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INTRODUCTION

Communication has revolutionized how people use computers. Local Area Networking (LAN) has enabled information and device sharing, moving the world from a centralized computing structure to a more distributed system approach. Networking has increased the usefulness of the computer by allowing it to share and exchange data seamlessly across a wide variety of physical technologies. We expect advances in mobile networking will similarly change the way people use mobile computers.

The most popular usage model for mobile computers today is based on nomadic computing. Mobile computers are used in stationary locations such as hotel rooms, airport clubs, or conference rooms. Accessing the Internet or communicating with the user's enterprise services is deliberate and infrequent. It depends on the connectivity options at each stationary location. Wireless Wide-Area Networking (WWAN) provides mobile users with another option.

Convenient On-Demand Communication
There are a variety of WWAN technologies and services available today [6]. The emergence of data services on wide-area cellular networks provides a new communication "gateway" for exploitation by mobile users. Wireless modems allow on-demand data connectivity using the cellular network. PC Card cellular modems or cables linking analog cellular phones to mobile computers have been available for several years, but the connection was prone to frequent errors and disconnections. However, the emergence of digital cellular data services on GSM and IS-95 has helped make connectivity more reliable. This paper focuses on data access through cellular handsets equipped with Bluetooth wireless technology.

Wide-area data services make it easier for the mobile user to instantly create a communication channel. This usage model is comparable to that of a home PC user except for the fact that the computer is more portable than the home PC, and the communication pipe is usually thinner, referred to as weak connectivity, and more expensive to use. Bluetooth wireless technology allows the PC to access the cellular network while the cellular handset is within range. There is no need for a cable, or even for taking the phone out of a purse or pocket.

Always On, Always Connected
On-demand communication tends to be deliberate, that is, the user always initiates the transfer of data. Tso used the phrase "the data I want finds me" and that is what the AOAC usage model does [9]. This is analogous to the enterprise LAN model where desktop users get e-mail notifications, video conferencing calls, and calendar reminders sent to them.

For the mobile user, AOAC means data continues to find the user even when the user is outside the enterprise. This requires enterprise applications to be able to find the mobile user or to be redirected via some proxy agent. In any case, some form of redirection mechanism is needed to locate the mobile PC.

Besides cost and bandwidth differences as mentioned in on-demand communication, AOAC for mobile PCs needs to take into account battery life. It may be fine to leave a desktop computer on and always connected to the Intranet, but leaving a notebook computer on and connected has serious implications for battery life. Therefore, schemes to enable AOAC technology tend to focus on "virtually connected" solutions where the mobile computer is sleeping the majority of the time to preserve the battery. As described later in this paper, Bluetooth wireless technology allows the phone to "wake up" the mobile computer and transfer data to it. This enables a number of future scenarios where data are delivered to briefcase-enclosed mobile computers while their users are carrying them down the street.

Proximity-Based Computing
Mobile computers have one very significant advantage over stationary desktop computers: they can travel with the user. This ability opens up a variety of new applications or location-based extensions to existing applications. Mobile computers may determine their relative position via a variety of methods including an attached Global Positioning System (GPS) device or a local cell site ID from a personal cellular phone . This information could be fed to Internet servers providing location-based services such as maps, restaurant guides, and tourist information. Bluetooth wireless technology allows the computer to extract location information from GPS devices and cellular handsets also equipped with the technology and pass this information on to applications running on the computer.

A natural extension to wide-area location-based services is local-area location-oriented services where communication is via information servers or conduits embedded into the local infrastructure. Usage models include local access points for Internet/FAX/voice services, conference room A/V remote control, Point-of-Sale (POS) transaction terminals in stores, electronic white and yellow pages in phone booths, and electronic menus and maps for nearby restaurants in transit terminals and hotels. For example, take the case of business users who have been working while disconnected. As they enter a restaurant, their computers recognize a public data access point is within range and its advertised rate is reasonable. Through a Bluetooth link to the user's cellular phone or Personal Digital Assistant (PDA), the computer sends a prompt to OK a file system synchronization operation. If the user positively acknowledges the operation on the phone or PDA, the computer connects to the data access point, and critical modified files are brought into synchronization with the files on the enterprise server where they can be backed up and made available to other users. To enhance enterprise security, all transferred data should be protected between the computer and the enterprise and should not depend on the security of the Bluetooth link.

In the proximity-based computing model, embedded services advertise themselves to mobile users. Users may either consciously or unconsciously decide to use them. In the example above, a user could create a profile that waives user authorization for activities deemed favorable enough. This type of unconscious proximity-based computing has the promise of simplifying a user's life by doing the rather obvious things automatically. Bluetooth technology supports this usage model by its wireless nature and ability to discover devices that are nearby.

Recap
To summarize, some of the differences between desktop and mobile computing usage models are as follows:

• WWAN provides weaker connectivity than LAN.
• WWAN has higher communication costs.
• Mobile computers have limited battery life.
• Mobile computers may take advantage of location-based applications.
• Proximity use benefits mobile users more than desktop users.