cs434/534: topics in networked (networking)...
TRANSCRIPT
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CS434/534: Topics in Networked (Networking) Systems
Wireless Foundation: Wireless Mesh Networks
Yang (Richard) YangComputer Science Department
Yale University208A Watson
Email: [email protected]
http://zoo.cs.yale.edu/classes/cs434/
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Outline❒ Recap❒ Wireless background
❍ Frequency domain❍ Modulation and demodulation❍ Wireless channels❍ Wireless PHY design❍ Wireless MAC design (one hop)
• wireless access problem and taxonomy• wireless resource partitioning dimensions• media access protocols
❍ From single-hop MAC to multi-hop mesh• wireless mesh network capacity• maximize mesh capacity
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Admin.❒ PS2 questions❒ Project first check point
❍ Due Thursday after break
3
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Recap: The Hidden Terminal Problem
BA C
❒ A is sending to B, but C cannot detect the transmission
❒ Therefore C sends to B
❒ In summary, A is “hidden” from C
DE
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Recap: 802.11 RTS-CTS-data-ACK
t
SIFS
DIFS
data
ACK
defer access
otherstations
receiver
senderdata
DIFS
new contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
10 usec
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Recap: 802.11 CSMA/CA
data
waitB1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31
B2 = 10busy
busy
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Recap: PIFS
tNAV
polledwirelessstations
point coordinator
NAV
PIFSD
USIFS
SIFSD
contentionperiod
contention free periodmediumbusy
D: downstream poll, or data from point coordinatorU: data from polled wireless station
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Recap: Wireless Link Access Control
❒ Problem: single shared medium, hence if two transmissions overlap on all dimensions [time, space, frequency, and code], then it is a collision
Slotted ALOHA Ethernet
Hidden-terminalCollision detection/prevention
Zigzag decoding
+CS+CD+EB
+CA+ACK+ RTS/CTS
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Recap: ZigZag Decoding
Exploits 802.11’s behavior❒ Retransmissions àSame packets collide again
❒ Senders use random jittersà Collisions start with interference-free bits
∆1 ∆2Pa
Pb
PaPb
Interference-free Bits
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ZigZag Algorithm
∆1 ∆2
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision
}
1
∆1 ≠∆2
1
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∆21
2
1∆1
How Does ZigZag Work?
∆1 ≠∆2 while (exists a chunk that is interference-free in one
collision and has interference in the other) {
decode and subtract from the other collision
}
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∆21
22∆1
How Does ZigZag Work?
3
∆1 ≠∆2 while (exists a chunk that is interference-free in one
collision and has interference in the other) {
decode and subtract from the other collision
}
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∆21
2 4∆1
How Does ZigZag Work?
3 3
∆1 ≠∆2 while (exists a chunk that is interference-free in one
collision and has interference in the other) {
decode and subtract from the other collision
}
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∆21
2 44∆1
How Does ZigZag Work?
3 5
∆1 ≠∆2 while (exists a chunk that is interference-free in one
collision and has interference in the other) {
decode and subtract from the other collision
}
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∆21
6∆1
How Does ZigZag Work?
3 5 5
2 4
∆1 ≠∆2 while (exists a chunk that is interference-free in one
collision and has interference in the other) {
decode and subtract from the other collision
}
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∆21
66∆1
How Does ZigZag Work?
2 4
3 5 7
∆1 ≠∆2 while (exists a chunk that is interference-free in one
collision and has interference in the other) {
decode and subtract from the other collision
}
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∆21
6 8∆1
How Does ZigZag Work?
2 4
3 5 77
∆1 ≠∆2
Delivered 2 packets in 2 timeslotsAs efficient as if the packets did not collide
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision
}
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Outline
❒ Admin. and recap❒ Media access control
❍ Slotted Aloha❍ Hidden terminal❍ Example: 802.11❍ Hidden terminal with 802.11 revisited
• Overall idea• Technical issues
– Collision detection
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Collision Detection: How does the AP know it is a Collision and Where the Second Packet Starts?
Time
∆
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Detecting Collisions and the Value of ∆
Time
AP received signal
Packets start with known preamble
AP correlates known preamble with signal
Correlation
Time
Correlate
∆
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Matching Collision
❒ Question to think offline: Given (P1 + P2(D)) and (P1’, P2’(D’)), how to determine that P1 = P1’ and P2 = P2’
PaPb
P’aP’b
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Outline
❒ Admin. and recap❒ Media access control
❍ Slotted Aloha❍ Hidden terminal❍ Example: 802.11❍ Hidden terminal with 802.11 revisited
• Overall idea• Technical issues
– Collision detection– Subtracting chunks
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How Does the AP Subtract the Signal?
• Channel’s attenuation or phase may change between collisions• Can’t simply subtract a chunk across collisions
Alice’s signal in first collision
Alice’s signal in second collision
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12
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Subtracting a Chunk
❒ Decode chunk into bits❍ Removes effects of channel during first collision
❒ Re-modulate bits to get channel-free signal
❒ Apply effect of channel during second collision❍ Use correlation to estimate channel despite
interference
12
12
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What if AP Makes a Mistake?
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∆1 ∆21 1
22
Bad News: Errors can propagate
3
Can we deal with these errors?
What if AP Makes a Mistake?
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∆1 ∆2
What if AP Makes a Mistake?
Good News: Temporal DiversityA bit is unlikely to be affected by noise in both collisions
Get two independent decodings
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Errors propagate differently in the two decodings
For each bit, AP picks the decoding that has a higher PHY confidence
Which decoded value should the AP pick?
∆1 ∆2
1 1
22
3
AP Decodes Backwards as well as Forwards
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Outline
❒ Admin. and recap❒ Media access control
❍ Slotted Aloha❍ Hidden terminal❍ Example: 802.11❍ Hidden terminal with 802.11 revisited
• Overall idea• Technical issues
– Collision detection– Subtracting chunks– Deployment
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Acknowledgement
❒ Use as much synchronous acknowledgement as possible for backward compatibility
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Implementation
• USRP Hardware• GNURadio software• Carrier Freq: 2.4-2.48GHz• BPSK modulation
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USRPsTestbed
• 10% HT,
• 10% partial HT,
• 80% perfectly sense each other
802.11a
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Throughput Comparison
802.11
Throughput
CD
F of
con
curre
nt fl
ow p
airs
Hidden Terminals
Partial Hidden Terminals
Perfectly Sense
00.10.20.30.40.50.60.70.80.9
1
0 0.5 1 1.5 2
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Throughput Comparison
ZigZag
Throughput
CD
F of
con
curre
nt fl
ow p
airs
802.11
Hidden Terminals get high throughput
00.10.20.30.40.50.60.70.80.9
1
0 0.5 1 1.5 2
ZigZag Exploits Capture Effect
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Capture Effect❒ Subtract Alice and combine Bob’s packet
across collisions to correct errors
∆1 ∆2Pa1
Pb
Pa2Pb
3 packets in 2 time slots à better than no collisions
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Summary
❒ Basic lesson:❍ Traditional thinking was on avoiding collisions❍ Zigzag changed the way of thinking: decoding
collisions
❒ Many extensions of the Zigzag idea: decoding collisions, instead of avoiding collisions❍ See Remap in Backup Slides❍ A good area for creative thinking
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Infrastructure Mode Wireless
APAP
AP wired networkAP: Access Point
Problems of infrastructure mode wireless networks?
802.11 by default operates in infrastructure mode.
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Mesh Networks
ad-hoc (mesh) mode
APAP
AP wired networkAP: Access Point
Benefits❍ fast and low-cost deployment❍ no central point of failure❍ no APs overhead for users
who can reach each other
Issues?
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Outline❒ Recap❒ Wireless background
❍ Frequency domain❍ Modulation and demodulation❍ Wireless channels❍ Wireless PHY design❍ Wireless MAC design (one hop)❍ From single-hop MAC to multi-hop mesh
• understanding: wireless mesh network capacity
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Capacity of Mesh Networks
❒ The question we study: how much traffic can a mesh wireless network carry, assuming an oracle to avoid the potential overhead of distributed synchronization (MAC)?
❒ Why study capacity?❍ learn the fundamental limits of mesh wireless
networks❍ separate the spatial reuse perspective and
system design perspective❍ gain insight for designing effective wireless
protocols
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Mesh Transmission Constraints
❒ transmissionsuccessful if there areno other transmitterswithin a distance (1+D)r of the receiver
receiver
sender
r(1+D)r
Interference constraint❒ a single half-duplex
transceiver at each node: ❍ either transmits or
receives❍ transmits to only one
receiver❍ receives from only one
sender
Radio interface constraint
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Model
❒Domain is a disk of unit area❒There are n nodes in the domain❒The transmission rate is W bits/sec
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Capacity of Mesh Wireless Network
❒ Consider two types of networks
❍ arbitrary networks: place nodes optimally to derive overall upper bound
❍ random network: nodes are placed randomly
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Outline❒ Recap❒ Wireless background
❍ Frequency domain❍ Modulation and demodulation❍ Wireless channels❍ Wireless PHY design❍ Wireless MAC design (one hop)❍ From single-hop MAC to multi-hop mesh
• understanding: wireless mesh network capacity– setting– arbitrary networks: place nodes optimally to derive overall upper
bound
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Transmission Model: Bit-time Perspective❒ Chop time into a total of WT bit-times in T
seconds❒ The transmission decision is made for each bit
bit time 2 bit time t bit time WTbit time 1
1
2
3
5
4
1
2
3
5
4
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Transmission Model: End-to-end Perspective❒ Assume the network sends a total of lT end-to-end
bits in T seconds❒ Assume the b-th bit makes a total of h(b) hops from the sender
to the receiver❒ Let rb
h denote the hop-length of the h-th hop of the b-th bit
2
1
2
lT
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47
Hop-Count Constraint
Since there are a total of WT bit-times, and during each bit-time there are at most n/2 simultaneous transmissions, we have
å=
£T
b
WTnbhl
1 2)(
2
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48
m
k
(1+D)r’
r’
Area Constraint❒ Consider two simultaneous
transmissions at a bit-time
i
j
(1+D)r
r
Djm + r >= Dim >= (1+D)r’
ÞDjm >= (r+r’) D /2
Djm + r’ >= Djk >= (1+D)r
½ Dr
½ Dr’
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49
Area Constraint
❒ For each transmissionwith distance r from sender to receiver, we draw a circle with radius ½ D r
❒ These circlesdo not overlap
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50
Area Constraint: Global Picture
2
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51
Area Constraint: Global Picture
sum over all circles, since each circle has at least ¼ of its area in the unit disk,
2
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Summary: Two Constraints
❒ transmissionsuccessful if there areno other transmitters
within a distance (1+D)r of the receiver
Interference constraint❒ a single half-duplex
transceiver at each node
Radio interface constraint
å=
£T
b
nWTbhl
1 2)(
21
)(
1
2 16)(D
£åå= = p
l WTrT
b
bh
h
hb
receiver
sender
r(1+D)r
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53
Capacity Bound åå==
£÷ø
öçè
æ n
ii
n
ii xnx
1
22
1Note:
Let L be the average (direct-line) distance for all lT end-to-end bits.
åå= =
£T
b
bh
h
hbrTL
l
l1
)(
1
ååååå= === =
££
®T
b
bh
h
hb
T
b
T
b
bh
h
hb rbhrTL
lll
l1
)(
1
2
11
)(
1)()(
nWTWTWTnTLD
=D
£
®
ppl 816
2 2
nWLD
£
®
pl 8 Discussion: what does the result mean?
å=
£T
b
nWTbhl
1 2)(
21
)(
1
2 16)(D
£åå= = p
l WTrT
b
bh
h
hb
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54
Discussion
❒ L depends on❍ traffic pattern (who needs to talk to whom) and ❍ positions of the end-to-end (application-level)
senders and (application-level) receivers❒ If end-to-end senders and receivers are
spread out throughout the network, L is large❍ per node capacity
❒ Otherwise, L will be small as network becomes denser
n1
µ
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Achieving Capacity: Example
❒ n/2 senders and receivers:
❒ lL=
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Results: Arbitrary Networks
Protocol Model
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Outline❒ Recap❒ Wireless background
❍ Frequency domain❍ Modulation and demodulation❍ Wireless channels❍ Wireless PHY design❍ Wireless MAC design (one hop)❍ From single-hop MAC to multi-hop mesh
• understanding: wireless mesh network capacity– setting– arbitrary networks: place nodes optimally to derive overall upper
bound– random networks: uniform distribution of nodes and
senders/receivers
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Uniform Random Networks
❒ Uniform distribution of n nodes
❒ n origin-destination (OD) pairs
❒ Each node chooses same power level P, and thus equal radius r(n)
❒ Equal throughput l(n) bits/sec for all ODpairs
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Random Networks: Required Bits
❒ Assume: average length of each OD pair is L
❒ Average number of hops:
❒ Total required bit transmissions per second to support l(n) :
)(nrL
)()(nrLnnl
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Random Networks: Offered Bits
❒ Required bit transmissions per second:
❒ What is the maximum number of transmissions (of bits) in one second?❍ space used per transmission (interference limited):
• at least ¼ p [Dr(n)/2]2 = pD2r2(n)/16
❍ number of simultaneous transmissions at most (interference limited):
❍ total bits per second
)()(nrLnnl
2222 )(16
16/)(1
nrnr D=
D pp
22 )(16nrW
Dp
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Random Networks: Capacity
Required ≤ offered
Þ
Þ
22 )(16
)()(
nrW
nrLnn
D£p
l
)( 16)( 2 nnrWLn
D£p
l
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62
Connectivity Constraint
❒ Need routes between origin-destination pairs - places a lower bound on transmit range r(n)
Not connected Connected
A D AD
To maintain connectivity with a high probability, requires r(n) on the order: n
nlog
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Random Networks: Capacity
Required ≤ offeredÞ
Þ
Þnn
Lnlog1)( µl
22 )(16
)()(
nrW
nrLnn
D£p
l
)( 16)( 2 nnrWLn
D£p
l
nnnr log)( µ
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Measurement
❒ Measured scaling law: throughput declines worse with n than theoretically predicted: 1/n1.68
❒ Remaining story line❍ wireless mesh networks
may have low scalability, and need techniques to increase capacity
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Improving Wireless Mesh Capacity
❒ transmissionsuccessful if there areno other transmitters
within a distance (1+D)r of the receiver
Interference constraint❒ a single half-duplex
transceiver at each node
Radio interface constraint
å=
£T
b
nWTbhl
1 2)(
21
)(
1
2 16)(D
£åå= = p
l WTrT
b
bh
h
hb
nWLD
£p
l 8 rate*distance
capacity:
Reduce LIncrease
W
Approx.optimal
Multipletransceivers
Reduceinterf. area