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doc.: IEEE /0086r0 Submission January 2004 Darwin Engwer, Nortel NetworksSlide 3 Scope of this presentation Considers existing standards: –1999 base, a, b, d, f, g and h Does not specifically address pending standards: –802.11e, i, j and n aspects of roaming not addressed: –criteria for triggering the roaming event
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January 2004
Darwin Engwer, Nortel Networks
Slide 1
doc.: IEEE 802.11-04/0086r0
Submission
Measurement of802.11 Roaming Intervals
Darwin EngwerNortel Networks
4655 Great America Pkwy, Santa Clara, CA 95054Phone: 408-495-7099
Fax: 408-495-5615e-Mail: [email protected]
January 2004
Darwin Engwer, Nortel Networks
Slide 2
doc.: IEEE 802.11-04/0086r0
Submission
Goals of this presentation
• define the components of an 802.11 system• define roaming in an 802.11 system• identify an initial set of roaming conditions for
consideration/ analysis• identify the start and end points of the roaming
interval• present test setups for effective roaming interval
measurements
• stimulate thought and discussion on handover topics ...
January 2004
Darwin Engwer, Nortel Networks
Slide 3
doc.: IEEE 802.11-04/0086r0
Submission
Scope of this presentation
• Considers existing 802.11 standards:– 1999 base, 802.11a, 802.11b, 802.11d, 802.11f, 802.11g and 802.11h
• Does not specifically address pending standards:– 802.11e, 802.11i, 802.11j and 802.11n
• aspects of roaming not addressed:– criteria for triggering the roaming event
January 2004
Darwin Engwer, Nortel Networks
Slide 4
doc.: IEEE 802.11-04/0086r0
Submission
Components of an 802.11 System
• Std 802.11- 1999 defines a station (STA)• further defines a subset of STAs called
Access Points (APs)– “STA acting as an AP”
• for clarity I further define a “STA that is NOT acting as an AP” as a Mobile Unit (MU).
January 2004
Darwin Engwer, Nortel Networks
Slide 5
doc.: IEEE 802.11-04/0086r0
Submission
802.11 System Layout
MU
Uplink to InfrastructureNetwork
AP#1SSID= “ACME”
AP#2SSID= “ACME”
January 2004
Darwin Engwer, Nortel Networks
Slide 6
doc.: IEEE 802.11-04/0086r0
Submission
Premises
• MU can only be associated with one AP at any given point in time (per clause number 5.4.2.2)
January 2004
Darwin Engwer, Nortel Networks
Slide 7
doc.: IEEE 802.11-04/0086r0
Submission
Roaming Definition
• Roaming occurs when an MU changes it’s association from one AP to another within the same ESS:
– i.e. the SSIDs of the two APs are identical– this is called a “BSS-transition” per clause 5.4.2.1.b– uses the reassociation service per clause 5.4.2.3
• Changing to an AP with a different SSID represents a change to a different network. This is a meta-case and is not discussed in this presentation. (“ESS-transition” per 5.4.2.1.c)
January 2004
Darwin Engwer, Nortel Networks
Slide 8
doc.: IEEE 802.11-04/0086r0
Submission
Roaming in an Ideal Network
MU MU
Uplink to InfrastructureNetwork
AP#1SSID= “ACME”
AP#2SSID= “ACME”
MU
January 2004
Darwin Engwer, Nortel Networks
Slide 9
doc.: IEEE 802.11-04/0086r0
Submission
Core Roaming Sequence
• 1. MU is communicating with AP#1• 2. MU moves to AP#2 coverage area• 3. stops communicating with AP#1• 4. starts commnicating with AP#2
• goal = measure time between events 3 and 4
January 2004
Darwin Engwer, Nortel Networks
Slide 10
doc.: IEEE 802.11-04/0086r0
Submission
But, it’s more complicated than that …
• networks are made of real components that can affect the outcome
January 2004
Darwin Engwer, Nortel Networks
Slide 11
doc.: IEEE 802.11-04/0086r0
Submission
Roaming in a HUB Network
HUB
Uplink to InfrastructureNetwork
AP#1SSID= “ACME”
AP#2SSID= “ACME”
MU MUMU
January 2004
Darwin Engwer, Nortel Networks
Slide 12
doc.: IEEE 802.11-04/0086r0
Submission
How does roaming work …
• consider the Message Sequence Chart (MSC) for roaming in a simple, hub-based network …
January 2004
Darwin Engwer, Nortel Networks
Slide 13
doc.: IEEE 802.11-04/0086r0
Submission
MSC: Establish Association with AP#1
MU AP1 AP2 Uplink
SCAN PHASE
JOIN PHASE
Auth RequestAck
Auth ReponseAck
Association RequestAck
Association ReponseAck
DATA
Ack
DATA
January 2004
Darwin Engwer, Nortel Networks
Slide 14
doc.: IEEE 802.11-04/0086r0
Submission
MSC: Roam to AP#2MU AP2 Uplink
Ack
DATADATA
SCAN PHASE
Re-Association Request
Ack
Re-Association ReponseAck
802.11f MOVE-Notify
802.11f MOVE-Response
Ack
DATADATA
AP1
JOIN PHASEAuth Request
AckAuth Reponse
Ack
Roaming event
t1
January 2004
Darwin Engwer, Nortel Networks
Slide 15
doc.: IEEE 802.11-04/0086r0
Submission
Important parts of the core roaming sequence:
• discovery of candidate APs• roaming event triggered• reassociate request to AP#2• handover from AP#1 to AP#2 [IAPP]
– was proprietary; now 802.11f defines an interoperable mechanism– ends the MU’s association with AP#1 - avoids “dangling association”– this helps AP#1differentiate between an MU that has wandered out of
range - AP#1 still tries to communicate - and an MU that has roamed to another AP - AP#1 does NOT try to communicate).
• AP#2 sends reassociate response
January 2004
Darwin Engwer, Nortel Networks
Slide 16
doc.: IEEE 802.11-04/0086r0
Submission
Reassociate Frame recap
• contains:– Capability Field– Listen Interval– Current [old] AP [MAC] address– SSID– Supported Rates
January 2004
Darwin Engwer, Nortel Networks
Slide 17
doc.: IEEE 802.11-04/0086r0
Submission
there are some alternatives
• these are “technically” allowed by the standard, but not recommended
• instead of reassociating with AP#2, the MU could perform a fresh association with AP#2
– in some cases this may be needed - such as when the MU has been completely out of range of the entire ESS for an extended period of time
– strongly discouraged since fresh association does properly support mobility (per clause 5.4.2)
January 2004
Darwin Engwer, Nortel Networks
Slide 18
doc.: IEEE 802.11-04/0086r0
Submission
… alternatives
• prior to re-associating with AP#2 the MU could send a disassociate request to AP#1
– allowed by the standard; intended to be used when changing ESS– physical aspects can make this approach troublesome when used within an
ESS• MU is trying to communicate over a known to be poor link (to AP#1)• MU and/ or AP#1 can get stuck retrying part of the disassociate frame
exchange sequence– OK for SSID changes– unreliable and error-prone in real networks– hence, not recommended within an ESS
January 2004
Darwin Engwer, Nortel Networks
Slide 19
doc.: IEEE 802.11-04/0086r0
Submission
… alternatives
• there are other techniques that are likely legal per the standard
• but, for fast roaming I recommend that focus be placed on the normal, preferred case: MU reassociates with AP#2
January 2004
Darwin Engwer, Nortel Networks
Slide 20
doc.: IEEE 802.11-04/0086r0
Submission
Recap - Roaming in a simple HUB Network
HUB
Uplink to InfrastructureNetwork
AP#1SSID= “ACME”
AP#2SSID= “ACME”
MU MUMU
January 2004
Darwin Engwer, Nortel Networks
Slide 21
doc.: IEEE 802.11-04/0086r0
Submission
But, it’s more complicated than that …
• a hub network doesn’t account for interceding delays or switching in a real network
January 2004
Darwin Engwer, Nortel Networks
Slide 22
doc.: IEEE 802.11-04/0086r0
Submission
Roaming in a Switched Network
SWITCH
Uplink to InfrastructureNetwork
AP#1SSID= “ACME”
AP#2SSID= “ACME”
P1 P2P3
MU MUMU
January 2004
Darwin Engwer, Nortel Networks
Slide 23
doc.: IEEE 802.11-04/0086r0
Submission
Roaming in a Switched Network
• need to consider switching delays• and, learning of MAC address-to-port mapping
– must be addressed in the absence of uplink traffic from the MU– even when there is uplink traffic there may be delays (until next uplink
packet is sent)– handled by the802.11f layer 2 update (XID) packet
• this packet is addressed at layer 2 as if it originated from the MU
January 2004
Darwin Engwer, Nortel Networks
Slide 24
doc.: IEEE 802.11-04/0086r0
Submission
MSC: Associating with AP#1MU AP2 UplinkAP1
Auth RequestAck
Auth ReponseAck
JOIN PHASE
SCAN PHASESwitch
Association RequestAck
Association ReponseAck
802.11f ADD-Notify
DATA
Ack
DATA802.11f L2 Update
January 2004
Darwin Engwer, Nortel Networks
Slide 25
doc.: IEEE 802.11-04/0086r0
Submission
MSC: Roaming in a Switched NetworkMU AP2 Uplink
DATA
Ack
DATA
SCAN PHASE
Re-Association RequestAck
Re-Association ResponseAck
802.11f MOVE-Notify802.11f MOVE-Notify802.11f L2 Update
802.11f MOVE-Resp 802.11f MOVE-Resp
DATA
Ack
DATARoaming event
SwitchAP1
JOIN PHASE
Auth RequestAck
Auth ReponseAck
January 2004
Darwin Engwer, Nortel Networks
Slide 26
doc.: IEEE 802.11-04/0086r0
Submission
Beginning of the Roaming Interval
• Defined: The last point in time when all network components know and agree upon the link path [to the MU].
• The relevant components are:– the MU– the AP (AP#1)– the infrastructure network (e.g. layer 2 switch)
• ...
January 2004
Darwin Engwer, Nortel Networks
Slide 27
doc.: IEEE 802.11-04/0086r0
Submission
Beginning of the Roaming Interval
• Is it:– the start of the Scan process? (i.e. MU sends probe request)
• No, MU could have scanned in advance• No, MU could be doing passive scanning
– the start of the Join process?• No, does not include any lost service due to possible scanning
– the reassociation request?• No, again may not include any lost service due to possible scanning
• From the MU’s perspective the last data frame received from AP#1 marks a definitive point in time when service via AP#1 was known to be good.
January 2004
Darwin Engwer, Nortel Networks
Slide 28
doc.: IEEE 802.11-04/0086r0
Submission
End of the Roaming Interval
• Defined: The point in time when all network components know and agree upon the new link path [to the MU].
• The relevant components are:– the MU– the old AP (AP#1)– the new AP (AP#2)– the infrastructure network (e.g. layer 2 switch)
• From the MU’s perspective the first data frame received from AP#2 marks a definitive point in time when service via AP#2 is known to be good.
January 2004
Darwin Engwer, Nortel Networks
Slide 29
doc.: IEEE 802.11-04/0086r0
Submission
Roaming Interval
• Defined: the end of service from AP#1 and the start of service from AP#2
• Beginning: last data frame successfully received from AP#1
• End: first data frame successfully received from AP#2
January 2004
Darwin Engwer, Nortel Networks
Slide 30
doc.: IEEE 802.11-04/0086r0
Submission
Determining the Roaming Interval
MU AP2 Uplink
DATA
Ack
DATA
SCAN PHASE
Re-Association RequestAck
Re-Association ResponseAck
802.11f MOVE-Notify802.11f MOVE-Notify802.11f L2 Update
802.11f MOVE-Resp 802.11f MOVE-Resp
DATA
Ack
DATARoaming
event
SwitchAP1
JOIN PHASE
Auth RequestAck
Auth ReponseAckRoaming
Interval
January 2004
Darwin Engwer, Nortel Networks
Slide 31
doc.: IEEE 802.11-04/0086r0
Submission
Determining the Roaming Interval
MU AP2 Uplink
DATA
Ack
DATA
SCAN PHASE
Re-Association RequestAck
Re-Association ResponseAck
802.11f MOVE-Notify802.11f MOVE-Notify802.11f L2 Update
802.11f MOVE-Resp 802.11f MOVE-Resp
DATA
Ack
DATA
SwitchAP1
JOIN PHASE
Auth RequestAck
Auth ReponseAckROAMINGRoaming
Interval
January 2004
Darwin Engwer, Nortel Networks
Slide 32
doc.: IEEE 802.11-04/0086r0
Submission
Measuring the Roaming Interval
• Now that the extent of the roaming interval is known, how can a test setup be configured in order to measure it?
• multi slides• building up to packet gen and sniffer
January 2004
Darwin Engwer, Nortel Networks
Slide 33
doc.: IEEE 802.11-04/0086r0
Submission
Measurement by Powering Down AP#1
P1 P2 P3
Traffic Generator
AP#1SSID= “ACME”
AP#2SSID= “ACME”
MU Sniffer
Switch
January 2004
Darwin Engwer, Nortel Networks
Slide 34
doc.: IEEE 802.11-04/0086r0
Submission
--- working point
January 2004
Darwin Engwer, Nortel Networks
Slide 35
doc.: IEEE 802.11-04/0086r0
Submission
Issues with Powering Down AP#1
• not representative of the real world:– not representative of real world RF conditions at roaming event
• NOT the more usual roaming AP-to-AP with some amount of coverage overlap– effects (and capabilities) of the interceding network are not considered
• no interceding delays or switching effects in the network– deals with AP failure (or completely out of range) case rather than real
seamless AP-to-AP roaming scenario• MU is not comparing 2 candidate APs and choosing one• it is just choosing the only AP available
– power down of AP#1 means the MU state machine is dealing with the case of a poor connection due to a complete end to the stream of beacons from AP#1
– APs cannot communicate with each other during roaming event
January 2004
Darwin Engwer, Nortel Networks
Slide 36
doc.: IEEE 802.11-04/0086r0
Submission
Analysis of Powerdown Measurement Setup
• roaming time measurement: power down setup is inadequate bcus:
– 1. not representative of real world RF conditions at roaming event– 2. not representative of infrastructure conditions at roaming event
• Desired setup is for the signal from AP#1 to fade rather than stop, so that at the roaming event it is still powered up an connected to infrastructure network.
• Also, need to ensure reassociate vs. fresh association
January 2004
Darwin Engwer, Nortel Networks
Slide 37
doc.: IEEE 802.11-04/0086r0
Submission
Measurement Using a “Cone of Silence”
P1 P2 P3
Traffic Generator
AP#1SSID= “ACME”
AP#2SSID= “ACME”
MU Sniffer
Switch
January 2004
Darwin Engwer, Nortel Networks
Slide 38
doc.: IEEE 802.11-04/0086r0
Submission
“Cone of Silence” Issues
• better but not as definitive as we would like• Murphy’s Law: when we need APs to be poor they are
amazingly good
January 2004
Darwin Engwer, Nortel Networks
Slide 39
doc.: IEEE 802.11-04/0086r0
Submission
Measurement Using an RF Attenuator on AP#1
P1 P2 P3
MU
AP#1SSID= “ACME”CH= 1
0-30 db
AP#2SSID= “ACME”CH= 11
3 db
Sniffer
Switch
January 2004
Darwin Engwer, Nortel Networks
Slide 40
doc.: IEEE 802.11-04/0086r0
Submission
Attenuator notes
• can be done using external antennas on the AP with an adjustable RF signal attenuator inline between the two
• could also be done using Tx power adjustment on AP#1
January 2004
Darwin Engwer, Nortel Networks
Slide 41
doc.: IEEE 802.11-04/0086r0
Submission
Recap - limits of the Roaming Interval
MU AP2 Uplink
DATA
Ack
DATA
SCAN PHASE
Re-Association RequestAck
Re-Association ResponseAck
802.11f MOVE-Notify802.11f MOVE-Notify802.11f L2 Update
802.11f MOVE-Resp 802.11f MOVE-Resp
DATA
Ack
DATA
SwitchAP1
JOIN PHASE
Auth RequestAck
Auth ReponseAckROAMINGRoaming
Interval
January 2004
Darwin Engwer, Nortel Networks
Slide 42
doc.: IEEE 802.11-04/0086r0
Submission
Measuring the Handover Interval
• Know when to start and end measuring• Know the test setup• How do we actually make the measurement?
• need a method to determine the end of service from AP#1 and the start of service from AP#2
• ...
January 2004
Darwin Engwer, Nortel Networks
Slide 43
doc.: IEEE 802.11-04/0086r0
Submission
Measuring the Handover Interval
• connect a packet generator (e.g. Chariot) to the switch• set the packet generator to send out [short] packets on
a continuous basis• set a fixed interval between packets• include a sequence number in every packet
January 2004
Darwin Engwer, Nortel Networks
Slide 44
doc.: IEEE 802.11-04/0086r0
Submission
Measurement Using a Packet Generator
P1 P2 P3
MU
Traffic Generator
AP#1SSID= “ACME”CH= 1
0-30 db
AP#2SSID= “ACME”CH= 11
3 db
Sniffer
Switch
January 2004
Darwin Engwer, Nortel Networks
Slide 45
doc.: IEEE 802.11-04/0086r0
Submission
Measuring the Handover Interval
• assume packets are sent every ‘n’ msec• by measuring the Roaming Interval (as defined earlier)
– time from last packet thru AP#1 to first packet thru AP#2• we can determine the Roaming Interval with an
accuracy of +/- n msec• or, restated, an accuracy of 2x ‘n’ msec
– (per Nyquist’s theory)• The sequence numbers in the packets will show if any
packets (and how many) were lost during the roaming interval
January 2004
Darwin Engwer, Nortel Networks
Slide 46
doc.: IEEE 802.11-04/0086r0
Submission
Summary
• defined the components of an 802.11 system• defined core roaming in an 802.11 system• identified an initial set of roaming conditions for
consideration/ analysis• identified the start and end points of the roaming
interval• presented test setups for effective roaming interval
measurements
January 2004
Darwin Engwer, Nortel Networks
Slide 47
doc.: IEEE 802.11-04/0086r0
Submission
Other Topics to Be Explored
• other cases to possibly consider:– inter-subnet layer 3 handover measurement - [need to describe setup too]– handover from one PHY type to another - within the same AP, e.g. 11b-
>11a, 11a->11b• must consider single MAC and multi MAC cases
January 2004
Darwin Engwer, Nortel Networks
Slide 48
doc.: IEEE 802.11-04/0086r0
Submission
Credits
• Haixiang He - artistic design and animation• Bob O’Hara - technical review
January 2004
Darwin Engwer, Nortel Networks
Slide 49
doc.: IEEE 802.11-04/0086r0
Submission
References
• Std 802.11-1999• Std 802.11F-2003• 11-03-0563-00-000i-tgi-4-way-handshake-timings.ppt,
Nick Petroni• Netwave Roaming Specification - 1995, Engwer
January 2004
Darwin Engwer, Nortel Networks
Slide 50
doc.: IEEE 802.11-04/0086r0
Submission
The End
January 2004
Darwin Engwer, Nortel Networks
Slide 51
doc.: IEEE 802.11-04/0086r0
Submission
Backup Slides
January 2004
Darwin Engwer, Nortel Networks
Slide 52
doc.: IEEE 802.11-04/0086r0
Submission
Roaming in Multi-PHY APs
DS
11a PHY
MAC
11b PHY
MAC
11g PHY
MAC
5 GHzRadio
2.4 GHzRadio
0x…1234 0x…1235 0x…1236
January 2004
Darwin Engwer, Nortel Networks
Slide 53
doc.: IEEE 802.11-04/0086r0
Submission
Roaming in Multi-PHY APs
DS
MAC
11a PHY 11b PHY 11g PHY
5 GHz 2.4 GHz Radio
0x…1234
January 2004
Darwin Engwer, Nortel Networks
Slide 54
doc.: IEEE 802.11-04/0086r0
Submission
Definitions
AP*MUMU
802.11 STA
* STA operating as an AP.
MU = Mobile Unit(Mobile STA)
January 2004
Darwin Engwer, Nortel Networks
Slide 55
doc.: IEEE 802.11-04/0086r0
Submission
Definitions
MU
802.11 STA
AP
January 2004
Darwin Engwer, Nortel Networks
Slide 56
doc.: IEEE 802.11-04/0086r0
Submission
Definitions
MMU
802.11 STA
APUMU
January 2004
Darwin Engwer, Nortel Networks
Slide 57
doc.: IEEE 802.11-04/0086r0
Submission
The End - Really