cs380y junior thesis1 the performance of tcp/ip over bluetooth chris snow supervisors: serguei...

CS380y Junior Thesis 1

The Performance of TCP/IP over Bluetooth

Chris Snow

Supervisors:

Serguei Primak, Electrical Engineering

Hanan Lutfiyya, Computer Science


Introduction

Ask your questions any time!

Who am I ? What is the point of this research?


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?


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!


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)


“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


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


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 …


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


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


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


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


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)


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


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


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….


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


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)


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


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


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


One channel results


Two channels – C1


Two channels - C2


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


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


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


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.


Questions?

You must have some questions to ask…. ?????

cs380y junior thesis1 the performance of tcp/ip over bluetooth chris snow supervisors: serguei...

Documents