A range of factors can affect the connection speed that subscribers actually achieve.
On the one hand, that speed can be influenced by the weakening of the signal on the xDSL networks. This attenuation is caused by the reduction in the strength of the signal between the subscriber and the DSL equipment in the exchange when the signal travels over the copper pair. The speed therefore depends on the distance between the subscriber and the DSL equipment.
This is only the case with VDSL. With fiber, the distance has no influence on the speed.
On the other hand, the achievable speed may vary over time due to overload. This is related to the effect of the shared use of the same bandwidth by different subscribers on the ISP’s network. The achievable speed can also be influenced by the management of the traffic and network capacity of the ISPs. As a result, there may be differences in performance between different platforms and between ISPs operating on the same platform.
Factors external to the networks of the ISPs can also affect the connection speed that is achievable for subscribers. We are thinking, for example, of the quality of the network within the home, the subscriber’s computer, the server to which the requests are sent, the quality of the subscriber’s wireless wifi network and the simultaneous sharing of the connection between different end devices and/or or applications. The ISPs have no direct control over these factors.
(Source: Decision of the BIPT Council of 4 December 2012 on Communication of the Speed of the Fixed Broadband Connection, p. 4)