规范 Vol2 的Part E，讲HCI command的第7章有几个command，看起来很有用，这里list下
7.5.1 Read Faild Contact Counter command
这个计数器(Failed_Contact_Counter，两个字节)是用来记录由于flush timeout到期（一般是因为空气中信号干扰比较多，数据包重传比较多）而flush掉的数据包的个数，在做bt music的时候经常看到smartphone送过来的数据包的序列号不连续，如果smartphone有用这个command来monitor数据包的状况的话，就比较容易看到这个现象。
When the 802.11 PAL receives an HCI_Read_Failed_Contact_Counter command it shall return the number of consecutive incidents in which the remote device didn’t respond after the flus>h timeout had expired, and the L2CAP packet that was currently being transmitted was automatically flushed. The Failed Contact Counter is specific to each logical link.
7.1.47 Truncated Page command
可以用来探测目标设备在不在可连接范围内？
The HCI_Truncated_Page command is used to page the BR/EDR Controller with the specified BD_ADDR and then abort the paging sequence after an ID response has been received. See [Vol >2] Part B, Section 8.3.3 for additional information.
复习下page的几个步骤，请看下面的两张图：
BLUETOOTH CORE SPECIFICATION Version 5.1 | Vol 2, Part E page 1112