1. Balancing Uplink and Downlink Delay of VoIP Traffic in 802.11 WLANs
Sangho Shin
Henning Schulzrinne
Columbia University
Department of Computer Science

2. Distribution Network (DS)
Motivation
VoIP in IEEE WLANs
Distribution Network (DS)
Access Point (AP)
STAs
May
1/30

3. Distribution Network (DS)
Motivation
VoIP in IEEE WLANs
Distribution Network (DS)
Access Point (AP)
STAs
May
1/30

4. Distribution Network (DS)
Motivation
VoIP in IEEE WLANs
Distribution Network (DS)
Access Point (AP)
STAs
May
1/30

5. 20 ms packetization interval (64kb/s)
Motivation
Downlink
Uplink
20 ms packetization interval (64kb/s)
May
2/30

6. Distribution Network (DS)
Motivation
When the channel is congested ….
Distribution Network (DS)
Queue
Queue
Queue
Queue
Queue
May
3/30

7. Distribution Network (DS)
Motivation
When the channel is congested ….
Distribution Network (DS)
Queue
Queue
Queue
Queue
Queue
May
3/30

8. Distribution Network (DS)
Motivation
When the channel is congested ….
Distribution Network (DS)
Queue
Queue
Queue
Queue
Queue
May
3/30

9. Distribution Network (DS)
Motivation
When the channel is congested ….
Distribution Network (DS)
Queue
Queue
Queue
Queue
Queue
May
3/30

10. Distribution Network (DS)
Motivation
When the channel is congested ….
Distribution Network (DS)
Queue
Queue
Queue
Queue
Queue
May
3/30

11. Motivation Solutions? Give a higher priority to the AP than STAs
Distribution Network (DS)
Queue
Queue
Queue
Queue
Queue
May
4/30

12. Outline Background Adaptive Priority Control (APC) Simulation results
Implementation Issues
Conclusions
Future work
May
5/30

13. Number of Backoff = (0, CWmin)
Background
Medium Access in DCF (Distributed Coordination Function)
Contention Window
Backoff
Slot time
DIFS
DIFS
Busy Medium
Frame
Defer Access
Number of Backoff = (0, CWmin)
May
6/30

14. Background How to control the priority of the AP?
Inter-Frame Spacing (IFS)
Contention Window (CW)
Contention Free Transmission
Contention Window
DIFS
DIFS
Busy Medium
Backoff
Frame
Defer Access
Slot time
May
7/30

15. Transmission Control Control Contention Window (CW) Frame3 Backoff
DIFS
Backoff
Frame1
Frame2
Number of Backoff = (0, CWmin)
Frame3
DIFS BO
Frame1
Frame2
Number of Backoff = (0, CWmin/2)
Remaining BO time BO BO
STA
DIFS
BO1
Defer Access
DIFS
BO2
Defer Access
DIFS BO
Frame AP
DIFS BO
Frame
DIFS BO
Frame
DIFS BO
Defer Access
May
8/30

16. Transmission Control Control Contention Window (CW)
It is hard to control the priority accurately.
Backoff time is randomly decided between (0, CWmin).
Shorter CW  Higher collision probability
Decrease the capacity.
May
9/30

17. Transmission Control Control IFS Frame3 Backoff Frame1 Frame2
DIFS
Backoff
Frame1
Frame2
IFS = DIFS
IFS = DIFS/2
Frame3
IFS
Backoff
Frame1
Frame2
Frame
STA1
STA2
Defer Access
DIFS BO
Defer Access
DIFS
Frame AP
Defer Access
IFS BO
Frame
May
10/30

18. Transmission Control Contention Free Transmission (CFB)
Control the number of frames (P)
Frame3
IFS
Frame1
Frame2
Frame3
IFS
Frame1
Frame2
P = 3
DIFS BO
Defer Access
IFS
Frame1
Frame2
DIFS BO
Defer Access
P = 2
Frame3
May
11/30

19. Transmission Control Contention Free Transmission (CFB)
Precise priority control
P  Priority
Transmitting three frames contention free  three times higher priority than other STAs.
No overhead
May
12/30

20. Adaptive Priority Control
Optimal priority of the AP to balance the uplink and downlink delay ?
Number of active wireless STAs
Semi-Adaptive Method
May
13/30

21. Semi-Adaptive
10 packets
P = 4
May

22. Semi-Adaptive
6 packets
P = 4
May

23. Adaptive Priority Control
Optimal priority of the AP to balance the uplink and downlink delay ?
Number of active wireless STAs
Semi-Adaptive Method
Simple
Adaptive to the change of the number of active wireless STAs.
Not adaptive to the change of the traffic volume of uplink and downlink.
May
13/30

24. Semi-Adaptive
10 packets
P = 4
2 packets
P = 4
May

25. Adaptive Priority Control
Optimal priority of the AP to balance the uplink and downlink delay ?
Uplink/Downlink delay ≈Queuing delay
The same packet processing time in Queue of the AP and STAs  The same queuing delay
Network
Queuing dely
Access delay
Propagation delay
May
14/30

26. Adaptive Priority Control
Optimal Priority = QAP/QSTA (QSTA>0)
May
15/30

27. APC
10 packets
P = 10 / 2 = 5
May

28. APC
10 packets
P = 10 / 2 = 5
May

29. APC
5 packets
P = 5/1 = 5
May

30. Adaptive Priority Control
Optimal Priority = QAP/QSTA
Simple
Adaptive to change of number of active STAs
Adaptive to change of uplink/downlink traffic volume
Results in the same packet processing time between the AP and STAs  same queuing delay in the AP and STAs  same uplink and downlink delay
May
15/30

31. 20 ms packetization interval (64kb/s)
Simulation Results
- DCF
20 ms packetization interval (64kb/s)
Downlink
Uplink
May
2/30

32. 20 ms packetization interval (64kb/s)
Semi-Adaptive Method
Simulation Results
20 ms packetization interval (64kb/s)
May
16/30

33. Simulation Results - APC
20 ms packetization interval (64kb/s)
May
17/30

34. Simulation Results APC vs Semi-Adaptive
20 ms packetization interval (64kb/s)
May
18/30

35. Simulation Results DCF vs APC
30%
Capacity = 28
Capacity = 35
20 ms packetization interval (64kb/s)
May
19/30

36. Simulation Results - APC
May
20/30

37. Simulation Results Semi-Adaptive Method
10ms + 20 ms packetization interval (64kb/s)
May
21/30

38. Simulation Results - APC
10ms + 20 ms packetization interval (64kb/s)
May
22/30

39. Simulation Results APC vs Semi-Adaptive Method
10ms + 20 ms packetization interval (64kb/s)
May
23/30

40. Simulation Results - APC
40 ms packetization interval (64kb/s)
May
24/30

41. Simulation Results - APC
20 ms + 40 ms packetization interval (64kb/s)
May
25/30

42. Implementation Issues
How can the AP know the queue size of nodes?
Add the queue size of each node to VoIP packets.
It requires the changes in clients
Estimate the average queue size of nodes
Queue size = Number of packets generated at APP layer – Number of packets transmitted
Number of packets transmitted ≈ Number of packets received
Number of packets generated at APP layer ≈ Number of active nodes x packetization interval
May
26/30

43. Implementation Issues APC with estimated Queue size
May
27/30

44. Implementation Issues
How to implement Contention Free Transmission?
IEEE e : Contention Free Bursty (CFB)
Change the CFB duration.
Wireless Media Extension (WME)
A subset of IEEE e
Implemented in many chipsets
Allows change of IFS, CFB duration
May
28/30

45. Conclusions
Uplink and downlink delay VoIP traffic in DCF are significantly unbalanced.
APC, in which AP transmits QAP/QNodes packets, balances the uplink and downlink delay.
APC improves the capacity for VoIP traffic from 28 calls to 35 calls, by 25%.
May
29/30

46. Future Work Integrate APC to IEEE 802.11e.
Measure the performance of APC with background traffic.
Measure the performance of APC with actual wireless nodes in ORBIT test-bed.
May
30/30

47. Thank you
Questions?
May

48. Distributed Network (DS)
Motivation
VoIP in IEEE WLANs
Distributed Network (DS)
May

49. Simulations VoIP traffic model ITU-T P59 Our Model 0.5 1.0 0.23 0.3
0.9
1.5
Parameter
Duration (s)
Rate (%)
Talk-spurt
1.004
38.53
Pause
1.587
61.47
Double-Talk
0.228
6.59
Mutual Silence
0.508
22.48
Duration (s)
Rate (%)
0.9
37.5
1.5
62.5
0.3
25.0
May