Christos Zarafetas's research while affiliated with University of Thessaly and other places

The adoption of Network Functions Virtualization (NFV) is considered as one of the enablers for a fully softwarized 5G architecture, that allows significantly higher flexibility for network service providers to instantiate services, conFigure and update them, as well as to truly realize multi-tenancy. The key enablers of the NFV technology lie in the evolution of virtualization technologies, such as the long established Virtual Machines, and the more recent paradigms of containers for micro-services, like docker and LXC. Such technologies allow the virtualization to span even to the wireless RAN, with fully softwarized base station architectures. Nevertheless, with the plethora of different virtualization technologies, several Virtual Infrastructure Managers (VIMs) and service orchestrators have emerged, each of them addressing different aspects for the provided services e.g. possible nomadic behaviour of the hosting computers, resource constrained devices hosting the services and services deployed for time critical services to name a few. In this work, we use a reference setup and experiment with some of the most widely adopted infrastructure managers, trying to experimentally derive and validate their differences under varying loads of traffic. Our results reveal the time needed for instantiating the same service function, for dockers, containers and Virtual Machines, for a different number of VNFs.

Network Functions Virtualization Management and Orchestration (NFV-MANO) provides a standardized approach for the management and effortless deployment of (virtual) services. Although NFV-MANO initially focused on the deployment of services over datacenters, the introduction of fully softwarized network architectures even for the wireless network creates fertile ground for the re-conception of the manner through which the underlying hardware is managed. In this paper, we consider the case of an open experimentation testbed, with focus on wireless networking, and adopt the Open Source MANO framework for provisioning virtual services on top of the experimentation equipment. We extend the Virtual Infrastructure Managers (VIMs) of the NFV-MANO architecture in two well-known frameworks (Openstack and OpenVIM) for the testbed in order to create and handle virtualized wireless network interfaces, hosted on the generic networking nodes of the testbed. Through our contributions, existing VNFs can be deployed over the testbed interconnected over wireless links, specified during the on-boarding phase. The extensions are introduced transparently to the existing operation of the platform, in order to allow the portability of network services and network functions to other instances as well. We focus on providing virtual functions inter-networked over wireless links, which traditionally are not handled by the framework, and allow easier interaction of the end-users with the testbed. We benchmark the framework in terms of performance and analyze the deployment case of a softwarized 5G Radio Access Network in a real testbed deployment.

5G networks bring increased flexibility for the operator at different levels. On one side, RAN disaggregation based on the Cloud-RAN concept allows the instantiation of base stations in an area based on demand, whereas on the other side NFV-MANO orchestration brings effortless delivery of services deployed over distributed infrastructure. In this paper, we demonstrate a disaggregated heterogeneous Cloud-RAN, consisting of cellular and WiFi infrastructure, deployed through the Open Source MANO orchestrator in the distributed infrastructure of a testbed. Through a pair of mmWave and WiFi redundant links, we backhaul the RAN and in case of a broken link, seamlessly switch technologies without affecting the traffic delivered to the end-user.

Cloud-RAN paves the way for flexible network management and control in the upcoming 5G and beyond networks. The base station disaggregation in different functional elements facilitates the incorporation of heterogeneous technologies in the user access network (e.g. 5G-NR, LTE, WiFi). Network densifica-tion and integration of heterogeneous technologies enables larger network capacity through the aggregation of multiple links, thus assisting the transition from the existing network infrastructure to innovative 5G networks. Nevertheless, as Ultra-Dense Heterogeneous Networks may operate in the same wireless spectrum, their performance potential may be hindered through the operation in overlapping frequencies. Thus, efficient coordination is required between the involved heterogeneous technologies. In this work, we consider a disaggregated base station setup, based on the current standards for 5G-NR, with capabilities to incorporate heterogeneous technologies for serving the UEs. We develop signalling between the heterogeneous Distributed Units and the Central Unit, and apply a spectrum coordination algorithm for optimal use of the wireless spectrum. We use OpenAirInterface as our development platform, and evaluate our results in a real testbed setup.

Network Functions Virtualization Management and Orchestration (NFV-MANO) aims to provide a common interface for technology developers and service operators for the instantiation of virtual network functions over generic equipment. Nevertheless, although NFV-MANO is currently targeting datacenter operations and the deployment of virtual services (through Virtual Machines or containers), it can easily extend to generic networking devices, and alter their operation based on the deployed network functions. In this paper, we consider the case of a wireless testbed, with focus on wireless networking, and adopt the OpenSourceMANO framework for provisioning of services on top of the equipment. We provide the necessary extensions to the framework in order to deploy services over virtualized wireless network interfaces, hosted on the generic networking nodes of the testbed. The extensions we focus on regard the provisioning of virtual functions over wireless links, that are traditionally not handled by the framework, and allow easier interaction of the end-users with the testbed.

Cloud-RAN based architectures are widely considered a fundamental part of 5G networks. As a consequence, in the upcoming standards for 5G RAN, disaggregating the RAN functionality between a Central Unit (CU) and multiple Distributed Units (DUs) is considered, addressing the splitting of the 5G protocol stack at the PDCP/RLC point. This split is expected to bring numerous advantages to mobile network operators, as through the isolation of the stack from the PDCP layer and upwards, the CU will be able to act as the Cloud-based convergence point among multiple heterogeneous technologies in the provisioned networks and hence able to serve multiple heterogeneous DUs. Moreover, data rate requirements for this type of split are not very demanding, thus allowing the IP-based transferring of data from the DU to CU and vice-versa. In this work, we propose, implement and evaluate a protocol for a Cloud-RAN based architecture allowing the selection and dynamic switching of different heterogeneous networks in the RAN. We rely on the open source OpenAirInterface platform and extend it to support data plane splitting of the LTE functionality, and the subsequent data injection to WiFi networks. We evaluate the platform using a real network setup, under several scenarios of network selection and different delay settings.

The wide proliferation of IEEE 802.11 compatible devices and the provisioning of costless Internet connectivity in most cases, have created fertile ground for investigating seamless client mobility and handoff management from a cellular technology to any wireless access point available. Although handoffs and client mobility are currently addressed by the IEEE 802.21 standard along with mobility management protocols such as Mobile IPv6, yet no remarkable efforts exist for the wide deployment of such solutions. Moreover, the adoption of such architectures requires considerable changes in the mobile node's networking stack. In this work, we propose a Software Defined Networking technology inspired scheme for managing client mobility among heterogeneous wireless networks, by adopting changes only on the network edges. Our solution is compatible with the existing IPv4 and IPv6 addressing solutions. By employing the OpenFlow technology on the border of our network with the Internet, we manage to keep both ends of the network aware of any topology changes, and thus preserve any already established connections, resulting in a seamless handoff process. We evaluate our technique in a real network setup, by employing WiFi and LTE technologies and benchmark it using higher layer protocols with multi-homing features, namely Stream Control Transmission Protocol and Multipath TCP.

The lessons already learned from the existing protocols operation are taken into deep consideration during the standardization activities of the potential technologies opted for the future 5th Generation mobile networks. Prior research on wireless technologies in general has clearly shown the need for open programmable experimental facilities which can be used for the implementation and evaluation of novel algorithms and ideas under real world settings, even directly comparable to existing technologies and methodologies. Nevertheless, provisioning of such testbed platforms mandates the respective tools which will enable access to the testbed resources and will expose the maximum possible flexibility in configuring them. In this work, we present our efforts in building such a facility, along with the tools and services that cope with such requirements. The facility upon which we build is the long-established NITOS wireless testbed, which is offering commercial as well as open source LTE components in a 24/7 basis.