DOCSIS Set-top Gateway (or DSG) is a specification describing how

out-of-band data In computer networking, out-of-band data is the data transferred through a stream that is independent from the main ''in-band'' data stream. An out-of-band data mechanism provides a conceptually independent channel, which allows any data sent via th ...

is delivered to a

cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...

set-top box. Cable

set-top box A set-top box (STB), also colloquially known as a cable box and historically television decoder, is an information appliance device that generally contains a TV-tuner input and displays output to a television set and an external source of sign ...

es need a reliable source of out of band data for information such as program guides, channel lineups, and updated code images.

Features

DSG is an extension of the

DOCSIS Data Over Cable Service Interface Specification (DOCSIS) is an international telecommunications standard that permits the addition of high-bandwidth data transfer to an existing cable television (CATV) system. It is used by many cable televisio ...

protocol governing

cable modem A cable modem is a type of network bridge that provides bi-directional data communication via radio frequency channels on a hybrid fibre-coaxial (HFC), radio frequency over glass (RFoG) and coaxial cable infrastructure. Cable modems are primaril ...

s, and applies to all versions of DOCSIS. The principal features of DSG are:

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 Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behaviour. Devices that typically ...

management information base A management information base (MIB) is a database used for managing the entities in a communication network. Most often associated with the Simple Network Management Protocol (SNMP), the term is also used more generically in contexts such as in ...

s (MIBs) to manage DSG devices. One MIB is used by the set-top, the other by the

cable modem termination system A cable modem termination system (CMTS) is a piece of equipment, typically located in a cable company's headend or hubsite, which is used to provide high speed data services, such as cable Internet or Voice over Internet Protocol, to cable subscr ...

(CMTS).

Architecture

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.''

References

{{Reflist Broadcasting standards Digital cable Set-top box