Carlos Belo

mWorld: A multiuser 3D virtual environment

mWorld integrates 3D virtual-scene generation tools and synchronous, asynchronous, and time-dependent data communication techniques to enable cooperative 3D design and visualization across networks.

Neurological teleconsultation for general practitioners.

A neurology teleconsulting network was implemented between a university hospital in Lisbon and five nearby health centres. PCs equipped for videoconferencing were installed, connected by ISDN lines at 128 kbit/s. Fifty-three general practitioners (GPs) were surveyed. The survey showed that the GPs had difficulties in using computers, but they had definite intentions to use teleconsultation for neurology cases and 83% of the respondents stated that they would probably use the technique. During the study, 90 neurology teleconsultations took place over 55 weeks. The average consultation rate was 1.6 teleconsultations per week (SD 1.3, range 0-6). The conferences lasted 10-45 min. Longer teleconsultations were mainly due to technical difficulties in using computers on the part of users with a low level of computer literacy. The patients were 42 males and 46 females, with a mean age of 38 years (SD 20, range 1-84); two patients were discussed twice. The benefits consisted mainly of advice on patient medication, diagnosis and the prevention of unnecessary specialist consultations or laboratory examinations. Doctor-doctor teleconsultation allows the rapid resolution of queries which otherwise cause stress to patients and increase the cost and complexity of care.

Explicit rate congestion control with binary notifications

This paper presents a new rate-based source adaptation algorithm for packet-switching networks. The proposed scheme relies on binary notifications from the network to avoid congestion. The network nodes are supposed to estimate a fair share of source rate either based on a stored per-flow information or using a stateless approach. Binary notifications are sent to the sources, reflecting a positive or negative difference between the source rate and the estimated fair share at the nodes. Based on these notifications, sources increase/decrease the current rates using an adaptive algorithm that performs better than the classical additive increase multiplicative decrease strategy. In particular, oscillations on the source rates are much smaller, even with large network delays. Simulation results are presented both for ATM ABR and Internet environments.

Explicit congestion control based on 1-bit probabilistic marking

It has been recognized that router-assisted congestion control based on explicit-rate feedback exhibits better performance than the implicit feedback mechanisms currently available on the Internet. However, the deployment of explicit-rate algorithms in the Internet poses some challenges, in particular, the requirement of multi-byte congestion notifications per packet. In this paper, we present a new mechanism for congestion control signaling, based on probabilistic marking of 1-bit feedback channels. The scheme is suitable for router algorithms that were originally devised for multi-byte explicit-rate marking. The proposed mechanism, which includes a source algorithm and a router marking procedure, removes the deployment constraints concerning multi-byte feedback while keeping the essential characteristics and performance of the router algorithms. We show, through simulation, that the performance of router-assisted control of sources subject to the proposed binary feedback is close to the performance obtained with sources subject to explicit-rate feedback.

A proposal for binary congestion control with rate-based sources and stateless routers

The paper presents new algorithms for rate-based congestion control. The network congestion status is made available to the sources by explicit binary notifications generated in the core nodes. Besides the associated control bandwidth reduction, binary notifications enable a simple implementation in heterogeneous network environments where switches/routers use different congestion control strategies. The source algorithm is adaptive and shows significant improvement on the level of oscillations as compared to classical binary schemes, e.g. multiplicative decrease/additive increase. An algorithm for the core routers is also proposed that achieves a max-min fair-share allocation of bandwidth without requiring the memorization of per-flow data. The simulation results presented show a good performance of the algorithms both for homogeneous (Internet) and heterogeneous (Internet/ATM) network environments.

Source rate encoding mechanisms for binary congestion control

This paper focus on congestion control for best-effort packet-switching networks, where congested routers use 1 bit per packet to communicate with sources. Sources adapt their rates according to the sequence of bits received. Routers do not keep per-flow information but perform selective marking based on the source rate value inserted in each packet. We propose a new strategy for source rate encoding in forward packets, directly applicable to existing network protocols (e.g. IP). The scheme supports differentiated classes with respect to rate allocation. We test, by simulation, this encoding mechanism as well as the performance of the router and source algorithms.