AudioCodes Mediant 800 - Page 153

CHAPTER13 Network
Mediant 800 Gateway & E-SBC | User's Manual
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).
These addresses:ports (local, STUN, TURN and any other network address) of the client are
termed "candidates". Each client sends its' candidates to the other in the SDP body of the INVITE
message. Peers then perform connectivity checks per candidate of the other peer, using STUN
binding requests sent on the RTP and RTCP ports. ICE tries each candidate and selects the one
that works (i.e., media can flow between the clients). The following figure shows a simple
illustration of ICE:
- 113 -