Fernando Cerdan

Optimizing TCP and RLC interaction in the UMTS radio access network

TCP, the dominant transport protocol for Internet applications, suffers severe performance degradation due to packet losses when a wireless link is present in the end-to-end path. For this reason, the 3G specification entity, 3GPP, has defined a reliable link layer protocol, RLC, to support packet switched services over UMTS. RLC provides error recovery in the radio access network by means of an ARQ algorithm. Early studies supported the benefit of using a reliable link protocol, while more recent studies outline new problems arising from RLC and TCP interaction and how to overcome them. This article describes the most relevant issues concerning TCP-RLC interaction and evaluates the most practical enhancement approaches, based on optimum parameter configuration at the transport and link layers. We devote special attention to RLC, whose specific configuration decisions are left to operators, and provide specific guidelines for setting its parameters. In addition, we propose two operational changes for enhancing the buffer management strategy, one of the main drawbacks of RLC

Subjective QoE analysis of VoIP applications in a wireless campus environment

The quality perceived by the user, a concept known as Quality of user Experience (QoE), is gaining momentum. Together with the efforts of providing appropriate architectures able to deliver the expected level of Quality of Service (QoS), users’ perception of the level of quality could play a key role in making a VoIP multimedia application to succeed or to fail. QoE can be measured from different approaches; subjective ones are based on polls while objective ones are mainly based on QoS network performance parameters. Although a subjective evaluation could be costly and time consuming, it gives the most accurate results. Moreover, wireless environments incorporate new challenges in terms of QoS and QoE because of inherent wireless communication characteristics. In this way, our contribution is twofold. First, we carry out a subjective QoE assessment for VoIP applications in a real wireless environment. Skype, Gizmo5, ooVoo, and Damaka are the selected VoIP applications under study. Next, we contrast QoE results with QoS evaluation from the captured video calls made during the poll. Results show that there is no precise match on both assessments, because of QoE parameters that cannot be inferred from QoS analysis, because of the different effect of some QoS parameters which prevail, or even because of the popularity of the VoIP application.

Internet measurements and data study over the regional

We present the most significant results studying the Internet network traffic measurements obtained in Ciez@net. Ciez@net is a citizen subnet located in the village of Cieza that belongs to the regional network of the autonomous community of Murcia in Spain. This subnet is one of the firsts pilot experiences of a digital city in Europe and the first in the region of Murcia. The goal is the seamless introduction of the information society in a medium-size population. Access to advanced electronic information services is stimulated or subsidized for effective penetration. These measurements will allow a qualitative and quantitative knowledge of the network traffic in order to achieve a most effective network resource provisioning and Internet traffic forecasting in a real scenario. A suitable dimensioning of the network as well as an adequate provisioning of quality of service to users may depend partially on these results. Measurements were taken from a frame relay link connecting Ciez@nets users to Internet through the main node located in Murcia city. We used a promiscuous network analyzer that avoids interfering with the network traffic. We report the results of the traffic load, network performance, percentage composition of traffic by protocol and type of application, and IP packet size distribution in both, up and down communications streams.

Maximal matching characterization of optical packet input-buffered wavelength routed switches

The shared wavelength path (SHWP) and the scattered wavelength path (SCWP) were proposed under the WASPNET (wavelength switched packet network) project. They are the envisaged operating mode competing alternatives for optical packet switching in the WDM networking context. A maximum matching model is presented to evaluate the performance of the input-buffered wavelength-routed (IB-WR) optical packet switch, under both SHWP and SCWP switching modes. We conduct a necessary set of modifications for this architecture, which was not originally conceived with a SCWP/SHWP scheme in mind. The characterization proposed is based on the establishment of independent maximum size matching or maximum weight matching optimization problems. It provides a method to calculate the optimum performance achievable by this architecture in terms of switch throughput and average delay, for both the SHWP and SCWP operational modes. The modeling may consider additional conditions to maintain, or not, the packet sequence. Besides that, our approximation establishes a framework for the study of simple scheduling processes which take benefit of the simple hardware requirements associated with IB-WR architecture. As an example, a sequential SHWP scheduler presented by IB-WR switch designers, and its SCWP evolution, which we present, are evaluated and compared.

Counters-based modified traffic conditioner

Traffic conditioners play a key role in implementing the Assured Service in the framework of the DiffServ approach. In this paper, we propose a traffic conditioner for the Internet Assured Service called Counters-Based Modified (CBM) that strictly guarantees target rates and performs a fair share of the excess bandwidth among TCP sources. The fairness in the outbound bandwidth distribution is met by probabilistically dropping OUT packets in the traffic conditioner. To determine the dropping probability of an OUT packet, some sort of signaling is needed. Although, it results more feasible than other proposed intelligent traffic conditioners. The CBM traffic conditioner is evaluated under different conditions by simulation using TCP Reno sources.

Unifying heterogeneous e-learning modalities in a single platform: CADI, a case study

Current e-learning forms are commonly based on improving the learning process through the enhancement of certain skills in students, such as collaborative, competitive or problem-based learning. However, it seems that there is still no e-learning formula that gathers the implementation of a number of more generic educational principles in a single e-learning system or platform. Therefore, this paper presents a web-based framework for the creation, development and implementation of heterogeneous learning environments called CADI. It offers a graphical user interface for students to interact with only the need for any generic WiFi compliant device. Its innovative design allows the instructor to select and combine the developed resources to create varied activities, providing content adapted to every single device. The usefulness of the system was tested by introducing a case study to a group of senior college students. The experience showed that the use of the system helped students to achieve better results in their evaluation. It also showed that the students had no problems interacting with it from the outset, promoting their active learning. Design, implementation and assessment results of the system are also presented and analyzed.

MUST, a multicast synchronous transfer application for fast intra-campus replications

In this paper, we introduce a new multicast application called MUST (multicast synchronous transfer) that allows the replication of hard disk partitions in a fast and efficient way. The main goal of MUST is to help system administrators to easily maintain and update all computers in a lab. MUST transmits its IP packets in multicast mode, avoiding the use of unnecessary bandwidth. Unfortunately, the well known TCP transport protocol cannot be used in this multicast environment since the number of TCP flow control windows (and consequently, the number of ACK) that server should process, grow exponentially. Therefore, we also outline an alternative. multicast transport protocol specifically designed for this application. This way, the number of acknowledge frames are drastically reduced, allowing the use of MUST application in a LAN and intra-campus (directly interconnected LANs via few routers). scenarios. The application has been programmed in C language using standard kernel routines, therefore, MUST can mount almost all existing file systems. In addition, we can distribute our application world wide without incurring legal conflicts.

Performance evaluation of profiler mechanisms for the Internet Assured Service

As Internet is rapidly growing and receiving traffic from multimedia applications that are sensitive to available bandwidth and delay experienced in the network, there is a strong need for quality of service (QoS) support. The Integrated and Differentiated Service models are two approaches for adding QoS to Internet. The Assured Service is an end-to-end service based on the Differentiated Service architecture. In this paper, we study and compare the performance of three profiler mechanisms to provide the guaranties of an Internet Assured Service. Two of them, TSW and Leaky Bucket are the most commonly used, and the third is a new Counter Based profiler, which is proposed in this paper. The study is done by simulation employing TCP RENO sources.

Optical packet switching fabrics comparison under SCWP/SHWP operational modes

Commercial applications of optical packet switching to backbone WDM networks are still far from reality due to hardware costs. The two envisaged alternatives for network control and operation of this type of networks is scattered wavelength path (SCWP) and shared wavelength path (SHWP). The advantages and drawbacks of both approaches involve many different issues. In this paper, the impact of SCWP and SHWP switching modes on performance and hardware costs is discussed and evaluated for three different prominent OPS switch fabric designs: KEOPS switch, space switch, and output-buffered wavelength-routed switch. To achieve it, we conduct a necessary set of modifications for these architectures, which were not originally conceived for SCWP/SHWP. This normalization process is unavoidable to provide a true comparison among the switch fabric designs, and to advance to a global comparison between SCWP and SHWP operation modes, that under knowledge of the authors do not exist yet.

The Application of PDAs and Smartphones in the Classroom as Instruments for Improvement of Learning Monitoring

The skill of students in the use of new technologies could provoke serious problems of lower attention toward studied contents in the classroom. Nevertheless, many advantages can be also highlighted from an educational point of view: high ability for the search of technical information, quicker exchange of information by keyboard interface, good communication resources for group works, etc. In this sense, the quality of teaching action can be remarkably improved by the application of educational platforms based on electronic devices such as PDA and Smartphones, as proved by the project of educative innovation that was carried out by the authors of this work. With the purpose of promoting the philosophy of continuous improvement into the teaching/learning process, these authors created a platform dedicated to facilitate the interactive communication between teacher and students inside the classroom, as a useful educative tool to provide a feedback about learning level achieved by students. The new technology is oriented to assist to the teacher by means of introduction of the Shewhart Cycle –or also named PDCA (Plan, Do, Check, Act) Cycle–, avoiding the waste of sheet and significant time losses during the educative practice. The results obtained regarding the course “Quality Engineering” that belongs to degree of Industrial Engineering are shown in this work, although similar benefits could be achieved for other courses contained in technical teachings.