AudioCodes Mediant 3000 User Manual page 171

User's Manual
address:port obtained from the first received SIP message.

NAT Is Used: NAT is always performed. The device considers the UA as located
behind NAT and always sends the media packets to the UA using the source address
obtained from the first media packet received from the UA. In this mode, the device
does not send any packets until it receives the first packet from the UA (in order to
obtain the IP address).

To enable NAT resolution using the First Incoming Packet mechanism:
1. Open the General Settings page (Configuration tab > VoIP menu > Media > General
Media Settings).
2. Set the 'NAT Mode' parameter (NATMode) to one of the following:
•
[0] Auto-Detect
•
[1] NAT Is Not Used
•
[2] NAT Is Used
3. Click Submit.
13.9.2.2.2 No-Op Packets
The device's No-Op packet support can be used to verify Real-Time Transport Protocol
(RTP) and T.38 connectivity, and to keep NAT bindings and Firewall pinholes open. The
No-Op packets are available for sending in RTP and T.38 formats.
You can control the activation of No-Op packets by using the ini file parameter
NoOpEnable. If No-Op packet transmission is activated, you can control the time interval in
which No-Op packets are sent in the case of silence (i.e., no RTP or T.38 traffic). This is
done using the ini file parameter NoOpInterval. For a description of the RTP No-Op ini file
parameters, see ''Networking Parameters'' on page 846.

RTP No-Op: The RTP No-Op support complies with IETF Internet-Draft draft-wing-
avt-rtp-noop-03 ("A No-Op Payload Format for RTP"). This IETF document defines a
No-Op payload format for RTP. The draft defines the RTP payload type as dynamic.
You can control the payload type with which the No-Op packets are sent. This is
performed using the RTPNoOpPayloadType ini parameter (see ''Networking
Parameters'' on page 846). The default payload type is 120.

T.38 No-Op: T.38 No-Op packets are sent only while a T.38 session is activated. Sent
packets are a duplication of the previously sent frame (including duplication of the
sequence number).
Note:
13.9.2.2.3 Fax Transmission behind NAT
The device supports transmission from fax machines (connected to the device) located
inside (behind) a Network Address Translation (NAT). Generally, the firewall blocks T.38
(and other) packets received from the WAN, unless the device behind the NAT sends at
least one IP packet from the LAN to the WAN through the firewall. If the firewall blocks T.38
packets sent from the termination IP fax, the fax fails.
To overcome this, the device sends No-Op ("no-signal") packets to open a pinhole in the
NAT for the answering fax machine. The originating fax does not wait for an answer, but
immediately starts sending T.38 packets to the terminating fax machine upon receipt of a
re-INVITE with T.38 only in the SDP, or T.38 and audio media in the SDP. This feature is
Version 7.0
Receipt of No-Op packets is always supported.
171
13. Network
Mediant 3000