ADSL (Asymmetric Digital Subscriber Line) is standardized such that the frequency bandwidth of regular telephony (below 4 KHz) on the access lines remains for telephony service. Broadband is transmitted on two other frequency bands; one of them is allocated to the low speed upstream channel (25 KHz to 138 KHz) and the other band is allocated to a high speed downstream channel (139 KHz to 1.1 MHz). The theoretical maximum bit rate of 8.1 Mbps is defined by the standard, but the bit rates, which can be achieved in practical implementations, depend on different parameters, for example, the distance between the household and the central, as the high frequency band of the copper line gets strongly attenuated as the distance increases. This distance dependency results in a situation whereby some households can simply not be reached by ADSL, even though they have access to PSTN infrastructures. Even in a country like Denmark, which has quite an advanced PSTN infrastructure, in mid-2004 about 5% of households could not be reached by any ADSL services and only 70% of the population could access a 2 Mbps connection.
As seen in the following, the new generations of DSL standards try to overcome these limitations and enable the DSL platform to be competitive in the future broadband market.