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DOMINO: Relative Scheduling in Enterprise Wireless LANs

DOMINO: Relative Scheduling in Enterprise Wireless LANs. Wenjie Zhou ( Co-Primary Author ), Dong Li ( Co-Primary Author ), Kannan Srinivasan , Prasun Sinha. Enterprise Networks. Internet. Important research topics: Channel assignment AP association Power adaptation Channel access.

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DOMINO: Relative Scheduling in Enterprise Wireless LANs

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  1. DOMINO: Relative Scheduling in Enterprise Wireless LANs WenjieZhou (Co-Primary Author), Dong Li (Co-Primary Author), KannanSrinivasan, PrasunSinha

  2. Enterprise Networks Internet • Important research topics: • Channel assignment • AP association • Power adaptation • Channel access Router … AP1 AP2 APN … Client3 ClientN Client1 Client2

  3. Channel Access Schemes in enterprise networks: • - Distributed Coordination Function (DCF) • - Downlink-only Centralized Schemes • - Fully Centralized Schemes

  4. Distributed Channel Access: DCF (WiFi) C2 C1 C3 Exposed Hidden Interfering nodes Flow direction AP2 AP1 AP3 • Pros: • - Simple to implement • - Robust to failures • Cons: • - Hidden and exposed terminal problems • - Low efficiency

  5. Downlink-only Centralized Schemes • CENTAUR (Mobicom’09): Downlink packets that could be sent simultaneously are forwarded to the APs at the same time. • OmniVoice (MobiHoc’11): Downlink packets are sent according to broadcast schedules. • Pros: • - No modification on clients • Cons: • - Downlink traffic only

  6. Fully Centralized Scheme • Schedule both uplink and downlink traffic C2 C1 C3 Interfering nodes Flow direction AP2 AP1 AP3 AP1 -> C1 C2 -> AP2 AP3 -> C3 Overall ~61% • Higher centralization higher throughput • Many theoretical work proposed • Challenges for fully centralized scheme: • Queue status of Clients • Time synchronization

  7. Domino • a practical platform to enable arbitrary centralized scheduling algorithms • without requiring tight time-synchronization

  8. DOMINO Outline • Rapid OFDM Polling (ROP) • Obtain the queue status of clients for uplink scheduling • Relative scheduling • Avoid tight time synchronization • Schedule converter • Create schedules for relative scheduling

  9. DOMINO Outline • Rapid OFDM Polling (ROP) • Obtain the queue status of clients for uplink scheduling • Relative scheduling • Avoid tight time synchronization • Schedule converter • Create schedules for relative scheduling

  10. ROP: Rapid OFDM Polling Question: How can we collect the queue status of clients efficiently? Solution: Concurrent transmission based on Orthogonal frequency-division multiplexing (OFDM) Central controller … AP1 AP2 APN … Client3 ClientN Client1 Client2

  11. ROP: Rapid OFDM Polling • Clients transmit queue status using subcarriers • Practical issues: • Freq offset • Time offset • Power mismatch • (details in paper) Client 1 Client 2 • Related work: • PAMAC (INFOCOM’09) • B2F (MobiCom’11)

  12. ROP collects the queue status of all clients with little overhead: - 40 μs (polling message) + 16 μs (OFDM symbol) - regular packet duration: 1000 μs - multiple regular transmissions/poll

  13. DOMINO Outline • Rapid OFDM Polling (ROP) • Obtain the queue status of clients • Relative scheduling • Avoid tight time synchronization • Schedule converter • Create schedule for relative scheduling

  14. Why time synchronization? AP2 C1 AP1---->C1 AP2---->C2 C2 AP1 AP3 C4 AP4---->C4 AP3---->C3 Slot 1 Slot 2 Interfering nodes AP-client association Currently transmitting C3 AP4 μs level synchronization required One Wi-Fi slot: 9 μs AP1 ---> C1: DataPacket ACK AP2---> C2: DataPacket ACK AP3 ---> C3: Collision! DataPacket ACK AP4 ---> C4: DataPacket Misalignment

  15. Current Time Synchronization Scheme Network Time Protocol (NTP), Precision Time Protocol (PTP), Reference-Broadcast Synchronization (RBS) (SIGOPS’02), Sourcesync(SIGCOMM’10): • Low accuracy; Or • Expensive hardware; Or • Low accuracy in large network.

  16. Can we avoid tight time synchronization?

  17. Relative Scheduling AP1---->C1 AP2---->C2 AP2 C1 AP4---->C4 AP3---->C3 Interfering nodes AP-client association Currently transmitting C2 AP1 AP3 C4 Slot 1 Slot 2 C3 AP4 AP1 ---> C1: DataPacket ACK AP2---> C2: DataPacket ACK AP3 ---> C3: DataPacket ACK AP4 ---> C4: DataPacket ACK

  18. Relative Scheduling AP2---->C2 AP1---->C1 AP2 C1 AP3---->C3 AP4---->C4 Interfering nodes AP-client association Currently transmitting C2 AP1 AP3 C4 Slot 2 Slot 3 C3 AP4 AP1 ---> C1: DataPacket ACK AP2---> C2: DataPacket ACK AP3 ---> C3: DataPacket ACK AP4 ---> C4: DataPacket ACK Transmission alignment achieved

  19. Node signatures as triggers: • A sequence of bits with a certain length • These sequences are orthogonal to each other • High detecting ratio even under interference • Experiment results: • 4 combined signatures can be decoded correctly • 4 transmissions can be triggered by one node

  20. Only APs know the schedules from the central controller How can we ask the clients to send the triggers?

  21. The combined signatures that should be sent by the AP A1: DataPacket SA3 SA2 S′ 1 slot SIFS C2 C1 C3 C1: SA3 S′ ACK The combined signatures that should be sent by the client A2 A3 A1 A special signature that notifies the start of transmission

  22. A1: ACK SA3 SA2 S′ 1 slot SIFS C2 C1 C3 C1: DataPacket SA3 S′ A2 A3 A1

  23. DOMINO Outline • Rapid OFDM Polling (ROP) • Obtain the queue status of clients • Relative scheduling • Avoid tight time synchronization • Schedule converter • Create schedule for relative scheduling

  24. Schedule Converter Requirements: • Every transmitter should be triggered • Polling packets should also be scheduled • Backup triggers should be included in case of transmission failure • Details in paper Arbitrary Schedule Relative Schedule ?

  25. Experiment USRP USRP USRP USRP >1.5X >3X >3X

  26. Trace Driven Simulation • Simulation Setup: • RSS trace collected from a 40 Wi-Fi nodes testbed • Randomly picked 10 APs and 2 clients per AP • Other schemes: • CENTAUR: Downlink traffic scheduled; using fixed backoffto align transmission • DCF: 802.11 standard (Wi-Fi)

  27. UDP Throughput & Delay 1.74X 0.5X Downlink traffic only

  28. UDP Throughput & Delay Heavy tail Low fairness 1.24X Uplink and downlink traffic

  29. TCP Throughput TCP ACK as regular packet 1.15X Downlink traffic only

  30. Conclusions Domino: a platform to enable centralized scheduling algorithms without requiring tight time-synchronization: • Queue information of clients are efficiently collected using one OFDM symbol • Nodes transmit relatively one after another instead of according to time stamps Future work: coexistence with existing Wi-Fi Thank you!

  31. Backup slides

  32. Why is CENTAUR behaving worse than DCF?

  33. UDP Throughput & fairness - 24%-74% throughput gain - High and stable fairness

  34. TCP Throughput & fairness • - 10%-15% throughput gain • - TCP ACK as regular packet • - High and stable fairness

  35. Evaluation: UDP & TCP Delay DCF: 2X higher - Queuing delay Similar delay performance: - Queuing delay - TCP congestion control

  36. UDP Throughput & Delay Heavy tail 2X 1.24X Uplink and downlink traffic

  37. TCP Throughput & Delay 1.15X Uplink traffic only

  38. Domino Solution Overview Contention Free Period Contention Period • Identify Hidden & Exposed Links • Construct link conflict map • Co-existence with current networks Slot 1 Slot 2 Slot 3 … Slot N AP1 → C1 Concurrent transmissions AP2 → C2 … APM → CM

  39. Evaluation: Misalignment Alignment achieved - Slot size: 9 μs> 2 μs

  40. Current Time Synchronization Scheme • Network Time Protocol (NTP): time accuracy of about 1ms in a quiet Ethernet network. • Precision Time Protocol (PTP): requires specialized and expensive hardware. • Reference-Broadcast Synchronization (RBS) (SIGOPS’02): synchronization accuracy decreases with network size. • Sourcesync (SIGCOMM’10): one collision domain

  41. Throughput Gain of network with 80 nodes ~58%

  42. ROP: Rapid OFDM Polling • Client TX queue state over subcarriers • Polling strategy FFTwindow Polling Packet From AP CP Client N N 1 slot Subchannel … … CP guard subcarriers CP Client 3 3 Client 2 CP 2 Client 1 CP 1 guard band subchannel 0 subchannel 11 subchannel 12 subchannel 23 guard band Client 0 CP 0 DC Time … … … … … … … … -109 -100 -9 -3 0 2 3 4 9 109 -128 -4 -2 -1 1 100 127

  43. ROP: Rapid OFDM Polling Subchannel FFTwindow Polling Packet From AP CP Client N N 1 slot … … CP CP Client 3 3 Client 2 CP 2 Client 1 CP 1 Client 0 CP 0 Time

  44. DOMINO: Example Batch 0 Batch 10 Batch 11 Slots Links 0 1 2 3 90 91 92 93 94 AP1 AP3 AP4 AP3 AP4 AP1 3 fake AP1 AP3 AP4 C4 C1 C3 C4 C3 C1 AP1 AP4 AP3 AP2 C1 AP2 AP2 AP2 C2 2 C2 AP2 1 C2 AP1 AP3 C4 24 s (a) (b) C3 AP4

  45. Subcarrier Separation TX2 (38dB, 99.9%, 3 sub) TX1 RX Tradeoff: - Less overhead - Higher decoding ratio

  46. Relative Scheduling: Node signatures as trigger • Node signature are orthogonal to each other,easier to detect.

  47. Existing work: MIFI

  48. Existing work: CENTAUR

  49. Domino Solution Overview • Identify Hidden & Exposed Links • Construct link conflict map • Co-existence with current networks • ROP: Rapid OFDM Polling • Relative Scheduling • Schedule Converter

  50. ROP: How it performs Decoded OFDM symbols of two clients at adjacent subchannels with guard interval. (30db diff. RSSI at AP)

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