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Dynamic Rate Adaptation in IEEE 802.11 WLANs

Dynamic Rate Adaptation in IEEE 802.11 WLANs

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Dynamic Rate Adaptation in IEEE 802.11 WLANs

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  1. Dynamic Rate Adaptation in IEEE 802.11 WLANs SongYiLin@ICT August 10, 2008

  2. References [1] On the Performance Characteristics of WLANs: Revisited (SIGMETRICS 2005) [2] CARA: Collision-Aware Rate Adaptation forIEEE 802.11 WLANs (INFOCOM 2006) [3] Robust Rate Adaptation for 802.11 Wireless Networks (MOBICOM 2006) [4] IEEE 802.11 Rate Adaptation: A Practical Approach (MSWiM 2004) [5] Link Adaptation Strategy for IEEE 802.11 WLAN via Received Signal Strength Measurement (ICC 2003)

  3. Outline • Introduction of Rate Adaptation in IEEE 802.11 WLANs • Existing Challenges • Development of Rate Adaptation Algorithms • Presentative old ones • New Kids in this area • My Opinion

  4. Outline • Introduction of Rate Adaptation in IEEE 802.11 WLANs • Existing Challenges • Development of Rate Adaptation Algorithms • Presentative old ones • New Kids in this area • My Opinion

  5. 802.11 MAC (with RTS/CTS on) RTS: 20 bytes in mac CTS: 14 bytes in mac [2]

  6. What is “Rate Adaptation” ? • The 802.11 a/b/g/n standards allow the use of multiple transmission rates • 802.11b, 4 rate options (1,2,5.5,11Mbps) • 802.11a, 8 rate options (6,9,12,18,24,36,48,54 Mbps) • 802.11g, 12 rate options (11a set + 11b set) • The method to select the transmission rate in real time is called “Rate Adaptation” • Rate adaptation is important yet unspecified by the 802.11 standards

  7. Why do we need “Rate Adaptation” ? Access Point MN

  8. Why do we need “Rate Adaptation” ? Access Point MN

  9. Why do we need “Rate Adaptation” ? SRN Access Point MN Distance Effects : attenuation fading interference

  10. Why do we need “Rate Adaptation” ? BPSK 1Mbps QPSK 2Mbps BPSK/QPSK/CCK Different modulation schemes Best throughput [5]

  11. How to adjust the rate ? • Rate adaptation plays a critical role to the throughput performance • Ideally, the transmission rate should be adjusted according to the …… Rate too high Rate too low Loss ratio increases under-utilize the capacity throughput decreases channel condition

  12. How to estimate channel condition ? • SNR of the channel • SNR of receiver…… • no feedback in 802.11…… • fluctuation of SNR…… • Gauging how well the currently chosen rate performs (Statistics: transmission loss/success) • not timely • affected by random collisions (unnecessary downshift) Modified frame sender receiver Symmetric link easy to implement

  13. Outline • Introduction of Rate Adaptation in IEEE 802.11 WLANs • Existing Challenges • Development of Rate Adaptation Algorithms • Presentative old ones • New Kids in this area • My Opinion

  14. Channel Dynamics • Wireless channel exhibits rich channel dynamics in practical scenarios • Random channel error • Mobility-induced change • Collisions induced by • Hidden-terminals • Multiple contending clients • Random collision • congestion

  15. Channel Dynamics • When should the transmission rate be updated? too quick: perform bad when channel conditions fluctuate acutely too slow: not in time (base on a relative long history) The algorithm should be adaptive…

  16. Equidistant Distribution MN1 Node Contention Effects: Collisions induced by: random backoff hidden terminal MN4 MN2 Access Point MN3

  17. Equidistant Distribution-random collisions means: unnecessary downshift [1]

  18. Equidistant Distribution-hidden terminal means: unnecessary downshift Hidden terminal broadcast packets at a mild rate of 0.379Mbps continuously while other nodes begin with 11Mbps [3]

  19. Non-equidistant Distribution Nodes Diversity Effects: collisions hidden terminal channel diversity link capture MN1 MN4 Access Point MN2 MN3 fairness

  20. Non-equidistant Distribution- fairness • Different kinds of fairness: • throughput fairness • time-share fairness • In single-rate networks: equivalent • In multi-rate networks: time-share fairness is the one to be concerned The poor-channel flows would consume more time and system resources

  21. Outline • Introduction of Rate Adaptation in IEEE 802.11 WLANs • Existing Challenges • Development of Rate Adaptation Algorithms • Presentative old ones • New Kids in this area • My Opinion

  22. Rate Adaptation Algorithms List 1997 ARF 1998 1999 2000 2001 RBAR 2002 OAR 2003 LA 2004 AARF MultiRateRetry AMRR 2005 ONOELD-ARF SampleRate 2006 CARARRAA 2007 AORA CHARM TBC ……

  23. Classification of existing algorithms Information channel estimation depends on: Open-loop Closed-loop Local channel estimation Packet transmission situation Beacon probe response RTS CTS ARF AARF MMR AMRR ONOE LD-ARF(NACK) SampleRate CARA RRAA RBAR OAR CARA RRAA LA CHARM AORA (idle slots) CHARM (noise)

  24. Classification of existing algorithms Rate Adaptation Processing: Estimation (channel conditions) Action (how to adjust) Which layer to use Which messages to use How to estimate sequential rate adjustment best rate adjustment MAC hybrid signal data mapping calculate PHY probe ARF AARF MRR AMRR ONOE LD-ARF CARA RRAA AORA RBAR LA SampleRate CHARM ARF/AARF MMR/AMMR LD-ARF SamleRate CARA、RRAA AORA、ONOE No probe deterministic statistical RBAR OAR CARA LD-ARF RRAA CHARM ARF/AARF MMR/AMMR LD-ARF SampleRate AORA CHARM ARF AARF LD-ARF MRR ONOE CARA AMRR SampleRate RRAA AORA CHARM RBAR OAR LA CHARM RBAR OAR LA、CHARM RRAA

  25. Trend of rate adaptation algorithms • easy to implement • can not react on the real time channel situation • suffer from random collisions • The SNR is obtained based on” Symmetric link”, which is not accurate… • One probe every 10 frames • Can react quickly to mobility • Too sensitive to probe failure • improve the upshift performance • Still suffer from random collision • not compliant with current 802.11 networks open-loop & statistics based: ARF close-loop & SNR based (rts/cts): RBAR/OAR statistics & SNR based & adaptive: LA statistics based & adaptive: AARF statistics based & adaptive & estimate transimission time for different rates: SampleRate statistics based & collisions avoid/detect: CARA、RRAA • Differentiate the reasons for packet loss… • Increasing load at some level… Each algorithm has its own Achille’s heel…

  26. Outline • Introduction of Rate Adaptation in IEEE 802.11 WLANs • Existing Challenges • Development of Rate Adaptation Algorithms • Presentative old ones • New Kids in this area • My Opinion

  27. Presentative old ones • ARF • RBAR • OAR • LA • AARF • SampleRate

  28. ARF- How does it work ? Use packet transmission situation to estimate the channel condition: • If two consecutive ACK frames are not received correctly, the second retry and subsequent transmissions are done at a lower rate and a timer is started. • When the number of successfully received ACKs reaches 10 or the timer goes off, a probe frame is sent at the next higher rate. However, if an ACK is NOT received for this frame, the rate is lowered back and the timer is restarted.

  29. ARF- Does it work well ? Advantages: • Compliant with 802.11 • All things can be done by the sender • Easy to implement Disadvantages: • Suffer from random collisions and hidden terminals • Constantly upshift try when channel condition is stable • Rate can only be adjusted step by step

  30. RBAR- How does it work ? Receivers control sender’s transmission rate: • RTS and CTS are modified to contain info on size and rate. • Uses analysis of RTS reception to estimate SNR and send choice back to sender in CTS. • Receiver picks rate based on pre-defined SNR thresholds.

  31. RBAR- Does it work well ? Advantages: • Rate can be adjusted according to the real time channel condition • Do not need to adjust the rate step by step • Will not suffer from random collisions and hidden terminals Disadvantages: • not 802.11 compatible (modified RTS/CTS) • The rate-SNR table is obtained based on a priori channel mode • SNR is not easy to get (most WLAN cards only have RSSI) • RTS/CTS is seldom used (only when the frame is too large…) • RTS/CTS introduce extra load

  32. OAR- How does it work ? Make full use of coherence times, provide time-share fairness: • Improvement based on RBAR. • Coherence times are durations for which mobile stations have better-than-average channels. • Grant the user during a coherence time a channel access time that allows multiple packet transmissions (Fragment mechanism in 802.11). The poor-channel flows would consume more time and system resources

  33. OAR- Does it work well ? Advantages: • Nodes with good channels can send more packets while providing time-share fairness to all the nodes • The same as RBAR… Disadvantages: • The same as RBAR…

  34. LA- How does it work ? Assume that the channel is symmetric: • Use RSSI to approximate SNR. • Use SNR of the sender to approximate SNR of the receiver. • Each node maintains 12 dynamic RSS thresholds. • The thresholds are updated depending on whether the transmission is successful . • Rate selection is based on both the RSS thresholds and number of retransmission attempts.

  35. LA- Does it work well ? Advantages: • 802.11 compatible. • RSSI is much more easy to get. • Rate can be adjusted according to the real time channel condition. • Do not need to adjust the rate step by step. • Can adjust the rate during network congestion. Disadvantages: • The symmetric assumption is dubitable. • RSSI is quite different from SNR. • suffer from random collisions and hidden terminals.

  36. AARF- How does it work ? Dynamic adjust the upshift threshold of ARF: • Improvement based on ARF. • To fix one existing problem of ARF (Constantly upshift try when channel condition is stable) ARF AARF

  37. AARF- Does it work well ? Advantages: • Compliant with 802.11 • All things can be done by the sender • Easy to implement Disadvantages: • Suffer from random collisions and hidden terminals • Rate can only be adjusted step by step • When channel condition gets better quickly…it can not react quickly on it

  38. SampleRate- How does it work ? Select rate by statistic information about the transmission time for each rate: • maintain the expected transmission time for each rate and update it after each transmission. (wnd=1s--10s) • A frame is transmitted at the rate that currently has the smallest expected transmission time. • sends one probe packet at another randomly selected rate every 10 frames. • Downshift its rate every 4 consecutive transmission failure.

  39. SampleRate- Does it work well ? Advantages: • React quickly to mobility • Do not always need to adjust the rate step by step • Compliant with 802.11 Disadvantages: • suffer from random collisions and hidden terminals (time window) • Can be very sensitive to probe failure

  40. Outline • Introduction of Rate Adaptation in IEEE 802.11 WLANs • Existing Challenges • Development of Rate Adaptation Algorithms • Presentative old ones • New Kids in this area • My Opinion

  41. New Kids in this area • CARA - Collision Aware Rate Adaptation • RRAA – Robust Rate Adaptation Algorithm

  42. CARA- the key idea The primary contribution is to differentiate frame collisions from frame transmission failures caused by channel error. • focus on rate down only, rate up is the same as that of ARF • Two methods for identifying collisions: • Adaptive RTS probing • Identifying collision via CCA detection

  43. CARA- Adaptive RTS Probing Assumptions: • All RTS transmission failures are due to collisions. • Transmission failure after RTS/CTS must be due to channel errors. • To reduce the signaling overhead, RTS probing that enables an RTS/CTS exchange ONLY when a data frame transmission fails.

  44. CARA-with default RTS Probing • Data frame transmitted without RTS/CTS. • If the transmission fails, RTS/CTS exchange is activated for the next retransmission. If this one fails again, then the rate is lowered. • If retransmission is successful, stay at the same rate and send next frame without RTS/CTS.

  45. CARA –Identifying collision via CCA detection If the wireless channel is busy while the expected ACK reception dose not start, the station conclude that a collision has just happened to its data transmission. A transmission failure detected by CCA to be a collision will not cause a RTS Probing [2]

  46. CARA- Performance evaluation 1 CARA-1: CARA with only default RTS Probing RTS/CTS: ARF scheme using RTS/CTS all the time [2]

  47. CARA- Performance evaluation 2 CARA-1: with only default RTS Probing CARA-2: with both default RTS Probing and CCA detecion [2]

  48. CARA-existing problems • When the channel condition is so bad that even the RTS can not be sent… CARA will be stuck there… • The network congestion can not be sensed, in which situation the rate should be downshifted… • when hidden terminals exist, it suffers from the drawback of RTS oscillation, which alternates on and off for RTS.

  49. RRAA- the key idea Short-term statistics to handle • random loss • mobility • drastic changes Adaptive RTS to handle • collision

  50. RRAA- loss estimation • Instead of single probe frame. • Uses a loss estimation window and computes the estimated loss ratio over the window (20-100ms). • Uses upper and lower loss threshold for each rate and estimated loss ratio to decide when to switch rates. • Otherwise, retain the current rate and continue sliding window