CS380y Junior Thesis1 The Performance of TCP/IP over Bluetooth Chris Snow Supervisors: Serguei Primak, Electrical Engineering Hanan Lutfiyya, Computer Science

CS380y Junior Thesis2 Introduction Ask your questions any time! Who am I ? What is the point of this research?

CS380y Junior Thesis3 Topics of Discussion A brief intro to Bluetooth Wireless channel modeling TCP/IP in a wireless environment Simulations and results So what does this all mean?

CS380y Junior Thesis4 Bluetooth A short range (usually 10m), low- power wireless protocol Used in mobile phones, PDAs, laptop computers, wireless headsets, ….. Allows for automatic detection and connection establishment The ultimate plug-and-play solution!

CS380y Junior Thesis5 Bluetooth usage scenario A mobile phone supporting data and a laptop computer With a Bluetooth connection, the laptop can use the phone’s data connection to get Internet access No cables needed! Just put the phone beside the computer (or even in your pocket – up to 10m)

CS380y Junior Thesis6 “In-depth” Bluetooth Devices form piconets, with 1 master and up to 7 slaves Any Bluetooth device can act as a master or a slave! Roles change… Devices can be part of more than one piconet at the same time Scatternets can be formed – collections of piconets

CS380y Junior Thesis7 Bluetooth Packets There are about 15 types of Bluetooth packets The important variations between types are: size (number of time slots) and use of forward error correction (FEC) The six important types are: 1,3, and 5 time slots, w/ and w/o FEC

CS380y Junior Thesis8 Wireless Channels The channel is whatever medium the message passes through between sender and receiver – in this case, the air, walls, buildings, etc. It is very difficult to characterize a wireless channel, because it is both spatially and temporally varying …

CS380y Junior Thesis9 Fading Channels The error pattern of the channel is not uniformly distributed Some event may cause increased loss in the channel for a period of time, then the channel state improves This is a fading channel – a major area of communications research

CS380y Junior Thesis10 Modeling Wireless Channels In order to simulate a fading channel, we need to develop a model This model should represent the quality of the channel as a function of one or more parameters A common model is the Gilbert- Elliot (GE) model

CS380y Junior Thesis11 The single channel Gilbert-Elliot Model In the GE model, the channel modeled as a two-state Markov process Either the channel is “good” or it is “bad” The model has several parameters: correlation, probability of a Good channel, and the probabilities of error given that we are in the Good or the Bad state

CS380y Junior Thesis12 Gilbert-Elliot (cont’d) The correlation relates successive packets in time. With non-zero correlation, the channel has some tendency to stay in the same state on transitions Every time a packet is received, the new state is computed (Good or Bad) and the packet reception is evaluated based on the relevant probability of error

CS380y Junior Thesis13 Two-channel GE model When we have two channels, we can model them in several ways: Uncorrelated – the two channels are completely unrelated (e.g. they are physically remote) Completely correlated – identical behaviour on the two channels (e.g. they pass through the same physical space)

CS380y Junior Thesis14 Two channels (cont’d) The third option is to provide some level of correlation between the channels This model defines 4 aggregate states based on the state of each channel (e.g. G1G2, G1B2, etc) We can then define the transition equations based on the correlation of each channel and the correlation between channels (alpha) The spatially correlated channel can be studied to examine the effect of different correlation levels

CS380y Junior Thesis15 TCP/IP The TCP protocol provides for connections between hosts on an IP network Includes many performance enhancers, including “sliding windows” and “back-off” The main problem: TCP/IP was designed for a wired network such as Ethernet

CS380y Junior Thesis16 TCP/IP with wireless In a wireless environment, many things cause packet loss TCP/IP’s mechanisms for dealing with failed transmits are not optimal for a wireless channel So why use TCP/IP ? Interoperability….

CS380y Junior Thesis17 The simulations I extended the Bluehoc package (an open-source Bluetooth simulator) to use the one- and two- channel models described above Then I studied the effects of different correlations, error rates, etc

CS380y Junior Thesis18 GE model in Bluehoc For performance reasons, the error model in Bluehoc is packet-based (either a packet is received, or it isn’t) The previous model just provided a table of probabilities of error, based on packet type and distance (calculated based on channel measurements by Rappaport)

CS380y Junior Thesis19 Fixing the GE model Different packet types and distances have different probabilities of error! The GE model assumes a fixed probability of error – we shouldn’t constantly change it This creates a big problem, because the error sequence now varies depending on the packet sequence ordering, which isn’t right

CS380y Junior Thesis20 The solution To fix this, we establish a baseline error rate (the maximum – 5 slot length, without FEC), which is fixed “Scaling” is done based on the actual distance and packet type Instead of the prob. of error in a bad state being 1, we modify it based on the packet type and distance

CS380y Junior Thesis21 Back to simulating Now that we have fixed the GE model, we can apply it to some simulations The main study involves an FTP connection between one master, and one or two slaves

CS380y Junior Thesis22 One channel results

CS380y Junior Thesis23 Two channels – C1

CS380y Junior Thesis24 Two channels - C2

CS380y Junior Thesis25 Correlation Sharing The previous two graphs show the throughput as a function of C2 correlation C1 correlation is fixed at 0 (i.e. the errors occur randomly) Note that the C2 performance doesn’t change very much However, the C1 performance looks much like the single-channel results

CS380y Junior Thesis26 Correlation Sharing 2 What happens here is that because the two channels are correlated (the alpha parameter), the state of C2 affects C1 Even though C1 has randomly occurring errors, the spatial correlation causes some portion of the C2 error stream to be superimposed on C1 This means that the effective correlation value for C1 isn’t actually zero C2 is not affected by correlation sharing, because C1 has zero correlation

CS380y Junior Thesis27 So what does this mean? In some circumstances, the performance predicted by a simple model may be completely different than the actual performance The use of a more realistic channel model (with non-uniform distribution of error) provides a more accurate model of performance TCP/IP performance over a Bluetooth link could be really bad, depending on the physical situation

CS380y Junior Thesis28 Acknowledgements Thanks to Hanan and Serguei for acting as supervisors! C. Snow, and S. Primak, “Performance Evaluation of TCP/IP in Bluetooth Based Systems”, Proc. IEEE VTC Spring ’02, in press.

CS380y Junior Thesis29 Questions? You must have some questions to ask…. ?????