1. VTS Robust Multicast/Broadcast Protocol
February 2008
doc.: IEEE /2376r0
May 2008
VTS Robust Multicast/Broadcast Protocol
Date:
Authors:
L.Chu Etc.
Liwen Chu, STMicroelectronics

2. February 2008
doc.: IEEE /2376r0
May 2008
Abstract
Video requires robust broadcast/multicast frame transmissions to guarantee QOS. Currently, it is difficult to provide robust broadcast/multicast frame transmissions in Some efforts [1--3] have been made to include a reservation-based medium access method in the n and s drafts. But to provide robust reserved medium access in broadcast/multicast transmission, a number of issues should be considered: different beacon intervals among OBSS; simplified negotiation protocol; etc. This contribution proposes the methods to provide robust reservation-based broadcast/multicast transmission. The proposed methods do not need any physical layer change.
L.Chu Etc.
Liwen Chu, STMicroelectronics

3. Reasons why Broadcast/Multicast is Unstable:
May 2008
Reasons why Broadcast/Multicast is Unstable:
Intra BSS collision:
Associated STAs transmit frames when an AP is transmitting broadcast/multicast frames.
Inter BSS collision (OBSS collision):
OBSS STAs/APs transmit frames when an AP is transmitting broadcast/multicast frames.
No feedback to indicate the correct reception of the broadcast/multicast frames
Methods to Broadcast/Multicast more Robust:
Decreasing the broadcast/multicast collision dramatically
Adding feedback to broadcast/multicast frames AP
Collision!
AP1
STA4
AP2
Collision!
STA1
STA3
STA1
STA3
STA6
STA2
STA2
STA5
Collision!
Inner BSS collision
Inter BSS (OBSS) collision
L.Chu Etc.

4. Reserved Transmission Time in 802.11
May 2008
Reserved Transmission Time in
HCCA
Reserved transmission time
Superframe [1]
PSMP (power save multi-poll) [2]
MDA (Mesh Deterministic Access) [3]
VTS (video transmission study group) has the opportunity to specify a simpler method than HCCA which would only support high priority (reserved periodic) downlink broadcast/multicast.
L.Chu Etc.

5. Reserved Broadcast/Multicast TXOP IE
May 2008
Reserved Broadcast/Multicast TXOP IE
A reserved broadcast/multicast TXOP IE in the beacon includes:
Active indication,
True if and only if TXOP is currently enabled,
Activation start time,
If “Active indication” (above) is false, this field indicates when the TXOP will be enabled. (Several beacons with “active indication” false may be needed to notify all the STAs in the BSS of the imminent TXOP.)
Destination broadcast/multicast address,
Service interval,
Offset to service interval,
Duration.
TBTT = Target Beacon Transmission Time
Reserved Broadcast/Multicast Tx
TBTT
Service Interval
Beacon Interval
Duration
Offset
L.Chu Etc.

6. Reserved Broadcast/Multicast Transmission
May 2008
Reserved Broadcast/Multicast Transmission
An AP reserves broadcast/multicast TXOP according to service requirements from the upper layers of the MAC.
How to schedule the broadcast/multicast TXOP is up to the implementation.
An AP broadcasts reserved broadcast/multicast TXOP in a beacon and/or in a broadcast/multicast TXOP reservation action frame.
The AP should be certain that all STAs associated with it receive the reserved broadcast/multicast TXOPs.
This means that a DTIM beacon should be used to broadcast the broadcast/multicast TXOP reservation if some STAs are in power saving mode.
TBTT
TBTT
TBTT
TBTT
TBTT
TBTT
TBTT
TBTT
TBTT
DTIM Beacon Interval
L.Chu Etc.

7. Intra BSS Broadcast/Multicast Collision Avoidance
May 2008
Intra BSS Broadcast/Multicast Collision Avoidance
An AP reserves periodical transmission time for its broadcast/multicast.
Only downlink reservation is required.
Non-AP STAs use unicast for their uplink transmissions.
An AP notifies the broadcast/multicast transmission time through management frames (i.e. beacon and/or a new action frame).
STAs associated with an AP set their NAVs to stop frame transmission during the broadcast/multicast transmission time.
L.Chu Etc.

8. February 2008
doc.: IEEE /2376r0
May 2008
Inter BSS Broadcast/Multicast Collision Avoidance: Reserved TXOP Selection
Two OBSS exchange broadcast/multicast TXOP information possibly through STA’s relayed action management frame.
Each AP records the difference of TSF values between it and its OBSS in order to calculate the overlapping reserved TXOP between itself and its OBSS.
When selecting new reserved broadcast/multicast TXOP, the interfered broadcast/multicast TXOP by its OBSS should not be used.
During the reserved TXOP, EDCA is used to contend the for medium access right.
Original reserved TXOP
New reserved TXOP
Duration
Offset
Service Interval
Service Interval
Service Interval
TBTT
Beacon Interval
TBTT
Duration
Offset
Service Interval
Service Interval
Beacon Interval
TBTT
TBTT
L.Chu Etc.
Liwen Chu, STMicroelectronics

9. May 2008
Inter BSS Broadcast/Multicast Collision Avoidance: Simultaneous Reservation
Two OBSS may reserve overlapping TXOP at almost the same time.
A random priority number is selected for each reserved TXOP. The OBSS will release overlapping TXOP with low priority and reselect a new TXOP.
Original reserved TXOP
New reserved TXOP
Duration
Service Interval
Service Interval
Offset
Service Interval
Beacon Interval
TBTT
TBTT
Duration
Service Interval
Offset
Beacon Interval
TBTT
TBTT
L.Chu Etc.

10. May 2008
Inter BSS Broadcast/Multicast Collision Avoidance: Medium Access in OBSS’s Reserved TXOP
If an AP receives the reserved TXOP IE from its OBSS AP directly,
The AP should not receive frames.
But the AP can transmit broadcast/multicast frames.
If an AP receives reserved TXOP IE from its OBSS STA directly,
The AP should neither transmit nor acknowledge received frames.
If a STA receives reserved TXOP IE from its OBSS AP directly,
The STA should not receive frames.
The STA’s associated AP should not broadcast frames during these TXOPs,
The STA’s associated AP should not multicast frames if the multicast address includes the STA.
If a STA receives reserved TXOP IE from its OBSS STA,
The STA should neither transmit nor acknowledge received frames.
The STA’s associated AP can broadcast/multicast frames during this TXOP.
If a STA always detects collided broadcast/multicast frames from its associated AP,
the STA shall notify the AP using an action frame,
the AP may re-allocate a new reserved TXOP to avoid broadcast/multicast collisions.
L.Chu Etc.

11. May 2008
Inter BSS Broadcast/Multicast Collision Avoidance: Different Beacon Interval Issue
Overlapping BSS’s may use different beacon intervals.
If this is the case, each AP’s beacon interval is used to select the service interval, offset, duration of reserved TXOP,
after some beacon intervals, two reserved TXOP may overlap with each other since OBSS’s TBTTs slide at different speeds.
So each AP’s beacon interval should not be used to select the reserved TXOP.
Original reserved TXOP
New reserved TXOP
Duration
Offset
Service Interval
Service Interval
Service Interval
Beacon Interval
TBTT
Beacon Interval
TBTT
Duration
Service Interval
Offset
TBTT
Beacon Interval
TBTT
L.Chu Etc.

12. May 2008
Inter BSS Broadcast/Multicast Collision Avoidance: Different Beacon Interval Issue (Cont’d)
The default beacon interval is used to select service interval, offset, duration of the reserved TXOP. The difference between TBTT and TDBTT should also be included in the reserved broadcast/multicast TXOP IE.
When a reserved service interval covers its own TBTT, the AP may extend the reserved service interval to compensate the beacon transmit time.
Original reserved TXOP
New reserved TXOP
TDBTT = Target Default Beacon Transmit Time
Duration
Service Interval
Service Interval
Offset
Service Interval
Beacon Interval
TDBTT
TDBTT
Duration
Service Interval
Offset
TDBTT
Beacon Interval
TDBTT
L.Chu Etc.

13. May 2008
Conclusions
A reservation-based broadcast/multicast TXOP method is proposed here.
EDCA is the medium access algorithm when these methods are used.
No physical layer change is required.
Inter-OBSS interference avoiding methods are proposed.
Robust broadcast/multicast transmission can be provided.
L.Chu Etc.

14. May 2008
Backup Slides
L.Chu Etc.

15. May 2008
Inter BSS Interference Analysis: STA Receiving Reserved TXOP IE from OBSS’s AP
STA4 and AP1 interfere with each other.
In the case that STA4 receives the reserved broadcast/multicast TXOP through management frames from its OBSS (for AP1), it is desirable that during the reserved time,
STA4 should not receive management frames, data frames and control frames (ACK etc) from its BSS, which also means
STA4 can not transmit unicast frames requiring ACK,
STA4 can transmit non-ACK frames,
AP2 should not broadcast frames,
AP2 should not multicast frames if the multicast address include STA4.
STA4 should relay the received AP1 broadcast/multicast reserved TXOP from AP1 to AP2 with an action frame.
STA2
AP2
AP1
STA4
STA6
STA1
STA5
L.Chu Etc.

16. May 2008
Inter BSS Interference Analysis: AP Receiving Reserved TXOP IE from OBSS’s STA
AP2 and STA3 interfere with each other.
In the case that AP2 receives the reserved broadcast/multicast TXOP through management frames from its OBSS (for STA3), it is desirable that during the reserved time,
AP2 should not transmit management frames, data frames, control frames (ACK etc.), which also means
AP2 can not acknowledge received unicast frames,
AP2 can receive non-ACK frames.
STA3 should forward the received AP1 broadcast/multicast reserved TXOP from AP1 to AP2.
AP1
AP2
STA3
STA1
STA6
STA2
STA5
L.Chu Etc.

17. May 2008
Inter BSS Interference Analysis: AP Receiving Reserved TXOP IE from OBSS’s AP
AP2 and AP1 interfere with each other.
In the case that AP2 receives the reserved broadcast/multicast transmission time through management frames from its OBSS (for AP1) , it is desirable that during the reserved time,
AP2 should not receive management frames, data frames, control frames (ACK etc) from its associated STAs, which also means
AP2 can transmit broadcast/multicast frames,
AP2 can transmit non-ACK unicast frames,
AP2 can not transmit unicast frames requiring ACK.
To fully protect video transmission, STA4 should relay the received periodic AP1 broadcast/multicast reserved TXOP from AP1 to AP2.
STA3
AP1
AP2
STA1
STA6
STA2
STA5
L.Chu Etc.

18. May 2008
Inter BSS Interference Analysis: STA Receiving Reserved TXOP IE from OBSS’s STA
STA4 and STA3 interfere with each other.
In the case that STA4 receives the reserved broadcast/multicast TXOP through management frames from its overlapping BS (for STA3), it is desirable that during the reserved time,
STA4 should not transmit management frames, data frames and control frames (ACK etc) frames, which also means
STA4 can not acknowledge received unicast frames,
STA4 can receive unicast no-ACK frames,
AP2 can transmit broadcast/multicast frames, and STA4 can receive broadcast/multicast frames.
STA3 should relay the received AP1 broadcast/multicast reserved TXOP from AP1 to STA4. And STA4 should relay the received AP1 broadcast/multicast reserved TXOP from STA3 to AP2.
AP1
STA4
AP2
STA3
STA1
STA6
STA2
STA5
L.Chu Etc.

19. May 2008
Reference
[1] n-stmicro-mac-partial-proposal-presentation, STMicroelectronics MAC Proposal for n CFP
[2] IEEE P802.11nTM/D3.00 Draft STANDARD for Information Technology- Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Amendment 4: Enhancements for Higher Throughput
[3] IEEE P802.11sTM/D1.08 Draft STANDARD for Information Technology- Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Amendment : Mesh Network
L.Chu Etc.