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The International Telecommunications Union Telecommunications Standardization Sector (ITU-T) has adopted two DOCSIS variants as international standards. DOCSIS 1.1 was ratified as ITU-T Recommendation J.112; subsequently, DOCSIS 2.0 was ratifed as ITU-T Recommendation J.122. DOCSIS 2.0/J.122 is backwards compatible with DOCSIS 1.1/J.112. As frequency allocation band plans differ between US and European CATV systems, DOCSIS standards have been modified for use in Europe; these are published under the name of "EuroDOCSIS". The main differences account for differing TV channel bandwidths; European cable channels conform to PAL TV standards and are 8 MHz wide, whereas in North-America cable channels conform to NTSC standards which specify 6 MHz. The wider bandwidth in EuroDOCSIS architectures permits 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). Japan employs other variants of DOCSIS. All DOCSIS technical documentation, to include lists of DOCSIS-certified equipment, is available at http://www.cablemodem.com. FEATURES DOCSIS provides great variety in options available at OSI layers 1 and 2, the Physical (PHY) and Media Access Control (MAC) layers.
Throughput: All of these features combined enable a total upstream throughput of 30.72 Mbit/s per channel (although the upstream speed in DOCSIS 1.0 and 1.1 is limited to 10 Mbit/s). All three versions of the DOCSIS standard support a downstream throughput of up to 38 Mbit/s per channel with 256-QAM (owing to 8 Mhz channel width, the EuroDOCSIS standard supports downstream throughput of up to 51 Mbit/s per channel). DOCSIS 3.0 Currently under development, DOCSIS 3.0 is expected to feature "channel bonding", which enables multiple downstream and upstream channels to be used together at the same time by a single subscriber. {Link without Title} EQUIPMENT A DOCSIS architecture includes two primary components: a cable modem (CM) located at the customer premises, and a Cable Modem Termination System (CMTS) located at the CATV headend. A typical CMTS is a device which hosts downstream and upstream ports (it is functionally similar to the DSLAM used in DSL systems). For duplex communication between CMTS and CM two physical ports are required (unlike Ethernet, where one port provides duplex communications). Because of the noise in the return (upstream) path, there are more upstream ports than downstream ports - the additional upstream ports provide ways of compensating for noisy lines (until DOCSIS 2.0, they were required to provide higher upstream speeds as well). Before a cable company can deploy DOCSIS 1.1 or above, it must upgrade its HFC network to support a return path for upstream traffic. Without a return path, the old DOCSIS 1.0 standard still allows use of data over cable system, by implementing the return path over regular phone lines, e.g. " Plain Old Telephone Service " (POTS). If the HFC is already 'two-way' or 'interactive', chances are high that DOCSIS 1.1 or higher can be implemented. The customer PC and associated peripherals are termed Customer Premise Equipment (CPE). The CPE are connected to the cable modem, which is in turn connected through the HFC network to the CMTS . The CMTS then routes traffic between the HFC and the Internet. Using the CMTS, the cable operator (or Multiple Service Operators - MSO) exercises full control over the cable modem's configuration; the CM configuration is changed to adjust for varying line conditions and customer service requirements. TRANSFER RATE Most DOCSIS cable modems have caps (restrictions) on upload and download rates. These are set by transferring a configuration file to the modem via TFTP when the modem first establishes a connection to the provider's equipment. As an example, Comcast , the largest cable provider in the United States , caps downstream bandwidth at 6 Mbit/s and upstream bandwidth at 384 Kbit/s (48 kB/s) for standard home connections. In some areas they are offering 8 Mbit/s downstream and 768 kbit/s (96 kB/s) upstream as a more expensive higher tier. These differing speed offerings are made possible by loading a different configuration file into the modem in different pricing tiers and regions. Another major cable provider, Cox Communications recently upgraded their base package to 6 MBit/s downstream and 512 kbit/s (64 kB/s) upstream in select markets, from their previous upgrade of 4MBit/s downstream and 512Kbit/s upstream. Their premier package allows downstream rates of 9 Mbit/s and upstream rates of 1 Mbit/s. In select markets (such as Northern Virginia ) they have increased these speeds further still, allowing 5 Mbit/s downstream and 2 Mbit/s upstream on their base package and 15 Mbit/s downstream and 2 Mbit/s upstream for the premier package. Cablevision , serving New Jersey, New York City, and Long Island, currently offers the fastest 'basic' cable internet service ( Optimum Online ) in the US with 15 Mbit/s Downstream and 2 Mbit/s upstream. A premium package offering 30 Mbit/s downstream and 2 Mbit/s upstream is also available, and a premier package offering symmetrical 50 Mbit/s downstream and 50 Mbit/s upstream (using a proprietary non-DOCSIS technology from Narad Networks) is currently in beta testing and is slated for release in 2006. The operator , locally known as UPC ) in Austria , Belgium , The Czech Republic , France , Hungary , Ireland , The Netherlands , Norway , Poland , Slovakia and Sweden offers the service '' Chello '' in ranges up to 36 Mbit/s downstream and 18 Mbit/s upstream. In New Zealand operator TelstraClear provides downstream speeds of 10Mbit/s and 2Mbit/s with an upstream speed of 2Mbit/s. One downstream channel can handle hundreds of 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. SEE ALSO EXTERNAL LINKS
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