Digital subscriber line

Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access. DSL service can be delivered simultaneously with wired telephone service on the same telephone line. This is possible because DSL uses higher frequency bands for data. On the customer premises, a DSL filter on each non-DSL outlet blocks any high-frequency interference to enable simultaneous use of the voice and DSL services.

The bit rate of consumer DSL services typically ranges from 256 kbit/s to over 100 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. Bit rates of 1 Gbit/s have been reached in trials, but most homes are likely to be limited to 500-800 Mbit/s. In ADSL, the data throughput in the upstream direction (the direction to the service provider) is lower, hence the designation of asymmetric service. In symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal. Researchers at Bell Labs have reached speeds of 10 Gbit/s, while delivering 1 Gbit/s symmetrical broadband access services using traditional copper telephone lines. These higher speeds are lab results, however. A 2012 survey found that "DSL continues to be the dominant technology for broadband access" with 364.1 million subscribers worldwide.

Data rate units

In telecommunications, data transfer rate is the average number of bits (bitrate), characters or symbols (baudrate), or blocks per unit time passing between equipment in a data transmission system. Most commonly, measurements of data transfer rate are reported in multiples of unit bits per second (bit/s) or occasionally in bytes per second (B/s). The data rates of modern residential high-speed Internet connections are most commonly expressed in multiples of bits per second, such as megabits per second (Mbit/s) or kilobits per second (kbit/s).

Standards for unit symbols and prefixes

Unit symbol

The ISQ symbols for the bit and byte are bit and B, respectively. In the context of data rate units, one byte consists of 8 bits, and is synonymous with the unit octet. The abbreviation bps is often used to mean bit/s, so that when a 1 Mbps connection is advertised, it usually means that the maximum achievable bandwidth is 1 Mbit/s (one million bits per second), which is 0.125 MB/s (megabyte per second), or about 0.1192 MiB/s (mebibyte per second). The Institute of Electrical and Electronics Engineers (IEEE) uses the symbol b for bit.