1. EDCA and BlockAck Extensions for Reliable Multicast/Broadcast Services
November 2007
12/7/2018May 2008
doc.: IEEE /2752r1
EDCA and BlockAck Extensions for Reliable Multicast/Broadcast Services
Date:
Authors:
Liwen Chu, et al.
Peter Ecclesine, Cisco Systems

2. November 2007
12/7/2018May 2008
doc.: IEEE /2752r1
Abstract
This contribution proposes EDCA and BlockAck extensions to improve link reliability for multicast/broadcast stream transmission
Liwen Chu, et al.
Peter Ecclesine, Cisco Systems

3. Outlines Problems for multicast in 802.11
November 2007
12/7/2018May 2008
doc.: IEEE /2752r1
Outlines
Problems for multicast in
RTS/CTS extensions for multicast protection
BlockAck extensions
Liwen Chu, et al.
Peter Ecclesine, Cisco Systems

4. Problems for Multicast/Broadcast in 802.11
12/7/2018May 2008
Problems for Multicast/Broadcast in
Collision in the BSS (intra-BSS Collision)
The associated STAs transmit when an AP is transmitting multicast/broadcast frames
Overlapping BSS (OBSS) Collision (Inter-BSS Collision)
The STAs/APs in OBSSes transmit when an AP is transmitting multicast/broadcast frames
Hidden node problem
No ACK
Collision S2 S1 S1
Collision S7
AP1
AP2 S5 S4 S3 S6 S3 S2
AP2 is a hidden node to AP1
Liwen Chu, et al.

5. RTS/CTS only for Unicast
RTS/CTS can alleviate the hidden node and OBSS problem by contending and reserve a TXOP before data frame transmissions in EDCA
However RTS/CTS is currently only for unicast in
For multicast, there is no RTS/CTS protection from hidden node and OBSS
TXOP
AIFS
Busy
Backoff
RTS
Data
Data
Data
Block-
AckReq
Source
SIFS
SIFS
CTS
Block-
Ack
Destination
Contention
Window
AIFS
Other
NAV (RTS)
NAV (CTS)
Liwen Chu, et al.

6. RTS/CTS Extensions for Multicast/Broadcast Protection
Definition: Reliable Multicast Broadcast Services (RMBS) capability elements
Carry in beacon, probe response, (re)association request frames
An AP may send multicast/broadcast RTS (MBRTS) to one or more stations to contend for a TXOP and to protect following multicast/broadcast transmissions
STAs may reply with MBCTS
MBRTS contains bitmap control and partial bitmap fields to specify a list of the reply-requested stations from which this MBRTS frame requests a MBCTS response.
The reply-requested STAs send MBCTS in the order that they are listed in the MBRTS bitmap field.
Liwen Chu, et al.

7. RMBS RTS/CTS Extensions
MB TXOP
Obtain TXOP
RMBS AP
MB-
RTS
Data
Data
Data
RMBS STA 1
MB-
CTS1
RMBS STA 2
MB-
CTS2
MB-
CTS3
RMBS STA 3
MBCTS
Transmission
Period (MCP)
MB receivers send MBCTS in the
order listed in MBRTS bitmap
RMBS STA 4
MB-
CTS4
RMBS AP assumes obtaining MB TXOP and sending pending MB data frames only if
Received all MBCTS from the intended STAs in the bitmap (or use other local policy)
If no AP Tx after MB-CTS transmission period (AP decides not using this TXOP)
STAs may reset their NAV
Note: RMBS = Reliable Multicast Broadcast Services
Liwen Chu, et al.

8. MBRTS and MBCTS Frame Format
Octets: 2 2 6 6 1
1-251 4
Frame
Control
Duration RA TA
MBRTS
Bitmap
Control
Partial Virtual Bitmap
FCS
Each bit in the bitmap identifies a STA by its association ID
from which a MBCTS response is requested.
Octets: 2 2 6 6 4
MBCTS
Frame Control
Duration RA TA
FCS
The TA field is the address of the STA that is transmitting the frame.
Liwen Chu, et al.

9. Multiple RTS/CTS Procedure
Multiple BlockAckReq/Blockack in a TXOP can be used as reliable multicast/broadcast communication.
But multiple BlockAck TXOP need protection before group transmission to avoid OBSS collision.
Before the Multiple BlockAck TXOP, the AP can select more than one multicast group member and use multiple RTS/CTS to protect the PSMP TXOP.
After receiving multicast RTS, multicast destinations return CTS based on the sequence indicated by the AP.
Liwen Chu, et al.

10. Multiple RTS/CTS Procedure (Cont’d)
Protected Group Transmission 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
AP2
STA4
STA6
STA5
AP1
STA1
STA2
STA3
AP3
STA7
STA8
STA9
Liwen Chu, et al.

11. PSMP Protection
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.
If not all requested STAs reply the PSMP, it is up to the AP to decide if it continues the following group transmission.
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, et al.

12. PSMP Protection (Cont’d)
Protected PSMP 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, et al.

13. Multicast BlockAckReq with Immediate Block Ack Policy
Multicast Block Ack exchange
Multicast data block
RMBS AP
Frame-exchange
for NAV Protection
Data
Data
Block
AckReq
Block
Ack
RMBS STA1
RMBS STA 2
Block
Ack
RMBS STA 3
Not included in the BlockAck-requested receiver list
RMBS BlockAckReq contains a list of BlockAck-requested receivers (in the form of receiver bitmap control and partial bitmap field)
The STAs in the list of BlockAck-requested receivers send their BlockAck in the order that they are specified in the partial bitmap of the RMBS BlockAckReq
Note: RMBS=Reliable Multicast Broadcast Services
Liwen Chu, et al.

14. Multicast BlockAck and Retransmission
BlockAckReq or BlockAck may be lost
If RSMP AP does not successfully receive the MBlockAcks from all the intended recipient STAs, then
retransmits the BlockAckReq with the remaining STAs in the receiver bitmap field from which the BlockAck have not received
the remaining STAs respond with an BlockAck in the order listed in the BlockAckReq receiver bitmap
this process is repeated until the AP receives BlockAcks from all the intended STAs or the number of retransmission attempts has reached to a limit MaxMBlockAckReqLimit.
After BlockAckReq and M-BlockAck exchanges, the AP determines from the information in the received BlockAck bitmap whether a MSDU or an A-MSDU needs to be retransmitted.
If a MSDU or an A-MSDU is lost (not correctly received by one or more intended multicast recipient STAs), the AP arranges the retransmissions of the lost MSDU or A-MSDU .
The retransmitted frames are sent multicast
After retransmitting the lost MSDU/A-MSDU and/or transmitting new MSDU/A-MSDU, the AP can initiate a new BlockAckReq and BlockAck exchange to obtain the receiving status of (re)transmitted MSDU/A-MSDU.
This retransmission process can be repeated for a lost MSDU/A-MSDU until all the intended recipient STAs that sent the BlockAck receive the MSDU/A-MSDU correctly or the lifetime of this MSDU/A-MSDU expires.
Liwen Chu, et al.

15. BlockAckReq Frame Extension
BAR Ack
Policy
Compressed
Bitmap
Multi-TID
RMBS
Reserved
TID_INFO
Frame
Control
Duration
/ID
BAR Receiver
Information RA TA
BAR Control
BAR
Information
FCS
BAR Receiver Bitmap Control
BAR Receiver Partial Bitmap
BAR receiver bitmap control subfield (1 octet): Bitmap Offset Value N = STA’s AID divided by 16
Bit 0 reserved and bits 1-7 Bitmap Offset
Each bit in the BAR receiver partial virtual bitmap subfield corresponds to a specific station
Bit i corresponds to the STA with an AID of 16 x N + i.
If bit i in the receiver bitmap field is set to 1, then the station with AID 16 x N + i is requested to reply to this BlockAckReq with BlockAck
Liwen Chu, et al.

16. BlockAck Frame Extension
BA Ack
Policy
Multi-TID
Compressed
Bitmap
RMBS
Reserved
TID_INFO
Frame
Control RA TA
BA Control BA
Information
Duration/ID
BA Address
FCS
BA Address: The BA Address field of the BlockAck frame contains the group address for which this BlockAck frame acknowledges, and is set to the value of TA field in the previously received BlockAckReq frame to which this BlockAck responds.
Liwen Chu, et al.

17. BlockAckReq & BlockAck Exchange Sequence with Delayed Policy and Ack
Multicast Block Ack Request and ACK
Delayed multicast Block Ack and ACK
Multicast data block
RMBS AP
(BAR Control, BAR Info, Receiver Info)
Frame-exchange
for NAV Protection
Data
Data
Block
AckReq
Ack
Ack
Block
Ack
Ack
RMBS STA 1
Ack
Block
Ack
RMBS STA 2
RMBS STA 3
Not included in the block ack-requested receiver list
Liwen Chu, et al.

18. Delayed multicast Block Ack Multicast Block Ack Request
M-BlockAckReq &M-BlockAck Exchange Sequence with Delayed Policy and No Ack
Delayed multicast Block Ack
Multicast data block
Multicast Block Ack Request
RMBS AP
Frame-exchange
for NAV Protection
Data
Data
Block
AckReq
Block
Ack
RMBS STA 1
Block
Ack
RMBS STA 2
RMBS STA 3
Not included in the blockack-requested receiver list
Liwen Chu, et al.

19. M-BlockAckReq & M-BlockAck Exchange Sequence with PSMP
RMBS AP
Downlink phase
PSMP
Multicast Data Block
Other multicast or unicast data blocks
Multi-cast Data
Multi-cast Data
Block
AckReq
Uni-cast data
STA 1 uplink data block
Uplink phase
Block
Ack
Up-link data
RMBS STA 1
STA 2 uplink data block
Block
Ack
RMBS STA 2
STA 3 uplink data block
Up-link data
RMBS STA 3
Not included in the blockack-requested receiver bitmap of M-BlockAckReq
Liwen Chu, et al.

20. Multicast/broadcast Joining Procedure
A STA uses the extended capability field to tell the associated AP if it supports reliable multicast/broadcast communication, and the protection methods that it supports.
An AP sends an Add multicast/broadcast blockAck (ADDMBBA) Request frame to the STA with the reliable multicast/broadcast capability to request to set up reliable multicast/broadcast transmission.
After receiving an ADDMBBA Request frame, the STA replies with an ADDMBBA Response frame to the AP to indicate if it will joins an reliable multicast/broadcast group.
Liwen Chu, et al.

21. Multicast/broadcast Deleting 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.
Liwen Chu, et al.

22. References
[1] IEEE /1972r13, “Draft PAR and 5 Criteria for Video Transport Stream (VTS) SG”
[2] IEEE P
[3] IEEE P802.11n Draft Standard D7.00
Liwen Chu, et al.

23. Thank You!
Questions?
Liwen Chu, et al.