Layouts globales

The target deployment context for DPEA is a university campus where downloaded learning content ultimately becomes distributed across various nodes within the campus network as depicted in Figure 4-1.

Figure 4-1: DPEA University Campus Setting

The DPEA architecture includes two major novel components: (a) a Performance Oriented

Adaptation Agent (POAA), (b) a DASH-based performance oriented Adaptive Video distribution solution (DAV). A high-level illustration of the DPEA architecture is presented in

Figure 4-2. The proposed solution does not modify content at remote servers and hence can potentially be used in conjunction with any remote host storing MPEG-DASH content.

Campus Network Campus Gateway Remote Server Personalised (Learning) System Viewing Device DASH Player DAV Gateway Local Host/ Viewing Device POAA DAV Client

Figure 4-2: High-level DPEA Architecture

4.1.1 POAA

The idea behind the POAA is to select for each Learning Object (LO) request the best performing network path to a server hosting the requested content, based on each server’s past performance. This chapter introduces two flavours of the Performance Oriented Adaptation

Learning (oPL) systems such as Open Adaptive Educational Hypermedia Systems (OAEHS) [202], [205] (see Section 3.4). The standalone oPOAA module directly communicates with the associated oPL system to enhance the content selection process. It deals with a variety of media types (e.g. text, images, animations, audio, and video) transported over UDP. It adds network performance aware adaptation to existing adaptive PL systems dealing with open corpus content residing on remote servers/repositories. The oPOAA architecture and underlying algorithms are described in Section 4.2.

As technology-enhanced learning systems matured, and with the growth in the number and types of inexpensive viewing devices and high throughput networks, educational video content has become the medium of choice for online systems. Furthermore, the MPEG-DASH [12] standard for HTTP-based video transmission supports bitrate adaptation at the client side. Therefore, given the shift towards educational video content and the recent MPEG-DASH standardisation, a DASH-aware POAA (dPOAA) [17], [18] is proposed. It focuses exclusively on TCP-transported video content for personalised systems where it performs intelligent selection across remote servers storing identical MPEG-DASH content. MPEG-DASH content is delivered as a sequence of video content segments (see Section 2.4.2) and is described by an MPD file (see Section 2.4.4) containing all required information for video playout, such as location and bitrate for each video segment. MPD files are downloaded by DASH players (see Section 2.4.5). Video segments (see Section A.2.1) may reside on multiple servers, in this case the MPD file contains multiple location URLs and a DASH player can choose between them. While the standard allows specification of multiple URLs, it does not dictate client-side selection algorithms. The dPOAA solution along with its selection algorithms are presented in Section 4.3. dPOAA calculates remote server ratings based on historical server performance information (e.g. throughput) and can be deployed as a Server Reputation Generator (see Section 4.4.2.2) to aid remote host selection during the DAV MPD creation process as described in Section 4.4.2.4. In the proposed setting, dPOAA is based on the campus gateway as indicated in Figure 4-1 and its output is used in the MPD creation process. However, dPOAA could also be used as a DASH player plugin independently of the DPEA architecture. In this case, dPOAA is deployed at the DASH player as described in Section 4.3.5.

4.1.2 DAV

The DASH-based performance oriented Adaptive Video distribution solution (DAV) considers user device characteristics and user profiles as well as the content already available locally to improve the content delivery process thereby increasing the overall viewing experience. While dPOAA aids remote server selection, DAV introduces access to locally stored content. DAV consists of two components: a DAV Gateway based on the campus gateway, and a DAV Client installed on high performing nodes within the campus network as indicated in Figure 4-1. The DAV Gateway dynamically constructs MPD files that contain URLs that point to both local and

remote hosts. The DAV Client component informs the DAV Gateway about its availability and about stored video segments. Each DAV Client is equipped with a simple, standard web server that serves cached MPEG-DASH content to other local nodes. Crucially, even devices without a DAV Client installed benefit from the DAV solution, as once provided with modified MPD files, they can request content both from remote servers and local hosts (nodes with DAV Client installed). DAV is presented in Section 4.4.

4.1.3 Solution Summary

Factors affecting playback under MPEG-DASH are indicated at the beginning of this chapter. A subset of these factors is considered in the DPEA solution and these factors are listed in Table 4-1. The emphasis was placed on the important factors that are either already available (e.g. user preference and user class are provided by the associated open PL system) or can be unobtrusively collected (e.g. throughput and RTT are measured by the DAV Gateway, device characteristics are determined based on the HTTP User-Agent header).

Factor Unit Source Consumer

(a) User-related Personal User preference and user class

N/A User Profiler (External PL System’s User Model)

MPD Builder (b) Network Performance

Connection throughput bits/second DAV Gateway oPOAA

dPOAA PL System

Delay (RTT) milliseconds DAV Gateway

oPOAA

dPOAA PL System (c) Viewing Device Hardware

Device Type Device Class Device Profiler MPD Builder

Connection characteristics wired/wireless Viewing Device MPD Builder

Screen resolution ranges Viewing Device DASH Player

Request rate number DAV Client (Heartbeat) DAV Gateway

Availability Boolean DAV Client (Heartbeat) DAV Gateway

(d) Viewing Device Software

Buffer content seconds DASH Player DASH Player

(e) Content

Segment bitrate bps Server (original MPD) MPD Builder

Segment length seconds Server (original MPD) MPD Builder

Segment ID segment list Server (original MPD) MPD Builder Content available locally segment list DAV Client Local Content

Elicitor

Table 4-2 summarises the link between proposed solutions and the service each provides.

Solution Service provided Utilised by Introduced in

oPOAA Server Rating oPL System Section 4.2.3

dPOAA Server Rating PL System, DPEA Section 4.3.3

DAV Gateway Host Selection MPD Builder Section 4.4.3.1

DAV Gateway MPD Building DAV and DASH Clients Section 4.4.3.2 DAV Client and Gateway Heartbeat Updates Local Content Elicitor Section 4.4.3.3

Table 4-2: DPEA Component, Mechanism, Consumer and Reference