Technical data

70
ASYNC/SYNC conversion module
The ASYNC/SYNC conversion module is inserted on top of OSI layer 3 and is used for automatic framing type
detection and conversion. Together with the B-channel autodetection, this module enables automatic
discrimination of the incoming call type and thus allows to set up a RAS server that is able to accept digital,
analog, wireless, and PIAFS calls on the same number by a standard ASYNC PPP application. See Incoming
RAS call type autodetection on page 69 for more information.
The ASYNC/SYNC conversion module is controlled by the AT+iF and AT+iG commands. If you select modem
profile 14 (AT&F14 or ATZ14), correct values will be set for most applications and B-channel protocol detection
will be enabled. See AT-command set on page 49 for more information.
At the beginning, the ASYNC/SYNC conversion module escapes every control character when it explodes a sync
frame because this is the initial state for PPP over asyncronous lines (expected by RNA). Thus 0xffffffff is set as
the initial receive ACCM. If the peer does not send an ACCM with its configuration request, a null ACCM added to
such a request by default, to prevent the overhead of escaping every control character.
The ASYNC/SYNC conversion module assumes that such frames will never contain unescaped control characters
for asynchronous RNA frames. Thus, 0x00000000 is set like the initial transmit ACCM. Because some routers
(for example Cisco and NetGW) reject an ACCM on synchronous links, the ASYNC/SYNC PPP conversion module
removes (but remembers) the ACCM from outgoing configuration requests by default.
Intelligent fax class 2 processing
The fax class 2 AT-command set provides an easy way to access fax-related functionality.
If you use fax class 2 for fax document processing, you need to be aware of the following problems, that might
occur:
• It is necessary to maintain a constant data stream between the application and the fax class 2 device. Any
interruption in this data stream will affect the quality of the fax document.
• It is necessary to deal with various low-level T.30 protocol settings like scan line time, compression, error
correction, etc.
• Not every application or device supports the command-set features provided by fax class 2. This limits the
functionality and may require modification of the existing application.
• The usage of fax document compression forces you to deal with compressed data (reception), or to be able
to generate compressed data stream on demand, or to provide documents in different compression formats
(transmission).
• The "classic" fax class 2 application is unable to deal with transmission speeds higher than 14400 bps. To be
able use V.34 fax transmission speeds of up to 33600 bps, the application needs to be modified.
This section explains how the Dialogic
®
Diva
®
Media Board can overcome these drawbacks and allows you to use
the fax class 2 AT-command set to process fax documents with a comparable level of reliability and flexibility as
a sophisticated high-level fax API.