Cog-Fi: A Cognitive Wi-Fi Channel Hopping Architecture for - PowerPoint Presentation

Slide1

Cog-Fi: A Cognitive Wi-Fi Channel Hopping Architecture for Urban MANETs

Sung

Chul

Choi and Mario Gerla

WONS 2012 PresentationSlide2

Motivation

2Slide3

Motivation

3Slide4

Network Model

4

Mobile node

Fixed Interfering sourceSlide5

Network Model

5

3

1

4

5

8

1

Network node

Interfering source

ch

Goal

:

Avoid

the channels used by interfering sources using a cognitive

multi-channel

scheme.Slide6

Problem Statement

Develop a multi-channel solution for P2P networks that operate in unlicensed bands, where external interference exists.

Requirements

Interfering sources should be avoided.

Yet, this must not impact P2P connectivity.

i.e., logical partition must be avoided.6

1

1

2

1

3

?

2

?

3

!

channel rendezvousSlide7

Problem Statement

Develop a multi-channel

scheme

for P2P networks that operate in unlicensed bands, where external interference exists.

RequirementsInterfering sources should be avoided.

Yet, this should not impact P2P connectivity.i.e., logical partition must be avoided.

7Slide8

Solution Preview

To avoid interfering sources:

Use Cognitive radio

technology to sense channel load and discover

lightly loaded

channelsTo maintain P2P network connectivity in spite of unpredictable interferers:Exploit multi channel diversity: a node can receive on multiple channels via Cognitive Channel Hopping Guarantee neighbor discovery and rendezvous in a finite # of steps(using the QUORUM set)

Design routing algorithm that accounts for “multichannel links” and Channel Hopping

8Slide9

Cognitive Channel Hopping

Cognitive Channel Hopping (CCH)

Single-radio

,

channel-hopping

solution in which each node picks its channels based on the load sensed on them9

t

f

t

f

t

f

t

f

t

f

t

fSlide10

CCH: Design Choice #1

Multi-radio nodes?

Pros

Can have each radio to tune to a different channel.

Enables simultaneous transmission and reception.

Can observe multiple channels simultaneously.ConsCostly.Device form factor.Power consumption.Requires a significant amount of low-layer changes.Assume single radio.

10Slide11

CCH: Design Choice #2

Channel coordination

Control channel-based explicit coordination?

A single point of failure.

Control channel saturation problem.

Control overhead.For single-radio nodes, time-synchronization is required.Use channel hopping.with distributed channel rendezvous.

11

1

3

?

2

?

3

!

channel rendezvousSlide12

CCH vs. Bluetooth Scatternet

12

Doesn't

Scatternet

already do this?

In CCH, channel hopping occurs at a much more slower rate -- tight time-synchronization is not required.In

Scatternet, each slave node syncs to its master's hopping sequence, and can only talk to its master(s). A direct link is

not established between two slaves. In CCH, pairwise rendezvous is achieved.

M

M

M

S

S

S

S

S

S

SSlide13

CCH: Protocol Operation

A node

x

periodically triggers

Channel Quality Assessment

(CQA).A channel availability vector a =

{

a1, …, a

|C|} is produced.

In this work, ai = 1 - [channel load in

i].Based on channel availabilities, x picks a channel set

Q = {q1

, …, qk}

from a predefined Quorum list (any two Q-sets have at least one common element)13

Example list of channel sets,

each with size

k

= 5

.

It picks the channel set with the highest

combined channel quality

, defined as:

C

= {0, 1, 2, … , 11, 12}Slide14

CCH: Protocol Operation

Given

Q

,

x

generates two hopping sequences, utx and

u

rx.14

12

9

3

1

0

12

9

3

1

0

12

9

3

1

0

12

9

3

1

0

12

9

3

1

0

12

9

3

1

0

0

12

9

3

1

1

0

12

9

3

3

1

0

12

9

9

3

1

0

12

M

tx

M

rx

Q

= {0, 1, 3, 9, 12}

12

9

3

1

0

12

9

3

1

0

12

9

3

1

0

3

1

0

…

12

9

3

1

0

0

12

9

3

1

1

0

12

9

3

0

12

9

…

u

tx

u

rx

k

= 5

|

u

tx

| = |

u

rx

| =

k

2

= 25Slide15

CCH: Channel Rendezvous Property

Claim

: A channel rendezvous of a pair of nodes is guaranteed to occur within

k

2

slots.15

Q

x = {0, 1, 2

}Q

y = {2, 3, 4}

Mtx(

x)

0

1

2

0

1

2

0

1

2

M

rx

(y

)

2

3

4

3

4

2

4

2

3

Q

x = {0, 1, 2

}

Qy = {2

, 3, 4}

0

1

2

0

1

2

0

1

2

2

3

4

3

4

2

4

2

3

2 appears in the same column, every row.

2 appears exactly once in each column.

By the property of a quorum system, there exists at least one common channel.

0

1

2

0

1

2

0

1

2

2

3

4

3

4

2

4

2

3

…

…

…

…

u

tx

(

x

)

u

rx

(

y

)Slide16

CCH: Channel Rendezvous Property

This still holds when two sequences are not in sync.

16

Q

x

= {0, 1,

2

}

Qy = {2, 3, 4}

Mtx

(x)

0

1

2

0

1

2

0

1

2

M

rx

(

y)

2

3

4

3

4

2

4

2

3

Q

x

= {0, 1, 2

}Q

y = {2

, 3, 4}

0

1

2

0

1

2

0

1

2

2

3

2

3

4

3

4

2

4

2 appears in the same column, every row.

2 appears exactly once in each column.

By the property of a quorum system, there exists at least one common channel.

0

1

2

0

1

2

0

1

2

2

3

4

3

4

2

4

2

3

…

…

…

…

u

tx

(

x

)

u

r

x

(

y

)Slide17

CCH: Channel Rendezvous Property

When the slot boundaries are not aligned (i.e.,

in a complete asynchrony)...

17

Q

x

= {0, 1, 2}

Qy = {2, 3, 4}

M

tx(x)

0

1

2

0

1

2

0

1

2

M

rx

(

y)

2

3

4

3

4

2

4

2

3

Q

x

= {0, 1,

2}

Qy = {

2

, 3, 4}

0

1

2

0

1

2

0

1

2

2

3

2

3

4

3

4

2

4

2 appears in every column.

2 appears exactly once in each column.

By the property of a quorum system, there exists at least one common channel.

0

1

2

0

1

2

0

1

2

2

3

4

3

4

2

4

2

3

…

…

…

…

u

tx

(

x

)

u

r

x

(

y

)

α

fraction of a slot

(1-

α

)

fraction of a slotSlide18

1

2

3

4

5

6

7

8

9

10

11

12

0

CCH: Protocol Operation

When

x

has no packet to transmit, it follows

u

rx

(

x

)

.

When

x

has packets

to transmit,

it follows

u

tx

(

x

)

to locate the neighbor.

A channel rendezvous is guaranteed within the length of

u

tx

(

x

),

k

2

.

18

time

channel quality assessment

channel quality assessment

Has packets to send to

y

.

tx

slot

r

x

slot

slot

No more packets to send.Slide19

CCH: Protocol Operation

Within a slot, a conventional RTS/CTS-based packet exchange is made.

By default, a slot is 10ms, enough to fit in tens of MAC frames.

Retransmissions occur within a slot

and

over multiple slots.19

time

x

y

backoff

RTS

CTS

DATA

ACKSlide20

CCH: Learning of u

tx

and

u

rx

Learning hopping sequencesEvery CCH frame includes information about the hopping sequences that the transmitter is using.

If node x has received a frame from y, it can later use its cache to predict which channel y will be without scanning channels with

utx(x

).20

y

xSlide21

CCH: MAC-level Broadcast

Broadcast function is critical in making upper-layer mechanisms to work (e.g., routing).

Not all neighbors are in the same channel as you!

21

1

1

3

2

4

1

2

?

?

?

?

Each broadcast frame is kept in a separate buffer and transmitted in the transmitting channel (specified in

u

tx

) in the beginning of the slot, for multiple slots.Slide22

CCH: Multi-channel Hidden Terminals

Channel rendezvous property breaks if:

Node

x

wants to send a frame to

y, so switches to x’s transmitting channel, while

y is also trying to transmit to another node

z, thus not in its receiving channel.22

y’s

rx: 1

z’s rx

: 2

tx

tx

x

y

y

’

s rx channel.

tx

rx

x

has a packet for

y.

where is

y?

y is involved in a transmission.

tx

y

z

2

1

x

?Slide23

CCH: Recovery

CCH mitigates

this effect by forcing each

node to "yield"

for a small amount of time after each frame transmission.

During a yield session, the node waits in its receiving channel.The yield length is dynamically adjusted based on the level of congestion the node sees.This simple

trick turns out to be very effective.

23

y’s rx: 1

z

’s rx: 2

y

z

2

1

x

?

tx

rx

tx

rx

x

y

y

’

s

rx

channel.

tx

rx

yield

yield

yield

y

’

s

rx

rx

x

has a packet for

y

.

x

finds

y

, and starts transmitting to

y

.

y

is involved in a transmission.Slide24

CCH: Recovery

A link is set up between two nodes x and y, and saturated CBR streams are configured between them.

24

x

y

y

x

Uni

-directional

B

i-directionalSlide25

Solution: Cog-Fi Architecture

Cog-Fi is a cross-layer architecture with these modules:

25

CCH

802.11 PHY

CH-LQSR

IP

PHY

MAC

Routing

Coordinate channel access.

Store and maintain channel status.

Make a routing decision.

Regular 802.11 PHY.

channel load, link rate, BER

SNR/BER

CH-LQSRSlide26

CH-LQSR: Motivation

Conventional

on demand routing protocols like AODV and DSR are not well-suited.

Problem 1: Not all hops are equal.

26

S

TSlide27

CH-LQSR: Motivation

Conventional, hop-count based routing protocols like AODV and DSR are not well-suited.

Problem 1: Not all hops are equal.

27

S

T

2

1

18Mbps

18Mbps

54Mbps

11MbpsSlide28

CH-LQSR: Motivation

Conventional, hop-count based routing protocols like AODV and DSR are not well-suited.

Problem 2: Broadcast does not occur simultaneously.

28

S

TSlide29

CH-LQSR: Motivation

Conventional, hop-count based routing protocols like AODV and DSR are not well-suited.

Problem 1: Not all hops are equal

.

Use the channel load and link rates to quantifying the quality of each hop, and factor this in when computing routes.

Problem 2: Broadcast does not occur simultaneously.Modify Route Discovery procedure.29Slide30

CH-LQSR: ETT

CH

Metric

Extend ETX and ETT [4, 5].

p

: prob. that the packet transmission is

not

successful:p

= 1 – (1 – pf) · (1 –

pr)

s(m)

: prob. that the packet is delivered at m-th attempt.

s(m) =

pm – 1 · (1 –

p)The expected transmission count (ETX) of link

e = (u, v) is:

Factoring in the link bandwidth and packet size, one can define the expected transmission time (ETT) of e as:

c: channel indexS

d: data packet sizeB: bandwidth (data rate) of the channel

30

v

u

e

p

f

and

p

r

, the forward and backward packet error probabilities, are computed based on the link BER reported from the PHY module.

B

, the bandwidth, is computed by taking the channel load and link rates into account.Slide31

CH-LQSR: ETT

CH

Metric

(cont'd. from the previous slide)

The multi-channel ETT of

e

,

ETT(

e), is: which is the avg. of

ETTc(e

) values over the channels in the channel set the receiver v is using.Finally,

Channel Hopping ETT of a path P is: 31

v

u

eSlide32

CH-LQSR: Protocol Operation

Extension of DSR

Route Discovery involving RREQ/RREP.

Once a route is discovered, source routing is used.

32

S

A

D

T

B

C

E

How do I reach T?

Here I am!Slide33

CH-LQSR: Protocol Operation

Extension of DSR

Route Discovery involving RREQ/RREP

33

S

A

D

T

B

C

E

RREQ(T)

Path: S

RREQ(T)

Path: S-A

ETT

CH

: 0.012

RREQ(T)

Path: S-A-B

ETT

CH

: 0.032

RREQ(T)

Path: S-A-B-C

ETT

CH

: 0.076

RouteCache

(E)

src

dest

ETT

CH

S

T

0.076

path

S-A-B-C

RREQ(T)

Path: S

RREQ(T)

Path: S-D

ETT

CH

: 0.011

RouteCache

(E)

src

dest

ETT

CH

S

T

0.011

path

S-DSlide34

Cog-Fi: Evaluation Setup

QualNet

4.5

CCH: implemented as a full-fledged MAC protocol.

CH-LQSR: implemented as a full-fledged routing protocol.

Channel environment13 orthogonal channels in the 5-GHz band.Interfering source: (

x,

y, tx_power,

channel, active_%).CCH parametersUse RBAR for rate adaptation [8], using 802.11a rates.Channel set

size k = 5.Channel switching delay: 80µs.Slot size: 10ms.CQA Period

: 3 seconds.34Slide35

Cog-Fi: Evaluation Setup

List of schemes compared for evaluation

35

Symbol

Description

CCH+CH-LQSR

Our Cog-Fi solution.

CCH+DSR

CCH with DSR.

CCH+AODV

CCH with AODV.

802.11a

Single-channel 802.11a, routed using DSR.

COG

A conventional cog radio scheme, with DSR.

RH+DSR

Random hopping and DSR.

time

[COG]

CONTROL

f

or single-radio nodes

communication

control

scanning

interferenceSlide36

Cog-Fi: 25-node (5x5) Grid Topology

5 saturated

1500-byte CBR

streams for 5 random node pairs.

36

…

…

…

…

…

…

40m

1

3

5

2

2

7

4

8Slide37

Cog-Fi: 100-node (10x10) Grid

5 saturated 1500-byte CBR streams for 5 random node pairs

.

37

…

…

…

…

…

…

3

0m

1

3

5

2

2

7

4

8Slide38

Cog-Fi: Route Request count

5 saturated 1500-byte CBR streams for 5 random node pairs.

38

…

…

…

…

…

…

40m

1

3

5

2

2

7

4

8

25-node gridSlide39

Summary

Goal: Devise a multi-channel multi-hop mechanism with the following requirements.

Interfering sources should be avoided

A CCH node employs a

cognitive radio-like channel sensing

to identify lightly loaded channels.The network connectivity must be maintained.Exploit multi channel diversity: a node can receiver on multiple channels via Cognitive Channel Hopping Guarantee neighbor discovery and rendezvous in a finite # of steps(using the QUORUM set)

Performance

is further improved by CH-LQSR, ie by using a link metric that factors in channel load and link rates.39Slide40

Thank You!

40