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

Distribution Network (DS) Motivation VoIP in IEEE 802.11 WLANs Distribution Network (DS) Access Point (AP) STAs May 2 2006 1/30

Distribution Network (DS) Motivation VoIP in IEEE 802.11 WLANs Distribution Network (DS) Access Point (AP) STAs May 2 2006 1/30

Distribution Network (DS) Motivation VoIP in IEEE 802.11 WLANs Distribution Network (DS) Access Point (AP) STAs May 2 2006 1/30

20 ms packetization interval (64kb/s) Motivation Downlink Uplink 20 ms packetization interval (64kb/s) May 2 2006 2/30

Distribution Network (DS) Motivation When the channel is congested …. Distribution Network (DS) Queue Queue Queue Queue Queue May 2 2006 3/30

Distribution Network (DS) Motivation When the channel is congested …. Distribution Network (DS) Queue Queue Queue Queue Queue May 2 2006 3/30

Distribution Network (DS) Motivation When the channel is congested …. Distribution Network (DS) Queue Queue Queue Queue Queue May 2 2006 3/30

Distribution Network (DS) Motivation When the channel is congested …. Distribution Network (DS) Queue Queue Queue Queue Queue May 2 2006 3/30

Distribution Network (DS) Motivation When the channel is congested …. Distribution Network (DS) Queue Queue Queue Queue Queue May 2 2006 3/30

Motivation Solutions? Give a higher priority to the AP than STAs Distribution Network (DS) Queue Queue Queue Queue Queue May 2 2006 4/30

Outline Background Adaptive Priority Control (APC) Simulation results Implementation Issues Conclusions Future work May 2 2006 5/30

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 2 2006 6/30

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 2 2006 7/30

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 2 2006 8/30

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 2 2006 9/30

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 2 2006 10/30

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 2 2006 11/30

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 2 2006 12/30

Adaptive Priority Control Optimal priority of the AP to balance the uplink and downlink delay ? Number of active wireless STAs Semi-Adaptive Method May 2 2006 13/30

Semi-Adaptive 10 packets P = 4 May 2 2006

Semi-Adaptive 6 packets P = 4 May 2 2006

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 2 2006 13/30

Semi-Adaptive 10 packets P = 4 2 packets P = 4 May 2 2006

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 2 2006 14/30

Adaptive Priority Control Optimal Priority = QAP/QSTA (QSTA>0) May 2 2006 15/30

APC 10 packets P = 10 / 2 = 5 May 2 2006

APC 10 packets P = 10 / 2 = 5 May 2 2006

APC 5 packets P = 5/1 = 5 May 2 2006

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 2 2006 15/30

20 ms packetization interval (64kb/s) Simulation Results - DCF 20 ms packetization interval (64kb/s) Downlink Uplink May 2 2006 2/30

20 ms packetization interval (64kb/s) Semi-Adaptive Method Simulation Results 20 ms packetization interval (64kb/s) May 2 2006 16/30

Simulation Results - APC 20 ms packetization interval (64kb/s) May 2 2006 17/30

Simulation Results APC vs Semi-Adaptive 20 ms packetization interval (64kb/s) May 2 2006 18/30

Simulation Results DCF vs APC 30% Capacity = 28 Capacity = 35 20 ms packetization interval (64kb/s) May 2 2006 19/30

Simulation Results - APC May 2 2006 20/30

Simulation Results Semi-Adaptive Method 10ms + 20 ms packetization interval (64kb/s) May 2 2006 21/30

Simulation Results - APC 10ms + 20 ms packetization interval (64kb/s) May 2 2006 22/30

Simulation Results APC vs Semi-Adaptive Method 10ms + 20 ms packetization interval (64kb/s) May 2 2006 23/30

Simulation Results - APC 40 ms packetization interval (64kb/s) May 2 2006 24/30

Simulation Results - APC 20 ms + 40 ms packetization interval (64kb/s) May 2 2006 25/30

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 2 2006 26/30

Implementation Issues APC with estimated Queue size May 2 2006 27/30

Implementation Issues How to implement Contention Free Transmission? IEEE 802.11e : Contention Free Bursty (CFB) Change the CFB duration. Wireless Media Extension (WME) A subset of IEEE 802.11e Implemented in many chipsets Allows change of IFS, CFB duration May 2 2006 28/30

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 2 2006 29/30

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 2 2006 30/30

Thank you Questions? May 2 2006

Distributed Network (DS) Motivation VoIP in IEEE 802.11 WLANs Distributed Network (DS) May 2 2006

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 2 2006