1. 802.11r Requirements Discussion
Month 2000
doc.: IEEE /xxx
May 2004
802.11r Requirements Discussion
Bob Beach, Symbol Technologies
Nancy Cam-Winget, Cisco Systems
Darwin Engwer, Nortel Networks
Haixiang He, Nortel Networks
Robert Love, LAN Connect Consultants
Jesse Walker, Intel Corporation
Jesse Walker et al
John Doe, His Company

2. May 2004
What is Fast Roaming?
Actually the first question is: what is roaming in general?
really does not talk about the roaming process at all
Protocols are defined but their usage is not
Roaming is a complex problem, fast roaming is even harder
We need a model for roaming before we can talk about requirements.
Jesse Walker et al

3. Roaming in the “Old Days”
May 2004
Roaming in the “Old Days”
Find an AP by listening to beacons and/or doing probes
The number of APs is small and all lead to the same wired network
Just pick one, maybe with the best RSSI value
Do Assoc Request/Response handshake
The new AP sends out an empty message that unassociates STA with the old AP if STA didn’t do it first (and no one did)
Security is “free”: Open or shared WEP keys
Don’t do “Authenticate” Sequence
All APs are trustworthy
If STA gets into trouble, start over again
Jesse Walker et al

4. Roaming now Find APs like before, but now:
May 2004
Roaming now
Find APs like before, but now:
There could be hundreds of them
Connected to different networks
Many of which have the same ESS
Some of which are rogues or honey pots
Getting “associated” is the easy part
But then comes “11i”, 802.1x, authentication,…
And then comes “11e”, TSPECs, ….
And after all that maybe STA changes its mind and decides that the old AP was better after all
And now it has to start over again on the old one
Who may not want you back!
Don’t even ask what happened to your data in this process
802.11r exists because i broke roaming
Jesse Walker et al

5. May 2004
The Plan
In order to make sense of “fast roaming” we need to define a model for roaming in general.
By having such a model we can partition the problem and hence discover what makes roaming fast or slow and how we can improve the roaming
Jesse Walker et al

6. May 2004
A Model for Roaming
The roaming process consists of four independent but related activities
Some run all the time, others run only on certain occasions
The model works for both unassociated and associated mobile units
Jesse Walker et al

7. The Roaming “Process” Build a “Data Base” of APs
May 2004
The Roaming “Process”
Build a “Data Base” of APs
Evaluate the current AP against all of the possibilities in the AP data base
Try to transition to a “better” AP
Post transition cleanup
Jesse Walker et al

8. Building the AP Data Base
May 2004
Building the AP Data Base
Mobile STAs build a data base of known APs
There could be hundreds of them
STA maintains a separate state machine for each AP
State machines not independent – STA can be associated with only one AP at a time
They get this information from several sources
Listening to beacons
Doing probe/probe response sequences
Configuration information
Information from “third parties”
The amount of information about any given AP can vary but the more the better
But the amount of data not be the same for all APs
The type of data may not be uniform for all APs
Jesse Walker et al

9. Interesting things about an AP
May 2004
Interesting things about an AP
ESSid, BSSid, Channel/Frequency
Security options, keys
Provider
Last/average received RSSI
Last Contact Time (local time and TSF)
Association “State”/History
Load information
Other useful things
“Bad experiences” (e.g. it’s a rogue, it refused my association)
Things that disqualify it (wrong security, wrong network)
Jesse Walker et al

10. The Information will change
May 2004
The Information will change
As the mobile STA moves, the set of available APs may change, as will their RSSI
The APs themselves can also change
Load will vary as Mobile STAs do different things
Channel/output power may change
The data base must be updated on a regular basis
The frequency depends on the state of the device as well as the what applications may be running
Jesse Walker et al

11. Evaluating the State of the World
May 2004
Evaluating the State of the World
Given the Mobile STA has built the AP data base, it must evaluate its current association against all the possibilities present in the AP data base
“Is the grass greener over there?”
The evaluation criteria is not constant but varies over time based on several factors such as:
State of current association
Application requirements
Jesse Walker et al

12. The most basic criteria
May 2004
The most basic criteria
Is there a current association?
If not,
Did things just change?
Was STA in the middle of something and really need one
Maybe its not important and STA should just give up for now
But when should STA try again??
Jesse Walker et al

13. Given STA is associated
May 2004
Given STA is associated
What does STA need from the association?
Basic connectivity (nothing going on, just listening to beacons)
Doing “plain old data transfer things”
Transactions, web browsing, file transfer
Want good transfer rate but will accept what I can get
Doing time critical things that are all or nothing
Voice is a good example – 95% of needed throughput is useless
Jesse Walker et al

14. How painful is it to roam?
May 2004
How painful is it to roam?
Which is more painful?
Staying where STA is now, or
Going through the roaming process?
If some target AP is in another SSID, then should STA expect a full authentication be required, and transition will not be fast?
If STA roams
Need to go through the process
It will take time
Will that disrupt current activity?
It may not succeed
STA might get a new IP address
Jesse Walker et al

15. May 2004
The Roaming Process
Having made the decision to roam, we now have to do it.
It will take time and it may not succeed
The process consists of at least three substeps
Failure is possible at any step
Jesse Walker et al

16. The Steps to a Successful Roam
May 2004
The Steps to a Successful Roam
The new AP accepts by association request
May get rejected for one reason or another
Successfully authenticate and get valid key information
Lots of failures possible here
Obtain the level of service needed
May be a non-issue or big issue
Jesse Walker et al

17. Its not over until its over
May 2004
Its not over until its over
A roam is not successful unless all steps are successful
A failure after basic association sequence can leave both AP and mobile STA in odd states
Am I associated or not??
is a little ambiguous on this point
“Association” is complete after initial handshake
But its not usable until after 11i and 11e is done
Jesse Walker et al

18. The final step: Cleanup
May 2004
The final step: Cleanup
After the step 3 is done, the Mobile STA is connected to the new AP
We still need to do things
Let the old AP know so that it stops forwarding data over the air interface to the STA
Let the old AP know to stop billing the STA
STA updates the local AP data base to reflect the new association
Jesse Walker et al

19. STA Model Per-AP Pre-Transition Discovery Per-AP Data Base
May 2004
STA Model
Per-AP Pre-Transition
Discovery
Per-AP Data Base
Per-AP Transition
Per-AP Post-Transition
Jesse Walker et al

20. Scope APs in the same ESS (same SSID)
May 2004
Scope
APs in the same ESS (same SSID)
APs might support multiple media, viz .11a, .11b, .11g
Jesse Walker et al

21. Requirements we will not address
May 2004
Requirements we will not address
Algorithms for determining which AP to chose or when to transition to another AP.
Except to exclude pathological solutions that lead network to collapse.
PHY layer specific issues
Others?
Jesse Walker et al

22. Requirements we will address
May 2004
Requirements we will address
Reliability of the transition process
QoS
Fall-Back mechanisms (to alternate slower mechanisms)
Handover algorithm between APs.
Define infrastructure conditions for fast transition
Requirement on Transition Time
Information collection flexibility (scanning)
PHY awareness and its implications
Define security implications and insure preservation of security level if required
Characterize transition to understand when the transition is complete
Others?
Jesse Walker et al

23. May 2004
Feedback?
Jesse Walker et al