1 A Bluetooth Link Markov Model: Simulation and Performance Evaluation under NS-2 CS215 - Computer Communication Networks - Winter 2001 Project March 22, 2001 Alessandro Bissacco Massimo Valla

2 Agenda BT Channel Markov Model description Implementation under NS-2 Simulated Environment Simulations Results Future Work

3 BT Packet Format PACKET TYPES Protected DM1, DM3, DM5 Unprotected DH1, DH3, DH5 p. head access codeheader payload 72bits 54 bits bits dataCRCFEC 8-16 bits 16 bits

4 What We are Modeling Radio channel propagation is characterized by three main parameters: Attenuation: free space loss, absorption by foliage, partitions Shadowing: obstacles between transmitter and receiver Multipath: due to the different phases on different paths master slave Indoor, fixed terminals moving obstacles

5 SNR Transitions and Markov Chain        S = Synchronization Failure (AC or HEAD error) B = Bad State (non zero residual bit error probability) G = Good State (totally error free condition)  time SNR (at receiver)   S B G q P p Q    e S = 1 eBeB e G = 0  

6 The BT Channel Model  The transition time T S of the Markov Chain is the BT bit time (T S =1 s)  The Markov Chain is initialized after each frequency hop (-> at each BT packet)  Each state of the Markov Chain corresponds to a bit-error probability e i : e i = Pr(bit error | Markov Chain state = i) We define: PDP = Pr(unrecoverable error in the HEADER or AC fields of the BT packet) P CRC = Pr(unrecoverable error in the PAYLOAD of the BT packet) PEP = Pr(unrecoverable error in the BT packet) PEP = PDP + (1-PDP)*P CRC PDP = P S P CRC depends on the packet type (protected, unprotected) and payload size. 3-state Discrete-Time Markov Chain   p q Q  P BSG

7 Error vector, Steady State Pr. and Transition Pr. The error vector e=[e S e B e G ] is: e S = 1, e G = 0 and e B = 2.5E-3 is obtained empirically from measured P CRC SSP: f  = p.d.f of SNR (t) The transition probabilities t i,j are computed using the SNR thresholds crossing rates:

8 P CRC for DHn packets There is an analytical formulation for P CRC for DHn packets: Where: L same as J with neg. sqr. root h b = 1 – e B N = BT payload length (in bits) for current packet

9 DTMC Model and Experimental Results

10 Wireless Phy BT MAC Channel NS-2 Implementation (1) Class BTWirelessPhy: public WirelessPhy receiver Wireless Phy Added to NS-2 BTWireless Drop packet if AC or Head Error Set error_ = 1 if PAYLOAD error BT MAC Wireless Phy sender

11 NS-2 Implementation (2) Pseudo-code: For each new incoming BT packet: based on SNR at the receiver, init. the MC: compute PDP = P S, P G, P B and all other parameters; sample a random number r 1 between 0 and 1; if r 1 < PDP then drop packet; else { using packet type (DM or DH) and payload length, compute P CRC ; sample a random number r 2 between 0 and 1; if r 2 < P CRC then error_ = 1; // packet will be dropped by the MAC layer send up packet to the MAC layer; }

12 Simulation Environment Simulation Parameters: Node distance 8 mt. Simulation time: 15 sec. Propagation Model: Free Space (NS-2 module) Traffic source: FTP (started at 1 sec.) TCP segments: 1,000 bytes BT buffer: 1,000 DH1 packets (i.e. 30,000 bytes) Various TCP versions: Tahoe, Reno, Westwood 01 masterslave

13 PEP vs Node Distance

14 TCP Tahoe

15 TCP Reno

16 TCP Westwood 8 7

17 Last Ack seen from receiver

18 Goodput (Tahoe, Reno, Westwood) (DH5 pkts)

19 TCP and UDP - Tahoe UDP: 600 Kbps

20 TCP and UDP - Westwood UDP: 600 Kbps

21 Future Work Deeper analysis of current simulation results Do more simulations to measure: packet drops rtxs delays and RTTs Simulations using Scatternets to increase RTT due to delays on gateways More simulations using different node distances to increase PEP Simulations with multiple TCP and UDP flows Thanks: Rohit Kapoor (NS-2 and BT MAC help) and Andrea Zanella (project mentor)