DOCSIS

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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, Netgear, 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. Because of 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.

Contents

[edit] 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 a mixture of deterministic access methods, specifically TDMA for DOCSIS 1.0/1.1 and both TDMA and S-CDMA for DOCSIS 2.0, with a limited use of contention for bandwidth requests. In contrast to the pure contention-based MAC CSMA/CD employed in Ethernet systems, DOCSIS systems experience few collisions. For DOCSIS 1.1 and above the MAC layer also includes extensive Quality of Service (QoS) features that help to efficiently support applications, for example Voice over IP, that have specific traffic requirements, such as low latency.

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 features IPv6 and channel bonding, which enables multiple downstream and upstream channels to be used together at the same time by a single subscriber. [1]

DOCSIS Downstream Upstream
1.0 38 Mbps 10 Mbps
2.0 40 Mbps 30 Mbps
3.0 160 Mbps 120 Mbps

[edit] 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.

[edit] 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.

One downstream channel can handle hundreds of cable modems. As the system grows, the CMTS can be upgraded with more downstream and 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 Mbps), 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[citation needed].

[edit] United States

  • Comcast, the largest cable provider in the United States, caps downstream bandwidth at 4 or 6 Mbps and upstream bandwidth at 384 Kbps (48 kB/s) for standard home connections. In some areas, they are offering 8 Mbps downstream and 768 Kbps (96 kB/s) upstream as a more expensive, yet speedier alternative. These differing speed options are made possible by loading a particular configuration file, for the respective pricing tier or region, into the modem.
  • Cox Communications, another major cable provider, recently upgraded its base package to 7 Mbps downstream and 512 Kbps (64 kB/s) upstream, in select markets, from its previous upgrade of 6 Mbps downstream and 512Kbps upstream. Its premier package allows downstream rates of 12 Mbps and upstream rates of 1 Mbps. In other markets, such as Fairfax, Virginia, where Verizon's FiOS provides high speed competition, the base package is 5 Mbps downstream and 2 Mbps upstream, with the premier package being 15 Mbps downstream and 2 Mbps upstream.
  • Time Warner Cable, through its regular Road Runner cable internet service speeds vary depending upon region. Some areas offering 7 Mbps downstream and 512 Kbps upstream, while others are at 5 Mbps downstream and 512 Kbps upstream. Its Road Runner Premium service also varies, with offers of speeds from 15 Mbps to 7 Mbps downstream, and 2 Mbps to 512 Kbps upstream.[2] [3]
  • Mediacom, the eigth largest cable provider in the United States, through its Mediacom Online service offers download speeds of up to 5 Mbps and upload speeds of up to 256 Kbps and for its premium service, Mediacom Online Max, it offers download speeds of up to 10 Mbps and upload speeds of 1 Mbps.[4]
  • CableOne, offers Residental with 1.5 Mbps download and 200 Kbps upload; Residental Plus with 3 Mbps download and 300 Kbps upload; Residental Preferred with 4 Mbps download and 400 Kbps upload; Residental PRO with 5 Mbps download and 500 Kbps upload; and SOHO with 5 Mbps download and 768 Kbps upload.
  • Charter Communications, basic service offers 3 Mbps download and 256 Kbps upload. Charter now offers premium speeds of up to 10 Mbps download and 1 Mbps upload.
  • Midcontinent Communications, serving South Dakota and parts of North Dakota, Minnesota, and Nebraska, offers three grades of service through it's MidcoNet cable internet service: Limited Internet service at 128 Kbps upload and download; Standard service with 8 Mbps download and 256 Kbps upload; and MidcoNet Max with unthrottled download speed (around the DOCSIS limit of 40 Mbps) and 512 Kbps upload.

[edit] Canada

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 are included below.

Service Provider Basic Standard Faster Fastest
Videotron 600 Kbps down, 128 Kbps up 7 Mbps down, 820 Kbps up 10.0 Mbps down, 900 Kbps up 20.0 Mbps down, 1.0 Mbps up
Cogeco 640 Kbps down, 150 Kbps up 10.0 Mbps down, 640 Kbps up 16.0 Mbps down, 1.0 Mbps up N/A
Rogers 1.0 Mbps down, 128 Kbps up 5.0 Mbps down, 384 Kbps up 6.0 Mbps down, 800 Kbps up N/A
Shaw 256 Kbps down, 128 Kbps up 5.0 Mbps down, 512 Kbps up 10.0 Mbps down, 1.0 Mbps up 25.0 Mbps down, 1.0 Mbps up
EastLink 256 Kbps down, 128 Kbps up 5.0 Mbps down, 1.0 Mbps up 10.0 Mbps down, 1.0 Mbps up N/A

[edit] Europe

Telewest Broadband and NTL are currently the only Cable Internet providers in the United Kingdom, and their premium packages both offer download speeds of up to 10 Megabits per second, with upload speeds of 384 Kilobits per second and 512 Kilobits per second respectively. However, the two rival companies began a merger in late 2005/early 2006, and aim to offer connection speeds of "50 to 100 Megabits per second" by the end of 2006. However, this does not look like a reality. Selected users subscribed to the 10Mbps are currently (October 2006) enjoying a trial upgrade to 20 Megabits per second download, 768 Kilobits per second upload.

The operator LGI (http://lgi.com/) (formerly UGC, locally known as UPC) is 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 Mbps downstream and 18 Mbps upstream.

[edit] Asia

In Singapore, Starhub offers download speeds of 4Mbps, 6.5Mbps and 30Mbps and upload speeds of 256kbps, 384kbps and 1MBps respectively. (4Mbps will be upgraded to 6Mbps & 6.5Mbps will be upgraded to 12Mbps on 28th Dec 2006) Upload speeds still remain unchanged.

[edit] Oceania

In Australia, Telstra Bigpond, offers Basic cable with 8 Mbps download and 128 Kbps upload; and Extreme with 17 Mbps download and 256 Kbps upload. OptusNet, offers one standard speed of 10 Mbps download and 256 Kbps upload. These cable incumbents in Australia are ramping up speeds to keep product parity with the more competitive ADSL2+ marketplace. Regional operator Neighbourhood Cable allows users to opt for a shaped plan at a lower cost (with speeds similar to those of ADSL providers) or a "uncapped" service, which promises download speeds of up to 30Mbps and upstream speeds of 2Mbps.

In New Zealand, operator TelstraClear provides downstream speeds of 10 Mbps, 4 Mbps and 2 Mbps, with all plans including an upstream speed of 2 Mbps.

[edit] 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 [ANSI/SCTE 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.


[edit] See also

[edit] External links

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