DOCSIS Set-top Gateway

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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.

Features[edit]

DSG is an extension of the DOCSIS protocol governing cable modems, and applies to all versions of DOCSIS.

The principal features of DSG are:

One-way operation[edit]

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[edit]

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.

Addition of new OSSI events[edit]

The DSG Specification extends the DOCSIS Open Source Software Institute (OSSI) error reporting and logging mechanism, adding standardized events pertaining to DSG operation.

Creating an out-of-band directory[edit]

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[edit]

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).

Architecture[edit]

A DSG network comprises three components; the Conditional Access Server generates a stream of out of band data. The DSG Agent (the CMTS) forwards the out of band data and publishes the out of band directory (the DCD); the DSG client on the set-top digests the DSG data.

The set-top device comprises three subcomponents; the DSG-capable cable modem is referred to as the DSG eCM (Embedded Cable Modem). A data consumer is called a DSG Client (there are generally many of these.) Finally, the eCM is configured and controlled by a component called the DSG Client Controller.

See also[edit]

References[edit]