1. January 2003 Joe Kwak InterDigital Communications Corporation
IEEE /100r1
doc: IEEE /225r0
March 2003
WLAN Handoff Scenarios Example handoffs with RRM measurements and network assistance
Joe Kwak
InterDigital Communications Corporation
Submission
Joe Kwak, InterDigital

2. Outline WLAN Handoff Functional Model
January 2003
IEEE /100r1
Outline
WLAN Handoff Functional Model
Functional analysis with/without network assistance
Message Sequences for Autonomous H/O
Message Sequences with Network Assisted H/O
Open Issues
Joe Kwak, InterDigital
Submission

3. WLAN H/O Functional Model
STA configures measurements and selects source Access Point (AP) from set of available neighbor APs.
STA configures cycle of neighbor measurements to continuously evaluate H/O options, continuously apply H/O decision algorithm.
When H/O is imminent, STA and network may prepare for scheduled H/O.
At H/O time, STA reassociates with destination AP.
AT H/O time, network source/target APs coordinate for IP reroute (IAPP) and data fragment routing.
Select/Configure
Measurements
Perform
Apply H/O
Decision
Algorithm
Handoff to
New AP
H/O
Yes No
Figure 1. STA Functions for AP Handoff
Joe Kwak, InterDigital
Submission

4. Scenario Goals
Demonstrate H/O functions in terms (present and future)
Indicate use of inter-RAT measurements between 11a and 11b for 11g devices
Indicate method to track signal quality of neighbor APs
Indicate desire to coordinate “seamless” H/O
Indicate differences between STA autonomous H/O and network assisted H/O
Detail high level Message Sequence Charts for autonomous and network assisted H/O
Scenario details reveal use of example, simplistic H/0 decision algorithm.
Indicate H/O support items which are out of scope for TGK.
Joe Kwak, InterDigital
Submission

5. Scenario Functional Analysis1
1. Initial association, using current methods, to select source AP.
2. Initial neighbor scanning to determine AP H/O options on all channels, all PHYs.
Auto: Passive scan all, then active scan idle channels; optional Beacon Report indicating link quality/received power to AP.
Net: AP sends Beacon_Scan_Request action frame to STA, Beacon Scan Report required.
3. Schedule continuous scan cycles. Use Power Save mode or NAV based non-contention periods to measure on other channels, on other PHYs.
Active PHY/Channels: short cycle period (20 second?), fast passive scan.
Auto: Continuous scanning with optional report cycle (1 minute?) with Beacon Report of all active PHY/channels.
Net: AP sends Beacon_Scan_Request with continuous scanning and periodic reporting (1 minute cycle?). Network may update neighbor list by reissuing Beacon_Scan_Request.
Inactive PHY/Channels: longer cycle period (5 min?), slower active scan. Active scan results used to update active PHY/channels list. Active scanning may only be initiated when number of active PHY/channels falls below threshold. 2 3
Joe Kwak, InterDigital
Submission

6. Scenario Functional Analysis (2 of 3)4
4a. Auto: Schedule continuous handoff decision evaluation.
Continuous statistics update with each beacon scan:
PSNI: LastVal, AverageVal, Variance, Trend.
RCPI: LastVal, AverageVal, Variance, Trend.
Apply H/O decision threshold after each beacon scan.
When H/O criteria are met, send H/O_Prep_Request.
Cross H/O Thresh
H/O trigger
Signal quality
Time
AP 3
AP 1
AP 4
AP 5
H/O Hysteresis
Cell load factor
AP1--->AP5
Joe Kwak, InterDigital
Submission

7. Scenario Functional Analysis (3 of 3)4
4b. Net: Schedule continuous handoff decision evaluation.
AP Updates neighbor list and provides cell load factors by reissuing Beacon_Scan_Request with continuous scanning and periodic reporting.
AP requests continuous measurement of STA’s UL load factor with periodic reporting.
STA may receive H/O_Prep_Request action frame from AP at any time.
STA may send H/O_Prep_Request action frame to AP at any time.
Cell load factor may be used to prevent H/O to certain or all cells.
5. Execute Synchronized_Handoff_Request Procedure
Source AP coordinates H/O with target AP. IP reroute for H/O is prepared at H/O_Prep time. Context transfer is prepared. Make-before-break switch?
Target AP pre-allocates required resources for STA.
Target AP may pre-authenticate, pre-authorize STA???
At H/O time, source AP reroutes IP for STA to target AP, transfers STA context.
At H/O time, STA changes PHY/channel and reassociates with target AP.
After H/O time, source AP forwards any undelivered data segments to target AP if decryption is still possible.
After H/O time, target AP reauthenticates and reauthorizes STA and reestablishes services with minimal (??) interuption. 5
Joe Kwak, InterDigital
Submission

8. Autonomous H/O Scenario Message Sequence Chart (1 of 2)
AP1-n
STA
APsource
Association_Request 1
Association_Response
Beacon_Scan_Request[NL]
Beacon_Scan_Response
Initial Neighbor Scan:
1. Passive scan all 11a chans
2. Active scan all inactive 11a chans
3. Passive scan all 11b chans
4. Active scan all inactive 11b chans
5. Off-channel scans not to interfere
with ongoing services
Power_Save_ON
Power_Save_Ack
Listen/Probe Chan1
Beacon or Probe Response 2
Listen/Probe Chan n
Beacon or Probe Response
Power_Save_OFF
Power_Save_Ack
Beacon_Scan_Report
Schedule Beacon Scans 3
Listen/Probe Chan 1
Beacon or Probe Response
Power_Save_On
Power_Save_Ack
Power_Save_OFF
Continuous Scan Cycles:
1. Short cycle active neighbor scans
2. Long cycle inactive chans
3. Update short/long PHY/chan lists
4. Apply H/O criteria after each short
cycle scan
5. Off-channel scans not to interfere
with ongoing services
Listen/Probe Chan 1
Beacon or Probe Response
Power_Save_On
Power_Save_Ack
Power_Save_OFF 4
Listen/Probe Chan 1
Beacon or Probe Response
Power_Save_On
Power_Save_Ack
Power_Save_OFF
Beacon_Scan_Report
Optional Message
Joe Kwak, InterDigital
Submission

9. Autonomous H/O Scenario Message Sequence Chart (2 of 2)
AP1-n
STA
APsource
When H/O criteria are met:
- STA may optionally notify APsource
for scheduled H/O, or
- STA reassociates without scheduling.
- If unscheduled H/O, APtarget uses
IAPP to transfer context and reroute.
- If scheduled, APtarget and APsource
coordinate at scheduled time.
H/O threshold crossed
H/O_Prep_Request[Apx,T]
H/O_Prep_Response
H/O_Prep_Request[Apx,T]
H/O_Prep_Response
at time=T
Reassociate_Request 5
Reassociate_Response
IAPP_Reassociate
H/O_Complete_Request
H/O_Complete _Response
Joe Kwak, InterDigital
Submission

10. Network Assisted H/O Scenario Message Sequence Chart (1 of 2)NM
AP1-n
STA
APsource
Beacon_Scan_Request[NL]
Beacon_Scan_Response
UL_Load_Request
UL_Load_Report
Power_Save_ON
Power_Save_Ack
Listen/Probe Chan1
Beacon or Probe Response 2
Initial Neighbor
Scan: same as Auto
Beacon_Scan_Report
Listen/Probe Chan n
Beacon or Probe Response
Beacon_Scan_Report
Beacon_Scan_Report
Power_Save_OFF
Power_Save_Ack
NL&CellLoad_Info
Beacon_Scan_Request[NL,Load,Hyst]
Beacon_Scan_Response 3
Schedule Beacon Scans
Continuous Scans,
H/O Eval: same as
Auto but with NM
supplied Cell Loads
and Hysteresis for
H/O evaluation
Listen/Probe Chan 1
Beacon or Probe Response
Power_Save_On
Power_Save_Ack
Power_Save_OFF 4
Listen/Probe Chan 1
Beacon or Probe Response
Power_Save_On
Power_Save_Ack
Power_Save_OFF
NL&CellLoad_Info
Beacon_Scan_Report
Beacon_Scan_Report
UL_Load_Report
Joe Kwak, InterDigital
Submission

11. Network Assisted H/O Scenario Message Sequence Chart (2 of 2)NM
AP1-n
STA
APsource
When H/O criteria
are met:
- STA will notify
APsource for
scheduled H/O, or
- NM will notify
scheduled H/O.
-All H/Os are
scheduled.
H/O threshold crossed
H/O threshold crossed OR
H/O_Prep_Request[Apx,T]
H/O_Prep_Request[Apx,T]
H/O_Prep_Response
H/O_Prep_Request[Apx,T]
H/O_Prep_Request[Apx,T]
H/O_Prep_Request[Apx,T]
H/O_Prep_Response 5
at time=T
Reassociate_Request
Reassociate_Response
H/O_Complete_Request
H/O_Complete _Response
Joe Kwak, InterDigital
Submission

12. New Items Needed to Support Handoff Scenarios
Action frame to perform Beacon_Scan_Request periodic measurements including:
short passive scan cycle of known neighbors (NL), periodic scan report
long active scan cycle on other channels, immediate beacon report on probe response
Supply STA hysteresis value for H/O decision
Supply Cell Loading factors for NL cells for H/O decision.
Measurement Report frame to provide Beacon_Scan_Report
Action frame for measurement of UL_load in STA.
Measurement Report frame to provide UL_Load_Report
Action frame for scheduled H/O_Preparation with APtarget, scheduled time for handoff, and other parameters.
Modifications to IAPP to support scheduled H/O
Action frame to perform H/O_Complete exchange to complete scheduled context transfer and to transfer any remnant data segments.
Network Manager (NM) access to Beacon_Scan_Report measurement data.
NM provision of Cell Load Factors and Hysteresis for H/O evaluation.
Items in red are out of scope for TGK
Joe Kwak, InterDigital
Submission

13. Open Issues
TGK is addressing only half of the specification changes for Handoff: the RRM measurement and reporting portion [STA-->Higher Layer Applications].
The network assistance interface to provide new H/O decision parameters [HL Applications-->STA] are out of scope for TGK.
Efforts are needed in WNG committee to initiate SG to begin work to complete this handoff interface.
Parallel efforts are underway at 802-level for Handoff Interface guidelines and may affect and TGK.
?Strawpoll to assess level of interest in starting now?
Joe Kwak, InterDigital
Submission