Bluetooth GATT Introduction

1. Introduction

GATT (Generic Attribute profile) describes a service framework that uses ATT
The framework defines the process (Procedure) and format of the service (Server) and service properties (characteristic)
Procedure defines characteristic discovery, read, write, notification (notifing), instructions (indicating)
and configure the characteristic broadcast

GATT can be used by application or other profiles
The protocol stack is shown below
---------------        ---------------
| Application | <----> | Application |
---------------        ---------------
|        Attribute | | Attribute |
| Protocol | <----> | Protocol |
---------------        ---------------
| L2cap |    <----> | L2cap |
---------------        ---------------
| Controller |  <----> | Controller |
---------------        ---------------

The GATT can be configured as the following two roles (role)
-Client: command, request initiator
-Server: command, request receiver

The role configuration instance is as follows

     _________ 
    / ======= \ 
   / __________\ 
  | ___________ |     Request      /\ 
  | | Computer | |   ---------->   // 
  | | ( Client) | |   <----------  //Sensor (Server) 
  | |_________| |     Response  // 
  \=____________/\/"" "" "" "" "" "" 
  \  
 /:::::::::::   

Computer is a temperature service client, sensor is a temperature service server
Computer to sensor procedure to read the value of sensor

The requirements for the lower levels of GATT are as follows
-Physical Link: Att created with gap Channel establishment Bearer
-GATT role: not dependent on the Coontroller character (Master/slave)
-Security: For Le,security Features (Authorization, authentication, encryption) is optional
For BR/EDR, encryption is mandatory.
-Multi-byte field in TX Order:gatt with least significant octet first (Little Endian) 2 content 2.1 configured broadcast

For Le physical links, during the server broadcast mode
Client via configured broadcast informs the server that it should add characteristic Value to advertising data

Method is the client setting of the specified bit bit
Broadcast frequency is part of the definition of service, characteristic behavior 2.2 GATT profile Hierarchy

GATT Specifies the structure of the data interaction (Structure)
This structure defines some basic elements, such as service, characteristic
These elements exist in the attribute

At the top of the GATT is profile,profile consisting of one or more services (service)
A service is made up of characteristics, or a reference to another service (Reference)
Characteristic contains a value that may contain information about the value 2.2.1 Service

A service is a collection of [data] and associated [behavior that accomplishes a specific function]/[attribute]
In GATT, a service is implemented by the service definition (defintion)
A service definition may contain reference services (Reference service), mandatory characteristic, and optional characteristic

In order to achieve compatibility with older versions, service definitions in the new version can only add new reference services or optional characteristic
Service definitions in the new version prohibit changing behavior from the old service definition

There are two types of server
-Primary Service: Services with basic functionality that can be included with other services and can be discovered through the Primary service discovery process
-Secondary service: Only for services referenced by Primary/other secondary service, high-level protocol

Interpreting a service that is primary or secondary services can be mandated by a high-level protocol 2.2.2 Included service

A included service is a way to refer to existing services.
The specific approach is to add a reference to the included service at the beginning of the services definition,
So the entire included service definition becomes part of the new service definition 2.2.3 characteristic

Characteristic is defined by the characteristic definition,
Contains a description of the characteristic declaration, characteristic attribute, value, value (Optional) 3 Service Interoperability Requirements 3.1 Service Definition

The service definition includes a service Declaration
may contain include definitions and characteristic definitions
Ends before the next service statement or when you arrive at maximum Attribute handle
The service definition renders the include definitions and characteristic definitions in the service definition on the service side based on the attribute handle order is considered part of the service
The order in the service definition is
Service Declaration ~ Include definitions (>=0) ~ Characteristic definitions (>=0)

Service declaration are as follows

Attribute Handle	Attribute Type	Attribute Value	Attribute Permission
0xNNNN	0x2800–uuid for <primary service> 0x2801–uuid for <secondary service>	16-bit Bluetooth UUID 128-bit UUID for Service	Read only, No Authentication, No Authorization

The rules are as follows:
-When multiple services are present
Services defined using the 16-bit Bluetooth UUID service should be grouped (as ordered)
Similarly, service-defined services that use the 128-bit uuid are also grouped
-a device or a high-level protocol may have multiple service definitions, while multiple service definitions contain the same services UUID
-All attributes on the server should contain a service declaration or a service definition exists
-Service definition on the server may be unordered; The client should not assume that the service definition of the server must be an orderly 3.2 Include definition

An include definition contains only one include Declaration

Include declaration as follows

Attribute Handle	Attribute Type	Attribute Value	Attribute Permission
0xNNNN	0x2802–uuid For<include>	Included Service Attribute Handle	End Group Handle	Service UUID	Read only, No Authentication, No Authorization

Where the UUID is only present when the 16-bit is a Bluetooth uuid

If the include definition (a) of a service is a reference to another server's include definition (B)
Then the include definition (B) should not refer to the Include definition (A), otherwise it is a circular reference (Circular Reference)

When a client detects a circular reference or detects nested include declarations to a greater level than it expects
Client should terminate this communication (ATT Bearer) 3.3 characteristic Definition

Characteristic definition contains the following content
-Characteristic Declaration:first
-Characteristic Value Declaration:second
-Characteristic descriptor Declarations (Optional): Last (with multiple order not critical)

Characteristic definitions in attribute handle order on the server

Each of the above Declaration is contained in a separate attribute 3.3.1 characteristic Declaration

Characteristic declaration as follows

Attribute Handle	Attribute Type	Attribute Value	Attribute Permission
0xNNNN	0x2803 –uuid for Characteristic	Characteristic Properties	Characteristic Value Attribute Handle	Characteristic Uuid	Read only, No Authentication, No Authorization

Where Attribute value is read-only
The Attribute value field is as follows

Attribute Value	Size	Description
Characteristic Properties	1 octets	Bit field of characteristic properties
Characteristic Value Handle	2 octets	Handle of the Attribute containing the value of this characteristic
Characteristic UUID	2/16 octets	16-bit Bluetooth uuid or 128-bit uuid for characteristic Value

A service may contain characteristic with the same characteristic UUID definitions

In a service definition, some mandatory characteristics should precede the include declarations and other optional characteristics
And the client should not think that characteristic is orderly.
Also, 16-bit bluetooth UUIDs and 128-bit bluetooth UUIDs should be grouped separately

Characteristic properties determines the following as a bit bit
-How to use characteristic value
-Characteristic descriptors access mode

The details of the characteristic Properties bit field are as follows
Notice: Multiple bits can be set simultaneously

Properties	Value	Description
Broadcast	0x01	If set, permits broadcasts of the characteristic Value Using characteristic Configuration descriptor
Read	0x02	If set, permits reads of the characteristic Value using procedures defined in Section 4.8
Write without Response	0x04	If set, permit writes of the characteristic Value Without response using procedures defined in section 4.9.1
Write	0x08	If set, permits writes of the characteristic Value With response using procedures defined in sections 4.9.3 or section 4.9.4
Notify	0x10	If set, permits notifications of a characteristic Value Without acknowledgement using the procedure Defined in section 4.10
Indicate	0x20	If set, permits indications of a characteristic Value With acknowledgement using the procedure Defined in section 4.11
Authenticated Signed writes	0x40	If set, permits signed writes to the characteristic Value Using the procedure defined in section 4.9.2
Extended Properties	0x80	If set, additional characteristic properties are Defined in the characteristic Extended Properties descriptor defined in section 3.3.3.1

3.3.2 characteristic Value Declaration

Characteristic Value declaration is as follows

Attribute Handle	Attribute Type	Attribute Value	Attribute Permissions
0xNNNN	0xuuuu–16-bit Bluetooth UUID Or 128-bit uuid for characteristic uuid	Characteristic Value	Higher layer Profile Or Implementation specific

3.3.3 Characteristic descriptor Declarations

Characteristic descriptor declarations contains characteristic value related information

GATT defines a series of standard characteristic descriptors for use in high-level protocols
High-level protocols can also define protocol-related characteristic descriptors

Characteristic descriptors on the service side is disorderly, the client should not be taken for granted
Characteristic descriptors Declarations permissions by a high-level protocol definition or protocol-related
The client should not be taken for granted to be readable

Characteristic descriptor declarations includes the following (see Specification for details)

-Characteristic Extended Properties
-Characteristic User Description
-Client characteristic Configuration
-Server characteristic Configuration
-Characteristic Presentation Format
-Characteristic Aggregate Format 3.4 Summary of GATT profile Attribute Types

Attribute Type	Uuid	Description
<primary service>	0x2800	Primary Service Declaration
<secondary service>	0x2801	Secondary Service Declaration
<Include>	0x2802	Include Declaration
<Characteristic>	0x2803	Characteristic Declaration
<characteristic Extended properties>	0x2900	Characteristic Extended Properties
<characteristic User description>	0x2901	Characteristic User Description descriptor
<client characteristic configuration>	0x2902	Client characteristic Configuration descriptor
<server characteristic configuration>	0x2903	Server characteristic Configuration descriptor
<characteristic format>	0x2904	Characteristic Format descriptor
<characteristic Aggregate format>	0x2905	Characteristic Aggregate Format descriptor

4. GATT Feature Requirements 4.1 Overview

11 items are defined in the GATT feature
1. Server Configuration
2. Primary Service Discovery
3. Relationship Discovery
4. Characteristic Discovery
5. Characteristic descriptor Discovery
6. Reading a characteristic Value
7. Writing a characteristic Value
8. Notification of a characteristic Value
9. Indication of a characteristic Value
Ten. Reading a characteristic descriptor
Writing a characteristic descriptor

Each Feature has a corresponding procedure and sub-procedure that describes how to use the ATT to implement the respective functions 4.2 Feature Support and Procedure Mapping

The detailed correspondence list is as follows table

No	Feature	Sub-procedure	Ref	Client Support	Server Support
1	Server Configuration	Exchange MTU	4.3.1	O	O
2	Primary Service Discovery	Discover all Primary Services	4.4.1	O	M
Discover Primary Services by Service UUID	4.4.2	O	M
3	Relationship Discovery	Find Included Services	4.5.1	O	M
4	Characteristic Discovery	Discover all characteristic of a Service	4.6.1	O	M
Discover characteristic by UUID	4.6.2	O	M
5	Characteristic Descriptor Discovery	Discover all characteristic descriptors	4.7.1	O	M
6	Characteristic Value Read	Read characteristic Value	4.8.1	O	M
Read Using characteristic UUID	4.8.1