1. Tyrrhenina Workshop, Sept. 2000 1 University of Padova Wireless Internet Access: the BT approach 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

2. Tyrrhenina Workshop, Sept. 2000 2 University of Padova Wireless Internet Access: the BT approach 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

3. Tyrrhenina Workshop, Sept. 2000 3 University of Padova Wireless Internet Access: the BT approach 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. active slave master parked slave standby slave1 slave2 slave3 master

4. Tyrrhenina Workshop, Sept. 2000 4 University of Padova Wireless Internet Access: the BT approach Bluetooth overview: Frequency Hopping 625  s t t master slave f(2k)f(2k+1)f(2k+2) 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.

5. Tyrrhenina Workshop, Sept. 2000 5 University of Padova Wireless Internet Access: the BT approach 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

6. Tyrrhenina Workshop, Sept. 2000 6 University of Padova Wireless Internet Access: the BT approach Bluetooth overview: multi-slot packets f(k) 625  s f(k+1)f(k+2)f(k+3)f(k+4) f(k+3)f(k+4)f(k) f(k+5) 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  )

7. Tyrrhenina Workshop, Sept. 2000 7 University of Padova Wireless Internet Access: the BT approach 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.

8. Tyrrhenina Workshop, Sept. 2000 8 University of Padova Wireless Internet Access: the BT approach 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. ACHEC access codepacket headerpayload 72 54 0-2745 CRC

9. Tyrrhenina Workshop, Sept. 2000 9 University of Padova Wireless Internet Access: the BT approach 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 Notebook Client Notebook Router FTP Server Ethernet Bluetooth TCP Snooper BT Statistics

10. Tyrrhenina Workshop, Sept. 2000 10 University of Padova Wireless Internet Access: the BT approach Packet Dropping Probability Packet Dropping Probability (PDP)Packet Dropping Probability (PDP) Probability of packet drop due to Access Code or Header Checksum failures. Packet Error Probability (PEP)Packet Error Probability (PEP) Probability of packet retransmission due to bad reception: Access Code or Header Checksum or CRC failures ACHEC CRC PDP PEP

11. Tyrrhenina Workshop, Sept. 2000 11 University of Padova Wireless Internet Access: the BT approach 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.

12. Tyrrhenina Workshop, Sept. 2000 12 University of Padova Wireless Internet Access: the BT approach Goodput VS Packet Drop Probability

13. Tyrrhenina Workshop, Sept. 2000 13 University of Padova Wireless Internet Access: the BT approach Segment Service Time

14. Tyrrhenina Workshop, Sept. 2000 14 University of Padova Wireless Internet Access: the BT approach End-to-end performance: transmission window

15. Tyrrhenina Workshop, Sept. 2000 15 University of Padova Wireless Internet Access: the BT approach End-to-end performance: spurious retransmission

16. Tyrrhenina Workshop, Sept. 2000 16 University of Padova Wireless Internet Access: the BT approach 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.

17. Tyrrhenina Workshop, Sept. 2000 17 University of Padova Wireless Internet Access: the BT approach 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.

18. Tyrrhenina Workshop, Sept. 2000 18 University of Padova Wireless Internet Access: the BT approach 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.

19. Tyrrhenina Workshop, Sept. 2000 19 University of Padova Wireless Internet Access: the BT approach PST, SST and RTT DH5 L2CAP Packet Datagram Source Destination BT link (ARQ S&W) L2CAP Packet Datagram PEP DH5 SST RTT PST

20. Tyrrhenina Workshop, Sept. 2000 20 University of Padova Wireless Internet Access: the BT approach 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.

21. Tyrrhenina Workshop, Sept. 2000 21 University of Padova Wireless Internet Access: the BT approach Packet Error Probabilities

22. Tyrrhenina Workshop, Sept. 2000 22 University of Padova Wireless Internet Access: the BT approach Packet Error Probability