主要介紹下藍牙協議棧（bluetooth stack） 串口協議（bluetooth SPP）Serial Port Profile 協議概念介紹。
本專欄文章我們會以連載的方式持續更新，本專欄計劃更新內容如下：
第一篇:藍牙綜合介紹 ，主要介紹藍牙的一些概念，產生背景，發展軌跡，市面藍牙介紹，以及藍牙開發板介紹。
第二篇:Transport層介紹,主要介紹藍牙協議棧跟藍牙芯片之前的硬件傳輸協議,比如基于UART的H4,H5,BCSP，基于USB的H2等
第三篇:傳統藍牙controller介紹，主要介紹傳統藍牙芯片的介紹，包括射頻層（RF），基帶層（baseband），鏈路管理層（LMP）等
第四篇:傳統藍牙host介紹，主要介紹傳統藍牙的協議棧，比如HCI,L2CAP,SDP,RFCOMM,HFP,SPP,HID,AVDTP,AVCTP,A2DP,AVRCP,OBEX,PBAP,MAP等等一系列的協議吧。
第五篇：低功耗藍牙controller介紹，主要介紹低功耗藍牙芯片，包括物理層（PHY），鏈路層（LL）
第六篇：低功耗藍牙host介紹，低功耗藍牙協議棧的介紹，包括HCI,L2CAP,ATT,GATT,SM等
第七篇：藍牙芯片介紹，主要介紹一些藍牙芯片的初始化流程，基于HCI vendor command的擴展
第八篇：附錄，主要介紹以上常用名詞的介紹以及一些特殊流程的介紹等。
另外，開發板如下所示，對于想學習藍牙協議棧的最好人手一套。以便更好的學習藍牙協議棧，相信我，學完這一套視頻你將擁有修改任何協議棧的能力（比如Linux下的bluez，Android下的bluedroid）。
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CSDN學院鏈接（進入選擇你想要學習的課程）：
藍牙交流扣扣群：970324688
Github代碼：
入手開發板：
藍牙學習目錄：
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SPP是Serial Port Profile（串口協議）的縮寫，其定義了使用藍牙進行RS232（或類似）串行電纜仿真的設備應使用的協議和過程。簡單來說就是在藍牙設備之間建立虛擬的串口進行數據通信。說白了就是可以兩個設備對端發送自定義數據，NOTED：蘋果不支持SPP協議。
目前SIG最新的版本是V1.2，之前更早的版本在官網只有V1.1可以下載到，但是這兩份文檔沒有功能上的差別，所以不做介紹
Device A (DevA) – This is the device that takes initiative to form a connection to another device
Device B (DevB) – This is the device that waits for another device to take initiative to connect.
說白了就是就是兩臺設備連接，誰發起的連接角色就是Device A,被連接的就是Device B,但是這個和其他協議不同，兩臺設備之前的code基本都是一樣，角色只是區分兩臺設備。
Device A是發起連接的主動房，所以Device A要至少支持Establish link and set up virtual serial connection.，Device B是被連接的乙方，所以要至少支持Accept link and establish virtual serial connection和Register Service record for application in local SDP database.以便Device A查詢服務并且連接，但是此協議漏掉最主要的兩個功能，發送數據，接收數據，或者SIG是覺得rfcomm做到了這個功能，所以不寫在SPP協議中吧，但是我還是決定要在SPP介紹收發功能。
This procedure refers to performing the steps necessary to establish a connection to an emulated serial port (or equivalent) in a remote device. The steps in this procedure are:
1）Submit a query using SDP to find out the RFCOMM Server channel number of the desired application in the remote device. This might include a browsing capability to let the user select among available ports (or services) in the peer device. Alternatively, if it is known exactly which service to contact, it is sufficient look up the necessary parameters using the Service Class ID associated with the desired?service.
2）Optionally, require authentication of the remote device to be performed. Also optionally, require encryption to be turned on.
3) ?Request a new L2CAP channel to the remote RFCOMM entity.
4) ?Initiate an RFCOMM session on the L2CAP channel.
5) ?Start a new data link connection on the RFCOMM session, using the aforementioned server channel number.After step 5, the virtual serial cable connection is ready to be used for communication between applications on both sides.
This procedure refers to taking part in the following steps:
1）If requested by the remote device, take part in authentication ?procedure and, upon
further request, turn on encryption.
2) ?Accept a new channel establishment indication from L2CAP.
3) ?Accept an RFCOMM session establishment on that channel.
4) ?Accept a new data link connection on the RFCOMM session. This may trigger a local request to authenticate the remote device and turn on ?encryption, if the user has required that for the emulated serial port being connected to (and authentication/encryption procedures have not already been carried out).
This procedure refers to registration of a service record for an emulated serial port (or equivalent) in the SDP database. This implies the existence of a Service Database, and the ability to respond to SDP queries.
NOTED：SPP只有安卓手機支持，并且部分安卓手機必須有SPP APP才可以連接，iphone手機不能直接SPP，但是又一個替代協議IAP，此協議需要買MFI chip才能用
我看下我們的SDP注冊：
static const uint8_t spp_service_record[] =
{
/* 1.ServiceClassIDList */
SDP_DES_SIZE8, 0x8,
SDP_UINT16, BT_ATTRIBUTE_SERVICE_CLASS_ID_LIST>>8&0xff, BT_ATTRIBUTE_SERVICE_CLASS_ID_LIST&0xff, /* Service class ID list attribute */
SDP_DES_SIZE8, 3,
SDP_UUID16, BT_SERVICE_CLASS_SERIAL_PORT>>8&0xff, BT_SERVICE_CLASS_SERIAL_PORT&0xff,
/* 2.ProtocolDescriptorList */
SDP_DES_SIZE8, 0x11,
SDP_UINT16, BT_ATTRIBUTE_PROTOCOL_DESCRIPTOR_LIST>>8&0xff, BT_ATTRIBUTE_PROTOCOL_DESCRIPTOR_LIST&0xff,/* Protocol descriptor list attribute */
SDP_DES_SIZE8, 0xc,
SDP_DES_SIZE8, 0x3,
SDP_UUID16, BT_PROTOCOL_L2CAP>>8&0xff, BT_PROTOCOL_L2CAP&0xff, /*L2CAP*/
SDP_DES_SIZE8, 0x5,
SDP_UUID16, BT_PROTOCOL_RFCOMM>>8&0xff, BT_PROTOCOL_RFCOMM&0xff, /*RFCOMM*/
SDP_UINT8, RFCOMM_SPP_SERVER_CHNL, /*RFCOMM channel*/
/* BluetoothProfileDescriptorList */
SDP_DES_SIZE8, 0xd,
SDP_UINT16, BT_ATTRIBUTE_BLUETOOTH_PROFILE_DESCRIPTOR_LIST>>8&0xff, BT_ATTRIBUTE_BLUETOOTH_PROFILE_DESCRIPTOR_LIST&0xff, /* profile descriptor List */
SDP_DES_SIZE8, 0x8,
SDP_DES_SIZE8,0x06,
SDP_UUID16,BT_SERVICE_CLASS_SERIAL_PORT>>8&0xff, BT_SERVICE_CLASS_SERIAL_PORT&0xff,
SDP_UINT16,0x01,0x02, /* V1.2 */
};?
