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Data Over Cable Service Interface Specification (DOCSIS) is an international standard developed by CableLabs and contributing companies that include: ARRIS, Broadcom, Cisco, Conexant, Correlant, Intel, Motorola, Terayon, and Texas Instruments. DOCSIS 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. Owing to increased demand for symmetric, real-time services such as IP telephony, DOCSIS was again revised to enhance upstream transmission speeds and Quality of Service capabilities; this revision - DOCSIS 2.0 - was released in January 2002.
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 changes were 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.
- Physical layer:
- Channel Width: DOCSIS 1.0/1.1 specified channel widths between 200 kHz and 3.2 MHz. DOCSIS 2.0 specifies 6.4 MHz, but is backward compatible to the earlier, narrower channel widths.
- Modulation: DOCSIS 1.0/1.1/2.0 specifies that 64-level or 256-level QAM (64-QAM or 256-QAM) be used for modulation of downstream data, and QPSK or 16-level QAM (16-QAM) be used for upstream modulation. DOCSIS 2.0 specifies 32-QAM, 64-QAM and 128-QAM also be available for upstream use.
- MAC layer: DOCSIS employs deterministic access methods, specifically TDMA for DOCSIS 1.0/1.1 and both TDMA and S-CDMA for DOCSIS 2.0. In contrast to the contention-based MAC CSMA/CD employed in Ethernet systems, DOCSIS systems experience few collisions.
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).
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. *
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 LGI (http://lgi.com/) (formerly UGC, 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.
Canadian cable operators offer varying levels of service based on different price points, though the actual marketing terms vary. Services offered by the major Canadian providers is included below.
| Service Provider | Basic | Standard | Pro |
|---|---|---|---|
| Rogers Cable | 1.0M down, 128k up | 5.0Mbit down, 384k up | 5.0M down, 800kbit up |
| Videotron | 600k down, 128k up | 5.1M down, 810k up | 10.0M down, 900k up |
| Shaw | 256k down | 5.0M down | 7.0M down, 1.0M up |
| Cogeco | 640k down, 128k up | 10.0M down, 600kbit up | 15.0M down, 1.0M up |
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.
Security
DOCSIS includes MAC layer security services in its Baseline Privacy Interface specifications. DOCSIS 1.0 utilized the initial Baseline Privacy Interface (BPI) specification. BPI was later improved with the release of the Baseline Privacy Interface Plus (BPI+) specification.
The intent of the BPI specifications is to describe MAC layer security services for DOCSIS CMTS to CM communications. BPI security goals are twofold:
- provide cable modem users with data privacy across the cable network
- provide cable service operators with service protection; i.e., prevent unauthorized users from gaining access to the network’s RF MAC services
BPI is intended to provide a level of data privacy across the shared medium cable network equal to or better than that provided by dedicated line network access services (analog modem or digital subscriber line). It does this by encrypting data flows between the CMTS and the CM.
The earlier BPI specification 22-2 had limited service protection because the underlying Key management protocol did not authenticate cable modems. BPI+ strengthened the service protection feature by adding digital certificate based authentication to its Key exchange protocol.
See also
External links
- CableLabs
- Everything You Need to Know About Next-gen Broadband
- Information about DOCSIS in German
- Cablevisor Real-time monitoring and collaboration for cable networks
- docsis.org
- DOCSIS Project docsis is a small program that can be used to generate binary configuration files for DOCSIS-compliant cable modems.
- Article: Le modem cable/VoIP Thomson THG520 sur Developpez.com
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