Scheduling Transmissions in 802.11 Networks

1. Scheduling Transmissions in 802.11 Networks Ananth Rao SAHARA Retreat, Jan 2004

2. Motivation • Multi-hop networks based on IEEE 802.11 standards have a lot of potential • Hardware is inexpensive • Many interesting applications • Recently, lots of thrust from the research community (MIT, MSR, Intel..) Internet Gateway

3. Problem • 802.11 MAC does not perform well for multi-hop networks • Real test-beds have been plagued with performance problems (Roofnet, MSR Testbed) • Throughput (poor contention resolution) • Fairness (longer routes get very low throughput) • Goal: Improve multi-hop 802.11 throughput and fairness

4. Constraints • Do not want to modify the MAC protocol • Takes a lot of time and effort to standardize • Must be compatible with the 802.11 MAC • Use readily available low-cost hardware • The only control we have is: When to ask the card to send a packet • Do some form of scheduling on top of 802.11 Use an “Overlay” for the MAC Layer

5. RTS Protocol High Data Rate Low Data Rate Overhead (1500 bytes) CTS 802.11a 54 Mbps 6 Mbps 38% DATA 802.11b 11 Mbps 1 Mbps 40% 802.11g 54Mbps 1 Mbps 70%?? ACK Overhead of Contention-Resolution Sender Receiver

6. I1 I2 Hidden Terminal Problem • CTS may not be received clearly for the following reasons • Node is within interference but outside communication range • Another transmission interferes with reception of CTS (loss rates as high as 60% seen in simulations) S R

7. Fairness Problems • The 802.11 MAC gives roughly equal number of transmission opportunities to competing stations • This results in undesirable outcomes when • Senders use different packet sizes • Senders are transmitting at different rates • Senders are forwarding traffic from other nodes

8. Different Data Rates R A B

9. Forwarding on Behalf of Others Ethernet 1/2 1/2 1/6 1/6 1/6 1/6 1/6 1/6 This problem cannot be solved by local scheduling or queue management algorithms like WFQ

10. Related Work and Challenges • Collision-free MACs • A Channel Access Scheme for Large Dense Packet Radio Networks (1998), Timothy J. Shepard • Channel Access Scheduling in Ad Hoc Networks with Unidirectional Links (2001), Lichun Bao, J.J. Garcia-Luna-Aceves • New Challenges • Accurate timing not possible at the software level • Devices don’t expose all information (eg. cannot carrier-sense and obtain result) • Senders from other networks might interfere – Polling messages might be lost • No changes to physical layer (spread spectrum techniques)

11. Time Slots on Top of 802.11 Time • Assume local synchronization of clocks • Use coarse-grained (compared to packet transmission times) time slots • Slots maybe • Available for contention • Assigned to a particular node • If the nodes queue goes empty, the rest of the slot is open to all 1 2 3 4 5 6 7 8 0 ms 24 ms 48 ms 72 ms Groups of 8 slots each of length 3ms

12. C C C C C C C C C C C Amortize the Cost of Contention Resolution • Nodes that transmitted successfully in the previous slot with index “i” own the slot with probability (1-p) • Cost is amortized because • A time-slot is much longer than a packet transmission • Nodes compete for an average of 1/p slots at a time • Orthogonal to method used to resolve contention for a slot Time 1 2 3 4 5 6 7 8 C C C 0 ms C C 24 ms 48 ms 72 ms

13. Synchronization of Clocks Every packet in encapsulated in a new header which contains a timestamp IP hdr …. ... MAC hdr ts tsAuth tsDist Initialize() myTsAuth = myNodeId; myTsDist = 0; Recv(Packet p) tDiff = estimatedTransitTime(p); if(p.tsAuth < myTsAuth || (p.tsAuth == myTsAuth && p.tsDist < myTsDist)) { myTime = p.ts + tDiff; myTsAuth = p.tsAuth; myTsDist = p.tsDist+1; }

14. Competing for and Relinquishing a Slot • Use 2-hop broadcasts to request a slot or to announce giving up a slot • The probability of winning a slot is based on the current # of slots owned and the weight of the competing node • Compete for a slot on • Receiving a relinquish message • Think slot is free and no packets are seen for 0.5ms after start of a slot • Immediate neighbors may stop the broadcast if it is somebody else’s slot

15. Simulation Results • Qualnet Network Simulator • Commercial software from www.scalable-networks.com • Packet level simulator similar to ns2, but faster and more scalable • Models collisions, interference and contention • Use 802.11a at 54 Mbps • 20 slots of 3 ms each, p=0.05

16. Performance in a Chain

17. Performance in a Multi-hop Network (Collisions)

18. Testbed • Hardware • ASUS Pundit barebones system • Celeron 2.4 Ghz, 256 MB • Netgear WAG511, 802.11a • Software • RH 9.0, Kernel 2.4.22 • Madwifi driver for Atheros • Click modular router

19. Mixed Queue FIFO SetTimeOffset LIFO SetOKSlots Click Architecture Push 1 Pull FromDevice DecapTimestamp ToDevice ContentionResolver TimeslotEnforcer EncapTimestamp Rest of the Router 1

20. Results (Testbed – Data Rates) With Scheduling Without Scheduling

21. Results (Testbed – Chain)

22. Conclusions and Future Work • Coarse-grained scheduling on top of 802.11 is a very powerful technique to • alleviate inefficiencies of the MAC protocol in resolving contention • overcome the lack of flexibility of assigning priorities to senders • Future work • Understand the performance problems in multi-hop networks better using the test-bed • Further refine the algorithms for allocation of slots and the implementaion