doc.: IEEE 802.11-08/0818r0

Submission

July 2008

Jing Zhu, Intel CorporationSlide 1

Managed Contention Access – A technique to improve Video Streaming Performance

Date: 2008-07-012

Name Affiliation Address Phone email Jing Zhu Intel

Corporation Hillsboro, OR USA +1 503 264

7073 jing.z.zhu@intel.com

Ganesh Venkatesan

Intel Corporation

Hillsboro, OR USA ganesh.venkatesan@intel.com

Adrian Stephens Intel Corporation

Authors:

doc.: IEEE 802.11-08/0818r0

Submission

July 2008

Jing Zhu, Intel CorporationSlide 2

Abstract

This submission describes Managed Contention Access (MCA), a technique that enables semi-deterministic access to 802.11 wireless medium.

doc.: IEEE 802.11-08/0818r0

Submission

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

doc.: IEEE 802.11-08/0818r0

Submission

Problem Statement: Increased Delay and JitterMaximum MSDU size: 2304 bytes (at 1Mbps) 18ms

1500 bytes (at 1Mbps) 12ms

Data ACKSTA1 (FTP)

STA2 (Video) Data ACK

Arrival

Delay (up to ~18ms)

General Mouse

Hi precision Mouse

Keyboard Voice Quality Headset

CD Quality headphones

Aggregate Throughput

< 8kbps < 8kbps < 8kbps 256 kbps 384kbps

Latency < 10ms < 3ms < 50ms < 30ms < 100ms

Delay Sensitive Throughput Sensitive

doc.: IEEE 802.11-08/0818r0

Submission

Problem Statement (cont’d): Reduced Sensitivity in Post-Absence Channel Access

Absence (power save, etc.)

TransmissionSTA

Data ACK

Data ACKPLCP

#2: CCA without preamble detectionmay not be sensitive enough to detect

NeighboringSTA

Data ACK

ACK

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

#1: ACK may be out of range of preamble detection (hidden)

DIFS + BO

doc.: IEEE 802.11-08/0818r0

Submission

How to manage contention access ?

doc.: IEEE 802.11-08/0818r0

Submission

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

doc.: IEEE 802.11-08/0818r0

Submission

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).

doc.: IEEE 802.11-08/0818r0

Submission

Limitations of EDCA-AC• “It is recommended that admission control not be required for the access

categories AC_BE and AC_BK.” (IEEE 802.11-2007)

• “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

doc.: IEEE 802.11-08/0818r0

Submission

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

MCA slot

MCA interval

DIFS Back-Off Slots Data ACKSIFSSTA

doc.: IEEE 802.11-08/0818r0

Submission

Managed Contention Access (Cont’d)

<first name> <last name>, Intel CorporationSlide 11

Beacon Interval

MCA Zone MCA ZoneMCA ZoneLegacy Zone

CTS-to-Self

MCA-Allowed

SIFS

MCA slot

MCA interval

Start Time MCA Period

NAVMCA-Unaware STA

MCA-Aware STA NAV

MCA Duration

doc.: IEEE 802.11-08/0818r0

Submission

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

<first name> <last name>, Intel CorporationSlide 12

doc.: IEEE 802.11-08/0818r0

Submission

Example: Mixed VoIP and Data

60ms(MCA Zone)

42.4ms(Legacy Zone)

102.4ms (Beacon Interval)

1ms 1ms 1ms 1ms 1ms… …

60 MCA slots, one TX per slot, 30% rTX ~20 VoIP (bidirectional) connections

Delay Bound = 50ms

doc.: IEEE 802.11-08/0818r0

Submission

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

doc.: IEEE 802.11-08/0818r0

Submission

MCA-Aware OBSS (cont’d)

• 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

BSS1

BSS2

OBSS

T (BSS2 MCA interval)

T (BSS1 MCA interval)d

d (OBSS MCA interval) T – d (OBSS MCA interval)

doc.: IEEE 802.11-08/0818r0

Submission

MCA-Unaware OBSS

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

doc.: IEEE 802.11-08/0818r0

Submission

MCA Violation

Problem:– 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.

doc.: IEEE 802.11-08/0818r0

Submission

Summary

Main Changes to IEEE 802.11 Standard• 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

doc.: IEEE 802.11-08/0818r0

Submission

Straw Poll #1

• Would you like to see a normative text proposal on the idea of Managed Contention Access?– Yes

– No

– Abstain