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 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.
Diva Media Boards provide a high-performance block-oriented IDI (ISDN Direct Interface) between the board hardware and the host CPU. The data transfer is performed via a BUS master DMA. This enables a reliable data transfer between the host CPU and the Diva Media Board memory that is not affected by the host CPU load. At the same time, using the BUS master DMA reduces the host CPU load.
The Dvia TTY interface does not perform data processing. It is only used to forward the data stream between the application and the IDI interface, i.e., the virtual or emulated TTY interface. The entire data processing is performed on the RISC CPU of the Diva Media Board. The reliability of the data stream is ensured by the board hardware through buffering (up to 64 Kbytes for every channel) and block-oriented data transfer (blocks of up to 2 Kbytes) via the BUS master DMA.
The FAX CLASS 2 application can ignore low-level T.30 protocol settings. The T.30 protocol stack that runs on the RISC CPU of the Diva Media Board is able to perform the required adjustment of transmission parameters to provide reliable and fast document transmission without requiring application intervention.
You can overrule the automatic T.30 protocol parameter adjustment with FAX CLASS 2 commands. If your application does not support the appropriate command set or the required features are not part of the FAX CLASS 2 command set, you can use Global Fax Configuration Options or Global Dialogic® Diva® TTY Configuration Options to overrule the automatic parameter adjustment.
SEP/SUB/PWD commands are used to address the document recipient and to select or protect the polled document.
The Diva Media Board's support for SEP/SUB/PWD is transparent to the application. The appropriate features can be activated via Global Dialogic® Diva® TTY Configuration Options without any application intervention. The received SEP/SUB/PWD frames are delivered to the application via "+FSA:", "+FPA:", and "+FPW:" indications.
You can control ECM support via the FAX CLASS 2 AT-command set or via Global Dialogic® Diva® TTY Configuration Options.
If you use global Diva TTY configuration options to enable ECM support, the Dialogic® Diva® Media Board will use ECM mode for document transfer, if supported by the opposite side.
Diva Media Boards use their internal memory to store document data. They retrieve data for ECM re-transmissions from this internal buffer (up to 64 Kbytes for every channel). This reduces the host CPU load and increases the reliability of the fax transmission.
Diva Media Boards use MR, MMR, T.6 fax document compression. In order to reduce transmission time, Diva Media Boards select the best compression algorithm supported by the opposite side. The Diva Media Board's RISC CPU is used to re-compress 1D-coded page data from the application to the format requested by the opposite side (transmission) and to convert received data to 1D-coded page data that is sent to the application (reception).
The re-compression process is handled internally by the board's RISC CPU and happens fully transparent to the application that deals only with 1D (MH) coded data.
You can adjust the compression-related T.30 protocol settings via Global Fax Configuration Options.
The V.34 fax support can be activated via Global Dialogic® Diva® TTY Configuration Options. If the Diva Media Board is able to establish a connection with a transmission speed higher than 14400 bps (V.34), it handles this transparent to the application. In order to avoid incompatibility with FAX CLASS 2 applications, the Diva Media Board never indicates transmission speeds higher than 14400 bps to the application. This means that transmission speeds higher than 14400 bps are mapped to 14400 bps.
You can use the divalogd Accounting Utility, which uses the Dialogic® Diva® Media Board Management Interface to get information on the real transmission speed and the used compression algorithm.