Post on 03-Jul-2018
Outline
Multiple rates in 802.11
Access scheme
Variants of 802.11 standard
Rates and modulation schemes
Performance anomaly of 802.11
Rate Adaptation algorithms
o ARF and AARF
o Sample Rate
o OAR (ERA)
o CARA / LDRA
Future directions
References
Multiple rates in 802.11
Initially only one base rate (1997).
Multirate enhancement since (1999)
Higher rates restricted by SNR.
Need for rate adaptation mechanisms.
Multitude of rate adaptation techniques, resulting from various metrics –
throughput, airtime fairness, power consumption, overall efficiency of network…
Algorithms include statistical approach, channel conditions measurement, loss
differentiation, machine learning, opportunistic attempts, ant colony
optimization...
Access scheme
RTS/CTS is an optional mechanism, introducing some
overhead, reducing chances of collisions.
Fragmentation mechanism.
Variants of 802.11 standard
802.11 (1997)
802.11a (1999)
802.11b (1999)
802.11g (2003)
802.11-2007 (a, b, d, e, F, g, h, i, j)
802.11n (2009)
802.11-2012 (k, r, y, n, w, p, z, v, u, s)
802.11ac (2012)
Rates and modulation schemes
Example for 802.11a, BER=10-5:
Rate (Mbp/s) SINR (dB) Modulation scheme Coding rate
6 6.02 BPSK 1/2
9 7.78 BPSK 3/4
12 9.03 QPSK 1/2
18 10.79 QPSK 3/4
24 17.04 16-QAM 1/2
36 18.80 16-QAM 3/4
48 24.05 64-QAM 2/3
54 24.56 64-QAM 3/4
Different modulation schemes and coding rates may produce non-linear
performance-SINR ratio. Assumption that next higher rate is “better".
Performance anomaly of 802.11
Degraded performance of hosts using higher rates in presence of hosts using
lower rates.
Basic CSMA/CA admission control gives all users equal access probability.
Users with lower rates occupy medium for longer periods of time than high-rate
users that transmit data packets in short time intervals.
Troublesome for rate adaptation algorithms without loss differentiation, it
resulted in various attempts at improving temporal fairness among users.
Rate adaptation algorithms
decrease rate decision increase rate decision RTS/CTS decision entiety Loss Diff. Type Year
ARF 2 missed frame 10 successful transmissions no sender STA - Loss 1997
AARF 2 missed frame x successful transmissions no sender STA - Loss 2004
AMRR +33% fail 10%- fail no sender STA - Loss 2004
ARC failure while cw >= OPTcw success while cw=< OPTcw no sender STA - Loss 2009
Onoe 50% frames lost in 1 second window success ratio (credit system) no sender STA - Loss 2004
RBAR channel conditions channel conditions yes receiver STA - SNR 2001
OAR channel conditions* channel conditions* yes* receiver STA* -* SNR* 2002
RSSLA RSS compared with adaptive Threshold RSS compared with adaptive Threshold no sender STA - RSS 2003
FAR channel conditions channel conditions yes hybrid - SNR 2005
SampleRate four succesive failures probing of promising rates no sender STA - Loss 2005
BARA channel conditions channel conditions no sender STA - SNR 2007
TA-ARA - when traffic can't be satisfied yes sender STA - SNR 2010
Minstrel probing of promising rates probing of promising rates no sender STA - Loss 2011
LD-ARF 2 missed frame due to channel fading 10 successful transmissions optional receiver STA + Loss 2005
CARA 2 fail 10 successful transmissions yes* sender STA + Loss 2006
RRAA loss ratio compared with current threshold loss ratio compared with current threshold yes * sender STA + Loss 2006
ERA frame missed due to channel fading x ACK (jak w AARF) no sender STA + Loss 2007
LDRA frame missed due to channel fading channel conditions no sender STA + SNR 2008
AARF-CD 2 fail 10 successful transmissions yes* sender STA + Loss 2008
SARA sukcesy i porażki w transmisji sukcesy i porażki w transmisji yes sender STA + Loss 2008
MutFed 2 missed frame channel conditions no hybrid + SNR 2010
MiSer optimal power consumption optimal power consumption yes sender STA - SNR 2003
Smart Sender probing and transmission history probing and transmission history no sender STA - Loss/SNR 2008
TFRC fairness criteria fairness criteria yes sender STA* - SNR 2009
ARF and AARF
Automatic Rate Fallback (1997), Adaptive Automatic Rate Fallback (2004).
ARF: Increase rate after 10 consecutive successful transmissions. If first
transmission at higher rate fails – return to previous rate. Decrease rate after 2
consecutive failed transmissions.
AARF: Increase rate after reaching T - threshold for rate increase. If first
transmission at higher rate fails – return to previous rate, double T. Decrease rate
after 2 consecutive failed transmissions.
Reduces unnecessary attempts in stable conditions.
Sample Rate
Keeps statistics for last 10-second period of transmission.
Periodically checks performance of other promising rates.
Decrease rate after 4 consecutive failures.
Every 10th frame, probe promising rates.
Transmission time of 1500 bytes in ideal channel = 1.873ms at 11Mbps, 2.976ms at 5.5Mbps, 6.834 at
2Mbps, 12.995ms at 1Mbps. If due to occasional failures, average transmission time at 11Mbps is 3.123ms,
then 5.5 Mbps will be probed as potentially better rate than 11Mbps.
OAR and ERA
Opportunistic Auto Rate (OAR), Effective Rate Adaptation (ERA).
OAR: “meta algorithm” (RBAR), aiming at airtime fairness. Using fragmentation
mechanism send base_rate/transmission_rate frames at once.
Example – 1 frame at 2Mbps, 5 frames at 11Mbps.
ERA uses fragmentation mechanism to determine cause of failed transmission.
Failed packet is divided into short and remaining fragment. If short fragment is
retransmitted at previous rate. If it’s successful, failure is attributed to collision. If
not, it’s retransmitted at basic rate. If this transmission is successful, previous
failure is attributed to channel fading. If not, it’s deemed to have been caused by
collision.
CARA and LDRA
Collision Aware Rate Adaptation (CARA), Loss Differentiation Rate Adaptation (LDRA)
CARA: Employs RTS/CTS mechanism, but only if ACK is missed. RTS exchange is
held at basic rate, and assumption is, that all RTS failures occur only due to
collision. So if it fails, collision is deduced and consecutive failure counter is not
incremented.
LDRA: Similar mechanism, but instead of RTS, it attempts at retransmission of lost
packet at basic rate to determine cause of failure.
Future directions
QoS supporting rate adaptation algorithm.
Devise algorithm that adapts rate according to both channel conditions and
traffic demands.
It would provide sufficient bit rate, delay, jitter, BER…
References
1. Lacage, M., Manshaei M.H., Turletti, T. “IEEE 802.11 Rate Adaptation: A Practical Approach”
2. Kamerman, A., Monteban, L. “WaveLAN-II: A High-Performance Wireless LAN for the Unlicensed Band”
3. Jongseok K., Seongkwan K. Sunghyun C., Daji Q. “CARA: Collision-Aware Rate Adaptation for IEEE 802.11
WLANs”
4. Biaz S. , Wu S. “ERA: An Efficient Rate Adaptation Algorithm with Fragmentation”
5. Wu S., Biaz S. “Loss Differentiated Rate Adaptation in Wireless Networks”
6. Sadeghi B., Kanodia V., Sabharwal A., Knightly E. “Opportunistic Media Access for Multirate Ad Hoc
Networks”
7. Heusse M., Rousseau F., Berger-Sabbatel G., Duda A. “Performance Anomaly of 802.11b”
8. John C. Bicket “Bit-rate Selection in Wireless Networks”
9. Labiod H., Afifi H., De Santis C. “Wi-Fi, Bluetooth, Zigbee and WiMax”
10. Ao X., Jiang, S., Tang L. “Traffic-aware Active Link Rate Adaptation via Power Control for Multi-hop Multi-
rate 802.11 Networks”