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PHY layer access misbehavior in WLAN networks. Master thesis presentation Radio Communication Systems, KTH Probir Khaskel Advisor: Olav Queseth & Examiner: Prof. Jens Zander. Outline. Problem definition System model Link adaptation Single cell system Multi-cell system - PowerPoint PPT Presentation
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PHY layer access PHY layer access misbehavior in WLAN misbehavior in WLAN
networksnetworks
Master thesis presentationRadio Communication Systems, KTH
Probir KhaskelAdvisor: Olav Queseth & Examiner: Prof. Jens Zander
2
OutlineOutline
Problem definition System model Link adaptation Single cell system Multi-cell system Game theoretical analysis Conclusion Future work
Ideal system performance System response in CCA level modification
Airport environment Infrastructure based network Time driven system in MATLAB 1 time slot = 1 OFDM symbol (4μ sec )
Players attitude Dynamic strategies for stable system
3
ProblemProblem Definition Definition
Is there any incentive to modify CCA level defined by the standard from the user’s point of view ?
Greedy users in a single network Modification in the PHY layer Frequent channel access by
modifying CCA threshold level
????
Does this modification have impact on the overall system performance (in terms of throughput) ?
Greedy users in a single network Modification in the PHY layer Frequent channel access by
modifying CCA threshold level
4
Greedy channel access in the Greedy channel access in the PHY layerPHY layer
Th
resh
old
T
hre
shold
(d
Bm
)(d
Bm
)
TimeTime
-82
-72
~
Channel is busy
-62 Channel is Idle
RF signal RF signal strengthstrength
Problem background
↑↑
5
System modelSystem model
Propagation model Traffic model and
packet System deployment Transmit power and
noise Capture model Channel
interference
Traffic model and packet Asynchronous and identical traffic from the upper layer
Fixed packet size (MPDU-256 bytes)
System deployment Single cell system with various
number of nodes Multicell system with 16 cells, re-
use factor 4, 3 nodes in each cell Transmit power and noise
Fixed maximum power of +23 dBm Noise level -95 dBm
Capture model
i
ij jjrx
iirx
NdP
dP
)(
)(
,
,
Channel interference
Co-channel interference Adjacent channel
interference is ignored
Propagation model
L (d)freeSpace = 32:44+ 20 ¢log10(fc) + 10 ¢n ¢log10(d)
L (d)fr eeSpace= 32:44+ 20 ¢log10(fc) + 10 ¢n ¢log10(d)
Lmw = L (d)fr eeSpace+ L s ¢n[n s + 5n s + 3 ¡ b]s +¾x
A. . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . ......................................... ........................Lmw = L (d)freeSpace+ L s ¢n
[n s + 5n s + 3 ¡ b]s +¾x
L (d)fr eeSpace= 32:44+ 20 ¢log10(fc) + 10 ¢n ¢log10(d)
6
IEEE 802.11a PHY and link IEEE 802.11a PHY and link adaptationadaptation
8 PHY modes with data rates ranging from 6 to 54 Mbps
Link Adaptation for data transmission is realized as MPDU-based fast link adaptation, placed closer to the air-
interface based on the estimated C/I at the receiver
Link Adaptation for ACK transmission is realized as receiver adopts the same PHY mode as the
corresponding received data packet
8 PHY modes with data rates ranging from 6 to 54 Mbps
Link Adaptation for data transmission is realized as MPDU-based fast link adaptation, placed closer to the air-
interface based on the estimated C/I at the receiver
Link Adaptation for ACK transmission is realized as receiver adopts the same PHY mode as the
corresponding received data packet
24.56
24.05
18.80
17.04
10.79
9.03
7.78
6.02
C/I (dB)
54 Mbps
48 Mbps
36 Mbps
24 Mbps
18 Mbps
12 Mbps
9 Mbps
6 Mbps
Data Rate
3/464-QAM8
2/364-QAM7
3/416-QAM6
1/216-QAM5
3/4QPSK4
1/2QPSK3
3/4BPSK2
1/2BPSK1
Code RateModulationMode
7
Single cell systemSingle cell system
8
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0.2
0.4
0.6
0.8
1
1.2
1.4
load vs. throughput
avg. offered load per station [Mbps]
avg.
thro
ughp
ut [M
bps]
6 STA
11 STA16 STA
21 STA
0 0.5 1 1.5 2 2.50
20
40
60
80
100
120
140
160
180
200load vs. delay
avg. offered load per station [Mbps]
avg.
del
ay [m
s]
For higher number of users, system gets saturated earlier
Single cell: Ideal system performance
System with higher number of users is less capable to support delay bounded QoS with increasing offered load
More number of users in the system, more throughput drops down from system capacity to saturation level
Number of collision also gets saturated in system saturation
load vs. collision
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.60
20
40
60
80
100
120
140
160
180
200
avg. offered load per station [Mbps]
avg.
no.
of c
ollis
ion
9
-85 -80 -75 -70 -65 -60 -55 -50 -45 -400
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6cslevel vs. throughput for various no. of nodes
cslevel [dBm]
avg.
thro
ughp
ut p
er u
ser
[Mbp
s]
5 nodes
10 nodes15 nodes
20 nodes
(-52, 1.51)
(-54, 0.719)
(-58, 0.352)
(-58, 0.206)
The less number of users in the system, the higher reachable CCA level
Single cell: Saturation analysis
10
0 5 10 15 20 250
5
10
15
20
25
-85 -80 -75 -70 -65 -60 -55 -50 -45 -400
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9cslevel vs. throughput; (D|S)
cslevel [dBm]
avg.
thro
ughp
ut p
er u
ser
[Mbp
s]
offered load per user:1.37 [Mbps]
G2: -82 dBm (timid)
G1: increasing (greedy)
G1: (-50, 0.857)
G2: (-50, 0.466)
-85 -80 -75 -70 -65 -60 -55 -50 -45 -400
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8cslevel vs. throughput; (S|D)
cslevel [dBm]
avg.
thro
ughp
ut p
er u
ser
[Mbp
s]
offered load per user:1.37 [Mbps]
G2: increasing (greedy)
G1: -82 dBm (timid)
G2: (-52, 0.761)
G1: (-52, 0.499)
Single cell: Greedy vs. timid users
11
load vs. throughput
0.2 0.4 0.6 0.8 1 1.2 1.4 1.60.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
avg. offered load per station [Mbps]
avg.
thro
ughp
ut [M
bps]
G2: -52 dBmG1: -50 dBmIdeal systemavg. system, (D|D)
System throughput decreases than that of the ideal system
Number of collision increases around eight times, however, delay performance does not deteriorate compare to throughput and collision
0 0.5 1 1.5 2 2.50
20
40
60
80
100
120
140
160
180
200load vs. delay
avg. offered load per station [Mbps]
avg.
del
ay [m
s]
G2: -52 dBmG1: -50 dBmIdeal systemavg. system, (D|D)
0 0.5 1 1.50
100
200
300
400
500
600
700
800
900
1000load vs. collision
avg. offered load per station [Mbps]
avg.
col
lisio
n
G2: -52 dBmG1: -50 dBmIdeal systemavg. system, (D|D)
Single cell: Greedy vs. greedy users
12
Current Nash Equilibrium is Pareto inefficient
(S|S) would be Pareto efficient NE
System performance is better when users follow the standard protocol
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1bargain domain of SSG, single cell system
payoff of G1, v1
payo
ff o
f G
2, v
2
(S|S)(0.719,0.719)
(D1|D2)(0.612,0.521)
(S|D2)(0.499,0.761)
(D1|S)(0.857,0.466)
S: -82 dBm
D1: -50 dBm
D2: -52 dBm
single cell: 11 STAG1: 3 nodesG2: 7 nodes
current NE is Pareto inefficient,(S|S) could be Pareto efficient NE
defection by G2 defection by G1
NashEquilibrium
Current Nash Equilibrium is Pareto inefficient
(S|S) would be Pareto efficient NE
Single cell: single stage game
G1↓ G2→ S D
S (0.719, 0.719) (0.499, 0.761)
D (0.857, 0.466) (0.612, 0.521)
13
Single cell: multi stage game
Users prefer to defect if δ<0.563, meaning that they are more likely to defect
Dynamic strategies: TFT, GRIM ensure a stable system
Discount factor, δ (delay bounded application dependent)
Anticipated payoff in stage t to player i
In an infinite game, payoff is computed as
14
Multi-cell systemMulti-cell system
15
System throughput out-perform the standard by cooperative modification of the CCA threshold
Collision reduces around three and a half times compare to the ideal system
-90 -80 -70 -60 -50 -40 -30 -200.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7cslevel vs. throughput; (C|C)
cslevel [dBm]
avg.
thro
ughp
ut p
er u
ser
[Mbp
s]
offered load per user:1.0 [Mbps]
(-68, 0.665)
(-82, 0.376)
-90 -80 -70 -60 -50 -40 -30 -2050
100
150
200
250
300
350
400
450
500cslevel vs. collision; (C|C)
cslevel [dBm]
avg.
no.
of c
ollis
ion
offered load per user:1 [Mbps]
(-82, 483)
(-68, 139)
Multi-cell: saturation analysis
16
G1↓ G2→ C D
C (0.665, 0.665) (0.327, 0.438)
D (0.727, 0.398) (0.366, 0.366)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1bargain domain of SSG; multi cell system
payoff of 1, v1
payo
ff o
f 2,
v2 (C|C)
(0.665,0.665)
(D|D)(0.366,0.366)
(C|D)(0.327,0.438)
(D|C)(0.727,0.398)
C: -68 dBmD: -44 dBm
multi cell: 48 nodesG1: 16 nodesG2: 32 nodes
defection by G1defection by G2
current NE is Pareto efficient
NashEquilibrium
SSG: Current Nash Equilibrium is Pareto efficient
MSG: TFT, GRIM dynamic strategies ensure a stable system
Users prefer to defect if δ<0.171, meaning that they are more likely to cooperate
Multi-cell: game analysis
17
ConclusionConclusion Single cell system
It’s possible to achieve higher throughput by modifying the CCA level
Any modification results in deterioration of the system performance
Multi-cell System
Adaptive modification of the CCA level gives a noticeable system improvement
A small group gains by further modification, the overall system performance deteriorates
Users are more likely to cooperate
Operators might be interested to have a control on the CCA level modification based on the network condition and update the users to adjust in a regular fashion
18
Future workFuture work Part of the received data of a collided packet could
be recoverable by smart decoding algorithm, which in tern could increase the system throughput by avoiding to retransmit the whole packet
Transmit Power Control (TPC) could increase system capacity by minimizing co-channel interference
In general, any misbehaving activities can be detected by collision counter. However, pinpointing a misbehaving user is a crucial task
Players’ assessment of others’ strategy by observed throughput might be a pitfall for system stability
19
Question Question
& &
Comments!Comments!
20
0.5630.171
δ →0 1
Higher preference of future payoff,
e.g. voice telephony
Lower preference of future payoff,
e.g. best effort type application
Discount factor, δ
Extra slides
21
Hidden Terminal ProblemHidden Terminal Problem
Hidden terminal
Station/Node
Access Point
Extra slides
22
Unlicensed Unlicensed frequencyfrequency bands bands
UPCS-Unlicensed Personal Communication Services [1.9GHz]
ISM-Industry, Science and Medicine [2.4-2.4835GHz]
UNII-Unlicensed National Information Infrastructure [5.15-5.825GHz] Why Why UnlicensedUnlicensed??
Promotes efficient spectrum sharing Further experimentation and innovation Mobility of wireless applications since no license
needed in new location
IntroductionIntroductionExtra slides