Question

我正在开发一个实时应用程序，该应用程序要求每隔5ms通过WiFi直接连接从从电话到主机（组所有者）传输约512字节的数据。平均传输延迟大约为15ms，这不是很好，但是最大的问题是我注意到一种相当一致的周期模式，即每2到3秒大约有100ms的延迟，再加上每分钟大约9秒的周期，吞吐量大大降低。请查看图表scatter graph of delay over time。我还附加了我的套接字编程代码。主机和从机内部时钟都在1ms错误内同步（不必担心时钟漂移）。请让我知道您是否也经历了这些结果，以及在此问题上是否可以做些改进。有趣的是，通常会有0个数据丢失，并且当一个数据包被延迟之前，所有数据包也都被延迟了（从图中很容易看出），即使这是UDP。我读到某个地方，即使使用UDP，也需要在MAC层确认数据包，这可能会导致大约100毫秒的延迟。

以下是从站（发送方）上的代码：

public class DataTransmissionServiceV3 {

public static final String TAG = "DataTransmissionService";

private long mClockOffset;
private DatagramChannel mDatagramChannel;
private boolean running;

public void start(InetAddress remoteAddress, long clockOffset) {
    if (!running) {
        running = true;
        mClockOffset = clockOffset;
        Thread t = new Thread(new TransmissionTask(remoteAddress));
        t.setPriority(Thread.MAX_PRIORITY);
        t.start();
    }
}

public void close() {
    running = false;
    if (mDatagramChannel != null) {
        mDatagramChannel.socket().close();
    }
}

private class TransmissionTask implements Runnable {

    private int mCount;
    private final ByteBuffer mByteBuffer = ByteBuffer.allocateDirect(
            Constants.DATA_PACKET_SIZE);
    private final byte[] mBytes = new byte[Constants.DATA_PACKET_SIZE];
    private final InetAddress mRemoteAddress;

    public TransmissionTask(final InetAddress remoteAddress) {
        mRemoteAddress = remoteAddress;
    }

    @Override
    public void run() {
        try {
            mDatagramChannel = DatagramChannel.open();
            mDatagramChannel.socket().connect(new InetSocketAddress(mRemoteAddress,
                    Constants.TRANS_MASTER_SERVER_PORT));
            mDatagramChannel.configureBlocking(false);
        } catch (IOException ioe) {
            Log.e(TAG, "Exception while connecting", ioe);
        }
        while (running) {
            TimeUtils.busyWait(5000000); // busy wait for 5ms
            send();
        }
    }

    private void send() {
        mCount++;
        DataPacket.setSequenceNumber(mBytes, mCount);
        DataPacket.setTimestamp(mBytes, TimeUtils.getTimestamp() + mClockOffset); // synchronized timestamp to master device to measure delay
        mByteBuffer.clear();
        mByteBuffer.put(mBytes);
        mByteBuffer.flip();
        try {
            mDatagramChannel.write(mByteBuffer);
        } catch (Exception e) {
            Log.e(TAG, "Exception while sending data packet", e);
            running = false;
        }
    }
}

}

以下是主机（接收方）上的代码：

public class DataTransmissionServerV3 {

public static final String TAG = "DataTransmissionServer";

private DatagramChannel mDatagramChannel;
private boolean mRunning;
private Handler mHandler;

public DataTransmissionServerV3(Handler handler) {
    mHandler = handler;
}

public void start(InetAddress localAddress) {
    if (!mRunning) {
        mRunning = true;
        Thread t = new Thread(new ReceiveDataTask(localAddress));
        t.setPriority(Thread.MAX_PRIORITY);
        t.start();
    }
}

public void close() {
    mRunning = false;
    if (mDatagramChannel != null) {
        mDatagramChannel.socket().close();
    }
}

private class ReceiveDataTask implements Runnable {

    private final InetAddress mLocalAddress;
    private final ByteBuffer mByteBuffer = ByteBuffer.allocateDirect(
            Constants.DATA_PACKET_SIZE);
    private final byte[] mBytes = new byte[Constants.DATA_PACKET_SIZE];
    private int mCount;

    public ReceiveDataTask(final InetAddress localAddress) {
        mLocalAddress = localAddress;
    }

    @Override
    public void run() {
        try {
            bind();
            while (mRunning) {
                receive();
            }
        } catch (IOException ioe) {
            Log.e(TAG, "Exception while binding datagram socket", ioe);
        }
    }

    private void bind() throws IOException {
        mDatagramChannel = DatagramChannel.open();
        mDatagramChannel.socket().bind(new InetSocketAddress(mLocalAddress,
                Constants.TRANS_MASTER_SERVER_PORT));
        mDatagramChannel.configureBlocking(true);
    }

    private boolean receive() {
        mByteBuffer.clear();
        try {
            SocketAddress isa = mDatagramChannel.receive(mByteBuffer);
            long t2 = TimeUtils.getTimestamp();
            if (isa != null) {
                mByteBuffer.flip();
                mByteBuffer.get(mBytes);
                mCount++;
                TransmissionStat stat = TransmissionStat.get(mBytes, mCount, t2);
                handlePacket(stat); // a statistic that is saved to file for later analysis (ignore this)
                return true;
            }
        } catch (IOException ioe) {
            Log.e(TAG, "Exception while receiving data", ioe);
            mRunning = false;
        }
        return false;
    }

    private void handlePacket(TransmissionStat stat) {
        Message msg = mHandler.obtainMessage();
        msg.what = Constants.TRANSMISSION_PACKET_RECEIVED_CODE;
        msg.obj = stat;
        msg.sendToTarget();
    }
}

减少使用UDP传输协议的Android WiFi直接通信中的抖动

0 个答案: