Version 7.2 161 Mediant 500L Gateway & E-SBC
User's Manual 12. Network
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).
To configure the No-Op packet feature:
1. Enable the feature, using the NoOpEnable ini file 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 ini file parameter.
3. For RTP No-Op packets, configure the payload type of the No-Op packets, using the
RTPNoOpPayloadType ini file parameter.
Note:
• The No-OP Packet feature requires DSP resources.
• Receipt of No-Op packets is always supported.
12.7.2.2.3 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, 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 packets are
enabled using the NoOpEnable and NoOpInterval parameters.
12.8 Robust Receipt of Media Streams by Media Latching
The Robust Media mechanism (or media latching) is an AudioCodes proprietary
mechanism to filter out unwanted media (RTP, RTCP, SRTP, SRTCP, and T.38) streams
that are sent to the same port number of the device. Media ports may receive additional
multiple unwanted media streams (from multiple sources of traffic) as result of traces of
previous calls, call control errors, or deliberate malicious attacks (e.g., Denial of Service).
When the device receives more than one media stream on the same port, the Robust
Media mechanism detects the valid media stream and ignores the rest. Thus, this can
prevent an established call been stolen by a malicious attacker on the media stream.
For the involved voice channel, the device latches on to the first stream of the first received
packet. All packets (of any media type) received from the same IP address and SSRC are
accepted (for T.38 packets, the device considers only the IP address). If the channel
receives subsequent packets from a non-latched source, the device can either ignore this
new stream and remain latched to the first original stream (IP address:port) or it can latch
on to this new stream. The media latch mode is configured using the
InboundMediaLatchMode parameter. If this mode is configured to latch on to new streams,
you also need to configure the following:
Minimum number of continuous media packets that need to be received from a
different source(s) before the channel can latch onto this new incoming stream.
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