Using Commodity Hardware Platform to Develop
and Evaluate CSMA Protocols
Amy (Mei-Hsuan) Lu
Peter Steenkiste and Tsuhan Chen
Carnegie Mellon University
1
Wireless Experiment is Hard

Simulators


Provide full control and
flexibility
 Can suffer from lack of
realism
Software Defined Radio

Fully programmable
 Considerable flexibility
 Steep learning curve
 Usually expensive
Host-based MAC
Simulators
Realism
Flexibility

SDRs
Alternative: Host-based MAC

Use commodity hardware
 Sufficient for certain experiments
2
Host-based MAC
Administrative tasks
User-define
Inter-frame
spacing
User-defined
Random
backoff
Userdefined
ACK
Userdefined
RTS/CTS
Host
Userdefined rexmissions
Driver
Inter data packet operations
RTS/CTS
exchange
Random
backoff
Inter-frame
spacing
HW
Rexmission
Control transactions
ACK
response
Card


Chips reveal flexibility to do so
Computing technologies advance
3
Outline

Design of FlexMAC

Challenges and Solutions

Precision of FlexMAC

Two Case Studies

Conclusion
4
Capability of FlexMAC
1. generate data packets and retransmissions
2. suppress ACK
channel busy
SIFS
RTS
CTS
DIFS
DIFS
DATA
ACK
4. modify backoff distribution
5. create transactions of
control messages
(flexible mode)
3. change inter-frame spacing
5
Challenges



Supporting Precise Scheduling
Handling Delay between Dependent Transmissions
Determining the Current State of Use of the Channel
6
Supporting Precise Scheduling
Host posts packets to the HW
HOST
t6
t0
Packet arrived at HW
t2
t1
t4
t3
Interrupt of TX status HW
transmission time
HW sends packets out
ACK arrived at HW
AIR
700
600
500

bus delay ≈ 20μs
histogram
bus delay
400
300
200
100
0
0
20
40
60
80
bus delay (usec)
100
120
7
Handling Delay between Dependent Transmissions
Host posts packets to the HW
HOST
t6
t0
Interrupt of TX status
Packet arrived at HW
t2
t3
t1
HW
t4
transmission time
HW sends packets out
ACK arrived at HW
AIR
histogram
1500
Interrupt latency
bus delay
≈ 30μs
1000
interrupt latency

500

0
0
20
40
60
80
100
interrupt latency (TX) (usec)
120
bus delay + interrupt latency
≈ 52μs ≈ 1x 802.11b DIFS
can afford dept. 802.11b data
packet transmissions
8
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
9
System Diagram
Network Interface (socket)
2. Dependentother
transmissions
packets
(e.g. re-TXs)
Frame
Dispatcher
1. Precise scheduling
NAV
Frame
Scheduler
Host
packet pool
self data packets
TX
Controller
frame rcvd
frame sent
ISR
Madwifi
HW
3. Current channel status
WNIC

ISR: Interrupt Service Routing

Run in the promiscuous mode
10
Disable retransmission and backoff
Precision of FlexMAC
Comparing a FlexMAC-based 802.11b implementation (SW MAC)
with the hardware-based implementation (HW MAC)
11
Throughput Comparison
sources
destinations
clear channel
TCP
UDP
2.5
2.5
SW MAC
SW MAC
HW MAC
2
Throughput (Mbps)
2
Throughput (Mbps)
HW MAC
1.5
1
1.5
1
0.5
0.5
0
0
1
2
3
1
Total:
6.30 Mbps
3
Session ID
Session ID
6.29 Mbps
2
5.91 Mbps
Total:
5.88 Mbps
12
Collision Ratios
sources
destinations
clear channel
13
Coexistence with HW MAC and FlexMAC
FlexMAC
FlexMAC
sources
destinations
HW MAC
clear channel
HW MAC
HW MAC
HW MAC
14
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
…
15
Case Study I: Opportunistic Relaying
freespace
Source
R
R
w/OR
w/oOR
Destination
S
w/OR
w/oOR
D
R
R
under fading
 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
16
rate
Case Study II: Fairness
rate
time
Temporal fairness
(equal share of transmission
time)
Ratio of TX rate =

Throughput fairness
(equal share of throughput)
=4
Temporal fairness: We can
manipulate backoff
distributions to grant the
green sender 4 consecutive
transmissions
17
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
18
Thank You!
19
Interoperable Mode
Administrative tasks
802.11b Inter data packet operations
Host
Rexmission
Random
backoff
Inter-frame
spacing
Driver
Control transactions
HW
RTS/CTS
exchange
ACK
response
Card


Chips reveal flexibility to do so
Computing technologies advance
20
Flexible Mode
Administrative tasks
802.11b/g Inter data packet operations Control transactions
RTS/CTS
exchange
Random
backoff
Inter-frame
spacing
Host
Rexmission
ACK
response
Driver
HW
Card


Chips reveal flexibility to do so
Computing technologies advance
21