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ZipTx: Harnessing Partial Packets in 802.11 Networks
Nate KushmanKate Ching-Ju Lin, Dina Katabi
Wasteful to throw away all correct bytes
Current wireless is all-or-nothingA packet that fails the CRC test is discarded
Much prior work trys to leverage partial packets• [JB07], [WKKS07], [KKBM08], [KKRL03], HARQ, …
But…..
I only care about my 802.11 laptop
• How do we build a driver that leverages partial packets?
• What are the throughput gains in 802.11 networks?
– Given 802.11’s auto-rate tries to avoid partial packets
Problem
Contributions• Characterizing the throughput gains of partial
packets in 802.11 networks with auto-rate: Indoor WLANS:• Auto-rate is effective at eliminating partial packets• Gains are limited to 20-25%
Challenged outdoor and mobile networks:• Auto-rate is ineffective at eliminating partial packets• Allows 2-3x throughput gains
• ZipTx, an 802.11 driver that collects these gains
Indoor Testbed• 35 nodes• Bitrates 6Mb/s 54 Mb/s• RSSI 1-34
Outdoor Testbed
Measurement Environment
• 25 nodes • Bitrates 1Mb/s 11 Mb/s• RSSI 1-12
Method• Configure hardware to pass up packets failing the CRC check• At each location, cycle between all bitrates• For each bitrate, compute correct-packet-throughput and
correct-byte-throughput
Why Does Rate Adaptation Matter?Say you can’t adapt the bit rate
Why Does Rate Adaptation Matter?Say you can’t adapt the bit rate
Correct Pkts at 48Mb/s
Why Does Rate Adaptation Matter?Say you can’t adapt the bit rate
Correct Pkts at 48Mb/sCorrect Bytes at 48Mb/s
No Rate Adaptation Dramatic Gains
35x
Why Does Rate Adaptation Matter?
Correct Pkts at 48Mb/sCorrect Bytes at 48Mb/s
Say you can adapt the bit rate
Gain is about 25%; Much smaller than no autorate
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Correct Pkts at 48Mb/sCorrect Bytes at 48Mb/sCorrect Pkts at 36Mb/s
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Envelope of Correct Pkts
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Envelope of Correct Pkts
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Envelope of Correct Pkts
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Envelope of Correct Pkts Envelope of Correct Bytes
Why Does Rate Adaptation Matter?Say you can adapt the bit rate
Rate Adaptation Limited Gain (about 25%)
Envelope of Correct Pkts Envelope of Correct Bytes
Where Do the Gains Come From?
Where Do the Gains Come From?
Where Do the Gains Come From?
Where Do the Gains Come From?
Where Do the Gains Come From?
Where Do the Gains Come From?
Where Do the Gains Come From?
Partial Packets allow an increase of one bit-rate
Where Do the Gains Come From?
Is a gain of 25% is all that partial packets can do for 802.11?
25% is the gain in Typical Indoor WLANs
But things look different for the more challenged outdoor environment
Gains in the Outdoor Environment
2x gain
Much larger gains outdoors
3.5x gain
Where Do the Outdoor Gains Come From?
Most Gains Are Not From Jumping Up Bit-Rates
Where Do the Outdoor Gains Come From?
In outdoors, auto-rate cannot avoid partial packets
What’s the fundamental difference?
ZipTx
For 802.11 gains vary from 25% up to 3x
How do we get them today?
Kernel
Driver
To Hardware
ZipTx Modifies the Madwifi Driver
Correct Partial PacketsAuto-Rate Maximizes
Correct bytes
From Application
Need to recover incorrect bits
A software-solution has to choose between: • Block-by-block CRC’s• Coding
How Does ZipTx Correct Partial Packets?
But, we don’t know which bits are incorrect!
Distributed Errors Coding is Better
How Much Coding?Need to look at percentages of byte-errors in packets90%
35% correct
60% partial pkts
~5% Erasures
• Correcting x errors requires 2x parity• We need 20% parity
Want No Overhead For Fully Correct Packets
How Does ZipTx Send Packets?No Ack
Still Requires 20% Overhead for 65% of Packets
How Does ZipTx Send Packets?
85% have less than 3% byte-
errors
Send a small amount of parity first, and only if still undecodable send the restOverhead = 0*.35 + .06*.5 + .2*.15 = 5%
Coded packets are piggybacked on next packet
Modified Auto-Rate• Leverage existing SampleRate algorithm• But maximize throughput after correcting
partial packets
Results
• Same Indoor/Outdoor testbed• Repeatedly choose a source and destination
and perform a 1 minute transfer• Compared Drivers:– Unmodified Madwifi– ZipTx
• Metric of comparison:
Throughput gain =
ThroughputDriver Unmodified
ThroughputZiptx
Experimental Setup
Typical Indoor WLANs
In indoor WLANs, ZipTx gain is 10-20%
Typical Indoor WLANs
Outdoor Environment
Outdoors, ZipTx average gain is about 2x
Outdoor Environment
Mobility Experiment
Repeatedly, walk down the hall and back
Ziptx provides auto-rate algorithm a margin of error
Mobility Experiment
Related Work
• Packet Recovery with Soft Information– [JB07], [WSKK07], and [KKBM08]
• Packet Recover with CRCs– [GJLA06], [DEV05], and [MB05]
• Packet Recover with Coding– HARQ and [KKRL03]
Contributions• Auto-rate matters:
Typical Indoor: 20-25% Challenged Outdoor: 2x-3x Challenged Mobile: 2x-3x
• Today’s 802.11 can collect these gains using ZipTx
Backup Slides
CPU Usage
CPU usage is low practical
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