Hicham KHALIFE, UPMC-LIP6

Location :

UPMC-LIP6 CNRS

104 avenue du president Kennedy
75016 Paris 

Room 847 

Abstract :

Wireless networks have experienced drastic evolutions in the last few years.
While many aspects related to ¿single cell¿ wireless networks were already
resolved, wireless multihop networks still suffer poor performances and limited
capacities. This thesis aims at providing acceptable quality of service in
terms of throughput and loss ratio to end users in various wireless multihop
environments. 

Today, access control in multihop wireless networks is mainly achieved through
the IEEE 802.11 protocol at the MAC layer. At the transport layer TCP, the
widely used protocol, adds an end-to-end control mechanism. Essentially, the
final goal of many if not all these techniques is to enhance either the global
system or a particular connection throughput. However, due to the numerous
challenges introduced by the wireless multihop, these two control levels, even
when used together, show numerous limitations. These new challenges originate
basically from the fact that the IEEE 802.11 and TCP protocols at the time of
their conception were not targeting the specific constraints added by the
wireless multihop environments. In fact, many aspects such as the interference,
the hidden nodes, the contentions and the lossy link properties that appeared
in the wireless multihop environments made the existing MAC and transport layer
control mechanisms become inefficient hence inappropriate. 

In this thesis, we propose three control mechanisms for wireless multihop
networks aiming to improve the achieved goodputs seen by a user. We first start
by presenting an analytical model to evaluate the hidden terminal effects in an
IEEE 802.11 network.  This model is then exploited to tune MAC layer control
parameters and we show that when this is done in conjunction with the TCP
control, throughput enhancement can be achieved. Second, since the wireless
medium gives broadcast for free, we exploit this feature to design HbH our
second control technique. Considering that not all data exchange entail a TCP
connection, with HbH we enforce the MAC layer control by using the broadcast
nature of the wireless medium. When using HbH, every node after the
transmission of a packet continues to listen on the channel to make sure that
its neighbor also transmitted the packet before the former decides to discard
it. HbH can be even used to detect and react to node failures in multihop
wireless networks due to mobility for instance. 

Third, we enlarge the spectrum of our research to look at multihop Cognitive
Radio Networks (CRNs). In this new emerging domain where users choose
opportunistically spectrum bands to use for their transmission, basic control
issues are still unsolved.  Especially, at the routing layer if the multihop
topology is built without accounting for the instability of the used channels
due to the primary nodes (licensed) dynamics we can get a close to null
throughput. For this reason, we propose the Most Probable Path routing protocol
MPP, one of the first routing solutions for multihop cognitive radio networks.
Our protocol is based on choosing a path and the set of channels to use between
the source node and the destination probabilistically based on estimating the
activity and the placement of primary nodes. Those must keep absolute priority
on the considered channels.  In this thesis our conducted studies are based on
both analytical models and simu- lations. 

keywords: IEEE 802.11, wireless multihop, control, cognitive radios

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