DOCSIS Set-top Gateway

DOCSIS Set-top Gateway (or DSG) is a specification describing how out-of-band data is delivered to a cable set-top box. Cable set-top boxes need a reliable source of out of band data for information such as program guides, channel lineups, and updated code images.

DSG is an extension of the DOCSIS protocol governing cable modems, and applies to all versions of DOCSIS. Like DOCSIS, DSG was developed by CableLabs and contributing companies, including Arris, BigBand Networks, Broadcom, Cisco, Comcast, Conexant, Correlant, Cox, General Instrument, Harmonic, Intel, Motorola, Netgear, Terayon, Time Warner Cable, and Texas Instruments.^[1]^[2]^[3]

Features

One-way operation

The original DOCSIS protocol supports only two way connectivity. A cable modem that is unable to acquire an upstream channel will give up and resume scanning for new channels. Likewise, persistent upstream errors will cause a cable modem to "reinitialize its MAC" and scan for new downstream channels. This behavior is appropriate for traditional cable modems, but not for cable set-top boxes. A cable set-top box still needs to acquire its out of band data even if the upstream channel is impaired.

The DSG specification introduced one way (downstream only) modes of operation. When upstream errors occur, the set-top enters a downstream-only state, periodically attempting to reacquire the upstream channel.

Defining how to recognize the correct downstream channel

Set-top out of band data is generally present only on certain downstream channels. The set-top needs a way to distinguish a valid downstream (containing the set-top's data) from an invalid one used only by standalone cable modems.

The DSG specification defines a special downstream keep-alive message so that the set-top can recognize an appropriate downstream channel.

Creating an out-of-band directory

The Advanced Mode of the DSG Specification introduces a special MAC Management message called the Downstream Channel Descriptor (DCD). The DCD provides a directory identifying the MAC and IP parameters associated with the out of band data streams.

Each data consumer is assigned a special Client Identifier that names the out of band data stream in the DCD.

SNMP MIBs

The DSG Specification creates two new SNMP management information bases (MIBs) to manage DSG devices. One MIB is used by the set-top, the other by the cable modem termination system (CMTS).

References

^ "Five Modem Makers' Systems Considered for Cable Data Specifications". CableLabs. September 23, 1996. Archived from the original on October 21, 2002.
^ "CableLabs Selects Broadcom and Terayon to Author Advanced Modem Technology Proposals". CableLabs. November 13, 1998. Archived from the original on October 11, 2013.
^ "Data-over-Cable Service Interface Specifications (DOCSIS)". Community.Cisco.com. Cisco Systems. March 1, 2019.

External links

DOCSIS DSG specifications

[1] "Five Modem Makers' Systems Considered for Cable Data Specifications". CableLabs. September 23, 1996. Archived from the original on October 21, 2002.

[2] "CableLabs Selects Broadcom and Terayon to Author Advanced Modem Technology Proposals". CableLabs. November 13, 1998. Archived from the original on October 11, 2013.

[3] "Data-over-Cable Service Interface Specifications (DOCSIS)". Community.Cisco.com. Cisco Systems. March 1, 2019.

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