DOCSIS
DOCSIS stands for Data Over Cable Service Interface Specification.
Introduction
DOCSIS is a non-vendor specific standard, defining 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 (CATV) system. Is it employed by many cable television operators to provide internet access (through IP) over their existing hybrid cable/coaxial (HFC) infrastructure. The first DOCSIS specification was version 1.0. A version 1.1 revision followed, and as of current we have DOCSIS version 2.0. All related documentation, including who is certified and qualified, as well as the documents covering all technical aspects of the DOCSIS is located at www.cablemodem.com.
The European version of DOCSIS is called EuroDOCSIS. The main difference is that in Europe cable channels are 8MHz wide (PAL), whereas in North-America cable channels are 6MHz 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 certain different DOCSIS flavors in Japan.
Features
DOCSIS uses the TDMA/SCDMA access method. This is different from the Ethernet system, in that DOCSIS systems experiences no collisions.
In DOCSIS, there are many variables on the OSI Layer 1 and 2 that can be configured, apart from access methods. 64QAM up to 256QAM are used for modulation of the downstream, and QPSK and 16QAM 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, higher orders of modulation (64QAM) and wider channels (6.4MHz). DOCSIS 2.0 also introduces Ingress Cancelation, which greatly improves throughput in the US. All these combinations give the customer an aggregate speed of 30Mbps to 57Mbps (raw speed) per each downstream channel. The upstream speed in DOCSIS 1.0/1.1 is limited to 5Mbps.
DOCSIS supports advanced Quality of Service QoS (from 1.1 version), where per-applications traffic throttling and shaping can be performed, and introduces VoIP with jitter and delay control.
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 in opposite to Ethernet, to provide bi-directional communication we need at least two phisical ports - downstream and upstream. Because of the noise in return path, there are more upstream ports than downstream ports i.e. 1 DS and 4US. Historicaly untill DOCSIS 2.0 US port couldn't match the speed of DS ports, but main reason why there are more US ports per DS is the noise.
Before a cable company can run DOCSIS it has to upgrade it's HFC network to support a return path (for upstream traffic). Without it, the old DOCSIS 1.0 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 MAC table and the downstream speed.
Transfer Rate
Typically for home consumers the downstream is capped (restricted) at 512kbps to 3mbps, and the upstream is capped at 256kbps, through a configuration file downloaded to the cable modem when it first establishes a connection to the provider end.
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 30mbps) by uploading their own configuration file to the cable modem. However, the CTMS can override this configuration file and can log any changes made. Uncapping is almost universally in violation of the ToS agreement.
