1

DOMINO: Relative Scheduling in Enterprise Wireless LANs

Wenjie Zhou (Co-Primary Author), Dong Li (Co-Primary Author),

Kannan Srinivasan, Prasun Sinha

2

Enterprise Networks

Important research topics:- Channel assignment- AP association- Power adaptation - Channel access

Internet

Router

AP1 AP2 APN

…

Client1 Client2

…

Client3 ClientN

3

Channel Access Schemes in enterprise networks:

- Distributed Coordination Function (DCF)- Downlink-only Centralized Schemes- Fully Centralized Schemes

4

Distributed Channel Access: DCF (WiFi)

Pros:- Simple to implement- Robust to failures

AP2 AP1 AP3

C2 C1 C3

ExposedHidden

Cons:- Hidden and exposed terminal problems- Low efficiency

Interfering nodes Flow direction

5

Downlink-only Centralized Schemes

• CENTAUR (Mobicom’09): Downlink packets that could be sent simultaneously are forwarded to the APs at the same time.

• OmniVoice (MobiHoc’11): Downlink packets are sent according to broadcast schedules.

Pros:- No modification on clients

Cons:- Downlink traffic only

6

0

4

8

12

16

20 DCF(Distributed)CENTAUR(Downlink-only)Omniscient(fully centralized)

Thro

ughp

ut (M

bps)

Fully Centralized Scheme

• Schedule both uplink and downlink traffic

AP1 -> C1 C2 -> AP2 AP3 -> C3 Overall

~61%

• Higher centralization higher throughput• Many theoretical work proposed• Challenges for fully centralized scheme:• Queue status of Clients• Time synchronization

AP2 AP1 AP3

C2 C1 C3

Interfering nodes Flow direction

7

Domino• a practical platform to enable arbitrary

centralized scheduling algorithms• without requiring tight time-synchronization

8

DOMINO Outline

• Rapid OFDM Polling (ROP)– Obtain the queue status of clients for uplink

scheduling

• Relative scheduling– Avoid tight time synchronization

• Schedule converter– Create schedules for relative scheduling

10

DOMINO Outline

• Rapid OFDM Polling (ROP)– Obtain the queue status of clients for uplink

scheduling

• Relative scheduling– Avoid tight time synchronization

• Schedule converter– Create schedules for relative scheduling

11

Question: How can we collect the queue status of clients efficiently?Solution: Concurrent transmission based on Orthogonal frequency-division multiplexing (OFDM)

Central controller

AP1 AP2 APN

…

Client1 Client2

…

Client3 ClientN

ROP: Rapid OFDM Polling

12

ROP: Rapid OFDM Polling

• Clients transmit queue status using subcarriers

Client 1 Client 2Practical issues:- Freq offset- Time offset- Power mismatch(details in paper)

Related work:- PAMAC (INFOCOM’09)- B2F (MobiCom’11)

14

ROP collects the queue status of all clients with little overhead: - 40 μs (polling message) + 16 μs (OFDM symbol) - regular packet duration: 1000 μs - multiple regular transmissions/poll

15

DOMINO Outline

• Rapid OFDM Polling (ROP)– Obtain the queue status of clients

• Relative scheduling– Avoid tight time synchronization

• Schedule converter– Create schedule for relative scheduling

16

AP1 ---> C1:

AP4 ---> C4:

AP2 ---> C2:

AP3 ---> C3:

Data Packet ACK

Data Packet ACK

Data Packet

Data Packet ACK

Misalignment

Collision!

AP1

C1AP2

C2 AP3

C3 AP4

C4

Slot 1

AP1---->C1

AP4---->C4

μs level synchronization requiredOne Wi-Fi slot: 9 μs

Why time synchronization?

Interfering nodes AP-client association Currently transmitting

Slot 2

AP2---->C2

AP3---->C3

17

Current Time Synchronization Scheme

Network Time Protocol (NTP), Precision Time Protocol (PTP), Reference-Broadcast Synchronization (RBS) (SIGOPS’02), Sourcesync (SIGCOMM’10):– Low accuracy; Or– Expensive hardware; Or– Low accuracy in large network.

18

Can we avoid tight time synchronization?

19

Relative Scheduling

AP1

C1AP2

C2 AP3

C3 AP4

C4

Slot 1 Slot 2

AP1---->C1

AP4---->C4

AP2---->C2

AP3---->C3

Data Packet ACK

Data Packet ACK

Data Packet ACK

Data Packet ACK

Interfering nodes AP-client association Currently transmitting

AP1 ---> C1:

AP4 ---> C4:

AP2 ---> C2:

AP3 ---> C3:

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Relative Scheduling

AP1

C1AP2

C2 AP3

C3 AP4

C4

Slot 2 Slot 3

AP1---->C1

AP4---->C4

AP2---->C2

AP3---->C3

Data Packet ACK

Data Packet ACK

Data Packet ACK

Data Packet ACK

Transmission alignment achieved

Interfering nodes AP-client association Currently transmitting

AP1 ---> C1:

AP4 ---> C4:

AP2 ---> C2:

AP3 ---> C3:

21

Node signatures as triggers:– A sequence of bits with a certain length – These sequences are orthogonal to each other– High detecting ratio even under interference– Experiment results:• 4 combined signatures can be decoded correctly• 4 transmissions can be triggered by one node

22

Only APs know the schedules from the central controller

How can we ask the clients to send the triggers?

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Data Packet SA3 SA2A1:

C1: ACK SA3

SIFS 1 slotS′

S′The combined signatures that should be sent by the client

The combined signatures that should be sent by the AP

A special signature that notifies the start of transmission

A2 A1 A3

C2 C1 C3

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A2 A1 A3

C2 C1 C3

Data Packet

SA2A1:

C1:

SA3

SA3

SIFS 1 slotS′

S′

ACK

25

DOMINO Outline

• Rapid OFDM Polling (ROP)– Obtain the queue status of clients

• Relative scheduling– Avoid tight time synchronization

• Schedule converter– Create schedule for relative scheduling

26

Schedule Converter

Requirements:– Every transmitter should be triggered– Polling packets should also be scheduled– Backup triggers should be included in case of

transmission failure– Details in paper

Arbitrary Schedule Relative Schedule?

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Experiment

Same Contention Domain

Hidden terminal Exposed terminal0123456789

10

4.25

5.42

9.18

2.76

1.62

2.72

DOMINO DCF

Thro

ughp

ut (K

bps)

>3X

>3X

>1.5X

USRP

USRP USRP

USRP

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Trace Driven Simulation

• Simulation Setup:– RSS trace collected from a 40 Wi-Fi nodes testbed– Randomly picked 10 APs and 2 clients per AP

• Other schemes:− CENTAUR: Downlink traffic scheduled; using fixed

backoff to align transmission− DCF: 802.11 standard (Wi-Fi)

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UDP Throughput & Delay

0 1 2 3 4 5 60

0.2

0.4

0.6

0.8

1

DOMINODCFCENTAUR

Throughput (Mbps)

CDF

(%)

Downlink traffic only

0

200000

400000

600000

800000

10000000

0.2

0.4

0.6

0.8

1

Packet delivery Delay (us)

CDF

(%)1.74X 0.5X

30

UDP Throughput & Delay

Uplink and downlink traffic

0 1 2 3 4 5 60

0.2

0.4

0.6

0.8

1

DOMINO DCF CENTAUR

Throughput (Mbps)

CDF

(%)

1.24X

Heavy tailLow fairness

31

TCP Throughput

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.2

0.4

0.6

0.8

1

DOMINO

CENTAUR

DCF

Throughput (Mbps)

CDF

(%)

Downlink traffic only

1.15XTCP ACK as regular packet

32

Conclusions

Domino: a platform to enable centralized scheduling algorithms without requiring tight time-synchronization:– Queue information of clients are efficiently collected using

one OFDM symbol– Nodes transmit relatively one after another instead of

according to time stamps

Future work: coexistence with existing Wi-Fi

Thank you!

33

Backup slides

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Why is CENTAUR behaving worse than DCF?

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UDP Throughput & fairness

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 190

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

DOMINOCENTAURDCF

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.50

5

10

15

20

25

30

35

40

DOMINOCENTAURDCF

- 24%-74% throughput gain- High and stable fairness

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TCP Throughput & fairness

- 10%-15% throughput gain- TCP ACK as regular packet

- High and stable fairness0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.50

5

10

15

20

25

30

DOMINOCENTAURDCF

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

DOMINOCENTAURDCF

370 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.50

50000

100000

150000

200000

250000

300000

350000

400000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.50

1000000

2000000

3000000

4000000

5000000

6000000

7000000

8000000

9000000

10000000

DOMINO

CENTAUR

DCF

Evaluation: UDP & TCP Delay

DCF: 2X higher - Queuing delay

Similar delay performance: - Queuing delay - TCP congestion control

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UDP Throughput & Delay

Uplink and downlink traffic

0 1 2 3 4 5 60

0.2

0.4

0.6

0.8

1

DOMINODCFCENTAUR

Throughput (Mbps)

CDF

(%)

0 100000000

0.2

0.4

0.6

0.8

1

Packet delivery Delay (us)

CDF

(%)

1.24X

Heavy tail

2X

39

TCP Throughput & Delay

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.2

0.4

0.6

0.8

1

DOMINO

CENTAUR

DCF

Throughput (Mbps)

CDF

(%)

Uplink traffic only

0 500000 10000000

0.2

0.4

0.6

0.8

1

Packet delivery Delay (us)

CDF

(%)

1.15X

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Domino Solution Overview

• Identify Hidden & Exposed Links– Construct link conflict map

• Co-existence with current networksContention Free Period Contention Period

Slot 1 Slot 2 Slot 3 … Slot N

AP1 → C1

AP2 → C2

…APM → CM

Concurrent transmissions

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Evaluation: Misalignment

0 1 2 3 4 50

5

10

15

20

25 20 us40 us60 us80 us

Slot index

Max

Tx

misa

lignm

ent (

us)

Alignment achieved - Slot size: 9 μs > 2 μs

42

Current Time Synchronization Scheme

• Network Time Protocol (NTP): time accuracy of about 1ms in a quiet Ethernet network.

• Precision Time Protocol (PTP): requires specialized and expensive hardware.

• Reference-Broadcast Synchronization (RBS) (SIGOPS’02): synchronization accuracy decreases with network size.

• Sourcesync (SIGCOMM’10): one collision domain

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Throughput Gain of network with 80 nodes

~58%

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ROP: Rapid OFDM Polling

• Client TX queue state over subcarriers

• Polling strategy

0 1 3 9 100 109 127-128 -109 -100

-9 -3 -1 2-2 4-4

DC

… … … ……………

guard band

guard band

subchannel 0

subchannel 11

subchannel 12

subchannel 23

guard subcarriers

Polling Packet FromAP

Client 0 Client 1 Client 2

Client N…

Client 3

1 slotSubchannel

Time0123…N

FFT window

CPCPCPCP

CPCP

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ROP: Rapid OFDM Polling

Polling Packet From AP

Client 0

Client 1

Client 2

Client N

…

Client 3

1 slot

Subchannel

Time

0

1

2

3

…N

FFT window

CP

CP

CP

CP

CP

CP

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DOMINO: Example

9291 93 9490

Batch 10 Batch 11SlotsLinks

AP1 AP1

AP1 C1

C1 AP1

AP2 AP2

AP2 C2

C2 AP2

AP3 AP3

AP3 C3

C3 AP3

AP4 AP4

AP4 C4

C4 AP4

0 1 2 3

Batch 0

1

2

24 s

fake 3

(a) (b)

AP1

C1AP2

C2 AP3

C3 AP4

C4

47

Subcarrier Separation

10 15 20 25 30 35 400

20

40

60

80

100

0

1

2

3

4

Difference in RSS (dB)

Corr

ect d

ecod

ing

ratio

(%) Separation

subcarriers

(38dB, 99.9%, 3 sub)

Tradeoff: - Less overhead - Higher decoding ratio

TX1

RX

TX2

48

Relative Scheduling: Node signatures as trigger

• Node signature are orthogonal to each other, easier to detect.

1 2 3 4 5 6 70

200

400

600

800

1000

1200

1 sender2 senders, same sig2 senders, different sig3 senders, same sig3 senders, different sig

Number of combined signatures

Det

ectio

n ra

tion

(%)

49

Existing work: MIFI

50

Existing work: CENTAUR

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Domino Solution Overview

• Identify Hidden & Exposed Links– Construct link conflict map

• Co-existence with current networks• ROP: Rapid OFDM Polling• Relative Scheduling• Schedule Converter

52

ROP: How it performs

Decoded OFDM symbols of two clients at adjacent subchannels with guard interval. (30db diff. RSSI at AP)

53

Relative Scheduling

AP1

C1AP2

C2 AP3

C3 AP4

C4

Slot 1 Slot 4Slot 3Slot 2

AP1---->C1

AP4---->C4

AP2---->C2

AP3---->C3

AP1---->C1

AP4---->C4

AP2---->C2

AP3---->C3

AP1C1 AP2C2

AP3C3 AP4C4

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Relative Scheduling: Node signature as trigger

• Node signature are orthogonal to each other, easier to detect.– AP to client

– Client to AP

Data Packet S1 S2AP:

Client: ACK S1

SIFS 1 slotS′

S′

Data Packet

ACK S2AP:

Client:

S1

S1

SIFS 1 slotS′

S′

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Relative Scheduling: How it performs

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Schedule Converter

Inbound & outbound Constraint

– 1<= Inbound <=2– Outbound <=4

In

boun

d Sender Receiver

Out

bo

und

Arbitrary Schedule Relative Schedule?

57

Schedule Converter

• Insert Fake Link– Saturate the network with fake links at each slot

• Retain Last Slot– Last slot of current schedule is used as the first

slot of next schedule

• Insert Polling slot– Insert polling slots between slots

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Evaluation: Experiment

SC HT ET0123456789

10

4.255.42

9.18

2.761.62

2.72

Throughput of USRP Prototype

DOMINO DCF

USRP

USRP

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Evaluation: Simulation

• Implement DCF, CENTAUR, DOMINO in NS3• 40 nodes with known RSSI trace (Stanford)

• Topology : APs, each AP has clients

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Evaluation: Misalignment

Vary wired latency from 20 to 80 (using T(10,2))

61

Evaluation: UPD & TCP

TCP and UDP throughput, delay and fairness of . The downlink data rate is fixed to 10 Mbps and the uplink data rate changes from 0 to 10 Mbps.

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Discussion

• Triggering may not easy

• Is fixed packet size good?

• Building conflict graph dynamically

• Low traffic, low efficiency

AP1

C1

AP2

C2

63

Challenge : Time Synchronization

• Network Time Protocol (NTP): time accuracy of about 1ms in a quiet Ethernet network.

• Precision Time Protocol (PTP): require specialized and expensive hardware.

• Reference-Broadcast Synchronization (RBS): accuracy decreases as network size increases.

64

Centralized Scheme

• Schedule both uplink and downlink traffic

AP2 AP1 AP3

C2 C1 C3

65

DOMINO: Example

9291 93 9490

Batch 10 Batch 11SlotsLinks

AP1 AP1

AP1 C1

C1 AP1

AP2 AP2

AP2 C2

C2 AP2

AP3 AP3

AP3 C3

C3 AP3

AP4 AP4

AP4 C4

C4 AP4

0 1 2 3

Batch 0

2

24 s

fake 3

AP1

C1AP2

C2 AP3

C3 AP4

C4

Polling packetData packet

1

66

Distributed Channel Access: DCF

Pros:- Simple to implement- Robust to failures

AP2 AP1 AP3

C2 C1 C3

Exposed Hidden

Cons:- Hidden and exposed terminal problems- Low efficiency

68

Enterprise Network

Internet

Central Server

AP1 AP2 APN

…

Client1 Client2 Client3 ClientM

…

Control Plane:

- Channel Assignment- Client Association- Power control

What about data plane?

69

Exposed and hidden terminals

C2

AP2

C1

AP1

C3

AP3

Exposed

Hidden

Centralized schedule could avoid hidden terminals while utilize exposed terminals

70

Expected Improvement

• DCF : purely distributed• CENTAUR : half-distributed, half centralized• DOMINO: centralized

DCF CENTAUR TRAIT

Throughput

DOMINO

centralization

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Design Overview

Internet

Central Server

AP1 AP2 APN

…

Client1 Client2 Client3 ClientM

…

Collector

Scheduler

Converter

queue size

time schedule

relative schedule

• Obtaining clients queue status• Identifying exposed and hidden links• Time synchronization

Challenges: