OBject EXchange

OBEX (abbreviation of OBject EXchange, also termed IrOBEX) is a communications protocol that facilitates the exchange of binary objects between devices. It is maintained by the Infrared Data Association but has also been adopted by the Bluetooth Special Interest Group and the SyncML wing of the Open Mobile Alliance (OMA). One of OBEX's earliest popular applications was in the Palm III. This PDA and its many successors use OBEX to exchange business cards, data, even applications.

Although OBEX was initially designed for infrared, it has now been adopted by Bluetooth, and is also used over RS-232, USB, and in devices such as Livescribe smartpens.

OBEX is similar in design and function to HTTP in providing the client with a reliable transport for connecting to a server and may then request or provide objects. But OBEX differs in many important respects:

OBEX works by exchanging objects, which are used for a variety of purposes: establishing the parameters of a connection, sending and requesting data, changing the current path or the attributes of a file.

Objects are fields and headers. As an example, the following may be the object used for requesting the phonebook from a mobile:

This object contains two fields (command and length) and two headers. The first field (command) specifies that is a request for data (GET). The second field is the total size of the object, including the two fields.

This object also contains two headers, specifically a "Connection ID" and a "Name". The first byte of each header is the header's name and its content type. In this case:

A possible response, containing the requested data, could be:

In this example, the phonebook is assumed short enough to be contained in a single response object. The only header has 0x49 as its identifier, meaning that it is an "End of Body", the last chunk of information (also the only one, in this case). The first two bits of 0x49 are 01, meaning that the content of this header is length-prefixed data: the two next bytes 0x00 0x2F tells the length of this data (in decimal, 47), the succeeding ones are the data, in this case a phonebook comprising only an empty vCard of 47 bytes.

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