Potential al Pitf tfalls ls o of f the he Mes essag age e in M Mes essag age M e Mec echa hanism sm in M Mode odern 8 n 802.11 Ne Networks Wei Wang, Wa Wai Kay L y Leon eong, and Ben Leong School of Computing, National University of Singapore
Wi-Fi is Ubiquitous School of 2 Computing
The Problem Message in Message Mechanism (MiM) MAC protocol ACK Interference Power Control School of 3 Computing
What is MiM? MESSAGE IN MESSAGE MECHANISM School of Computing
Conventional Receiver w/o MiM Frame B Higher RSSI RSSI Frame A Time School of 5 Computing
Reception of Conventional Receiver Both frames are lost Treated as noise Frame B RSSI corrupted CRC check fails Frame A Time School of 6 Computing
Message in Message (MiM) Higher signal dominates weaker signal Successfully Received Frame B RSSI Frame A Frame A knocked out Time School of 7 Computing
MiM is helpful 1. Salvaged otherwise lost frame Successfully Received Frame B Desired Frame RSSI Discarded Interfering Frame Frame A knocked out Time School of 8 Computing
MiM is helpful 1. Salvaged otherwise lost frame 2. Desired frame is lost Successfully Received Interfering Frame RSSI Discarded Desired Frame Time School of 9 Computing
MiM is helpful, at least no harm 1. Salvaged otherwise lost frame 2. Desired frame is lost Successfully Received Interfering Frame RSSI Discarded Desired Frame Time School of 10 Computing
However… Consider Aggregate MPDUs MAC Frames A-MPDU School of 11 Computing
However… Consider A-MPDU Interfering frame RSSI A-MPDU Time School of 12 Computing
However… Consider A-MPDU Without MiM RX: 3 Fail: 3 Interfering frame RSSI A-MPDU Time School of 13 Computing
However… Consider A-MPDU Key Insight: MiM can be harmful Without MiM RX: 3 Fail: 3 With MiM RX: 1 Fail: 5 Worse: No Block ACK Interfering frame RSSI A-MPDU gets knocked out A-MPDU Time School of 14 Computing
Why Use A-MPDU? • A-MPDU reduces TX overhead • Maximum A-MPDU size - 64 KB for 11n (equivalent to 40+ frames) - 1 MB for 11ac (600+ frames) • A tiny interfering frame (e.g. ACK) can destroy the whole A-MPDU School of 15 Computing
How Bad is it? SOMETIMES GOOD, SOMETIMES BAD School of Computing
What Can We Do? HOW TO EFFECTIVELY USE MIM School of Computing
Our Contributions 1. How bad is it? A: Study the impact of MiM on A-MPDUs 2. What can we do? A: Adaptive algorithm to enable/disable MiM School of 18 Computing
Studying the Impact of MiM Experimental set-up Interferer Sender Receiver ◦ Sender & Interferer out-of-range ◦ Receiver closer to Interferer School of 19 Computing
Studying the Impact of MiM Experimental set-up Interfering Frame Receiver Interferer Sender ◦ Sender & Interferer out-of-range ◦ Receiver closer to Interferer ◦ Sender sends an A-MPDU (w/o MAC retry) ◦ Interferer broadcast an Interfering Frame School of 20 Computing
Studying the Impact of MiM Experimental set-up Interfering Frame Receiver Interferer Sender ◦ Sender & Interferer out-of-range ◦ Receiver closer to Interferer ◦ Sender sends an A-MPDU (w/o MAC retry) ◦ Interferer broadcast an Interfering Frame ◦ Measure FDR School of 21 Computing
Ensure collision Immediately Tx A-MPDU Sender Poll Receiver Interfering Frame t Interferer Time t is uniformly distributed School of 22 Computing
Duration of A-MPDU Max duration limited by ath9k driver Max. 4 ms A-MPDU Time ≈ 3.8 ms School of 23 Computing
Size of A-MPDU (# frames) Depends on data rate 26 Mbps 8 frames Max. 6.5 Mbps 4 ms A-MPDU 2 frames Time ≈ 3.8 ms MCS Index 0 1 2 3 4 5 6 7 Data Rate (Mbps) 6.5 13 19.5 26 39 52 58.5 65 Frames 2 4 6 8 12 16 18 20 School of 24 Computing
The Detrimental Impact of MiM 1. Size of A-MPDU ◦ # Frames per A-MPDU 2. Length of Interference Frame ◦ Air-time duration 3. Channel Bonding ◦ Using adjacent channels School of 26 Computing
1. Size of A-MPDU? NUMBER OF FRAMES IN AN A-MPDU School of Computing
Impact of A-MPDU size A-MPDU of 2 frames Interfering Frame 60 μ s Time ≈ 3.8 ms 0.5 School of 28 Computing
Impact of A-MPDU size Interfering Frame 60 μ s A-MPDU of 4 frames Time ≈ 3.8 ms School of 29 Computing
Impact of A-MPDU size A-MPDU of 4 frames 0.25 School of 30 Computing
Impact of A-MPDU size Interfering Frame 60 μ s A-MPDU of 20 frames Time ≈ 3.8 ms More details in the paper School of 31 Computing
Frame Delivery Ratio Interfering Frame 60 μ s Time ≈ 3.8 ms School of 32 Computing
Frame Delivery Ratio Interfering Frame 600 μ s Interfering Frame 60 μ s Time ≈ 3.8 ms 0.9 0.5 School of 33 Computing
Frame Delivery Ratio Interfering Frame 600 μ s Time ≈ 3.8 ms 0.5 School of 34 Computing
2. Length of Interference Frame THE AIR-TIME DURATION School of Computing
Air-time of Interfering Frames T Time Intuition: ≈ 3.8 ms Without MiM, longer T more frames loss With MiM, T has no effect School of 36 Computing
How to set T T Time ≈ 3.8 ms 1. Vary frame length (# of bytes) 2. Vary data rate (bytes per sec) School of 37 Computing
Increasing Frame Length School of 38 Computing
Increasing Frame Length School of 39 Computing
Increasing Frame Length School of 40 Computing
Increasing Data Rate Air-time duration is what matters School of 41 Computing
Air-time Duration… in the Wild School of 42 Computing
Air-time Duration… in the Wild 170 μ s IPv6 Neighbor Discovery Protocol Median ≈ 30 μ s 20 μ s MAC ACK School of 43 Computing
Putting it in Perspective Be careful what you choose Suffer a large penalty in the wild School of 44 Computing
3. Channel Bonding USING ADJACENT CHANNELS School of Computing
Channel bonding Sender Receiver Interferer Interferer 20 MHz 40 MHz School of 50 Computing
Channel bonding: Case 1 Sender Receiver Interferer School of 51 Computing
Channel bonding: Case 2 Sender Receiver Interferer School of 52 Computing
Channel bonding: Case 3 Sender Receiver Interferer School of 53 Computing
Channel bonding: Case 4 Sender Receiver Interferer School of 54 Computing
Channel bonding: Case 5 Sender Receiver Interferer School of 55 Computing
Channel bonding Sender Case 1 Receiver Interferer Sender Case 2 Receiver Interferer Sender ≡ Sender Receiver Case 3 Receiver Interferer Interferer Sender Case 4 Receiver Interferer Sender Case 5 Receiver Interferer School of 56 Computing
Adjacent Channel Interference Sender Receiver Interferer School of 58 Computing
Adjacent Channel Interference 10 dB Threshold Sender Receiver Interferer Lesser More Interference Interference School of 59 Computing
Adjacent Channel Interference Sender Receiver Interferer School of 60 Computing
Adjacent Channel Interference Sender Receiver Receiver Interferer School of 61 Computing
Adjacent Channel Interference Sender Receiver Interferer School of 62 Computing
Adaptive MiM DECIDING WHEN TO ENABLE/DISABLE MIM School of Computing
Some Definitions Good Knock-out Successfully Received Desired Frame RSSI Interfering Frame Discarded Frame A knocked out Time Bad Knock-out Discarded Interfering Frame RSSI Discarded Desired Frame Time School of 64 Computing
Key Idea Count Good KO and Bad KO Compare No Disable MiM* Good > Bad Yes Periodically Enable MiM * CATCH Cannot count with MiM disabled School of 65 Computing
Evaluation Campus AP Experimental Set-up Interferer Position 2 Campus AP Equal signal strength Sender Position 3 Interference Position 1 is stronger Desired signal is stronger School of 66 Computing
Results w/o Adaptive MiM Sender < Interferer Sender > Interferer Sender = Interferer MiM detrimental MiM helpful MiM neutral School of 67 Computing
Results with Adaptive MiM Adaptive MiM always useful Sender < Interferer Sender > Interferer Sender = Interferer MiM detrimental MiM helpful MiM neutral School of 68 Computing
In Conclusion MiM not always helpful, can be harmful 1. Studied harmful effect of MiM ◦ on A-MPDUs ◦ 10 dB threshold ◦ Adjacent Channels 2. Adaptive MiM Algorithm ◦ Use MiM only when good ◦ Near optimal results School of 69 Computing
Future Work 1. Update the 802.11 MAC/PHY implementation in simulators like ns-3 2. Analytically model the effect of MiM on A- MPDU 3. Develop algorithm to dynamically adjust A- MPDU size School of 70 Computing
Thank You QUESTIONS? {weiwang|waikay|benleong}@comp.nus.edu.sg School of Computing
Recommend
More recommend
