GATT (Services and Characteristics) - Getting Started with Bluetooth Low Energy - page 71

Reading characteristics and descriptors is subject to the security permissions and the
server can deny permission if the connection's security level does not match the estab‐
lished requirements (see “Security” on page 72 ).
Writing Characteristics and Descriptors
To write to the value of a characteristic value or a descriptor, the client has the follow‐
ing choices:
Write characteristic value or descriptor
This feature is used to write to a characteristic value or descriptor. The client pro‐
vides a handle and the contents of the value (up to ATT_MTU-3 bytes, because the
handle and the ATT operation code are included in the packet with the data) and
the server will acknowledge the write operation with a response.
Write long characteristic value or descriptor
Similar to the read long characteristic value or descriptor feature, this permits a client
to write more than ATT_MTU-3 bytes of data into a server's characteristic value or
descriptor. It works by queueing several prepare write operations, each of which
includes an offset and the data itself, and then finally writing them all atomically
with an execute write operation.
Additionally, and only applicable for characteristic values, these features are available:
Write without response
This feature is the converse equivalent of notifications (detailed in “Server-Initiated
Updates” on page 72 ) and uses Write Command packets. Write Commands are un‐
acknowledged packets that include a handle and a value, and they can be sent at
any time and in any amount without any flow control mechanism kicking in (except
of course for the native Link Layer flow control, since all traffic is subject to it). The
server is free to silently discard them if it cannot process them or if the attribute
permissions prevent it from accepting it. The client will never know, but that is by
mutual agreement. The only way for the client to find out whether the value was
written is to read it after the fact.
Reliable writes
Similar to the read multiple characteristic values feature, when a client wants to
queue write operations to multiple characteristic values, it issues a final packet to
commit the pending write operations and execute them.
Writing characteristics and descriptors is subject to the security permissions, and the
server can deny permission if the connection's security level does not match the estab‐
lished requirements (see “Security” on page 72 ).