Elsevier

Computer Communications

Volume 33, Issue 14, 1 September 2010, Pages 1752-1766
Computer Communications

An efficient networking technique for synchronous e-learning platforms in corporate environments

https://doi.org/10.1016/j.comcom.2010.02.020Get rights and content

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.

Introduction

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.

Section snippets

Technological background

Delivery of audio and video data from instructor to learners, as well as the interchange of multimedia data, such as instant messages and shared whiteboard information, are common functions in synchronous e-learning activities. Audio and video streams stand out as being the most bandwidth-consuming data, imposing the most restrictive requirements on the underlying network.

The audio stream from the instructor carries his speech, while the video stream is useful to reinforce the learners’ sense

Analysis of related work

Before addressing the development of a new technique, a critical review of related work must be presented. A survey of the possible techniques for the implementation of group communication services was developed by El-Sayed et al. [4]. They grouped the techniques in several classes, although two main classes are usually considered for developing real-time group communication solutions: unicast/multicast reflectors and automatic overlay multicast (also called application layer multicast).

Architectural design of the networking platform

When designing a platform architecture for synchronous e-learning applications, several requirements must be met. First, the total amount of consumed network bandwidth must be as low as possible in order to reach a high number of participants and to share network resources with other types of traffic fairly. Second, the end-to-end latency introduced by data delivery should be low enough to permit real-time interactions between learners and the instructor. Finally, the platform must be resilient

RTP relay

The kernel of the proposed architectural design is an RTP relay, which is analyzed in this section. The RTP relay is a server that replicates incoming RTP data to all clients registered with the relay using both unicast and IP multicast delivery.

The main reason for the use of an RTP relay rather than a lower level UDP reflector is the set of rules that RTCP provides for maintaining membership in an RTP session. These rules can also be used for keeping track of the clients registered with the

Theoretical model of traffic in the proposed architecture

It is very common that participants in a synchronous e-learning activity interchange multiple types of data such as audio, video, annotations on a shared whiteboard, etc. Every type of data is delivered using its own RTP session. The streams sent by one participant must be delivered to the rest to enable collaboration. If participants send a predefined number of streams to the RTP session, it is possible to predict the RTP traffic that traverses each relay in the platform, regardless of the

Evaluation

Most of the previously analyzed ALMs [17], [19], [20], [21], [25], [24], [28] make use of simulations to test the efficiency of the management of the overlay network. This requires the utilization of specific metrics to evaluate the multicast distribution tree. The most commonly used metrics are defined in Ref. [17]. The stress of a physical link refers to the number of identical copies of a packet carried by a physical link. The Relative Delay Penalty (RDP) refers to the ratio of the delay

Conclusions

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

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