AudioCodes Mediant 4000 SBC User Manual page 145

CHAPTER 13 Network
No-Op Packets
The device can send No-Op packets to verify Real-Time Transport Protocol (RTP) and T.38
connectivity, and to keep NAT bindings and Firewall pinholes open. The No-Op packets can be
sent in RTP and T.38 formats:
■ 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"). The IETF document defines a No-Op payload
format for RTP. The draft defines the RTP payload type as dynamic. You can configure the
payload type as described in the following procedure (default 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).
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To configure the No-Op packet feature:
1. Enable the feature, using the [NoOpEnable] parameter.
2. Configure the time interval during which the device sends No-Op packets when silence occurs
(i.e., no RTP or T.38 traffic), using the [NoOpInterval] parameter.
3. For RTP No-Op packets, configure the payload type of the No-Op packets, using the
[RTPNoOpPayloadType] parameter.
The receipt of No-Op packets is always supported.
Fax Transmission behind NAT
The device supports transmission from fax machines (connected to the device) located inside
(behind) 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 problem, 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 configured using the
[T38FaxSessionImmediateStart] parameter. The No- Op packet feature is enabled using the
[NoOpEnable] and [NoOpInterval] parameters.
ICE Lite
The device supports Interactive Connectivity Establishment (ICE) Lite for SBC calls. ICE is a
methodology for NAT traversal, enabling VoIP interoperability across networks to work better
across NATs and firewalls. It employs Session Traversal Utilities for NAT (STUN) and Traversal
Using Relays around NAT (TURN) protocols to provide a peer with a public IP address and port
that can be used to connect to a remote peer.
In order for clients behind NATs and/or firewalls to send media (RTP) between one another, they
need to discover each others P address and port as seen by the "outside" world. If both peers are
located in different private networks behind a NAT, the peers must coordinate to determine the best
communication path between them.
ICE first tries to make a connection using the client's private local address. If that fails (which it will
for clients behind NAT), ICE obtains an external (public) address using a STUN server. If that fails,
traffic is routed through a TURN relay server (which has a public address).
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Mediant 4000 SBC | User's Manual