E-learning is a general term used to refer to computer-enhanced learning. The most notable advantages of e-learning are flexibility, convenience and the ability to work at our own pace. Other advantages include the ability to communicate with fellow classmates from around the country. However, a common problem of usual e-learning approach is the lack of face-to-face interaction with a teacher. The most obvious solution to overcome this problem is to include a real-time multipoint audio-video channel to provide synchronous communications between teacher and students. This paper starts from an implementation experience of an e-learning platform for synchronous interaction between the actors, keeping "low-cost" as the main requirement of the deployment, to give the possibility to any user to participate at home using a domestic Internet access and a base-level personal computer. This paper proposes a software-based platform to provide multipoint communications in a heterogeneous unicast/multicast environment. It is constituted by a number of S-MCUs (software MCUs). Each S-MCU runs on a low-cost computer and is implemented like a software router by using click. Design, implementation and performance issues are discussed by a comparative numerical analysis.

With the advent of multimedia traffic over the Internet several TCP-friendly protocols have been developed to improve UDP with congestion control mechanisms. The most promising TCP-friendly protocol is the TFRC. This paper addresses a novel analytical model for TFRC traffic sources. The model is able to capture TFRC behavior and its dependence on the network congestion state. Moreover, the model includes the TFRC loss probability estimation algorithm performed by the receiver. In order to keep the model manageable, whatever the number of sources and network routers, a fluid-flow approach is used.

Adaptive rate video encoding is required to maximize efficiency when wireless links are involved in the communication. In fact, wireless channels are characterized by high, time-varying bit error rates. To cope efficiently with this problem adaptive forward error correction schemes have been proposed. These schemes introduce an amount of redundancy dependent on the channel conditions. Accordingly, the bandwidth available at the application layer changes: it increases when channel conditions improve, and decreases when channel conditions worsen. Obviously, the encoding parameters must be tuned to adapt the video source transmission rate to the available bandwidth. This adaptation is achieved by means of appropriate feedback laws, which are relationships between the encoding parameters to be used and other variables representing the state of the system. An analytical framework is introduced which can be used for the design of the feedback laws. To this purpose both the channel and the video source are modeled by means of Markov models. The resulting model of the whole system is denoted as SBBP/SBBP/1/K. Analysis is derived which allows to evaluate the most significant performance measures and, therefore, to design optimal feedback laws.