Multi-Format Web Content Transcoding for Mobile Devices
J. Gabriel González S., Víctor J. Sosa S., Azucena Montes R., y J. Carlos Olivares R.Centro Nacional de Investigación y Desarrollo Tecnológico (cenidet)
The objective of this work is to allow users of mobile devices to visualize Web sites without mattering if Web sites were designed or not specifically for these types of devices; in that way, it is necessary to adapt Web contents to the limitations of each mobile device. In this paper, Web content transcoding mechanism “on-the-flight” is presented. This mechanism works by means of a simple inclusion of a HTTP header named X-Transform, which describes a transcoding format command .This header is generated by a client-side Proxy named GAP, which runs on mobile devices with Windows CE operating system (Pocket PC, Smartphone, among others). The transcoding header is interpreted by a server-side Proxy named GAT MT, that converts Web documents to WML, XHTML-MP, PDF, plain text, PostScript, XML and HTML adapted for mobile devices, making use of XSL technologies (XSLT and XSL-FO).
Keywords: Transcoding, Mobile Devices, Proxy, Windows CE, Pocket PC, Smartphone, XML, XSL, XSLT, XSL-FO, WML, XHTML-MP.
1. Introduction
In a globalize-world where the information is extremely important, accessing the information has become crucial. One of the mass media that has been developed more is the Internet (more specific the Web). In order to access to it, we need some computer devices.
At the moment, computer devices are more and more small, which have brought with themselves new paradigms like mobility and ubiquity of information.
One main issue that presents mobile devices is the lack of Web content specifically designed for thesetypes of devices (most of the content is designed for computer equipment with big screens, more colors, more power processing, etc.). As a consequence, users can only visualize a very small part of the Web and
making matters worse, this visualization does not occur in good terms because the navigability presents too much interaction for users.
In order to try to guarantee that all Web resources are available on mobile devices, we need some mechanism that turns these resources to others, which can be visualized in the best possible way in these devices. This mechanism is called transcoding.
Another mobile-device limitation is that the browsers for mobile devices accept a very limited rank of formats. Particularly, the mechanism described in this work turns resources that are in HTML to WML, XHTML-MP, PDF, PostScript, XML, plain text and a HTML reformatted version.
2. Transcoding
Transcoding is the process to turn a document from a format to another one [1]. In order to make the transformation the following technologies were used:
XSL (eXtensible Stylesheet Language) allows separating information of presentation. For example, in HTML (is not XML-based), the same document shows the data and the form like these data must bedisplayed. With XSL the presentation is structured in two forms: as the data must visualized: CSS (Cascade StyleSheet) and as the data must turn out to be: XSLT.
XSLT (eXtensible Stylesheet Language Transformation) allows transforming a document written in XML to another in XML. It does the data portable, allowing for example, that a XML file that represents a database scheme can be converted to HTML for making reports of the data contained in the document [2]
XSL-FO (eXtensible Stylesheet Language-Formmating Objects) allows making transformations to documents that are not XML-based (PDF, PostScript, plain text, etc.).
The idea behind Web content transcoding is a very old idea and it has been addressed in different ways. Generally, transcoding has been applied to the transformation of multimedia contents (audio and video). That is the case of video television transcoding: PAL to NTSC, or in the case of audio: WAV to MP3.
Few transcoders “on-the-flight” exist that are multiplatform. These are the case of proxies like Google wmlproxy that converts documents to WML. In other cases, there exist transformers online where users access to a Web site, introduce the URL resource that wish to transform and the system gives back the transformed page, that is the case of skweezer [3].
Some works have developed schemes that allow Web content transcoding “out-of-line”. For example, user download documents and when the documents are needed in another format, transformers like jTranscoder [4] are executed.
The current microbrowsers on mobile devices (e.g. Pocket Internet Explorer, Mozilla Minimo, Opera Mobile) make a basic type of transcoding; for example, they adapt documents to a single column to avoid the famous scrolling problem on mobile devices.
Another form to solve this problem is by having sites specially designed for a specific platform that is the case of the Google finder that has search interfaces for each platform and format (e.g. page for PDA, Cellular, iMode) or by having sites designed exclusively for this platform. This last idea entails to invest in administration expenses of a new site.
At the moment, some existing technologies offer that the same information on the Web can be visualized on different forms depending of the device, that is the case of ASP.NET and Java struts. In ASP.NET, a site is designed with MobileWebForms, when a device accesses to the site, the Web server detects the type of device and generates a new page that can be visualized in that device according to its displaying characteristics. Struts are based on MVC (Model-View-Controller) [5] architectonic design, which basically separates the program logic (model), the form in which the events and the user interaction with the system are managed (controller), and finally the form in which are the data (view). When we do independent the interface of the data and events, this interface can be adaptable to other devices maintaining intact the controller and the model.
With the UAProf (User Agent Profile) specification and CC/PP (Composite Capabilities/ Preferred Profile) are possible to send through a HTTP header request and /or an auxiliary RDF file, the information of the device profile, these allow to personalize and give back
better resources. Unfortunately, it does not present an option that indicates the type of format that is desired to turn the document (remember when access to a URL, the type of resource is indicated on implicit way; for example, http://www.cenidet.edu.mx/seleccion.htmlindicates that the resource is HTML).
At cenidet we have worked with XML technologies, concretely for the data formatting of databases to the Web [6], as well as transformation of Web contents. That is the case of [1], this one only makes the reformat document Web transcoding, reason why this work has been extended to accept diverse formats and easy to add more formats.
Reviewing specialized Literature we have not found a mechanism or tool that makes the work in the form which we are making it [7], [8], [9], [10], [11] y [12].
4. Methodology of solution
In order to solve this problem a client-side Proxy and a server-side Proxy services were implemented. The client-side Proxy is called GAP (Hoarding Manager for Pocket PCs) and the server-side Proxy is called GAT (Hoarding and Transcoding Manager) as it is in Figure 1.
The GAT is divided mainly in two mechanisms: MA (Hoarding Mechanism) and MT (Transcoding Mechanism). GAT MT is described in this paper.
GAT MT is developed in Java using the following APIs: xalan-j and xerces for the transformation; JAX and DOM for the XML document path; FOP, Avalon and Batik for the transformation of objects among others. In this tool, XML and XSLT technologies are used for documents transcoding; on the other hand, JTidy is used to generate a XHTML document from a HTML source code.
Figure 1.System architecture.
The GAP, on the other hand, manage the disconnection events that could be presented in the mobile devices, reason why it hoards resources into the local cache of the devices. Also, it generates and sends
the corresponding header to the GAT (see Table 1) for content transcoding, which we have named X-Transform (remember that specific HTTP headers that have not been standardized begin with an X capital letter).
Table 1.Example of a HTTP header request generated by GAP.
GET http://www.cenidet.edu.mx/ HTTP/1.0 Accept: */* UA-OS: Windows CE (Pocket PC) –Version 3.0UA-Color: Color16UA-Pixeles: 240x320UA-CPU: ARM SA1110UA-Voice: FalseUA-Language: Mozilla/2.0 Accept-Encoding: gzip, deflate User-Agent: Mozilla/2.0 (Compatible; MSIE 3.02; Windows CE; PPC; 240x320) Host: www.itmorelia.edu.mxProxy-Connection: Keep-Alive X-Transform: XHTML-MP; Complete
GAT MT interprets the format that contains the X-Transform header (if any, otherwise it assumes HTMLas the predetermined format) and if the device is a mobile device such as Windows CE (Pocket PC, Smartphone), Palm OS (Palm, Treo, Clie) and Epoc (Cellular with Symbian OS) the page is transformed.
The transcoding mechanism is very similar to the MVC architecture, Figure 2 shows the architecture. The model corresponds to the Web resource transformed to a XML file, the controller is the GAT MT that handles the form into which is going to be transform the document and finally, the view is represented by the document in all of the availableformats.
Web Resource GAT MT
HTML
XHTML-MP
WML
PDF
MODEL CONTROLLER
VIEW
XML
PostScript
Plain Text
Figure 2. MVC applied to the system.
The transformation process is the following one: The system firstly detects if the resource is in its cache
of transformed documents and if so, it gives back the document immediately (this avoids the system to do the process several times reason why the response time is improved). In case if the cache doesn’t contain the document, the system obtains it from the Internet, if the document is HTML; the JTidy tool denominated is used to transform it into a XHTML document. Base on this file, the system adds an especially header taking this XHTML code to transform it into a XML document. Having the XML document, the processing can be applied to it for reformatting, which consists of distilling all those characteristics that are not available on mobile devices (tables, frames, forms, etc.). Later, it groups elements with the same type (text, images and links).
Once having the distilled document, the system generates the corresponding style sheet to the wished format. In Table 2, there is an example of a style sheet in XSL-FO.
Table 2.Example of a style sheet in XSL-FO. <?xml version='1.0' encoding='iso-8859-1'?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:fo="http://www.w3.org/1999/XSL/Format"> <xsl:output method="xml" indent="yes"/> <xsl:template match="html"> <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format"> <fo:layout-master-set>…. <fo:block font-family="Helvetica" font-size="18pt" text-align="end" space-before="2cm"> GAT MT X-transform Convertidor de recursos Web a PDF </fo:block> <fo:block font-weight="bold" color='blue'> Prueba del transcodificador</fo:block> <fo:block font-size="12pt"> <xsl:value-of select="body" /><xsl:text> </xsl:text> <fo:block font-size="12pt" color='red'> * Imágenes *</fo:block> <fo:block><fo:external-graphic src='cenidet.jpg' height='1.4cm' width='2.0cm' scaling='uniform' /></fo:block> <fo:block font-size="12pt" color='red'> * Enlaces *<fo:basic-link external-destination='http://www.cenidet.edu.mx/~wm-serna‘ color='blue'>http://www.cenidet.edu.mx/~wm-serna/ </fo:basic-link> …</xsl:template></xsl:stylesheet>
Once generated the style sheet, the system proceedswith the transformation process using the xalan-j API
if the formats are XHTML-MP, WML or reformatted HTML and the FOP API if the formats are PDF, PostScript, XML or plain text. Once generated the resource, the cache mechanism gives back the resource to the client updating its cache list. In case of error, a page with the respective error is showed to the user.
It is very important indicates the resource type in the answer so the Web browsers are able to interpret the resource correctly. This can be appreciated in Table 3.
Table 3. Answer generated by the GAP. HTTP/1.0 200 Ok Server: cenidet GAP Content-Type: text/wmlContent-Length: 100 …<wml> … (content) </wml>
The conversion schemes are next in Figure 3 and Figure 4.
Figure 3.Activity diagram of the transcoding general process.
5. Results
The tests were made with the following equipment: a Pocket PC HP iPAQ rx3115 with Windows Mobile 2003, a Smartphone Motorola MPX 220 with Windows Mobile 2003. For the traditional devices
where GAT MT was executed, an HP Pavilion Entertainment Notebook PC dv1025/a (Intel Centrino to 1,7 Ghz. with 512 Mb. RAM) was used.
The test consisted of 100 sites visualized on all the formats (the Web address corpus was obtained of interviews and preferences of the users). The pages size average was 30,476.81 bytes (29.76 Kb) and 56.99 objects embedded in the main page.
Figure 4.Activity diagram of the transformation process.
Only 29% of the proven sites could be transformed in correct way, this happened because most of the documents are not possible to be transformed correctly to XML; that is to say, the immense majority of Web sites are badly formed and structured.
In Figure 5, ¡Error! No se encuentra el origen de la referencia. and Figure 6, show the transcoding results of the Web resources.
The transcoding results obtained are in Table 4. The times are given taking into account since the GAT MT receives the resource request, obtains it, transforms it and gives it back to the GAP.
Table 4.Times obtained for the transcoding process.Format Time in ms
PS 3,782.27 PDF 2,421.75 HTML 2,312.13 XML 1,794.17 TXT 1,767.06 XHTML-MP 1,574.34 WML 1,563.51
The transcoding average time was 2,173.60 ms, whereas the average without transcoding was 1.589,53 ms; the transcoding process was 36.74% slower.
In the transformation process, we could observe that it is proportional to the size of the main page
(that’s means, a big document produces a high time for the resource downloading and the processing of the document). Also we observed that between more elaborated is the style sheet more is the time of the transcoding process. In general the use of XSL-FO (PDF and PostScript) is slower than XSLT, this due to the visualization format complexity (PDF and PS require greater amount of elements to display information), in view of the fact that the elements extraction time, download time, etc. are similar.
Figure 7 shows the transcoding process times in each format.
Figure 5.Web resource transcoding using XSLT (reformatted HTML, WML and XHTML-MP
respectively).
6. Conclusions
With the present work, we determined that it is possible to transform Web documents (HTML) “on-the-flight” to other formats like WML, XHTML-MP, PDF, XML, PostScript, plain text and reformatted HTML for mobile devices; obtaining acceptable response times. With the work presented here, we have developed a tool that allows Web content transcoding “on-the-flight” into different formats (the user could extend a new transformation format only specifying the
code of the new style sheet) that can easily be adaptable to other presentation formats for mobile devices.
Figure 6.Web resource transcoding in PostScript format visualized in a desktop PC.
This work limits the transcoding of Web resources in its structural elements; that is to say, in its code. At the moment, it does not make image transcoding nor other structural elements transcoding (frames, tables, forms, audio and video). We are tried to make improvements to the system, so and as it can be seen in future works section. When making the Web resources transcoding, it is not needed to have different Web sites for each platform. This would save costs of maintenance when having a single Web site for infinity platforms. Reason why, we would be avoided the fragmentation of the Web and we would be in view of obtained a “One Web” that the W3C proposes. Also with this, we would be obtained the device independent visualization of the Web like the W3C also proposes.
Also it reduces the Web servers workload as a result of having a single site for platforms. It would be increased the market quota and the hearing of a Web site since as the pages can be visualized in different platforms. The bandwidth consumed by the servers is minor.
Other advantages obtained are on indirect way. When we do documents transcoding, the resources areadapted to the characteristics of the mobile devices, with this, we obtained less interaction with the user. When having a smaller interaction, we obtain significant savings in the batteries of the devices, time connection, since the user requires less time to move by the document. Also it agile the Web resource access if the resources transformed are in the device cache or in the transcoding Proxy cache.
The disadvantages that present this work are that the conversion is a process on line that depends on the size of the resource that is desired to transform, reason why to more size of the resource, more delay time for the user. This brings like consequence that the user is waiting long time hoping an answer without doing nothing, that consuming time connection and spend money.
It has not been taken into account the Web page accessibility problem on mobile devices. The main problem that we found was that Web pages internally are badly designed and the validation or conversion tools cannot solve these problems; for example, most of the Web browsers do not support some HTML tags and simply they ignore them. Another problem is that the users can write a Web page without order, which has given like result a problem that is called “HTML street”. It is of all known, that it is extremely difficult to do operations on languages that are not well structured. Reason why to make validation tools is extremely difficult. This problematic is detailed in [14].
7. Further work and another applications
This work has many areas of opportunity to grow. For example, in WML is not possible visualize images in the traditional image formats (jpeg, gif, etc.), these must be in a WBMP (Wireless BitMaP) format. Actually we are reviewing which changes can be made to transform these images.
On the other hand, in PDF files, the image paths are relatives in most of the cases, reason why at the time of generating the PDF document these cannot be visualized. In this part, it would be needed to create a parser that brings the resources that are inherent documents concretely the images, in this sense the process would become slower and harder to implement.
Figure 7. Comparative graph of the transcoding times.
Another improvement related to images would be to compress them, changing the size and the resolution so that they are less space. This is something difficult to implement but it would increase the navigation performance (perhaps it would make it slower). We must to remember that images transference represents 52% of the Internet load.
Another activity that is possible of implementing is the form to separate the style sheet in the code, this way to make the document conversion more extensile (also, we would have to modify the form to accede to the style sheet). The advantage to make this is that the style sheet from code wouldn’t be directly modified, every time those changes are reflected. This activity is highly recommendable to do it.
Another posible activity is to generate a single intermediate file, a generic kind of metamodel to avoid the great amount of repeated resources due to the diversity of existing caches. The problem would be again to generate the document, perhaps it would save the problem of getting the document and its intermediate reformats, but every time the documentwould have to become generated losing therefore the original concept of the system cache in the GAT.
A substantial improvement would be to try to read any Web document and transform it into another one. This problem is n-complexity because it is needed to create tools that read and transcodings in several formats. This is possible because transformation is made through XSL, which allows it to transform any XML document into any other document XML-based.
Another important modification consists of taking into account the devices restrictions and users preferences to make the transcoding since the scheme here proposed is made thinking about Pocket PC devices. Other future applications of this work consist of changing the mechanism or interaction model of the Web to an asynchronous non-interactive mechanism. One made the document transcoding, if the document format is text-based (plain text, XML) we would to send it through a SMS message. If the document is based on HTML or XHTML-MP, we would to send a special version with images, audio and video through MMS Message. For this reason, we need an asynchronous communication scheme, in where the user makes a transcoding request, the user becomes disconnected, the intermediary (Proxy) transforms the page into background and once finalized the process it sends in a the transformed page to the user. This scheme is a Push technology instead of classical Pull technology available on the Web.
Another possible improvement area would consist of the inverse process to databases report. The information obtained through transcoding process
could be stored in XML metamodel , this information is independent of the marked language. With this information we reconstruct the resource in the future. For example, if HTML or any marked language becomes obsolete and is replaced by another one, wewould have the content of the page and could transform later to other formats.
References
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Acknowledgements
I want to be thankful to the DGEST by the support granted with the extension of scholarship for Masters studies (SEP), scholarship holder: Juan Carlos Olivares Rojas.
