1. Multicast/Broadcast Communication With Acknowledge
February 2008
doc.: IEEE /2376r0
November 2008
Multicast/Broadcast Communication With Acknowledge
Date:
Authors:
Liwen Chu Etc.
Liwen Chu, STMicroelectronics

2. February 2008
doc.: IEEE /2376r0
November 2008
Abstract
Audio video requires reliable transmissions and deterministic delay.
Currently, it is difficult to provide reliable audio video transmissions in because of the poor support of broadcast/multicast transmission, and because EDCA cannot provide a deterministic delay guarantee.
This contribution proposes the use of blockAck based methods to provide reliable audio video transmission. The proposed methods do not require any physical layer modifications.
Liwen Chu Etc.
Liwen Chu, STMicroelectronics

3. MAC Enhancements in VTS PAR
November 2008
MAC Enhancements in VTS PAR
Graceful degradation of audio video streams when there is insufficient channel capacity, by enabling packet discarding without any requirement for deep packet inspection,
Increased robustness in OBSS environments, without the requirement for a centralised management entity,
Intra-Access Category prioritization of transport streams by modifying EDCA timing and parameter selection without any requirement for deep packet inspection,
Improved link reliability and low jitter characteristics for multicast/broadcast audio video streams,
Interworking with relevant 802.1AVB mechanisms (802.1Qat, 802.1Qav, 802.1AS)
Liwen Chu Etc.

4. Reasons why Broadcast/Multicast is Unreliable
November 2008
Reasons why Broadcast/Multicast is Unreliable
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 AP
Collision!
AP2
Collision!
AP1
STA4
STA1
STA3
STA1
STA2
STA3
STA6
STA2
STA5
Collision!
Inter BSS collision
Intra BSS (OBSS) collision
Liwen Chu Etc.

5. Methods to make Broadcast/Multicast Reliable
November 2008
Methods to make Broadcast/Multicast Reliable
Decrease the probability of a broadcast/multicast collision.
Add feedback to broadcast/multicast frames.
Liwen Chu Etc.

6. Feedback for a Single Multicast/broadcast Frame
November 2008
Feedback for a Single Multicast/broadcast Frame
An AP selects a single STA to feedback an ACK to the multicast/broadcast frame.
It is the most efficient, but least reliable solution.
An AP selects many STAs to feedback ACK frames and specifies the sequence for the replies (start offset, duration).
It is a less efficient solution, since more than one ACK will respond to a multicast/broadcast frame, but is more reliable.
Liwen Chu Etc.

7. Feedback for Multiple (BlockAck) Multicast/broadcast Frames
November 2008
Feedback for Multiple (BlockAck) Multicast/broadcast Frames
An AP must maintain a multicast/broadcast blockAck (MBBA) group (the AP and all STAs of the multicast/broadcast blockAck destinations are defined as a MBBA group).
STA joining procedure
STA leaving procedure
MBBA group teardown procedure
An AP must select STAs to reply with blockAcks using blockAck request.
An AP allocates the start offset and duration of the blockAck replies.
Liwen Chu Etc.

8. Multicast/broadcast Joining/Leaving Procedure
November 2008
Multicast/broadcast Joining/Leaving Procedure
A STA with the capability to join an MBBA group sends an Add multicast/broadcast blockAck (ADDMBBA) Request frame to the AP.
After receiving an ADDMBBA Request frame, an AP replies with an ADDMBBA Response frame to the STA with the capability to join an MBBA group in order to complete the joining.
If a STA wants to leave an MBBA group, it uses a Leave multicast/broadcast blockAck (LVMBBA) frame to tell an AP that it will leave the MBBA group.
Liwen Chu Etc.

9. MBBA Group Teardown Procedure
November 2008
MBBA Group Teardown Procedure
If an AP wants to tear down the group, it sends via unicast/multicast/broadcast a Leave multicast/broadcast blockAck (LVMBBA) frame to each STA in the MBBA group.
If all STAs have left a MBBA group, the AP tears down the MBBA group.
Liwen Chu Etc.

10. Multicast/Broadcast BlockAck Procedure
November 2008
Multicast/Broadcast BlockAck Procedure
Multicast/broadcast BlockAck only support the immediate blockAck policy.
An AP uses MBBA Request (MBBAReq) frames to request all or part of destination STAs to respond MBBA Ack frames
MBBAReq can be defined as following:
BlockAckReq with multicast/broadcast receiving address is MBBAReq.
BlockAckReq uses Bit 11 in BAR Control field to indicate weather this BlockAckReq is MBBAReq or normal (unicast) BlockAckReq.
A new control frame is allocated for MBBAReq.
MBBA Request frame indicates when a STA will reply a MBBA Ack frame
The following fields are required in MBBAReq in addition to the fields in BlockAckReq frame:
AID (STA identifier)
MBBA Ack start offset
The start offset is the offset relative to the start of MBBA Req frame
MBBA Ack duration
The duration is used for MBBA Ack transmission
Liwen Chu Etc.

11. Multicast/Broadcast BlockAck Example
November 2008
Multicast/Broadcast BlockAck Example
AP1, STA1, STA2, STA3 are in a MBBA group.
AP1
STA1
STA3
STA2
STA4
Multicast Data Frames
Backoff
MBBA
Req
AP1
MBBA
Ack
STA1
MBBA
Ack
STA2
MBBA
Ack
STA3
MBBA TXOP
Liwen Chu Etc.

12. Multicast/Broadcast BlockAck Retransmission Procedure
November 2008
Multicast/Broadcast BlockAck Retransmission Procedure
When many of the indicated STAs do not receive the multicast/broadcast frames correctly, an AP may use the MBBA mechanism to retransmit the multicast/broadcast frames.
A STA which receives the multicast/broadcast frames more than once will discard the repeated frames, but must reply with an MBBA Ack frame if requested by the AP.
When a few of the indicated STAs do not receive the multicast/broadcast frames correctly, an AP may use the normal blockAck procedure to retransmit the frames to the STA.
When only a few frames need to be retransmitted, an AP may send frames with normal ack policy.
Liwen Chu Etc.

13. Multicast/Broadcast BlockAck Retransmission Example
November 2008
Multicast/Broadcast BlockAck Retransmission Example
AP1, STA1, STA2, STA3 are in a MBBA group.
AP1
STA1
STA3
STA2
STA4
BA: BlockAck
Backoff
Backoff
MBBA
Req
AP1 BA
Req
Multicast Data Frames
MBBA
Ack
Multicast Data Frames
STA1
MBBA
Ack
STA2
MBBA
Ack
corrupted
STA3 BA
MBBA TXOP
Normal BlockAck for Retransmission
Liwen Chu Etc.

14. Possible MBBA Collision
November 2008
Possible MBBA Collision
Non protected MBBA TXOP may have a high chance to collide with other transmission.
BlockAck recommend to use RTS/CTS, HCCA etc. protection mechanism to protect the blockAck transmission.
STA7
Collision!
AP2
STA4
STA6
STA5
AP1
STA1
STA3
STA2
Backoff
Backoff
AP1
MBBA
Req BA
Req
Multicast Data Frames
MBBA
Ack
Multicast Data Frames
STA1
MBBA
Ack
STA2
Corrupted
MBBA Req
Corrupted
frames
STA3 BA
MBBA TXOP
Normal BlockAck for Retransmission
Backoff
Backoff
STA5
Liwen Chu Etc.

15. MBBA TXOP Protection Methods
November 2008
MBBA TXOP Protection Methods
Before the MBBA txop, an AP can select more than one MBBA group member and use MBBA Req/MBBA Ack to protect the MBBA TXOP.
MBBA TXOP
Backoff
MBBA
Req
MBBA
Req
AP1
MBBA
Ack
Multicast Data Frames
MBBA
Ack
STA1
MBBA
Ack
MBBA
Ack
STA2
MBBA
Ack
STA3
Protection Stage
Broadcast/Multicast Tx Stage
AP2
STA4
STA6
STA5
AP1
STA1
STA2
STA3
AP3
STA7
STA8
STA9
Liwen Chu Etc.

16. MBBA TXOP Protection Methods (Cont’d)
November 2008
MBBA TXOP Protection Methods (Cont’d)
Before the MBBA txop, an AP can select a MBBA group member and use RTS/CTS to protect the MBBA TXOP.
MBBA TXOP
Backoff
MBBA
Req
AP1
RTS
Multicast Data Frames
MBBA
Ack
STA1
MBBA
Ack
STA2
CTS
MBBA
Ack
STA3
Protection Stage
Broadcast/Multicast Tx Stage
AP2
STA4
STA6
STA5
AP1
STA1
STA2
STA3
Liwen Chu Etc.

17. MBBA TXOP Protection Methods (Cont’d)
November 2008
MBBA TXOP Protection Methods (Cont’d)
An AP selects one or more STAs that transmit protection frames in the protection stage to neighboring OBSS.
An k neighbor report or other method may be used to select STAs to transmit protection frames.
The selected STAs should cover all neighboring OBSS in order to minimize collisions.
Liwen Chu Etc.

18. PSMP TXOP Protection Methods
November 2008
PSMP TXOP Protection Methods
PSMP can be used as reliable multicast/broadcast communication.
But PSMP need protection before PSMP transmission to avoid OBSS collision.
Method 1: before the PSMP TXOP, an AP can select more than one multicast group member and use PSMP/Null frame (other protection frames may be used instead) to protect the PSMP TXOP.
No downlink data frames are transmitted.
The uplink duration only allows a null frame to be transmitted.
If there are buffered frames to the AP, the STA will respond with a null frame to request more uplink TXOP.
If there is no buffered frame, the STA will respond with a null frame to indicate no buffered frames.
Method 2: before the PSMP TXOP, an AP can select a multicast group member and use RTS/CTS (protection frame) to protect the PSMP TXOP.
Liwen Chu Etc.

19. PSMP TXOP Protection Methods (Cont’d)
November 2008
PSMP TXOP Protection Methods (Cont’d)
An AP selects one or more STAs that transmit protection frames in the protection stage to neighboring OBSS.
An k neighbor report or other method may be used to select STAs to transmit protection frames.
The selected STAs should cover all neighboring OBSS in order to minimize collisions.
Liwen Chu Etc.

20. PSMP TXOP Protection Methods (Cont’d)
November 2008
PSMP TXOP Protection Methods (Cont’d)
MBBA TXOP
Backoff
AP1
PSMP
PSMP
Null
frame
Multicast Data Frames
STA1
MTBA
STA1
Null
frame
STA2
MTBA
STA2
STA3
MTBA
STA3
Protection Stage
Broadcast/Multicast Tx Stage
AP2
STA4
STA6
STA5
AP1
STA1
STA2
STA3
AP3
STA7
STA8
STA9
Liwen Chu Etc.

21. PSMP TXOP Protection Methods (Cont’d)
November 2008
PSMP TXOP Protection Methods (Cont’d)
MBBA TXOP
Backoff
AP1
RTS
PSMP
Multicast Data Frames
STA1
MTBA
STA1
STA2
MTBA
STA2
CTS
STA3
MTBA
STA3
Protection Stage
Broadcast/Multicast Tx Stage
AP2
STA4
STA6
STA5
AP1
STA1
STA2
STA3
Liwen Chu Etc.

22. Multiple BlockAckReq/BlockAck (Leader) Based TXOP Protection Methods
November 2008
Multiple BlockAckReq/BlockAck (Leader) Based TXOP Protection Methods
Multiple BlockAckReq/Blockack in a TXOP can be used as reliable multicast/broadcast communication.
But multiple BlockAck TXOP need protection before PSMP transmission to avoid OBSS collision.
Method 1: before the Multiple BlockAck TXOP, the AP can select more than one multicast group member and use multiple RTS/CTS or modified RTS (multicast RTS)/CTS (other protection frames may be used instead) to protect the PSMP TXOP.
After receiving multicast RTS, multicast destinations return CTS based on the sequence indicated by the AP.
Method 2: before the Multiple BlockAck TXOP, the AP can select a multicast group member and use RTS/CTS (protection frame) to protect the Multiple BlockAck TXOP.
Liwen Chu Etc.

23. November 2008
Multiple BlockAckReq/BlockAck (Leader) Based TXOP Protection Methods (Cont’d)
An AP selects one or more STAs that transmit protection frames in the protection stage to neighboring OBSS.
An k neighbor report or other method may be used to select STAs to transmit protection frames.
The selected STAs should cover all neighboring OBSS in order to minimize collisions.
Liwen Chu Etc.

24. November 2008
Multiple BlockAckReq/BlockAck (Leader) Based TXOP Protection Methods (Cont’d)
MBBA TXOP
Backoff BA
Req BA
Req BA
Req
AP1
RTS
RTS
Multicast Data Frames
STA1
CTS BA
STA2
CTS BA
STA3 BA
Protection Stage
Broadcast/Multicast Tx Stage
Backoff
AP1
MRTS
AP2
STA4
STA6
STA5
AP1
STA1
STA2
STA3
AP3
STA7
STA8
STA9
STA1
CTS
STA2
CTS
STA3
Protection Stage
Liwen Chu Etc.

25. November 2008
Multiple BlockAckReq/BlockAck (Leader) Based TXOP Protection Methods (Cont’d)
MBBA TXOP
Backoff BA
Req BA
Req BA
Req
AP1
RTS
Multicast Data Frames
STA1 BA
STA2
CTS BA
STA3 BA
Protection Stage
Broadcast/Multicast Tx Stage
AP2
STA4
STA6
STA5
AP1
STA1
STA2
STA3
Liwen Chu Etc.

26. Protection Methods for Reserved Multicast TXOP
November 2008
Protection Methods for Reserved Multicast TXOP
An AP can reserve TXOP for video communication (HCCA etc.).
Before the multicast video transmission in a reserved TXOP, STAs selected by the AP can transmit CTS2Self to protect the video transmission.
An AP selects one or more STAs that transmit protection frames in the protection stage to neighboring OBSS.
An k neighbor report or other method may be used to select STAs to transmit protection frames.
The selected STAs should cover all neighboring OBSS in order to minimize collisions.
Liwen Chu Etc.

27. Multicast/Broadcast Sequence Number an BlockAck Buffer Release
November 2008
Multicast/Broadcast Sequence Number an BlockAck Buffer Release
A QoS AP assigns sequence number using a single modulo-4096 counter for multicast/Broadcast frames, management frames, non-QoS frames.
A recipient can not distinguish a lost multicast/broadcast data frame from a management frame, non-QoS frame or a frame of other group.
A transmitter may not be able to transmit a block of frames with the total number being equal to the negotiated buffer size for a group address.
A STA may have a unicast blockAck agreement and multicast/broadcast blockAck agreement for the same TID at the same time.
The reorder buffer of a multicast/broadcast blockAck should not be identified by TA/TID.
The blockAck buffer may not be released in time.
STA3
STA4 AP
STA1
STA2
Liwen Chu Etc.

28. Fast Multicast/Broadcast Buffer Release
November 2008
Fast Multicast/Broadcast Buffer Release
The reorder buffer of a multicast/broadcast blockAck should be identified by group address(RA)/TID.
An AP may send a BlockAckReq/MBBA/MTBA to shift the recipient’s starting sequence number past the hole in the SN space that is created by management frame, non-QoS frame, and/or frames of other group.
This can release multicast/broadcast buffer right away and allow the earliest possible passing of buffered frames up to the next MAC process.
This can guarantee that the transmitter and the receivers have the same start sequence number of the transmit buffer and the receiver buffer.
This can avoid the receiver wrongly discard multicast frames.
Liwen Chu Etc.

29. November 2008
Backup Slides
Liwen Chu Etc.

30. Delayed Multicast/Broadcast Buffer Release
November 2008
Delayed Multicast/Broadcast Buffer Release
The transmitting buffer size and receiving size of blockAck buffers are all 4.
STA1 and STA2 are one multicast group.
STA3 and STA4 are another multicast group. x x 3
STA4 3 x x 3
STA4
STA3
STA3 3 AP 1 2 3 4 AP 1 2 x 4
STA1 1 2 4
STA1
STA2 1 2 x 4
STA2 1 2 4
Liwen Chu Etc.

31. Delayed Multicast/Broadcast Buffer Release
November 2008
Delayed Multicast/Broadcast Buffer Release
The transmitting buffer size and receiving size of blockAck buffers are all 4.
STA1 and STA2 are one multicast group.
STA3 and STA4 are another multicast group. x x 3
STA4 3 x x 3
STA3
STA4
STA3 3 BA
Req AP 1 2 3 4 AP 1 2 x 4
STA1 1 2 4 BA
STA1
STA2 1 2 x 4
STA2 1 2 4
Liwen Chu Etc.

32. Delayed Multicast/Broadcast Buffer Release
November 2008
Delayed Multicast/Broadcast Buffer Release
The transmitting buffer size and receiving size of blockAck buffers are all 4.
STA1 and STA2 are one multicast group.
STA3 and STA4 are another multicast group. x x 3
STA1 does not sends frame4 to up layer since it does not know that frame3 is for another group!
STA4 3 x x 3
STA3
STA3
STA4 3 BA
Req AP 1 2 3 4 AP x 4
STA1 1 2 4 BA
STA1
STA2 1 2 x 4
STA2 1 2 4
Liwen Chu Etc.

33. Delayed Multicast/Broadcast Buffer Release
November 2008
Delayed Multicast/Broadcast Buffer Release
The transmitting buffer size and receiving size of blockAck buffers are all 4.
STA1 and STA2 are one multicast group.
STA3 and STA4 are another multicast group.
STA1 and STA2 do not sends frame4 to up layer since they do not know that
frame3 is for another group! x x 3
AP’s start sequence number for STA1/STA2 transmitting buffer is 5.
The starting sequence number in STA1’s and STA2’s receiving buffers are 3.
STA4 3 x x 3
STA3
STA3
STA4 3 BA
Req BA
Req AP 1 2 3 4 AP x 4
STA1 1 2 4 BA
STA1
STA2 x 4
STA2 1 2 4 BA
Liwen Chu Etc.

34. Delayed Fast Multicast/Broadcast Buffer Release
November 2008
Delayed Fast Multicast/Broadcast Buffer Release
The transmitting buffer size and receiving size of blockAck buffers are all 4.
STA1 and STA2 are one multicast group.
STA3 and STA4 are another multicast group. x x 3
AP’s start sequence number for STA1/STA2 transmitting buffer is 5.
The starting sequence number in STA1’s and STA2’s receiving buffers are 3.
STA4 3 x x 3
STA4
STA3
STA3 3 BA
Req BA
Req AP 1 2 3 4 5 AP x 4 5
STA1 1 2 4 BA 5
STA1
STA2 x 4 5
STA2 1 2 4 BA 5
Liwen Chu Etc.

35. Delayed Multicast/Broadcast Buffer Release
November 2008
Delayed Multicast/Broadcast Buffer Release
The transmitting buffer size and receiving size of blockAck buffers are all 4.
STA1 and STA2 are one multicast group.
STA3 and STA4 are another multicast group. x x 3
AP’s start sequence number for STA1/STA2 transmitting buffer is 5.
The starting sequence number in STA1’s and STA2’s receiving buffers are 3.
STA4 3 x x 3
STA3
STA4
STA3 3 BA
Req BA
Req AP 1 2 3 4 5 6 AP x 4 5 6
STA1 1 2 4 BA 5 6
STA1
STA2 x 4 5 6
STA2 1 2 4 BA 5 6
Liwen Chu Etc.

36. Delayed Multicast/Broadcast Buffer Release
November 2008
Delayed Multicast/Broadcast Buffer Release
The transmitting buffer size and receiving size of blockAck buffers are all 4.
STA1 and STA2 are one multicast group.
STA3 and STA4 are another multicast group. x x 3
AP’s start sequence number for STA1/STA2 transmitting buffer is 5.
The starting sequence number in STA1’s and STA2’s receiving buffers are 3.
STA4 3 x x 3
STA3
STA3
STA4 3 BA
Req BA
Req AP 1 2 3 4 5 6 7 AP x 4 5 6
Accepting it and adjusting starting SN
Or discarding it?
STA1 1 2 4 BA 5 6
STA1
STA2 x 4 5 6
Accepting it and adjusting starting SN
Or discarding it?
STA2 1 2 4 BA 5 6
Liwen Chu Etc.