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About the Network Diagnostic Tool (NDT)

With the Network Diagnostic Tool (NDT), the user measures the performance of his connection and, more specifically, receives statistical data relating to its most important quality features, such as available bandwidth to and from the computer and packet Round Trip Time (RTT) as well as jitter and percentage of packet loss.

These features may be assessed in a computer connected to the network using a client-programme (usually a java applet which runs through a web browser), which communicates with a specially designed server located at a central point of the network and to which a high capacity network connection is available. By analysing its communication with the user, the server is usually able to establish an accurate assessment of the afore-mentioned parameters, which is also provided to the user.

Using the NDT

When the NDT’s java applet appears, the user simply has to click on Start measurement to conduct a standard set of measurements. The measurement data [download and upload speed, Round Trip Time (RTT), packet loss, jitter] is gradually entered over the course of the measurement process. A typical process may last from 30 seconds to 1 minute. When the measurement is completed the message Measurement completed appears in the completed progress bar and the data has been entered.

Qualitative features of the connection

  • Download and upload speed. These measurements provide an accurate (as possible) assessment of available bandwidth in the computer where the measurement is being conducted. It should be noted that for these values as well as the others described here, the measurement is based on user-server communication. The assessment therefore relates to the entire “path” between them, not only the user’s physical connection to the network.

    Fast speeds usually mean high speed downloads for the user. Since measurement is conducted using the TCP protocol, other factors which affect this specific protocol also tend to be visible from this measurement. In most cases, the slowest connection on a path determines the maximum speed that can be achieved.
  • Round Τrip Τime (RTT). Defined as the time needed for a standard packet to go from user to server, as well as the time taken for a response to the packet to arrive back at the user. Clearly, since the measurement comprises both directions, it does not matter who sends the packet and who responds.

    The RTT is a great indicator of network quality, the significance of which is similar to that of upload/download speed. Lengthy RTTs may mean greater delays in interactive on-line games or other applications sensitive to this feature, such as telephony programmes (VoIP, Skype etc.). In order to calculate RTT as accurately as possible, dozens of uploads/responses are sent in both directions, from which a representative average is subsequently taken.
  • Packet Loss. In the vast majority of cases, packet loss means network congestion at some point of the path from user to server. It should be noted that even values of around 1% are an indication of a serious network malfunction.
  • Jitter. This feature shows the range of values around which RTT (see above) fluctuates. For example, a variation of around 3 milliseconds indicates the gap between the highest and the lowest value for packet upload/response time recorded during the measurement process. Jitter may often reveal possible network connection problems. However it should be taken into account that it is based on the extreme values from a large number of measurements and is consequently not always a reliable qualitative criterion.


The original version of the NDT’s entire graphical user interface can be called up in Greek by clicking on the Details option which leads to a new page. This page can be accessed at any time (even during measurement).

Measurement progress can be tracked in the Details window as one stage succeeds another. The tabs in the Details window have the following functions:

  • Statistics. Opens a new window entitled Detailed Statistics, where all the features of the connection for which the measurement was conducted are described in detail. This function provides the user with access to detailed features, such as whether the computer carries out TCP Window Scaling (and to what extent) or Selective Acknowledgement, if it is behind a router which changes the TCP MSS, the size of packet which can pass through without fragmentation.
  • More details. Activates the web100 variables window.

Possible problems and solutions

  • Window entitled Measurement Error. If the measurement process fails to complete, in most cases a window appears, informing the user of the nature of the error. In the example below, the server is temporarily down and the user is informed accordingly.

  • The measurement proceeds from 'start…' and after 2 minutes the message 'Server is busy. Please wait…' appears. In certain cases, other users may be occupying the server. It is recommended that a new measurement attempt be made after a few minutes.

  • The measurement continuously gets stuck on 'Step 1 of 4: Analysis of intermediate nodes'.

    In rare cases the measurement may not be completed. Reloading the page (Ctrl-R or F5) and trying again after a few minutes is recommended.