Performance issues & improvement on 802.11 MAC

1. Performance issues & improvement on 802.11 MAC • review of 802.11 MAC • performance issues • improvements • idle sense • an overlay approach • more …

2. An Overlay MAC Layer for 802.11 Networks Ananth Rao Ion Stoica UC Berkeley Mobisys 2005

3. Motivation • 802.11 hardware provides initial ease of deployability for many applications • mesh networks • long haul links • large Infrastructure Networks • these apps stretching 802.11 beyond its design goals (Wireless LANs) Internet Gateway

4. Problem 1: different data rates R A B

5. Problem 2: unpredictability 1 2 3 4 5

6. Problem 3: forwarding on behalf of others Ethernet 1/3 1/2 1/6 1/9 1/6 1/6 1/9 1/9 This problem cannot be solved by local scheduling or queue management algorithms like WFQ

7. 1M 11M A B C D E A B C D F Problems • 802.11 provides no control over resource allocation • Default allocation policy ill-suited for multi-hop networks • Bad fish problem • Forwarders get same share as others

8. Approaches • workarounds in routing/transport layer • easy to deploy • cannot address some issues • change/replace MAC • new protocols, new standard • more powerful, hard to deploy • overlay MAC layer (OML) • directly on top of 802.11 MAC • no need to change hardware • directly use interfaces exposed by 802.11 cards

9. Advantages of an overlay approach • easy to deploy • easy to modify • implemented in software • easy to modify for diverse requirements • tighter integration between MAC and upper layers • performance benefits • utilize information from higher layers

10. Bigger picture: overlay network

11. Bigger picture: Overlay network Focus at the application level

12. Even bigger picture: virtualization Virtualization of resources: powerful abstraction in systems engineering: • computing examples: virtual memory, virtual devices • virtual machines: e.g., java • IBM VM os from 1960’s/70’s • Networking examples: • connecting local heterogeneous networks • IP over ATM • overlay networks • VPN

13. Overlay MAC Layer (OML) design goals • efficient • fair or differentiated allocation • flexible and low cost • avoid modifying MAC

14. Overlay MAC Layer (OML): what can it control? • no control in upper layer • cannot decide when getting a packet • no control in MAC • cannot decide when packet is actually sent • can control only when to send packet to network card • packet scheduling policy: FIFO, strict priority scheduling, weighted fair queuing

15. 2 3 1 3 4 5 1 4 3 1 5 4 2 5 2 Detour: strict priority scheduling transmit highest priority queued packet • multiple classes, with different priorities • class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc.. • real world example: reservations versus walk-ins arrivals time packet service time departures

16. Detour: weighted fair queuing • each class gets weighted amount of service in each cycle • equal weight: Round Robin scheduling

17. OWL main idea: use TDMA-like schedule • divide time into slots • weighted slot allocation (WSA): allocate slots to nodes according to weighted fair queuing policy • assigns a weight to each node • allocate slots proportion to nodes’ weights -> weighted allocation • a slot is only assigned to one node in an interference region -> reduce packet loss

18. Questions • clock synchronization? • slot length? • interference region? • weighted slot allocation • how to choose weight? • decide a winner w/o communication?

19. Clock synchronization & slot size • Loose time synchronization • leader-based • slot size: 10 packets of maximum size • larger than clock synchronization error • larger than packet transmission time • as small as possible

20. Interference region • ideally node i applies WSA to all nodes that interfere with i • how to determine who interfere with me? • assume a node can interfere with all nodes within k-hop distance • only an approximation, not accurate • how to determine interference relationship is an active research!

21. Weighted Slot Allocation: decide winner w/o communication • each node uses pseudo-random function to generate a random number • Hi = H(ni, t) 1/wi • t: time slot, wi: weight of node ni • can generate random number for all nodes in the collision domain (e.g., 2-hop neighborhood) • the highest number wins

22. Evaluation methodology • Simulation in Qualnet • Implementation in Atheros Madwifi driver + Click router

23. Summary of results • Overhead: OML thruput comparable to native 802.11 • Reduced contention and retransmissions • Fairness: Fairness index for OML network much higher • A node’s share = # flows passing thru it • Limitations: Impact of mobility; Interference from native 802.11 clients

24. Simulation results • Similar throughput to 802.11 • Control overhead is small

25. Simulation results (cont.) • Improved fairness over standard 802.11 • Weight set to number of nodes in output queue

26. Summary • Coarse-grained scheduling on top of 802.11: • alleviate inefficiencies of the MAC protocol in resolving contention • overcome the lack of flexibility of assigning priorities to senders • Enables experiment with new scheduling and bandwidth management algorithms

27. Limitations • Interference from other 802.11 clients • Face incrementally deployment issues • Impact of mobility • Takes some time for newly joined nodes to get its proportional share • How to set weight? • How to know of weights of nodes in interference region (weights can be dynamic)?