Multiple Access and High Density 802.11 Wireless Access Networks

1. Multiple Access and High Density 802.11 Wireless Access Networks Dina Papagiannaki Intel Research Cambridge

2. Multiple Access • In broadcast environments we need a mechanism to coordinate access among devices (Ethernet, Wireless LANs, Cellular networks) • Every transmission is overheard by all other devices in range • Simultaneous transmissions lead to collisions that waste network resources • Two primary ways of mediating access: • Centralized • Distributed • Design goal: • Maximize the number of messages • Minimize a station’s waiting time Konstantina Papagiannaki

3. Centralized vs. Distributed • Centralized scheme • One node is assuming the role of the master node and determines the order by which slave nodes access the medium. • May lead to low medium utilization. • Distributed scheme • All nodes are equivalent and can talk to each other. • Need to coordinate access in order to avoid collisions. Konstantina Papagiannaki

4. Circuit-mode vs. packet-mode • Such a choice depends on the intended workload • Circuit-mode allocates part of the medium to a source for its exclusive use – cellular network • Packet-mode operates on a per-packet basis, more appropriate for bursty, non-persistent traffic types. Konstantina Papagiannaki

5. Further constraints • Spectrum scarcity • Radio channel impairments • Fading – degradation of the signal due to the environment • Multipath interference – reception of signal along multiple paths that may interfere and potentially cancel each other out • Hidden terminal – a transmission may not be overheard by all potentially interfering stations • Capture – the strongest signal at the receiver may be properly decoded (strongest sender has captured the receiver) Konstantina Papagiannaki

6. Applying those concepts to 802.11 wireless networking • IEEE 802.11 is used for Wireless LANs • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Three variations – 802.11b at 2.4GHz and 11 Mbps, 802.11g at 2.4 GHz and 54 Mbps, 802.11a at 5 GHz and 54 Mbps • Channel impairments dealt using rate adaptation • Different modulation and coding schemes employed that result in different effective transmission rates • Hidden terminal mitigation using Request to Send/Clear to Send (RTS/CTS) control frames Konstantina Papagiannaki

7. The 802.11 MAC protocol • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Before a transmission sender senses the medium • If the energy level lower than Clear Channel Assessment (CCA) threshold – medium idle • If not, medium busy • When the sender wishes to transmit it randomly draws a waiting time [0, CWmin] • Each idle slot allows the sender to reduce its CW by 1 slot • Upon each unacknowledged transmission the sender doubles its CW up to CWmax (back off) Konstantina Papagiannaki

8. Spectrum scarcity • 802.11 success primarily due to low cost and no licensing fees to use the 2.4 Ghz and 5 GHz bands • Small number of operating frequencies in 802.11b/g – slightly more in 802.11a • Sharing with non 802.11 devices (microwaves, cordless phones, BT devices, etc.) Konstantina Papagiannaki

9. The effect of contention • In a single contention domain each sender has an equal probability of accessing the medium • The greater the number of senders the smaller the throughput • Mechanisms for robustness to errors may lead to smaller effective transmission rates • Nominal transmission rate of 802.11a/g: 54 Mbps, effective ~30 Mbps, lowest encoding rate 1 Mbps, under contention even lower… Konstantina Papagiannaki

10. Research Challenges • Density of 802.11 APs increases in urban areas • Low cost/ease of deployment • No coordination in deployment • May feature manufacturer default settings • Campus and enterprise networks go wireless • Higher density could lead to better performance • Network management is an ART, especially due to medium dynamics • There is no equivalent to over-provisioning • Adding APs may be counter-productive Konstantina Papagiannaki

11. Self-organization in 802.11 networks • Tuneable knobs • AP frequency (frequency selection) • Association of clients to APs (user association) • Transmission power and CCA threshold (Power control/MAC layer tuning) • Performance evaluation • Requirements from existing platforms • Further Challenges Konstantina Papagiannaki

12. The problem statement client AP High density 802.11 wireless networks suffer from sub-optimal performance due to their static configuration (i.e. maximum transmission power, default operating channel, default aggressiveness to access the medium)! Konstantina Papagiannaki

13. Self-organization objectives • Seeking mechanisms that aim to optimize global performance using local information alone • Robust to changes in the medium and the topology • Can be implemented using today’s technology Develop fully decentralized algorithms for the self-organization of infrastructure 802.11 wireless networks Konstantina Papagiannaki

14. 3 facets to the problem • A.Frequency Selection by APs • Identify the appropriate frequency to use so as to minimize overall interference across the network • B.User association • When user association is flexible, balance the users across APs so as to maximize the long-term overall network capacity • C. Transmission Power and Aggressiveness to access the medium (CCA threshold) • Identify the appropriate level of transmission power and CCA threshold for the APs and clients so as to maximize overall network capacity Konstantina Papagiannaki

15. Frequency selection formulation Measure noise Measure power P1 P2 Konstantina Papagiannaki

16. A. Frequency Selection Minimize interference by operating on orthogonal frequencies. Minimize overlap when required to reuse a frequency. Konstantina Papagiannaki

17. User throughput • Channel access time • Aggregated transmission delay • Wireless channel quality Internet State of the art can lead to unnecessarily low throughput! Konstantina Papagiannaki

18. Analytical model • When wireless the bottleneck… • … traffic is downlink – APs are the only senders in the medium • … fully saturated traffic conditions – interference caused by APs does not depend on the #clients • All clients receive the same long-term throughput if rate adaptation employed • In a reference period of time T ET Konstantina Papagiannaki

19. User association formulation Konstantina Papagiannaki

20. B. User association Balance the user associations for minimal potential delay fairness. Users take into account the personal and social cost of different association rules. Konstantina Papagiannaki

21. Overall network fairness improved Mean:1428, variance:4378031 Mean:1559, variance: 627638 Konstantina Papagiannaki

22. Implementation on Intel 2915ABG • AP Capacity (APC) - MAC • Modify firmware to compute fraction of access time, i.e. number of busy slots in a reference period of time (M(a)) • Nominal capacity given by 11a/b/g (C(a)) • Aggregated transmission delay (ATD) – MAC/PHY • Modify firmware/ucode to measure amount of time between queueing the packet towards a client and the reception of the ACK (rate scaling, and retransmissions) • Keep a list of client MACs and delay, compute sum of delays • Transmission rate for new client approximated using RSSI - PHY • APC/ATD advertized through Beacon frames Konstantina Papagiannaki

23. Ch10 Ch10 Ch3 AP1 AP2 AP3 4 Mbps 4 Mbps 4 Mbps C1 C2 C3 Experimental Results Konstantina Papagiannaki

24. Power Control in 802.11 • Heterogeneous transmit powers across nodes can lead to node starvation! 1st order starvation We need to ensure that there is symmetry in the nodes’ contention domains. Konstantina Papagiannaki

25. What is the benefit of power control? • Reducing transmission power can reduce interference in the network • Increasing transmission power can improve client SINR thus allowing for higher transmission rates • There is a tradeoff between the amount of interference we introduce in the network and the additional throughput benefit at the client Konstantina Papagiannaki

26. Condition for starvation free power control • We need to ensure network symmetry • We have proven that for starvation-free power control we need to keep the product of CCA threshold and transmission power constant • CCA * P = C • The louder you are going to shout the more carefully you should listen for the nodes that whisper Konstantina Papagiannaki

27. How do we maximize network capacity? • We need to identify these values of C that result in the greatest transmission concurrency • We can optimize C using Gibbs sampling in order to maximize network capacity • Input: channel gains between APs, channel gains from AP to clients, number of clients per AP, transmission power • Output: APs select transmission power and CCA. Clients follow the setting of their AP. Konstantina Papagiannaki

28. Experimental Testbed Konstantina Papagiannaki

29. C. Power Control / CCA adaptation Tune power to offer the best transmission rate to the farthest client while not introducing excessive interference to neighboring co-channel devices. Adjust CCA to increase transmission concurrency across the network. Konstantina Papagiannaki

30. Experimental Results Gain Default CCA Client SS03: 149% 15% Client SS15: 228% 34% Client SS24: 112% 3% Konstantina Papagiannaki

31. Simulation Results (topology – 8 APs, 26 STAs, 802.11a, AP-STA: 3.5m) Konstantina Papagiannaki

32. Simulation Results (power, CCA) Konstantina Papagiannaki

33. Simulation Results (throughput) Konstantina Papagiannaki

34. Summary Results Gibbs also leads to the use of a smaller transmission power that can extend client’s lifetime Konstantina Papagiannaki

35. Implementation Requirements • AP Capacity (MAC) • Aggregated Transmission Delay (PHY/MAC) • Number of users • Worst Client Channel Gain (PHY) • Introduction of new Beacon fields (CCA, TxPower, auxiliary variables) • Channel Switch Announcements (802.11h/DFS/TPC) • Measurements (802.11k/802.11e) Konstantina Papagiannaki

36. Larger Scale Experimentation • SWAN testbed at William Gates Building • 80 Soekris dual mini-PCI boards • Intel 2915 ABG cards with modified ucode/firmware • PoE switches for ease of manageability • Investigation of benefits of the three different algorithms compared to the state of the art and their incremental benefits Konstantina Papagiannaki

37. Ground floor 1st floor 2nd floor Floorplans Konstantina Papagiannaki

38. More exciting problems…. Konstantina Papagiannaki

39. Community mesh networking Internet Konstantina Papagiannaki

40. Questions? Konstantina Papagiannaki

41. BACKUP Konstantina Papagiannaki