9.1 Definition of Characteristic Access Commands

When creating a connection with two Bluetooth Low Energy devices, one device plays the central role and the other plays a peripheral role. The peripheral device advertises to show its connectable status, whereas, the central device scans service advertisements, and if required, initiates a connection to the peripheral device.

After establishing the connection, both peers can initiate bonding. Related security keys are saved in PDS after successful bonding. Use these keys on the next connection to reduce the time elapsed on the security procedure.

The following figure illustrates that the Generic Attribute (GATT) profile defines a service framework using the Attribute (ATT) protocol. This framework defines procedures and formats of services and their characteristics. The following are the defined procedures:

Discovering
Reading
Writing
Notifying
Indicating characteristics

In other words, the operation with an arrow between the GATT client and GATT server.

A service definition contains a service declaration and includes definitions and characteristic definitions. All include definitions, and characteristic definitions contained within the service definition are considered to be part of the service.

The GATT profile specifies the structure in which profile data is exchanged. This structure defines basic elements, such as services and characteristics, used in a profile. All of the elements are contained by attributes. Attributes used in the attribute protocol are containers that carry this profile data.

The top level of the hierarchy is a profile. A profile is composed of one or more service necessary to fulfill a use case. A service is composed of characteristics. Each characteristic contains a value and contains optional information about the value. The service and characteristic and the components of the characteristic (in other words, value and descriptors) contain the profile data and are all stored in attributes on the server.

The user can identify each service and its characteristics by its UUID. The UUID takes either a 16-bit short form or a 128-bit long form. As specified in the Bluetooth core specifications, all Bluetooth SIG-adopted public services and characteristics have short UUIDs, whereas, the user-defined private UUIDs are in long form. For details for the Bluetooth SIG-adopted services and characteristics, refer to www.bluetooth.com/specifications/specs/.

The following table provides details about the accessibility of each characteristic, defined by an 8-bit characteristic property in bitmap format.

Table 9-1. Characteristic Properties
Property	Bitmap	Description
Extended Property⁽¹⁾	0b10000000	Additional property available
Authenticated Write⁽¹⁾	0b01000000	Write characteristic with authentication from client to server
Indicate	0b00100000	Indicate value of characteristic with acknowledgment from server to client
Notify	0b00010000	Notify value of characteristic without acknowledgment from server to client
Write	0b00001000	Write value of characteristic with acknowledgment from client to server
Write without response	0b00000100	Write value of characteristic without acknowledgment from client to server
Read	0b00000010	Read value of characteristic. Send the value from server to client
Broadcast⁽¹⁾	0b00000001	Broadcast value of characteristic
Note: Currently not supported in the RNBD451 module.

The following figure illustrates the GATT Service in the RNBD451 module. The GATT client can access the characteristics via ATT protocol in the GATT Server in the peripheral device. After establishing the connection, the client reads the GATT server service and characteristic UUIDs. The GATT client can access the characteristic values by using Write, Read, Indication and Notifications.

Write-REQ enables the client to update characteristic values on the peripheral’s GATT server. Perform the write requests by using the RNBD451 CHW commands. For more details on the GATT characteristic access commands, refer to 5.4.3 GATT Operation on Server Role and 5.4.4 GATT Operation on Client Role.

A Write-CMD message performs an unacknowledged write from a client to the server.

A client sends Read-REQ to read a characteristic value on the peripheral’s GATT server. Perform the read requests by using the RNBD451 CHR commands.

The server sends uninvited updates, such as notifications and indications to the client. The client must enable the notification and indication by configuring a Client Characteristic Configuration Descriptor (CCCD) to receive the updates. The RNBD451 module is using the CHW command to write a non-zero value to the CCCD. When the RNBD451 module GATT client role receives a notification, the %WC,hhhh,ddddddd% message is returned on UART in Command mode.