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G3 FACSIMILE COMMUNICATIONS
6.8.1 Preamble
The preamble is the signal that prefixes the binary signal, and is a series of
the flags for about 1 sec. A "flag" comprises eight bits "01111110" (7E
when expressed in Hex), and the preamble comprises about 37 continuous
flags. Synchronization of the MODEM (ITU-T V.21) for the procedure
signal must be performed by prefixing the binary signal. Also, in interna-
tional communications, the preamble guards against the loss of binary sig-
nals by an echo suppresser, for example.
6.8.2 Structure of binary coded information
Binary coded information comprises a data structure called an "HDLC
frame." "HDLC" stands for High level Data Link Control, a method of
achieving highly efficient and highly reliable transmission control proce-
dures. A feature of HDLC is a structure that checks for transmission errors
in the transmitted data itself. HDLC is stipulated by
transmission control procedure, and is also used in exchanging ISDN
packets. The structure of HDLC is called a "frame" because the informa-
tion is prefixed and appended with signals called "flags." The following
describes the names of each field in this structure and their content.
5–108
F
F
F
F
A
C
FCF
Fig. 5-80 Structure of Binary Coded Information
F
Flag sequence
Shows the beginning and the end of the frame, and is also used for
MODEM synchronization.
Format
01111110(=7E H)
Remarks
With the head, there are cases where there is 1 of this
signal and cases where there are 2 of this signal.
F
F
F
Approx. 1 sec
F: Flag sequence(01111110=7EH)
Fig. 5-79 Preamble
FIF
F
F
ISO
standards as one
FCS
F
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