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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012

Wireless Networking & Mobile Computing CS 752/852 - Spring 2012. Lec #7: MAC Multi-Rate. Tamer Nadeem Dept. of Computer Science. What is Data Rate ?. Number of bits that you transmit per unit time under a fixed energy budget Too many bits/s: Each bit has little energy -> Hi BER

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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012

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  1. Wireless Networking & Mobile ComputingCS 752/852 - Spring 2012 Lec #7: MAC Multi-Rate Tamer Nadeem • Dept. of Computer Science

  2. What is Data Rate ? Number of bits that you transmit per unit time under a fixed energy budget Too many bits/s: Each bit has little energy -> Hi BER Too few bits/s: Less BER but lower throughput

  3. Varying with time and space How do we choose the rate of modulation Some Basics • Friss’ Equation: Received Power • Bit error (p) for BPSK and QPSK : Channel Bandwidth SNR Data Rate Floor Noise

  4. 802.11b – Transmission rates

  5. Static Rates SINR time # Estimate a value of SINR # Then choose a corresponding rate that would transmit packets correctly most of the times # Failure in some cases of fading  live with it

  6. Adaptive Rate-Control SINR time # Observe the current value of SINR # Believe that current value is indicator of near-future value # Choose corresponding rate of modulation # Observe next value # Control rate if channel conditions have changed

  7. Impact Large-scale variation with distance (Path loss) 8 Mbps Path Loss Mean Throughput (Kbps) SNR (dB) 1 Mbps Distance (m) Distance (m)

  8. Question Which modulation scheme to choose? SNR (dB) SNR (dB) 2.4 GHz 2 m/s LOS Distance (m) Time (ms)

  9. Answer  Rate Adaptation • Dynamically choose the best modulation scheme for the channel conditions Desired Result Mean Throughput (Kbps) Distance (m)

  10. Is there a tradeoff ? Rate = 10 B C E A D

  11. Design Issues • How frequently must rate adaptation occur? • Signal can vary rapidly depending on: • carrier frequency • node speed • interference • etc. • For conventional hardware at pedestrian speeds, rate adaptation is feasible on a per-packet basis Coherence time of channel higher than transmission time

  12. RTS: 10 C 8 A B CTS: 8 D 10 Adaptation  At Which Layer ? • Cellular networks • Adaptation at the physical layer • Impractical for 802.11 in WLANs • For WLANs, rate adaptation best handled at MAC Why? RTS/CTS requires that the rate be known in advance Receiver Sender

  13. A B Who should select the data rate? • Collision is at the receiver • Channel conditions are only known at the receiver • SS, interference, noise, BER, etc. • The receiver is best positioned to select data rate

  14. Bigger Picture • Rate control has variety of implications • Any single MAC protocol solves part of the puzzle • Important to understand e2e implications • Does routing protocols get affected? • Does TCP get affected? • … • Good to make a start at the MAC layer • RBAR • OAR • Opportunistic Rate Control • …

  15. DATA 2 Mbps A B 2 Mbps Effective Range 1 Mbps Effective Range Lucent WaveLAN “Autorate Fallback” (ARF) • Sender decreases rate after • N consecutive ACKS are lost • Sender increases rate after • Y consecutive ACKS are receivedor • T secs have elapsed since last attempt

  16. Performance of ARF SNR (dB) • Slow to adapt to channel conditions • Choice of N, Y,T may not be best for all situations Time (s) Dropped Packets Rate (Mbps) Time (s) Failed to Increase Rate After Fade Attempted to Increase Rate During Fade

  17. RBAR Approach • Move the rate adaptation mechanism to the receiver • Better channel quality information = better rate selection • Utilize the RTS/CTS exchange to: • Provide the receiver with a signal to sample (RTS) • Carry feedback (data rate) to the sender (CTS)

  18. 1 Mbps C 2 Mbps RTS (2) 1 Mbps CTS (1) A B DATA (1) 1 Mbps D 2 Mbps Receiver-Based Autorate (RBAR) Protocol • RTS carries sender’s estimate of best rate • CTS carries receiver’s selection of the best rate • Nodes that hear RTS/CTS calculate reservation • If rates differ, special subheader in DATA packet updates nodes that overheard RTS

  19. Performance of RBAR SNR (dB) Time (s) Rate (Mbps) Time (s) RBAR Rate (Mbps) Time (s) ARF

  20. Question to the class • There are two types of fading • Short term fading • Long term fading • Under which fading is RBAR better than ARF ? • Under which fading is RBAR comparable to ARF ? • Think of some case when RBAR may be worse than ARF

  21. Implementation into 802.11

  22. Implementation into 802.11 PLCP Header

  23. Implementation into 802.11 Sequence Control Frame Control Duration DA FCS SA BSSID FCS Body • Encode data rate and packet length in duration field of frames • Rate can be changed by receiver • Length can be used to select rate • Reservations are calculated using encoded rate and length • New DATA frame type with Reservation Subheader (RSH) • Reservation fields protected by additional frame check sequence • RSH is sent at same rate as RTS/CTS • New frame is only needed when receiver suggests rate change Reservation Subheader (RSH) WHY

  24. Questions

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