ARA-1 Operations Manual
INTEROPERABILITY NOW
1-9
1.8.4
Deriving COR at the Distant Side
The options are different at the network interface; that is - how an ARA-1 determines that the
radio
at the other side of the network
is unsquelched. These are the options for deriving COR at
the unit’s network interface – how, in Figure 1-5, the
distant
ARA can determine that the
local
radio is unsquelched:
•
VOX: triggered by any audio input (audio coming over the network) above an adjustable
threshold level.
•
VMR: triggered by audio input
ONLY
if that audio has content (again above an adjustable
threshold), that correlates to human speech sounds and patterns. VMR can help prevent
false transmitter activation from background noise on the SIP connection (such as
someone breathing heavily into a SIP Phone handset mouthpiece, or someone using a SIP
Phone in a high ambient noise environment)
[Note: VMR requires significant processing resources. This is why VMR is supported
only on the network side and not on the radio side.]
•
RTP Header: makes use of an RTP extension header that JPS created for this purpose.
This is useful when there are JPS SIP devices at both ends (or the use of the SIP extension
header has been built in to another vendor’s equipment).
The JPS SIP devices also allow combinations of the above; these are useful when there
may be different types of SIP devices in the system. When a combination is invoked - for
example “
RTP VMR”
- the ARA-1 will use the RTP Header if it’s available
and the VMR function when it’s not.
•
Packet COR: Heeding the note below, this is the preferred method, and the combination
of Silence Suppression and Packet COR was made part of the NIST BSI specification
(JPS SIP interfaces are BSI compliant). With this method, the Local ARA -1, upon
determining that its associated local radio is squelched, will discontinue sending audio
packets (Silence Suppression). Regarding the Distant ARA's PTT function, it will activate
PTT to the distant radio only while receiving audio packets from the network.
Note: Packet COR should only be used if all SIP devices on the network use Silence
Suppression. If a link is made to any SIP device configured otherwise, that distant device
will continue to send audio packets over the network even when there is no valid audio
signal, and the PTT function of an ARA-1 set to Packet COR will therefore stay activated,
holding the local radio in transmit mode.