DOCSIS

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Data Over Cable Service Interface Specification (DOCSIS) (often pronounced /ˈdɒksɪs/) is an international telecommunications standard that 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 (see cable internet) over their existing hybrid fiber coaxial (HFC) infrastructure.

History

DOCSIS was developed by CableLabs and contributing companies including ARRIS, BigBand Networks, Broadcom, Cisco, Conexant, Correlant, Harmonic, Intel, Motorola, Netgear, Terayon, and Texas Instruments.

The first DOCSIS specification was version 1.0, issued in March 1997, with revision 1.1 (adding quality of service (QoS) capabilities) following in April 1999. Because of increased demand for symmetric services such as IP telephony, DOCSIS was revised to enhance upstream transmission speeds; DOCSIS 2.0 was released in December 2001. Most recently, the specification was revised to significantly increase transmissions speeds (this time both upstream and downstream) and introduce support for Internet Protocol version 6 (IPv6). This version, DOCSIS 3.0, was released in August 2006. Cross-version compatibility has been maintained across all versions of DOCSIS, with the devices falling back to the highest supported version in common between both endpoints: cable modem and cable modem termination system (CMTS) The reciprocal is not true, however: if one has a cable modem that is, for example, only capable of handling DOCSIS 1.0, and the system is running 2.0, the end user will not be able to use the highest speeds the cable network is capable of delivering.

Regional Variants

As frequency allocation band plans differ between U.S. 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 American cable channels conform to ATSC standards which specify 6 MHz. The wider bandwidth in EuroDOCSIS architectures permits more bandwidth to be allocated to the downstream data path (toward the user). EuroDOCSIS certification testing is executed by Excentis (formerly known as tComLabs), while DOCSIS certification testing is executed by CableLabs. Typically, customer premises equipment receives "certification", while CMTS equipment receives "qualification".

Most cable systems in Japan utilize the North American version of DOCSIS, while some employ a variant of DOCSIS that uses upstream channels that are based on a 9.216 MHz master clock (as opposed to 10.24 MHz used in DOCSIS/EuroDOCSIS) resulting in upstream channel widths that are a power-of-two division of 6 MHz (as opposed to 6.4 MHz in DOCSIS/EuroDOCSIS).

International Standards

The ITU Telecommunication Standardization Sector (ITU-T) has approved the various versions of DOCSIS as international standards. DOCSIS 1.0 was ratified as ITU-T Recommendation J.112 Annex B (1998), but it was superseded by DOCSIS 1.1 which was ratified as ITU-T Recommendation J.112 Annex B (2001). Subsequently, DOCSIS 2.0 was ratified as ITU-T Recommendation J.122. Most recently, DOCSIS 3.0 was ratified as ITU-T Recommendation J.222 (J.222.0, J.222.1, J.222.2, J.222.3).

Note: While ITU-T Recommendation J.112 Annex B corresponds to DOCSIS/EuroDOCSIS 1.1, Annex A describes an earlier European cable modem system ("DVB EuroModem") based on ATM transmission standards. Annex C describes a variant of DOCSIS 1.1 that is designed to operate in Japanese cable systems. The ITU-T Recommendation J.122 main body corresponds to DOCSIS 2.0, J.122 Annex F corresponds to EuroDOCSIS 2.0, and J.122 Annex J describes the Japanese variant of DOCSIS 2.0 (analogous to Annex C of J.112).

Features

DOCSIS provides great variety in options available at Open Systems Interconnection (OSI) layers 1 and 2, the physical and data link layers.

Physical layer
  • Channel width: DOCSIS 1.0/1.1 specified upstream channel widths between 200 kHz and 3.2 MHz. DOCSIS 2.0 also specifies 6.4 MHz, but can use the earlier, narrower channel widths for backward compatibility.
  • Modulation: All versions of DOCSIS specify 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 and 3.0 also specify 32-QAM, 64-QAM and 128-QAM also be available for upstream use.
Data link 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 and 3.0, with a limited use of contention for bandwidth requests. In contrast to the pure contention-based MAC CSMA/CD employed in older Ethernet systems (there is no contention in switched Ethernet), 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 that have specific traffic requirements such as low latency, e.g. voice over IP.
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 10.24 Mbit/s). All three versions of the DOCSIS standard support a downstream throughput of up to 42.88 Mbit/s per channel with 256-QAM (owing to 8 MHz channel width, the EuroDOCSIS standard supports downstream throughput of up to 55.62 Mbit/s per channel).

DOCSIS 3.0 features management over IPv6 and channel bonding, which enables multiple downstream and upstream channels to be used together at the same time by a single subscriber.[1]

The new 'DOCSIS 2.0 + IPv6' standard also supports IPv6, which may on the cable modem side only require a firmware upgrade[2][3]

Speed tables

Maximum raw throughput including overhead (maximum usable throughput without overhead)

Version Downstream Upstream
Channel configuration DOCSIS throughput EuroDOCSIS throughput Channel configuration Throughput
Minimum selectable number of channels Minimum number of channels that hardware must be able to support Selected number of channels Maximum number of channels Minimum selectable number of channels Minimum number of channels that hardware must be able to support Selected number of channels Maximum number of channels
1.x 1 1 1 1 42.88 (38) Mbit/s 55.62 (50) Mbit/s 1 1 1 1 10.24 (9) Mbit/s
2.0 1 1 1 1 42.88 (38) Mbit/s 55.62 (50) Mbit/s 1 1 1 1 30.72 (27) Mbit/s
3.0 1 4 m No maximum
defined		 m × 42.88 (m × 38) Mbit/s m × 55.62 (m × 50) Mbit/s 1 4 n No maximum
defined		 n × 30.72 (n × 27) Mbit/s

Common DOCSIS 3.0 speeds are listed in the table below.

Channel configuration Downstream throughput Upstream throughput
Number of downstream channels Number of upstream channels DOCSIS EuroDOCSIS
4 4 171.52 (152) Mbit/s 222.48 (200) Mbit/s 122.88 (108) Mbit/s
8 4 343.04 (304) Mbit/s 444.96 (400) Mbit/s 122.88 (108) Mbit/s

Note that the number of channels a cable system can support is dependent on how the cable system is set up. For example, the amount of available bandwidth in each direction, the width of the channels selected in the upstream direction, and hardware constraints constrain the maximum amount of channels in each direction.

Note that the maximum upstream speed on all versions of DOCSIS depends on the channel width, but the channel widths are independent of whether DOCSIS or EuroDOCSIS is used.

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 or node. Cable systems supporting on-demand programming use a hybrid fiber-coaxial system. Fiber optic lines bring digital signals to nodes in the system where they are converted into RF channels and modem signals on coaxial trunk lines.

A typical CMTS is a device which hosts downstream and upstream ports (it is functionally similar to the DSLAM used in DSL systems). While downstream and upstream communications travel on a shared coax line in the customer premises, and connect to a single F connector on the cable modem, it is typical for the CMTS to have separate F-connectors for downstream and for upstream communication. This allows flexibility for the cable operator. Because of the noise in the return (upstream) path, an upstream port is usually connected to a single neighborhood (fiber node), whereas a downstream port is usually shared across a small number of neighborhoods. Thus, there are generally more upstream ports than downstream ports on a CMTS. A typical CMTS has 4 or 6 upstream ports per downstream port.

Before a cable company can deploy DOCSIS 1.1 or above, it must upgrade its Hybrid fiber-coaxial (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-premises 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.

DOCSIS 2.0 is also used over microwave frequencies (10 GHz) in Ireland by Digiweb, using dedicated wireless links rather than HFC network. At each subscriber premises the ordinary CM is connected to an antenna box which converts to/from microwave frequencies and transmits/receives on 10 GHz. Each customer has a dedicated link but the transmitter mast must be in line of sight (mast sites are hilltop, ).

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 used by DOCSIS 1.1 & 2.0. Most recently, a number of enhancements to the Baseline Privacy Interface were added as part of DOCSIS 3.0, and the specification was renamed "Security" (SEC).

The intent of the BPI/SEC specifications is to describe MAC layer security services for DOCSIS CMTS to CM communications. BPI/SEC 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/SEC is intended to prevent cable users from listening to each other. It does this by encrypting data flows between the CMTS and the CM. BPI & BPI+ utilize 56-bit DES encryption, while SEC adds support for 128-bit AES. All versions provide for periodic key refreshes (at a period configured by the network operator) in order to increase the level of protection.

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 with a public key infrastructure to its Key exchange protocol.

Security in the DOCSIS network is vastly improved when only business critical communications are permitted, and end user communication to the network infrastructure is denied. Successful attacks often occur when the CMTS is configured for backwards compatibility with early pre-standard DOCSIS 1.1 modems which were "software upgradeable in the field," but did not include valid DOCSIS or EuroDOCSIS root certificates.

See also

References

  1. ^ http://www.cablelabs.com/news/pr/2006/06_pr_docsis30_080706.html
  2. ^ http://www.cablemodem.com/specifications/specifications20.html
  3. ^ http://www.rmv6tf.org/2008-IPv6-Summit-Presentations/Dan%20Torbet%20-%20IPv6andCablev2.pdf

External links

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