最近開始學習BT MESH系列協議
與傳統的BR/EDR和BLE相比，MESH core stack還是採用BLE，但是協議層卻是大變樣，具體架構如下：
BLE Core Spec將不會做爲主要介紹，主要介紹Mesh自身所定義的各個layer,由於翻譯水平有限，直接摘錄原文
（1）Model layer: defines models that are used to standardize the operation of typical user scenarios and
are defined in the Bluetooth Mesh Model specification or other higher layer specifications
（2）Foundation Model layer：defines the states, messages, and models required to configure and manage
a mesh network
（3）Access layer：defines how higher layer applications can use the upper transport layer. It defines the
format of the application data; it defines and controls the application data encryption and decryption
performed in the upper transport layer; and it checks whether the incoming application data has been
received in the context of the right network and application keys before forwarding it to the higher layer
（4）Upper transport layer：encrypts, decrypts, and authenticates application data and is designed to
provide confidentiality of access messages. It also defines how transport control messages are used to
manage the upper transport layer between nodes, including when used by the Friend feature
（5）Lower transport layer：defines how upper transport layer messages are segmented and reassembled
into multiple Lower Transport PDUs to deliver large upper transport layer messages to other nodes. It
also defines a single control message to manage segmentation and reassembly
（6）Network layer：defines how transport messages are addressed towards one or more elements. It
defines the network message format that allows Transport PDUs to be transported by the bearer layer.
The network layer decides whether to relay/forward messages, accept them for further processing, or
reject them. It also defines how a network message is encrypted and authenticated.
（7）Bearer layer：defines how network messages are transported between nodes. There are two bearers
defined, the advertising bearer and the GATT bearer. Additional bearers may be defined in the future