Collision-Free Asynchronous Multi-Channel Access in Ad Hoc Networks IEEE Globecom 2009, Hawaii

1

Collision-Free Asynchronous Multi-Channel Access in Ad Hoc

Networks IEEE Globecom 2009, Hawaii

University of California Santa Cruz*Palo Alto Research Center^

Duy Nguyen*, J.J. Garcia-Luna-Aceves*^, and Katia Obraczka*

2

Motivation for Multi-Channel MAC

• 3 non-overlapping 20MHz channels available in 2.4 GHz 802.11b/g/n

• 12 non-overlapping 20MHz channel available in 5Ghz of 802.11a

• 9 non-overlapping 40MHz channel in 5GHz of 802.11n

• Good bandwidth utilization

3

Challenges

• Hidden terminal problems• Using only a single transceiver: can only

transmit or receive but not both• How to make sure all neighbors aware of the

channel selection• Perception of available channel status is

different among nodes: – my neighbors’ views of available channel status is

different from mine (multi-hop networks)

4

Approaches

– Dedicated Control Channel (DCA[S.Wu and et])

• Dedicated control radio or channel for all control messages

– Split Phase (MMAC[J.So and N. Vaidya])

• Fixed periods divided into (i) channel negotiation phase on default channel & (ii) data transfer phase on negotiated channels

– Common Hopping (CHMA[A. Tzamaloukas and J.J Garcia-Luna-Aceves])

• All non-busy nodes follow a common, well-known channel hopping sequence -- the control channel changes.

– Parallel Rendezvous (SSCH[P. Bahl et] and McMAC[J. So et])

• Each node publishes its own channel hopping schedule

5

Dedicated Control Channel

Ch3

Ch2

Ch1

Time

Channel

Rts(2,3)

Cts(2)

Rsv(2)

Rts(3)

Cts(3)

Rsv(3)

Data . . . Ack

Data Ack

Rendezvous & contention occur on the control channel.

Legend: Node 1 Node 2 Note 3 Node 4

Node 1+2

Slide courtesy of H. Wilson So

6

Split-Phase

Ch2

Ch1

Ch0

Time

Channel

Hello(1,2,3)

Ack(1)

Rsv(1)

Channel negotiation on a common channel

Data AckRts Cts

Control Phase Data Transfer Phase

Data AckRtsCts

Hello(2,3) ...

Legend: Node 1 Node 2 Note 3 Node 4


7

Common Hopping

Ch2

Ch1

Ch0

Time

ChannelIdle nodes hop together in “common channel”

Ch3

1 2 3 4 5 6 7 8 9 10 11

Cts, Data, Ack

Enough for one RTS

RTS (c to d)

Legend: Node a Node b Note c Node d

RTS (b to a)


8

Parallel Rendezvous

t=1 2 3 4 5 6 ...

Ch 1

Ch 2

Ch 3

Ch 4

• Sender needs to know the home channel of the receiver

? ?


9

Parallel Rendezvous

t=1 2 3 4 5 6 7 8 9

Ch1

Ch2

Original schedule


10

Parallel Rendezvous

t=1 2 3 4 5 6 7 8 9

Ch1

Ch2

1. Data arrives 4. Hopping

resumes3. Hopping stopped during data transfer

2. RTS/ CTS/ Data

Original schedule


11

CSMA vs TDMA

TDMACSMA

# of Contenders

Channel Utilization

IDEAL

12

Yet another MAC?

• Current MACs are not sufficient: – CSMA of current IEEE 802.11 MAC

performance can be seriously degraded by the hidden terminal problems.

• Many current multi-channel MACs rely on synchronization

• Goal: To design a simple, asynchronous, and collision-free MAC with very minimal modifications to 802.11

13

Asynchronous Multi-Channel MAC (AM-MAC or “I’m MAC!”)

• Asynchronous Split Phase Approach

• Allows nodes to switch to rendezvous channel immediately once arrangement is made

• New and unique handshake is introduced to eliminate hidden terminal problems and guarantee collision freedom

14

AM-MAC: Assumptions

• N available orthogonal channels are of the same bandwidth.

• A single transceiver, can either transmit or receive but not both.

• Transmission time of RTS, CTS, ATS is • Maximum end-to-end propagation delay is • Switching delay is

'',',

15

AM-MAC: Basic mechanisms

• Borrow RTS/CTS and carrier sensing mechanism from 802.11

• Introduce ATS packet (Announce to Send)

• Additional fields:– RTS: available channel list, data time– CTS: selected channel, data time– ATS: selected channel, data time

16

Conditions for collision-free

MAXO TT

'','2 AM-MAC provides correct data channel acquisition provides that and

Let be the maximum channel observation time be the maximum data transmission time

AM-MAC is collision-free if

OTMAXT

17

RTS/CTS/ATS Based Access

• Duration field in RTS, CTS, ATS frames distribute Medium Reservation information which is stored in a Net Allocation Vector Net Allocation Vector (NAV)(NAV).

• Defer on either NAV or "CCA" indicating Medium BusyMedium Busy.

RTS

CTS Ack on channel n

Data on channel n

NAV

Src

Dest

Other

Defer Access RTS/CTS/ATS exchange continues

NAV

DIFS

ATS

ATS

18

AM-MAC Summary

• A sends RTS with available channels to B, assumes A had already met the observation time requirement.

• B replies with a CTS with the selected data channel to A, starts a timer for CTS so that upon expiration sends ATS

• On receiving CTS, A prepares to send ATS• Both A and B broadcast ATS with their intention

on data channel concurrently• A begins sending data to B on selected channels

19

B

C

E

A D

F

20

B

C

E

A D

F

RTS

Node hears RTS and backs off '''2

21

B

C

E

A D

F

CTS

Node hears CTS and backs off

''

22

B

C

E

A D

F

ATS

23

B

C

E

A D

F

DATA channel 2 RTS

24

B

C

E

A D

F

DATA channel 2 CTS

25

B

C

E

A D

F

DATA channel 2 ATS

26

B

C

E

A D

F

DATA channel 2DATA channel 1

27

R

B

S

A

ATS

Mutual Region

ZX

Y

28

R

B

S

A

ATS

Mutual Region

Z

X

Y

29

B

XSA

YR

30

B

XSA

YR

RTS arrives at B in error. B must back off for

RTS

'''2

RTS

31

B

XSA

YR

CTS arrives at X in error (X is aware of it because CTS is slightly longer than RTS). X backs off

CTS RTS

''

32

B

XSA

YR

X stays on the control channel.Y later switches to the data channel and, simply, times out and returns to the control channel.

ATS CTS

ATS

33

Simulation Models

• Simulation parameters from MMAC• Wireless LAN and Multi-hop scenarios• Channel bit-rate 3mb with CBR traffic• Transmission range approximately 250m• 3 or 4 channels where stated• Packet size of 512 bytes; Drop tail queue

length 50• 400x400, 1000x1000 topology

34

Aggregate Throughput 18 Flows

0

1000

2000

3000

4000

5000

6000

7000

1 10 100 1000

Packet Arrival Rate per Flow (packets/sec)

Ag

gre

gat

e T

hro

ug

hp

ut

(Kb

ps)

802.11

mmac

am-mac

Wireless LAN 36 nodes in 400x400 Throughput

35

Average Packet Delay, 18 Flows

0

2

4

6

1 10 100 1000

Packet Arrival Rate per Flow (pkts/sec)

Ave

rag

e P

acke

t D

elay

(se

c)

802.11

mmac

am-mac

Wireless LAN 36 nodes in 400x400

Delay

36

Aggregate Throughput 32 Flows

0

1000

2000

3000

4000

5000

6000

7000

1 10 100 1000


Ag

gre

gat

e T

hro

ug

hp

ut

(Kb

ps)

802.11

mmac

am-mac

Wireless LAN 64 nodes in 400x400Throughput

37

Average Packet Delay, 32 Flows

0

2

4

6

1 10 100 1000


Ave

rag

e P

acke

t D

elay

(se

c)

802.11

mmac

am-mac

Wireless LAN 64 nodes in 400x400

Delay

38

Aggregate Throughput 42 Flows, 3 channels, Multihop

0

1000

2000

3000

4000

5000

6000

7000

1 10 100 1000


Ag

gre

gat

e T

hro

ug

hp

ut

(Kb

ps)

802.11

mmac

am-mac

Multi-hop 121 nodes in 1000x1000 Throughput

39

Average Packet Delay, 42 Flows, 3 channels, Multihop

0

1

2

3

4

5

6

1 10 100 1000


Avera

ge P

acket D

ela

y (sec)

mmac

am-mac

802.11

Multi-hop 121 nodes in 1000x1000 Delay

40

Multi-hop 121 nodes in 1000x1000 Throughput

Aggregate Throughput 42 Flows, 4 channels, Multihop

0

1000

2000

3000

4000

5000

6000

7000

1 10 100 1000


Aggre

gat

e Thro

ughput (K

bps)

802.11

mmac

am-mac

41

Average Packet Delay, 42 Flows, 4 channels, Multihop

0

1

2

3

4

5

6

1 10 100 1000


Ave

rag

e P

acke

t D

elay

(se

c)

mmac

am-mac

802.11

Multi-hop 121 nodes in 1000x1000 Delay

42

Analytical Analysis Assumptions

• A finite population of N nodes

• Arrival of RTS is Poisson distributed

• Network is fully connected with the same number of neighbors

• Successful RTS and DATA occurs as a single event

• Packet length are independent and geometrically distributed

43

Analytical Throughput

• 3Mbps per-channel capacity

• propagation delay = 1/1000 of packet length

44

Conclusion

• We presented AM-MAC a novel solution to multi-channel medium access for single-transceiver nodes.

• AM-MAC employs a simple, yet efficient approach to collision-free data transmission over multiple channels without the need of temporal synchronization among nodes

Documents

Collision-Free Asynchronous Multi-Channel Access in Ad Hoc Networks IEEE Globecom 2009, Hawaii