1. November 2007
doc.: IEEE /2752r1
July 2008
Managed Contention Access – A technique to improve Video Streaming Performance
Date:
Authors:
Jing Zhu, Intel Corporation
Peter Ecclesine, Cisco Systems

2. November 2007
doc.: IEEE /2752r1
July 2008
Abstract
This submission describes Managed Contention Access (MCA), a technique that enables semi-deterministic access to wireless medium.
Jing Zhu, Intel Corporation
Peter Ecclesine, Cisco Systems

3. What is the problem? Intermittent Channel Access caused by
unsaturated / real-time traffic (video, voice, multimedia, etc.)
power saving & multi-radio
Contending STAs with saturate traffic
increased channel access time / jitter
reduced sensitivity and increased collisions
EDCA/HCCA does not work well in OBSS

4. Problem Statement: Increased Delay and Jitter
Maximum MSDU size: 2304 bytes (at 1Mbps)  18ms
1500 bytes (at 1Mbps)  12ms
Data
ACK
STA1 (FTP)
Delay (up to ~18ms)
Data
ACK
STA2 (Video)
Arrival
General Mouse
Hi precision Mouse
Keyboard
Voice Quality Headset
CD Quality headphones
Aggregate Throughput
< 8kbps
256 kbps
384kbps
Latency
< 10ms
< 3ms
< 50ms
< 30ms
< 100ms
Delay Sensitive
Throughput Sensitive

5. Problem Statement (cont’d): Reduced Sensitivity in Post-Absence Channel Access
(power save, etc.)
Transmission
DIFS + BO
STA
#1: ACK may be out of range of
preamble detection (hidden)
Data
ACK
ACK
Neighboring
STA
PLCP
Data
ACK
ACK
#2: CCA without preamble detection
may not be sensitive enough to detect
Data
ACK
#1 and #2 will cause the performance degradation of the whole BSS
Energy Detection Threshold = -62dBm
Preamble Detection = -82dBm
20dB loss of CCA sensitivity
90% CCA sensing rage reduction with path loss exponent = 2

6. How to manage contention access ?

7. State of the Art: EDCA Admission Control
Admissible Parameters
Description
G.711 (UDP)
Data (TCP)
Nominal MSDU Size
octets
200 (inter-arrival time:20ms)
1500
Minimum Service Interval
us (minimum interval between two sucessive SPs)
20ms
Maximum Service Interval
us (maximum interval between two successive SPs)
Inactivity Interval
us (minimum inactive time between successive Tx or Rx activities before the TS being deleted from HC)
Suspension Interval
(< Inactivity Interval)
us (minimum inactive time between successive Tx or Rx activities before the polling for this TS being stopped
Minimum Data Rate
bps (lowest data rate for transport of MSDU)
Mean Data Rate
bps (average data rate for transport of MSDU)
64Kbps
(<10% duty cycle)
100Kbps
(10% duty cycle)
Burst Size
Octets (maximum bytes of the MSDUs arriving at the peak data rate)
Minimum PHY Rate
desired minimum PHY rate
11Mbps
1 Mbps
Peak Data Rate
Maximum allowable data rate
Delay Bound
us (maximum time for a MSDU spent at the MAC layer)
50ms
Surplus Bandwidth Allowance
1.X (how much more bandwidth is allowed to be used for the TS)
1.3 (30% retransmissions)
1.3 (30% retransmission)
Medium Time
32us/s (the amount of time admitted to access the medium)
HC output

8. How “Medium Time” is used in EDCA admission control ?
Two state variables: admitted_time and used_time
Two parameters: medium_time and dot11EDCAAveragePeriod
Algorithm:
a) At dot11EDCAAveragingPeriod (default = 5) second intervals
used_time = max((used_time – admitted_time), 0)
b) After each successful or unsuccessful MPDU (re)transmission attempt,
used_time = used_time + MPDUExchangeTime
c) On receipt of a TSPEC element contained in a ADDTS Response frame indicating that the request has been accepted
admitted_time = admitted_time + dot11EDCAAveragingPeriod * (medium time of TSPEC).

9. Limitations of EDCA-AC
“It is recommended that admission control not be required for the access categories AC_BE and AC_BK.” (IEEE )
“Medium Time” (32us/s) usually too long to limit per-packet transmission time
TCP duty cycle =10% and dot11EDCAAveragingPeriod = 5 seconds, retransmission = 30% (Surplus Bandwidth Allowance = 1.3)
medium time = ceiling(10% x 1000 / 32) = 4
adimitted_time = 4 x 5 x 32 (us) = 640 ms
Difficult to choose “TXOP limit” to trade between MAC efficiency and latency: 10ms? or 1ms?
No restriction on OBSS traffic
No restriction on actual timing
 need something that can be controlled by AP but with minimal restrictions on AC_BE and AC_BK

10. Basic Idea: MCA-Aware Channel Access
MCA-aware channel access shall both start after the end of a MCA slot and end before the start of a MCA slot
STA
DIFS
Back-Off Slots
Data
SIFS
ACK
MCA slot
MCA interval

11. Managed Contention Access (Cont’d)
Beacon Interval
Start Time
MCA Period
Legacy
Zone
MCA Zone
MCA Zone
MCA Zone
MCA Duration
CTS-
to-Self
MCA-
Allowed
SIFS
MCA interval
MCA slot
MCA-Unaware STA
NAV
MCA-Aware STA
NAV
Slide 11
<first name> <last name>, Intel Corporation

12. Managed Contention Access
MCA Zone is divided into multiple MCA intervals started with a MCA slot with fixed duration
MCA slot is a short Guard Interval providing re-sychronization point for STAs contending the channel
At the start of a MCA Zone, the AP transmits
a CTS-to-Self (to protect the following MCA Zone)
Waits for SIFS period and
a MCA-Allowed frame
The CTS-to-Self shuts off legacy STAs
The MCA-Allowed permits the MCA-capable STAs to contend the channel despite of CTS-to-Self
Slide 12
<first name> <last name>, Intel Corporation

13. Example: Mixed VoIP and Data
60 MCA slots, one TX per slot, 30% rTX  ~20 VoIP (bidirectional) connections
102.4ms (Beacon Interval)
22.4ms
(Legacy Zone)
30ms
(MCA Zone)
20ms
(Legacy Zone)
30ms
(MCA Zone)
1ms
1ms
… …
1ms
1ms
1ms
MCA Configurations:
start time = 22.4ms,
MCA period = 50ms
MCA duration = 30ms
MCA interval = 1ms

14. MCA-Aware OBSS
MCA-slot schedule shall not be created (owned) by BSS that does not have AC_VI or AC_VO STAs
to reduce the number of different MCA-slot schedules in OBSS
OBSS MCA-Aware APs shall copy each other’s MCA-slot schedule
AP may reuse the existing MCA-slot schedule by other OBSS as much as possible
Owner: AP that generates a MCA-slot schedule
Follower: AP that copies other AP’s MCA-slot schedule, and needs to update when such information is changed over time due to clock drift.
STA may report MCA schedule of other OBSS if such information is not included in the Beacon.
OBSS APs may not be in each other’s range

15. MCA-Aware OBSS (cont’d)
T (BSS1 MCA interval)
BSS1
T (BSS2 MCA interval)
BSS2
d (OBSS MCA interval)
T – d (OBSS MCA interval)
OBSS
It is recommended that MCA-Aware OBSS use the same duration of MCA slot, and synchronize / align with each other as much as possible, i.e.,
d T-d

16. MCA-Unaware OBSS
CTS-to-Self will prevent MCA-Unaware OBSS to contend with MCA-aware STAs

17. MCA Violation Problem: Recommendation:
OBSS STA may still violate MCA rules unconsciously due to not receiving beacon or CTS-to-Self (collision, PS, etc.)
Recommendation:
detection: AP may detect it by counting the number of MCA slots being overlapped by transmissions. STA may also help detect it and report such event to AP
action: AP may modify MCA slot schedule or reduce / remove MCA zones completely.

18. Summary Main Changes to IEEE 802.11 Standard Benefit: Cost:
MCA-slot Aware Channel Access
MCA-Allowed Control Frame
New MCA IE in Beacon Frame
Benefit:
reliability, power efficiency, latency/ duty-cycle
OBSS support
Cost:
efficiency (total network throughput) loss
complexity

19. Straw Poll #1
Would you like to see a normative text proposal on the idea of Managed Contention Access?
Yes No
Abstain