11.6.2 Subscribing for Signals
Queries SHALL be subscribed by including a Subscription Element that defines the details of the subscription. The
Receiver of the subscription SHALL initially send an empty Response Message to the Sender. The Receiver of the
subscription SHALL send XJMF Signals whenever the conditions that were specified in the Subscription element are
met. Such a subscribed Query that requests multiple Signals is referred to as a "persistent channel".
Note: the Sender and Receiver roles for Signals are reversed compared with the initial subscription.
If a Controller that does not support persistent channels is queried to set up a persistent channel, it SHALL
answer the Query Message with a Response Message and set the @ReturnCode to "111".
Multiple Attributes of a Subscription Element are combined as a Boolean OR operation of these Attributes. For
instance, if @RepeatStep and @ObservationTarget are both specified, Messages fulfilling either of the
requirements are requested. If the Subscription Element contains only a URL, it is up to the emitting Controller to
define when to emit Messages.
11.6.3 Managing Persistent Channels.
Persistent Channels SHOULD be maintained even when a Device is powered off and powered on again.