4、BLE广播包解析：广播信道与数据信道、广播PDU类型、广播数据格式

各位同学，欢迎来到第四章。前面我们聊了BLE协议栈的架构，也看了抓包工具怎么用。今天咱们来点真格的——直接拆解一个广播包。

你想想看，一个蓝牙设备从开机到被手机发现，中间发生了什么？说白了，就是它在不停地“喊话”。这个喊话的过程，就是广播。我当年刚接触BLE时，觉得广播嘛，不就是发几个字节吗？结果一抓包，发现里面门道还挺多。今天我就把这几年的经验掰开揉碎讲给你听。

4.1 广播信道与数据信道

BLE在2.4GHz频段里干活，一共划了40个信道，编号0到39。但这40个信道不是平起平坐的，它们分成了两拨：广播信道和数据信道。

信道类型 信道编号 中心频率 用途
广播信道 37、38、39 2402 MHz、2426 MHz、2480 MHz 设备发现、广播、连接建立
数据信道 0~36（共37个） 2404~2478 MHz（间隔2 MHz） 连接后的数据收发

为什么广播信道只有3个？我刚开始也觉得奇怪，37个数据信道，才3个广播信道，够用吗？

其实这是故意的。BLE设计者希望广播设备能快速被找到，所以选了3个在频段上分布比较开的信道——37在最低端，38在中间，39在最高端。这样不管环境怎么干扰，总有一个信道能通。我在项目中遇到过，有些厂房里2.4GHz Wi-Fi特别多，信道38经常被挤爆，但37和39还能正常工作。这就是3个信道的妙处。

数据信道就简单了，连接建立之后，主设备和从设备在这37个信道上跳来跳去地通信，这叫跳频。跳频的好处是抗干扰，这个我们后面讲连接事件时再细说。

核心要点：广播只在37/38/39三个信道上进行。抓广播包时，你只需要监听这三个信道就够了。数据信道有37个，但那是连接之后的事。

4.2 广播PDU类型

广播包不是随便发的，它有一套严格的格式。我们先看广播信道PDU的类型。嗯，这里要注意，PDU就是Protocol Data Unit，协议数据单元，说白了就是“这一帧里到底装了什么消息”。

广播PDU主要分这么几类：

  • ADV_IND：可连接的非定向广播。最常用，手机扫描时看到的设备基本都是这种。
  • ADV_DIRECT_IND：可连接的定向广播。指定了目标设备地址，别人收到也不处理。
  • ADV_NONCONN_IND：不可连接的非定向广播。只发不收，适合广播者不想被连接的情况。
  • ADV_SCAN_IND：可扫描的非定向广播。允许别人发扫描请求来获取更多数据。
  • SCAN_REQ / SCAN_RSP：扫描请求和扫描响应。这是扫描者和广播者之间的“握手”。
  • CONNECT_REQ：连接请求。扫描者想跟广播者建立连接时发的。

我列个表，方便你对比记忆：

PDU类型 可连接 可扫描 定向 典型场景
ADV_IND 是 是 否 手机、手环、耳机广播
ADV_DIRECT_IND 是 否 是 快速重连
ADV_NONCONN_IND 否 否 否 信标（Beacon）
ADV_SCAN_IND 否 是 否 需要被动扫描获取额外数据

你可能会问：ADV_DIRECT_IND为什么不可扫描？因为定向广播的目标很明确，不需要别人来扫描。我曾经调试过一个手环快速重连的问题，设备发ADV_DIRECT_IND，手机就是收不到。后来发现是白名单没配对，地址对不上。这种坑，踩过一次就记住了。

实战技巧：抓包时，如果你看到大量ADV_DIRECT_IND包，说明设备在尝试快速重连。如果一直连不上，先检查目标地址对不对。

4.3 广播数据格式（AD Structure）

好了，现在我们知道广播包有哪些类型了。但广播包里到底装了啥？这就得看AD Structure了。

AD的全称是Advertising Data，广播数据。它的结构很简单，就是一个接一个的“数据块”，每个块叫一个AD Structure。格式如下：

+--------+--------+--------+--------+--------+
| Length |  AD Type |  AD Data ...
| (1字节) | (1字节)  | (Length-1字节)
+--------+--------+--------+--------+--------+

每个AD Structure由三部分组成：

  • Length：1字节，表示后面AD Type + AD Data的总长度。
  • AD Type：1字节，表示数据类型。比如0x01表示Flags，0x09表示Complete Local Name。
  • AD Data：Length-1字节，具体的数据内容。

举个例子，你抓到一个广播包，里面有一段数据：02 01 06 09 09 42 4C 45 5F 44 65 76 69 63 65。怎么解析？

我来手把手拆一遍：

  1. 第一个字节02：Length=2，说明这个AD Structure总长2字节。
  2. 第二个字节01：AD Type=0x01，这是Flags。
  3. 第三个字节06：AD Data，Flags的值是0x06，表示LE General Discoverable Mode + BR/EDR Not Supported。
  4. 第四个字节09：下一个AD Structure的Length=9。
  5. 第五个字节09：AD Type=0x09，Complete Local Name。
  6. 后面8个字节42 4C 45 5F 44 65 76 69 63 65：ASCII码，对应字符串"BLE_Device"。

你看，就这么简单。我刚开始手动解析时，经常数错字节。后来养成了一个习惯：先在纸上画格子，一格一个字节，标上Length、Type、Data，再对照着看。这个方法虽然土，但不容易出错。

注意：广播包的总长度不能超过31字节（PDU里最多31字节的有效载荷）。所以AD Structure不能太多，要精打细算。我曾经见过一个产品，把设备名称、服务UUID、厂商自定义数据全塞进去，结果超了，最后只能砍掉厂商数据。设计广播包时，一定要先算好预算。

4.4 常见AD Type一览

AD Type有很多种，但常用的就那几个。我列个表，你抓包时对照着看：

AD Type值 名称 说明
0x01 Flags 广播模式、BR/EDR支持等标志位
0x02 Incomplete List of 16-bit Service UUIDs 不完整的16位服务UUID列表
0x03 Complete List of 16-bit Service UUIDs 完整的16位服务UUID列表
0x08 Shortened Local Name 缩短的设备名称
0x09 Complete Local Name 完整的设备名称
0xFF Manufacturer Specific Data 厂商自定义数据

Flags这个字段特别重要。它的每一位都有含义：

  • Bit 0：LE Limited Discoverable Mode
  • Bit 1：LE General Discoverable Mode
  • Bit 2：BR/EDR Not Supported
  • Bit 3：Simultaneous LE and BR/EDR to Same Device Capable (Controller)
  • Bit 4：Simultaneous LE and BR/EDR to Same Device Capable (Host)

最常见的Flags值是0x06，表示“LE通用可发现模式 + 不支持BR/EDR”。你抓到的绝大多数BLE设备，Flags都是这个值。

4.5 广播包整体结构图

说了这么多，咱们用一张图把广播包的完整结构串起来。下面这张SVG图，是我自己画的，把广播信道PDU、MAC地址、AD Structure都标清楚了。

BLE广播包结构图 BLE广播包（ADV_IND）完整结构 广播信道PDU（Protocol Data Unit） PDU类型（4bit） | 保留（2bit） | 信道选择（2bit） | 发送地址 | 接收地址 | 广播数据 广播数据（Advertising Data） 最大31字节，由多个AD Structure组成 AD Structure 1 Length: 0x02 Type: 0x01 (Flags) Data: 0x06 AD Structure 2 Length: 0x09 Type: 0x09 (Name) Data: "BLE_Device" AD Structure 3 Length: 0x05 Type: 0xFF (厂商) Data: 厂商自定义 注意：所有AD Structure的Length之和 + 每个Structure的1字节Type + 1字节Length ≤ 31字节 超出部分会被接收端丢弃，所以设计广播包时要精打细算

这张图把广播包的层次关系画得很清楚。从上到下依次是：PDU层 → 广播数据层 → 具体的AD Structure。每个AD Structure都是独立的，你可以根据需要组合。

4.6 实战：抓包分析一个真实的广播包

光说不练假把式。咱们拿一个真实的抓包数据来练练手。

假设你用Wireshark抓到了一个广播包，原始数据是这样的（十六进制）：

04 3E 2B 02 01 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00
        

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