Wireless Networks Laboratory (WINET)Wireless Networks Laboratory (WINET)
Quality of Service Support in Wireless Networks
Hongqiang Zhaihttp://www.ecel.ufl.edu/~zhai Wireless Networks Laboratory
Department of Electrical and Computer EngineeringUniversity of Florida
In Collaboration with Dr. Xiang Chen and my advisor Professor Yuguang ``Michale’’ Fang
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Outline
Introduction
Performance analysis of the IEEE 802.11 MAC protocol
A call admission and rate control scheme
Conclusion and future research issues
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Wireless Landscape
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Wireless Local Area Networks/ Wi-Fi Hot Spots
Instant messaging Gaming over IP Voice over IP over Wi-Fi
Web traffic Email Streaming video
Next Call May Come from a Wireless Hot Spot
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Mobile Ad Hoc Networks and Wireless Mesh Networks
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Quality of Service (QoS) Requirements
Bandwidth
Delay and delay jitter
Packet loss rate
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Challenges
Unreliable physical channel Time-varying propagation characteristics Interference
Limited bandwidth
Limited processing power and battery life
Distributed control
Mobility
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Medium Access Control
Coordinate channel access Reduce collision Efficiently utilize the limited wireless bandwidth
A
B
C
D
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IEEE 802.11 Distributed Coordinate Function (DCF) MAC Protocol
Carrier sense multiple access with collision avoidance (CSMA/CA) Carrier sensing
Physical Carrier Sensing Virtual Carrier Sensing
Interframe Spacing (IFS) Short IFS (SIFS) < DCF IFS (DIFS)
DIFS
RTS
BackoffCTS
SIFS SIFS
DATA
SIFS
ACK
NAV(RTS)
NAV(CTS)
DIFS
RTS …
Backoff
Binary Exponential Backoff Randomly chosen from [0, CW] CW doubles in case of collision
Transmitter
Receiver
Others
B
A
ACK…
Request to send
Clear to send Acknowledge
DATA
Contention based MAC
Can it support QoS requirements of various applications?
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Previous Work on Performance Analysis of the IEEE 802.11 MAC Standard
Previous studies focus on saturated case Each device always has packets in the system and keeps
contending for the shared channel. Collision probability is very high Delay performance is very bad Only throughput and average delay have been derived.
Related work Bianchi, JSAC March 2000 Cali et al., IEEE/ACM Tran. Networking, Dec. 2000
QoS requirements of real-time services can not be guaranteed if there are many contending users?
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Previous Work on Supporting QoS in WLANs
Service differentiation Provide different channel access priorities for different
services by differentiating Contention window Interframe spacing (IFS)
IEEE 802.11e draft (based on 802.11b) Related work
Ada and Castelluccia, Infocom’01 (CW, IFS) Veres et al., JSAC Oct. 2001 (real-time measurement in virtual MAC) S.T. Sheu and T.F. Sheu, JSAC Oct. 2001 (real-time traffic periods) S. Mangold et al., Wireless Communications Dec. 2003 (802.11e)
Service differentiation is still not enough to meet the strict QoS requirements
Can the IEEE 802.11 MAC protocol do better than service differentiation?
Research issues
•Performance in both non-saturated and saturated case•Probability distribution of medium access delay
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MAC Service Time
123
23
12
3
3
Transmitqueue
MAC
Probability Generating Function (PGF) Pr{Ts=tsi}=pi (0 ≤ i < ∞)
MAC service time is discrete in value SIFS, DIFS, EIFS Backoff time is measured in time slots Packet to be transmitted is also discrete
in length
Packet arrival
0 1 20 1 2
0
( ) ...si s s s
S
t t t tT i
i
G Z p Z p Z p Z p Z
1
2
[ ] ' ( ) ' (1)
[ ] '' (1) ' (1) ' (1)
S S
S S S
S T Z T
S T T T
E T G Z G
VAR T G G G
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MAC Service Time
Generalized state transition diagram (GSTD) Mark the PGF of the transition time on each branch along with
the transition probability PGF of the transition time between two states is the
corresponding system transfer function
1Z start endp
(1 )p 2Z
Widely used method Calculate the average # of retransmissions NR = p/(1-p)
Average transition time is NR × τ1 + τ2=
2
1
(1 )( )
1ST
p ZG Z
pZ
1 2' (1)1ST
pG
p
1 21
p
p
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MAC Service Time of IEEE 802.11
0,0 0,1 0,2 0,W0-2 0,W0-1H(Z) H(Z) H(Z)
(p/Wj)C(Z)
1/W0
j,0 j,1 j,2 j,Wj-2 i,Wj-1H(Z) H(Z) H(Z)
(p/Wj+1)C(Z)
j-1,0
(p/Wj)C(Z)
,0 ,1 ,2 ,W-2 ,W-1H(Z) H(Z) H(Z)
pC(Z)
(p/W)C(Z)
(1-p)S(Z)
(1-p)S(Z)
(1-p)S(Z)
(1-p)S(Z)
start
end
State variable (j, k): j is the backoff stage, k is the backoff timerWj: the contention window at backoff stage jp: collision probability perceived by a node: maximum # of retransmissions
( )sTG Z
1
0 0
1
0
( ) ( ) / ; ( ) ( ),(0 )
( ) (1 ) ( ) ( ( )) ( ) ( ( )) ( )
i
S
iW ji i i jj j
iT ii
F Z H Z W E Z F Z i
G Z p S Z pC Z E Z pC Z E Z
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MAC Service Time of IEEE 802.11
MAC service time (ms)
PD
F
Observation:
When p is small, both the mean and standard deviation of MAC service time are small.
10-4
10-3
10-2
10-1
0
10
20
30
40
50
60
MeanStanard Deviation
Collision probability p
payload size = 8000 bits, with RTS/CTS
MA
C s
ervi
ce t
ime
(ms)
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Delay and Delay Variation
S W R S RT T T T T T
2[ ][ ] [ ] [ ] [ ]
2 [ ]S
S R SS
E TE T E T E T E T
E T
3 22
[ ] [ ] [ ]
5 [ ] [ ] [ ] ( )
12 [ ] 2 [ ]
S R
S SS
S S
VAR T VAR T VAR T
E T E TVAR T
E T E T
1
2
3Transmitqueue
MAC
Packet arrival
TW
TS
TR
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Network Throughput
CollisionSuccessful
transmission... ...Successful
transmission
Idle slot
(Tidl pi ) (Tcol pc ) (Tsuc ps )
s sucs
i idl s suc c col
ssuc
s suc c colb
i idl s suc c col
p TR
p T p T p T
datas R
T
p T p TR
p T p T p T
Channel utilization:
Normalized throughput :
Channel busyness ratio:
suc
col
T rts cts data ack 3sifs difs
T rts eifs
suc
col
T data ack sifs difs
T data eifs
With RTS/CTS
Without RTS/CTS
( )
( )
( )
ni
n 1s
c i s
n 1
p 1
p n 1
p 1 p p
p 1 1
n: # of nodes
: the prob. that a node transmits in any slot
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Network Throughput
10-3
10-2
10-1
100
0
0.2
0.4
0.6
0.8
1
n=5 n=10 n=300
p
n=5n=10n=300
channel busyness ratio
channel utilization
normalized throughput
Collision Probability p
Channel Busyness Ratio is an accurate, robust, and easily obtained sign of network status.
Maximum throughput with good delay performance
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Packet Loss Rate
Given the collision probability p, the MAC layer may drop the packet with the probability
( )d full fullP P 1 P p p
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
20
40
60
80
100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.2
0.4
0.6
0.8
1
Avg
. q
ueu
e le
ng
thP
kt l
oss
ra
te
Channel busyness ratio
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0 0.5 10
0.2
0.4
0.6
0.8
Nor
mal
ized
thro
ughp
ut
0 0.5 1
0.05
0.1
0.15
Mea
n of
del
ay (
s)
0 0.5 1
0.05
0.1
0.15
SD
of d
elay
(s)
SimulationAnalysis
SimulationAnalysis
SimulationAnalysis
Model Validation
Channel Busyness Ratio
The optimal operating point denoted by Umax
Simulation settings50 nodes, RTS/CTS mechanism is usedEach node has the same traffic rate.We monitor the performance at different traffic rates.
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Call Admission and Rate Control (CARC)
Classifier
Rate Control
MAC
Real-time trafficNonreal-time traffic
Upper layer traffic
Call Admission Control
Channel Busyness Ratio
Wireless channel
PriorityQueue
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Call Admission Control
Channel utilization/channel busyness ratio for a flow
Admission control test
, ,
, ,
A mean i mean rt
A peak i peak max
u u U
u u U
( ) * suc
Ru R T
L R: flow data rate (bps)
L: average packet length (bits)
Up to Urt (= γ Umax, 0<γ<1) can be assigned to real-time traffic
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Rate control
Notation: r: Channel resource r allocated to each node
Allowable channel time occupation ratio
tp: channel time for packet p Time that a successful transmission of packet p will last over the channel.
∆: scheduled interval Time between two consecutive packets that DRA passes to the MAC layer
br: channel busyness ratio brth = Umax
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Rate control
Initialization Procedure: r=rstart
Three-Phase Resource Allocation Mechanism: multiplicative-increase if underloaded, i.e., br < BM=α×brth
Additive-increase if moderately loaded, i.e., BM ≤ br < brth
Multiplicative-decrease if heavily loaded, i.e., br ≥ brth
thnew
brr r
br
pnew
tr r
r
thnew
brr r
br
, 0.95150
thstart
brr
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Theoretical Results of CARC
Convergence of Multiplicative-Increase Phase
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Theoretical Results of CARC
Convergence to Fairness Equilibrium
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Simulation Studies
Simulation settings in ns2 Channel rate = 11 Mbps Voice traffic with an on-off model
The on and off periods are exponentially distributed with an average value of 300 ms each.
During on periods, traffic rate is 32kb/s with a packet size of 160 bytes.
Greedy best effort traffic Saturated CBR traffic with a packet size of 1000bytes.
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Throughput and MAC delay
0 100 200 3002
3
4
5
Number of nodes
Thr
ough
put (
Mbp
s)
0 100 200 3000
200
400
600
800
Number of nodesM
AC
Del
ay (m
s)0 100 200 300
0
0.2
0.4
0.6
0.8
1
Number of nodes
Col
lisio
n pr
obab
ility
CARC802.11maximum
CARC802.11
DRA802.11
CARC improves the throughput by up to 71.62% with RTS/CTS, and by up to 157.32% without RTS/CTS
CARC achieves up to 95.5% of maximum throughput with and without RTS/CTS
Each node is a source of greedy traffic
0 100 200 3002
3
4
5
Number of nodes
Thr
ough
put (
Mbp
s)
0 100 200 3000
200
400
600
800
Number of nodes
MA
C D
elay
(ms)
0 100 200 3000
0.2
0.4
0.6
0.8
1
Number of nodes
Col
lisio
n pr
obab
ility
CARC802.11maximum
CARC802.11
DRA802.11
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0 100 200 300 400 5000
1
2
3
4
5
Thro
ughput (
Mbps)
Time (s)
Aggregate throughput
Individual user's throughput
CARC
0 100 200 300 400 5000
1
2
3
4
5
Thro
ughput (
Mbps)
Time (s)
Aggregate throughput
Individual user's throughput
802.11 Fairness
Higher aggregate throughput
Short term fairness
Fairness convergence speed: 0-2 s
300 305 3100
0.1
0.2
0.3
0.4CARC
Thr
ough
put
Time (s)
300 305 3100
0.1
0.2
0.3
0.4802.11
Thr
ough
put
Time (s)
A new greedy node joins the network every other 10 seconds
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0 200 400 600 800 100010
-3
10-2
10-1
100
101
102
Time (s)
De
lay
of v
oic
e p
ack
ets
(s)
Mean (802.11)SD (802.11)Mean (CARC)SD (CARC)
0 200 400 600 800 10000
0.2
0.4
0.6
0.8
1
1.2
1.4
Time (s)
802.11CARC
Th
rou
gh
pu
t of v
oic
e tr
affi
c (M
bp
s)
0 200 400 600 800 100010
-4
10-3
10-2
10-1
100
101
102
Time (s)
De
lay
of a
ll vo
ice
pa
cke
ts (
s)
802.11
CARC
Quality of Service for Voice Traffic
50 greedy nodesA new voice node joins the network every other 10 seconds.
97%ile 99%ile
0.0406 s 0.0811 s
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Conclusion
The IEEE 802.11 MAC protocol can support strict QoS requirements of real-time services while achieving maximum throughput.
Channel busyness ratio is a good network status indicator of the IEEE 802.11 systems.
An efficient call admission and rate control framework is proposed to provide QoS for real-time service and also to approach the maximum throughput.
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