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Cog-Fi: A Cognitive Wi-Fi Channel Hopping Architecture for Urban MANETs. Sung Chul Choi and Mario Gerla WONS 2012 Presentation. Motivation. Motivation. Network Model. Mobile node. Fixed Interfering source. Network Model. Network node. 8. ch. Interfering source. 3. 5. 1. 4. - PowerPoint PPT Presentation
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Cog-Fi: A Cognitive Wi-Fi Channel Hopping Architecture for Urban
MANETsSung Chul Choi and Mario Gerla
WONS 2012 Presentation
2
Motivation
3
Motivation
4
Network ModelMobile node
Fixed Interfering source
Network Model
5
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 20 1 20 1 2
Mrx(y)
2 3 43 4 24 2 3
Qx = {0, 1, 2}Qy = {2, 3, 4}
0 1 20 1 20 1 2
2 3 43 4 24 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 20 1 20 1 2
Mrx(y)
2 3 43 4 24 2 3
Qx = {0, 1, 2}Qy = {2, 3, 4}
0 1 20 1 20 1 2
2 3 23 4 34 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
xy
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 T2
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
vu e
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:
vu e
32
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!
33
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-A
ETTCH: 0.012 RREQ(T) Path: S-A-B
ETTCH: 0.032
RREQ(T) Path: S-A-B-CETTCH: 0.076
RouteCache(E)src des
t ETTCHS T 0.076
pathS-A-B-C
RREQ(T) Path: S
RREQ(T) Path: S-D
ETTCH: 0.011RouteCache(E)
src dest ETTCH
S T 0.011pathS-D
34
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.
35
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.
802.11a Single-channel 802.11a, routed using DSR.
COG A conventional cog radio scheme, with DSR.
RH+DSR Random hopping and DSR.
time
[COG]CONTROL
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!