private static void ShowNetworkTraffic()
{
    PerformanceCounterCategory performanceCounterCategory = new PerformanceCounterCategory("Network Interface");
    string instance = performanceCounterCategory.GetInstanceNames()[0]; // 1st NIC !
    PerformanceCounter performanceCounterSent = new PerformanceCounter("Network Interface", "Bytes Sent/sec", instance);
    PerformanceCounter performanceCounterReceived = new PerformanceCounter("Network Interface", "Bytes Received/sec", instance);

    for (int i = 0; i < 10; i++)
    {
        Console.WriteLine("bytes sent: {0}k\tbytes received: {1}k", performanceCounterSent.NextValue() / 1024, performanceCounterReceived.NextValue() / 1024);
        Thread.Sleep(500);
    }
}