Rockwell RFX144V24-S23, Руководство дизайнера

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RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
1070 9-1
9. T.30 IMPLEMENTATION
9.1 GENERAL
CCITT Recommendation T.30 details procedures for facsimile transmission over the PSTN. This standard describes how to
initiate, complete, and end a fax transmission. This section describes methods to set up host software to implement T.30.
A basic block diagram of a Group 3 facsimile machine is shown in Figure 9-1. The MONOFAX modem performs the
modulation/demodulation process. The fax machine manufacturer must implement the interface between the modem (T.30),
the data compression/decompression (T.4), and the interface to the scanner and printer.
There are five phases (A-E) to the T.30 facsimile protocol. Phase A is the call setup, in which both facsimile machines
connect to the line. Phase B is a pre-message procedure which consists of identification and command sections. The actual
high speed message transmission occurs during Phase C. This is followed by the post-message procedure or Phase D. Both
facsimile machines release the line in Phase E.
Figure 9-2 illustrates a typical Group 3 facsimile procedure. This example on T.30 describes a facsimile call where the calling
unit (originate) transmits a documents to a called unit (answer). Phase E is not included in this example since it is the call
release and both ends hang up.
9.1.1 Phase A
T.30 specifies that call establishment can be realized one of four ways. The four methods of call establishment are: manual-
to-manual, manual-to-automatic, automatic-to-manual, and automatic-to-automatic. Manual corresponds to operator or
human intervention while automatic means machine only. The explanation that follows describes an automatic-to-automatic
example.
The calling unit, or originating fax, first transmits a calling tone (CNG) to indicate it is a non-speech terminal.
Figure 9-3 describes how to set up the modem to generate a 1100 Hz (CNG) tone. Figure 9-4 describes how to set up the
modem to detect a 1100 Hz tone. The called unit, or answering fax, then responds with a called station ID (CED). Figure 9-5
and Figure 9-6 describe how to accomplish this task. The end of Phase A is signified after the called unit sends a 2100 Hz
(CED) tone and the calling unit has detected this tone. Some facsimile manufacturers do not configure the modem to detect
these tones. In this case, the modem looks for the preamble of flags (see phase B).
9.1.2 Phase B
The pre-message procedure consists of the handshake. One machine sends an identification signal and the other machine
responds with a command signal. A training check is sent at a high speed and the receiving machine informs the transmitting
machine if the training check was successful. This usually occurs at V.21 300 bps Frequency Shift Keying (FSK) modulation
in HDLC format.
HDLC stands for High level Data Link Control. It is a standard procedure used for data communications. HDLC is a bit-
oriented protocol (normally used in synchronous communications) that defines how the data being sent over the data link is
organized and arranged.
When using the HDLC protocol, the data is transmitted via frames. These frames organize the data into a format specified
by an ISO (International Standards Organization) standard that enables the transmitting and receiving station to synchronize
with each other. Figure 9-7 illustrates the HDLC frame structure used for the facsimile protocol.
The preamble is a series of HDLC Flags for one second ±15%. The purpose of the 7E flags is to condition the line. The flag
sequence defines the beginning and ending of a frame. The address field is required to provide identification for multi-point
addressing. For PSTN the format is 11111111. The control field's purpose is to provide the capability of encoding the
commands and responses. The format is 1100X000 (X=0 non-final frame; X=1 final frame).
The HDLC information field provides the specific information for the control and message interchange between the two
stations. In the fax protocol the format for the information field consists of two parts, the Facsimile Control Field (FCF) and
the Facsimile Information Field (FIF).
The FCF contains information regarding the type of information being exchanged and the position in the overall sequence.
The acronyms, functions, and format for FCF commands are defined in the T.30 Recommendation. The FIF contains
additional information which further clarifies the facsimile procedure. Some examples of some information communicated
with the FIF are: group capability, data rate, vertical resolution, coding scheme, recording width, recording length, and
minimum scan line time.
The Frame Check Sequence (FCS) follows the FIF. The modem automatically generates the FCS or Cyclic Redundancy
Check (CRC). The frame ends with an ending 7E flag. It is recommended that more than one ending flag be transmitted.