1. CARROM: Client Association, Rate and Redundancy Optimization for Wi-Fi Multicast Vishnu Navda, Ram Ramjee, Lenin Ravindranath (MSR India) Ranveer Chandra, Jitu Padhye (MSR Redmond) 1

2. Why Multicast? Today – Live video/audio streaming Potential – Windows/Virus Update – IPTV – Application Level Multicast leveraging IP Multicast 2

3. Wi-Fi Multicast Issues Data Sent only using Basic Rates (1/2 Mbps) – Resource under utilization Rate anomaly – Low rate client => poor throughput for everyone No back-off after collision – Unicast unfairness No Retransmissions – High Losses 3 Data Rate: 1 Mbps Access Point Multicast Group Clients

4. CARROM Architecture 4 Inter-AP optimizer Intra-AP optimizer Optimal Association Rate Adaptation Pseudo-Broadcast Adaptive FEC

5. Problem - Only basic rates Multicast sent at only basic rates (1/2Mbps) 1 Mbps 11 Mbps 5.5 Mbps 11 Mbps 1 Mbps Solution Rate adaptation 5.5 Mbps AP1 C1 C2 C3 5

6. Problem - Rate Anomaly 11 Mbps 1 Mbps 11Mbps Solution Control Association 6 C1 C2 C3 AP1 AP2Channel XChannel Y

7. Solution - Optimal Association Inter-AP optimizer – Collects feedback from clients – Determines optimal association – Inform clients about association – Inform APs about rate AP1 AP2 Inter-AP optimizer C1 C2 C3 7

8. Optimal Association Rate Matrix Metrics – Minimize sum of multicast air time across APs – Maximize average group throughput Optimized Association C1 - AP1 C2 - AP2 C3 - AP2 AP1AP2 C1 C2 C3 8

9. Optimal Association NP Hard problem Hierarchical Association Algorithm – Min Cut to Partition topology – LP solver – Brute force with N best APs when partition size is small Greedy Algorithm – Online – Associate clients/groups to that AP which minimizes impact to existing flows 9

10. Problem - Unicast unfairness When unicast and multicast co-exists – Unicast backs-off – Multicast doesn’t – Degrades unicast throughput Solution Pseudo-Broadcast approach Ref: MSWIM06 10

11. Solution - Pseudo-Broadcast AP unicast to one client Other clients listen in promiscuous mode Unicast fairness due to back-off Pseudo-broadcast Listening in promiscuous mode 11

12. Losses are uncorrelated Losses are uncorrelated across clients Solution Adaptive FEC Correlation between 2 users 12

13. Adaptive FEC Intra-AP optimizer – Pseudo-Broadcast Packet level feedback Weighted round-robin among clients Adapt FEC 13

14. Summary Optimal Association – Increase Multicast Throughput Pseudo-Broadcast – Unicast Fairness Multicast Rate Adaptation – Increase Multicast Throughput Adaptive FEC – Increase Reliability 14

15. Results – Optimal Association Optimal Association helps significantly reduce Wireless Utilization 15

16. Implementation Issues No modification – Optimal Association AP modification – Rate Adaptation – Adaptive FEC – Pseudo broadcast Missing Windows APIs – Associate to a AP with given BSSID – Listen in promiscuous mode 16