In terms of mobile and IP networks new innovative architectures and protocols have been proposed. These enable network operators to provide to customers connectivity through a variety of access network types (e.g. GERAN, UTRAN, wireless LAN, Bluetooth) and to support its functions over various types of mobile communication networks (e.g. GPRS, UMTS), as well as over wired networks (e.g., dial-up connections, wired LANs).

Wireless Local and Metropolitan Area Networks' Technologies

The purpose of the ad-hoc networks is the support of robust and efficient communication between mobile users over limited bandwidth wireless links without the presence of a central control unit. Routing capabilities are passed into mobile terminals, as each node forwards traffic that is destined to other users. These networks are required to support dynamic topologies that are often characterized by increased variance. The main challenge for the achievement of efficient operation of these networks is the limited bandwidth of the wireless links between the nodes. A wireless ad hoc sensor network consists of a number of sensors spread across a geographical area. Each sensor has wireless communication capability and some level of intelligence for signal processing and networking of the data. The efficient operation of a sensor network demands for low power consumption, self-organization capabilities, collaborative signal processing and querying ability.

Reconfigurability concerns the dynamic re-arrangement of functional entities in the communication and computing infrastructure for the support of flexible service provision. Additionally, it improves and enriches the overall user experience by enabling unprecedented services and substantially enhancing existing ones. Towards the introduction of mechanisms and systems to support reconfigurability in various layers (e.g. terminal or network side) several efforts have been undertaken. The CNL is among leading institutions in the reconfigurability domain.

Opportunistic Spectrum Access (OSA) is a novel paradigm aiming to address the issue of spectrum scarcity and meet the increasing demand for spectrum. It provides the capability to identify unused portions of licensed spectrum and share the wireless channel with licensed users in an opportunistic manner. The vision of OSA networks is to provide high bandwidth to mobile users using dynamic and efficient spectrum management techniques. OSA networks impose unique research challenges due to the high fluctuation in the available spectrum, as well as the diverse quality of service requirements of modern applications.

The vision of pervasive computing consists of the unnoticeable integration of computers with people's everyday activities. Computing and networking resources are distributed in physical environments, allowing the users to access information and computational resources anytime anywhere. The technology functions in the background, assisting users' current tasks posing the minimum possible distraction as the interactions with the system are pleasant and natural.Pervasive computing environments introduce new challenges for application development, due to the diversity and the dynamic nature of the devices involved.This vision, sometimes called ubiquitous computing or invisible computing, has inspired many researchers to work on new hardware, networking protocols, human-computer interactions, security and privacy, applications, and social implications.

The design of a protocol, algorithm or architecture that violates the traditional layered architecture is defined as a Cross-Layer Design. An architecture that follows the traditional discrete layers' structure separates the total networking activity in discrete layers and defines a hierarchy of services offered by each layer. This architecture does not allow the communication between non-adjacent layers. In order to satisfy the constantly increasing user QoS requirements, a large number of different performance enhancement mechanisms are introduced in almost all layers of the typical protocol stack. The completely independent operation of these mechanisms can lead to situations of inefficient operation, mainly in cases where conflicting actions are performed. To address this issue, the "violation" of the traditional architecture is proposed, with the adoption of Cross-Layer mechanisms that coordinate the operations of the individual adaptation mechanisms, and provide flexible and efficient solutions in different areas of interest, such as resource allocation, mobility management, QoS provision, security etc.

The protection of the communication and network resources is of vital importance for a networking environment. The key components of secure communication are confidentiality, authentication, message integrity and nonrepudiation and availability and access control. Network security does not only involve the protection, but also the detection of breaches of secure communication and attacks on the infrastructure, as well as the response to these attacks.

• End to end congestion control algorithms for rate adaptable continuous media streams.
• Playout adaptation algorithms for packet video receivers operating in jitter inducing best-effort networks.
• Caching for web and (large) multimedia objects: capacity optimization, object placement and replacement algorithms for large data objects.
• Issues relating to the design and the operation of Content Distribution Networks (CDNs).
• Mechanisms and protocols for provision of quality of service, such as efficient and strongly tied to scheduling disciplines call admission control algorithms.

• Video streaming applications (encoders and players) that deliver live MPEG-1, MPEG-2 or MPEG-4 streams over RTP/IP. Multicast IP and RSVP are supported.
• Video on Demand players that communicate with VoD servers via RTSP for access of multiple video and media files.
• Interactive multimedia-based environment based on SIP, RTSP, RTP and MPEG-4.
• Retrieval, display, process of DICOM images (Medical Images).

• Tele-teaching classrooms based on H.323 systems (terminals, MCU's, Gateways, Gatekeepers).
• Video streaming and Media on Demand services based on both open source (e.g., MBONE tools, Darwin server) and products (e.g., Real, Cisco, Microsoft, Optibase).
• Environment for synchronous and asynchronous e-learning.

The exploitation of location information presents a powerful new dimension to the range of information services that can be offered. By combining positional mechanisms with location sensitive user information, it is possible to offer truly customized personal communication services through the mobile phone or other mobile devices. In order to cope with the management and use of the location information in mobile communication systems the CNL introduces and develops an open and reconfigurable service provision architecture that exploits the location information to provide location aware service management and provision in 3G networks and beyond.

According to Tim Berners-Lee, inventor of the World Wide Web (WWW), the Semantic Web is an extension of the current Web, in which information is given well-defined meaning, better enabling computers and people to work in cooperation. The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries. For the Semantic Web to function, computers must have access to structured collections of information and sets of inference rules that they can use to conduct automated reasoning.

Large Scale Network Deployment (with different types of applications) demands a through Network Dimensioning in order to predict the behavior under varying traffic load. The dynamic behavior of networks is affected by the imposed Workload and the resource consumption. The characterization of the Workload requires adequate stochastic Modeling Techniques. Furthermore, the modeling of resource consumption in the specific nodes and links arises the need for adequate queuing models.

The ever increasing demand for more bandwidth and more sophisticated services poses the requirement to adapt the network architecture (and the respective control and management functions) in order to provide the end-to-end services through different types of network (e.g. UMTS, Wireless LAN, HIPERLAN, Bluetooth). The complexity of these types of networks has to be tackled in order to study not only the functional correctness, but also performance aspects by means of simulation studies.

The emergence of downloadable services and software reconfigurable networks and terminals in new communication era brings to front the requirement for new advanced testing approaches. Protocols, services and applications prior to their final release for downloading and installation have to be tested for meeting user and quality requirements, as well as ensuring conformity and interoperability into the hosting execution environment. Conformance testing captures the technical description of a specification to measure whether a service, application or protocol faithfully implements the specification. The testing result provides developers, users, and operators with increased levels of confidence in product quality and increases the probability of successful interoperability.

In CNL the SDL-based modeling techniques are used for specification of mobile communication protocols (Mobility Management, Resource Control, Call Handling). The performance evaluation of SDL based models with the QUEST tool has been applied in the area of ATM networks and UMTS based mobile radio networks. Furthermore, the discrete event simulation tool OPNET has been used for the performance evaluation of MAC protocols for wireless ATM, the resource management of wireless Internet based on WLAN/HIPERLAN 2, the ATM based infrastructure of the UMTS and the resource management of ATM based WLANs. Due to the convergence of different networking technologies the CNL studies the applicability of the above tools and potential combination of their features in an integrated modeling approach.