Allied Telesis AT-iMG634 - R2 User's Guide Page 628

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VoIP phone ports Port configuration
iMG/RG Software Reference Manual (Voice Service)
6-64
The signalling protocol (SIP) will negotiate the active codec based on the capabilities supported by the other
peer involved in the VoIP connection. In the case of local calls, the call control layer performs codec negotiation
locally.
The following table lists the codecs available on the iMG units.
6.3.1.3.1 T.38 support
The iMG is designed to support the transmission of T.30 fax signals using T.38 Internet Fax Protocol (IFP) pack-
ets.
Although T38 is listed the as a supported codec for the iMG family, T.38 is not in itself a codec, but rather a
technical solution to map FAX signals into a dedicated IP protocol - overriding the limitations (e.g. signal distor-
tion) that are present when faxes are sent using codecs designed for speech applications.
When T.38 support is enabled and a fax must be sent or received, the intelligent Multiservice Gateway tries
firstly to negotiate T.38 support with the called or calling end-point respectively. If this fails, the iMG automati-
cally falls-back to a non-compressed codec such as G711 A-law or
µ-law.
6.3.1.4 Voice quality management
To increase the voice/data quality additional parameters can be set on the voice system DSP. The following set-
tings are available on iMG models:
A fixed jitter buffer. Set to 120 ms with a jitter delay of 60 ms.
Separate TX and RX direction volume gain control. Adjustable between -48dB and +24dB.
ITU-T G.168 Line Echo Cancellation. Adjustable between 0 and 32 msec (a value of 0 disabled Line Echo
Cancellation).
Voice activity detection (VAD)/comfort noise generation (CNG).
TABLE 6-11 Codecs Available for iMGs
Codec Notes
g711a G.711 A law
g711u
G.711 µ law
g729ab G.729 Annex A and Annex B
g726-32 G.726 32kbps
T38 Media/codec negotiation for transmission of ITU-T T.30 fax signals via internet.
This is not an actual codec, but when specified in the codecs list indicates to the
iMG that ITU-T T.38 negotiation of media sessions & fall-back codecs should be
enabled.
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