روش شبکه سازی کارآمد برای پلت فرم آموزش الکترونیکی همگام در محیط های سازمانی

A synchronous e-learning platform constitutes an effective tool to implement training programs for human resources in large corporations. These organizations are usually widely dispersed in multiple sites, so the delivery of multimedia data over the whole corporation during an e-learning activity is a true challenge. Multimedia data is usually delivered using the RTP protocol. Although an RTP session can involve multiple participants and has no restriction on the underlying network, it is usually restricted to IP multicast scopes. However, IP multicast is rarely available throughout corporate networks. In this paper, an efficient delivery technique based on an overlay multicast network is proposed. The overlay is made up of multiple servers that operate at the RTP level and combine unicast and native multicast transport whenever possible. This technique has proved to be more efficient than a technique based exclusively on unicast delivery.

This paper presents a technique for real-time delivery of multimedia data between several user groups making efficient use of the network bandwidth when IP multicast is not globally available. The motivation for the development of this technique arises from the utilization of multimedia applications to support synchronous e-learning activities in geographically dispersed multi-national corporations. For security reasons, the employees of these corporations must always work within the corporate network. This network usually consists of several Local Area Networks (LANs), one in each site of the corporation, joined by the Internet. The bandwidth contracted by the corporation with one or several Internet Service Providers (ISPs) to connect the LANs of the various sites is usually fitted to the daily necessities of the corporation, without considering the additional load generated by an e-learning activity. Thus, any multimedia application operating with this kind of network must use the available bandwidth as efficiently as possible. Maximum efficiency is obtained when the multimedia information can be transmitted throughout the entire corporate network using IP multicast. However, the delivery of data using IP multicast reaching all the nodes of the network simultaneously is rarely feasible in world-wide corporate networks. The corporation could request its ISP to provide a temporal multicast service during e-learning activities between the sites where participants are located. However, if the sites are connected by different ISPs, enabling and later disabling the multicast service is cumbersome and expensive. In order to satisfy the training requirements of large corporations, we have developed a synchronous e-learning tool [1]. It provides the most common functionalities of this kind of tool, such as videoconference, instant messaging and presence control, shared whiteboard, annotation of slides and telepointers. Depending on the instructor’s wishes, these features can be used with one-way broadcast delivery from the instructor to the learners, or in a collaborative way from any one participant to the rest. For example, the instructor may use his telepointer to point to a relevant element on the shared whiteboard, while a learner may activate his telepointer to inquire about a specific element on the whiteboard. A snapshot of the synchronous e-learning tool can be seen in Fig. 1. All data must be delivered in real-time to enable instant interactions between participants, so the Real-time Transport Protocol (RTP) is used to transport all multimedia data. Separate RTP sessions are used to transmit each type of media, where each session represents an entity which is made up of all the participants. These sessions are not geographically or network scoped. That is, the RTP sessions are independent of the network layer. Ideally, IP multicast is available in the underlying network and every RTP session is associated to an IP multicast group, so participants must join the multicast groups in order to receive all the multimedia data. However, this is not always possible, as IP multicast is not available world-wide. The solution proposed assumes that IP multicast delivery can be easily enabled in the LAN of each site of the corporation. In this case, multiple servers can be deployed as proxies to emulate the behavior of a multicast network between participants located in different sites. Nevertheless, the path of data packets between two participants must traverse as few servers as possible to minimalize latency. Therefore, the proposed platform emulates the behavior of a global multicast network between the LANs using an overlay network tailored to the specific requirements of synchronous e-learning applications. On rare occasions participants who do not have IP multicast connectivity, or who join from outside the corporate network, participate in e-learning activities. Those from outside the corporate network join the network through VPN tunnels. The servers must forward traffic from the RTP sessions to these participants and vice versa. The remainder of this paper is organized as follows. Technological background about multimedia data delivery using RTP is presented in Section 2. In Section 3, related work on existing techniques for group communication is discussed. The proposed architectural design of the networking platform is addressed in Section 4. In Section 5, the design and operation of the RTP relay used in the proposed networking platform are explained. The theoretical model of the traffic supported by the relays in the platform is presented in Section 6. In Section 7, the validation of the theoretical model and the performance evaluation of the platform are presented. Finally, Section 8 contains the concluding remarks and outlines future work.

In this paper an architectural network design to support synchronous e-learning activities in large corporations dispersed in multiple sites is proposed. The design implies an overlay multicast network based on multiple RTP relays, each one located at a specific site of the corporation. This approach supposes that IP multicast connectivity is enabled or can be easily enabled within a site. The solution combines unicast and IP multicast delivery techniques. IP multicast is used within a site to communicate all participants with the relay, and unicast is used to communicate one relay to its peer relays, deployed in a full-mesh topology. The developed RTP relay is an application-level intermediate system that forwards data between several multicast groups and other relays in an RTP session. Therefore, it extends the scope of an RTP session, which is no longer limited to an IP multicast island. The relay uses multicast transport whenever possible, and unicast transport only when IP multicast is not available. Thus, this solution is optimal and the bandwidth consumption minimum, as opposed to solutions based on an unicast star topology. The platform has been tested to estimate its scalability and performance. The results conclude that the platform scales better than a solution based exclusively on unicast connections. The relay mesh can easily support enough participants and a significant number of sites, for e-learning activities. Nevertheless, scalability is related to the dispersion of participants. The more concentrated the participants, the more scalable the platform. However, the relay and the platform developed have some drawbacks. The main drawback of the platform is the lack of an autonomic architectural discovery protocol. Relays must be configured for each e-learning activity ad hoc, that is, all relays must be informed of their peer relays in the mesh. Moreover, if a relay goes down, the whole site which it serves will be disconnected from the rest of the e-learning activity. Future work will be oriented to autonomic deployment of the overlay network based on RTP relays. Thus, network configuration will be completely transparent for applications and users.