DOCSIS
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DOCSIS stands for Data Over Cable Service Interface Specification.
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Introduction
DOCSIS is a standard developed by CableLabs which defines the communications and operation support interface requirements for a data over cable system. It permits the addition of high-speed data transfer to an existing cable TV (CATV) system. It is employed by many cable television operators to provide Internet access over their existing hybrid fibre coaxial (HFC) infrastructure. The first DOCSIS specification was version 1.0, issued in March of 1997, with revision 1.1 following in April of 1999. The DOCSIS standard is currently at version 2.0, released in January of 2002. All documentation, including lists of DOCSIS certified equipment, as well as the documents covering all technical aspects of the DOCSIS is located at www.cablemodem.com[1] (http://www.cablemodem.com).
The European version of DOCSIS is called EuroDOCSIS. The main difference is that in Europe cable channels are 8 MHz wide (PAL), whereas in North-America cable channels are 6 MHz wide (NTSC). This translates to permitting more bandwidth to be allocated to the downstream data path (taken from a user's point of view, downstream is used to download data, while upstream is used to upload data). There are also different DOCSIS flavors in Japan.
Features
DOCSIS uses the TDMA/SCDMA access method. This is different from the Ethernet system, in that DOCSIS systems experience few collisions.
In DOCSIS, there are many variations of the OSI Layers 1 and 2 that can be configured, apart from access methods. 64- or 256-level QAM is used for modulation of downstream data, and QPSK and 16-level QAM are used for the upstream modulation. Channel width can range from 400kHz to 3.2MHz in DOCSIS 1.0/1.1. DOCSIS 2.0 brings more options to the upstream, including higher orders of modulation (64-level QAM) and wider channels (6.4 MHz). DOCSIS 2.0 also introduces Ingress Cancellation, which greatly improves throughput. All of these improvements combine to deliver a total upstream throughput of 30.72 Mbit/s per channel. The upstream speed in DOCSIS 1.0 is limited to 5 Mbit/s, and 10 Mbit/s in DOCSIS 1.1. All three versions of the DOCSIS standard support a downstream throughput of up to 38 Mbit/s per channel with 256-level QAM. The EuroDOCSIS standard supports downstream throughput of up to 51 Mbit/s per channel (due to the 8MHz channel width).
Equipment
Typical Cable Modem Termination System (CMTS) (the cable company head-end equipment, equivalent to the DSLAM in DSL technology) is a device which hosts downstream and upstream ports. This means that, unlike Ethernet, to provide bi-directional communication we need at least two physical ports - downstream and upstream. Because of the noise in return path, there are more upstream ports than downstream ports. Until DOCSIS 2.0, upstream ports couldn't transfer data as fast as downstream ports, but the main reason why upstream ports outnumber downstream ports is line noise.
Before a cable company can run DOCSIS, it must upgrade its HFC network to support a return path for upstream traffic. Without it, the old DOCSIS 1.0 standard still allows use of data over cable system, by implementing the return path using the plain old telephone service (POTS). If the HFC is already 'two-way' or 'interactive', chances are high that DOCSIS can be implemented.
The customer PC and associated peripherals are termed Customer Premise Equipment (CPE). It's connected to the cable modem, which is in turn connected through the HFC network to the CMTS. The CMTS will then route traffic between the cable network and the Internet. Cable operators (sometimes called Multiple Service Operators - MSOs) have full control of the cable modem's configuration.
Transfer rate
Typically for home consumers the downstream is capped (restricted) through a configuration file downloaded to the cable modem via TFTP when it first establishes a connection to the provider end. Comcast, the largest cable provider in the United States, caps downstream bandwidth at 4 Mbit/s and upstream bandwidth at 384 kbit/s for standard home connections.
One downstream channel can handle up to 1000 cable modems. As the system grows, the CMTS can be upgraded with more downstream/upstream ports. If the HFC network is vast, the CMTS can be grouped into hubs for efficient management.
Some users have attempted to override the bandwidth cap and gain access to the full bandwidth of the system (often as much as 30 Mbit/s) by uploading their own configuration file to the cable modem, a process called uncapping. Uncapping is almost always a violation of the Terms of Service agreement and the law.
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