Improved Access Point Selection

MobiSys2006

Outline

• INTRODUCTION

• FIELD STUDY

• VIRGIL

• EVALUATION

• CONCLUSION

INTRODUCTION

INTRODUCTION

• Strongest signal strength(SSS)

• Presents Virgil– quickly associates to each, and – runs a battery of tests to estimate the quality

INTRODUCTION

• Legal and Security Issues– if users cannot trust their network access points,

end-to-end encryption is the only reliable way.

FIELD STUDY

1. Finds one of the APs in a list

2. Scans for all available APs and chooses with the strongest signal strength(SSS).

FIELD STUDY

1. A new AP ?

2. Unusable or usable AP

3. Quality of Internet connection ?

FIELD STUDY

VIRGIL

1. Scan for all available APs2. Test each unencrypted AP in the scan set

– Get AP properties (SSID, MAC address, signal strength, etc.)– Try to get DHCP address from AP– If successful, probe the AP– Store test results in a local database

3. Select the “best” AP, based on test results

VIRGIL-Probing an AP

• “Quality” is highly subjective– Bandwidth– Port traffic– Round-trip-time

• By spawning a thread.

VIRGIL-Probing an AP

• To aid in AP testing, use a set of reference servers.

• Virgil tests the status of a handcompiled list of 45 common port numbers.

• For UDP ports, Virgil simply sends the nonce. – (nonce+1) - “open”. – something different - “redirected”. – nothing before a timeout expires - “closed”.

VIRGIL-Leveraging History

VIRGIL-Leveraging History

• Keep database to improve performance, but not repeatedly rescanning.

• Updates its timestamp field.

• Every 30 minutes, but this is a configurable value.

VIRGIL-Choosing the “best” AP

• Based on these “test results”,

• By “open”,– (1) the AP granted a DHCP address to the

client. – (2) at least one port was found to be open.

EVALUATION

1. How much more often does Virgil let users be connected, than simply selecting on signal strength?

2. How much better are the connections that Virgil finds?

3. How beneficial is tracking AP history?

4. Is Virgil’s overhead reasonable such that it is useful to users?

EVALUATION

EVALUATION-Connection Time and Quality

EVALUATION-History

EVALUATION-Overhead

• Client Overhead

EVALUATION

• Reference Server Overhead

CONCLUSION

• An automatic AP discovery and selection system.

• Quickly associates

• Runs a battery of tests for use

• By estimating the bandwidth and round-trip-time

• Probe for blocked or redirected ports