using commodity hardware platform to develop and evaluate csma protocols
DESCRIPTION
Using Commodity Hardware Platform to Develop and Evaluate CSMA Protocols. Amy (Mei-Hsuan) Lu Peter Steenkiste and Tsuhan Chen Carnegie Mellon University. Wireless Experiment is Hard. Simulators Provide full control and flexibility Can suffer from lack of realism. Software Defined Radio - PowerPoint PPT PresentationTRANSCRIPT
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Using Commodity Hardware Platform to Developand Evaluate CSMA Protocols
Amy (Mei-Hsuan) Lu
Peter Steenkiste and Tsuhan Chen
Carnegie Mellon University
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Software Defined Radio Fully programmable Considerable flexibility Steep learning curve Usually expensive
Simulators Provide full control and
flexibility Can suffer from lack of
realism
Flexibility Realism
Simulators SDRsHost-based MAC
Alternative: Host-based MAC Use commodity hardware Sufficient for certain experiments
Wireless Experiment is Hard
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Host-based MACH
ost
HW
Driver
Card
Administrative tasks
Chips reveal flexibility to do so Computing technologies advance
Randombackoff
Re-xmission
Inter-framespacing
Inter data packet operations
ACKresponse
RTS/CTSexchange
Control transactions
User-defined Randombackoff
User-defined re-xmis
sions User-defined ACK
User- defined
RTS/CTSUser-define Inter-frame
spacing
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Outline
Design of FlexMAC
Challenges and Solutions
Precision of FlexMAC
Two Case Studies
Conclusion
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Capability of FlexMAC
RTS
CTS
DATA
ACK
1. generate data packets and retransmissions
DIFS
channel busy
DIFS
5. create transactions of control messages(flexible mode)
3. change inter-frame spacing
4. modify backoff distribution
SIFS
2. suppress ACK
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Challenges
Supporting Precise Scheduling Handling Delay between Dependent Transmissions Determining the Current State of Use of the Channel
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transmission time
Supporting Precise Scheduling
AIR
HW
HOSTHost posts packets to the HW
Packet arrived at HW
HW sends packets out
Interrupt of TX status
t0
t1
t2
t6
ACK arrived at HW
t4t3
bus delay
0 20 40 60 80 100 1200
100
200
300
400
500
600
700
bus delay (usec)
bus delay ≈ 20μs hist
ogra
m
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transmission time
Handling Delay between Dependent Transmissions
AIR
HW
HOSTHost posts packets to the HW
Packet arrived at HW
HW sends packets out
Interrupt of TX statust0
t1
t2
t6
ACK arrived at HW
t4t3
interrupt latency
bus delay + interrupt latency
≈ 52μs ≈ 1x 802.11b DIFS can afford dept. 802.11b data p
acket transmissions
bus delay
0 20 40 60 80 100 1200
500
1000
1500
interrupt latency (TX) (usec)
hist
ogra
m
Interrupt latency
≈ 30μs
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Determining the Current State of Use of the Channel
Most CSMA protocols require backoff freezing when the channel is in use
Virtual carrier sense NAV is obtainable from headers of overhearing packets
Physical carrier sense Leverage on-chip registers to obtain the current status of
the channel
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Network Interface (socket)
Frame Dispatcher
TXController
Frame Scheduler
ISR
WNIC
Ho
st
pa
cket
po
ol
HW
frame sent
self data packets
NAV
other packets
Madwifi
System Diagram
frame rcvd
Run in the promiscuous mode Disable retransmission and backoff
2. Dependent transmissions (e.g. re-TXs)
1. Precise scheduling
ISR: Interrupt Service Routing
3. Current channel status
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Precision of FlexMAC
Comparing a FlexMAC-based 802.11b implementation (SW MAC) with the hardware-based implementation (HW MAC)
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UDP
0
0.5
1
1.5
2
2.5
1 2 3
Session ID
Th
rou
gh
pu
t (M
bp
s)
SW MAC HW MAC
TCP
0
0.5
1
1.5
2
2.5
1 2 3
Session ID
Th
rou
gh
pu
t (M
bp
s)
SW MAC HW MAC
Total:6.29 Mbps
6.30 MbpsTotal:
5.91 Mbps
5.88 Mbps
clear channel
Throughput Comparison
sources destinations
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Collision Ratios
clear channel
sources destinations
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Coexistence with HW MAC and FlexMAC
clear channel
sources destinationsFlexMAC FlexMAC
HW MAC HW MAC
HW MACHW MAC
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Possible Use of FlexMAC
Link-layer multi-hop relaying Fairness provision Performance improvement Differentiated services Rate adaption Network security (e.g. address spoofing) Selfish nodes detection and isolation Unequal error protection for streaming video Adaptive FEC codes to deal with channel dynamics …
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Idle nodes (relays) coexist in the network
Relay(s) overhear xmissions from S to D
Relay(s) retransmit on behalf of S upon failed xmissions The closest relay that overheard the packet has a higher chance to transmit
R
R
R R
Case Study I: Opportunistic Relaying
Destination
DS
Source
freespace
w/oOR
freespace
w/ORw/oOR
under fading
freespace
w/ORw/oOR
w/ORw/oOR
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Case Study II: Fairness
Temporal fairness: We can manipulate backoff distributions to grant the green sender 4 consecutive transmissions
Throughput fairness (equal share of
throughput)time
rate
Temporal fairness (equal share of transmission
time)
rate
= 4Ratio of TX rate =
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Conclusion
FlexMAC provides a flexible platform for research in CSMA wireless protocols
FlexMAC performs closely to HW implementations
Two case studies are given to demonstrate the usefulness of FlexMAC
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Thank You!
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Interoperable ModeH
ost
HW
Driver
Card
Administrative tasks
Chips reveal flexibility to do so Computing technologies advance
Randombackoff
Re-xmission
Inter-framespacing
802.11b Inter data packet operations
ACKresponse
RTS/CTSexchange
Control transactions
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Flexible ModeH
ost
HW
Driver
Card
Administrative tasks
Chips reveal flexibility to do so Computing technologies advance
Randombackoff
Re-xmission
Inter-framespacing
802.11b/g Inter data packet operations
ACKresponse
RTS/CTSexchange
Control transactions