Athanasios Vaios

On the Effects of Cooperation in DTNs

In a delay tolerant network (DTN) the nodes may behave autonomously deciding on their own whether to implement or not the rules of a routing algorithm. In this paper, the effects of node cooperation (or lack of it) are explored for three well-known routing algorithms proposed for DTNs with respect to the message delivery delay and the transmission overhead incurred until message delivery or the termination of the message spreading process. The results show that the sensitivity of the algorithms to the cooperation degree can be high, to the point of making them inferior to algorithms they typically outperform under a fully cooperative environment. Finally, it is demonstrated how a simple mechanism that incorporates the cooperation degree can help improve effectiveness.

Study of two-hop message spreading in DTNs

In this paper, a delay tolerant network environment is considered where the source is in full control of the two-hop spreading mechanism by setting key parameters such as the number of copies allowed to be spread in the network and the delay bound of the messages. The analysis allows for a differentiation between the source of the message and the intermediate nodes (in terms of e.g. transmission power or speed). Analytical expressions for the cumulative distribution function (cdf) of the delivery delay and the induced overhead are extracted, taking into account the fact that the source node may continue spreading copies after the message delivery. In addition, a fairly accurate approximate expression for the cdf of the delivery delay is also derived and validated through simulations.

A centralized ad-hoc network architecture (CANA) based on enhanced HiperLAN/2

In ad-hoc networks, where a user can enter, leave or move inside the network with no need for prior configuration, the support of multimedia applications that require very high bit-rates, is a challenging problem. Here, a centralized ad-hoc network architecture (CANA) is proposed, capable of efficiently supporting those applications in low mobility environments, while at the same time a standard wireless LAN environment is maintained for fast moving users. CANA is based on an enhanced HiperLAN/2 protocol architecture [ETR0230002 V0.2.0, 1999-04] [Johnsson, M., 1999], (even though this is not mandatory) that supports a dual mode of operation at 5 GHz and 60 GHz. In this system architecture, several ad-hoc specific functionalities are included, such as neighborhood discovery, clustering and routing. Among them, switching between different modes of operation has a large impact on the achievable performance of CANA.

Approximate analysis of LRU in the case of short term correlations

One of the most widely considered cache replacement policies is least recently used (LRU) based on which many other policies have been developed. LRU has been studied analytically in the literature under the assumption that the object requests are independent. However, such an assumption does not seem to be in agreement with recent studies of Web-traces, which indicate the existence of short term correlations among the requests. This paper introduces an approximate analysis that fairly accurately predicts the hit ratio of the LRU policy in the case of short term correlations. The approximation approach is based on the relation between the working set model and LRU, while the request generation process is assumed to follow a recently proposed model for Web-traces, which captures short term correlations among the requests. The accuracy of the introduced approximate analysis is validated for synthetic as well as real Web-traces.

On the performance of two-hop message spreading in DTNs

In this paper, a Delay Tolerant Network environment is considered where the source is in full control of the two-hop spreading mechanism by setting key parameters such as the number of copies allowed to be spread in the network and the delay bound of the messages. The introduced analysis allows for a differentiation between the source of the message and the intermediate nodes (in terms of e.g. transmission power, speed or cooperation degree). Analytical expressions for the cumulative distribution function (cdf) of the delivery delay and the induced overhead are extracted, taking into account the fact that the source node may continue spreading copies after the message delivery. In addition, a fairly accurate approximate expression for the cdf of the delivery delay is also derived and validated through simulations.

Analysis of a topology control paradigm in WLAN/WPAN environments

The coordinated coexistence of WLANs and WPANs in a dual-mode network is a recently introduced idea and is expected to increase the overall system performance by allowing for the efficient cooperation of both WLANs and WPANs. Topology control (e.g., power control, smart antennas, and different frequency channels) needs to be employed to allow for the simultaneous operation of both modes. In this paper, different frequency channels that allow for high data rates within a small transmission range are considered in order to create multiple WPAN environments inside a WLAN cell. The latter environment requires the support of a second mode of operation which introduces additional overhead that may degrade the overall system performance. Certain conditions, under which system performance improvement is achievable, are established here. In particular, an analytical mobility model for WPAN environments is proposed and employed in the analytical studies. It is shown that the system may be effective when node mobility is low and the traffic load among nodes is high. The corresponding upper and lower bounds on mobility and traffic are also analytically derived. Simulation results for a variety of scenarios support the claims and expectations of the aforementioned analysis and demonstrate that performance improvement is possible when WLANs and WPANs coexist and cooperate in a network.

IST Project: BroadWay – The Way to Broadband Access at 60 GHz

BroadWay (IST-2001-32686) is an Information Society Technologies project. It is a three-year project, which started in 2002, and it is partly funded by the Commission of the European Community. The partners involved in the project are the Motorola Labs (France), TNO-FEL (Netherlands), Intracom and the National & Kapodistrian University of Athens (Greece), IMST, IRK and Dresden University of Technology (Germany) and FARRAN (Ireland).

A dual-band HiperLAN/2-based architecture for indoor hotspot applications

The 1ST BroadWay project, [M. de Courville et al., June 2002], introduces the ad-hoc networking paradigm in the traditional 5 GHz HiperLAN/2. The ad-hoc networking paradigm is employed at the 60 GHz frequency band, which allows for a high transmission rate communication. The dual mode of operation primarily aims at offloading the 5 GHz HiperLAN/2 cell in very dense urban deployments (high traffic needs and number of users), while the peculiarities of the BroadWay system induce modifications on the existing HiperLAN/2 system and make the development of a routing scheme a rather challenging task. The centralized ad-hoc network architecture (CANA) is described as a means to efficiently support multimedia applications that require very high bit rates in low mobility environments; several ad-hoc specific functionalities are included such as neighborhood discovery, clustering and route selection. BroadWay introduces modifications to the MAC of HiperLAN/2 regarding new messages and framing considerations at both frequency bands to cater for the new requirements imposed by the dual mode of operation.

Triggering wireless network flashes

Recent advances in wireless technology make high data rate communication in short ranges feasible. The network instances of short-range paths that may achieve high data rates are defined as network flashes in this paper. The characteristics and the efficiency of network flashes are discussed here.

Layered architecture and modules of CANA supporting dual mode HiperLAN/2

The centralized ad-hoc network architecture (CANA) is an efficient way to support dual mode systems, [Oikonomou, K, et al., Sept. 2003]. A dual mode of operation has been introduced in the traditional 5 GHz HiperLAN/2 to allow data off-loading by using resources of an ad-hoc network created at 60 GHz (the dual mode concept was introduced by the IST BroadWay project, IST-2001-32686). The off-loading is possible using CANA but certain changes in the standard HiperLAN/2 protocol stack are required and certain algorithms need to be incorporated. In this paper, the new enhanced protocol stack of HiperLAN/2 that supports CANA is presented as well as key algorithms that need to be developed and whose performance is crucial for the efficient utilization of the system resources. These algorithms can also be implemented for any other WLAN in order to support a second mode of operation. Simulations shed light into the trade-offs induced by the dual mode of operation and the way the network resources may efficiently be utilized under certain conditions.