📄 7_3_04.txt
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5i'
ANNEX A
(to Recommendation T.30)
Procedure for G3 document facsimile transmission in the general
switched telephone network incorporating error correction
A.1 Introduction
A.1.1 This Annex is intended to apply to document facsimile
apparatus covered by Recommendation T.4, Annex A. It describes the
procedures and signals to be used where facsimile equipment incor-
porates error correction capabilities. When existing equipment is
operating in a non-CCITT manner, they shall not interfere with
equipment operating in accordance with the Series T Recommendation.
A.1.2 Use of this Annex is optional
A.1.3 Outline of the error correction method
The error correction method described in this Annex is based
on the half-duplex page selective repeat ARQ (automatic repeat
request) technique.
An HDLC frame structure is utilized for all binary coded fac-
simile message procedures.
The transmitting terminal can decide to use either 256 or 64
octets for the frame size by using DCS command. The receiving ter-
minal must be able to receive 256 and 64 octets of frame size.
The transmitting station divides the coded data specified in
Recommendation T.4, S 4 into a number of frames and transmits them
with each frame number.
When the previous message has not been satisfactorily
received, the receiving station transmits PPR response to indicate
that the frames specified in the associated facsimile information
field are required to be retransmitted.
When PPR is received, the transmitting station retransmits the
requested frames specified in PPR information field.
When PPR is received four times for the same block, either the
EOR command is transmitted for end of retransmission or CTC (con-
tinue to correct) command is sent for continuous retransmission.
In the case of continuous retransmission, the modem speed may
fall back or continue at the same speed in accordance with the
decision of the transmitting terminal.
A.2 Definitions
A.2.1 The signals and definitions used in the error correction
procedure are as defined in the main body of this Recommendation
unless specified otherwise.
A.2.2 Frame formats of RCP frame and FCD frame for the
in-message procedure are defined in Recommendation T.4, Annex A.
A.2.3 Relations between a page, blocks, partial pages and
frames
One page of coded data as specified in Recommendation T.4, S 4
is divided into a number of blocks. The block contains a number of
frames. A partial page is defined as one transmitted block or a
number of retransmitted frames.
A.2.4 Block size
The block size is defined as the maximum number of frames that
can be sent by the transmitter before receiving the response.
A.3 Block size and frame size
A.3.1 For T.4 error correction mode, a transmitting terminal
indicates frame size by using DCS signal.
A.3.2 The following values of frame size are applicable: 256
or 64 octets. These values of frame size do not include either FCF
or frame number octet. Therefore, the total length of the HDLC
information field including both the FCF and the frame number octet
is as follows: 258 or 66 octets.
A.3.3 The receiving terminal must have the following condi-
tion:
frame size: 256 or 64 octets.
block size: 256 frames.
A.3.4 The transmitting terminal may send the block whose size
is less than 256 frames at the end of each page. This block is
called a short block
A.3.5 The frame size should not be changed during a transmis-
sion of one page. In order to change the frame size, indication of
mode change should be made using PPS | (mu | OM or EOR | (mu | OM
command at the page boundary.
A.4 Information field | (see also S 5.3.6)
The HDLC information field is of variable length and contains
the specific information for the control and message interchange
between two facsimile stations. In this Recommendation it is
divided into two parts, the facsimile control field (FCF) and the
facsimile information field (FIF).
1) Facsimile control field (FCF) - The facsimile
control field is defined to be the first 8 bits or 16 bits of the
HDLC information field. FCF of 16 bits should be applied only for
the optional T.4 error correction mode. This field contains the
complete information regarding the type of information being
exchanged and the position in the overall sequence. The bit assign-
ments within the FCF are as follows:
Where X appears as the first bit of FCF, X will be defined
as follows:
- X is set to 1 by the station which receives a
valid DIS signal;
- X is set to 0 by the station which receives a
valid and appropriate response to a DIS signal;
- X will remain unchanged until the station again
enters the beginning of phase B.
2) Facsimile information field (FIF) - In many
cases the FCF will be followed by the transmission of additional
8-bit octets to further clarify the facsimile procedure. This
information for the basic binary coded system would consist of the
definition of the information in DIS, DCS, DTC, CSI, CIG, TSI, NSC,
NSF, NSS, CTC, PPS and PPR signals.
A.4.1 Command to receive | (see also S 5.3.6.1.3)
From the transmitter to the receiver.
Format: X100 XXXX
1) Continue to correct (CTC) - This command indi-
cates that the transmitting station shall continue to correct the
previous message. (This is a response to the 4th PPR received, and
indicates that the transmitting station shall immediately send the
requested frames specified in PPR information field.)
When the transmitter receives PPR four times, the modem
speed may fall back or continue the previous transmission speed
using CTC command.
This command should have the FIF of 2 octets, which
corresponds to the bits No. 1-16 of DCS standard command (See
Table 2/T.30). The receiving terminal uses only the bits No. 11-14
to determine the data signalling rate.
Format: X100 1000
A.4.2 Pre-message response signals | (see also S
5.3.6.1.4)
From the receiver to the transmitter.
Format: X010 XXXX
1) Response for continue to correct (CTR) - This
signal is the digital response to CTC signal, so that the receiving
terminal can accept the contents included in CTC signal.
Format: X010 0011
A.4.3 Post message commands | (see also S 5.3.6.1.6)
From the transmitter to the receiver.
Format: X111 XXXX
1) Partial page signal (PPS) - This command indi-
cates the end of a partial page or a complete page of facsimile
information and also indicates to return to the beginning of
phase B or C upon receipt of MCF.
Format: X111 1101
The frame construction of PPS command and transmission
order of bits included in I1-I3 are shown in Figure A-1/T.30.
Figure A-1/T.30, (N), p.
2) End of retransmission (EOR) - This command
indicates that the transmitter decides to terminate the retransmis-
sion of error frames in the previous partial page and to transmit
the next block upon receipt of ERR response.
Format: X111 0011
The frame construction of EOR command is shown in
Figure A-2/T.30.
Figure A-2/T.30, (N), p.
3) Receive ready (RR) - This command is used to
ask for the status of the receiver.
Format: X111 0110
Note 1 - This command is defined for flow control.
Note 2 - For flow control method, make reference to S A.5.
A.4.4 Post-message responses | (see also S 5.3.6.1.7)
From the receiver to the transmitter.
Format: X011 XXXX
1) Partial page request (PPR) - This signal indi-
cates that the previous message has not been satisfactorily
received and that the frames specified in the associated facsimile
information field are required to be retransmitted.
Format: X011 1101
The facsimile information field of the PPR signal is a fixed
length of 256 bits, each bit corresponds to an FCD frame i.e., the
first bit to the first frame etc. For FCD frames which are received
correctly, the corresponding bit in the PPR information field will
be set to "0"; those that are received incorrectly or not received
will have their bit set to "1".
If more than one PPR signal is transmitted, the bit
corresponding to an FCD frame which has been received correctly
must always be set to "0".
The frame construction of PPR response is shown in
Figure A-3/T.30.
The process of an error correction is shown in
Figure A-4/T.30.
Note 1 - The number of frames in a partial page is less than
or equal to 256 frames. Therefore, in some circumstances there may
be extra bits that do not correspond to any frames. These bits are
set to "1" (see Figure A-5/T.30).
Note 2 - The first bit in the FIF corresponds to the first
frame (frame No. 0).
2) Receive not ready (RNR) - This signal is used
to indicate that the receiver is not ready to receive more data.
Format: X011 0111
Note 1 - This signal is defined for flow control.
Note 2 - For flow control, make reference to S A.5.
3) Response for end of retransmission (ERR) -
This signal is the digital response to EOR signal.
Format: X011 1000
Figure A-3/T.30, (N), p.
Figure A-4/T.30, (N), p.
Figure A-5/T.30, (N), p.
A.5 Flow control procedure
A.5.1 Flow control in the transmitting station is made by con-
tinuous flag transmission between frames or before the first frame.
A.5.2 The maximum transmission time of flags should be less
than the value of timer T1.
A.5.3 In case of transmission on a noisy channel, a long flag
sequence may be destroyed by noise. Therefore, it is recommended
that the receiver implement a control procedure to discard invalid
frames which are obtained from erroneous flag sequences.
A.5.4 Flow control in the receiving station is made using
RR/RNR signals as shown in Figure A-6/T.30.
A.5.4.1 Inactivity timer T5 is defined as follows:
T5 = 60 s _ 5 s.
Note - As the use of the T5 timer reduces transmission effi-
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