An efficient networking technique for synchronous e-learning platforms in corporate environments Juan C. Granda * , Daniel F. García, Pelayo Nuño, Francisco J. Suárez Department of Computer Science, University of Oviedo, Campus de Viesques s/n, 33204 Gijón, Spain article info Article history: Available online 24 February 2010 Keywords: Synchronous e-learning Multimedia networking Overlay multicast Modeling and measurement Network scalability abstract 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. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction This paper presents a technique for real-time delivery of multi- media 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 syn- chronous 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 sev- eral Internet Service Providers (ISPs) to connect the LANs of the various sites is usually fitted to the daily necessities of the corpo- ration, 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 possi- ble. Maximum efficiency is obtained when the multimedia infor- mation 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 mul- ticast service during e-learning activities between the sites where participants are located. However, if the sites are connected by dif- ferent ISPs, enabling and later disabling the multicast service is cumbersome and expensive. In order to satisfy the training requirements of large corpora- tions, we have developed a synchronous e-learning tool [1]. It pro- vides 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 col- laborative way from any one participant to the rest. For example, the instructor may use his telepointer to point to a relevant ele- ment 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 inter- actions between participants, so the Real-time Transport Protocol (RTP) is used to transport all multimedia data. Separate RTP ses- sions 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. 0140-3664/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2010.02.020 * Corresponding author. Address: Office 2.5.6, Edif. Polivalente, Campus de Viesques s/n, 33204 Gijón, Spain. Tel.: +34 985 18 26 38; fax: +34 985 18 19 86. E-mail addresses: jcgranda@uniovi.es (J.C. Granda), dfgarcia@uniovi.es (D.F. García), uo141061@uniovi.es (P. Nuño), fjsuarez@uniovi.es (F.J. Suárez). Computer Communications 33 (2010) 1752–1766 Contents lists available at ScienceDirect Computer Communications journal homepage: www.elsevier.com/locate/comcom