1、 STD-ITU-T RECMN V-43-ENGL 3998 = V86259L Ob50893 2V3 D i NTERNATIONAL TELECOM M U N ICATION UN ION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU SERIES V: DATA COMMUNICATION OVER THE TELEPHONE NETWORK Error control v.43 (02/98) Data flow control ITU-T Recommendation V.43 (Previously CCITT R
2、ecommendation) STDmITU-T RECMN VOq3-ENGL 1998 W 4862591 0650872 L8T = ITU-T V-SERIES RECOMMENDATIONS DATA COMMUNICATION OVER THE TELEPHONE NETWORK II II General v. 1 -v.9 Interfaces and voiceband modems II Wideband modems v. I eV. 34 v.35-v.39 Transmission quality and maintenance Simultaneous transm
3、ission of data and other signals Interworking with other networks Interface layer specifications for data communication Control procedures Modems on digital circuits v.50-v.59 V.60-V.99 v. I 00-v.199 V.200-V.249 V.250-V.299 V.300-V.399 For firther details, please refer to ITU-T List of Recommendatio
4、ns. - STDmITU-T RECMN VoY3-ENGL 3998 9 4862593 Ob50893 03b 9 ITU-T RECOMMENDATION V.43 DATA FLOW CONTROL Summary This Recommendation I International Technical Report provides guidance for the choice of an appropriate method for, and the implementation of, data flow control capabilities in DTEs and D
5、CEs. It also coordinates information from other Recommendations and International Standards and provides tutorial material on these flow control techniques. Source ITU-T Recommendation V.43 was prepared by ITU-T Study Group 16 (1997-2000) and was approved under the WTSC Resolution No. 1 procedure on
6、 the 6* of February 1998. This Recommendation is technically identical with the ISOAEC International Technical Report No. 15294 - STD-ITU-T RECMN V.43-ENGL 1998 4862593 0650894 T52 FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommu
7、nications. The IT Telecommunication Standardization Sector (ITU-T) is a permanent organ of the IT. The ITU-T 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 World Telec
8、ommunication Standardization Conference (WTSC), which 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 ITU-T is covered by the procedure laid down in
9、 WTSC Resolution No. 1. In some areas of information technology which fali within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication
10、administration and a recognized operating agency. INTELLECTUAL PROPERTY RIGHTS The ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validi
11、ty or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, the ITU had not received notice of intellectual property, protected by patents, which may be re
12、quired to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. O ITU 1998 All rights reserved. No part of this publication may be reproduced or utilized in any form o
13、r by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. Recommendation V.43 (02198) 11 STD-ITU-T RECMN V.43-ENGL 1998 m 48b2591 Ob50895 999 m CONTENTS Page 1 Scope 2 References 3 Abbreviations . 4 4.1 Data flow control methods . Met
14、hods for flow control of transmitted data . 4.1 . 1 4.1.2 Synchronous mode of operation Methods for flow control of received data . 4.2.1 Asynchronous mode of operation . 4.2.2 Synchronous mode of operation . Asynchronous mode of operation 4.2 1 1 1 2 2 3 Recommendation V.43 (02/98) iii STD-ITU-T RE
15、CMN V.lI3-ENGL 1998 YB62591 Ob50896 825 Introduction When using DCEs incorporating data compression and/or error correction, it is essential that the DCE have some method of controlling the flow of data from the DTE (a similar requirement applies for the associated DES; see below). This is because t
16、he degree of compression obtained will vary fi-om moment to moment, and the buffers in the DCE may fill up during periods of lower compression, or while correction of transmission errors is active. Likewise, during periods of high compression, the DTE may become overloaded with the amount of incomin
17、g data and may not be able to process this data properly without means of controlling the flow of data from the DCE. There are many methods of flow control in existence, and DCE and DTE designers should ensure that they provide methods suitable for the associated DTE and DCE, respectively, to be use
18、d. This Recommendation aims at giving guidelines in order to assist DTE and DCE designers in their tasks. It lists several mechanisms which are known to operate successfully with DTEs and DCEs although no single mechanism will operate with all DTEs and DCEs, and some DTEs may not respond to any of t
19、he mechanisms described. Both the synchronous and the asynchronous modes of operation are addressed. NOTE - The guidelines given in this Recommendation I International Technical Report may not be exhaustive. iv Recommendation V.43 (02/98) Recommendation V.43 DATA FLOW CONTROL (Geneva, I998) 1 Scope
20、This Recommendation I International Technical Report provides guidance for the choice of an appropriate method for, and the implementation of, data flow control capabilities in DTEs and DCEs. It also coordinates information from other Recommendations and International Standards and provides tutorial
21、 material on these flow control techniques. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations
22、 and other references are subject to revision; all users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly publi
23、shed. 11 ITU-T Recommendation V.24 (1 996), List of definitions fur interchange circuits between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE). 3 Abbreviations This Recommendation uses the following abbreviation: IRA International Reference Alphabet 4 Data flow control m
24、ethods The guidance is provided for: - - each of which is subdivided according to asynchronous and synchronous modes of operation. It is assumed that the asynchronous mode of operation in the context of this Recommendation I International Technical Report is accomplished using synchronous DCEs incor
25、porating asynchronous-to-synchronous conversion according to e.g. Recommendation V. 14 or V.42. This Recommendation 1 International Technical Report restricts itself to the description of methods for flow control that exist locally between a DTE and a DCE. Methods for end-to-end flow control involvi
26、ng the two DTEs or the two DCEs concerned are beyond the scope of this Recommendation 1 International Technical Report. At present this Recommendation I International Technical Report describes only flow control methods that operate at the physical layer of the OS1 Reference Model. In the context of
27、 this Recommendation I International Technical Report, the use of DCl/DC3 characters is also understood as to fall in this category. Methods that involve the DCE implementing Layer 2 or higher layer protocols, for example HDLC, are for further study. flow control of transmitted data; flow control of
28、 received data, Recommendation V.43 (02/98) 1 STDmITU-T RECMN V-43-ENGL 1998 4862591 Ob50898 bTB 4.1 The control function of the DCE shall be capable of indicating to the DTE a temporary inability to accept data on circuit 103, Transmitted Data (DCE not-ready condition). Upon receiving such an indic
29、ation, the DE shall ideally complete transmission of any partially transmitted character (asynchronous mode of operation) or frame (synchronous mode of operation) and then cease transmitting data on circuit 103 and clamp circuit 103 to binary 1. When the DCE not-ready condition is cleared, the DTE m
30、ay resume the transmission of data on circuit 103. Figure 1 depicts the interchange circuits at the DTE-DCE interface and the functionalities inside the DTE and the DCE, respectively, which may be used, as appropriate, for the flow control of Methods for flow control of transmitted data transmitted
31、data. DTE DCE Data from higher layer output function detection 1 03 106 SIA 112 A 1 i4 S 104 Flow control of transmitted data Control function IA 1 DC3DCI Line - Line - A Applicable for the asynchronous mode of operation S Applicable for the synchronous mode of operation T16W450-97 Figure 1N.43 4.1.
32、1 Asynchronous mode of operation Two standard methods exist. 4.1.1.1 a) Flow control by use of V.24 interchange circuits Use of circuit 106 - Ready for sending The DCE not-ready condition is indicated by turning circuit 106 OFF, and cleared by turning circuit 106 ON. This method should be the prefer
33、red one because it is unambiguous and is applicable to any kind of data communication. However, many DTEs will not immediately recognize the OFF condition on circuit 106 and will not cease transmitting at the end of the present character. Instead, these DTEs will complete the present frame which may
34、 be up to some thousand octets long, and only then detect the OFF condition on circuit 106. 2 Recommendation V.43 (02198) STD-ITU-T RECMN VD43-ENGL 3998 a 4862573 Ob50899 534 m It is therefore suggested that the remaining buffer capacity inside the DCE be kept large enough to cope with this conditio
35、n. A reasonable remaining buffer size suggested may be between 2000 and 4000 octets, so that the minimum total buffer size may be between 4000 octets (DCE not-ready condition is indicated to the DTE when the buffer is half-full) and 10 O00 octets (DCE not-ready condition is indicated to the DTE when
36、 about 80% of the buffer is full). NOTE - This assumes that a remaining buffer size of about 2000 octets is needed. For a DTE to be able to interwork with DCEs incorporating data compression and/or error correction it is advisable that the DTE cyclically check the condition of circuit 106 and, upon
37、recognition of the OFF condition on circuit 106, stop the transmission of data as soon as possible. Use of circuit 112 - Data signalling rate selector (DCE source) The DCE may, by controlling circuit 1 12, select one of two data signalling rates in the DTE to coincide with the data signalling rate i
38、n use in the DCE. The ON condition on this circuit selects the higher rate, the OFF condition selects the lower rate. b) 4.1.1.2 The DCE not-ready condition is indicated by transmitting a DC3 character (IRA character 1/3), and cleared by transmitting a DCl character (IRA character ll), on circuit 10
39、4. This method should not be applied when there is a risk that binary characters that appear in the data stream may be confused with DC3 and DCl characters, which would result in a malfunction of the system. Schemes that indicate in binary file transfer whether a DC3 or a DC1 character that the DTE
40、detects in the data transmitted on circuit 104 either is generated by the DCE for the purpose of flow control, or is a binary character and part of the data received from the remote DTE, are for further study. Flow control by use of DC3/DC1 characters 4.1.2 Synchronous mode of operation A variety of
41、 methods is conceivable for the synchronous mode of operation. 4.1.2.1 Flow control by changing the rate of the transmitter signal element timing (DCE source) When the transmit clock signal is sourced from the DCE (circuit 114), it may be possible to slow the data flow from the DTE by reducing the c
42、lock rate. The clock rate should be halved by the DCE and, if that action is insufficient, the rate should be halved again, and so on. The clock rate should be increased again as soon as conditions in the DCE have improved. The change to a different rate has to occur while circuit 114 is in the OFF
43、condition. NOTE - Some DTEs use narrow-band phase-locked loops in their received clock circuitry and may not be able to respond to this mechanism. 4.1.2.2 This method halts the clock signal on circuit 1 14, thus causing the DTE to stop sending. Circuit 1 14 may be held in the OFF condition for a lim
44、ited period of time. The duration of the OFF condition shall be an integer multiple of the length of a signal element at the normal (maximum) signalling rate. Signalling on this circuit may be resumed at a different rate, as specified above. NOTE 1 - Some DTEs may - with or without a certain time-ou
45、t - raise an alarm under this condition. Flow control by stopping the transmitter signal element timing signal CE source) Recommendation V.43 (02/98) 3 STDOITU-T RECMN V.43-ENGL 3998 W 4862593 0650900 086 c Mux AA DC3/DC 1 Control Data to function higher layer 4 4 NOTE 2 - The recommended maximum pe
46、rmissible duration of an OFF condition on circuit 114 is for further study. Line i 03 DC3/DC1 detection b A DC3DCl t 133 NS 111 NS 128 S 104 Flow control of received data c b output function Line 4 4.1.2.3 Circuit 106 (Ready for sending) may be turned OFF when the buffers in the DCE are almost full
47、(i.e. to a specified degree). This method of operation follows the definition of circuit 106 in Recommendation V.24. The considerations concerning the DCE buffer size given in 4.1.1.1 a) apply. If the transmit clock is sourced from the DTE (circuit 1 13), this may be the only usable method. Flow con
48、trol by use of circuit 106 4.2 The DTE shall be capable of indicating to the DCE a temporary inability to accept data on circuit 104, Received Data (DTE not-ready condition). Upon receiving such an indication, the DCE shall complete the delivery of any partially transmitted character (asynchronous m
49、ode of operation) or frame (synchronous mode of operation) and then cease delivering data to the DTE on circuit 104 and clamp circuit 104 to binary 1. When the DTE not-ready condition is cleared, the DCE may resume the delivery of data on circuit 104. NOTE - The data whose flow is temporarily stopped or slowed down is originated from the remote DTE. In order to control the flow of that data so that no data is lost, the local DCE will either need to provide a received data buffer whose size cannot be specified in this Recommendation I Internat
