Francesco Vacirca

Large-scale RTT measurements from an operational UMTS/GPRS network

In this paper we present some observations about TCP RTT as captured in the live traffic of an operational GPRS/UMTS network. RTT samples are extracted from traces collected by passive monitoring the Gn interfaces of one of the major providers in Austria, EU. We compare results for GPRS and UMTS, and expose some methodological issues involved in performing this type of analysis, for instance the potential bias introduced by heavy users. We also explore the correlation of RTT with time-of-day. The primary motivation for this research is to gain a better understanding of the dominant causes that shape the behaviour and the distribution of RTT across the TCP connections that populate a GPRS/UMTS network. The results presented here are useful for the scientific community, for instance to set more realistic simulation scenarios for other research activities. In addition, RTT measurements as obtained by large-scale passive monitoring can be used in combination with other indicators to build a summary indicator of the performances experienced by the TCP users at the scale of the whole network, i.e. a global RTT-based KPI (key performance indicator).

Optimal design of hybrid FEC/ARQ schemes for TCP over wireless links with Rayleigh fading

In this paper, we investigate interactions between TCP and wireless hybrid FEC/ARQ schemes. The aim is to understand what is the best configuration of the wireless link protocol in order to guarantee TCP performance and channel efficiency. Interactions between TCP and different link layer mechanisms are evaluated by means of an analytic model that reproduces: 1) a Rayleigh fading channel with FEC coding, 2) a generic selective repeat ARQ Protocol, and 3) the TCP behavior in a wired-cum-wireless network scenario. The analytic model is validated-by means of ns-based simulations. The analysis represents a contribution to the optimal design of link layer parameters of wireless networks crossed by TCP/IP traffic. The main findings can be summarized as follows: 1) fully reliable ARQ protocols are the best choice for both TCP performance and wireless link efficiency and 2) optimal values of FEC redundancy degree from the point of view of energy efficiency maximizes TCP performance as well.

Bottleneck detection in UMTS via TCP passive monitoring: a real case

In this work we address the problem of inferring the presence of a bottleneck from passive measurement in the UMTS core network. The study is based on one month of packet traces collected in the core network of mobilkom austria AG & Co KG, the leading mobile telecommunications provider in Austria, EU. During the measurement period a bottleneck link in the UMTS core network was revealed and removed, therefore the traces enable the accurate analysis and comparison of the traffic behavior in the two network conditions. The proposed approach exploits statistics of estimated TCP performance parameters (e.g. RTT, re-transmissions) in order to build a set of bottleneck indicators. We show that such statistics are volatile due to the presence of few top users, but this effect can be counteracted with a simple filtering method. Results show that the frequency of re-transmission events is a powerful indicator for the specific type of bottleneck under study, and it can be used to provide early warning about future occurrences of similar events. This application is particularly important for operational UMTS networks nowadays, since the traffic volumes and composition is still under evolution.

On the effects of ARQ mechanisms on TCP performance in wireless environments

In this paper we investigate the interaction between TCP and wireless ARQ mechanisms. The aim is to understand what is the best reliability degree of the wireless link in order to guarantee TCP performance. For this purpose, we first develop a Markov model for a selective repeat ARQ protocol, widely used in the current wireless environments. Secondly, we design a cross-layer algorithm that, by exploiting the proposed model, can adapt the number of link layer transmission attempts to the end-to-end packet loss rate perceived by TCP. The interaction between TCP and link layer is evaluated in a specific case study (TCP over 3G radio access) by means of simulations carried out by using a very detailed UMTS-TDD simulator based on us. The deployment of the link layer Markov model and of the proposed algorithm allows us to derive some interesting conclusions about the design of retransmission protocols in TCP/IP network environments.

An algorithm to detect TCP spurious timeouts and its application to operational UMTS/GPRS networks

This paper proposes an algorithm to identify TCP spurious retransmission timeouts by post processing of packet traces monitored in operational networks. The operational principles of the algorithm and the assumptions behind its design are explained in detail as well as the situations in which the algorithm is prone to inaccuracies. By extensive measurements in a lab testbed using realistic round trip time characteristics as observed in operational wireless networks and FTP-like as well as Web-like traffic generators, it is shown that the algorithm is accurate in detection of spurious retransmission timeouts. Subsequently, the algorithm is applied to real traffic traces captured at several interfaces of an operational UMTS and GPRS network to analyze the frequency of spurious retransmission timeouts as well as the spurious timeout probability dependent on the load situation in the network and the flow size. This investigation, to our best knowledge the first on large scale TCP traffic traces monitored in an operational UMTS network, shows that spurious timeouts are infrequent events in the considered UMTS as well as the GPRS network. Among other findings, it is additionally shown that the ratio between spurious timeouts and other congestion recovery events experienced by TCP flows is low, indicating a negligible impact of spurious timeouts on TCP performance.

Diagnosis of capacity bottlenecks via passive monitoring in 3G networks: An empirical analysis

In this work we address the problem of inferring the presence of a capacity bottleneck from passive measurements in a 3G mobile network. The study is based on one month of packet traces collected in the UMTS core network of mobilkom austria AG & Co KG, the leading mobile telecommunications provider in Austria, EU. During the measurement period a bottleneck link in the UMTS core network was revealed and removed, therefore the traces enable the accurate analysis and comparison of the traffic behavior in the two network conditions: with and without a capacity bottleneck. Two approaches to bottleneck detection are investigated. The first one is based on the signal analysis of the marginal rate distribution of the traffic aggregate along one day cycle. Since TCP-controlled traffic dominates the overall traffic mix, the presence of a bottleneck strains the aggregate rate distribution and compresses it against the capacity limit during the peak hour. The second approach is based on the analysis of several TCP performance parameters, e.g. estimated frequency of retransmissions. Such statistics are unstable due to the presence of few top users, but this effect can be counteracted with simple filtering methods. Both approaches are validated via simulations. Our results show that both approaches can be used to provide early warning about future occurrences of capacity bottlenecks, and can complement other existing monitoring tools in the operation of a production network.

Traffic monitoring and analysis in 3G networks: lessons learned from the METAWIN project

A 3G network is a magnificently complex object embedded in a highly heterogeneous and ever-changing usage environment. It combines the functional complexity of the wireless cellular paradigm with the protocol dynamics of TCP/IP networks. Understanding such an environment is more urgent and at the same time more difficult than for legacy 2G networks. Continuous traffic monitoring by means of an advanced system, coupled with routine expert-driven traffic analysis, provides an in-depth understanding of the status and performances of the network as well as of the statistical behaviour of the user population. Such knowledge allows for a better engineering and operation practice of the whole network, and specifically the early detection of hidden risks and emerging troubles. Furthermore, the exploitation of certain TCP/IP dynamic behaviour, particularly the TCP control-loop, coupled with information extracted from the 3GPP layers, provides a cost-effective means to monitor the status of the whole network without requiring access to all network elements. In this article the main lessons are summarized learned from a two-year research activity on traffic monitoring and analysis on top of an operational 3G network.Ein 3G-Mobilfunknetz stellt ein extrem komplexes Gebilde dar, das in ein stark heterogenes und ständig wechselndes Umfeld eingebettet ist. In ihm werden komplexe Funktionen der Mobilfunkübertragung mit TCP/IP-Protokollen verbunden. Ein tiefes Verständnis dieses Systems ist dringend erforderlich, aber sehr schwierig im Vergleich zu vorhandenen 2G-Netzen. Nur die kontinuierliche Beobachtung dieses modernen Übertragungssystems erlaubt es, sowohl die Leistungsfähigkeit des Netzes zu verstehen als auch das statistische Benutzerverhalten zu interpretieren. Die daraus gewonnenen Erkenntnisse begünstigen sowohl eine verbesserte Netzplanung als auch einen optimalen Betrieb und gestatten es darüber hinaus, verborgene Risiken und vorhandene Schwierigkeiten sehr früh zu erkennen. Insbesondere erlaubt die Beobachtung bestimmter TCP/IP-Dynamik gemeinsam mit der Information, die aus den 3GPP-Schichten gewonnen wird, eine kostengünstige Möglichkeit, den Zustand des Gesamtnetzes zu erfassen ohne jedes Einzelelement des Netzes zu erfassen. In diesem Artikel sind die wesentlichen Erkenntnisse zusammengefasst, die in Folge eines zweijährigen Forschungsprojektes zur Beobachtung und Analyse eines 3G-Mobilfunknetzes gewonnen wurden.

Architectures and protocols for mobile computing applications: a reconfigurable approach

This work deals with reconfigurable control functions and protocols for supporting mobile computing applications in heterogeneous wireless systems like cellular networks and WLANs. The control functions are implemented in a software module, named Reconfigurable Access module for MObile computiNg applications (RAMON), placed in mobile and/or base stations. RAMON operates on abstract models of the main communication functions of a wireless systems (e.g., transmission over the radio channel, coding end error recovery, capacity sharing and packet scheduling, handover, congestion control, etc.). RAMON algorithms are programmed with reference to the abstract models, independently of specific radio and network technologies. RAMON interactions with a specific wireless access system are conceptually defined by means of parameters that can be measured and controlled, so that the general logic of the module can be posed on top of each system within the constraints and the flexibility provided by the system itself. The implementation of this architectural paradigm requires the definition of specific software adaptation modules between RAMON and each specific system. The reconfigurability of RAMON is exploited to adapt and select the algorithms on the basis of user/application requirements. The paper describes the RAMON architectural model and its control algorithms. Specific examples of adaptation modules are also provided. A selection of performance results achieved by a simulator implementing the RAMON module, the adaptation modules and the main communication functions of UMTS, 802.11 and Bluetooth are shown.

End-to-end evaluation of WWW and file transfer performance for UMTS-TDD

Wireless data is deemed as a major booster of next generation wireless networks. In this context, the support of Internet applications based on the TCP still presents several open issues. The aim of this work is to evaluate the performance of TCP data transfer over the UMTS TD-CDMA air interface, by means of a rather detailed simulation model. The evaluation includes both bulk FTP like data transfer and interactive client-server (WWW) traffic. The distinctive point of view is exploring the interaction between application and transport level protocol functions and lower layers protocols over the radio interface. The interplay of radio access and fixed core network impairments is analyzed as well. Major results deal with balancing the error recovery performance of the RLC against the radio channel correlation and the fixed network impairments.

Characterization of Service Times Burstiness of IEEE 802.11 DCF

First order metrics (throughput, average delay) of the IEEE 802.11 DCF MAC protocol have been extensively analyzed. The service process of the same protocol has not received the same attention, although it is known that it might be bursty. We develop a simple and accurate Markov model that allows a complete characterization of the service process of an 802.11 BSS. A major result of simulations is that correlation between consecutive service times is negligible, hence the service process can be safely described as a renewal process. The analytic model highlights that service burstiness lies essentially in the doubling of the contention window after a collision up to very large values. The trade-off between average throughput and service burstiness is obtained from the model.