3.3. Structure of a Fax Call

• 3.3.1. Phase A - Set Up Fax Call
• 3.3.2. Phase B - Pre-Message Procedure
• 3.3.3. Phase C - Transmit Message
• 3.3.4. Phase D - Post-Message Procedure
• 3.3.5. Phase E - Release Fax Call

T.30 is an ITU-T recommendation that specifies a fax communications protocol for Group 3 fax. This recommendation describes how to establish and terminate communications between Group 3 fax machines. ITU-T is the International Telecommunication Union, a United Nations agency that develops and recommends international telecommunications standards.

The recommendation specifies five separate phases in a fax call or session. These phases are implemented using Dialogic fax library API functions along with fax library data structures that accomplish the proper negotiation of each phase.

The five consecutive phases indicating the flow of a fax session are:

Phase A - set up fax call (begin fax session)

Phase B - pre-message procedure

Phase C - transmit message

Phase D - post-message procedure

Phase E - release fax call and disconnect (end fax session)

3.3.1. Phase A - Set Up Fax Call

Phase A establishes communication between two stations, caller and called. This usually begins with a request for service and, in some cases, dialing the other station.

NOTE:

The connection between the caller and called stations is implemented by functions other than fax library API functions. For analog SCbus and stand-alone configurations, see the Voice Software Reference - Programmer's Guide; for digital SCbus configurations, see the Digital Network Interface Software Reference.

Once the line connection with the called party is established, the following takes place during Phase A:

• fax tone detection
• digital handshake detection

The caller station typically sends an 1,100 Hz tone known as the CNG tone. The called station responds with a 2,100 Hz tone, the CED.

In preparation for Phase B, the caller station is initially given fax transmitter status, and the called station is initially given fax receiver status. (The initial fax state for the fax application must be set by the application prior to issuing the first send or receive function of a fax session.)

3.3.2. Phase B - Pre-Message Procedure

Phase B is used by the transmitter and receiver to negotiate the parameters for sending and receiving the fax document/page, such as polling, type of data, transmission speed, resolution, width and more.

In this phase, the receiver identifies its capabilities to the transmitter, and the parameter values used depend on the capabilities of the receiver.

3.3.3. Phase C - Transmit Message

Phase C transmits the fax document page based on the parameters negotiated between the caller and called applications in Phase B.

3.3.4. Phase D - Post-Message Procedure

Phase D defines a continuation value to indicate to the receiver what to do after the transfer of the fax document/page is completed.

The fax send function transmits data from various sources with independently defined page and document boundaries. Phase D continuation values allow multiple send functions to be linked together to transmit data from many document file sources, building a fax transmission dynamically.

Phase D continuation values are used in a table of DF_IOTT structures to indicate Phase D commands to the receiver. The DF_IOTT table defines parameters for transmitting one or more files containing fax data. Each DF_IOTT table entry contains parameters describing the characteristics of the fax data to be sent. For details on DF_IOTT, see Section 7.6. DF_IOTT -- Fax Transmit Data Description on page 109.

3.3.5. Phase E - Release Fax Call

Phase E releases the fax call. The caller station sends a disconnect signal (DCN) and both fax stations disconnect from the phone line.

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