Information Technology Reference
In-Depth Information
for a terminal to declare all its capabilities; it need only declare those that it
wishes to be used.
A terminal may indicate its capabilities to transmit. A terminal that receives
transmission capabilities from the remote terminal may request a particular
mode to be transmitted to it. A terminal indicates that it does not want its
transmission mode to be controlled by the remote terminal by sending no
transmission capabilities.
3.2.2.7 Round Trip Delay Determination Signaling Entity
It may be useful in some applications to have knowledge of the round trip
delay between a transmit terminal and a receiving terminal. A mechanism
is provided to measure this round trip delay. This mechanism may also be
useful as a means to detect whether the remote terminal is still functioning.
3.2.2.8 Maintenance Loops Signaling Entity
Procedures are specified to establish maintenance loops. It is possible to
specify the loop of a single logical channel either as a digital loop or decoded
loop, and the loop of the whole multiplex.
3.2.2.9 Commands and Indications
Commands and indications are provided for various purposes: video/audio
active/inactive signals to inform the user, and a fast update request for
source switching in multipoint applications are some examples. Neither com-
mands nor indications elicit response messages from the remote terminal.
Commands force an action at the remote terminal while indications merely
provide information and do not force any action.
A command is defined to allow the bit rate of logical channels and the
whole multiplex to be controlled from the remote terminal. This has a num-
ber of purposes: interworking with terminals using multiplexes in which
only a finite number of bit rates are available, multipoint applications where
the rates from different sources should be matched, and flow control in con-
gested networks.
3.2.2.10 Abstract Syntax Notation One (ASN.1) and X.691
In H.245, the message syntax is defined using a notation called Abstract
Syntax Notation One (ASN.1). The ASN.1 defines a number of simple data
types and specifies a notation for referencing these types and for specifying
values of these types.
The ASN.1 notations can be applied whenever it is necessary to
define the abstract syntax of information without constraining how the
information is encoded for transmission in any way. It is particularly, but
