Wireless Networks Laboratory (WINET) Quality of Service Support in Wireless Networks Hongqiang Zhai...

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

http://www.ecel.ufl.edu/~zhai


2

Outline

Introduction

Performance analysis of the IEEE 802.11 MAC protocol

A call admission and rate control scheme

Conclusion and future research issues


3

Wireless Landscape


4

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


5

Mobile Ad Hoc Networks and Wireless Mesh Networks


6

Quality of Service (QoS) Requirements

Bandwidth

Delay and delay jitter

Packet loss rate


7

Challenges

Unreliable physical channel Time-varying propagation characteristics Interference

Limited bandwidth

Limited processing power and battery life

Distributed control

Mobility


8

Medium Access Control

Coordinate channel access Reduce collision Efficiently utilize the limited wireless bandwidth

A

B

C

D


9

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?


10

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?


11

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


12

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


13

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


14

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


15

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)


16

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


17

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


18

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


19

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


20

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.


21

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


22

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


23

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


24

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


25

Theoretical Results of CARC

Convergence of Multiplicative-Increase Phase


26

Theoretical Results of CARC

Convergence to Fairness Equilibrium


27

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.


28

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


29

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


30

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


31

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.

Wireless Networks Laboratory (WINET) Quality of Service Support in Wireless Networks Hongqiang Zhai...

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