1. HCF and EDCF Simulations
October 2001
October 2001
HCF and EDCF Simulations
Aman Singla
Greg Chesson
Atheros Communications, Inc.
Aman Singla, Atheros

2. Overview Simulations Analysis
October 2001
Overview
Simulations
First simulation results for both contention-based (EDCF) and polling-based (HCF) channel access methods
Study the performance and efficiency of the two methods under the same scenarios
Analysis
Understand the operational behavior of EDCF and HCF
Summary and Implications
Aman Singla, Atheros

3. October 2001
Simulations
Aman Singla, Atheros

4. Background Down-link characteristics are equivalent
October 2001
Background
Down-link characteristics are equivalent
EAP (HC) always has preferential access _ EAP can transmit at pifs for both EDCF / HCF
Concentrate on the up-link differences
EDCF: contention-based access
HCF: contention-free polled access
Concentrate on guaranteed service scenarios
Admission Control for all QoS streams
Uniform 10% PER for all frames
Channel degrades rapidly above 10% PER to make PHY rate backoff more effective
Aman Singla, Atheros

5. Background (continued)
October 2001
Background (continued)
Overload the channel with background traffic
best-effort traffic is not admission controlled
A realistic, non-trivial HCF scheduler
Uses all available information
Reacts within sifs to data arrival and queue state information
Implemented various heuristics and used the best performing
Report results for a 36Mb/s 11a PHY
Statistical principles apply at all PHY rates
Aman Singla, Atheros

6. Simulations Scenario I
October 2001
Simulations
Scenario I
Study the effect of number of QoS stations on latency and jitter
Scenario II
Study the QoS capacity and efficiency of the two channel access methods
Scenario III
Telephony case study
Aman Singla, Atheros

7. QoS Streams - Traffic Model
October 2001
QoS Streams - Traffic Model
CBR streams with the inter-packet arrival interval based on a normal distribution around a period
variance = period/4
mean = period
Aman Singla, Atheros

8. Simulations – I (constant load)
October 2001
Simulations – I (constant load)
Scenario
PHY = 36 Mb/s (11a), 10% PER
Background traffic backlogged queues 1500 Byte packets
QoS traffic Fixed Mb/s 2048 Byte packets
Experiment
Vary the # of ESTAs applying the QoS load
2 streams 2 ms period,
4/8/16 streams 4/8/16 ms period,
32 streams 32 ms period
Objective
Study effect of changing number of ESTAs on end-to-end latency and jitter for QoS traffic
Aman Singla, Atheros

9. EDCF – Latency Distribution
October 2001
EDCF – Latency Distribution
80% of packets have end-to-end
latency <= 2.5ms
Aman Singla, Atheros

10. HCF – Latency Distribution
October 2001
HCF – Latency Distribution
2 streams
32 streams
Aman Singla, Atheros

11. Conclusions – Simulations I
October 2001
Conclusions – Simulations I
EDCF is resilient to the number of stations applying the QoS load
This constant QoS load study shows that collision rate is largely independent of the number of stations
HCF scheduler phase contributes significantly to end-to-end latency
Scheduler phase: timing difference between poll arrival and data arrival
Aman Singla, Atheros

12. Simulations – II (varying load)
October 2001
Simulations – II (varying load)
Scenario
PHY = 36 Mb/s (11a), 10% PER
Background traffic backlogged queues 1500 Byte packets
QoS traffic streams (ESTAs) CWMin/Max for EDCF = 7/15
Experiment
Vary the packet size and period (applied load) of the QoS traffic
a) 4 x ms b) 4 x ms
c) 4 x ms d) 4 x ms
e) 4 x ms f ) 4 x ms
Objective
Compare channel capacity and efficiency for EDCF/HCF at similar performance levels
Aman Singla, Atheros

13. Latency Distribution (2048 Byte pkts)
October 2001
Latency Distribution (2048 Byte pkts)
Aman Singla, Atheros

14. Latency Distribution (1500 Byte pkts)
October 2001
Latency Distribution (1500 Byte pkts)
Aman Singla, Atheros

15. Latency Distribution (1024 Byte pkts)
October 2001
Latency Distribution (1024 Byte pkts)
Aman Singla, Atheros

16. Latency Distribution (512 Byte pkts)
October 2001
Latency Distribution (512 Byte pkts)
Aman Singla, Atheros

17. Latency Distribution (256 Byte pkts)
October 2001
Latency Distribution (256 Byte pkts)
Aman Singla, Atheros

18. Latency Distribution (128 Byte pkts)
October 2001
Latency Distribution (128 Byte pkts)
Aman Singla, Atheros

19. Efficiency October 2001 Efficiency is measured by the amount
QoS load for both EDCF / HCF
Efficiency is measured by the amount
of background load supported
Background load using EDCF
Background load using HCF
Aman Singla, Atheros

20. Conclusions – Simulations II
October 2001
Conclusions – Simulations II
EDCF and HCF demonstrate comparable latency and jitter performance
EDCF and HCF support similar levels of background traffic (are equally efficient) in the presence of the same QoS load for these case studies
EDCF and HCF have similar QoS capacity
QoS capacity is the maximal QoS load that can be supported for a particular set of service guarantees
Latency and jitter degradation for EDCF and HCF occur at essentially the same QoS loads - i.e. loads greater than the QoS load curve on Slide 19
The contention-free period for HCF on Slide 19 is 90+%
Aman Singla, Atheros

21. Simulations – III (telephony case)
October 2001
Simulations – III (telephony case)
Scenario
PHY = 36 Mb/s (11a), 10% PER
Background traffic backlogged queues 1500 Byte packets
QoS traffic uni-directional (up-link) telephony streams Each stream = ms CWMin/Max for EDCF = 7/15
Objective
Case study comparing EDCF and HCF
Aman Singla, Atheros

22. Latency Distribution (36 phones)
October 2001
Latency Distribution (36 phones)
EDCF delivers 4.71 Mb/s
of background traffic x
HCF uses 75% of channel time for CFP
Delivers 4.77 Mb/s of background traffic
Aman Singla, Atheros

23. Conclusions – Simulations III
October 2001
Conclusions – Simulations III
EDCF provides better latency and jitter in this case study
EDCF and HCF show similar efficiency (surplus bandwidth for background traffic) within the parameters of this case study
EDCF and HCF have similar QoS capacity for this experiment
HCF’s CFP duty cycle is 75%
Latency exceeds 10ms with more phones
Latency increase is caused by contention for EDCF, or scheduler conflicts for HCF
Aman Singla, Atheros

24. October 2001
Analysis
Aman Singla, Atheros

25. Operational behavior for EDCF
October 2001
October 2001
Operational behavior for EDCF
AIFS isolates contention between TCs
Traffic from one TC contends mostly against traffic within the same TC
AIFS projects an image of a lightly loaded network to higher priority applications by
Separating contention/arbitration for higher priority traffic from contention/arbitration for lower priority traffic
Deferring contention/arbitration for lower priority traffic until after contention/arbitration for higher priority traffic
Transmission
Contention/arbitration for high
priority access
Contention for low priority access
AIFS
Aman Singla, Atheros

26. Operational behavior for EDCF (cont.)
October 2001
Operational behavior for EDCF (cont.)
Admission Control is used to control the QoS load
Controlled load _ controlled contention
Contention as a function of
the number of stations
Contention at excessive loads
Contention at low loads
Aman Singla, Atheros

27. Operational behavior for HCF polling
October 2001
Operational behavior for HCF polling
Performance depends on the scheduler
Contention free channel access
Contention exists instead in the scheduler
Scheduled Polling
Phase difference between poll generation and data arrival contributes to latency
Error control
Channel error recovery contributes to scheduler complexity and overhead
Admission control is needed
Controlled load _ feasible schedule
Aman Singla, Atheros

28. Factors affecting latency
October 2001
Factors affecting latency
Packet size, PHY rate, etc.
Channel arbitration or scheduling artifacts
Background traffic
Channel packet errors
Per-packet latency
Aman Singla, Atheros

29. Summary (I) Latency and jitter are a reality of the wireless network
October 2001
Summary (I)
Latency and jitter are a reality of the wireless network
Access method is just one contributing factor
Controlled access does not guarantee fixed or low latency
Similar latency and jitter for EDCF and HCF
Service guarantees will be probabilistic
Service guarantees will need Tx and Rx buffers
Simulation studies help determine buffer sizes
Similar buffer requirements for EDCF and HCF
CBR source
TX fifo
Wireless
channel
RX fifo
CBR sink
Aman Singla, Atheros

30. October 2001
Summary (II)
Service guarantees can be provided only on some limited QoS load
Admission Control is required for both EDCF and HCF
EDCF and HCF both work well within the limits
EDCF and HCF both fail when QoS load exceeds the limit
The QoS load limits for both EDCF and HCF are substantially the same
~90% of maximal throughput may be used for QoS load
TCP congestion control arrives at the same operating point
Remaining channel may be used for background (best-effort) traffic with equal efficiency for both EDCF and HCF
Aman Singla, Atheros