MOBILE MIDDLEWARE
The term middleware refers to the software layer between the operating system and the distributed applications that interact via the networks. The primary mission of a middleware layer is to hide the underlying networked environment's complexity by insulating applications from explicit protocol handling disjoint memories, data replication, network faults, and parallelism (Geihs, 2001). Mobile middleware translates requests from mobile stations to a host computer and adapts content from the host to the mobile station (Saha, Jamtgaard, & Villasenor, 2001).
WAP and i-mode
According to an article at Eurotechnology.com (Eurotechnology, n.d.), 60% of the world's wireless Internet users use i-mode, 39% use WAP, and 1% use Palm middleware. Table 3 compares i-mode and WAP along with details of each.
| ‚ | WAP | i-mode |
|---|---|---|
| Developer | WAP Forum | NTT DoCoMo |
| Function | A protocol | A complete mobile Internet service |
| Host Language | WML (Wireless Markup Language) | CHTML (Compact HTML) |
| Major Technology | WAP Gateway | TCP/IP modifications |
| Key Features | Widely adopted and flexible | Highest number of users and easy to use |
WAP (Wireless Application Protocol)
WAP (http://www.wapforum.org) is an open global specification that allows users with mobile stations to easily access and interact with information and services instantly. It is a very flexible standard including most wireless networks, which comprise CDPD, CDMA, GSM, PDC, PHS, TDMA, FLEX, ReFLEX, iDEN, TETRA, DECT, DataTAC, Mobitex, and GRPS. It is supported by most operating systems and was specifically engineered for mobile stations, including Palm OS, EPOC, Windows CE, FLEXOS, OS/9, and JavaOS. The most important technology applied by WAP is probably the WAP Gateway, which translates requests from the WAP protocol stack to the WWW stack, so they can be submitted to Web servers. For example, requests from mobile stations are sent as a URL through the network to the WAP Gateway; responses are sent from the Web server to the WAP Gateway in HTML and are then translated to WML and sent to the mobile stations. Although WAP supports HTML and XML, its host language is WML (Wireless Markup Language), which is a markup language based on XML that is intended for use in specifying content and user interfaces for mobile stations. WAP also supports WMLScript, which is similar to JavaScript but makes minimal demands on memory and CPU power because it does not contain many of the unnecessary functions found in other scripting languages.
i-mode
i-mode (http://www.nttdocomo.com/) is the full- color , always-on, and packet-switched Internet service for cellular phones offered by NTT DoCoMo. Introduced in February 1999, it has attracted over 36 million subscribers worldwide. With i-mode, cellular phone users can easily access more than 62,000 Internet sites, as well as specialized services such as e-mail, online shopping and banking, ticket reservations , and personalized ringing melodies that can be downloaded for their phones. The i-mode network structure not only provides access to i-mode and i-mode-compatible contents through the Internet but also provides access through a dedicated leased-line circuit for added security. It is the only network in the world that now allows subscribers continuous access to the Internet via cellular phones. Users are charged based on the volume of data transmitted rather than the amount of time spent connected. In spring 2001, NTT DoCoMo introduced its next -generation mobile system, based on wideband CDMA (W-CDMA), which can support speeds of 384 kbps or faster, allowing users to download videos and other bandwidth- intensive content with its high-speed packet data communications.
Implementation
Both WAP and i-mode are built on top of existing network protocols such as Internet Protocol (IP) and Transmission Control Protocol (TCP). IP provides a network routing service for upper layer protocols like TCP, which transports data reliably between two end parties of a network connection. This reliable data delivery service is crucial to the success of transactions in mobile commerce systems. In a wireless environment, IP and TCP require significant modification in order to adapt to features like mobility and radio communication.
Mobile IP
The Mobile IP (Internet Engineering Task Force, 2003) defines enhancements that permit Internet Protocol (IP) nodes ( hosts and routers) using either IPv4 or IPv6 to seamlessly "roam" among IP subnetworks and media types. It supports transparency above the IP layer, including the maintenance of active TCP connections and UDP port bindings. Two types of mobile-IP capable router, home agent (HA) and foreign agent (FA), are defined to assist routing when the mobile node is away from its home network. All datagrams destined for the mobile node are intercepted by HA and tunneled to FA. FA then delivers these packets to the mobile node through a care-of-address established when the mobile node is attached to FA.
TCP for mobile networks
Transmission Control Protocol (TCP) was designed for reliable data transport on wired networks and its parameters have been fine- tuned for such environments. As a result, when it is applied directly to mobile networks, TCP performs poorly due to factors such as error-prone wireless channels, frequent handoffs, and disconnections. In order to optimize reliable data transport performance, a number of variants of TCP have been suggested for mobile networks. An idea proposed by Yavatkar and Bhagawat (1994) was to split the path between the mobile node and the fixed node into two separate sub-paths: one over the wireless links and the other over the wired links. This approach limits the TCP performance degradation in a "short" wireless link connection. The proposed "packet caching" scheme (Balakrishnan, Seshan, Amir, & Katz, 1995) tries to reduce the TCP retransmission overhead due to handoff. The "fast retransmission" scheme (Caceres & Iftode, 1996) utilizes the fast retransmission option immediately after handoff is completed and shows smooth TCP performance during handoff .
