Search(query, AP BSSID, GSM ID)

Access Point AP1BSSID: 00:11:22:33:44:55

GSM Tower GSM1Tower ID: XX1 Server

Post-its in the air

Post-its in the air Scan for landmarks Post (tagged with landmarks) Search (giving landmarks) Retrieve post-its in the air

Challenges What if a landmark moves?

Move posts to Neighborhood How to find the neighborhood?

Radio map

Radio Map Periodically clients report Access Points and GSM towers

seen Build a graph

Nodes are APs/GSM Towers Edge between two nodes if they are reported together Node weights and Edge Weights Delete nodes and edges based on their weights

GSM

WiFi

WiFi

WiFi

WiFi

WiFiWi

Fi WiFi

GSM

WiFi

WiFi

WiFi

WiFi

WiFi

WiFi Wi

Fi

Buddies Near Me

GSM

WiFi WiFi

WiFi

WiFi

WiFiWiFi

GSM

WiFi

Buddies who were here You are visiting a new place

Need to know information and past experience of people

Find friends who visited this place in the past Find them online

Ask queries

Location Sensitive Search Search for Coffee

Gives nearby Coffee shops info Search results ranked based on nearness

Comments and Reviews Read others reviews Post comments

Reminders in the air Location based reminders

Post reminders in the air Remind me when I cross this place next time

Remind me to buy milk when I am near the market Remind me office work only when I am in the office

Neighborhood based reminders Remind me when one or more friends are nearby at a

location Remind me to give Prashanth Rs 100 when he comes

nearby

Other Applications Location based profiles Location based games, Game pairing Dating service Enhanced presence

Landmark based services Summary No absolute location required

Leverage Landmarks and Neighborhoods Client needs to be connected Stateful Number of applications

Submitted to Ubicomp 2008 Joint work with Mohit Gupta, Prashanth Mohan

Thinkweek paper (Brooks was here.. So was Red)

Benefits• Does not depend on GPS• Lesser power• Works inside buildings, malls

• No regular war driving necessary (as is in the case of WiFi, GSM triangulation)

• Readily deployable• No operator support required

Questions

Code in the air Post code in the air Devices crossing that location will execute

them Harnessing other devices

Example Monitor temperature at a place continuously Post the following code in the air

use sensor(themometer)temp = themometer.getTemperature()send(temp, “http://leninr/tempService”)