Laila Daniel will defend her thesis "Cross-layer Assisted TCP Algorithms for Vertical Handoff" on Saturday,

20 November 2010 at 10.00. in Auditorium XII in the Main Building. Her opponent is Professor Xiaoming Fu,

Georg-August-University of Göttingen, Germany, and her custos Professor Jussi Kangasharju.

 Popular abstract:

Increasingly we use our mobile devices (e.g., mobile phones, laptops) for diverse applications such as
reading e-mail, browsing the web, downloading and listening to music, playing games and for making payments
for products and services irrespective of our location and mobility by connecting the mobile device to
the Internet anytime anywhere.

Access networks that enable a mobile device to connect to the Internet use diverse technologies and differ
widely in their characteristics. Mobile devices inherently use wireless access networks by means of radios
for Internet connectivity. For example, Wireless Local Area Network (WLAN) is a fast network that can be used
inside a building whereas General Packet Radio Service (GPRS) is comparatively slower and can span a city or
a country and even beyond. A mobile device with multiple radio interfaces can changeover to any of the several
access networks depending on its location or the application needs. Handoff refers to this changeover and it
is known as vertical handoff when the underlying technologies of the two access networks are different.

TCP, a data communication software, which resides both at the sender and the receiver of data, delivers the
application data reliably and also adjusts its sending rate depending on the availability of the resources
in the Internet. TCP behaviour depends on the characteristics of the end-to-end path and in particular the
bottleneck link, the link with minimum capacity in the path. A wireless link that connects the mobile device
to the Internet is often the bottleneck link and the abrupt change in its characteristics due to a vertical
handoff significantly affects TCP performance and consequently the application quality.

The focus of this thesis is to study TCP behaviour in a vertical handoff and to devise algorithms to improve
its handoff performance using cross-layer notification to convey information about the characteristics of the
access links involved in the handoff. Our first study is in the WLAN-GPRS environment with minimum information to
TCP about the handoff and the results show that TCP performance can be significantly improved. Subsequently we
enlarge the scope of the study to cover a more general case using rough estimates of the access link parameters.
The evaluation of the algorithms is based on simulation experiments with a wide range of access networks showing
that TCP performance in handoff can be significantly improved using this approach. Our algorithms can be useful
to devise solutions for the real world scenarios.