Kimon P. Kontovasilis

FlexRAN: A Flexible and Programmable Platform for Software-Defined Radio Access Networks

Although the radio access network (RAN) part of mobile networks offers a significant opportunity for benefiting from the use of SDN ideas, this opportunity is largely untapped due to the lack of a software-defined RAN (SD-RAN) platform. We fill this void with FlexRAN, a flexible and programmable SD-RAN platform that separates the RAN control and data planes through a new, custom-tailored southbound API. Aided by virtualized control functions and control delegation features, FlexRAN provides a flexible control plane designed with support for real-time RAN control applications, flexibility to realize various degrees of coordination among RAN infrastructure entities, and programmability to adapt control over time and easier evolution to the future following SDN/NFV principles. We implement FlexRAN as an extension to a modified version of the OpenAirInterface LTE platform, with evaluation results indicating the feasibility of using FlexRAN under the stringent time constraints posed by the RAN. To demonstrate the effectiveness of FlexRAN as an SD-RAN platform and highlight its applicability for a diverse set of use cases, we present three network services deployed over FlexRAN focusing on interference management, mobile edge computing and RAN sharing.

Bursty traffic modeling and efficient analysis algorithms via fluid-flow models for ATM IBCN

In this paper fluid models for heterogeneous multiplexed traffic are considered. First, some extensions to the general theory applicable to superposed, time-reversible Markovian Rate Processes are given. These refer to the connection between performance metrics, the consideration for singular systems and the continuity of the solution, with respect to the system parameters. The general framework is then carried over to the heterogeneous multiplexing of ON/OFF sources. By combining the general theory with the special structure of the ON/OFF sources several important facets of this structure are highlighted. As a result, more powerful methods that improve computation speed, stability and ease of implementation are produced. More specifically, the numerical part of the method is reduced to a solution of a nonlinear equation per system eigenvalue. The solution is obtainable through a variant of the (locally quadratically convergent) Newton method. For this method, easily computable starting values that guarantee convergence are given. In addition, explicit expressions for the eigenvectors are provided with the potentially unstable quantities factored-out. The paper also provides explicit and stably computable formulae for upper bounds to the coefficients of the spectral components, present in the expressions for the performance measures of interest. Moreover, the paper proves a partial ordering property for the system eigenvalues and presents an algorithm that performs full ordering on-line. This, in many cases, results in a great reduction to the amount of computation, without any significant loss of precision. Lastly, the particular case of heterogeneity where the differences are only identified in the rates within bursts is seen to have features resembling homogeneous systems. The possibility to substitute an “equivalent” homogeneous system of reduced order, for the original heterogeneous one is addressed.

Interactions between intelligent multimodal terminals and a network management system in a B3G context

This work addresses aspects relevant to the optimal joint exploitation of heterogeneous wireless networks in an ‘all-IP’ integrated infrastructure, frequently called ‘Beyond 3G Systems’ (B3G) or composite radio networks. After briefly reviewing general architectural aspects pertaining to such systems, the paper focuses to the management functionality possessed by B3G systems and highlights the benefits that stem from sharing intelligence among the hybrid network (through a network management system) and the multimodal wireless terminals. In particular, the paper discusses an appropriate signaling protocol for the exchange of information between management modules at the network and the wireless terminal, investigates associated performance aspects and correlates the relevant findings with measurement results obtained from experiments over a prototype B3G network testbed. Copyright

Statistical multiplexing, bandwidth allocation strategies and connection admission control in ATM networks

This paper gives a brief description and the main results of work done within the RACE 1022 project on bandwidth allocation and Connection Admission Control in ATM-based networks. Additionally, and before presenting these two main issues, some important facets of statistical multiplexing are highlighted. The key question which is answered by a bandwidth allocation algorithm is “how much bandwidth is required by a group of connections (e.g., within a VP), with certain traffic characteristics, in a certain multiplexing environment”. Answers to this question are given for specific traffic classes and mixing conditions. The advantages and the limitations of the proposed strategies are indicated. In cases where the proposed bandwidth allocation strategies fail to fulfil the specified objectives (e.g., in highly heterogeneous traffic mixes), sophisticated CAC algorithms are required. In particular, the hierarchically organised CAC strategy implemented within the RACE 1022 ATD Technology Testbed will be described in detail. This CAC scheme is based on a simple real-time processing algorithm which provides a quick acceptance decision. This decision will be refined by a precise but numerically complex background algorithm. Simulation studies confirm the potential of this concept.

Measurement and Statistical Analysis of Asymmetric Multipoint Videoconference Traffic in IP Networks

The paper contributes results on the modeling of videoconferencing traffic over IP networks. The study is based on extensive data, gathered by tracing the actual packet exchange during a comprehensive set of realistic teleconferencing sessions over an asymmetric platform, in which commercial H.261-compliant terminal clients were communicating through a Multipoint Control Unit (MCU) at ‘continuous presence’ mode. Analysis of the data suggests that the video traffic from the client terminals can always be represented at the frame level as a stationary stochastic process with an autocorrelation function of exponentially fast decay and a marginal frame size distribution of approximately Gamma form. The video traffic from the MCU to the clients is again stationary and with exponentially decaying correlations, while the corresponding marginal frame-size PDF has the form of an appropriately weighted sum of Gamma components, the number of terms in the sum always being equal to the number of conferring terminals. The paper discusses methods for correctly matching the parameters of the modeling components to the data and for combining these components into complete traffic models that have been proposed in the literature.

A novel Effective Capacity-based framework for providing statistical QoS guarantees in IEEE 802.11 WLANs

This article proposes a performance model of the IEEE 802.11 MAC layer that employs the notion of Effective Capacity. In particular, the paper establishes that an IEEE 802.11 mobile station can be regarded as a Semi-Markovian bursty server of the On/Off type, with known distributions for the On and Off periods, and subsequently applies known results for Semi-Markovian models to derive the Effective Capacity function of this On/Off server. The general Effective Bandwidth/Capacity theory can then be used for computing buffer overflow probabilities and for employing simple traffic control policies to enforce related QoS guarantees. The policies guarantee a soft bound on the buffer overflow probability and are suitable for real-time traffic control over WLANs. The Effective Capacity model of IEEE 802.11 stations is originally developed by assuming that the other competing stations are saturated. This is a conservative assumption that becomes very accurate in a highly loaded network. Subsequently, the model is adapted to encompass lightly loaded networks as well. In the adapted model, each mobile station directly measures a few model parameters, instead of calculating them on the basis of the saturation assumption, and uses these measurements in the computation of its Effective Capacity function. The theoretical results are checked against simulations, validating the appropriateness of the model.

ATM traffic engineering for ABR service provisioning

The Available Bit Rate (ABR) service is being designed as a low-cost transport service over ATM, which will be using the bandwidth left available after servicing connections of another, high-priority class. The implementation of the ABR service requires large buffers at each multiplexing/switching stage to keep cell-loss rates down to a minimum, and a feedback mechanism from the network to the terminals in order for the latter to adjust their traffic profiles according to the prevailing congestion conditions. Thus, an enhanced set of traffic control functions is necessary to support this new service. In this paper the main traffic analysis and control problems related with the ABR service are addressed, modelled and answered on the basis of effective rates defined for the multiplexed connections. Emphasis is given to a simple CAC scheme which consists in allocating peak rates to the high-priority class and effective rates to the ABR class. An adaptive shaping mechanism is then required to enforce the contracted effective rates for the ABR streams. Producing ON/OFF streams facilitates the control functions by allowing the use of approximate closedform calculations.

Bursty traffic modelling and multiplexing performance analysis in ATM networks: a three moment approach

Modelling of hyper-exponential ON/OFF traffic streams by a four-state Markov Modulated Rate Process (MMRP) is considered as a means of obtaining a sufficiently accurate, yet computationally tractable analytical model for ATM traffic. A general formulation of the proposed model along with the associated multiplexing analysis is first developed. Then, a special time-reversible version of the model that arises naturally in many practical configurations of interest is analysed, which enables the modelling and the parameter assessment of the ON and OFF phases independently as two-stage, Erlang-type processes. Application examples with comparison of analysis, simulation and experimental results derived on an ATM testbed are given.

On the energy requirements of vertical handover operations: Measurement-based results for the IEEE 802.21 framework

The availability of multiple collocated wireless networks and the multi-radio capabilities of contemporary wireless terminals open the potential for optimized wireless connectivity over the heterogeneous wireless environment. Vertical Handover (VHO) operations play a central role in such a setting, enabling the dynamic association of wireless users with the most appropriate wireless networks among those available, towards improved effectiveness according to a host of relevant criteria. At the same time, the efficiency of executing the VHO operations themselves is also a factor affecting the overall system effectiveness. Although several aspects of VHO operations have been investigated in the literature, energy efficiency has not received so much attention yet. Towards closing this gap, the paper investigates the energy requirements associated with the execution of the VHO, by expressing the relevant operations as a sequence of elementary actions and determining the energy expenditure at the involved mobile node for each such action. The results address VHO operations compliant to the widely accepted IEEE 802.21 framework and were obtained through actual measurements taken on a prototype heterogeneous network testbed, employing Linux-powered mobile devices and making use of the Advanced Configuration and Power Interface (ACPI) subsystem for taking energy consumption measurements on them.

Effective-capacity-based stochastic delay guarantees for systems with time-varying servers, with an application to IEEE 802.11 WLANs

Many network applications rely on stochastic QoS guarantees. With respect to loss-related performance, the effective bandwidth/capacity theory has proved useful for calculating loss probabilities in queues with complex input and server processes and for formulating simple admission control tests to ensure associated QoS guarantees. This success has motivated the application of the theory for delay-related QoS too. However, up until now this application has been justified only heuristically for queues with variable service rate. The paper fills this gap by establishing rigorously that the effective bandwidth/capacity theory may be used for the asymptotically correct calculation and enforcement of delay tail-probabilities in systems with variable rate servers too. Subsequently, the paper applies the general results to IEEE 802.11 WLANs, by representing each IEEE 802.11 station as an On/Off server and employing the effective capacity function for this model. Comparison of analytical results with simulation validates the effectiveness of the On/Off IEEE 802.11 model for delay-related QoS, complementing earlier results on loss-related performance.