1. Setting of DTIM Interval for MCCA
May 2011
doc.: IEEE /0687r0
May 2011
Setting of DTIM Interval for MCCA
Date:
Authors:
Michael Bahr, Siemens AG
Michael Bahr, Siemens AG

2. May 2011
doc.: IEEE /0687r0
May 2011
Abstract
The setting of the DTIM interval in MCCA to a limited set of 19 absolute values unnecessarily restricts the flexibility for many applications that would benefit greatly from MCCA, for instance, VoIP, industrial WLAN networks, or emerging usage scenarios.
The choice of the DTIM interval directly influences the possible beacon intervals and the possible times between two subsequent MCCAOPs of the same MCCAOP reservation. These are important parameters in applications requiring some determinism for the transmission as well as optimized duty cycles such as industrial communication.
This presentation describes a simple mechanism that allows the flexible and application specific setting of the DTIM interval in MCCA where the DTIM interval has the same properties as with the limited number of absolute values (same DTIM interval or multiples by power of 2). This will support many existing and emerging applications such as industrial WLAN networks.
Michael Bahr, Siemens AG
Michael Bahr, Siemens AG

3. Problems to Be Solved ensure interoperability with the use of MCCA
May 2011
Problems to Be Solved
ensure interoperability with the use of MCCA
enable a wide range of applications (existing, emerging, and future) to use MCCA by providing a flexible configuration of MCCAOP reservations
allow any time (in multiples of 32µs) between subsequent MCCAOPs of the same reservation
allow any beacon interval in order to optimize the duty cycle and to maximize the lifetime of battery-powered devices
enable an easy check of overlapping MCCA reservations
Michael Bahr, Siemens AG

4. DTIM Interval for MCCA Current setting (D11.0): Proposed setting:
May 2011
DTIM Interval for MCCA
Current setting (D11.0):
“Mesh STAs that use MCCA shall use a DTIM interval with a duration of 2^n * 100 TU with n a non-negative integer less than or equal to 18.”
This allows only 19 DTIM interval values leading to a limited number of beacon intervals and a limited number of distances between subsequent MCCAOPs of an MCCA reservation.
Proposed setting:
Mesh STA that establishes a new MBSS selects DTIM interval (Beacon Interval and DTIM Count) as required by application
Mesh STAs that become a member of this MBSS select a DTIM interval that is either the same or a multiple / quotient by a power of 2 of the discovered DTIM interval (that has been selected by the mesh STA that established this MBSS)
This achieves the goals of the current setting: DTIM intervals are of the form 2^n * m TU and do not shift with respect to each other and an easy check for overlapping MCCAOPs is possible
Michael Bahr, Siemens AG

5. DTIM Interval DTIM Interval = Beacon Interval * DTIM Period
May 2011
DTIM Interval
DTIM Interval = Beacon Interval * DTIM Period
setting the DTIM Interval defines the possible Beacon Intervals
setting the Beacon Interval defines the possible DTIM Intervals
time_d = DTIM Interval / MCCAOP Periodicity
time_d represents the time between two subsequent MCCAOPs of the same MCCAOP reservation
setting the DTIM Interval defines the possible times between two subsequent MCCAOPs of the same MCCAOP reservation (time_d)
setting the time_d defines the possible DTIM Interval – MCCAOP Periodicity combinations
Michael Bahr, Siemens AG

6. Industrial Wireless Communication
May 2011
Industrial Wireless Communication
very diverse applications and usage scenarios
production of gummi candy bears, cars, chemicals; sewage systems; process monitoring; environmental monitoring; factory automation, industrial automation; sensor networks, …
main requirements for communication
reliability, determinism, energy efficiency
to different degrees according to application or usage scenario
important configuration / design parameters
some reservation-based, timeslot-based superframe
beacon interval, time between periodic transmissions (time_d)
values have to be derived from the real-world application and can be very diverse
optimization of duty cycle for energy-efficiency
MCCA would be useful here for using WLAN mesh networks
requires flexibility in setting the DTIM interval
beacon interval and time_d are configured  DTIM interval follows from this
Note: An MBSS would use the same* DTIM interval for all its member mesh STAs since this improves network capacity and makes the handling of MCCAOP reservations easier. However, it should be possible to set this (same*) DTIM interval to any value.
* same means the same or a multiple so that each larger is a multiple of each smaller
Michael Bahr, Siemens AG

7. Beacon Interval DTIM Interval = Beacon Interval * DTIM Period
May 2011
Beacon Interval
DTIM Interval = Beacon Interval * DTIM Period
current setting: DTIM Interval = 2^n * 100 TU (0  n  18) only 43 beacon intervals possible out of (0.066%) only 29 beacon intervals > 100ms only 19 beacon intervals > 1s
1, 2, 4, 5, 8, 10, 16, 20, 25, 32, 40, 50, 64, 80, 100, 128, 160, 200, 256, 320, 400, 512, 640, 800, 1024, 1280, 1600, 2048, 2560, 3200, 4096, 5120, 6400, 8192, 10240, 12800, 16384, 20480, 25600, 32768, 40960, 51200, 65536
proposed setting:
allows to choose any beacon interval at the mesh STA establishing the MBSS. This defines the (set of) DTIM interval(s) to be used in the MBSS.
mesh STA becoming a member of the MBSS choose a beacon interval that is derived from the discovered DTIM interval
Michael Bahr, Siemens AG

8. time_d: time between subsequent MCCAOPs of the same MCCAOP reservation
May 2011
time_d: time between subsequent MCCAOPs of the same MCCAOP reservation
time_d = DTIM Interval / MCCAOP Periodicity
current setting: DTIM Interval = 2^n * 100 TU (0  n  18) only 2431 specific time_ds possible
proposed setting:
allows to choose any suitable set of time_ds that is possible with a (set of) DTIM intervals at the mesh STA establishing the MBSS. This defines the (set of) DTIM interval(s) to be used in the MBSS.
mesh STA becoming a member of the MBSS choose a DTIM interval that is derived from the discovered DTIM interval giving them the same set of possible time_ds
Michael Bahr, Siemens AG

9. Some reasons for more than 19 absolute DTIM values
May 2011
Some reasons for more than 19 absolute DTIM values
Mesh Density
every mesh STA beacons
in order to avoid network congestion by beacons, the beacon interval should be configured depending on the density of mesh STAs
in wireless mesh networks, beacon interval is usually 1s, up to 5s
in current DTIM interval setting, some of the possible beacon intervals (1024, 1280, 1600, 2048, 2560, 3200, 4096, 5120 TU) may require a large DTIM interval (the smallest DTIM interval for 1024 and 5120 is TU)
this limits the dynamics of the mesh network
Optimization of duty cycle by Beacon Interval and time_d (for instance, in sensor networks)
duty cycle has direct impact on battery lifetime  optimization
beacon important part of duty cycle
time_d between MCCAOPs also influences duty cycle
beacon interval and time_d should be set to optimal value wrt. application and energy efficiency
a difference of 10 s in the beacon interval or time_d might well mean a difference of a month in battery lifetime
maintenance cost issue in installations with many (sensor) nodes
regulations may require a guaranteed lifetime
Michael Bahr, Siemens AG

10. Proposed selection of DTIM interval for mesh BSS with MCCA (I)
May 2011
Proposed selection of DTIM interval for mesh BSS with MCCA (I)
at mesh STA 1, dot11MCCAActivated = true
mesh STA 1 does not find suitable candidate peer mesh STA for its mesh profile
establishes new mesh BSS
DTIM interval of its choice (very likely to be configured by some external entity)
DTIM = m
(e.g TU)
mesh
STA 1
Michael Bahr, Siemens AG

11. Proposed selection of DTIM interval for mesh BSS with MCCA (II)
May 2011
Proposed selection of DTIM interval for mesh BSS with MCCA (II)
at mesh STA 2, dot11MCCAActivated = true
mesh STA 2 does find mesh STA 1 as suitable candidate peer mesh STA for its mesh profile
mesh STA 2 will become a member of the mesh BSS
DTIM interval is set to either
the same as the DTIM interval of mesh STA 1 (e.g )
a multiple of the DTIM interval of mesh STA 1 with a factor of 2^n (e.g , 40000, …)
a fraction of the DTIM interval of mesh STA 1 with a divisor of 2^n (e.g. 5000, 2500, …)
DTIM = m
(e.g TU)
mesh
STA 1
DTIM = m / 2
(e.g TU)
mesh
STA 2
Michael Bahr, Siemens AG

12. Proposed selection of DTIM interval for mesh BSS with MCCA (III)
May 2011
Proposed selection of DTIM interval for mesh BSS with MCCA (III)
mesh
STA 3
DTIM = m * 4
(e.g TU)
at mesh STA 3, dot11MCCAActivated = true
mesh STA 3 does find mesh STA 1 as suitable candidate peer mesh STA for its mesh profile
mesh STA 3 will become a member of the mesh BSS
DTIM interval is set to either
the same as the DTIM interval of mesh STA 1 (e.g )
a multiple of the DTIM interval of mesh STA 1 with a factor of 2^n (e.g , 40000, …)
a fraction of the DTIM interval of mesh STA 1 with a divisor of 2^n (e.g. 5000, 2500, …)
DTIM = m
(e.g TU)
mesh
STA 1
mesh
STA 2
DTIM = m / 2
(e.g TU)
Michael Bahr, Siemens AG

13. Proposed selection of DTIM interval for mesh BSS with MCCA (IV)
May 2011
Proposed selection of DTIM interval for mesh BSS with MCCA (IV)
mesh
STA 3
DTIM = m * 4
(e.g TU)
at mesh STA 4, dot11MCCAActivated = true
mesh STA 4 does find mesh STA 2 as suitable candidate peer mesh STA for its mesh profile
mesh STA 4 will become a member of the mesh BSS
DTIM interval is set to either
the same as the DTIM interval of mesh STA 2 (e.g. 5000)
a multiple of the DTIM interval of mesh STA 2 with a factor of 2^n (e.g , 20000, …)
a fraction of the DTIM interval of mesh STA 2 with a divisor of 2^n (e.g. 2500, 1250, …)
DTIM = m
(e.g TU)
mesh
STA 1
mesh
STA 2
DTIM = m / 2
(e.g TU)
mesh
STA 4
DTIM = m * 4
(e.g TU)
Michael Bahr, Siemens AG

14. Flexibility of usage of MCCA
May 2011
Flexibility of usage of MCCA
The property „that the starting times of the reservations do not change relative to each other between consecutive DTIM intervals“ has to be retained.
Property is achieved by having DTIM intervals in the MBSS of the from m * k^n with m being the smallest DTIM interval, k the same natural number (suggestion is to fix k to 2), n a non-negative number.
The proposed DTIM interval setting allows a more flexible setting of the DTIM interval with * DTIM Period possible values compared to 19 possible values with the current setting.
IEEE is dis- covering new application areas:
smart metering IEEE ah
low power IEEE chipsets
proposed specification
all mesh STA with MCCA in an MBSS have arbitrary DTIM intervals that are the same or multiples by 2^n
current specification
fixed set of 19 DTIM values
Michael Bahr, Siemens AG

15. Necessary text changes (D10.0)
May 2011
Necessary text changes (D10.0)
Change „Mesh STAs that use MCCA shall use a DTIM interval with a duration of 2n x 100 TU with n being a non-negative integer less than or equal to 18.“ in clause 9.9a.3.1 General (page 115, lines 29-32) to „Mesh STAs that use MCCA shall use a common DTIM interval or DTIM intervals that are multiples where the factors are a power of 2.“
Change „NOTE—The DTIM interval of the form 2n x 100 TU has been chosen to ensure that the starting times of the reservations do not change relative to each other between consecutive DTIM intervals. The restriction that n be less than or equal to 18 has been chosen to be compatible with the maximum DTIM interval and so that the range of the MCCAOP Offset in the reservation, see (MCCAOP Reservation field), is compatible with the maximal DTIM interval length.“ in clause 9.9a.3.1 General (page 115, lines 34-38) to „ NOTE—The DTIM interval of this form has been chosen to ensure that the starting times of the reservations do not change relative to each other between consecutive DTIM intervals.“
remove „When dot11MCCAActivated is true, the mesh STA shall choose a DTIM interval with a duration of 2n x 100 TU with n a non-negative integer less than or equal to 18.“ in clause 11C TBTT selection (page 281, lines 21-23)
insert text from next slide in clause 11C C.2.8 Establishing or becoming a member of a mesh BSS (based on 11-11/521r4), complete instructions are provided in 11-11/0560r0.
Michael Bahr, Siemens AG

16. Necessary text changes in Mesh Discovery
May 2011
Necessary text changes in Mesh Discovery
Michael Bahr, Siemens AG

17. May 2011
Conclusion
current setting of DTIM interval value (19 absolute values only) has some limitation to some application areas of WLAN mesh network
industrial communication, other emerging and future application areas
choice of DTIM interval defines the possible beacon intervals and times time_d between MCCAOPs of the same MCCAOP reservation (both with limited set of possible values)
some applications need to configure beacon interval and time_d and want to set DTIM interval in the MBSS accordingly (DTIM interval of form m*2^n)
Proposed DTIM interval setting supports the flexibility of costumized and usage specific DTIM and MCCA settings while still maintaining the property that the starting times of the reservations do not change relative to each other between consecutive DTIM intervals
mesh station with MCCA enabled that establishes new mesh BSS sets DTIM value with no restrictions according to the needs of the application / usage scenario
mesh stations with MCCA enabled that become a member of the mesh BSS set their DTIM value according to the requirement based on the DTIM interval of the chosen candidate peer mesh STA
provides enough flexibility to support a wide set of applications and usage scenarios as well as emerging and future applications and ensures the preferred characteristics of the DTIM interval in MCCA (same or multiple with factor 2^n)
Michael Bahr, Siemens AG

18. May 2011
Motion
Move to adopt the text changes provided in 11-11/0560ryyy as resolution to CIDs xxx with resolution status „Principle“ and resolution detail „Text provided in submission 11-11/0560ryyy“
Michael Bahr, Siemens AG

19. References IEEE 802.11s Draft Standard, D11.0
May 2011
References
IEEE s Draft Standard, D11.0
11-11/0xxxry Comments of 4th Recirculation Sponsor Ballot on IEEE s D11.0
11-11/554r2 „Setting of DTIM Interval for MCCA”, April 2011
11-11/0xxxry “text for changes”, May 2011
Michael Bahr, Siemens AG