1. GAPA - Efficient, More Reliable Multicast
September 2006
doc.: IEEE /1458r0
May 2008
GAPA - Efficient, More Reliable Multicast
Date:
Authors:
Hart, Qian (Cisco Systems)
Joonsuk Kim, Broadcom Corp.

2. Requirements for Multicast Video
May 2008
Requirements for Multicast Video
Requirements
Very low PLR
Low delay and delay jitter
Multiple transmissions per beacon period
Compatible with legacy STAs
Duplicate detection
Objectives
Efficiency (since video throughput can be high)
Feedback for rate adaptation
Compatible with power save
Hart, Qian (Cisco Systems)

3. Few candidate solutions meet these requirements
May 2008
Few candidate solutions meet these requirements
(1) Existing multicast
No retries, no Acks, no inputs to rate adaptation, sent infrequently
Doesn’t work without retries (3 retries assumed)
Hart, Qian (Cisco Systems)

4. Few candidate solutions meet these requirements
May 2008
Few candidate solutions meet these requirements
Retransmitting multicast frames a fixed number of times
Inefficient if all receivers receive the frame correctly the first time
No feedback for rate adaptation
Legacy STAs cannot be guaranteed to perform duplicate detection and discarding correctly
(2) Multicast-to-unicast conversion
Increases delay for later receivers
Increases delay jitter for later receivers
Multiplies the number of packets over the air for an already high-throughput application
Can’t be used with bridges if MC-ness not preserved
Retransmitting multicast frames with a delayed Block Ack policy
Block Acks must contend for the medium
Hart, Qian (Cisco Systems)

5. May 2008
Proposed Solution “Group-Addressed PSMP Ack” (GAPA) is More Efficient and More Reliable
(3) Transmit multicast frames via PSMP
PSMP bursts comprise:
PSMP sequences, which in turn comprise:
Downlink phase – for MC data
Uplink phase – for PSMP Acks to MC data
The first PSMP sequence
Sends the MC data in the DTT
Retrieves scheduled ACKs in the UTT
Subsequent PSMP sequences are used for MC data retries if needed
All within the same TXOP
Scheduled PSMP may make sense for some applications
The benefits of GAPA are orthogonal to using scheduling to avoid collisions. Likely both problems need to be solved via complementary proposals
Hart, Qian (Cisco Systems)

6. Three Main Schemes Illustrated
September 2006
doc.: IEEE /1458r0
May 2008
Three Main Schemes Illustrated
Data
(1) MC
(2) MC2UC
Data
Data
Data
ACK1
ACK2
ACK3
(3) GAPA
Data
ACK1
ACK2
ACK3
Delay
Hart, Qian (Cisco Systems)
Joonsuk Kim, Broadcom Corp.

7. Anticipated Characteristics of the Three Schemes
September 2006
doc.: IEEE /1458r0
May 2008
Anticipated Characteristics of the Three Schemes
Multicast
Unicast
PSMP
Data frames repeated No
(>=1) x #dest
1+#retries
ACK
Yes
Packet Loss Rate
Too high to meet requirement
Low
Delay and delay jitter
Too high
Capacity
High
Medium
Rate
The lowest in the MC group
Individual rate for each
The lowest of the unAcked members of the MC group
Complexity
Hart, Qian (Cisco Systems)
Joonsuk Kim, Broadcom Corp.

8. Preliminary Simulations
September 2006
doc.: IEEE /1458r0
May 2008
Preliminary Simulations
N x [3 video devices at [24, 54, 130] Mbps in a single BSS with a fixed PER]
Low throughput video (max 360 kbps)
For a delay limit of 20 ms, GAPA allows for a 7 times increase in capacity over MC2UC!
(Courtesy of Luke Qian)
ms)
Hart, Qian (Cisco Systems)
Joonsuk Kim, Broadcom Corp.

9. GAPA is compatible with legacy (= pre-11aa)
May 2008
GAPA is compatible with legacy (= pre-11aa)
Since we cannot depend upon legacy’s ability to perform duplicate detection with MC (see 9.2.9), then retries need to be hidden from legacy
GAPA is hidden from legacy by sending GAPA transmissions to a different MC MAC address
GAPA-capable STAs request GAPA for selected MC TSPECs
The accepted TSPEC response includes an alternative, AP-allocated, otherwise-unused MC address (called GAPA MC MAC address)
If legacy TSPECs are part of the MC group, frames are (also) sent via legacy MC & addressed to the normal MC address
Ignored by STAs with GAPA TSPECs
If GAPA TSPECs are part of the MC group, frames are (also) sent via GAPA & addressed to that MC group’s GAPA MC MAC address
GAPA BC is sent to a special MAC address (TBD – e.g. ff-ff-ff-ff-ff-fe)
BA agreement (rather than TSPEC) is likely sufficient [08/0615r0]
GAPA MC duplicate detection is provided by:
Same sequence number for all retries
Hart, Qian (Cisco Systems)

10. GAPA can be robust to lost STAs
May 2008
GAPA can be robust to lost STAs
If retries are exhausted for a specific STA say a threshold number of PSMP bursts in a row, possibly that STA has roamed away without disassociating / been turned off / etc.
Call this a lost STA
The AP may disassociate the lost STA, or decline that lost STA’s BA agreement/TSPEC, or no longer send retries if only lost STAs have not Acked the MC data
Hart, Qian (Cisco Systems)

11. Summary of GAPA Benefits
May 2008
Summary of GAPA Benefits
All STAs receive the data simultaneously
Reduced delay and delay jitter
Data is sent once and only retried when necessary
Better capacity
Acks are scheduled as efficiently as possible
Better capacity, reliability, and enables rate adaptation
Compatible with power saving mechanisms
Compatible with legacy MC/BC
GAPA provides for duplicate detection
GAPA is complementary to collision-avoidance scheduling mechanisms
Hart, Qian (Cisco Systems)

12. May 2008
Questions ?
Hart, Qian (Cisco Systems)

13. Strawpoll Would you support the GAPA scheme in 11aa? May 2008
Hart, Qian (Cisco Systems)