Allied Telesis AT-iMG634 - R2 User's Guide Page 685

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DTMF-RELAY Common VoIP attributes: QoS, Media and DTMF-Relay
6-121
iMG/RG Software Reference Manual (Voice Service)
When VoIP MGCP protocol is used, the lowest ports pair is assigned to the first configured end-point, the sub-
sequent pair is assigned to the second configured end-point and so on.
When SIP protocol is used, ports pair are used in a round robin fashion.
iMG/RG/iBG devices allow to specify a specific the RTP packetization time as timeframe between to consecutive
RTP packets.
Packetization time could be negotiated at runtime during the call establishment phase.
The value specified via CLI is the value that is normally advertized by the VoIP protocol (via VoIP signalling mes-
sages) and that it’s used when is not negotiated during the call setup.
6.4.2.1 Media Timeout
Only when VoIP SIPprotocol is used, it's also possible set iMG/RG/iBG devices to detect if an incoming RTP flow
is still present or not (e.g. the other end-point was abruptly disconnected or network has critical problems)
forcing the call release if no RTP packet flow has been detected for the current call for a time longer than the
specified observation period.
6.4.3 DTMF-RELAY
iMG/RG/iBG devices support DTMF relay for the transmission of DTMF dialpad tones.
DTMF tones can be transmitted in-band through the RTP media stream as normal voice or can be transmitted
via RTP stream accordingly to RFC2833 or can even being transmitted via VoIP specific messages.
Using DTMF in-band transmission is reliable only when the media stream uses uncompressed codec like G.711u
or G.711a. When compressed codecs like G.729 or G.726 are used, the distortion produced by the codec
compression algorithm does not allow a clear transmission of the DTMF tones that could be not intelligible on
the far end.
In this case iMG/RG/iBG devices allow to use RFC2833 Telephone events or SIP NOTIFY or MGCP NTFY mes-
sages for DTMF transmission.
In case of SIP protocol, it’s possible force the system to automatically select the proper DTMF relay mode
depending on the negotiated codec.
In case of MGCP protocol, because call properties are controlled by the call agent, the selection of the proper
DTMF relay mode is demanded to the call agent logic.
6.4.4 Functional Differences for Common VoIP attributes in Product Categories
The table below is intended to identify what is common amongst the product families - as well as where there
are differences - to highlight those differences. To determine which family your device belongs to - please refer
to the preface.
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