20.4.2. IPDA (NCUI/STMI) 
In IPDA, two main fax connections can be distinguished:
- DMC (NCUI/STMI as endpoint) and
- master calls.
Master / Slave Connection
The master connection is controlled by the host system, whereas the
slave connection (DMC) is controlled by the board itself.
Since IPDA master connections are not controlled by H323, IPDA endpoints
detect T.38 with tone evaluation. Each IPDA endpoint sends a message
to the DNIL in the SWU stating that T.38 is possible. If this is the
case, DMC is rejected. When the DNIL has received the messages, a T.38
path switch message is sent to the endpoints.
Restrictions
- If no T.38 resources are available, G.711 is used as an alternative. The resource limitation is signaled to the DNIL. This restriction applies to all hardware gateways.
- QoS Monitoring
-
QoS monitoring on the T.38 (former voice) channel is adapted to T.38 to that effect that T.38 is always regarded as "good quality". Hence, no BAD_QUALITY messages are generated.
- Bandwidth Management
-
The consumed bandwidth depends on the selected redundancy factor and the fax transmission speed. There is no specific bandwidth management for T.38; it will be counted like G.711, with 30ms packet size.This restriction also applies to connections other than IPDA.