Guaranteed Quality-of-Service Access to IEEE 802.11 Wireless LANs Dr-Jiunn Deng Department of Electrical Engineering National Taiwan University, Taipei,

Guaranteed Quality-of-Service Guaranteed Quality-of-Service Access to IEEE 802.11 Access to IEEE 802.11

Wireless LANsWireless LANs

Dr-Jiunn DengDr-Jiunn Deng

Department of Electrical EngineeringDepartment of Electrical EngineeringNational Taiwan University, Taipei, Taiwan, R.O.C.National Taiwan University, Taipei, Taiwan, R.O.C.

Email: Email: [email protected] 29, 2002October 29, 2002

The next wave of the InternetThe next wave of the Internet

The Net's founders predict its future:"Nomadic computing, providing access while you're on the road so that the Internet services you see when you're someplace else are no different than what you have back in your office.“ --Leonard Kleinrock

"Radio-based links into the Net will be very typical. If you have a question, you'll whip out your Palm Pilot with a radio link and go on the Net and pull the data out.“ --Vinton Cerf

"Many sites in the research community will have access at gigabyte speed to the Internet. You'll see the increasing introduction of wireless access, so people don't have to feel tethered to the Net. And we're going to see increasing content.“ --Robert Kahn"The Internet will become the pervasive network for the world's telecom traffic. Voice and video will transfer over to it in the next five to 10 years. Clearly, you're going to have video on demand, radio or TV, that can have millions of different sources or special subjects that (small numbers) care about.“ --Lawrence Roberts

Towards Multimedia Oriented Towards Multimedia Oriented Mobile Systems and providing Mobile Systems and providing

““Anytime Anywhere Anyform” Anytime Anywhere Anyform” Information SystemsInformation Systems

Status of specified wireless networking Status of specified wireless networking technologiestechnologies

IEEE 802.11 – keep growing

Bluetooth – will see

The rest – some need to be watched, most never take off

802.11 WLAN Architecture802.11 WLAN Architecture

Wired NetworkWired Network

Basic Service SetBasic Service Set

APAP

DestinationDestination

RTSRTS

CTSCTS

DataData

ACKACK

NAV (RTS)

NAV (CTS)

NAV (Data)

DIFS

SIFS

SIFS

SIFS

Defer access

DIFS

DIFS

DIFS

iranf22() TimeSlot_Backoff time:

DCF (CSMA/CA)DCF (CSMA/CA)

CW

CW

CW

Backoff time started

SourceSource

OtherOther

MAC ArchitectureMAC Architecture

Contention-Contention-freefreeServiceService

ContentionContentionServiceService

MACMACExtenExtentt

Point CoordinationPoint CoordinationFunction (PCF)Function (PCF)

Distributed Coordination Function (DCF)Distributed Coordination Function (DCF)

PCFPCF

CFP CP CFP CP

SuperframeSuperframe SuperframeSuperframe

Stretched DCF periodStretched DCF period

BeaconBeacon CF-PollCF-Poll

SIFS

SIFS

Sta-to-StaSta-to-Sta

SIFS

ACKACKCF-PollCF-Poll

SIFS

Sta-to-StaSta-to-Sta

SIFS

ACKACK

PIFS PIFS

CF-EndCF-End

Contention Free PeriodContention Free Period Contention PeriodContention Period

Contention Free Period Repetition IntervalContention Free Period Repetition Interval

NAV

PIFS

Packet-switched solutions that take advantage of silences in a given voice call by multiplexing voice data from other calls are more bandwidth-efficient than circuit-switched solutions

In wireless networks, where bandwidth is more constrained

DCF can not support service discipline of integrated multimedia traffic since it does not include any priority and access policy

PCF mode offers a “packet-switched connection-oriented” service, which is well suited for telephony traffic

In order to poll the stations an AP must maintain a polling list, which is implementation dependent

MotivationMotivation

The use of packet-switched techniques

for carrying multimedia traffic in

802.11 WLANs are indeed needed

Bandwidth management and QoSBandwidth management and QoS IETF groups are working on proposals including

RSVP, Differentiated Services, and Integrated Services to provide better QOS control in IP networks

Principle 1: Marking of packets is needed to distinguish between different classes

Principle 2: provide protection (isolation) for one class from other classes

Principle 3: It is desirable to use resources as efficiently as possible

Principle 4: Application flow declares its needs, network may block call if it cannot satisfy the needs

Wired NetworkWired Network

Basic Service SetBasic Service Set

APAP

Should we supported these functionalities in..

CORE?

AP

EDGE?CLIENT?

Delay in packet-switched networks

nodalprocessing

transmission

propagation

ondsmicrobits

kbits

sec8sec/10

89

ondsmilim

km

sec6.16sec/103

50008

A

B

queueing

packets608.160001010)106.16108( 3936

Enforcing priority for RAToo support priority, we change the backoff time generation function

inim kranf 22() iranf 22()

Consecutive times (i) Backoff slot numbers

Types of requests (k, m, n)

1st 2nd 3 rd 4 th

Real-time handoff traffic( 0, 1, 1 )

0 – 3 0 -7 0 – 15 0 – 31

Admitted inactivated video traffic( 1, 1, 1 )

4 – 7 8 - 15 16 – 31 32 - 63

Non-real-time handoff trafficNew request traffic

( 2, 2, 1 ) 8 – 15 16 - 31 32- 63 64 – 127

Adaptive contention window The collision avoidance strategy in DCF avoids long ac

cess delays when the load is light, but it causes a high collision probability and channel utilization in degraded in bursty arrival or congested scenarios

In this paper, we propose an adaptive contention window mechanism to dynamically expand and contract the contention window size according to the current load and achieve the theoretical capacity limits.

Define the utilization factor The value of utilization factor provides a lower

bound to the actual number of stations trying to access the channel during the last contention window

imbS

2

is a tight upper bound of in a system operating with the optimal channel utilization level

By fixing a given value for the frame size, the value of is almost constant

can be used as a measure of the network contention level when the network utilizes the optimal contention window size corresponding to the ongoing network and traffic configuration

We double the size of contention window when the utilization factor exceeds , but we halve the size of contention window when the utilization factor becomes less than , rather then

Adaptive contention window

optpM

optpM

optpM

optpM

optpM 25.0 optpM 5.0

Packet scheduling policy in CFP

Token buffer for voice traffic

Token buffer for video traffic

Contention Free PeriodContention Free PeriodContention PeriodContention Period

Contention Free Period Repetition IntervalContention Free Period Repetition Interval

PACKET-TRANSMISSIONPACKET-TRANSMISSIONCHANNELCHANNEL

Packet-Transmission PermissionPacket-Transmission Permission

PIGGY-BACKINGPIGGY-BACKINGOF REQUESTOF REQUEST

CSMA/CACSMA/CAwith with

ENFORCING PRIORITYENFORCING PRIORITYfor RA for RA

RequestRequest

Packet flow Control flow Request flow

Packet scheduling policy in CFP1) The PBS first scans the token buffers of voice sources according to the

preset priority order. If a token is found, it removes one token from this token buffer and transmits a packet for this voice source. Then, the PBS generates next token for this voice source after second if the piggybacking was set while transmitting the packet, where is the time to transmit a packet.

2) If no tokens are found in the token buffers of voice sources, the PBS continues to scan the token buffers for video sources according to the preset priority order. If a token is found, it transmits a packet for this video source. And it will not remove the token if the piggybacking was set while transmitting this packet. If the piggybacking was not set and it is not the last packet (End-of-File) either, the PBS removes the token, and then generates next token for this video source after seconds. But if this admitted inactivated video source contended successfully within seconds, there is not any toke be generated by PBS automatically.

3) If there is no token found in all token buffers. The AP uses the CF-END frame to announce the end of the contention free period and the maximum time interval of following contention period.

ptpc tr )1(

Admission Control for voice traffic Let , If and for all =1,2…, , then all the pack

ets generated by new-call voice sources meet their jitter constraints.

Furthermore, if and for sources which is handoffed from other cells, then the packet generated by the source after handoff meets its jitter constraint.

1

1

i

kp

ci

ckpi t

r

rt

cni ,...,1

ACKSIFSPacketSIFSCFPollPIFSt p

cii r1 ii i cn

ciii r1 iii thi

thi

Let , , , ,

and , where .

If and for all , then the delay

constraints are satisfied for all the new-call video sources. Furthermore, if for source which is handoff from other cells, then the packet generated by the source after handoff meets its delay constraint.

Admission Control for video traffic

)1(0

_

cp nt

cn

icipv rtr

1

_

0 )1(_

jpj t vjpvj rtr _

1

0

_

1

1

*

0

_

*

1

)(

j

kvk

j

kkvkp

j

kk

jj

r

drtd

vnj ,...,1

10

_

vn

kvkr

jj dd * j

jjjj dd * thj

thj

Adaptive Bandwidth Allocation Strategy

Channel Ifor

new-call/handoff voice/video traffic

Channel IIfor

handoff voice/video traffic

Channel IIIfor

data traffic

Contention Free PeriodContention Free Period Contention PeriodContention Period

Contention Free Period Repetition IntervalContention Free Period Repetition Interval

CFP-channel I CFP-channel II CP-channel III

Adaptive Bandwidth Allocation Strategy

IF monitored dropping probability > threshold_D THEN IF bandwidth utilization < THEN size of allocated bandwidth II= min {max {size of allocated bandwidth I, size of allocated bandwidth II} up_ , total bandwidth } ELSE size of allocated bandwidth II= min {max {size of allocated bandwidth I, size of allocated bandwidth II} up_ , total bandwidth threshold_up_II }ELSE

Adaptive Bandwidth Allocation Strategy (cont.)

IF monitored blocking probability > threshold_B THEN IF bandwidth utilization < THEN size of allocated bandwidth I= min {size of allocated bandwidth I up_ , total bandwidth threshold.1_up_I } ELSE size of allocated bandwidth I= min {size of allocated bandwidth I up_ , total bandwidththreshold.2_up_I } ELSE IF bandwidth utilization < THEN size of allocated bandwidth II= max {size of allocated bandwidth II down_ , total bandwidththreshold_down_II } size of allocated bandwidth I= max {size of allocated bandwidth I down_ , total bandwidththreshold_down_I }

Conclusions The design of priority-sensitive network protocols con

tinues to be an important problem Broadband wireless links constitute a subclass where

prioritization is key to optimizing overall performance We proposed a pragmatic non-preemptive priority ba

sed access control scheme built on well-know protocols and offered easily implemented and yet flexible criteria for traffic prioritization in a wireless environment.

Various QoS requirements are needed in the future Multilevel priorities, bandwidth allocation, connectio

n admission control, and traffic policing all need to be considered together in the future networks

Reference

D. J. Deng and R. S. Chang, “A Priority Scheme for IEEE 802.11 DCF Access Method,” IEICE Trans. CommunIEICE Trans. Commun., vol. E82-B, no. 1, pp. 96-102, January 1999.

D. J. Deng and R. S. Chang, “A Non-Preemptive Priority Based Access Control Scheme for Broadband Ad-Hoc Wireless ATM Local Area Networks,” IEEE Journal on Selected Areas of CommunicatIEEE Journal on Selected Areas of Communicationsions, Vol. 18, no. 9, Sep. 2000, pp. 1731-1739.