INTERNATIONAL  TELECOMMUNICATION  UNION	

CCITT	V.17
THE  INTERNATIONAL
TELEGRAPH  AND  TELEPHONE
CONSULTATIVE  COMMITTEE

DATA  COMMUNICATION
OVER  THE  TELEPHONE  NETWORK

A  2-WIRE  MODEM  FOR  FACSIMILE
APPLICATIONS  WITH  RATES  UP  
TO  14 400  bit/s

Recommendation  V.17

	Geneva, 1991	

Printed in Switzerland

FOREWORD
	The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
	The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988).
	Recommendation V.17 was prepared by Study Group XVII and was approved under the Resolution No. 2 procedure on the 22 of February 1991.

___________________

CCITT  NOTE
	In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication Administration and a recognized private operating agency.

ã  ITU  1991
All rights reserved. No part of this publication may 0 be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.

PAGE BLANCHE

Recommendation V.17
Recommendation V.17
A  2 -WIRE  MODEM  FOR  FACSIMILE  APPLICATIONS  WITH
RATES  UP  TO  14 400  bit/s
1	Introduction
	This Recommendation defines the modulation methods and operating sequences for a modem intended only for use in high speed facsimile applications.
	Appropriate T-Series Recommendations should be consulted regarding operating procedures and other features employed in facsimile transmission applications, as these differ from those recommended for high speed modems for general applications.
	The modem has the following principal characteristics:
a)	Provision for half duplex operation at data signalling rates of:
—	14 400 bit/s synchronous,
—	12 000 bit/s synchronous,
—	9600 bit/s synchronous,
—	7200 bit/s synchronous;
b)	Quadrature amplitude modulation with synchronous line transmission at 2400 symbols per second.
c)	Inclusion of data scramblers, adaptive equalizers and eight-state trellis coding.
d)	Two sequences for training and synchronization: long train and resync.
2	Line signals
2.1	Carrier frequency
	The channel carrier frequency is 1800 ± 1 Hz. The receiver must be able to operate with received frequency offsets of up to ± 7 Hz.
2.2	Modulation
	The modulation rate shall be 2400 ± 0.01% symbols per second.
2.3	Signal element codings
2.3.1	Signal element codings for 14 400 bit/s
	The scrambled data stream to be transmitted is divided into groups of six consecutive data bits, which are ordered according to their time of occurrence. As shown in Figure 1/V.17, the first two bits in each group, Q1n and Q2n (where n designates the sequence number of the group) are first differentially encoded into Y1n and Y2n according to Table 1/V.17.

Figure 1/V.17  =  13.5 cm

TABLE 1/V.17
Differential coding for use with trellis coding
Input		Previous outputs		Outputs		
Q1n	Q2n	Y1n—1	Y2n—1	Y1n	Y2n	
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1	0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1	0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1	0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1	0
0
1
1
0
0
1
1
1
1
0
0
1
1
0
0	0
1
0
1
1
0
1
0
0
1
1
0
1
0
0
1	

	The two differentially encoded bits Y1n and Y2n are used as inputs to a systematic convolutional encoder which generates a redundant bit Y0n. This redundant bit and the six information-carrying bits Y1n, Y2n, Q3n, Q4n, Q5n and Q6n are then mapped into the coordinates of the signal element to be transmitted according to the signal space diagram shown in Figure 2/V.17.
Figure 2/V.17  =  21 cm

2.3.2	Signal element codings for 12 000 bit/s
	The scrambled data stream to be transmitted is divided into groups of five consecutive data bits, which are ordered according to their time of occurrence. As shown in Figure 1/V.17, the first two bits in each group, Q1n and Q2n (where n designates the sequence number of the group) are first differentially encoded into Y1n and Y2n according to Table 1/V.17.
	The two differentially encoded bits Y1n and Y2n are used as inputs to a systematic convolutional encoder which generates a redundant bit Y0n. This redundant bit and the five information-carrying bits Y1n, Y2n, Q3n, Q4n and Q5n are then mapped into the coordinates of the signal element to be transmitted according to the signal space diagram shown in Figure 3/V.17.
Figure 3/V.17  =  18.5 cm

2.3.3	Signal element codings for 9600 bit/s
	The scrambled data stream to be transmitted is divided into groups of four consecutive data bits, which are ordered according to their time of occurrence. As shown in Figure 1/V.17, the first two bits in each group, Q1n and Q2n (where n designates the sequence number of the group) are first differentially encoded into Y1n and Y2n according to Table 1/V.17.
	The two differentially encoded bits Y1n and Y2n are used as inputs to a systematic convolutional encoder which generates a redundant bit Y0n. This redundant bit and the four information-carrying bits Y1n, Y2n, Q3n and Q4n are then mapped into the coordinates of the signal element to be transmitted, according to the signal space diagram shown in Figure 4/V.17.
Figure 4/V.17  =  13 cm

2.3.4	Signal element codings for 7200 bit/s
	The scrambled data stream to be transmitted is divided into groups of three consecutive data bits, which are ordered according to their time of occurrence. As shown in Figure 1/V.17, the first two bits in each group, Q1n and Q2n (where n designates the sequence number of the group) are first differentially encoded into Y1n and Y2n according to Table 1/V.17.

	The two differentially encoded bits Y1n and Y2n are used as inputs to a systematic convolutional encoder which generates a redundant bit Y0n. This redundant bit and the three information-carrying bits Y1n, Y2n and Q3n are then mapped into the coordinates of the signal element to be transmitted according to the signal space diagram shown in Figure 5/V.17.
Figure 5/V.17  =  11 cm

2.4	Transmitted spectra
	With continuous binary ONEs applied to the input of the scrambler, the transmitted energy density at 600 Hz and 3000 Hz should be attenuated by 4.5 ± 2.5 dB with respect to the maximum energy density between 600 Hz and 3000 Hz.
3	Interchange circuits
3.1	List of interchange circuits
	References in the Recommendation to V.24 interchange circuit numbers are intended to refer to the functional equivalent of such circuits and are not intended to imply the physical implementation of such circuits. For example, references to circuit 103 should be understood to refer to the functional equivalent of circuit 103 (see Table 2/V.17).
TABLE 2/V.17
Interchange circuits
Number	Description	
102
103
104
105
106
107
108/1 or
108/2
109
114
115
125	Signal ground or common return
Transmitted data
Received data
Request to send
Ready for sending
Data set ready
Connect data set to line (Note)
Data terminal ready (Note)
Data channel received line signal detector
Transmitter signal element timing (DCE source)
Receiver signal element timing (DCE source)
Calling indicator	
Note — This circuit shall be capable of operating as circuit 108/1 or circuit 108/2.

3.2	Transmit data
	The modem shall accept data from the facsimile control function on circuit 103; the data on circuit 103 shall be under the control of circuit 114.
3.3	Receive data
	The modem shall pass data to the facsimile control function on circuit 104; data on circuit 104 shall be under the control of circuit 115.
3.4	Timing arrangements
	Clocks shall be included in the modem to provide the facsimile control function with transmitter element timing on circuit 114 and receiver signal element timing on circuit 115.
3.5	Data rate control
	This shall be provided by a connection between the modem and the facsimile control function; the nature of this connection is beyond the scope of this Recommendation.
3.6	Circuits 106 and 109 response times
	After the training and synchronizing sequences defined in § 5, circuit 106 shall follow OFF to ON or ON to OFF transitions of circuit 105 within 3.5 ms. The OFF to ON transition of circuit 109 is part of the training sequence specified in § 5. Circuit 109 shall turn OFF 30 to 50 ms after the level of the received signal appearing at the line terminal of the modem falls below the relevant threshold defined in § 3.7. Following a dropout, after the initial handshake, circuit 109 shall turn ON 40 to 205 ms after the level of the received signal appearing at the line terminal of the modem exceeds the relevant threshold defined in § 3.7.
3.7	Circuit 109 threshold
> —43 dBm ON.
> —48 dBm OFF.
	The condition of circuit 109 between the ON and OFF levels is not specified except that the signal detector shall exhibit a hysteresis action, such that the level at which the OFF to ON transition occurs shall be at least 2 dB greater than that for the ON to OFF transition.
	Circuit 109 thresholds are specified at the input to the modem when receiving scrambled binary ONEs.
	Administrations are permitted to change these thresholds where transmission conditions are known.
	Note — Circuit 109 ON to OFF response time should be suitably chosen within the specified limits to ensure that all valid data bits have appeared on circuit 104.
3.8	Clamping
	The DCE shall hold, where implemented, circuit 104 in the binary ONE condition and circuit 109 in the OFF condition when circuit 105 is in the ON condition and, where required to protect circuit 104 from false signals, for a period of 150 ± 25 ms following the ON to OFF transition on circuit 105. The use of this additional delay is optional, based on system considerations.
4	Scrambler and descrambler
	The modem shall use a self-synchronizing scrambler/descrambler with the generator polynomial:

	At the transmitter, the scrambler shall effectively divide the message data sequence by the generating polynomial. The coefficients of the quotient of this division, taken in descending order, form the data sequence which shall appear at the output of the scrambler. At the receiver, the received data sequence shall be multiplied by the scrambler generating polynomial to recover the message sequence.
5	Operating sequences
5.1	Training and synchronizing sequences
	Two separate sequences of training and synchronizing signals are defined in Table 3/V.17.
	The long train sequence is for initial establishment of a connection or for retraining when needed.
	The resync. sequence is for resynchronization after a successful long train.
TABLE 3/V.17
Training and synchronizing signals
	Segment 1	Segment 2	Segment 3	Segment 4			
	ABAB alternations	Equalizer training signal	Bridge signal	Scrambled ONEs	Total symbol interval	Approximate time (ms)	
Long train	256	2976	64	48	3344	1393	
Resync.	256	2938	64	48	3342	1142	

5.1.1	Segment 1: ABAB alternations
	This segment consists of alternations between states A and B as shown in Figures 2/V.17 to 5/V.17.
5.1.2	Segment 2: equalizer training signal
	This segment consists of the sequential transmission of four signal elements A, B, C, and D as shown in Figures 2/V.17 to 5/V.17.
	The equalizer conditioning pattern is a pseudo-random sequence at 4800 bit/s generated by the 
1 + x—18 + x—23 data scrambler. During segment 2, any differential quadrant encoding is disabled and the scrambled dibits are encoded as shown in Table 4/V.17.
	With a binary ONE applied to the input, the initial scrambler state shall be selected to produce the following scrambler output pattern and corresponding signal elements:
TABLE 4/V.17
Encoding for four phase training signal
00
C	01
D	00
C	01
D	00
C	01
D	00
C	01
D	00
C	01
D	00
C	01
D	10
B	01
D	10
B	01
D	
Segment 2
Dibit	Signal state	
00
01
11
10	C
D
A
B	

5.1.3	Segment 3: bridge signal
	This segment, which is used only during an initial long train, consists of a 16-bit binary sequence transmitted eight times. The sequence as defined in Table 5/V.17 is scrambled, and transmitted at 4800 bit/s using the signal elements A, B, C, and D as defined in Figures 2/V.17 to 5/V.17.
TABLE 5/V.17
Segment 3: Bit designations
B	B	B	B	B	B	B	B	B	B	B	B	B	B	B	B	
0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	
0	0	0	0	0	0	0	1	0	0	0	1	0	0	0	1	
Note 1 — The B0 bit is the first bit in the data stream as it enters the scambler.
Note 2 — Any use of bits 4-6, 8-10, 12-14 would be for further study. Some existing equipment may set one or more of these bits to binary ONE; such bits shall be ignored.

	The dibits are differentially encoded as defined in Table 6/V.17.
	The differential encoder shall be initialized using the final symbol of the previous segment. The first two bits and subsequent dibits of each 16-bit sequence shall be encoded as one signal state.
TABLE 6/V.17
Segment 3: Dibit encoding
Dibit	Phase change		Previous output/output				
00
01
10
11	+ 90
0
180
— 90	degrees
degrees
degrees
degrees	A/B, B/C, C/D, D/A
A/A, B/B, C/C, D/D
A/C, B/D, C/A, D/B
A/D, B/A, C/B, D/C				

5.1.4	Segment 4
	Scrambled binary ONEs shall be sent at the channel data bit rate.
	For the long train sequence, the differential encoder shall be initialized using the first symbol of segment 3.
	For the short train sequence, the differential encoder shall be initialized using the last symbol of segment 2.
	The convolutional encoder initial state shall be initialized to zero.

	The scrambler shall be clocked at the bit rate and the scrambler output sequence encoded as defined in § 4. The initial scrambler state is that state produced by the last symbol interval of the previous segment.
	The duration of segment 4 is 48 symbol intervals. At the end of segment 4, circuit 106 is turned ON and data are applied to the input of the data scrambler.
	Circuit 109 shall be turned ON during the reception of segment 4.
5.2	Turn OFF sequence
	After an ON to OFF transition of circuit 105, the line signals emitted after remaining data or the end of the training check signal during retrain procedure have been transmitted are shown in Table 7/V.17.
TABLE 7/V.17
Turn OFF sequence
Segment A	Segment B			
Continuous scrambled ONEs	No transmitted energy	Total of segments	Approximate
 time	
32 SI	48 SI	80 SI	33 ms	
SI denotes Symbol intervals
Note — If an OFF to ON transition of circuit 105 occurs during the turn OFF sequence, it will not be taken into account until the end of the turn OFF sequence.

5.3	Talker echo protection (TEP) signal
	A TEP signal may, optionally, be transmitted prior to the transmission of training and synchronization sequences. The TEP signal shall consist of an unmodulated carrier for a duration of 185 to 200 ms followed by a silent period of 20 to 25 ms.
	When used, the TEP signal shall be considered as part of the training sequences.
	Alternative methods for achieving the intended benefits of a TEP signal are for further study.