What is the length limit for BLE characteristics?

Question

I am reading the Bluetooth Core Spec. v 4.2 document, but I can't find any information about the characteristic length limit.

• What is the length limit for characteristic of BLE services?

Solution

The limit is set by what is possible for an attribute value, see the ATT part (Vol 3, Part F, 3.2.9):

The maximum length of an attribute value shall be 512 octets.

You should also consider what could be sent in one packet, which is 20 bytes (ATT_MTU - 3). With 20 bytes or less you can use simple transfer procedures and save power.

Comments

[Guo Xingmin]

The default MTU is 20 and this value can be changed to max 512, then the GATT value can be set as 512-3 byte. use 20 bytes or less does not save power actually if you want to transfer byte more than 20.