1. Dynamic Rate Adaptation in IEEE 802.11 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] Robust Rate Adaptation for Wireless Networks (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 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) [2] 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
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 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
Ideally, the transmission rate should be adjusted according to the channel condition
[5]

11. under-utilize the capacity
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
Rate too high → loss ratio increases
Rate too low → under-utilize the capacity
channel condition

12. How to estimate channel condition ?
SNR of the channel
SNR of receiver……
no feedback in ……
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
Rate too high → loss ratio increases
Rate too low → under-utilize the capacity
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
Access
Point
MN2
基础设置模式下，所有节点都跟AP保持同样距离
MN3

17. Equidistant Distribution- random collisions
means:
unnecessary
downshift
基础设置模式下，所有节点都跟AP保持同样距离
[1]

18. Equidistant Distribution- hidden terminal
means:
unnecessary
downshift
基础设置模式下，所有节点都跟AP保持同样距离
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
由于不同的节点的链路状况是不同的。因而需要采用不同的发送速率以达到最小的包误码率。
而当节点采用不同的发送速率发送同样大小的数据包时，需要的时间长短是不一样的。
高速率发送的快一些。
因而在复杂的网络环境中，各个节点的差异比较大的时候，不能单纯的追求各个节点之间的throughput fairness。
time-share fairness 才是更加重要的。
可以让高速率的节点获得信道的时候，连续多发几个包（按照比例。。。比方说1Mbps的为基准。1Mbps的发一个包，2Mbps的发两个包，11Mbps的发送11个包。。。依次类推）。
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 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
Packet transmission
situation
RTS
CTS
Beacon
probe response
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
MMR/AMMR
LD-ARF
SamleRate
CARA、RRAA
AORA、ONOE
ARF
AARF
MRR
AMRR
ONOE
LD-ARF
CARA
RRAA
AORA
RBAR LA
SampleRate
CHARM 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
One probe every 10 frames
Can react quickly to mobility
Too sensitive to probe failure
The SNR is obtained based on” Symmetric link”, which is not accurate…
improve the upshift performance
Still suffer from random collision
not compliant with current networks
Differentiate the reasons for packet loss…
Increasing load at some level…
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: CARA、RRAA

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: Disadvantages:
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 pre-defined SNR thresholds.

31. RBAR- Does it work well ? Advantages: Disadvantages:
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.
Grant the user during a coherence time a channel access time that allows multiple packet transmissions (Fragment mechanism in ).
The poor-channel flows would consume more time and system resources

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

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: Disadvantages: 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: Disadvantages:
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 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 Adaptive RTS
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- PMTL and PORI We set PMTL = αP*(R), α ≥ 1 PORI = PMTL(R+) / 2
Decrease the rate when expected throughput is more than PMTL
α = 1.25>1, to anticipate certain level of losses at R_
PORI = PMTL(R+) / 2
increase the rate when expected throughput is less than PORI
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. loss ratio thresholds:
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 ExampleNo
collision
Collision may occur
More packets are sent with RTS if
the collision level is high

56. RRAA- Performance evaluation
Compared with ARF、AARF、SampleRate
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 802.11 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?