March 2009
doc.: IEEE 802.11-09/0247r0
Quasi-reliable Multicast
Date: 2009-03-02
Authors:
Name
Affiliations Address
Jochen Miroll Saarland
University
Submission
Phone
email
Campus C6 3,
+49 681 302 6546
66123 Saarbruecken,
Germany
Slide 1
[email protected]
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
Abstract
• Reliable Multicast
• Quasi-reliable Multicast/Broadcast paradigm
• Use-cases for delay driven quasi-reliability
Submission
Slide 2
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
Glossary
• Multicast Broadcast in 802.11 (shared medium)
• PHY rate FEC and modulation
– Determines 802.11 throughput
• E.g. FEC 2/3, 64-QAM => 54Mbps
– Reciprocal: Determines „air time“ for each frame (individually)
• VoIP
• H.264
Submission
Internet telephony, also teleconferences
Advanced video coding according to MPEG
Slide 3
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
Reliable Multicast
•
Multicast and reliability
– Reliability in 802.11 Multicast/Broadcast can be done
•
Using sequence numbers and ACK scheme
– But should it be a requirement for 802.11aa?
•
Reliable Multicast similar to 802.11 unicast
– ACK received from every Multicast client for a seq.#?
– If at least one ACK (from some client) is missing
•
Submission
Condition for retransmission
Slide 4
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
Quasi-reliability paradigm
•
Reliable Multicast
– What are typical use-cases for perfect reliability in Multicast?
•
Instead: Quasi-reliability
– 802.11 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
•
•
•
Submission
Perfectly reliable for none
Not strictly reliable for many
Unreliable for some
Slide 5
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
Use-cases
• What kinds of applications does 802.11aa target?
• Applications for wireless Multicast seem to be
1. Loss tolerant
cf. VoIP, H.264 error concealment, etc.
2. Delay constrained
cf. VoIP, live-TV, gaming, etc.
Submission
Slide 6
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
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?
Submission
Slide 7
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
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 802.11 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)
Submission
Slide 8
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
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
Submission
Slide 9
Jochen Miroll, Saarland University
March 2009
doc.: IEEE 802.11-09/0247r0
References
• Multicast MAC Extensions for high rate real-time
traffic in Wireless LANs
Submission
Slide 10
Jochen Miroll, Saarland University