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Data over cable service interface specification (DOCSIS) is an internationally recognized standard allowing high speed data transfer on existing cable TV systems (CATVSs) used by many cable operators to provide Internet access to their customers through a cable modem. The latest version of the standard also supports high definition televisions (HDTVs).
The standard was approved by the International Telecommunication Union (ITU) in 1998.
The first version of DOCSIS was issued in March 1997. Later it was enhanced in response to increased demand for symmetric services such as IP telephony. Version 2.0 was released in December 2001. In August 2006 version 3.0 was released with significantly increased transmission speeds, both upstream and downstream. This version also included support for Internet protocol version 6 (IPv6). Cross-version compatibility has been maintained. However, the transmission speed will only be at the speed of the oldest version. As of late 2010, the fastest downloads were 120 Mbit/s (and 20 Mbit/s upload) in Canada, with the next fastest 107 Mbits/s in the US.
All cable modems today are in compliance with DOCSIS, which exists in two versions named DOCSIS for the USA and EuroDOCSIS for Europe. These were required because of the differences between the two systems: the European PAL (phase alternating line), which is an analog color television encoding system; and the USA’s analog NTSC (National Television System Committee), which is the first widely adopted broadcast color system. In June 2009, NTSC switched to the digital ATSC (Advanced Television Systems Committee) in the USA. Each operates on different radio frequency (RF) channels, 6 MHz in the USA and 8 MHz for Europe. Except for some systems in Japan, nearly all today’s cable modem systems use a version of DOCSIS.
Using layers 1 and 2 of the Open Systems Interconnection (OSI) layers, DOCSIS enables upstream transmission speeds of 30.72 Mbits/s per 6 MHz channel and 10.24 Mbits/s per 3 MHz channel. All three versions (USA, Europe and Japan) enable downstream speeds of up to 42.88 Mbit/s per 6 MHz channel (USA) or 55.62 Mbit/s per 8 MHz channel (Europe). However, with hardware to support the minimum number of four coupled channels, DOCSIS 3.0 has maximum possible downstream/upstream speeds of 160/120 Mbit/s.