EDCF-- Not “Simple and Good Enough”

1. EDCF--Not “Simple and Good Enough” Jin-Meng Ho, Sid Schrum, Khaled Turki, Yonghe Liu, and Matthew B. Shoemake Texas Instruments Incorporated 12500 TI Blvd. Dallas, Texas 75243 (214) 480-1994 (Ho) jinmengho@ti.com

2. Introduction • Draft EDCF was adopted in March 2001 under the persuasion of being “simple and good enough”. • Experience since then indicates otherwise: • EDCF is complex. • EDCF itself is more complicated than polled HCF and DCF. • EDCF burdens ESTAs with another access method . • EDCF itself is inferior to polled HCF and even to DCF in providing significant QoS. • EDCF disturbs polled HCF and adds uncertainties and jitters to HC scheduling. DCF Polled HCF ESTA EDCF DCF Polled HCF EDCF

3. Outline • This document examines two aspects: • Implementation complexity and weaknesses of EDCF when compared toHCF at both EAP/HC and WSTAs. • Delay and throughput performance of EDCF vs. HCF • and EDCF vs. DCF. EAP/HC WSTA EAP/HC WSTA EDCF Polled HCF DCF Polled HCF Delays: EDCF DCF Polled HCF Throughputs: EDCF

4. EDCF Complexity at HC • The HC is required to estimate the traffic load for each potential traffic category to provide correct AIFS, CWmin, and PF access parameters. • This is not a trivial task: • It needs clear distinction among idle, success, and collision events on a WM. • It needs a workable algorithm for translating the estimated loads into specific AIFS, CWmin, and PF values--no such algorithm has been published yet. ... Traffic Category 1 Traffic Category 8 ... Estimating Loads: Idles Successes Collisions Idles Successes Collisions ... Calculating Access Parameters: AIFS CWmin PF AIFS CWmin PF

5. EDCF Complexity at WSTA • The WSTA is required to implement multiple backoff times based on different AIFS, CWmin, and PF values. • This is not a simple task: • The contention window is not set independently by the station. • The backoff may not be binary exponential. • Internal collision may occur and need be identified and resolved. Truncated Binary Exponential CWmin CW CW Backoff Backoff = 0 Backoff DIFS DIFS WM Busy DIFS Backoff Decrements ESTA Transmits Backoff Decrements

6. EDCF Complexity • The ESTA is required to transmit by both polling and EDCF. • This is not a smooth task: • The ESTA may need to pull a frame out of an “EDCF queue” for transmission in response to a poll arrival. • The ESTA may need to pull a frame out of an “HCF queue” for transmission in response to a timer expiration. • This is not a straightforward task: • What happens to the backoff timer (and the corresponding CW) running with a waiting frame which was sent in response to a poll prior to the timer expiration? • How is the CW set or reset for a frame transmitted unsuccessfully via polling? • How is the retry limit incremented for a frame that was (re)transmitted by both EDCF and HCF(polling)?  x x Frame EDCF Frame EDCF Transmits Transmits x x Polled HCF Polled HCF  x x Frame Polled HCF Frame Polled HCF Transmits Transmits x x EDCF EDCF

7. HCF (Polling) Complexity • The HC conducts its access using PIFS or a single backoff timer. • The HC may implement a scheduling algorithm for polling in software. • The scheduling algorithm may be improved over time by software upgrade. • The WSTA transmits only when receiving a poll. Scheduling EAP/HC Polls PIFS Transmits WSTA

8. EDCF Access • AIFS may not be used in providing priority differentiation. • AIFS  PIFS may cause collisions between the WSTAs and the HC. • The HC accesses the WM using PIFS + a random backoff. • AIFS > DIFS gives an access priority lower than DCF. • Starvation of low priority traffic results. • CWmin has a small room in providing priority differentiation. • CWmin  15 (802.11b) or CWmin  7 (802.11a) is needed to achieve higher priority than DCF. • CWmin = 7 may lead to repeated collisions even at moderate loads. • CWmin = 3 may lead to repeated collisions even at low loads. • PF has no theoretical justification in its use. AIFS SIFS PIFS DIFS Slot CWmin 3 7 15 (802.11a) 31 (802.11b) 63

9. EDCF -- More • EDCF -- Good Enough QoS? • Achieves effective priority differentiation for no more than two traffic categories. • Provides lower priority service to low-priority traffic than DCF. • Could starve best-effort traffic. • EDCF does not provide traffic delivery guarantees. • EDCF -- Express Lane to QoS? • AIFS and CWmin may be used in providing priority differentiation because the number of legacy 802.11 stations in the market may be ignored. • About 9 million 802.11b stations have been deployed, with another 6 million expected before IEEE 802.11e compliant stations hit the market. • The argument that ESTAs will outnumber STAs over time is against the motivation that EDCF will be a vehicle to QoS near term.

10. EDCF Performance • Model simulation are quite different from real operations. • Computer simulations benefit from both foresight and hindsight: • Correct access parameters may be chosen based on assumed traffic patterns (unless blind adaptation algorithms were developed). • Parameter values may be adjusted for optimal performance. • Actual operations may only guess and fail: • The HC does not know if any, and what, type of traffic is arriving to the stations it is coordinating but relies on probabilistic prediction. • Making inaccurate load predictions or translating them into incorrect access parameter values leads EDCF to performing even worse than DCF. • Capture effects make actual performance worse than simulation results. • Significant capture has occurred in real tests with DCF. • Capture would destroy fairness for EDCF access by traffic of the same priority.

11. EDCF Performance -- More • EDCF is vulnerable to capture. • Some ESTAs may always win the contention. • Some ESTAs may always lose the contention. • 5dB difference in received SNR assures capture to occur. • 5db difference in received SNR arises readily from radio path differences. • Collisions are still unavoidable for contention by EDCF. • Delays and jitters are increased. • Channel and station throughputs are decreased. • No admission or policing control is enforced by EDCF. • Low-priority traffic squeezes high-priority traffic. • Strong QoS support is compromised.

12. HCF (Polling) Performance • HC scheduling may benefit from QoS signaling. • Transmission of traffic from the EAP/HC and polling of WSTAs for their transmission may be done according to the preset traffic specifications. • HC polling may also use deterministic feedback. • TXOPs are given to WSTAs requesting additional bandwidth. • HC policing may enhance customer experience and security. • Good behavior is protected by prohibiting bad behavior. • Admission control may be exercised to provide better QoS. • Greedy low-priority traffic is prevented from entering the network and squeezing ongoing high-priority traffic. • Contention-free transfer may not be susceptible to capture. • QoS and fairness may be guaranteed.

13. Simulation--STA Impact on ESTA Traffic & Service Scenario • One EAP, one ESTA, and one STA. • One bi-directional voice stream between EAP and ESTA. • CBR, 64 Kbps. • AIFS = DIFS, CWmin = 15. • One uni-directional data stream from STA to EAP. • VBR, mean rate = 5 Mbps. • AIFS = DIFS, CWmin = 31. EAP Voice Data STA ESTA

14. Simulation--STA Impact on ESTA Voice Delay 70 Voice Delay EAP -> ESTA Voice Delay ESTA -> EAP 60 50 40 Voice Delay (ms) 30 20 10 0 0 1 2 3 4 5 6 7 8 9 10 Simulation Time (sec) Legacy traffic contending

15. Simulation--EDCF vs. DCF Traffic & Service Scenario • One EAP (ap), eight ESTAs (sta1-sta8), and one STA (sta9). • One bi-directional data stream between EAP and each ESTA. • VBR, 400 bytes per frame, one frame per 10 ms on average. • AIFS = PIFS (i.e., contention-free transfer) for all eight downstreams. • AIFS = DIFS, CWmin = 7 (1st priority) • for four upstreams (from sta1, …, sta4). • AIFS = DIFS, CWmin = 15 (2nd priority) • for four upstreams (from sta5, … sta8). • One uni-directional data stream • from STA to EAP. • VBR, 400 bytes per frame, • one frame per 10 ms on average. • AIFS = DIFS, CWmin = 31.

16. Simulation-- EDCF vs. DCF Delay Performance (Upstreams) EDCF: CWmin = 31 EDCF: CWmin = 15 DCF EDCF: CWmin = 7

17. Simulation--EDCF vs. HCF Traffic & Service Scenario • Same as in EDCF vs. DCF simulation. • Simulation with EDCF same as for EDCF vs. DCF simulation. • Simulation with HCF: • HC-scheduled contention-free transfer • for traffic between EAP and ESTAs. • Earliest Deadline First (EDF) based scheduling. • 30 ms target delay set for 1st priority. • 40 ms target delay set for 2nd priority. • DCF-contention transmission • for traffic from STA to EAP.

18. Simulation-- EDCF vs. HCF Delay Performance (Priority Upstreams) EDCF: Low Priority EDCF: High Priority HCF

19. Simulation-- EDCF vs. HCF Delay Performance (Best-Effort Upstream) HCF EDCF HCF

20. Simulation-- EDCF vs. DCF & HCF Throughput Performance (Priority Upstreams) HCF EDCF DCF

21. Simulation-- EDCF vs. DCF & HCF Throughput Performance (Best-Effort Upstream) DCF HCF EDCF

22. Observations • With EDCF, the presence of a single legacy station severely degrades the performance of priority traffic . • Low-priority traffic fares worse with EDCF than with DCF. • Best-effort traffic is essentially starved with EDCF. • EDCF becomes unstable with moderate traffic. • HCF without EDCF can effectively guarantee QoS where EDCF fails. • HCF without EDCF achieves higher throughput than EDCF alone.