Study Goals & Talk summary

1. Study Goals & Talk summary • Aims • Evaluation of FTP performance over Bluetooth (BT) radio link in different environmental conditions • Influence of BT radio packet format on system performance • Outline • TCP over wireless link • Bluetooth overview • Methodology of analysis & Performance metrics • Main results • Conclusions & Future Work Wireless Internet Access: the BT approach

2. TCP over Wireless Links • A hard coexistence • TCP is tuned to work well in wired networks • Wireless Link can produce packet losses not related to congestion • These events may trigger useless congestion reaction mechanisms, resulting in sub-optimal performances • The “Link Layer” solution • Idea: • providing radio link reliability by using local retransmissions • Drawback: • possibility of bad interaction between TCP and Link Layer retransmission mechanisms Wireless Internet Access: the BT approach

3. master active slave parked slave standby Bluetooth overview: piconet • Two up to eight Bluetooth units sharing the same channel form a piconet. • In each piconet, a unit acts as master. • Channel access is organised on the bases of a centralised polling scheme. slave2 slave3 master slave1 Wireless Internet Access: the BT approach

4. f(2k) f(2k+1) f(2k+2) master t slave t 625 ms Bluetooth overview: Frequency Hopping • Each piconet is characterised by a pseudo-random frequency hopping sequence, imposed by master. • All the units in the same piconet hop synchronously. • Time is divided into slots of 625 s; each slot corresponds to a different hop frequency. • Consecutive packets are transmitted on different RF carriers. Wireless Internet Access: the BT approach

5. A B B X C MASTER NAK ACK G F H SLAVE 1 Z Z SLAVE 2 Bluetooth overview: ARQ scheme • An Automatic Retransmission Query (ARQ) mechanism grants the reliability of asynchronous data traffic (ACL) • 1-bit fast ACK/NAK • 1-bit sequence number • header piggy-backing Wireless Internet Access: the BT approach

6. 625 s f(k) f(k+1) f(k+2) f(k+3) f(k+4) f(k+5) f(k) f(k+3) f(k+4) f(k+5) f(k) f(k+5) Bluetooth overview: multi-slot packets • A baseband packet can extend over one, three or five consecutive slots. • The carrier frequency remains unchanged for the whole packet duration. • Multi-slot packets reduce bandwidth losses due to packet header and PLL settling time (220) Wireless Internet Access: the BT approach

7. Bluetooth overview: DH & DM packets • Asynchronous data packet can be optionally protected by a 2/3 Forward Error Correction (FEC). • Protected packet formats realise medium data rate and are noted with DM. • Unprotected packet formats realise higher payload capacity but are more subject to errors. They are noted with DH. Wireless Internet Access: the BT approach

8. 54 72 0-2745 AC HEC CRC access code packet header payload Bluetooth overview: packet format • Access Code (AC) • All packet exchanged in a piconet have the same AC. • Packets that don’t satisfy AC test are immediately discarded. • Packet Header • Contains, among other information, slave active member receiver address, ARQ flags, payload format, header checksum field (HEC). • If the HEC test fails, the packet is immediately discarded. • Payload • If the CRC test fails, the packet is negative acknowledged. Wireless Internet Access: the BT approach

9. Client Router FTP Server Bluetooth Ethernet Notebook Notebook BT Statistics TCP Snooper Methodology of Analysis • Bluetooth radio link connects a nomadic client to a FTP server. • Snooping programs collect end-to-end and point-to-point transmission statistics. • A series of large bulk data transfers have been performed, with notebook in different positions. • Data collected have been analysed to extract system performances. Measurement Platform Wireless Internet Access: the BT approach

10. Packet Dropping Probability • Packet Dropping Probability (PDP) • Probability of packet drop due to Access Code or Header Checksum failures. • Packet Error Probability (PEP) • Probability of packet retransmission due to bad reception: Access Code or Header Checksum or CRC failures AC HEC CRC PDP PEP Wireless Internet Access: the BT approach

11. Performance indexes • Radio Link performance metrics • Goodput: • average number of bit transmitted successfully in master to slave direction, in the unit of time. • End-to-end performance metrics • Segment Service Time (SST): • time employed by the BT entities to transmit a TCP segment through the radio link. • TCP sender transmission window size. Wireless Internet Access: the BT approach

12. Goodput VS Packet Drop Probability Wireless Internet Access: the BT approach

13. Segment Service Time Wireless Internet Access: the BT approach

14. End-to-end performance: transmission window Wireless Internet Access: the BT approach

15. End-to-end performance: spurious retransmission Wireless Internet Access: the BT approach

16. Conclusions • Longest unprotected packet format (DH5) realises the highest performance in almost all the situations considered. • FEC protected packets overrun unprotected ones only in particularly hostile channel conditions. • Mean and standard deviation of the Segment Service Time grow rapidly when PDP moves near one. • In general, TCP well follows RTT oscillations except when PDP changes drastically during the transmission. Wireless Internet Access: the BT approach

17. Open Issues and Future Work • Definition of a mathematical model for the Bluetooth radio connection. • Study of methods to protect TCP sender against drastic variations of environmental conditions. • Performance analysis in systems with more than two units per piconet. • Study of hand-off problem between piconets. • Routing algorithms for scatternet. Wireless Internet Access: the BT approach

18. Segment Service Time: measurement problems • At the moment, we cannot directly measure the SST, because • probing programs suffer of some drawbacks: • master & slave statistics are collected independently, • probing time is not always constant; • we are not able to distinguish radio packets belonging to the same TCP segment. • We can estimate the SST statistic by using traditional queue theory. Wireless Internet Access: the BT approach

19. PST, SST and RTT Destination Source RTT Datagram Datagram SST L2CAP Packet L2CAP Packet DH5 DH5 DH5 DH5 DH5 DH5 DH5 DH5 PST BT link (ARQ S&W) PEP Wireless Internet Access: the BT approach

20. Segment Service Time: statistic estimation • Let PST be the number of transmissions attempts until positive acknowledgement. • PST is a modified geometric random variable, with mean 1/PEP: • Let N be the number of radio packets needed to carry a whole TCP segment. • Since each packet requires PST transmission, SST is given by the sum of N i.i.d. random variables: • Hence, SST results a random variable with modified Pascal distribution. Wireless Internet Access: the BT approach

21. Packet Error Probabilities Wireless Internet Access: the BT approach

22. Packet Error Probability Wireless Internet Access: the BT approach