Enabling TDMA for Todays Wireless LANs Zhice Yang 1 *, Jiansong - - PowerPoint PPT Presentation

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Enabling TDMA for Todays Wireless LANs Zhice Yang 1 *, Jiansong - - PowerPoint PPT Presentation

Nov 12, 2022 373 likes •818 views

Enabling TDMA for Todays Wireless LANs Zhice Yang 1 *, Jiansong Zhang 2 *, Kun Tan 2 , Qian Zhang 1 , Yongguang Zhang 2 1 CSE, Hong Kong University of Science and Technology 2 Microsoft Research Asia * Co-Primary Author Motivation AP1 AP2

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SLIDE 1

Enabling TDMA for Today’s Wireless LANs

Zhice Yang1*, Jiansong Zhang2*, Kun Tan2, Qian Zhang1 , Yongguang Zhang2

1CSE, Hong Kong University of Science and Technology 2Microsoft Research Asia *Co-Primary Author

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SLIDE 2

Motivation

ClientA AP1 AP2 ClientB

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SLIDE 3

Motivation

ClientA AP1 AP2 ClientB ClientC

!?

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SLIDE 4

Motivation

ClientA AP1 AP2 ClientB ClientC

!?

AP1 B B B B AP2 C C C C Time

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SLIDE 5

Motivation

  • Limitations of Distributed Coordination Function

(DCF) in Current Wireless LANs

  • Weak Interference Management
  • Inefficient Channel Access
  • Lacking Guarantee in QoS
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SLIDE 6

Motivation

  • Limitations of Distributed Coordination Function

(DCF) in Current Wireless LANs

  • Weak Interference Management
  • Inefficient Channel Access
  • Lacking Guarantee in QoS
  • Demands for Higher-Efficiency Wireless Network
  • Proliferation of Wireless Devices
  • Emerging Network QoS Sensitive Applications

Conflict

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SLIDE 7

How to Fill the Gap?

ClientA AP1 AP2 ClientB ClientC

AP1 B B B B AP2 C C C C Time

Backhaul Network

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SLIDE 8

How to Fill the Gap? -- TDMA

  • Arrange the transmission of all the wireless packets

in the air to

  • Manage interfering transmissions
  • Reduce contention overhead
  • Provide priorities for QoS transmissions
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SLIDE 9

Research Question

  • Arrange the transmission of all the wireless packets

in the air to

  • Manage interfering transmissions
  • Reduce contention overhead
  • Provide priorities for QoS transmissions

Is TDMA possible with commodity WLAN devices?

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SLIDE 10

Feasibility for TDMA in WLAN

  • Time Synchronization
  • Backhaul network
  • Scheduling
  • Central controller

Exist

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SLIDE 11

ClientA AP1 AP2 ClientB ClientC

Backhaul Network

Architecture Borrowed from SDN

ClientA

Ethernet

AP1

OpenTDMF

AP2

OpenTDMF

ClientB ClientC OpenTDMF Controller

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SLIDE 12

Architecture Borrowed from SDN

10 … 11 12 … Time

ClientA

Ethernet

AP1

OpenTDMF

AP2

OpenTDMF

ClientB ClientC OpenTDMF Controller

AP2<->ClientC in 10 slots C

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SLIDE 13

Architecture Borrowed from SDN

10 … 11 12 … Time

ClientA

Ethernet

AP1

OpenTDMF

AP2

OpenTDMF

ClientB ClientC OpenTDMF Controller

AP2<->ClientC in 12 slots C

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SLIDE 14

Architecture Borrowed from SDN

10 … 11 12 … Time

ClientA

Ethernet

AP1

OpenTDMF

AP2

OpenTDMF

ClientB ClientC OpenTDMF Controller

AP2<->ClientC in 10,12 AP1<->ClientB in 11

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SLIDE 15

Architecture Borrowed from SDN

10 … 11 12 … Time

ClientA

Ethernet

AP1

OpenTDMF

AP2

OpenTDMF

ClientB ClientC OpenTDMF Controller

AP1 AP2 A A B B A A A C C C C C A A C AP1<->ClientB in 11 AP2<->ClientC in 10,12

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SLIDE 16

Challenges

  • Time Synchronization

~10𝜈𝑡

Commodity WLAN devices lack means for accurate synchronization

10 11 12 Local Time of AP1: 10 11 12 Local Time of AP2:

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SLIDE 17

Challenges

  • Time Synchronization
  • Uplink Scheduling Enforcement

Commodity WLAN devices lack means for accurate synchronization Commodity WLAN devices is designed for distributed access and determines channel access independently

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SLIDE 18

Outline

  • OpenTDMF Design
  • Time Synchronization
  • Uplink Scheduling Enforcement
  • Experiment Results
  • Scheduling Examples
  • Conclusion
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SLIDE 19

Outline

  • OpenTDMF Design
  • Time Synchronization
  • Uplink Scheduling Enforcement
  • Experiment Results
  • Scheduling Examples
  • Conclusion
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SLIDE 20

Backhaul Time Synchronization

  • Using IEEE 1588 Precise Time Protocol (PTP) to

Synchronize the Wired APs

  • Assumption: The network delay is symmetrical
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SLIDE 21

The Problem

  • Large Variation

TP-Link 4900 AP Switch Chip SoC Notebook Desktop

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SLIDE 22

Analyze the Problem

  • The Architecture of the Commodity AP Introduces

Variance in the Delay Measurement

Wireless Access Point Switch Chip SoC Port0 Port3 Port1 Port6 NIC0 NIC1 PCIE LAN/WAN Wireless NIC Data PTP

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SLIDE 23

Analyze the Problem

  • The Architecture of the Commodity AP Introduces

Variance in the Delay Measurement

Wireless Access Point Switch Chip SoC Port0 Port3 Port1 Port6 NIC0 NIC1 PCIE LAN/WAN Wireless NIC Data PTP

Delayed!

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SLIDE 24

Our Solution

Data Data PTP Before Port0 After Port0 Delayed PTP Packet Normal PTP Packet Data Data PTP Data Data PTP Data Data PTP 𝑈∆ == 𝑈𝑒𝑏𝑢𝑏 𝑈∆ > 𝑈𝑒𝑏𝑢𝑏

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SLIDE 25

Scheduling Enforcement

  • Use busy waiting to ensure accurate software timer

event

  • Use transmission gate handler in WiFi chip to

ensure accurate transmission control

10 … 11 … Scheduling Time Slot Scheduled Timer Event Rx Sirq Processing Real Timer Event

Waiting

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SLIDE 26

Outline

  • OpenTDMF Design
  • Time Synchronization
  • Uplink Scheduling Enforcement
  • Experiment Results
  • Scheduling Examples
  • Conclusion
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SLIDE 27

Uplink Control

  • Polling for Uplink Transmission
  • Poll packet is a normal packet with a poll flag in the

control filed of the MAC header

  • Clients response the poll packet with the uplink data
  • Uplink can be treated as downlink

Poll Data ACK Poll+Data Data ACK SIFS Time

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SLIDE 28

First Transmission Problem

  • AP needs to know about the packet queue

information in clients to schedule polling

  • Clients piggyback queue information in every uplink

packet

  • AP doesn’t know when the client want to transmit

the first uplink packet

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SLIDE 29

Group Polling

  • Group polling for the first transmission
  • Group poll packet is a poll packet with group address

Client1 Client2 Client3 Client4 Random SIFS +0 slots +3 slots +1 slots +4 slots

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SLIDE 30

Group Polling

  • Group polling for the first transmission
  • Group poll packet is a poll packet with group address

Group Poll Client1 Info Client3 Info Client2 Info Client4 Info Time Client1 Client2 Client3 Client4 Random SIFS +0 slots +3 slots +1 slots +4 slots

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SLIDE 31

Group Polling

  • Group polling for the first transmission
  • Group poll packet is a poll packet with group address

Group Poll Client1 Info Client3 Info Client2 Info Client4 Info Time Client1 Client2 Client3 Client4 Random SIFS +0 slots +3 slots +1 slots +4 slots

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SLIDE 32

Group Polling

  • Group polling for the first transmission
  • Group poll packet is a poll packet with group address

Group Poll Client1 Info Client3 Info Client2 Info Client4 Info Client1 Client2 Client3 Client4 Random SIFS +0 slots +3 slots +1 slots +4 slots Time

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SLIDE 33

Group Polling

  • Group polling for the first transmission
  • Group poll packet is a poll packet with group address

Group Poll Client1 Info Client3 Info Client2 Info Client4 Info Client1 Client2 Client3 Client4 Random SIFS +0 slots +3 slots +1 slots +4 slots Time

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SLIDE 34

Outline

  • OpenTDMF Design
  • Time Synchronization
  • Uplink Scheduling Enforcement
  • Experiment Results
  • Scheduling Examples
  • Conclusion
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SLIDE 35

Implementation

  • Implemented with TP Link 4900 with Atheros

AR9381 and AR9580 WiFi Chip

  • Modified ath9k driver
  • Modified linuxptp program
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SLIDE 36

Timing Error

AP1 AP2 ClientA ClientB AP3 ClientC

Backhaul Network

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SLIDE 37

Uplink Efficiency

30%

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SLIDE 38

Outline

  • OpenTDMF Design
  • Time Synchronization
  • Uplink Scheduling Enforcement
  • Experiment Results
  • Scheduling Examples
  • Conclusion
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SLIDE 39

Topology and Policy

AP1 AP2 ClientA ClientB ClientC

Knowledge of the Controller

AP1<->B conflicts with AP2<->C & AP1<->B requires high priority

Control Policy for AP1 Control Policy for AP2

Flow ID Time Slots Priority AP1<->B 1,2 mod 3 High AP1<->A ALL Normal Flow ID Time Slots Priority AP2<->C 0 mod 3 Normal

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SLIDE 40

AP1 AP2 ClientA ClientB ClientC

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SLIDE 41

Outline

  • OpenTDMF Design
  • Time Synchronization
  • Uplink Scheduling Enforcement
  • Experiment Results
  • Scheduling Examples
  • Conclusion
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SLIDE 42

Conclusion

  • Thoughtful study of accurate synchronization in

commodity AP

  • Enable polling based uplink transmission in

commodity WiFi chips

  • Build the OpenTDMF system and validate the

feasibility of TDMA in commodity WLANs.

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SLIDE 43

Thank you !