The Landscape of Pervasive & Mobile Computing Standards Sumi Helal Synthesis Lectures on Mobile and Pervasive Computing Preface

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14.2 Mobile Web Paradigms

14.1 Introduction

Mobile and wireless communication networks are taking the desktop Internet experience beyond office or home networking environments, the differences that used to exist between a wireless and wired Internet are narrowing and getting closer to become one. These differences have been decreasing as telecommunication service providers enable 3G networks, allowing web multimedia content which has been exclusively provided thru WAN pipes is now as pervasive as high-bandwidth content to mobile devices. This experience is enhanced with mobile devices supporting IEEE 802.11 wireless access which is also as pervasive as in coffee shops, museums, restaurants and universities. In the United States, telecommunication carriers such as T-Mobile, AT&T, and Sprint provide 3G plans with speeds of up to 3 Mbps covering wide metropolitan areas. For instance, South Florida is covered from Jupiter to the FL Keys entirely .

As a consequence of wireless bandwidth and multimedia evolution of handsets, the mobile web can no longer be different and, does not have to be different to standard desktop environments. In the past, WAP (Wireless Applications Protocol) standards and WML (Wireless Markup Language) content were the choice; in fact WAP is still popular with many of the mobile carriers ^14.1. However, text-based Internet is continuously becoming an obsolete choice for rich multimedia mobile phones with CPUs, usually ARM-based, with clocks ranging from 100 to 250 MHz and memory footprints of 128 MB.

14.2 Mobile Web Paradigms

It's then expected that mobile content is continuously evolving to a richer experience in which standard HTML pages are delivered and formatted to fit a mobile device screen. Web applications on the other hand are increasingly popular as well as interactive HTML experiences. Currently, there are several mobile browsers providing an experience similar to that of the desktop. These new mobile browsers are being designed using two paradigms:

Sever transcoding, in this approach, a mobile client requesting a URL is sent to a proxy server on the Internet, usually residing at the carrier premises. The proxy server, on behalf of the client, makes the request to the encoded server on the URL. HTML content retrieved could then be converted into a special or proprietary format, which is rendered by the requesting mobile browser. In general, these rendering engines are proprietary, and reside at the server. The mobile browser may also include additional compression to support low-bandwidth scenarios.
Direct delivery: The mobile device renders HTML directly from a web server or a local file. This approach is seen today on desktops and is becoming the preferred choice on mobile browsers. A rendering engine (e.g. Webkit ^14.2) is embedded as part of the device firmware and in many cases these HTML rendering engines are open source. A similar approach has been followed by other browsers such as Pocket Internet Explorer (or Pocket IE) which relies on rendering Compact HTML content. Webkit on the other hand, provides support for standard Javascript, AJAX (Asynchronous Javascript and XML), CSS (Cascading Style Sheets) , and HTML -- in general, what is defined thus far as Web 2.0 ^14.3

Table 14.1 Mobile Browsers Paradigms

Server Transcoding	Direct Delivery
Server in the middle required Rendering done at the server, images or PNGs are delivered to the handset Multimedia is handled at the server May use non-standard Rendering engines A simple thin-client approach can be implemented in several platforms Upgrades done at the server may not require reprovisioning		Direct access to the internet Javascript, CSS, HTML support May require plug-ins for multimedia content Standard rendering engines Model used on Android, iPhones; may require higher computation hardware

In the server transcoding paradigm, mobile browsers such as Opera browser and Skyfire ^14.4, are written for native platforms like Java Micro-Edition (J2ME) and Windows Mobile. These two browsers provide full page rendering and visualization for mobile devices supporting J2ME and Windows Mobile. Their rendering engines use server-based computing power to generate compressed image files that are displayed by the mobile browser. Because these images are preprocessed at the server, content is irrelevant to the mobile browser. Therefore, RTSP (Real-time Streaming Protocol) videos and complex HTML content can be rendered on any handset.

In general, server-transcoding requires the implementation and management of a server infrastructure, and many web applications may not work with this approach due to their reliance to AJAX or secure mechanisms unsuitable for mobile clients. Additionally, concerns over a single point of failure start to become more apparent, especially in scenarios that could potentially limit the amount of requests made from a server, such as web-casted sporting event, or secure mobile banking applications. In either case, scalability and browser lever security need to be addressed.

In a different approach to transcoding, mobile content is delivered directly to the handset, where it is both rendered and processed using local resources. Today, several mobile browsers take advantage of this paradigm through the Webkit engine, including mobile Safari (iPhone and iPod touch) ^14.4 , and more recently Google Android ^14.5. In the following section, HTML rendering engines are briefly explained.

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