Ubiquitous Computing Center A Rate-Adaptive MAC Protocol for Multi-hop Wireless Networks 황 태 호

 Gavin Holland  Texas A&M University  Nitin Vaidya  Texas A&M University  Department of Electrical and Computer Engineering Co- Director, Illinois Center for Wireless Systems Research Professor  Paramvir Bahl  Microsoft Research  ACM SIGMOBILE July 2001, Rome, Italy 2

Introduction - 1  in WLAN (IEEE )  Devices can transmit at 11 Mbps, with 54 Mbps  Number of encoded bits per symbol  Data rate  Modulation in mobile wireless networks  path loss, fading, interference  SNR, BER variations  Support Multi-Modulation scheme  BPSK  QPSK  QAM16  QAM64  QAM256  Tradeoff emerges between modulation schemes.  The higher the data rate, the higher the BER  Figure 1, Figure 2 3

Introduction - 2 4

Rate Adaptation  Dynamically switching data rates to match the channel conditions  Two Aspect  Channel quality estimation  Measuring Signal Strength, Symbol error rate, etc  Prediction of future quality  Rate selection  Channel Quality Prediction  Threshold selection  Minimize the delay between prediction and selection 5

Previous Work on rate adaption  Ref. [19]. Dual Channel Slotted ALOHA  Separate control channel  Receiver feedback to sender  Ref. [15]. Auto Rate Fallback(ARF, )  Lucent’s WaveLAN II  The sender selects the best rate based on previous tx data.  Ref [9]. Adaptive Transmission Protocol  Selects based on cached per-link information  Separate transmit receive tables  Maintained by exchanging control packet(RTS/CTS)  Cellular network  Channel quality estimation by the receiver  Rate selection by the sender using the feedback  Reside at the physical layer (symbol-by-symbol)  Improper to MAC based on contention access 6

Motivation  ARF Protocol  Receiver  Channel Quality Estimation  Rate Selection 7

Overview of IEEE  Src sends a data packet to Dst  Transmission using one of basic rate set  All node can demodulate the RTS/CTS packets  Virtual carrier sense  RTS includes D RTS  CTS includes D CTS  NAV  Network Allocation Vector  The aggregate duration of time that medium is pre sumed to be busy 8

Receiver-Based Autorate (RBAR) Protocol  The receiver selects the appropriate rate for the data packet during the RTS/CTS exchange  More accurate rate selection  Smaller overhead for the channel quality estimation  In control packet  Instead of D RTS,D CTS  modulation rate and packet size  Src  chooses a data rate based on some heuristic method  Send RTS  Dst  Estimate the channel condition  Send CTS  Node A, B  Calculates the duration  Update NAV  Reservation SubHeader (RSH) in the MAC header of the data packet 9

Incorporation of RBAR into  Data Packet  Header Check Sequence  RTS/CTS  Rate and Length  PLCP header  RSH rate 10

Simulation Environment  NS-2  Extensions from the CMU Monarch project for modeling mobile ad hoc networks  Number of traffic generators  PHY/MAC/Networking stacks  Addition  Detailed MAC and PHY models  Modulation and rate adaption  Rayleigh fading simulator  Interfaces Intersil Prism II chipset  IEEE , DSSS radio,  Observation  Hot the individual rate adaption protocols reacted to the changing channel conditions 11

Simulation – ARF model  Rate selection  If no ACKs for two consecutive data packets, DOWN Rate  If received ACKs for ten consecutive data packets, UP Rate and timer cancelled  If timer expired, UP Rate  Relatively insensitive to choice of timeout 12

Simulation – RBAR  Rate selection  Simple threshold based technique  Estimate : SNR of RTS  Select : (BER) ≤ 1E-5, highest data rate 13

Simulation – Error Model 1  Jake’s method  Simulation of Rayleigh fading  A finite number of oscillators with Doppler shifted frequencies  Instantaneous gain 14

Simulation – Error Model 2  Log-distance path loss model  Friis free space propagation model  Noise model 15 n : path loss exponent k : Boltzmann’s constant T : temperature (in Kelvin) B T : bandwidth

Simulation – Error Model 3  Computed Bit Error Rate  BPSK, QPSK  M-ary QAM  E b /N 0 : bit energy to noise ratio  For gain, Coherence time  For noise,  Adjusting SNR 16

Simulation – Network Configuration  Configuration 1  Two node  One of the nodes was fixed position, the other traveled along a direct-line path (300m)  Configuration 2  20 nodes  Random waypoint mobility  Random speed : 2, 4, 6, 8, 10 m/s  1500 x 300 m 2  DSR (Dynamic source routing) Protocol  Average of 30 times 17

Performance Evaluation  Overhead of RSH 18

Slow Changing Channel Conditions  Configuration 1  0 ~ 300m, by 5m  60s, Tx UDP packets(1460 bytes) 19

Fast Changing Channel Conditions  Experiment 1  Configuration 1  Mean node speed : 2, 4, 6, 8, 10 m/s  Single UDP Connection  Performance improvement from 6% (10m/s) to 20% (2m/s)  Experiment 2  Single TCP Connection 20

Fast Changing Channel Conditions 21

Impact of Variable Traffic Sources  Configuration 1  Bursty data sources  Pareto distribution 22

Multi-hop Performance  Configuration 2 23

Future work & Conclusion  Basic Access mode in  Not used the RTS/CTS protocol  Hybrid scheme conditional RTS/CTS  When ACKs are lost  When Long packet size  Proposed RBAR  Optimizing performance WLAN 24