rohan murty harvard university jitendra padhye, ranveer chandra, alec wolman, and brian zill...
TRANSCRIPT
1
Designing High Performance Enterprise Wi-Fi Networks
Rohan Murty Harvard University
Jitendra Padhye, Ranveer Chandra, Alec Wolman, and Brian Zill Microsoft Research
2
Trends in Enterprise Wi-Fi Networks
Increased adoption and usage [Forrester]
Culture of mobility: Users tend to use Wi-Fi even when wired connections are available [Gartner, Forrester, Economist]
Move towards an all wireless officeUsers want wire-like performance from wireless networks
3
Capacity of Conventional Corporate WLANs
Corporate WLAN Study: 12 users < 1 Mbps each
4
Characteristics of Conventional Corporate WLANs
Focus on coverage Fewer APs than clients Clients talk to APs far away; worsens rate anomaly
Clients pick APs to associate with Use RSSI of beacon packets Agnostic to channel load at APs
Lack adaptive behavior No load balancing; fixed channel assignments Congestion and hotspots worsen
5
DenseAP
Focus on capacity Lots of APs; densely deployed Clients can talk to APs near by; mitigates rate
anomaly
Infrastructure picks client-AP associations Global view of network conditions (channel load,
interference, etc.)
Adaptability Load balance associations; Dynamic channel
assignment Redistributes load away from local hotspots
6
DenseAP is Practical
No client modifications Works with legacy clients Changes limited to the infrastructure Easy to deploy
Self-managing
7
DenseAP Central Controller (DC)
DenseAP System Architecture
AssociationsChannel AssignmentsLoad Balancing
DenseAP Nodes (DAPs)
Commands to DenseAP
nodes
Summarized Data from DenseAP nodes
Summarized
Data Wired NetworkCommand
s
Interface with clientsSend summaries to DC
8
Key Challenges
Controlling Associations Mechanisms Policy
Dynamic Channel Assignment Mechanism Policy
Load Balancing Mechanism Policy
9
Probe Request
Probe Request
Probe Request
ACL
ACL
ACL00:09:5B:5A:1F:4F
Association Control in DenseAP
10
ACL
ACL
ACL00:09:5B:5A:1F:4F
Probe Request MAC = 00:09:5B:5A:1F:4F
RSSI = 30
Probe RequestMAC = 00:09:5B:5A:1F:4F
RSSI = 42
Probe Request MAC = 00:09:5B:5A:1F:4F
RSSI = 40
Association Control in DenseAP
11
Accept Client
ACL00:09:5B:5A:1
F:4F
ACL
ACL00:09:5B:5A:1F:4F
Association Control in DenseAP
Client only sees one DAP at any given time
Probe Response
12
Association Policy
What is the quality of a connection between a client and a DAP? (rate)
How busy is the medium around each DAP?
Overall goal: Associate client with a DAP
that will yield good throughput
13
A Metric for DAP Selection
Expected Transmission-
Rate (Mbps)
Available Capacity (AC)
(Mbps)
Free Air Time(%)
X=
Probe Request
Pro
be R
eq
uest
Prob
e Req
uest
Free air time = 0.35
DAP2
DAP1
DAP3
RSSI = 20
RSSI = 10
RSSI = 30
Free air time = 0.45
Free air time = 0.22
DAP Free Air-
Time
RSSI
DAP1 0.35 20
DAP2 0.22 10
DAP3 0.45 30
Accept Client
Probe Response
DAP Free Air-Time
RSSI
Ex. Tx-Rate
AC
DAP1
0.35 20 18 6.3
DAP2
0.22 10 6 1.32
DAP3
0.45 30 48 21.6
15
RateMap: Estimating Expected Transmission-Rate
Correlation between RSSI of Probe Request
packets Avg. throughput between
a DAP-client pair
Rough approximation - ordering of DAPs
Online profiling method that builds RSSI to data-rate estimates Upload and RSSI correlation
= 0.71Download and RSSI correlation = 0.61
0-9 10 - 19
20 - 29
30 - 39
40+0
2
4
6
8
10
12
14
16
18Upload Download
RSSI Values
Avera
ge T
hro
ug
hp
ut
(Mb
ps)
16
Estimating Free Air Time
Estimate how busy is the medium around at a DAP
Technique similar to ProbeGap* Measure time taken to
finish a packet transmission
Estimates match up closely with offered traffic load
*Lakshminarayan et al., 2004
*Vasudevan et al., 2005
0 20 40 60 80 100
0
20
40
60
80
100
Actual Load (%)
Esti
mate
d L
oad
(%
)
17
Channel Assignment
Integrated into the association process
DAPs not discovered by clients don’t need channels
A DAP is assigned a channel only when it goes from being passive (no clients) to active (services at least one client) Central controller assigns channel with
least load
18
Re-evaluating Associations
So far, associations when a new client joins the network
No association is perfect Client traffic demands change Local hotspots created
19
Load Balancing
Central controller monitors load on every DAP
When channel load on a DAP crosses a certain threshold Client causing most load is determined Moved to less loaded DAP nearby Ensure client continues to get at least as much
available capacity at the new DAP
Load balancing achieved via handoffs Use association control; manipulate ACLs on DAPs
20
Results
21
UP
DN
DN
UP
30003000
30013001
30023002
30043004
30053005
30063006
3012SHUOCHEN
3012
3014ARUNKU3014
3016CHADV3016
3018BRADDAN
3018
3019EMREK3019
30203020
30213021
30223022
3024YMWANG
3024
30263026
30283028
30303030
3032MFORNEY
3032 30333033
3034RAMANC3034
3035V-DOAVELV-JONAHAV-TEDKAO
3035
30373037
30393039
30403040
3042V-JWING
3042
30443044
3046DESNEY3046
3048DCR30483052
3052
31003100
3102MARYCZ3102
310331033104
GGR3104
3106GREGSMI
3106
3108BRIANME
3108
31103110
3112SZELISKI
3112
3114NURIA3114
3116MATTU3116
3118BROWN3118
3120SWINDER
3120
31233123
31253125
31303130
31313131
3134STEEDLY
3134
3136SBKANG3136
31383138
3139T-LUCASK
3139
31403140
31413141
31433143
31453145
31473147
31493149
31513151
31533153
3154MCOHEN
3154
3156ERUDOLPH
3156
31593159
31603160
31613161
31633163
31653165
31673167
3172LARRYZ3172
3174A-HGOOD
3174
3176PETERJ3176
3178CRAIGVI
3178
31793179
31803180
31813181
32003200
32013201
32033203
32043204
32053205
3206KENTOY
3206
32073207
3208ANANDAN
3208320932093210
3210
32113211
3212LORIMAC
3212
32133213
3214CLOOP3214
32153215
3216MARCEL3216
3217BLINN3217
32183218
32193219
32203220
32223222
3224HHOPPE3224
3226JOHNSNY
3226
3228BGUENTER
3228
32303230
32323232
32333233
3234V-AVD3234
3235BODHIP3235
3237CYL
3237
32393239
32403240
32423242
3244JFAY3244
3246GEORGP3246
3248SDRUCKER
3248
3252WONG3252
33003300
3302JGEMMELL
3302
33033303
3304JDUNAGAN
3304
3305MZH3305
3306BZILL3306
3307DMALTZ3307
3308DANSIMON
330833093309
33103310
3312RICHDR3312
3314HOWELL
3314
3316SAGARWAL
3316
3318HELENW
3318
3320PADHYE3320
332133213323
3323
33253325
33293329
33303330
33313331 3333
3333
3334PADMANAB
3334
3336LIUJ3336
3338JELSON3338
3339SUMANN
3339
33403340
33413341
3343RANVEER
3343
3347T-ANDRES
3347
33513351
3354ZHAO3354
33563356
3359JOHNDO
3359
33603360
3363RATUL3363
33713371
3372LORCH3372
3374BAHL3374
3376ALECW3376
33773377
3378BOLOSKY
3378
33793379
33803380
CH31CH31
CH32CH32
CH33CH33
CI32CI32
EL 31EL 31
EL 32EL 32
STR31STR31
STR32STR32
(0,0)
(98,32)
Testbed
1 Corp AP
24 DAPs
24 Clients
802.11 a/bg
22
Results: Roadmap
Performance Density Channels Intelligent Association
Load Balancing
23
2 4 6 8 10 1202468
101214161820
DenseAP Corporate WLAN
Number of Clients
Per-
Clien
t Th
rou
gp
ut
(Mb
ps)
2 4 6 8 10 1202468
101214161820
Corporate WLAN
Number of Clients
Per-
Clien
t Th
rou
gp
ut
(Mb
ps)
Overall DenseAP Performance: 802.11a
Gains due to• More channels• DAP density• Intelligent
associations
1250% gain
Why?
24
Exploring the impact of density
Put all DAPs on the same channel
Factors out Channels Intelligent Associations: same load on all DAPs
Single out impact of Density
25
Impact of Density: Using only 1 channel
Higher density provides better performance
1 2 3 4 5 60
5
10
15
20
Corporate WLAN
Number of Clients
Avg
. P
er-
Clien
t Th
rou
gh
pu
t (M
bp
s)
26
Is intelligent association control necessary?
27
Why does intelligent association matter?
Client-Driven Disable intelligent association control Let clients pick DAP to associate with (conventional
WLANs)
Compare with DenseAP
Factors out Channels Density
Single out impact of Intelligent association
28
2 4 6 8 10 1202468
101214161820
Client-Driven DenseAP
Number of Clients
Per-
Clien
t Th
rou
gh
pu
t (M
bp
s)
Necessity of the Association Policy
Intelligent association policy is necessary
160% gain
29
Load Balancing
30
0 50 100 150 200 250 3000
5
10
15
20
25
Client 3
Client 1
Client 2
Time (s)
Th
rou
gh
pu
t (M
bp
s)
Load Balancing
Client 1
moved
Client 1
improves
Clients 2 & 3
improve
Client 2
moved
31
Other Details and Results in the Paper
Load balancing algorithm and mechanism
Mobility
Performance Fewer DAPs Fewer channels 802.11g …..
Scalability
32
Related Work
Plenty of prior work on static channel assignment, power control and associations Each studied each aspect in isolation Require client modifications [Ramani and Savage,
Infocom 2005] SMARTA [Ahmed et al., CoNext 2006]
Examines channel and power control Increase overall network capacity Does not consider associations, load balancing
MDG [Broustis et al., MOBICOM 2007] Identified tuning channel, power and associations Studies the order in which these knobs must be tuned Requires client modifications
33
Overall Contributions
Practical system How do density, intelligent association,
and more channels affect capacity? Adaptive system
Future directions Impact of hidden terminals Heterogeneous mix of client traffic patterns Other backhauls: e.g. Wireless, powerline