1、I l INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMU N CATION STAN DARD IZATION SECTOR OF ITU SERIES V: DATA COMMUNICATION OVER THE TELEPHONE NETWORK Interfaces and voiceband modems V.25 (1 0/96) Automatic answering equipment and general procedures for automatic calling equipment on the general
2、 switched telephone network including procedures for disabling of echo control devices for both manually and automatically established calls ITU-T Recommendation V.25 (Previously CCITT Recommendation) ITU-T V-SERIES RECOMMENDATIONS DATA COMMUNICATION OVER THE TELEPHONE NETWORK 3 - Wideband modems 4
3、- Errorcontrol 5 - Transmission quality and maintenance 6 - Interworking with other networks Forjurther details, please refer io ITU-T List of Recommendations. STD*ITU-T RECMN V.25-ENGL L99b Li8b257L Ob28293 L5Li FOREWORD The IT-T (Telecommunication Standardization Sector) is a permanent organ of th
4、e International Telecommunication Union (IT). The IT-T is responsible for studying technical, operating and tariff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), whic
5、h meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the IT-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993).
6、ITU-T Recommendation V.25, was revised by IT-T Study Group 14 (1993-1996) and was approved by the WTSC (Geneva, October 9-18,1996). NOTES 1. telecommunication administration and a recognized operating agency. In this Recommendation, the expression “Administrationy is used for conciseness to indicate
7、 both a 2. follows: The status of annexes and appendices attached to the Series V Recommendations should be interpreted as - - an annex to a Recommendation forms an integral part of the Recommendation; an appendkc to a Recommendation does not form part of the Recommendation and only provides some co
8、mplementary explanation or information specific to that Recommendation. O ITU 1997 All rights reserved. No part of this publication may 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. R
9、ecommendation V.25 (10/96) 1 1 2 3 4 5 6 7 8 9 10 11 CONTENTS Scope Abbreviations and definitions Interface procedures at call-originating data station Interface procedure at answering data station Manually controlled modem calling automatically controlled modem Automatically controlled calling mode
10、m calling manually controlled modem Proposed line procedures . Disabling of echo suppressors in the case of manually operated data stations Notification to ordinary telephone users Manual selection of automatic answering, data mode and voice mode . 2 1 O0 Hz tone recognition . References . 11 Recomm
11、endation V.25 (10196) Page 1 2 3 6 6 7 7 8 8 8 8 9 STD-ITU-T RECMN V-25-ENGL 199b 48b257L Ob28295 T27 Recommendation V.25 AUTOMATIC ANSWERING EQUIPMENT AND GENERAL PROCEDURES FOR AUTOMATIC CALLING EQUIPMENT ON THE GENERAL SWITCHED TELEPHONE NETWORK INCLUDING PROCEDURES FOR DISABLING OF ECHO CONTROL
12、DEVICES FOR BOTH MANUALLY AND AUTOMATICALLY ESTABLISHED CALLS (Mar dei Plata, 1968; amended at Geneva, 1972 and 1976; revised at Malaga-Torremolinos in 1984 and at Geneva in 1996) 1 Scope 1.1 This Recommendation is concerned with the setting-up of a data connection when Data Circuit-terminating Equi
13、pment (DCE) comprising automatic answering equipment is used over international circuits. Automatic calling procedures are defined in Recommendations V.25 bis and V.25 ter. NOTE - Up to and including the 1988 (Blue Book) version, this Recommendation, in addition to what is outlined herein, specified
14、 parallel automatic calling equipment using the 200-Series interchange circuits defined in Recommendation V.24, and an associated automatic call Set-up protocol. It is the opinion of the ITU-T that both this type of equipment and the associated protocol are out of date, and that more modem technique
15、s and protocols are specified in Recommendations V.25 bis and V.25 ter. It is therefore not recommended to use this technique for new designs. Automatic answering equipment used within any single Members area or between two Members by bilateral agreement is not necessarily constrained by the procedu
16、res specified herein. In particular, the use of the 2 100 Hz answering tone, as described in this Recommendation, could be substituted by another tone when the equipment is used over circuits not equipped with echo control devices. Similarly, the calling tone could be omitted by bilateral agreements
17、, but attention is drawn to clauses 7 and 8 below. in addition, the provisions for echo canceller disabling and for a “calling station response” prior to the temination of the answer tone are optional and only applicable to Data Circuit-terminating Equipment (DCE) for which the relevant V-Series Rec
18、ommendation specifically calls for such provision(s). 1.2 This Recommendation describes the sequence of events involved in establishing a connection between an automatic calling data station) and an automatic answering data station for V-Series Recommendations modems specified for general switched t
19、elephone network operations. Consideration is given only to: a) b) the events which affect the interfaces between the data terminal equipment and the data circuit- terminating equipment at the answering data station; and the events on the line during establishment of a data call. Interactions within
20、 the data circuit-terminating equipment are not considered, since such consideration is unnecessary for purposes of intemational standardization. 1.3 The proposed procedures are intended to be suitable for four types of calls, namely: a) b) c) d) automatic calling data station to automatic answering
21、 data station; manual calling data station to automatic answering data station; automatic calling data station to manual answering data station; disabling of echo suppressors in the case of manual calling data stations. 1) In this Recommendation, the term “data station” is synonymous with the term “
22、terminal installation for data transmission” i. Recommendation V.25 (10/96) 1 1.4 The data terminal equipment is responsible for: a) during call establishment: i) ensuring that the data circuit-terminating equipment is available for operation; ii) providing the telephone number; iii) deciding to aba
23、ndon the call if it is unsuccessfully completed; b) after call is established: i) establishing identities; ii) exchanging such traffic as is appropriate; iii) initiating disconnect at calling and answering station. 2 Abbreviations and definitions The following abbreviations are used in this Recommen
24、dation: DCE Data Circuit-terminating Equipment DTE Data Terminal Equipment The following definitions apply to this Recommendation: 2.1 calling tone: The tone transmitted from the calling end. The calling tone consists of a series of interrupted 1300 Hz f 15 Hz signals (bursts). ON for a duration of
25、not less than 0.5 s and not more than 0.7 s and OFF for a duration of not less than 1.5 s and not more than 2.0 s.*) 3, 2.2 answering tone: The tone transmitted from the called end. NOTES 1 - The answering tone is an uninterrupted 21 O0 f 15 Hz tone with a duration, except when truncated as describe
26、d in 4.3, of 3.3 t 0.7 s. 2 - Recommendation V.8 specifies, for the purpose of starting sessions of data transmission, an amplitude modulated answering tone, ANSam. 2.3 phase reversals: Reversals (180“) in the phase of the answering tone at intervals of 425 to 475 ms. The reversal in phase shall be
27、accomplished such that the phase is within 180 f 1 O degrees in 1 ms, and that the amplitude of the answering tone is not more than 3 dB below its steady state value for more than 400 p. 2.4 starting signal: Binary 1, synchronizing signal or equalizer training signal, as specified in the relevant V-
28、Series Reommendation.) 2.5 calling station response: A tone or signal transmitted from the calling DCE in response to its detection, as defined in this Recommendation, of answering tone.4). 5)9 6, 2.6 automatic calling: A procedure by which a DTE, by use of V.24 interchange circuits, may instruct a
29、DCE to perform the call establishment function. The transmission, fom DTE to DCE, of each digit to be dialled is achieved on interchange circuit 103 (see Recommendations V.25 bis and V.25 ler). 2, The calling tone and calling station response should not contain power in the band 2100 f 250 Hz. 3, Th
30、e power levels of the signals specified in this Recommendation shall conform to the levels specified in Recommendation V.2. 4, The calling tone and calling station response should not contain power in the band 2100 f 250 Hz. 5, The power levels of the signals specified in this Recommendation shall c
31、onform to the levels specified in Recommendation V.2. 6, The specification of the calling station response and the timing of its transmission are the subject of the individual V-Series Recommendation for the DCE involved. The specifications in this Recommendation cover only limitations on its transm
32、ission during call establishment. 2 Recommendation V.25 (10196) STD*ITU-T RECMN V.25-ENGL 179b 48b2571 Ob28297 8TT M 3 Interface procedures at cai-originating data station The description of the complete interface procedures for call initiation at a call-originating data station is not part of this
33、Recommendation. Details may be found in Recommendations V.25 bis and V.25 fer. Event 3.1 The DTE instructs the DCE to establish a call, and specifies the number to be dialled. Subsequently, circuit 10812 is in the ON condition (circuit 108/2 may be turned in the ON condition at any time up to and in
34、cluding event 3.6). 3.2 For half-duplex modems, DTE puts circuit 105 ON if the calling end wishes to transmit first. Circuit 105 can be placed ON at any time up to and including event 3.1 O. 3.3 The DCE goes “off-hook”. 3.4 3.5 3.6 3.7 The telephone system puts dial tone on line.) The DCE dials the
35、wanted telephone number. The DTE tunis circuit 108/2 ON, ifnot previously ON. If implemented, the DCE transmits the interrupted calling tone to line, as shown in Figures 1,2 and 3. DTE, CT 107 ON Calling tone r0.5-0.7 S7: 1.5-2 s r0.5-0.7 Y r-i L-J II 3.3 I 0.7 s Answering tone 2100 Hz 0.9 s t t t N
36、etwork inter- Not sufficient to suppressors action time detection time I II FIGURE 1N.25 Timing of line signais 7, Some counties apply the second dial tone to the line after the initial digit is transferred. Gap for recognizing the end of the 2100 Hz answering tone I I Ti 403050-97/dOl Recommendatio
37、n V.25 (10/96) 3 Calling tone burst truncateda) 4 r400ms Calling station m Timing at calling Calling tone data station Answering data station connected to line 1 Calling station response detected for 2 100 ms - I ANS=) Timing at answering data station / / / / / / / / / / / / / / / DCEunder / CT 107
38、ONb) Answer modem line signal 75 f 20 ms Silent interval 5 4.0 sd) T1403060-97/d02 a) If ANS is detected during a calling tone burst, the burst may be truncated. If it is not truncated, the calling station response must be delayed until at least 400 ms afier the end of the burst. b, See 3.10 for exc
39、eption. c, ANS denotes the answer tone. d, If a calling station response is not received, the answer tone shall continue for 3.3 f 0.7 seconds. FIGURE 2N.25 Timing of line signais - Optional calling station response 3.8 a) If the call is answered by a data station, then the answering tone is receive
40、d by the calling DCE. Echo suppressors and echo cancellers are disabled during coincidence of a silent period in the interrupted calling tone (event 3.7) with the answering tone. The answering tone shall not activate circuits 104 and 109. b) The subsequent action of the DCE and the DTE if no answeri
41、ng tone is detected is beyond the scope of this Recommendation. Details may be found in Recommendations V.25 bis and V.25 ter. 3.9 When an answering tone has been received by the DCE for a period of 100 to 600 ms, the interrupted calling tone is discontinued by the DCE as shown in Figures 1, 2 and 3
42、. The DCE transfers control of the connection to circuit 10812. 4 Recommendation V.25 (10/96) STD-ITU-T RECMN V.25-ENGL L77b 48b259L b28299 b72 m Calling tone burst truncateda) Timing at Calling tone calling data burst station I for 2 100 ms/ Answering data station connected to line 1 I/ Calling sta
43、tion response detected for 2 100 rns I DCEunder control of 1 theDTE, CT 107 ONb) Timing at - Silent interval ST k- 75 i: 20 rns T T T = 450 i 25 ms I -1 T1403070-97ld03 5 4.0 sd) If ANS is detected during a calling tone burst, the burst may be truncated. If it is not truncated, the calling station r
44、esponse must be delayed until at least 1 second after the end of the burst. ANS denotes the answer tone. received. b, See 3.10 for exception. dl The answer tone duration must be at least 2.6 seconds if a calling station response is not denotes the answer tone with its phase reversed. FIGURE 3N.25 Ti
45、ming of tine signals, optional provision for echo canceller disabling and for calling station response The DCE may, as shown in Figures 2 and 3, transmit the calling station response following the detection of a continuation of the answering tone for a period of at least 400 ms after the transmissio
46、n of the calling tone is terminated. As indicated in Figure 3, the required duration (2 1 s) of the continuous 2100 Hz period, which must follow the termination of the calling tone, is longer if the answering tone includes phase reversals to disable echo cancellers. 3.10 The DCE examines the line to
47、 determine the end of the answering tone. The DCE detects an absence of the answering tone for 75 f 20 ms, and then turns circuit 107 ON:*) i) If circuit 105 is ON, the starting signal is put on the line. After its delay as specified in the appropriate V-Series Recommendation, circuit 106 comes ON,
48、and the DTE can then transmit data. ii) If circuit 105 is OFF, the incoming starting signal is recognized, and &er its delay as specified in the appropriate V-Series Recommendation, the DCE turns circuit 109 ON to allow the examination of circuit 104 by the DTE. iii) For the duplex modem case, where
49、 circuit 105 is not used, the starting signal is put on the line after circuit 107 has been turned ON. The DCE then turns circuits 109 and 106 ON after a delay as specified in the appropriate V-Series Recommendation. *) For some DCEs requiring extended training sequences, the associated V-Series Recommendation may speciQ that circuit 107 be turned ON at some later time, during the handshake sequence, which is more consistent with the specification in Recommenda- tion V.24 of circuit i 07. Recommendation V.25 (10/96) 5 3.11 Details may be found in Recommenda
