1. MDA Simulation Study: MDAOP Stretching and Other Concerns
September 2007
doc.: IEEE /2537r0
September 2007
MDA Simulation Study: MDAOP Stretching and Other Concerns
Date:
Authors:
Ye Chen, Motorola
Ye Chen, Motorola

2. September 2007
doc.: IEEE /2537r0
September 2007
Abstract
This document investigates challenges involved in restricting channel access to occur during deterministic intervals (e.g. an MDAOP), and explores the benefits of adding flexibility. Results from simulation experiments are presented to illustrate the issues involved. Additionally, modification to MDA channel access to address these issues are reviewed.
Ye Chen, Motorola
Ye Chen, Motorola

3. September 2007
Comments Received
Comments received about the deterministic channel access mechanism specified in MDA include:
Concerns about restriction that MDA TXOPs are not allowed to cross MDAOP boundaries, and how these restrictions complicate retransmission procedures
Concerns about needing to reserve excess time so every MDAOP can accommodate one or more retransmissions
Questions raised about whether time taken to access the channel should count towards the MDAOP length
Questions about whether the use of a single set of channel access parameters both within and after the MDAOP should be prohibited
Ye Chen, Motorola

4. September 2007
What have we done?
Started looking into the details of MDA channel access using a simulation scenario to better understand
Do the start of MDA TXOPs frequently get delayed?
Do MDA TXOPs generally finish in time (e.g. before the end of the reserved MDAOP)?
If we were to enforce the rule that MDA TXOPs must finish in time, how much excess time should each MDAOP include
Note: this is not a question for which we would expect to find a simple answer
Ye Chen, Motorola

5. September 2007
How did we do it?
Started with MDA simulation model described in 11/07-356r0
Modified the model to collect some new MDA statistics
Collected results for mixture of MDA and station traffic
Mesh AP2 / Mesh Portal
Mesh AP7
Mesh AP3
Mesh AP1
Mesh AP6
Wired DS
Ye Chen, Motorola

6. New Statistics? Looking at two error cases: Normal MDAOP
September 2007
New Statistics?
Looking at two error cases:
MDAOP
TxOP
Delayed Start Time
Delayed in Completion time
Normal MDAOP
Case 1: Stretched MDAOP
MDAOP
TxOP
Delayed Start Time
Delayed in Completion time
Case 2: Missed MDAOP
Ye Chen, Motorola

7. Results Delayed Start Time Percentage of Missed/Stretched MDAOPs
September 2007
Results
Delayed Start Time
Percentage of Missed/Stretched MDAOPs
Delay in Completion Time
Ye Chen, Motorola

8. Delayed Start Time (Portal AP2)
September 2007
Delayed Start Time (Portal AP2)
Each MDAOP contains 90 µs (10 slots) of extra time
If Start Delay > 90 µs, then it’s a stretched MDAOP
Reserved MDAOP = 340 µs
If Start Delay > 340 µs, then it’s a missed MDAOP
Even at low loads (e.g. 5 VoIP STA), the MDAOP is often stretched; issue for delay sensitive traffic
Ye Chen, Motorola

9. Missed/Stretched MDAOPs (AP2 Portal)
September 2007
Missed/Stretched MDAOPs (AP2 Portal)
A significant number of the TXOPs do not complete on time; issue for delay sensitive traffic
Ye Chen, Motorola

10. Delay in Completion Time (AP2 Portal)
September 2007
Delay in Completion Time (AP2 Portal)
Completion time can get pushed out due to:
Delays in start time
Hidden terminals
Retransmissions
Numbers shown are averages; size of MDAOP would need to be lengthened even more to keep losses low
Ye Chen, Motorola

11. Discussion Results suggest issues with current MDA specification
September 2007
Discussion
Results suggest issues with current MDA specification
Start times and completion times are frequently delayed
A significant percentage of MDAOPs are either stretched or missed
Factor preventing on-time completion include
STA traffic, congestion, hidden terminals, retransmissions
Changes recommended
Recommend modifying MDA channel access to accommodate and work around conditions that prevent on-time completion
Ye Chen, Motorola

12. Proposal Continue Negotiating MDAOP times with neighbors
September 2007
Proposal
Continue Negotiating MDAOP times with neighbors
Use existing MDAOD setup and advertisement signaling
Creation of schedule reduces collisions between mesh points that might be hidden from one another
Modify MDA contention mechanism
Restriction to complete transmissions within an MDAOP were in place to
Avoid misuse of the MDA contention parameters
Avoid fairness issues with nodes not deploying MDA
Avoid contending for channel during the neighbors’ MDAOP
Largest gain in performance due to scheduling, not specialized contention parameters
Revert to utilizing EDCA during and after MDAOP
Ye Chen, Motorola

13. Details of Proposed Changes
September 2007
Details of Proposed Changes
Modify 9.21 and specifically to
Transmit using EDCA channel access parameters associated with each frame instead of employing a special set of MDA parameters
Replace restriction that
(old) TXOP must complete within an MDOP, with
(new) if MDAOP stretching or a missed MDAOP occurs then channel access is suspended during any neighbor’s MDAOPs
Reuse EDCA retransmit procedures to recover from channel errors, subject to restriction above
Continue to use existing MDAOD setup and advertisement signaling with enhanced access scheme
Document 11-07/2536r0 describes suggested changes to the s draft 1.06 to address comments 1045, 1300, and 1538
Ye Chen, Motorola

14. September 2007
Questions?
Ye Chen, Motorola

15. September 2007
Backup
Ye Chen, Motorola

16. Current MDAOP Definition Modified MDAOP Definition
September 2007
Case Study
MDAOP Schedule
MDAOP1
MDAOP2
MDAOP3
Current MDAOP Definition
Dstart1
Dstart2
Channel busy
Tx 3
Tx2
Time
Tstart1
Tstart2
Channel error
Modified MDAOP Definition
Dstart1
Dstart2
Channel busy
Tx 1
Tx2
Tx 3
reTx
Time
Channel error
The MP with missed/stretched MDAOP can finish the MDA transmission if possible instead of waiting for the next MDAOP
The same contention parameters within/outside the MDAOP simplifies the transition in/out MDAOP (no sudden contention parameter changes)
Ye Chen, Motorola

17. CDF of Delayed Start time (AP2 Portal)
September 2007
CDF of Delayed Start time (AP2 Portal)
For 8 voice STAs, 5% of the frames have a delayed start time that exceeds 340 µs (missed MDAOP)
For 13 voice STAs, 20% of the frames have a delayed start time in excess of 340 µs. (missed MDAOP)
8 Voice STAs
13 Voice STAs
340 µs ms
Ye Chen, Motorola

18. CDF of Delayed Completion Time (AP2 Portal)
September 2007
CDF of Delayed Completion Time (AP2 Portal)
For 8 voice STAs, 10% of the frames have a completion time delay time that exceeds 400 us.
For 13 voice STAs, 10% of the frames have a completion time that exceeds 1.5 ms
The maximum delay in completion time for 8 voice STAs loading is about 2 ms. For 13 voice STAs loading it could be a long as 8 ms
8 Voice STAs
13 Voice STAs ms
These trends can impact delay sensitive traffic
Ye Chen, Motorola