improving quality of service for voip traffic in ieee 802.11 wireless networks

12/13/2006

Improving Quality of Improving Quality of Service Service for VoIP Traffic in IEEE for VoIP Traffic in IEEE 802.11 Wireless 802.11 Wireless NetworksNetworks

Sangho ShinHenning Schulzrinne

IntroductionIntroduction Increased Usage of VoIP service over

wireless networks Widely deployed WLANs

Shopping Malls Coffee shops Streets Parks

Wi-Fi phones New service plans

Sprint – cellular and Wi-Fi Limited QoS support in 802.11 WLANs

IEEE 802.11e ?

[skyhook]

Map of APs in Manhattan

OutlineOutline

Overview of QoS problems My earlier work for the problems Fair resource distribution b/w

uplink and downlink using APC Call Admission Control with QP-CAT Conclusion Future work

Framework of VoIP over Framework of VoIP over WLANsWLANs

AP AP

Router Router

160.38.x.x 128.59.x.x

InternetInternet

PBX

Wireless client

Introduction

QoS problems of VoIP serviceQoS problems of VoIP service

The network disruption during L2/L3 handoff Fast L2/L3 handoff

Limited VoIP capacity Dynamic PCF (DPCF)

Unbalanced uplink and downlink delay Adaptive Priority Control (APC)

Significant deterioration of QoS in the case of channel congestion Call admission control using QP-CAT

Introduction

OutlineOutline


uplink and downlink (APC) Call Admission Control with QP-

CAT. Conclusion Future work

Seamless layer-2 handoff Fast layer-3 handoff Dynamic PCF

Fast layer-2 handoff Fast layer-2 handoff [26][26] (1/2)(1/2)

Earlier work

AP

Router

Wireless client All APs

New APAuthentication request

Authentication response

Association request

Association response

Probe request (broadcast)

Probe responses

AP

L2 handoff trigger

Authentication delay

Probe delay

Association delay

300m ~ 1s

2 ms

2 ms

[26] Sangho Shin, Andrea G. Forte, Anshuman Singh Rawat, and Henning Schulzrinne. Reducing MAC layer handoff latency in IEEE 802.11 wireless LANs. In ACM MobiWac '04, pages 19~26, New York, NY

0

100

200

300

400

500

Original SelectiveScanning

Caching

Total handoff timems

Fast layer-2 handoff Fast layer-2 handoff (2/2)(2/2)

Selective Scanning Scan non-overlapping channels

first Channel 1, 6, and 11 in 802.11b Reduces the scanning time to

1/3 Caching

Motivated by locality Store the scanned AP

information in a cache Use it in the future handoffs

Handoff without scanning Reduces the total handoff time

to 4 ms No changes in the

infrastructure Practical solution!

Earlier work

Experiments in 802.11b WLANs

Fast layer-3 handoff Fast layer-3 handoff [9][9] (1/2)(1/2)

Earlier work

AP AP

Router Router

160.38.x.x 128.59.x.x

L2 handoff L3 handoff

Subnet change detection A few minutes

New IP acquisition- DHCP 1 second

DHCP Discover

DHCP Offer

DHCP Request

DHCP ACK

DAD

DHCP procedure

[9] Andrea Forte, Sangho Shin, and Henning Schulzrinne. Improving layer 3 handoff delay in IEEE 802.11 wireless networks. In WICON, Aug 2006.

DAD: Duplicate Address Detection

0

200

400

600

800

1000

Original TempIP Best

Total handoff timeExperiments in 802.11b

ms

Fast layer-3 handoff Fast layer-3 handoff (2/2)(2/2)

Fast subnet change detection Broadcast a bogus DHCP request

The DHCP server responds with DHCP NACK

Check the IP address of the DHCP server

Temporary IP address Scan potentially unused IP

addresses in the new subnet Transmit multiple ARP packets

Pick a non-responded IP address as a temporary IP address

Use it until a new IP address is assigned by the DHCP server

Revisits the previous subnets IP address lease not expired

Use the old IP address!

Earlier work

ARP

DHCP Discover

timeout

Update sessionsDHCP Offer

DHCP RequestUpdate sessions

DHCP Ack

DHCP Request

DHCP NACKSubnet change

L2handoff

18030

[18] Takehiro Kawata, Sangho Shin, and Andrea G. Forte. Using dynamic PCF to improve the capacity for VoIP traffic in IEEE 802.11 networks. In WCNC, IEEE, vol 3, pages 13~17, Mar 2005.

Dynamic PCF (DPCF) Dynamic PCF (DPCF) [18] (1/2)[18] (1/2)

PCF (Point Coordination Function) Polling based media access

No contention, no collision Polling overhead

No data to transmit Unnecessary polls waste bandwidth

Big overhead, considering the small VoIP packet size.

Earlier work

Contention Free Period (CFP) Contention Period (CP)

Contention Free Repetition Interval

DCF

poll poll Poll+data poll pollBeacon CF-End

data data data Null Null

Polling overhead

Transmission Rate (Mb/s)

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 7 8 9 10 11 12

Nu

mb

er o

f V

oIP

Flo

ws

DCFPCFDPCF

13

23

28

7

DCF

3024

PCF

7

14

37

28

DPCF

Capacity for 64kb/s VBR VoIP traffic

32%

Dynamic Polling List Store only “active” nodes

Higher priority for VoIP

Simulation results in 802.11b

0 1 2 3Number of Data Sessions

Delay (90th%tile) and throughput of 28 VoIP sources and data traffic

0

500

1000

1500

2000

2500

3000

Th

rou

gh

pu

t (k

b/s

)

0

100

200

300

400

500

600

En

d-t

o-E

nd

De

lay

(m

s)

Dynamic PCF (DPCF) Dynamic PCF (DPCF) (2/2)(2/2)

CFP CP

Polling List 1 23 45678

1 3 8

65

Null NullACK

1 2 3 4

ACK

7 8

7

ACK

567

Queue

CFP CP

Dynamic Polling List1 3 8

1 3 8

1 2 3

65

ACKACK

7

ACK

Earlier work

DCF DCF DCF DCF

25

400

487

545

DPCF DPCF DPCF DPCF

11 22 26 29

PCF

PCF PCF PCF

18

67

183

312

Data throughput

VoIP throughput

OutlineOutline


uplink and downlink using APC in DCF

Call Admission Control with QP-CAT Conclusion Future work

Motivation Adaptive Priority Control Simulation results Related work Conclusion

MotivationMotivation Big gap between

uplink and downlink delay

Why? - DCF The same

chance to transmit frames among nodes

In VoIP traffic, the AP has more packets to transmit than each client

APC

Solution ?

Give a higher priority to the AP

0

100

200

300

400

500

600

26 27 28 29 30 31 32 33 34

Number of VoIP sources

De

lay

(ms)

Uplink (90th%tile)

Downlink (90th%tile)

Uplink (AVG)

Downlink (AVG)

DCF = Distributed Coordination Function

64kb/s VBR VoIP traffic802.11b 11 Mb/svia simulations

Control Contention Window (CW) Shorter CW smaller backoff time higher priority Hard to control

Backoff = random (0, CW) Higher collision rate

Control Inter-Frame Spacing (IFS) Increase IFS lower priority Not accurate because of BO Increase the delay due to the increased IFS

How to control the priority? How to control the priority? (1/2)(1/2)

APC

Node ANode B

backoffNode B

Node A

Tx in DCF

BO frame

DIFS

Channel DIFS = DCF Inter-Frame Spacing

Priority of node A is 3 times of node B

How to control the priority? How to control the priority? (2/2)(2/2)

Contention Free Transmission (CFT) Transmit frames w/o backoff Control the number of frames to be

sent contention free

Accurate priority control No overhead Reduce the overall transmission time

Improve the capacity

APC

Node ANode B

Optimal priority of the AP Optimal priority of the AP ((PP))

Intuitive method P the number of wireless clients (N) Adapts to change in the number of VoIP

sources Not adapts to change in the traffic volume

APC P QAP/QNodes

QAP is the number of packets in the queue of the AP QNodes is the average number of packets in the

queue all nodes Adapts to instant change of uplink and

downlink traffic load

APC

APC – example 1APC – example 1

AP

DS

queue queue queue queue

queue

APC

QAP = 4QNode = 1

P = 4/1 = 4


AP

DS


queue

APC

QAP = 4QNode = 2

P = 4/2 = 2


AP

DS


queue

APC

QAP = 2QNode = 1

P = 2/1 = 2

Simulation results of APC Simulation results of APC (1/3)(1/3)

APC

0

100

200

300

400

500

600

26 27 28 29 30 31 32 33 34


De

lay

(ms)

Uplink (90th%tile)


Uplink (AVG)

Downlink (AVG)

Threshold

Capacity

DCF

0

100

200

300

400

500

600

30 31 32 33 34 35 36 37


De

lay

(ms)

Uplink (90th%tile)


Uplink (AVG)

Downlink (AVG)

Threshold

Capacity

APC

28 Calls 35 calls (25%)

802.11b11Mb/s64kb/s VBR traffic20ms pkt intvl0.39 activity ratio


APC

90th%tile delay of VoIP traffic (35 calls)


0

50

100

150

200

250

20 21 22 23 24 25


Del

ay (

ms)

Uplink (90th%tile)


Uplink (AVG)

Downlink (AVG)

10ms + 20ms Packetization Interval

0

50

100

150

200

250

38 39 40 41 42 43 44 45 46


De

lay

(ms)

Uplink (90th%tile)


Uplink (AVG)

Downlink (AVG)

20ms + 40ms Packetization Interval

APC

Related workRelated work Many papers about fairness in throughput ([Deng

et. al. IEICE Trans. Comm.], [Barry et. al. Infocom ’01], [Aad Infocom ’01], [Tickoo et. al. Infocom ’04])

In VoIP traffic, Jain’s Fairness Index is almost 1 regardless of the big gap

Wang et al. [AINA ’04] - New Fair MAC Fairness between clients Clients transmit all packets consecutively at most for

Maximum Transmission Time (MTT) Improved only a few ms low uplink delay

Casetti et al. [PIMRC ’04] Based on EDCA Found a fixed optimal CW by simulations Improved the capacity by 15% Not a global solution for all traffic types

APC

ConclusionConclusion Uplink and downlink delay of VoIP traffic in

DCF are significantly unbalanced Unfair resource distribution b/w uplink and downlink

APC balances the uplink and downlink delay

very well by allowing the AP to transmit QAP/QNodes packets using Contention Free Transmission (CFT)

APC improves the capacity for VoIP traffic from 28 calls to 35 calls, by 25%

APC

Future workFuture work

Implement APC in 802.11e and evaluate it with various background traffic

Implement APC using the Mad-Wifi driver and evaluate the performance in the OBRIT test-bed

Improve APC so that it does not require client information

APC

OutlineOutline


uplink and downlink using APC Call Admission Control with QP-CAT Conclusion Future work

Motivation and requirements Metric for admission control Queue size Prediction using CAT Simulation results Related work Conclusion

MotivationMotivation When channel is congested, delay of all VoIP

flows significantly increases

Experimental results in the ORBIT test-bed

•64kb/s CBR VoIP traffic•802.11b •11Mb/s•20ms packet interval

Requirements for CACRequirements for CAC Accurate (Guarantee QoS of existing flows)

Need a good metric for QoS Fast

Users should not wait for a long time to call

Efficient Minimum waste of bandwidth

Flexible (extensible) Many types of VoIP traffic need to be

supported

Admission Control using QP-CAT

Metric Metric (1/3)(1/3)

Queue size of the AP (The number of packets in the queue of the AP)

Correlation b/w the queue size and downlink


Experimental results in the ORBIT test-bed

802.11b11 Mb/s64 kb/s CBR20 ms Pkt Intvl


Theoretical model

TTT

TAP

Q

TQ

DQDDQD

DQQD

DDD

)1(

1

Q

D

D

D

T

Q

Downlink delay

Queuing delay

Transmission delay

Processing time of the AP

Queue size of the AP


8.0TDAccording to computation using 802.11b parameters

Errors according to the delayCumulative Distribution Function of errors


Errors of the model


Queue size Prediction (QP)Queue size Prediction (QP) How to use the metric ?

When the queue size goes beyond a threshold, then reject the future calls ?

Too late QoS already deteriorated Cannot disconnect already admitted

flows See the future!

Need to predict the queue size in advance


Admission Control using QP-CATComputation of Additional Computation of Additional Transmission (CAT) Transmission (CAT) (1/5)(1/5)

Basic concepts Emulate the additional VoIP flows

Predict the queue size

Emulate VoIP traffic

Packets from a new flow

Compute Additional Transmission

channel

Actual packets

Additional transmission

Decrease the queue size

Predict the future queue size

+

current packets

additional packets

CAT CAT (2/5)(2/5)

Emulation of VoIP flows Two counters: DnCounter, UpCounter Follow the same behavior of VoIP

flows Increase the counters every

packetization interval of the flows

Decrement the counters alternatively

20ms time

DnCounter++UpCounter++



20ms

Example : 20ms packetization interval


CAT CAT (3/5)(3/5)

Computation of transmission time of a VoIP frame (Tt)

DIFSbackoff

Tv

SIFSACK frame

VoIP packet

Tb

TACK

Tt

PLCP MAC IP UDPRTP Voice data

Tt = DIFS + Tb + Tv + SIFS + TACK


CAT CAT (4/5)(4/5)

Computing Additional Transmission (np)

t

cp T

Tn

Tc= t2 - t1

DnCounter-- UpCounter--

Busy medium Tt TtTr

tpcr TnTT


t1 t2

CAT CAT (5/5)(5/5)

Additional considerations Collision Deferral Serialization of downlink packets

16 calls

17 calls + 1 call16 calls + 1 call

17 calls

17 Calls 18 Calls

Simulation results Simulation results (1/3)(1/3)

Another call is added

32 kb/s VoIP traffic with 20ms packetization interval

14 calls + 1 call 15 calls + 1 call

14 calls 15 calls

15 calls 16 calls



29 calls + 1 call 30 calls + 1 call

29 calls 30 calls

30 calls 31 calls



Related work Related work (1/3)(1/3)

Numerical or theoretical approaches Yang Xiao et al. [Communication Magazine ‘04 ]

802.11e EDCA based access control Compute available bandwidth using TXOPs and

announce it to clients Guarantee bandwidth, but not delay Applicable

Video traffic only Pong et al. [Globecom’03]

Estimate available bandwidth using an analytical model

Check if the requested BW available by changing CW/TXOP

The assumption of the analytical model is far from real environment

Kuo et al [Globecom’03] Pure analytical model based Expected bandwidth and delay are computed

using an analytical model


CBR/CUE - Sachin et al. [Globecom’03] and Zhai et al. [QShine04 ]

New metric : Channel Utilization Estimate (CUE)/Channel Business Ratio (CBR) = fraction of time per time unit needed to transmit the flow

CUE per flow is computed using the average Tx rate of each flow

Clients compute the CBR from their Tx rate and send it to the AP regularly.

Compare the remaining CUE and the requested CUE Assume 15% of wasted bandwidth due to collision or

fluctuation 0.85 max total CUE Actual probing

Metric: delay and packet loss Used for wired networks Very accurate and simple Waste a certain amount of bandwidth


ComparisonApproaches

Metric Assumption

Adapts to channel

Waste of BW

Extensibility

Theoreticalapproaches

CW/TXOPComputed bandwidth

Saturated channel

No Low Good

CUE/CBR CUECBR

Max CU(Measured in advance)

No(Fixed Max CU)

Middle(Reserved BW for collisions)

Good

Actual Probing

Delay/packet loss

No Yes High(Probing flow)

Bad

QP-CAT Queue size of the AP

No Yes Low Good

ConclusionConclusion The queue size of the AP is a good

indicator for QoS of all existing calls

QP-CAT can predict the future queue size of the AP very well

We can perform call admission control using QP-CAT accurately and efficiently

Future workFuture work

Evaluate the QP-CAT using various types of background traffic

Implement call admission control framework using QP-CAT

Evaluate the efficiency of CAC with QP-CAT using actual call arrival rate and call duration

ContributionsContributions APC

Introduced a New dynamic priority control method, CFT and showed it is better than others (CW, IFS).

Verified that the optimal priority of the AP for balancing uplink and downlink delay is QAP/QNode, using CFT.

Increased the VoIP capacity by 25% QP-CAT

Verified the linear correlation between the number of packets in the queue of the AP and downlink delay of VoIP traffic via experiments

Enabled the AP to predict the future downlink delay accurately using CAT

Protect the QoS of the existing VoIP traffic in fluctuating channel conditions, minimizing the waste of bandwidth

TimetableTimetableTimeline Work Progress

~ Dec. 2006

Implementation in the QualNet simulator and evaluation of APC and QP-CAT

Completed

Jan. 2007 ~ Apply QP-CAT to call admission control and evaluate the efficiency

Mar. 2007 ~

Evaluate the call admission control with QP-CAT with various types of background traffic

Apr. 2007 ~

Integrate APC with 802.11e using the QualNet simulator

May. 2007 ~

Evaluate APC with various types of background traffic

Jun. 2007 ~ Implement APC with the MadWifi driver and evaluate it in the ORBIT test-bed

Aug. 2007 ~

Introduce and implement a new APC that does not use the queue size of nodes

Oct. 2007 ~

Start to write my thesis

Feb. 2008 Defend my thesis

Thank you

References References (1/3)(1/3)

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