integrated qos for wireless packet networks
TRANSCRIPT
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Integrated QOS for WirelessPacket Networks
Javier Gomez and Andrew T. CampbellDepartment Of Electrical Engineering
Columbia University
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Problem
Channel impairments time scale
packet
flow/session time
modulation FEC Interleaving?
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Is this necessary?
Slow/fast fading
out of range
low power
interference
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Problems
• Flow/session adaptation mechanisms
Channel impairmentstime scale
packet flow/session
Time scale
modulation FEC Interleaving prediction compensation Adaptation
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Channel Prediction
• Advantage: Efficient use of bandwidth.
• Problem: Is it possible to predict the state of the channel?
– Previous work
• The state of the channel is known• The channel remains in bad state for 200 milliseconds
• The predictor is 100 percent accurate
• factors in channel prediction:• behavior of the channel• prediction strategy• packet size
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Predictor
CTS
RTS
ACK
DATA
Base Station Mobile host
Transmission
Prediction
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Complex interactions...
rts cts data ack
Examples :
• packet
Bad channel
Good channel
time
•Wireless channel is modeledas a two state Markov process•Is this a valid model ?• Can the results using thismodel be generalized?
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Prediction accuracy
• Channel• good 10:1 bad
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Without prediction
• Flow test: 500 kbps
• Channel :
– good: 20,000 bytes
– bad: 8,000 bytes
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With prediction
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Compensator
• Problem: Prediction does not compensate mobiles that deferredtransmission
• Solution: The scheduler must compensate mobiles deferringtransmission
• Previous work
• Assumed perfect channel prediction
• complex solutions, WRR• Try to minimize the latency bounds
• Our solution
• Simple, DRR• minimize latency bound is difficult
– The channel is beyond control
– The MAC (IEEE 802.11) makes latency worst
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WRR
• Quantum size (QS): Weight of each session• Deficit counter (DC): Credit record of each session
200250 100
200 100
350100
100
100 100
100
300 200
DC QS
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Compensator
• Quantum size (QS): Weight of each session
• Deficit counter (DC): Credit record of each session• Compensation Counter (CC): maintains the compensation credit
200250 100
150 100
250100
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100 100
150
300 100
DC QS
0
400
200
CC
100
200
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Compensator
• Problem 1: How much credit should a flow receive after transmissionwas deferred?
• Solution: Compensate according to the load– low credit when little load– a quantum size for heavy load
200250 100 400 150
DC QS
400
CC
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Compensator
• Problem 2: How much compensation in one round?
• Solution:
• Use available bandwidth if possible
• Bound maximum compensation if system is heavy loaded
200250 100
150 100
250100
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100 100
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300 100
DC QS
0
400
200
CC
100
200
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Compensator
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Compensator
• Fairness:
• The compensator achieves fairness unless:• Channel prediction fails• Good state periods are not long
enough• Buffer overflows and packets are
dropped
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Bounds...
• Channel: Good 10:1 Bad
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Adaptation
• Do selecting dropping.
• Applications can transmit with different levels of hierarchy– Examples
• MPEG-1 (I frame, P frame, B frame)• MPEG-2 (Base layer, Enhancement layers)• MPEG-4 (multiple objects)
• When the buffer is about to overflow, drop low priority packetsfirst
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Adaptation
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Drop mark...
• Tradeoff:
• If the average buffer occupancy is large, then set thedrop mark near the head of the buffer
• if the average buffer occupancy is small, then set thedrop mark near the head of the buffer
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Conclusions/Future Work
• Channel prediction, compensation and adaptation need towork in unison.
• We have shown the limitations of:– Channel prediction– Compensation– flow adaptation
• Implement this framework in a testbed– New 802.11 radios/freeBSD