To Beam or to Stream:
Satellite-based vs. Streaming-based Infrastructure for Distance Education

Saraswathi Krithivasan, Sridhar Iyer
KR School of Information Technology
Indian Institute of Technology, Bombay
{saras, sri}@it.iitb.ac.in

Abstract

Reaching participants across geographically dispersed locations is the primary goal of any distance education program. To disseminate multimedia content, in addition to the wide reach, ensuring good quality of video and audio reception at the receiving stations is critical. Satellite based solutions offer wide reach as well as scalability with a recurring cost component which can be subsidized across a number of participating centers. Streaming live or recorded lectures over the Internet is another option to provide reach and flexibility to participants. This paper examines these two technology options for effective dissemination of multimedia content to end-users. The implemented Distance Education Program (DEP) network at Indian Institute of Technology, Bombay (IITB) is taken as an example to study the various aspects of the satellite network as well as the viability of a parallel streaming solution over the Internet. The two technologies are critically compared across three dimensions – providing the QoS (Quality of Service) requirements, scalability, and cost effectiveness.

Section 1: Introduction
Bringing quality education to a wide audience dispersed across a geographical area is the main motivation for any distance learning initiative. Indian Institute of Technology Bombay (IITB), has successfully deployed a satellite-based model of Distance Education Program (DEP) [1] from 2002. This is currently being augmented with a streaming media model over the Internet. The DEP is designed to:

•	Provide rich multimedia content – typically the classroom lecture is captured with appropriate slides and notes written on the white board.
•	Transmit lectures synchronously according to a predetermined schedule, to emulate a live classroom environment.
•	Ensure scalability in terms of effective utilization of the resources to serve multiple end-users and enable them to participate with minimal cost.

In addition, DEP also has requirements such as live interaction between the participants and the faculty during the lecture, and flexibility in terms of accessibility to the lectures.

To achieve these goals, the following general framework is used:
1.	All the lectures originate from a central site.
2.	The lectures are received by a set of geographically dispersed end points, synchronously.
3.	The program is managed (processes such as registration, local support etc.) in a distributed manner.

Given the above framework, it is important to ensure that the lectures are received with acceptable quality and without interruption. Quality of Service requirements of a multimedia application can be categorized broadly along three dimensions: bandwidth, data loss, and timing. Of these, bandwidth and timing play critical roles for the application to be effective. It can be argued that for a widely deployed multimedia application such as distance education, the following parameters are critical:
1.	Providing the QoS requirements
2.	Scalability 
3.	Cost effectiveness
In this paper, we critically examine the suitability of two technologies:  (1) based on a satellite network and  (2) based on streaming over the Internet. While the satellite network is functional at IITB, with nine Remote Centers (RCs), the Internet streaming experiment is being piloted in the Spring 2004 semester. Section 2 examines the implemented satellite network and its aspects. Section 3 discusses issues related to streaming over the existing Internet and the model for our deployment. In Section 4, we compare the two technologies across the three dimensions. Section 5 provides the summary and conclusions.

Section 2: A satellite based synchronous model for interactive distance education

Leveraging the wide reach of a satellite network, education can be brought to remote locations in a cost effective way. A Very Small Aperture Terminal (VSAT) is a device used to receive satellite transmissions. VSATs are low cost micro stations having small (0.55-1.2 m) antenna, capable of a power output of about 1 watt. Typically, the VSATs do not have enough power to communicate directly with each other. A “hub”, which is a special ground station with a high gain antenna, is used to relay traffic between the VSATs [6]. 

In the DEP model [8] [9], lectures are transmitted from a central location to multiple RCs simultaneously according to a pre-defined schedule. The synchronous transmission provides participants the feel of a classroom, even though some flexibility is lost due to participants traveling to the RCs.

The model provides the added benefit of live interaction between the participants and faculty. The mechanism to provide interaction is briefly outlined below:
•	Lectures from the central site are synchronously transmitted to the various RCs.
•	A typical classroom at each RC has thirty to forty students viewing the lectures, which are projected on to a large screen. 
•	Any participant from any of the RCs has the freedom to ask a question during the lecture. The desire to ask a question is communicated to the faculty through a video-conferencing software.
•	The faculty may grant the floor to the RC, in which case the question being asked is heard by the faculty as well as the participants at all the other RCs. Subsequently the floor is taken back by the faculty, the question is answered and the lecture continues.

The network architecture for such the DEP model is illustrated below. Figure 1 provides the architecture at the Central site whereas Figure 2 illustrates the architecture at the RC.

Figure 1: Network architecture at the central site

Figure 2: Network architecture at the Remote Center
The network uses VSAT technology in a novel manner, having two channels with different access mechanisms [4], to achieve interaction between the participants and faculty without incurring additional bandwidth costs. A fixed bandwidth of 512 kbps is used for data transfer, through the use of a DAMA channel. A separate 16 kbps channel is used for the control messages. These control messages, flow through the central hub, using a FTDMA access scheme to poll the RCs.
The central site initiates an interactive multicast [3] session, which is joined by one or more RCs. As the session initiator, the central site becomes the MCU (Master Control Unit) or the chair for the session, having complete control over the multicast session. At connection establishment, the dedicated 512 kbps is assigned to the central site, which transmits the lectures to the RCs in a live, synchronous mode. When a RC registers a request for floor, if the MCU decides to grant the floor (reallocate the bandwidth to the requesting RC), it generates a control message. This message is then communicated to the hub, where the reallocation is effected. At this point, the RC becomes the transmitting station, which uses the 512 kbps channel to communicate to all the other centers, including the central site. 
The novelty of the solution is that the architecture allows for interaction, which provides the participants the feel of an actual classroom without the need for multiple high bandwidth channels. As the dedicated 512 kbps channel is reassigned to the centers for live interaction based on the MCU’s recommendation, scalability is achieved. Since the transmission at any point in time is from a single point to multipoint, the bandwidth requirement remains constant, irrespective of the number of participating RCs. Thus, this model allows expansion to a large number of RCs without additional bandwidth costs, resulting in a cost effective solution.
In summary, the satellite based DEP model exhibits the following features:
•	By using dedicated bandwidth of 512 kbps, the model provides QoS guarantees. Even though the satellite imposes a minimum end-to-end delay of 250 ms, the delay is constant and the quality of reception is not jeopardized. 
•	As the satellite supports multicast, scalability is achieved, allowing for addition of centers without incurring additional bandwidth costs.
•	The cost of the 512 kbps channel can be amortized over the number of RCs. However, there is an additional cost incurred at each RC for the receiving equipment.
Section 3: Distance Education using media streaming over the Internet
Streaming generally refers to the delivery of time-based media where media frames have to be played in order and in real time, without having to download the entire file. Considering that the Internet provides several advantages including geographical reach and flexibility, it provides an important option for streaming lectures in the context of distance education. While it is possible to provide on demand streaming [7], which allows flexibility of timing to the participants, this option is not considered here, since synchronous transmission is a requirement with the defined DEP framework. 
In DEP, lectures are streamed according to a predetermined schedule and authorized end-users can join the session from any convenient computer. The steps involved in streaming the multi media files are briefly outlined below [5]:
•	The live media captured using a video camera is passed through an encoder and a packetizer before it is streamed with the use of a server. The live session can also be captured on appropriate storage medium for future use. 
•	When a user starts to play streamed media by specifying the source URL, the media plug-in sends a request to the server. The server responds by sending the multimedia content to the client computer. 
•	A client joins a session either by a unicast or a multicast mode, by making use of the predetermined broadcast schedule. 
•	At the client end, typically a media player client provides the functions of reassembly and decoding and displays the media file for the user. 
Figure 3 illustrates a simple model for streaming stored media files over the Internet.

Figure 3: Architecture for streaming over the Internet
If unicast streams are generated to serve each user, the bandwidth requirement from the streaming server to the end-user increases proportionally with the number of end-users, posing constraints on scalability and cost effectiveness. In many cases, an Internet Service Provider (ISP) with widely deployed resources can best handle streaming to multiple clients. While the multi media content is generated at the source site, the streaming server is typically located at the ISP’s premises. The ISP provides routing services and user management services. By leveraging the multicasting services at the ISP, the problems with bandwidth and delay can be overcome.  
In summary, the streaming based model provides flexibility to the end-user, with minimal infrastructure requirement. However, we note that: 
•	Quality of service guarantees are possible only if the ISP guarantees minimum bandwidth and bounded delay per end-user.
•	Scalability can be achieved provided the ISP supports multicast streaming.
•	This model can be cost effective, provided the QoS guarantees and scalability are achieved.  
Section 4: Comparison of technologies
In this section, the two technology options are compared across the three dimensions, vis-à-vis, providing the QoS requirements, scalability, and cost effectiveness in the context of distance education. 
1. Providing the QoS requirements: The main parameters for assessing QoS are [2]:
 
•	Assured bandwidth – providing a channel for delivering the multimedia data in a timely manner.
•	Bounded loss/error rate – assuring uninterrupted communication by keeping the number of packets lost/corrupted to a tolerable level. 
•	Minimized delay jitter – reducing variability in inter packet arrival delay, to ensure smooth reception.

Satellite systems can provide for dedicated bandwidth. Satellite IP multicasting can simultaneously deliver content to a virtually unlimited number of end-user locations at speeds up to 40 Mbps [10]. Because of the dedicated links in the end-to-end paths, satellite-based dissemination guarantees a worst-case delay of 250 ms and the variability from this worst case is small. Although satellite channels are prone to loss due to attenuation, the loss rates are within the acceptable bounds to assure good quality of reception at the RCs.
On the other hand, Internet based systems are best effort. Presently, end-to-end Internet delay is roughly 200 ms, under low traffic scenario. In the absence of multicast support, to send a stream to hundred recipients over the Internet would require hundred separate and identical connections, each of which will consume bandwidth and server resources. As these streams may traverse different routes, they may arrive at different times, contributing to high loss rates and jitter. 
Thus, serving a large number of end users with acceptable quality is achievable through a satellite-based architecture; the same requires significant support from the ISP for streaming-based architecture. 
2. Scalability
Satellite-based dissemination is scalable because of its multicast nature. Satellite technology naturally accommodates network and bandwidth expansion. Also, in the implemented DEP model, the bandwidth requirements remain constant irrespective of the increase in the number of RCs. 

If multicasting is enabled, internet-based dissemination is also scalable. However, because in today’s Internet, multicasting is not widely deployed, bandwidth requirement for streaming the lectures (using unicast streams) increases proportionally with the number of end users. When the ISP’s national backbone is multicast enabled, serving multiple end users from the ISP’s local Points of Presence (PoPs), which are on the backbone, renders a scalable solution.

3. Cost effectiveness

Deploying a VSAT network is very expensive, as it is infrastructure intensive. For example, the DEP VSAT infrastructure at the central site costs around Rs. 6 million, while the cost for an RC is around Rs. 1.5 million. There is also a recurring annual cost (Rs. 3.5 million in our case), for the dedicated bandwidth. While the bandwidth cost can be amortized across the RCs, given the high costs, recuperating the initial investment can take several years.

In the streaming model, the main costs involved are in the streaming server (around Rs. 0.5 million, in our case) and in the recurring bandwidth (around Rs. 0.8 million for a 2 Mbps leased line). A reasonable quality multimedia stream requires at least 141 kbps. So we can support at most fourteen unicast streams from the server. As the number of end users increases, the bandwidth requirement, and hence the recurring costs, increase proportionally. 

Section 5: Summary and conclusions

While a satellite based solution for multimedia dissemination is expensive to implement, it provides the reach and reliability of service, crucial for distance education. This architecture is suitable for mass deployment with multiple courses. However, cost effectiveness is achieved only with a large number of RCs and a significant number of participants per RC. The differentiating feature of the satellite network architecture is that it supports live interaction. While the model places a constraint on the participant to travel to a RC to attend a lecture, compromising flexibility, the live interaction greatly enhances the effectiveness of the program. 

The Internet allows easy access to the end-user and a less expensive streaming based solution for the central site. Streaming to a large number of end-users on a scheduled basis has not been tested enough to understand the scalability implications and outcomes. In any case it requires the support of the ISPs, for multicast functionality and priority service architectures. Although streaming-based architecture provides flexibility to the end-user, interaction during the lecture is hard and typically needs a parallel mechanism (such as chat, etc.). 

We feel that in order to be most effective, a distance education program could use the satellite based model to reach various cities and enable streaming within each city (with the support of ISP) to provide guaranteed QoS. By leveraging the inherent features of these technologies, the primary goal of distance education -- reaching geographically dispersed participants -- can be truly achieved.

References
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