Marco Happenhofer

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.

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.

Quality of signalling: A new concept for evaluating the performance of non-INVITE SIP transactions

Providing Quality of Service (QoS) and Quality of Experience (QoE) in connection with media data like audio or video is of key importance for the evolution of future packet-based networks. Especially in telecommunications, the subjective service quality as perceived by the end user depends amongst other parameters on the setup success rate as well as on the delay for session setup, which is measured end to end. In this paper, we introduce the new concept of “Quality of Signaling” (QoSg) and present an analysis based on Session Initiation Protocol (SIP) networks, where messages traverse usually several hops (SIP proxies) and several connections. SIP introduces the concept of transactions that are managed hop-by-hop and form the basic building blocks for SIP signaling. We argue that the end-to-end service quality strongly depends on the performance of these transactions and provide a comprehensive analytic, simulative and measurement-based performance evaluation of single SIP transactions.

Measurement-Based Analysis of Head-of-Line Blocking for SIP over TCP

The Transmission Control Protocol (TCP) is a transport protocol which delivers packets strictly in order. For signaling protocols which send multiple independent requests within one single TCP session, the loss of a TCP packet may therefore lead to increased delay for later signaling requests. This phenomenon is known as Head-of-Line Blocking (HoLB) and for instance has led to the standardization of the Stream Control Transmission Protocol (SCTP) as a transport protocol for SS7 over IP. In this paper, we will discuss the effects of HoLB for another signaling protocol, i.e. the Session Initiation Protocol (SIP). We introduce a two layer measurement methodology which allows differentiating several types of delay (e.g. due to congestion control, transmission and HoLB), and discuss the resulting correlations between delay, loss and HoLB duration for TCP.

An Architectural and Evaluative Review of Implicit and Explicit SIP Overload Handling

Last years trend to migrate circuit-switched voice networks to packet switched Internet Protocol IP based networks has favored wide deployment of Session Initiation Protocol SIP based systems and networks. As a reaction to large-scale SIP deployment experiences in the field and the need to implement high availability and reliability within these new networks, the focus of SIP extension standardization has shifted from adding new SIP signaling functionality to operational and maintenance aspects, a particular importance being attributed to overload control. Overload denotes a situation in which the traffic injected into a system exceeds the systems designed capacity. The authors present a detailed categorization of overload architectures and outline main reasons why SIP-based networks are at high risk to collapse when operating at overload. Using measurements in a real SIP infrastructure this paper compares the performance of two overload protection schemes, namely implicit and explicit overload protection, against the performance of non-protected systems. The measurement results recommend overload protection as a mandatory component of commercial SIP deployments to safeguard operation and prevent system collapse in case of overload.

Collapse Detection and Avoidance for SIP Architectures

Abstract The Session Initiation Protocol (SIP) relies on timer-based message retransmission for safeguarding reliable message transfer when deployed on top of the unreliable User Datagram Protocol (UDP) over IP. In this paper we present a detailed impact analysis of SIP timers onto the functionality of a system and its capabilities to recover from overload situations. The results of our event-based SIP simulations demonstrate that message retransmissions originating from a minor short-term overload can force a system into a deterministic congestion collapse when using default SIP timer settings. A recovery from this severe system overload situation is highly difficult or impossible, even if the system load is reduced substantially afterward. Our performance evaluation shows that an increase of timer T1s value significantly enhances the stability, robustness and the ability of systems to handle overload, whereas the resulting increase of response times is relatively small and overall system responsiveness can even improve in some cases. We propose an algorithm for implicit collapse detection as solution for dynamic timer T1 optimization. Based on monitoring of system load and pending transaction counts, our algorithm enables intermediate SIP proxies to detect congestion in a very early phase, allowing them to counteract in time, i.e., to modify timers and reject new system load in order to prevent the system from collapse.

The SHIP: A SIP to HTTP Interaction Protocol

IMS is capable of providing a wide range of services. As a result, terminal software becomes more and more complex to deliver network intelligence to user applications. Currently mobile terminal software needs to be permanently updated so that the latest network services and functionality can be delivered to the user. In the Internet, browser based user interfaces assure that an interface is made available to the user which offers the latest services in the net immediately. Our approach combines the benefits of the Session Initiation Protocol (SIP) and those of the HTTP protocol to bring the same type of user interfacing to IMS. SIP (IMS) realizes authentication, session management, charging and Quality of Service (QoS), HTTP provides access to Internet services and allows the user interface of an application to run on a mobile terminal while processing and orchestration is done on the server. A SHIP enabled IMS client only needs to handle data transport and session management via SIP, HTTP and RTP and render streaming media, HTML and Javascript. SHIP allows new kinds of applications, which combine audio, video and data within a single multimedia session.

A study of SIP proxy load patterns

We present sensing mechanisms that can be used for implicit high-load and overload detection in SIP networks. By means of measurements and simulations we highlight the characteristics of SIP proxy servers for different load situations and using different transport protocols. Each protocol yields to distinctive patterns that encourage deriving an algorithm that is able to estimate a downstream servers current load.

BIQINI – A Flow-Based QoS Enforcement Architecture for NGN Services

Novel mobile cellular access network technologies like Long Term Evolution (LTE) promise capacities exceeding the ones of existing 3G networks by at least one order of magnitude. This evolution will enable the deployment of services which, due to their capacity requirements, are currently restricted to fixed access networks. On the other hand, packet-switched-only architectures raise the need for a reliable and accurate management of these high access capacities, particularly service-specific Quality of Service (QoS) enforcement, in order to prioritize real-time (voice) services and safeguard a satisfactory Quality of Experience (QoE) to the user.

Cashflow: A Channel-Oriented, Credit-Based Virtual Currency System for Establishing Fairness in Ad-Hoc Networks with Selfish Nodes

Cooperation in ad hoc networks cannot be taken for granted when nodes belong to distinct authoritative domains. In this paper we present Cashflow, a virtual currency system to motivate nodes to participate in ad hoc networks and to prevent selfishness. This system is different to previously proposed virtual currency systems in that it uses a channel concept for data transmission as well as a market system for pricing. The combination of channel and market concept results in a number of positive system characteristics. For instance, Cashflow provides implicit access regulation and load balancing mechanisms to avoid overload situations in the network. Additionally it considers node context for data transmission and pricing. Cashflow also leaves the decision about the participation degree to the user, which is an important but frequently neglected topic for the user acceptance of virtual currency systems.