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

2. Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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. Page 3 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Some Basics Floor Noise Data Rate Received Power Channel Bandwidth Bit error (p) for BPSK and QPSK : SNR Friss’ Equation: Varying with time and space How do we choose the rate of modulation

4. Page 4 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 802.11b – Transmission rates

5. Page 5 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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. Page 6 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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. Page 7 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Impact Large-scale variation with distance (Path loss) SNR (dB) Distance (m) Mean Throughput (Kbps) Path Loss 1 Mbps 8 Mbps

8. Page 8 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Question SNR (dB) Time (ms) Distance (m) 2.4 GHz 2 m/s LOS Which modulation scheme to choose?

9. Page 9 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Answer  Rate Adaptation Dynamically choose the best modulation scheme for the channel conditions Mean Throughput (Kbps) Distance (m) Desired Result

10. Page 10 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Is there a tradeoff ? CE A D B Rate = 10

11. Page 11 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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. Page 12 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Cellular networks Adaptation at the physical layer Impractical for 802.11 in WLANs For WLANs, rate adaptation best handled at MAC Adaptation  At Which Layer ? D C BA CTS: 8 RTS: 10 10 8 Sender Receiver RTS/CTS requires that the rate be known in advance Why?

13. Page 13 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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 A B

14. Page 14 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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. Page 15 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Lucent WaveLAN “Autorate Fallback” (ARF) Sender decreases rate after N consecutive ACKS are lost Sender increases rate after Y consecutive ACKS are received or T secs have elapsed since last attempt B A DATA 2 Mbps Effective Range 1 Mbps Effective Range

16. Page 16 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Performance of ARF Time (s) Rate (Mbps) SNR (dB) Time (s) – Slow to adapt to channel conditions – Choice of N, Y, T may not be best for all situations Attempted to Increase Rate During Fade Dropped Packets Failed to Increase Rate After Fade

17. Page 17 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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. Page 18 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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 Receiver-Based Autorate (RBAR) Protocol C BA CTS (1) RTS (2) 2 Mbps 1 Mbps D DATA (1) 2 Mbps 1 Mbps

19. Page 19 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Performance of RBAR Time (s) SNR (dB) Time (s) Rate (Mbps) Time (s) RBAR ARF

20. Page 20 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 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. Page 21 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Implementation into 802.11

22. Page 22 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Implementation into 802.11 PLCP Header

23. Page 23 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Implementation into 802.11 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 Frame Control Duration DA SABSSID Sequence Control Body FCS Reservation Subheader (RSH) WHY

24. Page 24 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Questions