Dynamic Rate Adaptation in IEEE WLANs

Transcription

1 Dynamic Rate Adaptation in IEEE WLANs August 10, 2008

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

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

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

6 What is Rate Adaptation? The a/b/g/n standards allow the use of multiple transmission rates b, 4 rate options (1,2,5.5,11Mbps) a, 8 rate options (6,9,12,18,24,36,48,54 Mbps) g, 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 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? Access Point SRN Distance Effects : attenuation fading interference MN

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

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

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

13 Outline Introduction of Rate Adaptation in IEEE 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 Node Contention Effects: MN1 Collisions induced by: random backoff MN4 Access Point MN2 hidden terminal 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 MN4 Access Point MN1 MN2 fairness MN3

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 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 RBAR 2002 OAR 2003 LA 2004 AARF MultiRateRetry AMRR 2005 ONOE LD-ARF SampleRate 2006 CARA RRAA 2007 AORA CHARM TBC

23 Classification of existing algorithms Information channel estimation depends on: Open-loop Closed-loop Local channel estimation LA CHARM AORA (idle slots) Packet transmission situation ARF AARF MMR AMRR ONOE LD-ARF(NACK) SampleRate CARA RRAA RTS CTS RBAR OAR CARA RRAA Beacon probe response 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 PHY MAC hybrid signal data mapping calculate RBAR OAR ARF/AARF MMR/AMMR LD-ARF SamleRate CARARRAA AORAONOE LACHARM RBAR OAR CARA LD-ARF RRAA CHARM probe ARF/AARF MMR/AMMR LD-ARF SampleRate AORA CHARM No probe RRAA RBAR OAR LA CHARM deterministic statistical ARF AARF LD-ARF MRR ONOE CARA AMRR SampleRate RRAA AORA CHARM ARF AARF MRR AMRR ONOE LD-ARF CARA RRAA AORA RBAR LA SampleRate CHARM

25 Trend of rate adaptation algorithms easy to implement The One improve SNR not probe compliant is the every can obtained upshift Differentiate not react on 10 with frames based current on the Symmetric performance the real time reasons link, for channel Can which packet react situation Still networks is suffer not loss quickly to mobility from suffer accurate random Increasing from collision random load collisions Too at some sensitive level to probe failure Each algorithm has its own Achille s heel 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: CARARRAA

26 Outline Introduction of Rate Adaptation in IEEE 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 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 predefined 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 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. The poor-channel flows would consume more time and system resources Grant the user during a coherence time a channel access time that allows multiple packet transmissions (Fragment mechanism in ).

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: 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 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 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 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

51 RRAA- Critical Loss Ratio (P*) For any rate R, let the next lower rate be R_ and the next higher rate be R+. With a loss ratio of P*, the throughput at R becomes the same as the loss-free throughput at R_.

52 RRAA- P MTL and P ORI We set P MTL = αp*(r), α 1 Decrease the rate when expected throughput is more than P MTL α = 1.25>1, to anticipate certain level of losses at R_ P ORI = P MTL (R+) / 2 increase the rate when expected throughput is less than P ORI The loss ratio at the current rate R has to be small enough such that the rate increase not quickly jump back to R

53 RRAA- Rate change loss ratio thresholds: [3]

54 RRAA- Adaptive RTS Filter Selective use of RTS/CTS Tradeoff between overhead and benefits of RTS RTSwnd (RTScounter) RTSwnd is initially set to be 0. Window is increased by one when last frame lost without RTS (potentially due to a collision) When the last frame was lost with RTS or succeeded without RTS, RTSwnd is halved (assume no collision involved).

55 RRAA- Adaptive RTS Example Collision may occur No collision More packets are sent with RTS if the collision level is high

56 RRAA- Performance evaluation Compared with ARFAARFSampleRate Static client case: Throughput gains 0.3% ~ 67.4% Mobile client case Throughput gains 10.0% ~ 27.6% Hidden-station case Throughput gains 74% ~ 101% [3]

57 RRAA- existing problems How to decide the window size under different situation The network congestion can not be sensed, when turn on RTS/CTS [3]

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

59 My Opinion The ability to differentiate the reasons for a frame loss. Link error (SNR) Random collisions/hidden terminals Network congestion (available bandwidth? CCA? Idle slots?) The speed of the adaptation (be adaptive in different situation? What happens when introduce handoffs here?) The interaction between link-layer rate adaptation and high-layer (TCP) rate adaptation An Evaluation model

60 Thanks Questions?