Joachim Fabini

The Importance of Being Really Random: Methodological Aspects of IP-Layer 2G and 3G Network Delay Assessment

The accurateness of round-trip- and one way delay measurements for 2G and 3G networks rely to a much larger extent on employing a sound methodological framework than this is the case for other types of networks. Typical mobile access networks differ significantly from core networks, most prominently with respect to delay. In this paper we present payload-dependent delay measurement results for public 2G and 3G networks which illustrate that accurate IP-layer delay measurements in mobile networks must use high sample counts and randomness in start times for uplink and for downlink. Most notably this concerns ICMP round-trip delay measurements which, due to the synchronization of ICMP requests with the network clock when leaving the mobile networks uplink, fail to meet the random start time criterion for ICMP replies in the downlink (or vice-versa). This synchronization effect leads to significant clustering of one-way delay values for the reply leg and causes minor delay differences in the core network to have potential significant impact on ICMP round-trip delays. Therefore, highly accurate simulations and emulations must model uplink and downlink of time-slotted networks as two interrelated links based on a common timebase.

Terminal-Centric Location Services for the IP Multimedia Subsystem

Location-based services in 3G and beyond networks depend on high-accuracy location data. Todays 2G and 3G access networks determine user location in a network-centric manner. The accuracy of these location mechanisms depends on the radio infrastructure near the users geographical position and is typically limited to hundreds or thousands of meters. This precision is inadequate for typical location-based services like city maps or route planners. In this paper we present a terminal-centric location enabler that integrates seamless with the existing IP Multimedia Subsystem presence architecture and interoperates with network-centric location mechanisms. We argue that the user terminal itself, equipped with location sources like GPS receivers, can determine the geographical position with an optimum of accuracy. In consequence, the user terminal should be used as the primary source for location information. We extend the existing and well-known concept of presence by defining location as a type of presence information that is of interest to users. Terminal-based triggers and filters reduce the amount of traffic on the air interface and enable a scalable location architecture. We present the implementation of a J2ME-based mobile terminal prototype, which uses GPS as location source and interfaces with IMS location services.

The Illusion of Being Deterministic --- Application-Level Considerations on Delay in 3G HSPA Networks

The delay experienced by mobile applications in HSPA networks depends to a large extent on highly dynamical global context like, e.g., cell load or algorithms and thresholds governing radio resource scheduling, and on local context like, e.g., user-generated load or load history. These complex uncertainty factors are outside of an applications sphere of influence and result in applications perceiving HSPA link behavior as non-deterministic and non-reproducible. This paper analyzes accurate round-trip and one-way delay measurement results for three public HSPA networks to demonstrate the high degree of network non-determinism which mobile applications are likely to encounter in practice, particularly significant payload-dependence and halved delay on slightly increased user-generated link load. We argue that current HSPA radio link schedulers, relying on instantaneous user load as decision criterion for channel capacity allocation, neglect real-time application requirements. Cross-layer optimization is one solution which enables deterministic scheduler decisions based on application requirements.

Location-based assisted handover for the IP Multimedia Subsystem

The IP Multimedia Subsystem (IMS) is one promising candidate for merging currently deployed circuit-switched and packet-switched voice and data networks into a service-based All-IP architecture. One requirement common to all of these Next Generation Network (NGN) architectures is seamless user mobility - a mandatory feature in order to match with the tremendous success of todays circuit-switched GSM voice networks. In addition most NGN architectures mandate the support for heterogeneous access technologies like WLAN, WiMAX, GPRS/EDGE, UMTS/HSDPA which raises the need for seamless vertical handover between these access technologies. Despite ongoing standardisation and technology developments like Generic Access Network (GAN), Voice Call Continuity (VCC) or Media Independent Handover (MIH, IEEE 802.21) we argue that there is currently no solution to enable seamless and application-transparent vertical handover in the IMS. In this paper, we review vertical handover standards for IMS and propose a novel vertical handover architecture which relies on geographical location and global network information as primary trigger or support for handover initiation. The underlying Session Initiation Protocol (SIP) notification framework - a Terminal-Centric IMS Location Enabler (LE) - is based on the key concept that location information is one specific kind of presence information which is optimally provisioned by mobile devices: the terminal knows best where it is located. The Location Enabler solicits notifications from the mobile device on specific events - e.g., entering or leaving specific geographical areas - thus dramatically reducing the amount of data transferred over the radio interface when compared to tracking or polling. The SIP registration and Re-Invite message flow is used to convey all handover-relevant information between the IMS core network and the mobile terminal. Using accurate access network measurement results the paper presents a performance estimation of the proposed vertical handover architecture with respect to handover duration and required handover area width at different velocities.

Delay Measurement Methodology Revisited: Time-Slotted Randomness Cancellation

The operation of todays networked applications and protocols depends to a large extent on accurate end-to-end delay measurements and estimations. Appropriate methodologies can significantly improve quality, accuracy, and representativeness of delay samples acquired through measurements. This paper focuses on limitations of state-of-the-art end-to-end delay measurement methodologies. Its central observation is that the first time-slotted link in a measurement path cancels start-time randomness of delay measurement samples. When leaving this first link, all measurement samples are time-synchronized with each other and potentially with global time modulo link period. Because of this effect, which the paper introduces as time-slotted randomness cancellation effect, random sampling is not possible beyond the first time-slotted link of measurement paths. End-to-end measurements, therefore, fail to capture the full delay range of subsequent time-slotted links in the path, measurement results being limited to a specific session setup. Following a detailed discussion of theoretical models, the paper proposes a novel delay measurement methodology that adds artificial delay functionality to intermediate network nodes. Measurement packet headers include random seeds that are used by compatible ingress nodes of subsequent time-slotted network segments to regenerate start-time randomness. Samples acquired with this measurement methodology can assess a network paths full delay range. Practical applicability of the presented concept and methodology is demonstrated by a prototype implementation that assesses delay in public mobile cellular networks. Measurement results presented in this paper confirm that the proposed concept and methodology is generally applicable and that randomness regeneration can significantly improve quality and representativeness of delay measurement samples.

A cross-layer route discovery strategy for virtual currency systems in mobile ad hoc networks

New challenges arise for mobile ad hoc networks when their nodes belong to different authorities. Cooperation and fair behavior cannot be taken for granted, but are essential for wide distribution of this network type. One solution to this problem is the integration of rewarding schemes using virtual currency systems. Nodes are compensated for their service to stimulate cooperation, and payment for used services prevents selfishness and free riders. Besides, integrated payment systems can make mobile ad hoc networks attractive for a number of business scenarios. Most of todays payment systems are realized as a service in mobile ad hoc networks and consequently do not profit from deep integration into routing mechanism. As a solution to this drawback, we present in this paper a new cross layer optimized route discovery scheme for reactive routing protocols, particularly designed to fit the needs of payment systems. Using a distributed route pre-selection technique in combination with artificial delaying of route discovery packets, the performance of the protocol was optimized which will be shown using simulations. The proposed route discovery scheme can be used by a number of reactive routing protocols. Exemplarily, we present a realization based on dynamic source routing (DSR). Additionally we discuss other application scenarios that can profit by using the concepts presented in this paper.

Resilience and Security: A Qualitative Survey of Urban Smart Grid Architectures

Smart grids require information and communication technology (ICT) in order to control dynamics in the power grid. However, adding ICT creates additional entry points in vulnerable hard- and software, increasing the attack surface, and provides distribution paths that can be used by malware for attacks. This paper provides a qualitative evaluation of smart grid architectures for urban environments, comparing four topology types based on six quality indicators: resource control, security, resilience, quality of service, compatibility, and cost. The impact of each power grid topology on the applicability of ICT components in communication topologies is also considered. We summarize the benefits and drawbacks of each topology with a focus on the implementation of decentralized and self-organizing structures.

Content-aware scheduling for video streaming over HSDPA networks

In this article we propose the implementation of a content-aware cross-layer optimization for video streaming in HSDPA networks. The video packets are marked at the IP layer based on the impact on the quality of experience at the application layer. In the core network these packets are mapped onto multiple logical paths with different quality of service accordingly to their marking. The proposed approach complies with the 3GPP releases newer than R99. It has been tested with an HSDPA system level simulator and H-264 video traces. The results of simulations show that the proposed content aware scheduling delivers over 1 dB Y-PSNR improvement compared to the standard round robin.

Security Challenges for Wide Area Monitoring in Smart Grids

Wide Area Monitoring Systems (WAMS) improve situational awareness in the electric grid. They support planning and optimizing of grid operations and provide valuable information to prevent critical incidents. Communication demands for WAMS have been elevated by the variety of applications that rely on measurement data from distributed sensors. Besides bounds on tolerated end-to-end latencies for some applications, security is a major concern in todays Wide Area Monitoring Systems. We review recent approaches for WAMS communication and point out security challenges that need to be addressed in future communication solutions for WAMS.ZusammenfassungWide Area Monitoring-Systeme (WAMS) formen Netze aus verteilten Sensoren zur Überwachung von intelligenten Stromnetzen (Smart Grids). Sie unterstützen Planung und Optimierung von Prozessen im Elektrizitätsnetz und liefern wertvolle Informationen zur Prävention von kritischen Ereignissen. Viele Anwendungen benötigen aktuelle Messdaten, um auf neue Situationen in intelligenten Stromnetzen reagieren zu können. Damit steigen auch die Kommunikationsanforderungen für Wide Area Monitoring-Systeme. Neben Anforderungen bezüglich der maximal tolerierbaren Ende-zu-Ende-Verzögerungszeiten sind heutzutage vor allem Sicherheitsmechanismen von hoher Bedeutung für Wide Area Monitoring-Systeme. Wir vergleichen verschiedene Ansätze für WAMS-Kommunikation und zeigen, welche Sicherheitsanforderungen in zukünftigen Kommunikationslösungen für WAMS berücksichtigt werden müssen.

Packet delay measurements in reactive IP networks

Discussions of how to measure the performance of computer networks for various applications have been ongoing for over twenty years in the area of network research. The continual increase of data traffic volume has reached a point at which solving any network problem by over-provisioning is not suitable. The quest for alternatives makes it vital to have well-defined metrics for evaluating and sustaining the performance of networks.