DOCSIS Set-top Gateway
Encyclopedia
DOCSIS Set-top Gateway is a specification describing how out of band data is delivered to a cable
set-top box. Cable set-top box
es need a reliable source of out of band data for information such as program guides, channel lineups, and updated code images.
protocol governing cable modem
s, and applies equally to all versions of DOCSIS.
The principal features of DSG are:
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.
The DSG specification defines a special downstream keep-alive message so that the set-top can recognize an appropriate downstream channel.
OSSI error reporting and logging mechanism, adding standardized events pertaining to DSG operation.
Each data consumer is assigned a special Client Identifier that names the out of band data stream in the DCD.
MIBs to manage DSG devices. One MIB is used by the set-top, the other by the CMTS.
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.
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
set-top box. Cable set-top box
Set-top box
A set-top box or set-top unit is an information appliance device that generally contains a tuner and connects to a television set and an external source of signal, turning the signal into content which is then displayed on the television screen or other display device.-History:Before the...
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 DOCSISDOCSIS
Data Over Cable Service Interface Specification is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV system...
protocol governing cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
s, and applies equally to all versions of DOCSIS.
The principal features of DSG are:
One Way Operation
The original DOCSISDOCSIS
Data Over Cable Service Interface Specification is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV system...
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.
Addition of New OSSI Events
The DSG Specification extends the DOCSISDOCSIS
Data Over Cable Service Interface Specification is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV system...
OSSI error reporting and logging mechanism, adding standardized events pertaining to DSG operation.
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 SNMPSimple Network Management Protocol
Simple Network Management Protocol is an "Internet-standard protocol for managing devices on IP networks. Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more." It is used mostly in network management systems to monitor...
MIBs to manage DSG devices. One MIB is used by the set-top, the other by the 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 digest 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.