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DOCSIS

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Data Over Cable Service Interface Specification (DOCSIS) 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. [1]

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 parts of New Jersey, Brooklyn, The Bronx, Long Island, Westchester, and Connecticut 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.

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
Videotron 600k down, 128k up 5.1M down, 810k up 20.0M down, 1.0M up
Cogeco 640k down, 128k up 10.0M down, 600k up 16.0M down, 1.0M up
Rogers Cable 1.0M down, 128k up 5.0Mbit down, 384k up 6.0M down, 800k up
Bell 128k down 1.0M down 5.0M down, 1.0M up
Shaw 256k down 5.0M down 7.0M down, 1.0M up

Telewest Broadband and NTL are currently the only Cable Internet providers in the United Kingdom, and their premium packages both offer download speeds of upto 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 seems highly unlikely as neither company has upgraded its network to DOCSIS2 or DOCSIS3 as of yet.

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 lawŠablona:Citation needed.

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. -->

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Citováno z „https://cs.wikipedia.org/w/index.php?title=DOCSIS&oldid=665327“