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

MANETs

Sung Chul Choi and Mario GerlaWONS 2012 Presentation

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Motivation

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Motivation

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Network Model

Mobile node

Fixed Interfering source

Network Model

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3

1 4

5

8

1

Network node

Interfering sourcech

• Goal: Avoid the channels used by interfering sources using a cognitive multi-channel scheme.

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Solution Preview To avoid interfering sources:

Use Cognitive radio technology to sense channel load and discover lightly loaded channels

To 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

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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 them

t

f t

f t

f

t

ft

f

t

f

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)

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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}

CCH: Protocol Operation Given Q, x generates two hopping

sequences, utx and urx.

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129310

129310

129310

129310

129310

129310

012931

101293

310129

931012

Mtx Mrx

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

129310 129310 129310 310 …

129310 012931 101293 0129 …

utx

urx

k = 5

|utx| = |urx| = k2 = 25

CCH: Channel Rendezvous Property Claim: A channel rendezvous of a pair of

nodes is guaranteed to occur within k2 slots.

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Qx = {0, 1, 2}

Qy = {2, 3, 4}

Mtx(x)

0 1 2

0 1 2

0 1 2

Mrx(y)

2 3 4

3 4 2

4 2 3

Qx = {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

… …

……

utx(x)

urx(y)

CCH: Channel Rendezvous Property This still holds when two sequences are

not in sync.

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Qx = {0, 1, 2}

Qy = {2, 3, 4}

Mtx(x)

0 1 2

0 1 2

0 1 2

Mrx(y)

2 3 4

3 4 2

4 2 3

Qx = {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 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

… …

……

utx(x)

urx(y)

123456789101112

0

CCH: Protocol Operation When x has no packet to transmit, it follows urx(x).

When x has packets to transmit, it follows utx(x) to locate the neighbor.

A channel rendezvous is guaranteed within the length of utx(x), k2.

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time

channel quality assessment

channel quality assessment

Has packets to send to y.

tx slotrx slot

slot

No more packets to send.

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.

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time

x

y

backoffRTS

CTS

DATA

ACK

CCH: Learning of utx and urx

Learning hopping sequences Every 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).

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yx

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!

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11

3 2

4

12?

?

??

Each broadcast frame is kept in a separate buffer and transmitted in the transmitting channel (specified in utx) in the beginning of the slot, for multiple slots.

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Solution: Cog-Fi Architecture Cog-Fi is a cross-layer architecture with these

modules:

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-LQSR

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CH-LQSR: Motivation Conventional on demand routing

protocols like AODV and DSR are not well-suited. Problem 1: Not all hops are equal.

S

T

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CH-LQSR: Motivation Conventional, hop-count based routing

protocols like AODV and DSR are not well-suited. Problem 1: Not all hops are equal.

S T

2

1 18Mbps

18Mbps

54Mbps

11Mbps

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CH-LQSR: Motivation Conventional, hop-count based routing

protocols like AODV and DSR are not well-suited. Problem 2: Broadcast does not occur

simultaneously.

S

T

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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.

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CH-LQSR: ETTCH 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 index• Sd: data packet size

• B: bandwidth (data rate) of the channel

vue

pf and pr, 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.

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CH-LQSR: ETTCH 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:

vue

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CH-LQSR: Protocol Operation Extension of DSR

Route Discovery involving RREQ/RREP. Once a route is discovered, source routing is

used.

S

A

D

T

B

C

E

How do I reach T?

Here I am!

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CH-LQSR: Protocol Operation Extension of DSR

Route Discovery involving RREQ/RREP

S

A

D

T

B

C

E

RREQ(T)

Path: S

RREQ(T)

Path: S-AETTCH: 0.012

RREQ(T)

Path: S-A-BETTCH: 0.032

RREQ(T)

Path: S-A-B-CETTCH: 0.076

RouteCache(E)src

dest

ETTCH

S T 0.076path

S-A-B-C

RREQ(T)

Path: S

RREQ(T)

Path: S-DETTCH: 0.011

RouteCache(E)src

dest

ETTCH

S T 0.011pathS-D

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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 environment

13 orthogonal channels in the 5-GHz band. Interfering source: (x, y, tx_power, channel, active_%).

CCH parameters Use 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.

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Cog-Fi: Evaluation Setup List of schemes compared for evaluation

Symbol Description

CCH+CH-LQSR Our Cog-Fi solution.

CCH+DSR CCH with DSR.

CCH+AODV CCH with AODV.

802.11aSingle-channel 802.11a, routed using DSR.

COGA conventional cog radio scheme, with DSR.

RH+DSR Random hopping and DSR.

time

[COG]CONTROL

for single-radio nodes

communication

control

scanning

interference

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Cog-Fi: 25-node (5x5) Grid Topology 5 saturated 1500-byte CBR streams for 5 random node

pairs.

…

…

…

… … …

40m

1

3

5

2

2

7

4 8

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Cog-Fi: 100-node (10x10) Grid 5 saturated 1500-byte CBR streams for 5 random node

pairs.

…

…

…

… … …

30m

1

3

5

2

2

7

4 8

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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.

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Thank You!