Digital Subscriber Line - DSL
Digital Subscriber Line is a family of technologies of broadband Internet access. Typical download speeds range from 128 kbps to 50 Mbps, depending on DSL technology used in the country. For ADSL upload speed is lower than the speed of retrieval, but for SDL technology these speeds are symmetrical. Local connection of the Public Switched Telephone Network was designed for voice communication and signalling, as the oldest, basic telephony services of the Plain Old Telephone Services. The concept of data was not then known. For economic reasons, the telephone system sends speech signal in limited frequency range between 300 and 3400 Hz - this frequency band is sufficient for good understanding of human speech. Services that use modems connected to the telephone network are limited by the bandwidth of telephone channel. The voice is sampled with 8000 Hz frequency, coded in the form of 8-bit signal and forwarded as 64 kbps data stream. According to the Nyquist theorem, this allows for the transmission of the signal with maximum frequency at 4000 Hz. Higher frequencies are not passed through the telephone network (are filtered out) because the sound does not carry at these frequencies any useful information about the speech. The transmition medium (copper cable) connecting most subscribers to the telephone exchange is capable to transmit frequencies higher than the rate of 3.4 kHz, this is the limit of POTS. This limit may be higher depending on the distance and connection quality. DSL uses higher unused frequencies by creating channels with width of 4312.5 Hz each, starting between 10 and 100 kHz, depending on the system configuration. Allocation of these channels continues at higher and higher frequencies (for ADSL up to 1.1 MHz) until the new channels will be considered unfit for use. Each channel is extended to make it suitable for use on more than one route, as is the case with POTS.