WiFi设计原理（源码分析）

最近一直在研究WiFi, 从AP层到Framework层， 后来同事介绍了一篇好文章， 如下（转载于http://stulog.com/?post=194），和大家共同分享下。

(添加红色部分应该能使大家更易理解）

 

 

 

初始化
在 SystemServer 启动的时候,会生成一个 ConnectivityService 的实例,
              try {
                    Log.i(TAG, "Starting Connectivity Service.");
                    ServiceManager.addService(Context.CONNECTIVITY_SERVICE, new
ConnectivityService(context));
              } catch (Throwable e) {
                    Log.e(TAG, "Failure starting Connectivity Service", e);
              }
ConnectivityService 的构造函数会创建 WifiService,
         case ConnectivityManager.TYPE_WIFI:

                if (DBG) Slog.v(TAG, "Starting Wifi Service.");                WifiStateTracker wst = new WifiStateTracker(context, mHandler);                WifiService wifiService = new WifiService(context, wst);                ServiceManager.addService(Context.WIFI_SERVICE, wifiService);                wifiService.startWifi();   //在boot的时候就会执行startWifi....                mNetTrackers[ConnectivityManager.TYPE_WIFI] = wst;                wst.startMonitoring();

WifiStateTracker 会创建 WifiMonitor 接收来自底层的事件,WifiService 和 WifiMonitor 是整
个模块的核心。

（1）WifiService 负责启动关闭 wpa_supplicant、启动关闭 WifiMonitor 监视线程
和把命令下发给 wpa_supplicant

 

在boot的时候会执行下面代码：

 

/**     * Check if Wi-Fi needs to be enabled and start     * if needed     *     * This function is used only at boot time     */    public void startWifi() {        /* Start if Wi-Fi is enabled or the saved state indicates Wi-Fi was on */        boolean wifiEnabled = !isAirplaneModeOn()                && (getPersistedWifiEnabled() || testAndClearWifiSavedState());        Slog.i(TAG, "WifiService starting up with Wi-Fi " +                (wifiEnabled ? "enabled" : "disabled"));        setWifiEnabled(wifiEnabled);    }

 

 

（2） WifiMonitor 负责从 wpa_supplicant 接收事件通知。

刚开始看不太懂这句话， 后来看源码，找到了下面这么句：

在WifiMonitor.java里有 for (;;) {

                String eventStr = WifiNative.waitForEvent();

............................

}

// 贴些WifiMonitor接收wpa_supplicant的log你就更容易明白了

I/wpa_supplicant( 1885): CTRL-EVENT-STATE-CHANGE id=0 state=5

V/WifiMonitor( 1721): Event [CTRL-EVENT-STATE-CHANGE id=0 state=5]I/wpa_supplicant( 1885): CTRL-EVENT-STATE-CHANGE id=0 state=5V/WifiMonitor( 1721): Event [CTRL-EVENT-STATE-CHANGE id=0 state=5]I/wpa_supplicant( 1885): CTRL-EVENT-STATE-CHANGE id=0 state=5V/WifiMonitor( 1721): Event [CTRL-EVENT-STATE-CHANGE id=0 state=5]I/wpa_supplicant( 1885): CTRL-EVENT-STATE-CHANGE id=0 state=6I/wpa_supplicant( 1885): CTRL-EVENT-STATE-CHANGE id=0 state=6I/wpa_supplicant( 1885): WPA: Key negotiation completed with 68:ef:bd:81:92:80 [PTK=TKIP GTK=TKIP]I/wpa_supplicant( 1885): CTRL-EVENT-STATE-CHANGE id=0 state=7I/wpa_supplicant( 1885): CTRL-EVENT-CONNECTED - Connection to 68:ef:bd:81:92:80 completed (auth) [id=0 id_str=]V/WifiMonitor( 1721): Event [CTRL-EVENT-STATE-CHANGE id=0 state=6]V/WifiMonitor( 1721): Event [CTRL-EVENT-STATE-CHANGE id=0 state=6]V/WifiMonitor( 1721): Event [WPA: Key negotiation completed with 68:ef:bd:81:92:80 [PTK=TKIP GTK=TKIP]]V/WifiMonitor( 1721): Event [CTRL-EVENT-STATE-CHANGE id=0 state=7]V/WifiMonitor( 1721): Event [CTRL-EVENT-CONNECTED - Connection to 68:ef:bd:81:92:80 completed (auth) [id=0 id_str=]]

连接 AP
1. 使能 WIFI
WirelessSettings 在初始化的时候配置了由 WifiEnabler 来处理 Wifi 按钮,
     private void initToggles() {
          mWifiEnabler = new WifiEnabler(
                       this,
                       (WifiManager) getSystemService(WIFI_SERVICE),
                       (CheckBoxPreference) findPreference(KEY_TOGGLE_WIFI));
当用户按下 Wifi 按钮后,               Android 会调用 WifiEnabler 的 onPreferenceChange,    再由 WifiEnabler
调用 WifiManager 的 setWifiEnabled 接口函数,通过 AIDL,实际调用的是 WifiService 的
setWifiEnabled 函数,WifiService 接着向自身发送一条 MESSAGE_ENABLE_WIFI 消息,在
处理该消息的代码中做真正的使能工作:首先装载 WIFI 内核模块(该模块的位置硬编码为
"/system/lib/modules/wlan.ko" ), 然 后 启 动 wpa_supplicant ( 配 置 文 件 硬 编 码 为
"/data/misc/wifi/wpa_supplicant.conf") 再通过 WifiStateTracker 来启动 WifiMonitor 中的监视
                                           ,
线程。
     private boolean setWifiEnabledBlocking(boolean enable) {
          final      int     eventualWifiState   =     enable    ?   WIFI_STATE_ENABLED     :
WIFI_STATE_DISABLED;
          updateWifiState(enable ? WIFI_STATE_ENABLING : WIFI_STATE_DISABLING);
          if (enable) {
                if (!WifiNative.loadDriver()) {
                       Log.e(TAG, "Failed to load Wi-Fi driver.");
                       updateWifiState(WIFI_STATE_UNKNOWN);
                       return false;
                }
                if (!WifiNative.startSupplicant()) {
                       WifiNative.unloadDriver();
                       Log.e(TAG, "Failed to start supplicant daemon.");
                       updateWifiState(WIFI_STATE_UNKNOWN);
                       return false;
                }
                mWifiStateTracker.startEventLoop();
          }
             // Success!
             persistWifiEnabled(enable);
             updateWifiState(eventualWifiState);
             return true;
      }
当使能成功后,会广播发送 WIFI_STATE_CHANGED_ACTION 这个 Intent 通知外界 WIFI
已 经 成 功 使 能 了 。 WifiEnabler 创 建 的 时 候 就 会 向 Android 注 册 接 收
WIFI_STATE_CHANGED_ACTION,因此它会收到该 Intent,从而开始扫描。
          private void handleWifiStateChanged(int wifiState) {
             if (wifiState == WIFI_STATE_ENABLED) {
                  loadConfiguredAccessPoints();
                  attemptScan();
             }
2. 查找 AP
扫描的入口函数是 WifiService 的 startScan,它其实也就是往 wpa_supplicant 发送 SCAN 命
令。
static jboolean android_net_wifi_scanCommand(JNIEnv* env, jobject clazz)
{
      jboolean result;
      // Ignore any error from setting the scan mode.
      // The scan will still work.
      (void)doBooleanCommand("DRIVER SCAN-ACTIVE", "OK");
      result = doBooleanCommand("SCAN", "OK");
      (void)doBooleanCommand("DRIVER SCAN-PASSIVE", "OK");
      return result;
}
当 wpa_supplicant 处理完 SCAN 命令后,它会向控制通道发送事件通知扫描完成,从而
wifi_wait_for_event 函数会接收到该事件,由此 WifiMonitor 中的 MonitorThread 会被执行来
出来这个事件,
             void handleEvent(int event, String remainder) {
                        case SCAN_RESULTS:
                             mWifiStateTracker.notifyScanResultsAvailable();
                             break;
WifiStateTracker 则接着广播发送 SCAN_RESULTS_AVAILABLE_ACTION 这个 Intent
                  case EVENT_SCAN_RESULTS_AVAILABLE:
                        mContext.sendBroadcast(new
Intent(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
WifiLayer 注册了接收 SCAN_RESULTS_AVAILABLE_ACTION 这个 Intent,所以它的相关
处理函数 handleScanResultsAvailable 会被调用,在该函数中,先会去拿到 SCAN 的结果(最
终是往 wpa_supplicant 发送 SCAN_RESULT 命令并读取返回值来实现的)                             ,
                List<ScanResult> list = mWifiManager.getScanResults();
对每一个扫描返回的 AP,WifiLayer 会调用 WifiSettings 的 onAccessPointSetChanged 函数,
从而最终把该 AP 加到 GUI 显示列表中。
     public void onAccessPointSetChanged(AccessPointState ap, boolean added) {
          AccessPointPreference pref = mAps.get(ap);
          if (added) {
                if (pref == null) {
                      pref = new AccessPointPreference(this, ap);
                      mAps.put(ap, pref);
                } else {
                      pref.setEnabled(true);
                }
                mApCategory.addPreference(pref);
          }
     }
3. 配置 AP 参数
当用户在 WifiSettings 界面上选择了一个 AP 后,会显示配置 AP 参数的一个对话框,
     public boolean onPreferenceTreeClick(PreferenceScreen preferenceScreen, Preference
preference) {
          if (preference instanceof AccessPointPreference) {
                AccessPointState             state           =         ((AccessPointPreference)
preference).getAccessPointState();
                showAccessPointDialog(state, AccessPointDialog.MODE_INFO);
          }
     }
4. 连接
当用户在 AcessPointDialog 中选择好加密方式和输入密钥之后,再点击连接按钮,Android
就会去连接这个 AP。
     private void handleConnect() {
          String password = getEnteredPassword();
          if (!TextUtils.isEmpty(password)) {
                mState.setPassword(password);
          }
          mWifiLayer.connectToNetwork(mState);
     }
WifiLayer 会先检测这个 AP 是不是之前被配置过,这个是通过向 wpa_supplicant 发送
LIST_NETWORK 命令并且比较返回值来实现的,
          // Need WifiConfiguration for the AP
          WifiConfiguration config = findConfiguredNetwork(state);
如果 wpa_supplicant 没有这个 AP 的配置信息,                     则会向 wpa_supplicant 发送 ADD_NETWORK
命令来添加该 AP,
          if (config == null) {
                // Connecting for the first time, need to create it
                config                             =                     addConfiguration(state,
ADD_CONFIGURATION_ENABLE|ADD_CONFIGURATION_SAVE);
          }
ADD_NETWORK 命 令 会 返 回 一 个 ID , WifiLayer 再 用 这 个 返 回 的 ID 作 为 参 数 向
wpa_supplicant 发送 ENABLE_NETWORK 命令,从而让 wpa_supplicant 去连接该 AP。
          // Make sure that network is enabled, and disable others
          mReenableApsOnNetworkStateChange = true;
          if (!mWifiManager.enableNetwork(state.networkId, true)) {
                Log.e(TAG, "Could not enable network ID " + state.networkId);
                error(R.string.error_connecting);
                return false;
          }
5. 配置 IP 地址
当 wpa_supplicant 成功连接上 AP 之后,它会向控制通道发送事件通知连接上 AP 了,从而
wifi_wait_for_event 函数会接收到该事件,由此 WifiMonitor 中的 MonitorThread 会被执行来
出来这个事件,
          void handleEvent(int event, String remainder) {
                     case CONNECTED:
                           handleNetworkStateChange(NetworkInfo.DetailedState.CONNECTED,
remainder);
                           break;
WifiMonitor 再调用 WifiStateTracker 的 notifyStateChange,WifiStateTracker 则接着会往自身
发送 EVENT_DHCP_START 消息来启动 DHCP 去获取 IP 地址,
     private void handleConnectedState() {
          setPollTimer();
          mLastSignalLevel = -1;
          if (!mHaveIPAddress && !mObtainingIPAddress) {
                mObtainingIPAddress = true;
                mDhcpTarget.obtainMessage(EVENT_DHCP_START).sendToTarget();
          }
     }
然后再广播发送 NETWORK_STATE_CHANGED_ACTION 这个 Intent
                case EVENT_NETWORK_STATE_CHANGED:
                     if (result.state != DetailedState.DISCONNECTED || !mDisconnectPending) {
                           intent                                 =                        new
Intent(WifiManager.NETWORK_STATE_CHANGED_ACTION);
                           intent.putExtra(WifiManager.EXTRA_NETWORK_INFO,
mNetworkInfo);
                          if (result.BSSID != null)
                                intent.putExtra(WifiManager.EXTRA_BSSID, result.BSSID);
                          mContext.sendStickyBroadcast(intent);
                     }
                     break;
WifiLayer 注册了接收 NETWORK_STATE_CHANGED_ACTION 这个 Intent,所以它的相关
处理函数 handleNetworkStateChanged 会被调用,
当 DHCP 拿到 IP 地址之后,会再发送 EVENT_DHCP_SUCCEEDED 消息,
     private class DhcpHandler extends Handler {
          public void handleMessage(Message msg) {
                switch (msg.what) {
                     case EVENT_DHCP_START:
                          if (NetworkUtils.runDhcp(mInterfaceName, mDhcpInfo)) {
                                event = EVENT_DHCP_SUCCEEDED;
                                                      }
WifiLayer 处 理 EVENT_DHCP_SUCCEEDED 消 息 , 会 再 次 广 播 发 送
NETWORK_STATE_CHANGED_ACTION 这个 Intent,这次带上完整的 IP 地址信息。
                case EVENT_DHCP_SUCCEEDED:
                     mWifiInfo.setIpAddress(mDhcpInfo.ipAddress);
                     setDetailedState(DetailedState.CONNECTED);
                     intent                                 =                           new
Intent(WifiManager.NETWORK_STATE_CHANGED_ACTION);
                     intent.putExtra(WifiManager.EXTRA_NETWORK_INFO, mNetworkInfo);
                     mContext.sendStickyBroadcast(intent);
                     break;
至此为止,整个连接过程完成。
问题:
     目前的实现不支持 Ad-hoc 方式。