Data Over Cable Service Interface Specification

from Wikipedia, the free encyclopedia

Data Over Cable Service Interface Specification ( DOCSIS ) is a specification developed by Cable Labs around 1997 for interfaces of cable modems and associated peripheral devices , which was ratified by the ITU in March 1998 (ITU-T Recommendation J.112). DOCSIS is a standard that defines the requirements for data transmission in a broadband cable network. The most important application area of ​​DOCSIS is the fast transmission of data over existing cable television networks . It is thus in direct competition with DSL and, in comparison, enables considerably higher ranges through the use of coaxial cables (instead of the unshielded copper wire pair for telephone connections) and considerably higher data transmission rates of up to 10 thanks to the conceptual design based on frequency division multiplex transmissions (see graphic) Gbit / s in download.

DOCSIS range compared to DSL variants (according to Unitymedia )

development

In the early days of the Internet it was for ISP is not possible or economically, its own infrastructure for the Internet to build, such as fiber to the end user . Instead, the existing infrastructure (initially telephone lines , later also radio cable connections, cellular networks and other technologies) was upgraded and converted for this purpose. DOCSIS was introduced as a competing technical platform and expands the radio cable connection to include data services such as the dominant Internet, which since DOCSIS 2.0 can also be used for real-time communication such as voice over cable . This creates additional added value on the customer side and new business models on the provider side, so-called triple play offerings (broadcasting, Internet, IP telephony ).

DOCSIS 1.0

DOCSIS scored in the receive direction (English " downstream ") has a useful data transfer rate of about 38 Mbit / s (EuroDOCSIS: 50 Mbit / s) per channel in the frequency range of 50 MHz to about 862 MHz, and in the transmission direction (engl. " Upstream ") a usable Data transfer rate of around 27 Mbit / s per channel in the frequency range from 5 MHz to 65 MHz.

DOCSIS 2.0

The successor specification DOCSIS 2.0 was presented in 2002 against the background of the increasing demand for symmetrical, real-time data services such as IP telephony. DOCSIS 2.0 supports a frequency spectrum from 88 MHz to 860 MHz in the downstream and (5 MHz) 30 MHz to 42 MHz in the upstream.

DOCSIS 3.0

The DOCSIS 3.0 specification was published in August 2006. By bundling up to 32 channels with 256- QAM (38 or 50 Mbit / s per channel), speeds of 1216 or 1600 Mbit / s can be achieved via EuroDOCSIS in the receiving direction (downstream). In the sending direction (upstream), bundling up to 8 channels with 256-QAM (27 or 30 Mbit / s per channel) can achieve 216 or 240 Mbit / s via EuroDOCSIS. DOCSIS 3.0 supports a frequency spectrum from 108 MHz to 1002 MHz in the downstream and 30 MHz to 85 MHz in the upstream as well as IPv6 .

DOCSIS 3.1

The DOCSIS 3.1 specification was published in October 2013 and supports data rates of up to 10 Gbit / s in the downstream and 1 Gbit / s in the upstream. This is achieved with 4096- QAM and 20 kHz to 50 kHz wide carriers with Orthogonal Frequency Division Multiplex Coding (OFDM). These carriers can be combined within a frequency spectrum, which in the downstream is at least 24 MHz and max. 192 MHz wide. DOCSIS 3.1 supports a frequency spectrum of up to 1.8 GHz in the downstream and 5 MHz to 204 MHz in the upstream (further permissible upstream split frequencies: 65/85/117 MHz) as well as IPv6 .

DOCSIS 4.0

The DOCSIS 4.0 specification was published in March 2020 and supports data rates of up to 10 Gbit / s in the downstream and 6 Gbit / s in the upstream. In addition, DOCSIS 4.0 includes the new full duplex and extended spectrum functions.

DOCSIS vs. EuroDOCSIS

Due to the different television systems, the frequencies in the US American and European cable networks are divided differently. Due to the PAL television system used in Europe , the bandwidths were set at 8 MHz; from the US NTSC tradition, however, they were originally 6 MHz. For this reason, the DOCSIS specifications were adapted for the European market and named EuroDOCSIS.

Due to the larger channel bandwidths, EuroDOCSIS allows a higher data rate in the downstream per individual channel (compared to the American DOCSIS). However, since several channels are connected together anyway, the same data rates can also be achieved overall over the same overall bandwidths. Are z. B. 3 EuroDOCSIS channels connected together at 8 MHz each, this results in a total bandwidth of 24 MHz; just as much as when 4 channels of 6 MHz each are interconnected with DOCSIS. When cable network providers in Europe now use DOCSIS (and not EuroDOCSIS) as an exception, they take advantage of the fact that the American DOCSIS results in a slightly more flexible frequency band allocation due to the narrower channel bandwidths.

distribution

DSL is the older and better known Internet access. DOCSIS is often equated or compared with DSL ("DSL speed") due to the significantly higher data rates compared to dial-up connections (cf. narrowband communication ). However, both terms define different access technologies under the umbrella term broadband internet access .

Cable modems working with the DOCSIS standard are particularly widespread in France, Austria, Switzerland and the USA as well as in metropolitan areas in other industrialized countries . In Germany, the number of DOCSIS providers is limited due to the historically complicated division of the network levels of the cable networks . Unitymedia in particular, but also Vodafone Kabel Deutschland, exploits the possible bandwidths within the framework of the DOCSIS specification. However, the availability and acceptance of DOCSIS offers in Germany has been growing rapidly since 2009.

In 2017, however, it was still problematic for cable network operators to obtain modems for the newer DOCSIS standard 3.1.

implementation

DOCSIS in the OSI layer model

OSI layer model

DOCSIS is located in OSI layers 1 and 2 and thus provides the platform for the transfer and backup of data.

On the physical level, DOCSIS 1.0 provides transmission bandwidths of 0.2 MHz to 3.2 MHz and DOCSIS 2.0 up to 6.4 MHz. DOCSIS 2.0 is fully downward compatible with DOCSIS 1.0. The data to be transmitted is modulated with quadrature amplitude modulation (64-QAM or 256-QAM for the downstream and 16-QAM or QPSK for the upstream; DOCSIS 2.0 also provides 32-QAM, 64-QAM and 128-QAM for the upstream) .

At Layer 2, DOCSIS uses a combination of time division multiplex and code division multiplex ( flexible time division multiplex , TDMA).

technology

The DOCSIS architecture essentially consists of two components: a cable modem on the customer side and a cable modem termination system (CMTS) in the provider's cable headend .

The CMTS consists of several downstream and upstream modulators that each form a port and establish the connection to the cable modem, similar to a DSLAM in a DSL network. Since the upstream and downstream are each located in different frequency ranges, two physical ports are always required for an existing duplex connection. A WAN interface creates the connection between the CMTS and the IP - Backbone .

The subscriber end devices ( customer premises equipment ) such as computers or telephones are connected to the cable modem via Ethernet , USB or ATA and also to the IP backbone via the CMTS.

Via the Media Access Control (MAC) implemented by DOCSIS , the cable network operator can configure the cable modem, regulate the transmission bandwidths and activate or deactivate certain services. Since information sent in a broadband cable network is potentially transmitted to every connected unit (and thus also to other cable modems), the privacy of customers is also guaranteed by MAC encryption .

literature

  • Andres Keller: Data transmission in the cable network (DOCSIS via Hybrid-Fiber-Coax), Springer, 10.2004, ISBN 978-3-540-22501-0

Web links

Individual evidence

  1. ↑ http://www.zdnet.de/88299079/avm-bringt-fritzbox-7590-und-fritzbox-6590-cable-in-den-handel/?inf_by=59650d49681db85b618b489a 32x8 channel bundling via DOCSIS 3.0
  2. ↑ http://www.cablelabs.com/specs/specification-search/?cat=docsis&scat=docsis-3-0 DOCSIS 3.0 specification
  3. ↑ http://www.cablelabs.com/specs/specification-search/?cat=docsis&scat=docsis-3-1 DOCSIS 3.1 specification
  4. ↑ https://www.cablelabs.com/on-the-path-to-10g-cablelabs-publishes-docsis-4-0-specification
  5. ↑ https://www.golem.de/news/fruehstart-kabelnetzbetreiber-findet-keine-modems-fuer-docsis-3-1-1705-128074.html golem.de: Cable network operator cannot find modems for Docsis 3.1