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Both GATT and ATT can work only with attributes, so for clients and servers to interact, all information must be organized in this form.Ĭonceptually, attributes are always located on the server and accessed (and potentially modified) by the client. They are addressable pieces of information that can contain relevant user data (or metadata) about the structure and grouping of the different attributes contained within the server.
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In these cases, you’ll need to use the full 128-bit UUID value at all times.Īttributes are the smallest data entity defined by GATT (and ATT).
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Shortening is not available for UUIDs that are not derived from the Bluetooth Base UUID (commonly called vendor-specific UUIDs). But if your application needs its own, either because the ones offered by the SIG do not cover your requirements or because you want to implement a new use case not previously considered in the profile specifications, you can generate them using the ITU’s UUID generation page. The SIG provides (shortened) UUIDs for all the types, services, and profiles that it defines and specifies. To reconstruct the full 128-bit UUID from the shortened version, insert the 16- or 32-bit short value (indicated by xxxxxxxx, including leading zeros) into the Bluetooth Base UUID: xxxxxxxx-0000-1000-8000-00805F9B34FB These shortened formats can be used only with UUIDs that are defined in the Bluetooth specification (i.e., that are listed by the Bluetooth SIG as standard Bluetooth UUIDs). X.667, alternatively known as ISO/IEC 9834-8:2005.įor efficiency, and because 16 bytes would take a large chunk of the 27-byte data payload length of the Link Layer, the BLE specification adds two additional UUID formats: 16-bit and 32-bit UUIDs. UUIDs are used in many protocols and applications other than Bluetooth, and their format, usage, and generation is specified in ITU-T Rec. That means that both a GAP central and a GAP peripheral can act as a GATT client or server, or even act as both at the same time.Ī universally unique identifier (UUID) is a 128-bit (16 bytes) number that is guaranteed (or has a high probability) to be globally unique. It is worth mentioning once more that GATT roles are both completely independent of GAP roles (see “Roles”) and also concurrently compatible with each other. Every BLE device sold must include at least a basic GATT server that can respond to client requests, even if only to return an error response. It also sends server-initiated updates when configured to do so, and it is the role responsible for storing and making the user data available to the client, organized in attributes. It receives requests from a client and sends responses back. The GATT server corresponds to the ATT server discussed in “Attribute Protocol (ATT)”.
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After completing service discovery, it can then start reading and writing attributes found in the server, as well as receiving server-initiated updates. The GATT client does not know anything in advance about the server’s attributes, so it must first inquire about the presence and nature of those attributes by performing service discovery. It sends requests to a server and receives responses (and server-initiated updates) from it. The GATT client corresponds to the ATT client discussed in “Attribute Protocol (ATT)”. This determines many fundamental aspects of GATT discussed in this chapter.Īs with any other protocol or profile in the Bluetooth specification, GATT starts by defining the roles that interacting devices can adopt: Client This data is organized hierarchically in sections called services, which group conceptually related pieces of user data called characteristics. GATT uses the Attribute Protocol (detailed in “Attribute Protocol (ATT)”) as its transport protocol to exchange data between devices. This makes GATT a key section of the BLE specification, because every single item of data relevant to applications and users must be formatted, packed, and sent according to its rules. All standard BLE profiles are therefore based on GATT and must comply with it to operate correctly. GATT also provides the reference framework for all GATT-based profiles (discussed in “SIG-defined GATT-based profiles”), which cover precise use cases and ensure interoperability between devices from different vendors. In contrast with GAP ( Chapter 3), which defines the low-level interactions with devices, GATT deals only with actual data transfer procedures and formats.
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The Generic Attribute Profile (GATT) establishes in detail how to exchange all profile and user data over a BLE connection.
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