ieee 802.11e : edca - dtic.upf.edubbellalt/mcn_docs/mcn-wlans-edca.pdf · ieee 802.11e mac model...

B. Bellalta

Mobile Communication Networks

IEEE 802.11e : EDCA

Mobile Communications Networks

Scenario

STA

STA

STA

STAAP

Server Fixed Network

Server

Server

STA

STAAP

Upwnlink TCP flows

Downlink TCP flows

What is the WLAN cellperformance with bidirectional TCP traffic?


3

VoIP over WLANs

• Voice over IP– Economical alternative to

traditional telephony.

• Wireless LANs– Broadband access network to

internet.

• VoIP over WLANs.– A market opportunity.

• Complement/Substitute of current cellular networks.

– Technical limitations.

178,000 new Skype users per day!

August 2003


4

VoIP over WLANs

• Voice over IP– Economical alternative to

traditional telephony.

• Wireless LANs– Broadband access network to

internet.

• VoIP over WLANs.– A market opportunity.

• Complement/Substitute of current cellular networks.

– Technical limitations.

178,000 new Skype users per day!

August 2003


5

VoIP over WLANs

• Limitations for VoIPoWLAN (and other realtime services)– Protocol overheads.– Capacityvarying channels (multirate).– Random Access MAC.

• Bandwidth penalty due to the distributed access.– Unfairnes Downlink/Uplink.

• Difficult coexistence between heterogeneous (rigid and elastic) flows.– Handoff / Roaming between WLANs.


6

Example: Rdata = 2 Mbps, Rbasic = 1 Mbps, VoIP codec: G.729 (8 Kbps)– Downlink/Uplink TCP flows ↑ simultaneous VoIP calls ↓

IEEE Solution: EDCA (Enhanced Distributed Channel Access): • Traffic differentiation at MAC layer + CAC optional.

• EDCA is too conservative.• Static Traffic Differentiation without Admission Control.

VoIP over WLANs

VoIP call

VoIP call

ACK packets

TCP packets

ACK packets

TCP packets

TCP packets

ACK packets


7

Multimedia Traffic Flows

• General Classification (queue occupation). – Elastic flows (TCPbased):

• use all available bandwidth, fair with other elastic flows.• saturated source/queue with : B ≈ L/X, ρ =1

– Streaming/Rigid flows (UDPbased): • use only the bandwidth required.• finiteload (unsaturated) source/queue with: B ≈ ρ L/X, ρ ≤ 1

Rigid Traffic (UDP)

User Perception

B

B

• Video/audio streaming• Voice over IP

• Web (HTTP)• FTP, email, ...

Multimedia

Elastic Traffic (TCP)User Perception

MinimumBandwidth

Good

Bad

Better

Worst


The DCF/EDCA model


9

The scenario

AP

MN

MN

MN

MN

MN

MN

MN

MN

MN and AP share the channel using a random access MAC protocol.

B=100 Kbps

B=200 Kbps

B=80 Kbps

B=120 Kbps

B=450 Kbps

B=max. av.

B=max. av.

B=max. av.

Be=max. av.

MN1

MN2


10

BUSY

BUSY

BO

IEEE 802.11 MAC Model (DCF)

• Each node is modelled by a M/M/1/K queue with packet arrival rate λ k and service time X.– Assumption: Poisson arrivals and service time exponentially distributed.– The model accounts for: queuing delays, packet losses.

t

t

MN1

MN2

X11/λ 1

X2

packet arrival(HOL)

packet arrival(queued) packet departure

DATA ACK BO DATA ACK

packet departure

X1

EQ1:Queueing delay

BO DATA ACKBUSY BO

backoff suspension

BO

BO

COLLISION

COLLISION

Queue empty

1/λ 2

K

Xλ k


11

BUSY

BUSY

BO

IEEE 802.11 MAC Model (DCF)

• Each node is modelled by a M/M/1/K queue with packet arrival rate λ k and service time X.– Assumption: Poisson arrivals and service time exponentially distributed.– The model accounts for: queuing delays, packet losses.

t

t

MN1

MN2

X11/λ 1

X2

packet arrival(HOL)



packet departure

X1

EQ1:Queueing delay

BO DATA ACKBUSY BO

backoff suspension

BO

BO

COLLISION

COLLISION

Queue empty

1/λ 2

K

Xλ k


12

DCF EDCA

BUSY

BUSY

BO t

t

MN1

MN2

X11/λ 1

X2

packet arrival(HOL)



packet departure

X1

EQ1:Queueing delay

BO DATA ACKBUSY BO

backoff suspension (succ.)

BO

BO

COLLISION

COLLISION

Queue empty

1/λ 2

tBO BUSY

MN3

backoff suspension (coll.)

empty slot

AIFS

BEB

TXOP


13

IEEE 802.11e MAC Model (EDCA) : TXOP• Multiple packet transmission each time a MN/AP gets the channel

– The overall network throughput increases (S=Data/Time [bps])– Less wasted time in random access.

BO DATA ACK

BO DATA ACKbusy

BO busy

BO DATA ACK

BO DATA ACKbusy

BO busy

DATA ACK

BO DATA ACK

GAIN

BO DATA ACKBO busy

BO DATA ACK

Packet arrival

Packet arrival

Packet arrival (Queued)


Packet arrival


Packet arrival

Packet arrival (Queued) Service Time (X)

MN 1

MN 2

MN 1

MN 2

GAIN

Service Time (X)

t

t

t

t

No TXOP

TXOP


14

IEEE 802.11e MAC Model (EDCA) : TXOP

• Each user is modeled as an M/G[1,B]/1 queue (bulk service time queue)– TXOP (Burst length): maximum consecutive number of frames transmitted at each

successful attempt.

0 1 2 i=B j K

• Stationary (equil.) distribution.• Blocking Probability.• Queue occuppation.• Queuing delay.

• Departure distribution.• Average TXOP length• Average TXOP duration

Renewal Analysis


15

IEEE 802.11e MAC Model (EDCA): AIFS

• AIFS

BOBUSY BO tBO BUSY

Extra blocked slots due to AIFS

BUSYBUSYBUSY

BOBUSY BO tBO BUSYBUSY BUSY BUSY

Aggregate load from other nodes

MN1

MN1

Service Time ↑Transmission Prob. ↓


16

IEEE 802.11e MAC Model (EDCA): EB, p

• Expected BackOFF slots

• Cond. Collision Probability

• Prioritizing a flow: TXOP ↑ AIFS ↓ CWmin↓

• This model is able to capture the impact of the other flows with different MAC parameters.


17

Wireless Scenario

• SingleHop AdHoc network.• No hiddenterminals and ideal channelconditions.• Channel access based on the DCF (BA and RTS/CTS).• Two types of flows: streaming (UPDlike) and elastic (TCPlike).

3

2

n1

n

i1


18

Results

• Singlecell WLAN (adhoc)– Two types of flows:

• S1 (streaming ~ unsaturated)– Bs = 100 Kbps– L = 400 bytes

• E1 (elastic ~ saturated)– Be (network dependant)– L = 1500 bytes

• Default WLAN MAC parameters– 0 S1 flows.

• Solution (1):– AIFS ↑ (elastic flows, A=3)

• 8 S1 flows– The TCP throughput is reduced.

S1 saturation point


19

Model Validation

• Singlecell WLAN (adhoc) – Two types of flows:

• S1 (streaming ~ unsaturated)– Bs = 100 Kbps– L = 400 bytes

• E1 (elastic ~ saturated)– Be (network dependant)– L = 1500 bytes

• Solution (2):– TXOP S1 ↑ (=4)

• 7 S1 flows.– CWmin,E1 ↑ (=64)

• 6 S1 flows.


20

Call Admission Control


21

Problem statement: Providing QoS

• Flowlevel Admission Control. • Adaptive selection of MAC parameters.


22

Problem statement: Providing QoS

• Flowlevel Admission Control. • Adaptive selection of MAC parameters.

random access MAC(Performance)=f(MAC parameters, number of nodes, traffic profile each node)

non-linear relation

maximize performance (under the QoS)


23

The available bandwidth estimation problem

nn

BandwidthBandwidth

Bandwidthpenalty

Flow rejected Flow rejected

... how to decide if a flow can be admited? We have to compute or to testif the new flow is possible.

Ideal Channel Sharing Random Access Channel Sharing

... the bandwidth penalty due to the distributed access depends on the number of flows active and the traffic profile of each flow.

Availablebandwidth Available

bandwidth


24

Multirate Problems on VoIP capacity


25

Multirate wireless

AP

11Mbps

11Mbps

11Mbps

11Mbps

etc…


26

Multirate wireless

AP11Mbps

11Mbps

11Mbps

etc…

1Mbps


27

Problem Statement

Distribution of VoIP flows in a cell (G.711 codec)

While total cell capacity is around 12 calls when all of them transmit at 11Mbps…

…this capacity falls with any change from fast to slow flows


28

Multihop (Mesh) NetworksVoIP capacity


29

VoIP Capacity in a mesh network

MP MP MP MP

VoIP calls (G.279)

Single channel

Channel Contention (MAC)

hidden nodes

D. Niculescu. S. Ganguly, K. Kim, R. Izmailov; “Performance of VoIP in a 802.11 Wireless Mesh Network”. IEEE Infocom 2006, Barcelona, Spain.

2 Mbps


30

VoIP Capacity in a mesh network

MP MP MP MP

VoIP calls

Multiple channels(Two channels)

Channel Contention (MAC) ↓

hidden nodes ↓

MP MP MP MP

VoIP calls

NO Channel Contention (MAC)

NO hidden nodes

A

BMultiple channels(Three channels)


31

VoIP Capacity in a mesh networkD. Niculescu. S. Ganguly, K. Kim, R. Izmailov; “Performance of VoIP in a 802.11 Wireless Mesh Network”. IEEE Infocom 2006, Barcelona, Spain.


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