dynamic rate adaptation in ieee 802.11 wlans -...

Dynamic Rate Adaptation in IEEE 802.11 WLANs

SongYiLin@ICT

August 10, 2008

References

[1] On the Performance Characteristics of WLANs: Revisited (SIGMETRICS 2005)

[2] CARA: Collision-Aware Rate Adaptation for IEEE 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)

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

802.11 MAC (with RTS/CTS on)

[2]

RTS: 20 bytes in mac

CTS: 14 bytes in mac

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

Why do we need “Rate Adaptation” ?

Access Point MN


Access Point MN

Distance Effects : attenuation

fading interference

SRN


[5]

BPSK 1Mbps QPSK 2Mbps

BPSK/QPSK/CCK Different modulation schemes

Best throughput

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 ……

throughput decreases

Rate too high

Loss ratio increases

Rate too low

under-utilize the capacity

channel condition

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) easy to implement

receiver sender

Modified frame

Symmetric link

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

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…

Equidistant Distribution

Access Point MN2

MN1

MN4

MN3

Node Contention Effects:

Collisions induced by: random backoff hidden terminal

Equidistant Distribution- random collisions

[1]

means: unnecessary downshift

Equidistant Distribution- hidden terminal

[3]

means: unnecessary downshift

Hidden terminal broadcast packets at a mild rate of 0.379Mbps continuously while other nodes begin with 11Mbps

Non-equidistant Distribution

Access Point

MN3

Nodes Diversity Effects:

collisions hidden terminal channel diversity link capture

MN1

MN2

MN4

fairness

Non-equidistant Distribution- fairness

The poor-channel flows would consume more time and system resources

 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

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 ……

Classification of existing algorithms

Open-loop

Local channel estimation

Closed-loop

Packet transmission situation

RTS CTS

Beacon probe response

Information channel estimation depends on:

LA CHARM

AORA (idle slots)

ARF AARF MMR AMRR ONOE

LD-ARF(NACK) SampleRate

CARA RRAA

RBAR OAR CARA RRAA

CHARM (noise)

Classification of existing algorithms

Estimation (channel conditions)

Which layer to use

Action (how to adjust)

Which messages to use

How to estimate

sequential rate

adjustment

best rate

adjustment

PHY MAC hybrid data signal

probe No probe

mapping calculate

deterministic statistical

Rate Adaptation Processing:

RBAR OAR

ARF/AARF MMR/AMMR

LD-ARF SamleRate

CARA、RRAA AORA、ONOE

LA、CHARM

RBAR OAR CARA

LD-ARF RRAA

CHARM

ARF/AARF MMR/AMMR

LD-ARF SampleRate

AORA CHARM

RRAA

RBAR OAR LA

CHARM

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

Trend of rate adaptation algorithms

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

•  easy to implement •  can not react on

the real time channel situation

•  suffer from random collisions

•  not compliant with current 802.11 networks

•  The SNR is obtained based on” Symmetric link”, which is not accurate…

•  improve the upshift performance

•  Still suffer from random collision

•  One probe every 10 frames

•  Can react quickly to mobility

•  Too sensitive to probe failure

•  Differentiate the reasons for packet loss…

•  Increasing load at some level…

Each algorithm has its own Achille’s heel…

Presentative old ones   ARF   RBAR   OAR   LA   AARF   SampleRate

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.

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

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.

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

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

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…

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.

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.

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

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

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.

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

New Kids in this area

 CARA - Collision Aware Rate Adaptation

 RRAA – Robust Rate Adaptation Algorithm

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

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.

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.

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.

[2] A transmission failure detected by CCA to be a collision will not cause a RTS Probing

CARA- Performance evaluation 1

[2]

CARA-1: CARA with only default RTS Probing RTS/CTS: ARF scheme using RTS/CTS all the time

CARA- Performance evaluation 2

[2]

CARA-1: with only default RTS Probing CARA-2: with both default RTS Probing and CCA detecion

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.

RRAA- the key idea

Short-term statistics to handle •  random loss •  mobility •  drastic changes

Adaptive RTS to handle

•  collision

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

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_.

RRAA- PMTL and PORI

 We set PMTL = αP*(R), α ≥ 1 •  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

RRAA- Rate change

[3]

loss ratio thresholds:

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).

RRAA- Adaptive RTS Example

Collision may occur

No collision

More packets are sent with RTS if the collision level is high

RRAA- Performance evaluation

[3]

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%

RRAA- existing problems

[3]

 How to decide the window size under different situation

 The network congestion can not be sensed, when turn on RTS/CTS

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

Thanks……

Questions?

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