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Multi-Touch-Tisch aus dem Institut für Informatik, Foto: Universität Paderborn, Fotografin: Judith Kraft
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Multi-Touch-Tisch aus dem Institut für Informatik, Foto: Universität Paderborn, Fotografin: Judith Kraft

Prof. Dr. Holger Karl

Kontakt
Vita
Publikationen
Prof. Dr. Holger Karl

Rechnernetze

Fachgruppeninhaber - Professor

Telefon:
+49 5251 60-5375
Telefon:
60-5376
Fax:
+49 5251 60-5377
Büro:
O3.158 (Karte)
Sprechzeiten:

Mo 13-14

Web:
Besucher:
Pohlweg 51
33098 Paderborn
Postanschrift:
Warburger Str. 100
33098 Paderborn

01.10.2004 - heute

Professur Rechnernetze, UPB

2000 - 2004

Wissenschaftlicher Assistent TU Berlin

Arbeitsgruppe Telekommunikationsnetze, Prof. Wolisz

1996 - 1999

Promotion an der Humboldt-Universität zu Berlin

Title: Responsive Cluster Computing

1990 - 1996

Studium Informatik, Universität Karlsruhe


Liste im Research Information System öffnen

Specifying and Analyzing Virtual Network Services Using Queuing Petri Nets

S.B. Schneider, A. Sharma, H. Karl, H. Wehrheim, in: 2019 IFIP/IEEE International Symposium on Integrated Network Management (IM), 2019

For optimal placement and orchestration of network services, it is crucial that their structure and semantics are specified clearly and comprehensively and are available to an orchestrator. Existing specification approaches are either ambiguous or miss important aspects regarding the behavior of virtual network functions (VNFs) forming a service. We propose to formally and unambiguously specify the behavior of these functions and services using Queuing Petri Nets (QPNs). QPNs are an established method that allows to express queuing, synchronization, stochastically distributed processing delays, and changing traffic volume and characteristics at each VNF. With QPNs, multiple VNFs can be connected to complete network services in any structure, even specifying bidirectional network services containing loops. We discuss how management and orchestration systems can benefit from our clear and comprehensive specification approach, leading to better placement of VNFs and improved Quality of Service. Another benefit of formally specifying network services with QPNs are diverse analysis options, which allow valuable insights such as the distribution of end-to-end delay. We propose a tool-based workflow that supports the specification of network services and the automatic generation of corresponding simulation code to enable an in-depth analysis of their behavior and performance.


MARVELO: Wireless Virtual Network Embedding for Overlay Graphs with Loops

H. Afifi, S. Auroux, H. Karl, Proc. of IEEE Wireless Communications and Networking Conference (WCNC), 2018


Scaling and Placing Bidirectional Services with Stateful Virtual and Physical Network Functions

S. Dräxler, S.B. Schneider, H. Karl, in: 4th IEEE International Conference on Network Softwarization (NetSoft 2018), 2018

Network function virtualization requires scaling and placement, deciding the number and the location of function instances. Current approaches are limited in flexibility and practical applicability. Specifically, we study dynamic, single-step, joint scaling and placement of network services with bidirectional flows traversing Physical or Virtual Network Functions (VNFs) and returning to their sources. We develop models to support stateful components and legacy network functions with fixed locations in these network services as well as the possibility of reusing VNFs across network services. We formalize the problem of jointly scaling and placing such network services as a mixed- integer linear program (MILP). We show that this problem is NP-complete and also present a heuristic algorithm to find good solutions in short time. In an extensive evaluation with realistic scenarios, we investigate the capabilities of the two approaches.


Let the state follow its flows: An SDN-based flow handover protocol to support state migration

M. Peuster, H. Küttner, H. Karl, in: 4th IEEE International Conference on Network Softwarization (NetSoft 2018), 2018

Dynamically steering flows through virtualized net- work function instances is a key enabler for elastic, on-demand deployments of virtualized network functions. This becomes par- ticular challenging when stateful functions are involved, necessi- tating state management. The problem with existing solutions is that they typically embrace state migration and flow rerouting jointly, imposing a huge set of requirements on the on-boarded VNFs, e.g., solution-specific state management interfaces. In this paper, we introduce the seamless handover proto- col (SHarP). It provides an easy-to-use, loss-less, and order- preserving flow rerouting mechanism that is not fixed to a single state management approach. This allows VNF vendors to implement or use the state management solution of their choice. SHarP supports these solutions with additional information when flows are migrated. Further, we show how SHarP significantly reduces the buffer usage at a central (SDN) controller, which is a typical bottleneck in existing solutions. Our experiments show that SHarP uses a constant amount of controller buffer, irrespective of the time taken to migrate the VNF state.


Joint Orchestration of Cloud-Based Microservices and Virtual Network Functions

H. Razzaghi Kouchaksaraei, H. Karl, in: The Ninth International Conference on Cloud Computing, GRIDs, and Virtualization CLOUD COMPUTING, 2018


Containernet 2.0: A Rapid Prototyping Platform for Hybrid Service Function Chains

M. Peuster, J.. Kampmeyer, H. Karl, in: 4th IEEE International Conference on Network Softwarization (NetSoft 2018), 2018

Developing a virtualized network service does not only involve the implementation and configuration of the network functions it is composed of but also its integration and test with management solutions that will control the service in its production environment. These integration tasks require testbeds that offer the needed network function virtualization infrastructure~(NFVI), like OpenStack, introducing a lot of management and maintenance overheads. Such testbed setups become even more complicated when the multi point-of-presence~(PoP) case, with multiple infrastructure installations, is considered. In this demo, we showcase an emulation platform that executes containerized network services in user-defined multi-PoP topologies. The platform does not only allow network service developers to locally test their services but also to connect real-world management and orchestration solutions to the emulated PoPs. During our interactive demonstration we focus on the integration between the emulated infrastructure and state-of-the-art orchestration solutions like SONATA or OSM.


5G OS: Control and Orchestration of Services on Multi-Domain Heterogeneous 5G Infrastructures

S. Dräxler, H. Karl, H. Razzaghi Kouchaksaraei, A. Machwe, C. Dent-Young, K. Katsalis, K. Samdanis, in: 27th European Conference on Networks and Communications (EUCNC 2018), 2018


Understand your chains and keep your deadlines: Introducing time-constrained profiling for NFV

M. Peuster, H. Karl, in: IEEE/IFIP 14th International Conference on Network and Service Management (CNSM), IEEE/IFIP, 2018


JASPER: Joint Optimization of Scaling, Placement, and Routing of Virtual Network Services

S. Dräxler, H. Karl, Z.A. Mann, IEEE Transactions on Network and Service Management (2018)

To adapt to continuously changing workloads in networks, components of the running network services may need to be replicated (scaling the network service) and allocated to physical resources (placement) dynamically, also necessitating dynamic re-routing of flows between service components. In this paper, we propose JASPER, a fully automated approach to jointly optimizing scaling, placement, and routing for complex network services, consisting of multiple (virtualized) components. JASPER handles multiple network services that share the same substrate network; services can be dynamically added or removed and dynamic workload changes are handled. Our approach lets service designers specify their services on a high level of abstraction using service templates. JASPER automatically makes scaling, placement and routing decisions, enabling quick reaction to changes. We formalize the problem, analyze its complexity, and develop two algorithms to solve it. Extensive empirical results show the applicability and effectiveness of the proposed approach.


Emulation-based Smoke Testing of NFV Orchestrators in Large Multi-PoP Environments

M. Peuster, M. Marchetti, G. Garcia de Blas, H. Karl, in: European Conference on Networks and Communications (EuCNC), 2018

Management and orchestration~(MANO) systems are the key components of future large-scale NFV environments. They will manage resources of hundreds or even thousands of NFV infrastructure installations, so called points of presence~(PoP). Such scenarios need to be automatically tested during the development phase of a MANO system. This task becomes very challenging because large-scale NFV testbeds are hard to maintain, too expensive, or simply not available. In this paper, we present a multi-PoP NFV infrastructure emulation platform that enables automated, large-scale testing of MANO stacks. We show that our platform can easily emulate hundreds of PoPs on a single physical machine and reduces the setup time of a test PoP by a factor of 232x compared to a DevStack-based test PoP installation. Further, we present a case study in which we test ETSI's Open Source MANO~(OSM) against our proposed system to gain insights about OSM's behaviour in large-scale NFV deployments.


Modelling Time-Limited Capacity of a Wireless Channel as aMarkov Reward Process

B. Shiferaw Heyi, H. Karl, Proc. of IEEE Wireless Communications and Networking Conference (WCNC), 2018


Towards Predicting Resource Demands and Performance of Distributed Cloud Services

S. Dräxler, M. Peuster, M. Illian, H. Karl, 2018

Understanding the behavior of distributed cloud service components in different load situations is important for efficient and automatic management and orchestration of these services. For this purpose and for practical research in distributed cloud computing in general, there is need for benchmarks and experimental data. In this paper, we describe our experiments for characterizing the relationship between resource demands of application components and the expected performance of applica- tions. We present initial results for predicting the interdependence between resource demands and performance characteristics using support vector regression and polynomial regression models. The data gathered from our experiments is publicly available.


Programmable and Flexible Management and Orchestration of Virtualized Network Functions

H. Razzaghi Kouchaksaraei, S. Dräxler, M. Peuster, H. Karl, IEEE, 2018

Supporting the vast variety of network services’ management and orchestration requirements is one of the main challenges that Network Function Virtualization (NFV) is dealing with. While general management requirements such as Virtual Network Function (VNF) resource requirements can be specified by the service developers using service descriptors, specific management operations like VNF-specific configuration cannot be performed by these descriptors. On the other hand, it is inefficient and also very challenging for Management and Orchestration (MANO) frameworks to provide all specific-management operations for every individual network service and their constituent VNFs. To mitigate this issue, we propose the use of service-specific programs called Specific Managers (SMs) that can customize management and orchestration of network services and also extend the capability of MANO frameworks to support per-service management and orchestration. The results of our evaluation show that the higher flexibility and programmability enabled by SMs improve the performance of the service performance and also utilises the service provider resources more efficiently.


Distributed Placement of Virtualized Control Applications in Mobile Backhaul Networks

S. Auroux, H. Karl, Proc. of IEEE Wireless Communications and Networking Conference (WCNC), 2018

DOI


Pishahang: Joint Orchestration of Network Function Chains and Distributed Cloud Applications

H. Razzaghi Kouchaksaraei, T. Dierich, H. Karl, 2018

Developing cloud applications using a microservice architecture allows their functional blocks to be distributed and deployed on multiple Cloud infrastructures. This enables service providers to mix and match Cloud-based microservices and Virtual Network Functions (VNFs) that are provided by Network Function Virtualization (NFV). Provisioning complex services containing VNFs and Cloud-based microservices across NFV and cloud infrastructures can enhance service quality, reduce latency, and optimise cost. This can be provided by an orchestration system that can handle cross-ecosystem dependencies. To this end, we implemented Pishahang that is a framework for jointly managing and orchestrating virtual network functions and Cloud-based microservices. During the demo, we deploy several complex services to demonstrate features provided by Pishahang to support management and orchestration of complex services.


DeepCAS: A Deep Reinforcement Learning Algorithm for Control-Aware Scheduling

B. Demirel, A. Ramaswamy, D. Quevedo, H. Karl, 2018

DOI


Generating Resource and Performance Models for Service Function Chains: The Video Streaming Case

S. Dräxler, M. Peuster, M. Illian, H. Karl, in: 4th IEEE International Conference on Network Softwarization (NetSoft 2018), 2018



A flexible multi-pop infrastructure emulator for carrier-grade MANO systems

M. Peuster, S. Dräxler, H. Razzaghi Kouchaksaraei, S.v. Rossem, W. Tavernier, H. Karl, in: 2017 IEEE Conference on Network Softwarization (NetSoft), IEEE, 2017

Developing a virtualized network service does not only involve the implementation and configuration of the network functions it is composed of but also its integration and test with management solutions that will control the service in its production environment. These integration tasks require testbeds that offer the needed network function virtualization infrastructure (NFVI), like OpenStack, introducing a lot of management and maintenance overheads. Such testbed setups become even more complicated when the multi point-of-presence (PoP) case, with multiple infrastructure installations, is considered. In this demo, we showcase an emulation platform that executes containerized network services in user-defined multi-PoP topologies. The platform does not only allow network service developers to locally test their services but also to connect realworld management and orchestration solutions to the emulated PoPs. During our interactive demonstration we focus on the integration between the emulated infrastructure and state-of-theart orchestration solutions like SONATA or OSM.


Minimizing downtimes: Using dynamic reconfiguration and state management in SDN

A. Schwabe, E. Rojas, H. Karl, in: 2017 {IEEE} Conference on Network Softwarization, NetSoft 2017, Bologna, Italy, July 3-7, 2017, 2017, pp. 1--5

DOI


Joint Optimization of Scaling and Placement of Virtual Network Services

S. Dräxler, H. Karl, Z.A. Mann, in: Proceedings of the 17th IEEE/ACM International Symposium on Cluster, Cloud and Grid Computing (CCGrid 2017), 2017

Management of complex network services requires flexible and efficient service provisioning as well as optimized handling of continuous changes in the workload of the service.To adapt to changes in the demand, service components need to be replicated (scaling) and allocated to physical resources (placement) dynamically. In this paper, we propose a fullyautomated approach to the joint optimization problem of scaling and placement, enabling quick reaction to changes. We formalize the problem, analyze its complexity, and develop two algorithms to solve it. Extensive empirical results show the applicability andeffectiveness of the proposed approach.


Profile Your Chains, Not Functions. Automated Network Service Profiling in DevOps Environments

M. Peuster, H. Karl, in: IEEE Conference on Network Function Virtualisation and Software Defined Networks (NFV-SDN), 2017

Benchmarking and profiling virtual network functions (VNFs) generates input knowledge for resource management decisions taken by management and orchestration systems. Such VNFs are usually not executed in isolation but are often deployed as part of a service function chain (SFC) that connects single functions into complex structures. To manage such chains, isolated performance profiles of single functions have to be combined to get insights into the overall behavior of an SFC. This becomes particularly challenging in highly agile DevOps environments in which profiling processes need to be fully automated and detailed insights about a chain's internal structures are not always available. In this paper, we introduce a fully automatable, flexible, and platform-agnostic profiling system that allows to profile entire SFCs at once. This obviates manual modeling procedures to combine profiling results from single VNFs to reflect SFC performance. We use a case study with different SFC configurations to show that it is hard to model the resulting SFC performance based on single-VNF measurements and that performance interactions between real, non-trivial functions that are deployed in a chain exist.


Response-Time-Optimised Service Deployment: MILP Formulations of Piece-wise Linear Functions Approximating Non-linear Bivariate Mixed-integer Functions

M. Keller, H. Karl, IEEE Transactions on Network and Service Management (2017), pp. 121--135

A current trend in networking and cloud computing is to provide compute resources at widely distributed sites; this is exemplified by developments such as Network Function Virtualisation. This paves the way for wide-area service deployments with improved service quality: e.g. user-perceived response times can be reduced by offering services at nearby sites. But always assigning users to the nearest site can be a bad decision if this site is already highly utilised. This paper formalises two related decisions of allocating compute resources at different sites and assigning users to them with the goal of minimising the response times while the total number of resources to be allocated is limited – a non-linear capacitated Facility Location Problem with integrated queuing systems. To efficiently handle its non-linearity, we introduce five linear problem linearisations and adapt the currently best heuristic for a similar scenario to our scenario. All six approaches are compared in experiments for solution quality and solving time. Surprisingly, our best optimisation formulation outperforms the heuristic in both time and quality. Additionally, we evaluate the influence of distributions of available compute resources in the network on the response time: The time was halved for some configurations. The presented formulation techniques for our problem linearisations are applicable to a broader optimisation domain.


A flexible multi-pop infrastructure emulator for carrier-grade MANO systems

M. Peuster, S. Dräxler, H. Razzaghi Kouchaksaraei, S. van Rossem, W. Tavernier, H. Karl, in: IEEE Conference on Network Softwarization, NetSoft 2017, Bologna, Italy, July 3-7, 2017, 2017, pp. 1--3

Developing a virtualized network service does not only involve the implementation and configuration of the network functions it is composed of but also its integration and test with management solutions that will control the service in its production environment. These integration tasks require testbeds that offer the needed network function virtualization infrastructure~(NFVI), like OpenStack, introducing a lot of management and maintenance overheads. Such testbed setups become even more complicated when the multi point-of-presence~(PoP) case, with multiple infrastructure installations, is considered. In this demo, we showcase an emulation platform that executes containerized network services in user-defined multi-PoP topologies. The platform does not only allow network service developers to locally test their services but also to connect real-world management and orchestration solutions to the emulated PoPs. During our interactive demonstration we focus on the integration between the emulated infrastructure and state-of-the-art orchestration solutions like SONATA or OSM.


Specification, Composition, and Placement of Network Services with Flexible Structures

S. Dräxler, H. Karl, International Journal of Network Management (2017), pp. 1--16

Network function virtualization and software-defined networking allow services consisting of virtual network functions to be designed and implemented with great flexibility by facilitating automatic deployments, migrations, and reconfigurations for services and their components. For extended flexibility, we go beyond seeing services as a fixed chain of functions. We define the service structure in a flexible way that enables changing the order of functions in case the functionality of the service is not influenced by this, and propose a YANG data model for expressing this flexibility. Flexible structures allow the network orchestration system to choose the optimal composition of service components that for example gives the best results for placement of services in the network. When number of flexible services and number of components in each service increase, combinatorial explosion limits the practical use of this flexibility. In this paper, we describe a selection heuristic that gives a Pareto set of the possible compositions of a service as well as possible combinations of different services, with respect to different optimization objectives. Moreover, we present a heuristic algorithm for placement of a combination of services, which aims at placing service components along shortest paths that have enough capacity for accommodating the services. By applying these solutions, we show that allowing flexibility in the service structure is feasible.


SONATA: Service programming and orchestration for virtualized software networks

S. Dräxler, H. Karl, M. Peuster, H. Razzaghi Kouchaksaraei, M. Bredel, J. Lessmann, T. Soenen, W. Tavernier, S. Mendel-Brin, G. Xilouris, in: 2017 IEEE International Conference on Communications Workshops (ICC Workshops), IEEE, 2017

In conventional large-scale networks, creation and management of network services are costly and complex tasks that often consume a lot of resources, including time and manpower. Network softwarization and network function virtualization have been introduced to tackle these problems, aiming at decreasing costs and complexity of implementing new services, maintaining the implemented services, and managing available resources in service provisioning platforms and underlying infrastructures. To experience the full potential of these approaches, innovative development support tools and service provisioning environments are needed. To answer these needs, we introduce the architecture of the open-source SONATA system, a service programming, orchestration, and management framework. We present a development toolchain for virtualized network services, fully integrated with a service platform and orchestration system. We introduce the modular and flexible architecture of our system and discuss its main components and features, such as function- and service-specific managers that allow fine-grained service management, slicing support to facilitate multi-tenancy, recursiveness for improved scalability, and full-featured DevOps support.


SONATA: Service Programming and Orchestration for Virtualised Software Networks

S. Dräxler, H. Karl, H. Razzaghi Kouchaksaraei, M. Peuster, M. Bredel, J. Lessmann, T. Soenen, W. Tavernier, S. Mendel-Brin, G. Xilouris, in: Communications Workshops (ICC Workshops), 2017 IEEE International Conference on, 2017

DOI


Assessing Genetic Algorithms for Placing Flow Processing-aware Control Applications

S. Auroux, S. Scholz, H. Karl, in: Proc. European Wireless, 2017


E-State: Distributed state management in elastic network function deployments

M. Peuster, H. Karl, in: IEEE NetSoft Conference and Workshops, NetSoft 2016, Seoul, South Korea, June 6-10, 2016, 2016, pp. 6--10

Elastic deployments of virtualized network functions~(VNF) can automatically scale the amount of used resources in relation to their workload. This is often done by starting new VNF instances or stopping old ones. A problem of these scale operations is that most network functions are stateful and their internal state is not automatically migrated when traffic is redistributed in the deployment. As a result, mechanisms are needed to exchange or migrate internal network function state between VNF instances. This paper presents a state management framework that creates a logically distributed state store on top of elastically deployed virtual network functions. We also introduce a novel programming model that provides both a local and a global view of the state to each VNF instance. We discuss the integration of our framework into existing network function virtualization architectures and compare the performance of our prototype to a centralized and a distributed state store solution.


DevOps for network function virtualisation: an architectural approach

H. Karl, S. Dräxler, M. Peuster, A. Galis, M. Bredel, A. Ramos, J. Martrat, M.S. Siddiqui, S. van Rossem, W. Tavernier, G. Xilouris, Transactions on Emerging Telecommunications Technologies (2016), pp. 1206-1215

The Service Programming and Orchestration for Virtualised Software Networks (SONATA) project targets both the flexible programmability of software networks and the optimisation of their deployments by means of integrating Development and Operations in order to accelerate industry adoption of software networks and reduce time-to-market for networked services. SONATA supports network function chaining and orchestration, making service platforms modular and easier to customise to the needs of different service providers, and introduces a specialised Development and Operations model for supporting developers.


DCT²Gen: A traffic generator for data centers

P. Wette, H. Karl, Computer Communications (2016), pp. 45--58

DOI


MeDICINE: Rapid Prototyping of Production-Ready Network Services in Multi-PoP Environments

M. Peuster, H. Karl, S. van Rossem, in: IEEE Conference on Network Function Virtualization and Software Defined Networks (NFV-SDN), 2016

Virtualized network services consisting of multiple individual network functions are already today deployed across multiple sites, so called multi-PoP (points of presence) environments. This allows to improve service performance by optimizing its placement in the network. But prototyping and testing of these complex distributed software systems becomes extremely challenging. The reason is that not only the network service as such has to be tested but also its integration with management and orchestration systems. Existing solutions, like simulators, basic network emulators, or local cloud testbeds, do not support all aspects of these tasks. To this end, we introduce MeDICINE, a novel NFV prototyping platform that is able to execute production-ready network functions, provided as software containers, in an emulated multi-PoP environment. These network functions can be controlled by any third-party management and orchestration system that connects to our platform through standard interfaces. Based on this, a developer can use our platform to prototype and test complex network services in a realistic environment running on his laptop.


Composition of SDN applications: Options/challenges for real implementations

A. Schwabe, P. A. Aranda-Gutierrez, H. Karl, in: Proceedings of the 2016 Applied Networking Research Workshop, {ANRW} 2016, Berlin, Germany, July 16, 2016, 2016, pp. 26--31

DOI


Understand Your Chains: Towards Performance Profile-Based Network Service Management

M. Peuster, H. Karl, in: Fifth European Workshop on Software-Defined Networks, EWSDN 2016, Den Haag, The Netherlands, October 10-11, 2016, 2016, pp. 7--12

Allocating resources to virtualized network functions and services to meet service level agreements is a challenging task for NFV management and orchestration systems. This becomes even more challenging when agile development methodologies, like DevOps, are applied. In such scenarios, management and orchestration systems are continuously facing new versions of functions and services which makes it hard to decide how much resources have to be allocated to them to provide the expected service performance. One solution for this problem is to support resource allocation decisions with performance behavior information obtained by profiling techniques applied to such network functions and services. In this position paper, we analyze and discuss the components needed to generate such performance behavior information within the NFV DevOps workflow. We also outline research questions that identify open issues and missing pieces for a fully integrated NFV profiling solution. Further, we introduce a novel profiling mechanism that is able to profile virtualized network functions and entire network service chains under different resource constraints before they are deployed on production infrastructure.


Joint real-time scheduling and interference coordination for wireless factory automation

S. Auroux, D. Parruca, H. Karl, in: 27th IEEE Annual International Symposium on Personal, Indoor, and Mobile Radio Communications, {PIMRC} 2016, Valencia, Spain, September 4-8, 2016, 2016, pp. 1--6

DOI


Delayed (de-)activation in servers with a sleep mode

M. Herlich, N. Bredenbals, H. Karl, Sustainable Computing: Informatics and Systems (2016), pp. 48-55

DOI


Reusability of software-defined networking applications: {A} runtime, multi-controller approach

R. Doriguzzi Corin, P. A. Aranda-Gutierrez, E. Rojas, H. Karl, E. Salvadori, in: 12th International Conference on Network and Service Management, {CNSM} 2016, Montreal, QC, Canada, October 31 - Nov. 4, 2016, 2016, pp. 209--215

DOI


Demonstrating on-demand cell switching with a two-layer mobile network testbed

M. Peuster, H. Karl, A. Enrico Redondi, A. Capone, in: IEEE Conference on Computer Communications Workshops, INFOCOM Workshops 2016, San Francisco, CA, USA, April 10-14, 2016, 2016, pp. 1015--1016

Traditional cellular networks are forced to remain active regardless of the actual amount of traffic that is currently produced/requested, with a clear waste of energy. Two-layer mobile networks with separated signalling and data layers have been recently proposed for energy savings in future implementations. These networks are able to switch off unneeded data cells completely while maintaining full coverage with their signalling cells, thus saving energy. In this demonstration, we showcase a testbed that uses Wi-Fi access points to emulate small cells of the data layer and a publicly available cellular connection as the signalling layer. We use off-the-shelf Android smartphones with an ad-hoc networking management module and a MultiPath TCP-enabled kernel to manage the Wi-Fi and cellular interfaces simultaneously. The testbed is used to demonstrate the general feasibility of this layered architecture and to facilitate experiments with network-wide resource optimization.


Placement of Services with Flexible Structures Specified by a YANG Data Model

S. Dräxler, H. Karl, in: Proceedings of the 2nd International IEEE Conference on Network Softwarization (NetSoft), 2016, pp. 184--192

Network function virtualization and software-defined networking allow services consisting of virtual network functions to be designed and implemented with great flexibility by facilitating automatic deployments, migrations, and reconfigurations for services and their components. For extended flexibility, we go beyond seeing services as a fixed chain of functions. We present a YANG model for describing the service structure in deployment requests in a flexible way that enables changing the order of functions in case the order of traversing them does not affect the functionality of the service. Upon receiving such requests, the network orchestration system can choose the optimal composition of service components that gives the best results for placement of services in the network. This introduces new complexities to the placement problem by greatly increasing the number of possible ways a service can be composed. In this paper, we describe a heuristic solution that selects a Pareto set of the possible compositions of a service as well as possible combinations of different services, with respect to different resource requirements of the services. Our evaluations show that the selected combinations consist of representative samples of possible structures and requirements and therefore, can result in optimal or close-to-optimal placement results.



HybridTE: Traffic Engineering for Very Low-Cost Software-Defined Data-Center Networks

P. Wette, H. Karl, in: Proceedings of the 4th European Workshop on Software Defined Networks (EWSDN 2015), 2015, pp. 1--7

The size of modern data centers is constantly increasing. As it is not economic to interconnect all machines in the data center using a full-bisection-bandwidth network, techniques have to be developed to increase the efficiency of data-center networks. The Software-Defined Network paradigm opened the door for centralized traffic engineering (TE) in such environments. Up to now, there were already a number of TE proposals for SDN-controlled data centers that all work very well. However, these techniques either use a high amount of flow table entries or a high flow installation rate that overwhelms available switching hardware, or they require custom or very expensive end-of-line equipment to be usable in practice. We present HybridTE, a TE technique that uses (uncertain) information about large flows. Using this extra information, our technique has very low hardware requirements while maintaining better performance than existing TE techniques. This enables us to build very low-cost, high performance data-center networks.


Anticipatory Download Scheduling in Wireless Video Streaming with Uncertain Data Rate Prediction

M. Dräxler, J. Blobel, H. Karl, in: 8th IFIP Wireless and Mobile Networking Conference, WMNC 2015, Munich, Germany, October 5-7, 2015, 2015, pp. 136--143

DOI


SmarterPhones: Anticipatory Download Scheduling for Wireless Video Streaming

M. Dräxler, J. Blobel, P. Dreimann, S. Valentin, H. Karl, in: Proceedings of the 2nd International Conference on Networked Systems (NetSys), 2015, pp. 1--8

Video streaming is in high demand by mobile users. In cellular networks, however, the unreliable wireless channel leads to two major problems. Poor channel states degrade video quality and interrupt the playback when a user cannot sufficiently fill its local playout buffer: buffer underruns occur. In contrast, good channel conditions cause common greedy buffering schemes to buffer too much data. Such over-buffering wastes expensive wireless channel capacity. Assuming that we can anticipate future data rates, we plan the quality and download time of video segments ahead. This anticipatory download scheduling avoids buffer underruns by downloading a large number of segments before a drop in available data rate occurs, without wasting wireless capacity by excessive buffering.We developed a practical anticipatory scheduling algorithm for segmented video streaming protocols (e.g., HLS or MPEG DASH). Simulation results and testbed measurements show that our solution essentially eliminates playback interruptions without significantly decreasing video quality.


On Network Application representation and controller independence in {SDN

P. A. Aranda-Gutierrez, H. Karl, E. Rojas, A. Leckey, in: 2015 European Conference on Networks and Communications, EuCNC 2015, Paris, France, June 29 - July 2, 2015, 2015, pp. 429--433

DOI


Extending Hadoop's Yarn Scheduler Load Simulator with a highly realistic network & traffic model

P. Wette, A. Schwabe, M. Splietker, H. Karl, in: Proceedings of the 1st {IEEE} Conference on Network Softwarization, NetSoft 2015, London, United Kingdom, April 13-17, 2015, 2015, pp. 1--2

DOI


An Architecture for Energy-aware On-demand Mobile Network Management

M. Peuster, H. Karl, in: Proceedings of the 5th Workshop on All Things Cellular: Operations, Applications and Challenges, 2015


SynRace: Decentralized Load-Adaptive Multi-path Routing without Collecting Statistics

A. Schwabe, H. Karl, in: Proceedings of the 4th European Workshop on Software Defined Networks (EWSDN 2015), 2015, pp. 37-42

Multi-rooted trees are becoming the norm for modern data-center networks. In these networks, scalable flow routing is challenging owing to vast number of flows. Current approaches either employ a central controller that can have scalability issues or a scalable decentralized algorithm only considering local information. In this paper we present a new decentralized approach to least-congested path routing in software-defined data center networks that has neither of these issues: By duplicating the initial (or SYN) packet of a flow and estimating the data rate of multiple flows in parallel, we exploit TCP’s habit to fill buffers to find the least congested path. We show that our algorithm significantly improves flow completion time without the need for a central controller or specialized hardware.


Error characterization of multi-access point WSNs in an aircraft cabin

J. Blanckenstein, C. Nardin, J. Klaue, H. Karl, in: {IEEE} International Conference on Communication, {ICC} 2015, London, United Kingdom, June 8-12, 2015, Workshop Proceedings, 2015, pp. 2363--2368

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Flexible reassignment of flow processing-aware controllers in future wireless networks

S. Auroux, H. Karl, in: 26th IEEE Annual International Symposium on Personal, Indoor, and Mobile Radio Communications, PIMRC 2015, Hong Kong, China, August 30 - September 2, 2015, 2015, pp. 1850--1855

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Topology model to generate realistic latency for simulations

A. Schwabe, H. Karl, in: 2015 IEEE International Conference on Communications, ICC 2015, London, United Kingdom, June 8-12, 2015, 2015, pp. 6122--6127

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Efficient flow processing-aware controller placement in future wireless networks

S. Auroux, H. Karl, in: 2015 IEEE Wireless Communications and Networking Conference, WCNC 2015, New Orleans, LA, USA, March 9-12, 2015, 2015, pp. 1787--1792

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Energy-efficient clock synchronization using wake-up receivers

J. Blanckenstein, H. Karl, in: 22nd International Conference on Software, Telecommunications and Computer Networks, SoftCOM 2014, Split, Croatia, September 17-19, 2014, 2014, pp. 408--413

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Adding Geographical Embedding to AS Topology Generation

A. Schwabe, H. Karl, in: CoRR, 2014


Power model design for ICT systems – A generic approach

F. Beister, M. Dräxler, J. Aelken, H. Karl, Computer Communications (2014), pp. 77-85

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Analytic quantification of outage probability and radiated power of cooperative base stations

M. Herlich, H. Karl, in: 12th International Symposium on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks (WiOpt) 2014, IEEE, 2014

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Incorporating feedback from application layer into routing and wavelength assignment algorithms

P. Wette, H. Karl, in: 2013 IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS), IEEE, 2014

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Using Application Layer Knowledge in Routing and Wavelength Assignment Algorithms

P. Wette, H. Karl, in: Proceedings of the IEEE International Conference on Communications 2014, 2014, pp. 3270-3276

Preemptive Routing and Wavelength Assignment (RWA) algorithms preempt established lightpaths in case notenough resources are available to set up a new lightpath in aWavelength Division Multiplexing (WDM) network. The selectionof lightpaths to be preempted relies on internal decisions of theRWA algorithm. Thus, if dedicated properties of the networktopology are required by the applications running on the network,these requirements have to be known to the RWA algorithm.We present a family of preemptive RWA algorithms for WDMnetworks. These algorithms have two distinguishing features: a)they can handle dynamic traffic by on-the-fly reconfiguration,and b) users can give feedback for reconfiguration decisions andthus influence the preemption decision of the RWA algorithm,leading to networks which adapt directly to application needs.This is different from traffic engineering where the network is(slowly) adapted to observed traffic patterns.Our algorithms handle various WDM network configurationsincluding networks consisting of heterogeneous WDM hardware.To this end, we are using the layered graph approach togetherwith a newly developed graph model that is used to determineconflicting lightpaths.


Specifying and Placing Chains of Virtual Network Functions

S. Dräxler, M. Keller, H. Karl, in: Proceedings of the 3rd International Conference on Cloud Networking (CloudNet), 2014, pp. 7-13

Network appliances perform different functions on network flows and constitute an important part of an operator’s network. Normally, a set of chained network functions process network flows. Following the trend of virtualization of networks, virtualization of the network functions has also become a topic of interest. We define a model for formalizing the chaining of network functions using a context-free language. We process deployment requests and construct virtual network function graphs that can be mapped to the network. We describe the mapping as a Mixed Integer Quadratically Constrained Program (MIQCP) for finding the placement of the network functions and chaining them together considering the limited network resources and requirements of the functions. We have performed a Pareto set analysis to investigate the possible trade-offs between different optimization objectives.


Response Time-Optimized Distributed Cloud Resource Allocation

M. Keller, H. Karl, in: Proceedings of the SIGCOMM workshop on Distributed cloud computing, 2014, pp. 47--52

In the near future many more compute resources will be available at different geographical locations. To minimize the response time of requests, application servers closer to the user can hence be used to shorten network round trip times. However, this advantage is neutralized if the used data centre is highly loaded as the processing time of re- quests is important as well. We model the request response time as the network round trip time plus the processing time at a data centre.We present a capacitated facility location problem formal- ization where the processing time is modelled as the sojourn time of a queueing model. We discuss the Pareto trade-off between the number of used data centres and the resulting response time. For example, using fewer data centres could cut expenses but results in high utilization, high response time, and smaller revenues.Previous work presented a non-linear cost function. We prove its convexity and exploit this property in two ways: First, we transform the convex model into a linear model while controlling the maximum approximation error. Sec- ond, we used a convex solver instead of a slower non-linear solver. Numerical results on network topologies exemplify our work.


Template Embedding: Using Application Architecture to Allocate Resources in Distributed Clouds

M. Keller, C. Robbert, H. Karl, in: Proceedings of 7th International Conference on Utility and Cloud Computing (UCC), 2014, pp. 387--395

In distributed cloud computing, application deployment across multiple sites can improve quality of service. Recent research developed algorithms to find optimal locations for virtual machines. However, those algorithms assume to have either single-tier applications or a fixed number of virtual machines – a strong simplification of reality. This paper investigates the placement and scaling of complex application architectures. An application is dynamically scaled to fit both the current demand situation and the currently available infrastructure resources. We compare two approaches: The first one is based on virtual network embedding. The second approach is a novel method called Template Embedding. It is based on a hierarchical 1-allocation hub flow problem and combines applica- tion scaling and embedding in one step. Extensive experiments on 43200 network configurations showed that Template Embedding outperforms virtual network embedding in all cases in three metrics: success rate, solution quality, and runtime. This positive result shows that template embedding is a promising approach for distributed cloud resource allocation.


A Game-Theoretic Approach to the Financial Benefits of Infrastructure-as-a-Service

J. Künsemöller, H. Karl, Future Generation Computer Systems (2014), pp. 44--52

Financial benefits are an important factor when cloud infrastructure is considered to meet processing demand. The dynamics of on-demand pricing and service usage are investigated in a two-stage game model for a monopoly Infrastructure-as-a-Service (IaaS) market. The possibility of hybrid clouds (public clouds plus own infrastructure) turns out to be essential in order that not only the provider but also the clients have significant benefits from on-demand services. Even if the client meets all demand in the public cloud, the threat of building a hybrid cloud keeps the instance price low. This is not the case when reserved instances are offered as well. Parameters like load profiles and economies of scale have a huge effect on likely future pricing and on a cost-optimal split-up of client demand between either a client’s own data center and a public cloud service or between reserved and on-demand cloud instances.


Power model design for ICT systems -- A generic approach

F. Beister, M. Dräxler, J. Aelken, H. Karl, Computer Communications (2014), pp. 77--85

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Flow processing-aware controller placement in wireless DenseNets

S. Auroux, H. Karl, in: 25th IEEE Annual International Symposium on Personal, Indoor, and Mobile Radio Communication, {PIMRC} 2014, Washington DC, USA, September 2-5, 2014, 2014, pp. 1294--1299

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Predicting mobile video inter-download times with Hidden Markov Models

F. Beister, H. Karl, in: IEEE 10th International Conference on Wireless and Mobile Computing, Networking and Communications, WiMob 2014, Larnaca, Cyprus, October 8-10, 2014, 2014, pp. 359--364

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MaxiNet: Distributed Emulation of Software-Defined Networks

P. Wette, M. Dräxler, A. Schwabe, F. Wallaschek, M.H. Zahraee, H. Karl, in: Proceedings of the 2014 IFIP Networking Conference (Networking 2014), 2014, pp. 1-9

Network emulations are widely used for testing novel network protocols and routing algorithms in realistic scenarios. Up to now, there is no emulation tool that is able to emulate large software-defined data center networks that consist of several thousand nodes. Mininet is the most common tool to emulate Software-Defined Networks of several hundred nodes. We extend Mininet to span an emulated network over several physical machines, making it possible to emulate networks of several thousand nodes on just a handful of physical machines. This enables us to emulate, e.g., large data center networks. To test this approach, we additionally introduce a traffic generator for data center traffic. Since there are no data center traffic traces publicly available we use the results of two recent traffic studies to create synthetic traffic. We show the design and discuss some challenges we had in building our traffic generator. As a showcase for our work we emulated a data center consisting of 3200 hosts on a cluster of only 12 physical machines. We show the resulting workloads and the trade-offs involved.


Anticipatory power cycling of mobile network equipment for high demand multimedia traffic

M. Dräxler, P. Dreimann, H. Karl, in: IEEE Online Conference on Green Communications, OnlineGreenComm 2014, November 12-14, 2014, 2014, pp. 1--7

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Specifying and Placing Chains of Virtual Network Functions

S. Mehraghdam, M. Keller, H. Karl, in: CoRR, 2014


Provider Competition in Infrastructure-as-a-Service

J. Künsemöller, S. Brangewitz, H. Karl, C. Haake, in: Proceedings of the 2014 IEEE International Conference on Services Computing (SCC), 2014, pp. 203-210

This paper explores how cloud provider competition influences instance pricing in an IaaS (Infrastructure-as-a-Service) market. When reserved instance pricing includes an on-demand price component in addition to a reservation fee (two-part tariffs), different providers might offer different price combinations, where the client’s choice depends on its load profile. We investigate a duopoly of providers and analyze stable market prices in two-part tariffs. Further, we study offers that allow a specified amount of included usage (three-part tariffs). Neither two-part nor three-part tariffs produce an equilibrium market outcome other than a service pricing that equals production cost, i.e., complex price structures do not significantly affect the results from ordinary Bertrand competition.



Network of Information (NetInf) - An information-centric networking architecture

C. Dannewitz, D. Kutscher, B. Ohlman, S. Farrell, B. Ahlgren, H. Karl, Computer Communications (2013), pp. 721--735

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A topology-aware adaptive deployment framework for elastic applications

M. Keller, M. Peuster, C. Robbert, H. Karl, in: 17th International Conference on Intelligence in Next Generation Networks, {ICIN} 2013, Venice, Italy, October 15-16, 2013, 2013, pp. 61--69

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A Local Heuristic for Latency-Optimized Distributed Cloud Deployment

M. Keller, S. Pawlik, P. Pietrzyk, H. Karl, in: Proceedings of the 6th International Conference on Utility and Cloud Computing (UCC) workshop on Distributed cloud computing, 2013, pp. 429-434

In Distributed Cloud Computing, applications are deployed across many data centres at topologically diverse locations to improved network-related quality of service (QoS). As we focus on interactive applications, we minimize the latency between users and an application by allocating Cloud resources nearby the customers. Allocating resources at all locations will result in the best latency but also in the highest expenses. So we need to find an optimal subset of locations which reduces the latency but also the expenses – the facility location problem (FLP). In addition, we consider resource capacity restrictions, as a resource can only serve a limited amount of users. An FLP can be globally solved. Additionally, we propose a local, distributed heuristic. This heuristic is running within the network and does not depend on a global component. No distributed, local approximations for the capacitated FLP have been proposed so far due to the complexity of the problem. We compared the heuristic with an optimal solution obtained from a mixed integer program for different network topologies. We investigated the influence of different parameters like overall resource utilization or different latency weights.


Improving Cooperative Transmission Feasibility by Network Reconfiguration in Limited Backhaul Networks

M. Dräxler, T. Biermann, H. Karl, International Journal of Wireless Information Networks (2013), pp. 183--194

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Energy-Efficient Assignment of User Equipment to Cooperative Base Stations

M. Herlich, H. Karl, in: ISWCS 2013, The Tenth International Symposium on Wireless Communication Systems, Ilmenau, Germany, August 27-30, 2013, 2013, pp. 1--5


OpenNetInf - prototyping an information-centric Network Architecture

C. Dannewitz, M. Herlich, H. Karl, in: 37th Annual IEEE Conference on Local Computer Networks -- Workshops, IEEE, 2013

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Incorporating feedback from application layer into routing and wavelength assignment algorithms

P. Wette, H. Karl, in: Proceedings of the 32nd IEEE International Conference on Computer Communications (INFOCOM), 2013, pp. 51-52

Preemptive Routing and Wavelength Assignment (RWA) algorithms preempt established lightpaths in case not enough resources are available to set up a new lightpath in a Wavelength Division Multiplexing (WDM) network. The selection of lightpaths to be preempted relies on internal decisions of the RWA algorithm. Thus, if dedicated properties of the network topology are required by the applications running on the network, these requirements have to be known to the RWA algorithm.Otherwise it might happen that by preempting a particular lightpath these requirements are violated. If, however, these requirements include parametersknown only at the nodes running the application, the RWA algorithm cannot evaluate the requirements. For this reason an RWA algorithm is needed which incorporates feedback from the application layer in the preemption decisions.This work proposes a simple interface along with an algorithm for computing and selecting preemption candidates in case a lightpath cannot be established. We reason about the necessity of using information from the application layer in the RWA and present two example applications which benefit from this idea.


Trade-off between latency and coverage in cooperative radio access networks

T. Hohenberger, M. Herlich, H. Karl, in: IEEE International Conference on Advanced Networks and Telecommunications Systems, ANTS 2013, Kattankulathur, India, December 15-18, 2013, 2013, pp. 1--6

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Cross-layer scheduling for multi-quality video streaming in cellular wireless networks

M. Dräxler, H. Karl, in: 2013 9th International Wireless Communications and Mobile Computing Conference, IWCMC 2013, Sardinia, Italy, July 1-5, 2013, 2013, pp. 1181--1186

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NetIDE: First Steps towards an Integrated Development Environment for Portable Network Apps

F. M. Facca, E. Salvadori, H. Karl, D. R. Lopez, P. Arranda Gutierrez, D. Kostic, R. Riggio, in: Second European Workshop on Software Defined Networks, EWSDN 2013, Berlin, Germany, October 10-11, 2013, 2013, pp. 105--110

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Which Flows Are Hiding Behind My Wildcard Rule? Adding Packet Sampling to OpenFlow

P. Wette, H. Karl, in: Proceedings of the ACM SIGCOMM '13, 2013, pp. 541-542

In OpenFlow [1], multiple switches share the same control plane which is centralized atwhat is called the OpenFlow controller. A switch only consists of a forwarding plane. Rules for forwarding individual packets (called ow entries in OpenFlow) are pushed from the controller to the switches. In a network with a high arrival rate of new ows, such as in a data center, the control trac between the switch and controller can become very high. As a consequence, routing of new ows will be slow. One way to reduce control trac is to use wildcarded ow entries. Wildcard ow entries can be used to create default routes in the network. However, since switches do not keep track of ows covered by a wildcard ow entry, the controller no longer has knowledge about individual ows. To nd out about these individual ows we propose an extension to the current OpenFlow standard to enable packet sampling of wildcard ow entries.


A Scalable Redundant TDMA Protocol for High-Density WSNs Inside an Aircraft

J. Blanckenstein, J. Garcia-Jimenez, J. Klaue, H. Karl, in: Lecture Notes in Electrical Engineering, Springer International Publishing, 2013, pp. 165-177

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On the Quality of Selfish Virtual Topology Reconfiguration in IP-over-WDM Networks

P. Wette, H. Karl, in: Proceedings of the 19th IEEE International Workshop on Local and Metropolitan Area Networks (IEEE LANMAN), 2013, pp. 1 - 6

The process of planning a virtual topology for a Wavelength Devision Multiplexing (WDM) network is called Virtual Topology Design (VTD). The goal of VTD is to find a virtual topology that supports forwarding the expected traffic without congestion. In networks with fluctuating, high traffic demands, it can happen that no single topology fits all changing traffic demands occurring over a longer time. Thus, during operation, the virtual topology has to be reconfigured. Since modern networks tend to be large, VTD algorithms have to scale well with increasing network size, requiring distributed algorithms. Existing distributed VTD algorithms, however, react too slowly on congestion for the real-time reconfiguration of large networks. We propose Selfish Virtual Topology Reconfiguration (SVTR) as a new algorithm for distributed VTD. It combines reconfiguring the virtual topology and routing through a Software Defined Network (SDN). SVTR is used for online, on-the-fly network reconfiguration. Its integrated routing and WDM reconfiguration keeps connection disruption due to network reconfiguration to a minimum and is able to react very quickly to traffic pattern changes. SVTR works by iteratively adapting the virtual topology to the observed traffic patterns without global traffic information and without future traffic estimations. We evaluated SVTR by simulation and found that it significantly lowers congestion in realistic networks and high load scenarios.


The playground of Wireless Dense networks of the future

C. Cicconetti, A. Morelli, M. Dräxler, H. Karl, V. Mancuso, V. Sciancalepore, R. Gupta, A. de la Oliva, M. Isabel Sanchez, P. Serrano, L. Roullet, in: 2013 Future Network & Mobile Summit, Lisboa, Portugal, July 3-5, 2013, 2013, pp. 1--4


Anticipatory Buffer Control and Quality Selection for Wireless Video Streaming

M. Dräxler, J. Blobel, P. Dreimann, S. Valentin, H. Karl, in: CoRR, 2013


Efficiency of On-Path and Off-Path Caching Strategies in Information Centric Networks

M. Dräxler, H. Karl, in: 2012 IEEE International Conference on Green Computing and Communications, Conference on Internet of Things, and Conference on Cyber, Physical and Social Computing, GreenCom/iThings/CPSCom 2012, Besancon, France, November 20-23, 2012, 2012, pp. 581--587

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Segment-based packet combining: how to schedule a dense relayer cluster?

A. Willig, H. Karl, D. Kipnis, Wireless Networks (2012), pp. 199--213

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OpenNetInf - prototyping an information-centric Network Architecture

C. Dannewitz, M. Herlich, H. Karl, in: 37th Annual IEEE Conference on Local Computer Networks, Workshop Proceedings, Clearwater Beach, FL, USA, October 22-25, 2012, 2012, pp. 1061--1069

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Using OMNeT++ for energy optimization simulations in mobile core networks

M. Dräxler, F. Beister, S. Kruska, J. Aelken, H. Karl, in: International ICST Conference on Simulation Tools and Techniques, SIMUTOOLS '12, Sirmione-Desenzano, Italy, March 19-23, 2012, 2012, pp. 157--165

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Average and competitive analysis of latency and power consumption of a queuing system with a sleep mode

M. Herlich, H. Karl, in: Proceedings of the 3rd International Conference on Energy-Efficient Computing and Networking, e-Energy'12, Madrid, Spain, May 9-11, 2012, 2012, pp. 14

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Cooperating base station set selection and network reconfiguration in limited backhaul networks

M. Dräxler, T. Biermann, H. Karl, W. Kellerer, in: 23rd IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC 2012, Sydney, Australia, September 9-12, 2012, 2012, pp. 1383--1389

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Quantization techniques for accurate soft message combining

T. Volkhausen, K. Schinköthe, H. Karl, in: 2012 IEEE Wireless Communications and Networking Conference, WCNC 2012, Paris, France, April 1-4, 2012, 2012, pp. 575--580

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Energy Efficient Clustering using a Wake-up Receiver

J. Blanckenstein, J. Klaue, H. Karl, in: European Wireless 2012 - 18th European Conference 2012, April 18-20, 2012, Poznan, Poland., 2012


Efficient cooperative relaying in wireless multi-hop networks with commodity WiFi hardware

T. Volkhausen, K. Dridger, H. S. Lichte, H. Karl, in: 10th International Symposium on Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks (WiOpt), Paderborn, Germany, May 14-18, 2012, 2012, pp. 299--304



On Local Separation of Processing and Storage in Infrastructure-as-a-Service

J. Künsemöller, H. Karl, in: Economics of Grids, Clouds, Systems, and Services - 9th International Conference, GECON 2012, Berlin, Germany, November 27-28, 2012. Proceedings, 2012, pp. 125--138

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Research challenges towards the Future Internet

M. Conti, S. Chong, S. Fdida, W. Jia, H. Karl, Y. Lin, P. M{\, M. Maier, R. Molva, S. Uhlig, M. Zukerman, Computer Communications (2011), pp. 2115--2134

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