Version 6.6 695 October 2014
Installation & Operation Manual 41. PSTN Gateway and IP-to-IP Applications
Parameter
Name
Type Provisioning
Type
Description
CNG Mode Enum
Disable, Relay, EventsOnly
Instant Comfort Noise Generation Mode
Disable - The originating
gateway doesn’t detect
CNG, the CNG signal
passes transparently to the
remote side.
Relay - CNG is detected on
the originating side. CNG
packets are sent to the
remote side according to
T.38 (if IsFaxUsed=1), a Re-
INVITE message isn’t sent
and the fax session starts by
the terminating gateway.
This option is useful (for
example) when the
originating gateway is
located behind a firewall that
blocks incoming T.38
packets on ports that have
not yet received T.38
packets from the internal
network (i.e., originating
gateway).
Events Only - CNG is
detected on the originating
side. The CNG signal passes
transparently to the remote
side and fax session is
started by the originating
side using Re-INVITE
message. Usually T.38 fax
session starts when the
‘preamble’ signal is detected
by the answering side. Some
SIP gateways don’t support
the detection of this fax
signal on the answering side,
thus, for these cases it is
possible to configure the
gateways to start the T.38
fax session when the CNG
tone is detected by the
originating side. However,
this mode is not
recommended.
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