Pervasive Client Technologies: WAP and MMS
WAP browser and MMS messaging client are two technology pillars supported by all Nokia Developer Platforms. We cover the basics of those two technologies in this section.
A WAP browser works pretty much the same way as the HTML Web browser on a desktop PC. The user interacts with the remote application server by following dynamic links and submitting forms. The handset renders the content provided by the server. All the application logic is processed on the server side.
Although mobile and desktop browser applications share the same application model, the actual network architecture and markup languages are different. We check out those differences in the next several sections.
While an HTML Web browser can make direct HTTP connections to the server, the WAP browser must go through a gateway server to connect to the general TCP/IP Internet. The WAP infrastructure is illustrated in Figure 2-11. The gateway converts data packets from the wireless network to TCP/IP format and then forwards them onto the wired Internet, and vice versa.
Figure 2-11 The WAP network infrastructure.
From the Web application developer's point of view, however, the gateway is almost completely transparent. All the developer needs to do is set up a normal HTTP server to serve the markup pages and other media objects. HTTP headers, including cookies and authentication credentials, pass through the gateway transparently. The gateway also handles encrypted HTTPS connections automatically.
For developers, the biggest difference between an HTML Web application and a WAP wireless application is the different markup languages. Most mobile browsers support the Wireless Markup Language (WML), and all Nokia Series 40 and 60 devices support the WML specification. A core element in WML is <card>. Unlike HTML, where one page corresponds to one screen, one WML download page can contain a deck of cards denoted by the <card> tag. Each card corresponds to one screen and mobile device, and the user can navigate between cards using internal reference links. The cards help to break long content into several screens without requiring multiple round trips to fetch them one by one from the server. For example, the WML snippet below shows a deck of WML cards in one page, and Figure 2-12 shows how it looks on a device. The <do> tag maps a text label to a soft key. When the user presses on the soft key, the browser navigates to the page or card URL specified in the enclosed <go> tag.
<?xml version='1.0'?> <!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.2//EN" "http://www.wapforum.org/DTD/wml_1.2.xml"> <wml> <card id="Name" title="Enter Name"> <do type="accept" label="SayHello"> <go href="#Hello"/> </do> <p>Please enter your name: <input type="text" name="name"/> </p> </card> <card id="Hello" title="Say Hello"> <p>Hello, $(name)</p> </card> </wml>
Figure 2-12 A deck of WML cards displayed on a cell phone screen.
The XHTML markup language is developed by the W3C to replace HTML. It is HTML-defined as an XML document with cleaner and stricter syntax. Series 60 devices and some Series 40 devices feature dual-mode WAP browsers that support both WML and XHTML. The browser conforms to the XHTML Mobile Profile (MP) specification, which contains a subset of most widely used XHTML tags. A key benefit of the dual-mode browser is that it allows users to access the vast amount of Web content out there on the wired Internet. The XHTML browser also supports WAP cascading style sheets (CSS) for styling.
Details about the WAP infrastructure, applications, markup languages, and Nokia device browsers can be found in Chapter 15, "Browser Applications."
An MMS message is analogous to an email message on the wired Internet. It contains a text body and any number of multimedia file attachments. The MMS client in Nokia Series 40 and 60 devices supports all popular attachment types, including JPEG, GIF, PNG, and MIDI. Some devices support advanced formats such as AMR TrueTone audio and 3GPP mobile video clips. You can send an MMS message to any MMS-enabled phone or ordinary email address. The message is delivered as follows:
- The sender composes a message and sends it to the carrier's Multimedia Messaging Service Center (MMSC).
- The MMSC forwards the message to the recipient carrier's MMSC or email server via the wired Internet.
- The message is delivered to the recipient's phone or email inbox.
As we can see, the MMSC is central to the MMS architecture. We can write applications that connect to the MMSC directly over the wired Internet and send automated messages to a large number of users (see Figure 2-13).
Figure 2-13 The MMS network architecture.
An interactive MMS application functions like an automated email information service. It works as follows:
- The user requests an application action by sending messages to the server.
- The server returns the results via messages delivered to the phone.
- The user then makes a further request by replying to that message.
This process goes on until the user stops replying to the message, thereby ending the session (see Figure 2-14).
Figure 2-14 The MMS application interaction diagram.
In addition to the text and multimedia components, the MMS message can also include a presentation component written in a special XML format called Synchronized Multimedia Integration Language (SMIL), which is also a W3C standard. A SMIL document contains time sequence instructions on how to display the attached multimedia components. The following SMIL example code instructs the MMS client to display image demo.gif and text demo.txt simultaneously on different parts of the screen for four seconds. At the same time, the client should play the demo.midi audio file.
<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE smil PUBLIC "-//W3C//DTD SMIL 2.0//EN" "http://www.w3.org/2001/SMIL20/SMIL20.dtd"> <smil xmlns="http://www.w3.org/2001/SMIL20/Language"> <head> <layout> <root-layout width="320" height="240" title="Demo"/> <region id="Image" width="150" height="60" left="0" top="0"/> <region id="Text" width="150" height="35" left="0" top="70"/> </layout> </head> <body> <par dur="4s"> <img src="demo.gif" region="Image"/> <text src="demo.txt" region="Text"/> <audio src="demo.midi"/> </par> </body> </smil>
Not all devices support the SMIL component in MMS messages. Some earlier Series 40 devices ignore the SMIL attachment altogether but still allow the user to access other attachments in the MMS message. More details of the MMS infrastructure, applications, and SMIL are available in Chapter 14, "Multimedia Messaging Service."
The Thin-Client Application Paradigm
The WAP and MMS applications both run on servers. The handsets merely render the content and capture user interaction. This is commonly known as the thin-client application paradigm. It is a proven success in the Internet-based applications. Key advantages of this thin-client application model include the following:
The clients are pervasively available. WAP browsers are almost universally supported by all device manufacturers and network carriers. The SMS and MMS messaging services are also widely available throughout the world. Several factors contribute to the pervasiveness of those technologies:
- Since the device only handles presentation, it does not require much processing power. WAP browsers and messaging clients can be implemented on small, low-end devices with high sales volumes and long battery lives.
- Since WAP has been around for a long time, most wireless data networks are well equipped to handle WAP traffic reliably. That makes thin-client applications available all over the world.
- WML, XHTML, SMIL, and MIME attachments are standard technologies with a huge installed base worldwide. Most compatibility problems have been worked out over the years.
Thin-client applications and developers are readily available.
- The Web application and email application models are well known to today's Internet developers. They can easily migrate their skills to the new wireless arena.
- A large number of Web applications are available today. It is relatively easy to make changes to their presentation layer so that they generate WML pages instead of HTML pages.
- Thin-client applications are installed and deployed on the server end. There are no complex and costly provisioning process, license management, security update, and so forth.
However, a crucial disadvantage of the thin-client paradigm is that it requires the mobile device to be always connected. Today's wireless data networks are slow, unreliable, and expensive. They cover only limited areas. Those limitations have severely hindered the adoption of thin-client applications. To get around the network problem, we have to rely on the other two pillars in the Nokia Developer Platforms: J2ME and Symbian C++.