Configuring and Troubleshooting Fax and Modem Calls in AOS
Configuring and Troubleshooting Fax and Modem Calls in AOS
The following guide explains how to configure and troubleshoot fax/modem calls over a VoIP
network using T1/E1/analog based voice trunks and users. There are numerous factors to
consider when sending or receiving fax/modem calls over a VoIP network. Fortunately, some of
these factors can be eliminated or mitigated by changing the configuration on the ADTRAN IP
Business Gateway (IPBG).
Configuration
There are two different methods for provisioning T1/E1/analog based trunks and analog voice
users to handle fax/modem calls appropriately. The first method is by having the unit
dynamically provision the settings, based on its ability to detect fax/modem tones during a call.
The other method is through static provisioning, where various settings such as the audio codec,
echo cancellation mode, and jitter buffer settings are manually provisioned. The following
sections cover each method in more detail.
Dynamic Provisioning
Dynamic provisioning is done through a feature called modem passthrough. Modem passthrough
works by automatically adjusting how the call is setup when certain fax/modem tones are
detected. This can be beneficial for users or trunks where voice calls are also present. Table 1
below lists the fax/modem tones that modem passthrough will detect:
Table 1: Fax and Modem Tones
T.30 CNG Series of interrupted 1100±38 Hz tone with 4000±600 samples on and
24000±3600 samples off.
V.21 Preamble Periodic sequence of 1850±12 Hz / 1650±12 Hz / 1850±12 Hz tones
having durations of 26 / 156 / 26 samples and repeating 38±6 times.
V.25 ANS
T.30 CED
Uninterrupted 2100±15 Hz tone with 26400±5600 samples on.
V.25 ANS with
Phase Reversal
ANS/CED tone with periodic phase reversals (180°±10°) at intervals of
3600±200 samples. At least two phase reversals are required to disable
echo cancellation.
V.8 ANSam Uninterrupted 2100±1 Hz tone with 20% amplitude modulation by a
15±0.1 Hz tone.
V.8 ANSam with
Phase Reversal
ANSam tone with periodic phase reversals (180°±10°) at intervals of
3600±200 samples. At least two
