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1. How accurate is the measurement conducted using the NDT?

The NDT allows broadband subscribers to accurately measure the qualitative features of their connections and presents them in an impartial manner. It should, however, be noted that:

  • Any measurement may be affected by various external factors (e.g. wired/wireless interface to the router, cross-traffic from other applications, memory and processing capabilities of user’s computer, or structured cabling in the house).
  • The user is wholly liable when using the tools.
  • ΕΕΤΤ is not liable for any problems or faults which may arise during usage of the tools.

To ensure a reliable measurement it is essential that (a) no other application is “running” on the user’s computer or local network, which would generate network traffic and (b) there should be no form of temporary network problem on the path to the server.

It should be noted that if other computers are also generating traffic on the user’s local network, this may negatively affect the accuracy of a measurement. The user in any case is advised to ensure that the network connection is as inactive as possible before proceeding to measure.

2. Can a computer using a WiFi connection be used for the measurement?

Using WiFi may affect the accuracy of the measurement. For the most widely used protocol, 802.11g, depending on the quality of the signal, the available WiFi bandwidth varies from 6 to 54 Mbps, and the wireless part of the connection operates in half-duplex mode. Therefore, if there are other devices connecting to the WiFi router (e.g. a smartphone), or the signal is weak, the WiFi speed could be lower than the speed of the wired interface, and as a result the overall speed would be limited by the WiFi speed. This rarely occurs in the case of a wired connection, where the bandwidth is always 100Mbps full duplex (if not more). It is therefore advisable to measure from a computer connected by cable to the domestic router.

3. What factors negatively affect the quality of a measurement?

Firewalls, heavy use of the computer’s central server and even faults on the computer network cable, may affect measurement quality.

4. Does the measurement include the overhead from TCP headers, IP headers etc.?

No. The measurement uses TCP/IP as underlying protocols, but does not include this overhead in the throughput calculation. This results in lower speed, but is closer to the throughput an application would see.

5. What is the range of variation for the overhead of a broadband connection?

It isn’t the same in all cases. It varies according to the WAN protocol used (e.g. PPP or HDLC), as well as application overhead, overhead for encryption, etc.

6. Let’s suppose, for example, that a user has bought a package with 24 Mbps downlink speed. What result would his measurement be expected to produce in the downlink?

The user will always get less than the nominal speed he/she purchased as the nominal speed is only a theoretical limit at L2 (Data link layer). The actual speed of connection to the Internet depends on many factors, such as the distance from the local exchange or cabinet, noise levels, the degree of congestion in the provider’s network, as well as the internal network at the subscriber’s premises (wired/wireless connection to the router, existence of other devices in the same local network, cross-traffic from other applications).

Interested users can visit for more details on the variation of the downlink connection speed depending on the distance from the exchange, for an ADSL line.

Nevertheless, it is very difficult to properly account for all factors that affect Internet speed and predict the expected speed without conducting any measurements.

7. What protocol does the NDT use for its measurements?

The NDT employs the ndt7 protocol over TCP. For detailes, please, refer to the ndt7 protocol specification.

8. How are Round Trip Time (RTT), packet loss and jitter calculated?

By analyzing the measurement packets (data and acknowledgement) sent during the client user computer-server communication. The user sees the average values based on all the exchanged packets. The difference between the two extreme delay values constitutes the delay variation (jitter), whilst packets that were lost and had to be retransmitted are included in the percentage loss.