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.
Likewise, persistent upstream errors will cause a cable modem to "reinitialize its MAC" and scan for new downstream channels.
A cable set-top box still needs to acquire its out of band data even if the upstream channel is impaired.
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.