VoIP Voice and Fax Signal Processing

14.14 HDLC MESSAGES IN ECM

The ECM aspects are given in Section 14.4.3. In this section, the ECM payload-specific overview is given. The G3 facsimile machine uses a basic HDLC frame structure, including the flag sequences, address field, control field, information field, and frame check sequence for transmitting the image data in ECM mode. The HDLC structure for a partial page is shown in Fig. 14.14(a). The details on HDLC FCD and RCP frames are listed in Fig. 14.14(b).

In ECM mode, every new partial page block starts with a series of synchronization flags for a nominal duration of 200 ms. The synchronization flags are used prior to the first HDLC frame. These flags are transmitted for about 1 second ±20% for the V.21 facsimile messages procedures. Similar to the V.21 HDLC frame, an 8-bit flag sequence is used to denote the beginning and end of an HDLC frame. The flag sequence is also used to establish bit and frame synchronization.

Synchronization sequence flag: This byte is part of the synchronization sequence. The synchronization training sequence is a series of flag sequences for nominal 200 ± 100 ms.

Address field: The address field is the same for both V.21 facsimile messages and T.4 ECM data of value: 0xFF.

Control field: The 8-bit HDLC control field provides the capability of encoding the command unique to the facsimile message procedure. Format: 1100 X000, where the bit X is set to 0 for the T.4 FCD frame and return to control for the partial page (RCP) frame.

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