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Leading-Edge Java
The Web Within an Arm's Reach
Near-Field Communication with JSR 257
by Frank Sommers
October 5, 2005

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Go Contactless

The Java Contactless Communication API, developed through the JCP as JSR 257, addresses the communication needs of a significant subset of the D2D Web: Devices that reside within about an arm's reach of where you are[2]. If you look around, you can probably observe devices that already have some form of built-in capability to communicate with other devices within that short radius. For instance, your notebook computer may have the ability to communicate with your cell phone or PDA via Bluetooth. Or your PDA may be able to sync up with your desktop computer using infrared light.

These devices employ contactless communication: communication that requires no wires, and is limited in range from a few inches to a few feet. Also termed near-field communication, contactless communication technologies include radio-frequency identification (RFID), optical barcode readers, and even infrared communication.

Contactless communication capability is built into an increasing variety of physical objects, such as credit cards, PDAs, mobile phones, airport kiosks, and even packaging materials, facilitating peer-to-peer interaction between these objects and a remote reader. The city of Taipei's public transportation system, for instance, started to use near-field communication technology to facilitate the purchase of bus and train tickets from mobile phones equipped with near-field communication transmitters. The fare is billed to the customer's mobile phone account. More recently, some credit cards include near-field communication capabilities to offer contactless monetary transactions, eliminating the need to swipe a credit card at the point of purchase.

A special form of contactless communication, RFID tags attached to physical objects respond to radio frequencies emitted from a reader device, and transmit globally universal manufacturer and product codes[3]. Combined with sensors, such tags may also transmit detailed product- and time-specific information, such as a product's current temperature and location. RFID tags are designed for high-volume, inexpensive manufacture, and thus promise ubiquitous near-field communication access to information about all sorts of physical objects that currently have no place on the Web. For example, while waiting in line to purchase a movie ticket, an RFID tag embedded in the movie poster could allow your cell phone to automatically connect to a URL recorded in the tag, and download and play a movie preview.

Bootstrapping the D2D Web

While wire-based communication is often initiated at the moment a cable connects two or more devices, wireless communication technologies rely on bootstrapping mechanisms specific to each technology. Having to deal with differing bootstrapping technologies presents a challenge to developers wanting to harness that special corner of the D2D Web. For instance, establishing a Bluetooth connection between two Bluetooth-enabled devices requires a priori "pairing" of the devices. That requirement diminishes usability, since the user has to first arrange the communication between the devices before software could take advantage of that connection. A similar situation exists with other wireless and near-field communication technologies, as anyone regularly visiting wireless hotspots could confirm.

A more desirable situation would hide that communication bootstrapping from the user, and even from applications built to exploit contactless communication. A key value of the Java Contactless Communication API is that it provides that bootstrapping abstraction to Java applications. Applications on a JSR 257-enabled device can register with a DiscoveryManager to receive notifications when other near-field communication-enabled devices come into proximity. Based on the type of event, an application can query that newly discovered device and retrieve data from it.

Since devices taking advantage of contactless communication often have but scarce memory and processing resources, applications may also register with a special PushRegistry, and request that they be "awakened" when certain discovery events are received. That alleviates the need for applications interested in near-field communication events to continuously stay resident in memory.

For those familiar with Jini and RMI, the Contactless Communication API may sound much like a pared down version of those technologies. The Jini API even has a DiscoveryManager class to facilitate service discovery[4]. In addition, Jini's initial vision depicted communication networks formed spontaneously by Jini-enabled devices. The crucial difference between Jini and the Contactless Communication API is that Jini's discovery protocol centers around services. Indeed, services in a Jini network are defined via Java service interfaces, and the Jini discovery protocols locate and initiate communication with services based on the interfaces those service implement.

The devices the Contactless Communication API aims to discover and interact with cannot be called services in the Jini sense. Instead, JSR 257's focus is the discovery and retrieval of tiny pieces of data from devices, which is often all such devices can offer. Once the Contactless Communication API retrieves and processes those small data items, it may typically hand that data to other applications via events, perhaps even triggering such applications to load into memory as a result of those events. Higher-level applications can interpret that data in application-specific ways, and take actions, if needed.

Some of those actions may involve further communication with the newly discovered device, using a higher-level communication protocol, such as Bluetooth, for example. The Contactless Communication API helps bootstrap that higher-level communication by transferring just enough data from one device to another to seed the higher-level protocol. Conceptually, that bootstrapping is similar to how the Dynamic Host Configuration Protocol (DHCP) helps bootstrap higher-level TCP/IP-based communication between hosts.

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