1. Quasi-reliable Multicast
March 2009
doc.: IEEE /0247r0
March 2009
Quasi-reliable Multicast
Date:
Authors:
Jochen Miroll, Saarland University
Jochen Miroll, Saarland University

2. Abstract Reliable Multicast
March 2009
doc.: IEEE /0247r0
March 2009
Abstract
Reliable Multicast
Quasi-reliable Multicast/Broadcast paradigm
Use-cases for delay driven quasi-reliability
Jochen Miroll, Saarland University
Jochen Miroll, Saarland University

3. Glossary Multicast Broadcast in 802.11 (shared medium)
March 2009
Glossary
Multicast Broadcast in (shared medium)
PHY rate FEC and modulation
Determines throughput
E.g. FEC 2/3, 64-QAM => 54Mbps
Reciprocal: Determines „air time“ for each frame (individually)
VoIP Internet telephony, also teleconferences
H.264 Advanced video coding according to MPEG
Jochen Miroll, Saarland University

4. Reliable Multicast Multicast and reliability
March 2009
Reliable Multicast
Multicast and reliability
Reliability in Multicast/Broadcast can be done
Using sequence numbers and ACK scheme
But should it be a requirement for aa?
Reliable Multicast similar to unicast
ACK received from every Multicast client for a seq.#?
If at least one ACK (from some client) is missing
Condition for retransmission
Jochen Miroll, Saarland University

5. Quasi-reliability paradigm
March 2009
Quasi-reliability paradigm
Reliable Multicast
What are typical use-cases for perfect reliability in Multicast?
Instead: Quasi-reliability
Multicast essentially is Broadcast
Typical Broadcast (e.g. TV) is not perfectly reliable
Loosely speaking, traditional broadcast becomes less reliable with increasing distance from the base station (cf. analogue TV and DVB)
Quasi-reliable shall refer to
Perfectly reliable for none
Not strictly reliable for many
Unreliable for some
Jochen Miroll, Saarland University

6. Use-cases What kinds of applications does 802.11aa target?
March 2009
Use-cases
What kinds of applications does aa target?
Applications for wireless Multicast seem to be
Loss tolerant
cf. VoIP, H.264 error concealment, etc.
Delay constrained
cf. VoIP, live-TV, gaming, etc.
Jochen Miroll, Saarland University

7. Multicast retransmissions
March 2009
Multicast retransmissions
Assumption: at least one ACK missing
Do a retransmission round?
Yes: Retransmit at the same or some lower PHY rate?
No: Reliability is violated!
After how many rounds (1,2,3,4,...) should we stop?
Too many rounds will stall the network
How do we define a retransmission round?
Repeat the lost frame(s) at the same PHY rate
Repeat the lost frame(s) e.g. twice at the same PHY rate
Repeat the lost frame(s) at a lower PHY rate
How much time does it take?
Jochen Miroll, Saarland University

8. 11aa Multicast error correction
March 2009
11aa Multicast error correction
The 11aa error correction should primarily
Correct errors, i.e. retransmit packets
Correct errors for many/most/some of the clients
Never stall (like unicast or TCP/IP does)
What we think is reasonable for 11aa error correction
Quasi-reliability
Perfect reliability will stall the network in some scenario
Guarantee to stay below some max. retransmission delay
Possible since we know how long a frame at a certain PHY rate will take to be transmitted
Limit the number of retransmission rounds
Calculate delay in advance (using PHY rate and # of rounds)
Jochen Miroll, Saarland University

9. Questions Does 11aa target perfect reliability?
March 2009
Questions
Does 11aa target perfect reliability?
TGaa: Is quasi-reliability an option?
Or maybe perfect reliability should be optional
Delay driven quasi-reliability
Stop retransmissions when a certain delay is exceeded
ACK-based approach? can be done
NACK/leader-based approach? can be done
May be more efficient than Block-ACK
We have simulation data in case TGaa is interested
Jochen Miroll, Saarland University

10. March 2009
References
Multicast MAC Extensions for high rate real-time traffic in Wireless LANs
Jochen Miroll, Saarland University