Gerasimos Dimitriadis

Seamless continuity of real-time video across UMTS and WLAN networks: challenges and performance evaluation

This article addresses several challenges related to the evolution toward seamless interworking of wireless LAN and 3G cellular networks. The main objective is to evaluate the conditions and restrictions under which seamless continuity of video sessions across the two networks is feasible. For this purpose, we formulate a number of practical interworking scenarios, where UMTS subscribers with ongoing real-time video sessions hand over to WLAN, and we study the feasibility of seamless continuity by means of simulation. We particularly quantify the maximum number of UMTS subscribers that can be admitted to the WLAN, subject to maintaining the same level of UMTS QoS and respecting the WLAN policies. Our results indicate that the WLAN can support seamless continuity of video sessions for only a limited number of UMTS subscribers, which depends on the applied WLAN policy, access parameters, and QoS requirements. In addition to this study, we do address several other issues that are equally important to seamless session continuity, such as the QoS discrepancies across UMTS and WLAN, the vertical handover details, and various means of access control and differentiation between regular WLAN data users and UMTS subscribers. The framework for discussing these issues is created by considering a practical UMTS/WLAN interworking architecture.

Satellite handover techniques for LEO networks

Low earth orbit satellite constellations could play an important role in future mobile communication networks due to their characteristics, such as global coverage and low propagation delays. However, because of the non-stationarity of the satellites, a call may be subjected to handovers, which can be cell or satellite handovers. Quite many techniques have been proposed in the literature dealing with the cell handover issue. In this paper, a satellite handover procedure is proposed, that investigates and exploits the partial satellite diversity (namely, the existing common coverage area between contiguous satellites) in order to provide an efficient handover strategy, based always on a tradeoff of blocking and forced termination probabilities for a fair treatment of new and handover calls. Three different criteria were examined for the selection of a satellite. Each one of them could be applied either to new or handover calls, therefore we investigated nine different service schemes. A simulation tool was implemented in order to compare the different service schemes and simulation results are presented at the end of the paper.

Comparative performance evaluation of EDCF and EY-NPMA protocols

Medium access represents one of the most critical building blocks regarding the performance of a wireless LAN. In this letter, we compare the performance of two well-known medium access control protocols specifically developed for the wireless environment, EDCF and EY-NPMA. To our knowledge, it is the first time that these two quality-of-service (QoS) aware medium access schemes are compared. Furthermore, we propose a novel scheme for medium access based on EY-NPMA, that demonstrates reduced overhead compared to the base protocol and better utilization of the common medium. The conclusions of this paper are based on extensive simulation trials.

Two Alternative Schemes to EY-NPMA for Medium Access in High Bitrate Wireless LANs

The growing penetration of WLANs in the market, as well as the wider spectrum of applications that these kinds of networks are called to support, demands the introduction of new methods for efficient medium access. Today, the two dominating paradigms in medium access control are those of dynamic assignment and contention based ones. EY-NPMA is a contention based scheme that exhibits very low collision rates, while being quality of service aware. In this paper, we propose a solution to the highly criticised drawback of the EY-NPMA protocol — the increased overhead. Towards this end, we propose and evaluate the performance of two medium access schemes that are based on EY-NPMA and compare them to the base protocol. The theoretical analysis of the proposed schemes shows that they make better utilization of the common medium. It also allows us to determine the optimal working parameters for each one. Furthermore, extended simulation trials validate these results and confirm the good characteristics of the proposed protocols.

On the Performance of the HSLS Routing Protocol for Mobile Ad hoc Networks

The area of mobile ad hoc networks has recently attracted much scientific interest, as a very appealing research area with many open issues and still unsolved problems. One of the main issues that concerns researchers is the development of routing algorithms that present good performance and face a hostile environment. Many routing protocols have been proposed, attempting to minimize routing overhead, or to reduce the energy consumed by nodes in order to maximize their lifetime. A critical issue, though, is the development of routing protocols that have the ability to maintain their good characteristics at an acceptable level as the network population grows, an ability known as scalability. FSR, ZRP, HierLS and FSLS protocol family are only a sample of scalable algorithms that have been proposed so far. The HSLS protocol is a member of the FSLS family that is proved to scale the best among the algorithms of the FSLS protocol family. In this paper we propose a mechanism to enhance the already good characteristics of the HSLS protocol aiming at the reduction of routing overhead of the original protocol. This new scheme, which we called AFHSLS, exploits the so-called border nodes, in order to deliver routing packets to their destinations. The new algorithm is proved through simulations to significantly reduce routing overhead, with minor or practically no effect on other metrics, such as packet delivery ratio and delay of data packets.

Clustered fisheye state routing for ad hoc wireless networks

This paper presents a routing scheme based on fisheye state routing (FSR), which employs a clustering framework in order to reduce the redundancy in the broadcasts of topology information. Clustered fisheye state routing (CFSR) consumes less bandwidth by restricting the propagation of routing control messages in paths formed by alternating gateways and clusterheads, and allowing the gateways to selectively include routing table entries in their control messages.

Two-Hop Polling: An Access Scheme for Clustered, Multihop Ad hoc Networks

In multiple channel environments, clustering provides a convenient framework for channel access and bandwidth allocation. Many clustering schemes, however, demand that terminals may communicate directly only if they share a common clusterhead. This requirement deactivates otherwise helpful links; those between nodes that belong to different clusters (intercluster links). Links between nodes that belong to different clusters constitute a distributed gateway. In this paper, we evaluate the importance of distributed gateways for two different clustering schemes and propose a novel access scheme for clustered environments using the link-cluster architecture, called two-hop polling (2HP). Two-hop polling manages to utilize intercluster links, leading to better connectivity and throughput.

Variable yield for adaptive EY-NPMA

Previous studies have shown that significant gains can be achieved, when the parameters of the EY-NPMA protocol are updated on the fly. When EY-NPMA adapts dynamically to the offered load, improved figures both in throughput and access delay are observed. The estimation of the number of contending nodes, which is necessary for the calculation of the optimal parameters of EY-NPMA, also provides room for further enhancement of the protocol through the linking of the backoff distribution of the yield phase with the outcome of the elimination phase. In this work, a mechanism that implements this concept is proposed, with simulation results documenting its positive characteristics.

Adaptive EY-NPMA: A medium access protocol for wireless LANs

Wireless local area networks have known an increasing popularity during the past few years. However, as new user applications diverge from the traditional data-centric model, the introduction of efficient, QoS aware medium access methods becomes of utmost importance. EY-NPMA is a medium access protocol belonging to the contention paradigm that provides support for service differentiation and low collision rates. In this paper, we address a shortcoming of EY-NPMA as indicated by previous studies, namely the insensitivity of the protocol to different working conditions. In this work, we study and evaluate a mechanism that allows a network employing EY-NPMA to adapt its operating parameters according to the offered load. Simulation studies further document and confirm the positive characteristics of the proposed mechanism.

On the Performance of a Medium Access Scheme Based on EY-NPMA in a High Bitrate Wireless LAN

Wireless LANs provide an efficient and inexpensive way for the creation of networks that do not constrain the users’ mobility. Furthermore, advanced physical layer techniques have allowed wireless LANs to support bitrates that until recently were attainable only in wired networks. The growing penetration of WLANs in the market, as well as the wider spectrum of applications that these kinds of networks are called to support, demands the introduction of new methods for efficient medium access. In this paper, we propose and evaluate the performance of a medium access scheme that is based on the EY-NPMA protocol for medium access, showing through simulation the good characteristics of the proposed scheme.