Android7.0 Framework telephony中数据业务链接错误处理一般分3种情况：
1. SETUP_DATA_CALL 时返回错误
2. Modem上报DATA_CALL_LIST包含错误码或者链接中断
3. 一段时间内没有上下行数据（TX/RX）

下面以第一种情况为例分析:
通过DcTracker.setDataEnabled-->dcac.bringUp-->DataConnection.DcInactiveState.onConnect-->RILJ向MODEM发送SETUP_DATA_CALL的信令后,
由MODEM返回结果,逐层返回DcTracker中的case DctConstants.EVENT_DATA_SETUP_COMPLETE:
            case DctConstants.EVENT_DATA_SETUP_COMPLETE:
                onDataSetupComplete((AsyncResult) msg.obj);


    /**
     * A SETUP (aka bringUp) has completed, possibly with an error. If
     * there is an error this method will call {@link #onDataSetupCompleteError}.
     */
    private void onDataSetupComplete(AsyncResult ar) {

        DcFailCause cause = DcFailCause.UNKNOWN;
        boolean handleError = false;
        ApnContext apnContext = getValidApnContext(ar, "onDataSetupComplete");

        if (apnContext == null) return;

        if (ar.exception == null) {
    //链接成功
    ...
        } else {
            cause = (DcFailCause) (ar.result);
            if (DBG) {
                ApnSetting apn = apnContext.getApnSetting();
                log(String.format("onDataSetupComplete: error apn=%s cause=%s",
                        (apn == null ? "unknown" : apn.apn), cause));
            }
            if (cause.isEventLoggable()) {
                // Log this failure to the Event Logs.
                int cid = getCellLocationId();
                EventLog.writeEvent(EventLogTags.PDP_SETUP_FAIL,
                        cause.ordinal(), cid, TelephonyManager.getDefault().getNetworkType());
            }
            ApnSetting apn = apnContext.getApnSetting();
            mPhone.notifyPreciseDataConnectionFailed(apnContext.getReason(),
                    apnContext.getApnType(), apn != null ? apn.apn : "unknown", cause.toString());

            // Compose broadcast intent send to the specific carrier signaling receivers
            Intent intent = new Intent(TelephonyIntents
                    .ACTION_CARRIER_SIGNAL_REQUEST_NETWORK_FAILED);
            intent.putExtra(TelephonyIntents.EXTRA_ERROR_CODE_KEY, cause.getErrorCode());
            intent.putExtra(TelephonyIntents.EXTRA_APN_TYPE_KEY, apnContext.getApnType());
            mPhone.getCarrierSignalAgent().notifyCarrierSignalReceivers(intent);

            if (cause.isRestartRadioFail() || apnContext.restartOnError(cause.getErrorCode())) {
                if (DBG) log("Modem restarted.");
                sendRestartRadio();
            }

            // If the data call failure cause is a permanent failure, we mark the APN as permanent
            // failed.
            if (isPermanentFail(cause)) {
                log("cause = " + cause + ", mark apn as permanent failed. apn = " + apn);
                apnContext.markApnPermanentFailed(apn);
            }

            handleError = true;
        }

        if (handleError) {
       //继续处理链接错误,有条件的重试
            onDataSetupCompleteError(ar);
        }

        /* If flag is set to false after SETUP_DATA_CALL is invoked, we need
         * to clean data connections.
         */
        if (!mDataEnabledSettings.isInternalDataEnabled()) {
            cleanUpAllConnections(Phone.REASON_DATA_DISABLED);
        }

    }




    /**
     * Error has occurred during the SETUP {aka bringUP} request and the DCT
     * should either try the next waiting APN or start over from the
     * beginning if the list is empty. Between each SETUP request there will
     * be a delay defined by {@link #getApnDelay()}.
     */
    private void onDataSetupCompleteError(AsyncResult ar) {

        ApnContext apnContext = getValidApnContext(ar, "onDataSetupCompleteError");

        if (apnContext == null) return;

        long delay = apnContext.getDelayForNextApn(mFailFast);

        // Check if we need to retry or not.
        if (delay >= 0) {
            if (DBG) log("onDataSetupCompleteError: Try next APN. delay = " + delay);
            apnContext.setState(DctConstants.State.SCANNING);
            // Wait a bit before trying the next APN, so that
            // we're not tying up the RIL command channel

        //在delay时间后发起重试链接
            startAlarmForReconnect(delay, apnContext);
        } else {
            // If we are not going to retry any APN, set this APN context to failed state.
            // This would be the final state of a data connection.
            apnContext.setState(DctConstants.State.FAILED);
            mPhone.notifyDataConnection(Phone.REASON_APN_FAILED, apnContext.getApnType());
            apnContext.setDataConnectionAc(null);
            log("onDataSetupCompleteError: Stop retrying APNs.");
        }
    }




    /**
     * IDLE: ready to start data connection setup, default state
     * CONNECTING: state of issued startPppd() but not finish yet
     * SCANNING: data connection fails with one apn but other apns are available
     *           ready to start data connection on other apns (before INITING)
     * CONNECTED: IP connection is setup
     * DISCONNECTING: Connection.disconnect() has been called, but PDP
     *                context is not yet deactivated
     * FAILED: data connection fail for all apns settings
     * RETRYING: data connection failed but we're going to retry.
     *
     * getDataConnectionState() maps State to DataState
     *      FAILED or IDLE : DISCONNECTED
     *      RETRYING or CONNECTING or SCANNING: CONNECTING
     *      CONNECTED : CONNECTED or DISCONNECTING
     */  
    public enum State {  
        IDLE,  
        CONNECTING,  
        SCANNING,  
        CONNECTED,  
        DISCONNECTING,  
        FAILED,  
        RETRYING  
    }