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, WAP 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:
HTTP is normally layered above a TCP/IP link. OBEX can also be, but is commonly implemented on an IrLAP/IrLMP/Tiny TP stack on an IrDA device. In Bluetooth, OBEX is implemented on a Baseband/ACL/L2CAP/RFCOMM stack. Other such "bindings" of OBEX are possible, such as over USB.
HTTP uses human-readable text, but OBEX uses binary-formatted type-length-value triplets named "Headers" to exchange information about a request or an object. These are much easier to parse by resource-limited devices.
HTTP transactions are inherently stateless; generally a HTTP client opens a connection, makes a single request, receives its response, and either closes the connection or makes other unrelated requests. In OBEX, a single transport connection may bear many related operations. In fact, recent additions to the OBEX specification allow an abruptly closed transaction to be resumed with all state information intact.
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:
0xCB means that this header is a "Connection ID", a number obtained previously; the two highest-order bits of 0xCB are 11, and this pair specifies that this as a 4-byte quantity;
the first byte of the second header is 0x01; this byte identifies this header as a "Name" one; the first two bits of 0x01 are 00, meaning that the content of this header is a null-terminated unicode string (in UCS-2 form), prefixed by the number of bytes it is made of (0x00 0x1e).
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.
This example shows a single GET command and its response, the only headers involved being connection id, name and end-of-body. Before issuing it, a CONNECT command should have been sent for establishing some parameters of the connection, including the connection id. Other commands are: put, setpath, action, abort, disconnect. Some other notable headers include: type, time, description, target.
After the client (e.g., computer) connects to the server (e.g., mobile), a typical session consists in the client sending a number of objects and getting their responses from the server. As an example:
CONNECT: one of the fields specifies the largest size of packets the client can receive; a TARGET header specifies the kind of service the client is expecting (file-browsing, sync-ml, phonebook access); the server answer with its maximal packet length, the connection id, and other data
GET: the client requests a file, specifying the connection id, the file name and/or its type; the server answer with the file content, or just a part of it; in the latter case, the client has to send other GET objects to obtain the rest of the file
SETPATH: the client tells the server to switch to a different file folder, specifying the connection id and the folder name in two headers
GET: the client request a listing of the folder content by sending an object with the connection id and an appropriate TYPE header (e.g., "x-obex/folder-listing" for file transfer, "x-bt/vcard-listing" for phonebook access)
PUT: the client sends a file to the server; if it is too large to fit into a single packet, the server will request the next part with a CONTINUE response
DISCONNECT: the client informs the server that is closing the session
The exchange may differ significantly depending on the service. For example, SyncML does not use SETPATH, while a OBEX push is made of just CONNECT (without a TARGET header), PUT and an optional DISCONNECT.
The following protocols runs over OBEX, or have bindings to do so:
OBEX Push: used for transferring a file from the originator of the request to the recipient; a CONNECTION object containing no target is sent, then PUT is used to transfer the file
OBEX File Transfer Protocol: used to store and retrieve files, similar to FTP; the target header of the CONNECTION object is {0xF9, 0xEC, 0x7B, 0xC4, 0x95, 0x3C, 0x11, 0xD2, 0x98, 0x4E, 0x52, 0x54, 0x00, 0xDC, 0x9E, 0x09}; the response contains the connection id to use in subsequent GET, PUT, SETPATH and ACTION object
Phonebook Access: similar to file transfer, but uses a target {0x79, 0x61, 0x35, 0xF0, 0xF0, 0xC5, 0x11, 0xD8, 0x09, 0x66, 0x08, 0x00, 0x20, 0x0C, 0x9A, 0x66}; phonebook entries can be listed (with various possible orderings and filters) and retrieved from certain directories under telecom/ using GET and SETPATH
IrMC: used to exchange phonebooks entries, calendar notes, messages, etc.; in its connectionless form, a single PUT is used to transfer data; otherwise, various files and folders within telecom/ can be retrieved or pushed; a target header {'I', 'R', 'M', 'C', '-', 'S', 'Y', 'N', 'C'} may be used in GET requests to differentiate the kind of indexing used
SyncML: used to synchronize phonebooks, calendars, notes and other data; in its OBEX binding, the target of the CONNECT object is {'S', 'Y', 'N', 'C', 'M', 'L', '-', 'S', 'Y', 'N', 'C'}; a session then consists in a sequence of PUT-GET pairs where nameless XML or WBXML files are sent and received, in turn
Optional package javax.obex in Java APIs for Bluetooth provides an implementation of OBEX in Java.
OpenObex is an open-source implementation of OBEX in C. It provides functions for connecting over IrDA, Bluetooth, USB and TCP/IP, building objects and handling received data. An example schema of a client application is:
Objects are sent by OBEX_Request. After calling OBEX_HandleInput, received data is processed in the callback function (which was specified when calling OBEX_Init). The callback function can determine whether the response has been completely received, and therefore whether the main program can exit from the while loop it is executing.
PyOBEX provides partial support for OBEX in Python.
OBEX is the foundation for many higher-layer "profiles":
All Palms since Palm III, except the Palm Pre, Palm Pre Plus, Palm Pixi and Palm Pixi Plus.
Most Sharp, Motorola, Samsung, Sony Ericsson, HTC and Nokia phones with infrared or Bluetooth port
LG EnV Touch (VX11000)
Many other PDAs since 2003
Many other phones with infrared or Bluetooth port
Android devices in version 2.1 and above
Windows Phone 7.8 and 8 devices (limited to the transferring of pictures, music and videos via a 'Bluetooth Share' app).
