BLE ATT: Attributes, Handles, Read, Write and MTU

Attribute and Handle

Each Attribute has a unique Handle in the Attribute table of the same server, a Type or UUID, access level and Value. Handle is a 16-bit number and specifies the place of the Attribute in the table. Handle is not guaranteed constant between firmware versions; The professional client discovers the service and does not guess the number.

Read and Write

Read Request sends a Handle and the server returns a Read Response. Write Request has success or error response, but Write Command or Write Without Response does not require ATT response for faster response. The choice of operation should be based on the importance of authentication, data rate and loss tolerance.

Error Response

If the Handle does not exist, the authorization is insufficient, or the length is invalid, the server sends an Error Response containing the Request Opcode, Handle, and Error Code. Logging these three values ​​is usually faster than guessing the cause of the error. Do not confuse security error with pairing; Characteristic may require Encryption.

MTU and fragmentation

ATT MTU is the largest PDU size in ATT. The default amount is small and both parties can negotiate a larger amount. The useful payload of each operation is a few bytes less than the MTU, because Opcode and Handle also take up space. Increasing MTU without sufficient data length and buffer does not always increase throughput.

Read and Notification from the byte

Handle is the ID of an Attribute in the table of the same server; Do not assign a new independent table to each connection. Valid handles are in the range 0x0001 to 0xFFFF. After changing the GATT database, the client cache should also remain valid according to the protocol mechanisms.

Read Request:  0A 25 00
Read Response: 0B C4 09
Notification:  1B 25 00 C4 09

In this hypothetical example, Handle value is 0x0025 and data is 0x09C4 i.e. 2500. Just by convention in this example, 2500 is equivalent to 25.00 degrees; ATT itself does not specify the temperature unit. The Read response does not reload the Handle.

The effective MTU is the result of negotiation between the two parties. If one supports 185 and the other supports 247, the agreed value cannot exceed 185; Enlarging the MTU also does not take the place of checking the Data Length and buffer.