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1、| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | BRITISH STANDARD BS ISO/IEC TR 15294:2000
2、ICS 33.040.40; 35.200 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW Information technology Methods for Data flow Control at Synchronous and Asynchronous DTE-DCE Interfaces Licensed Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c)
3、BSI This British Standard, having been prepared under the direction of the DISC Committee, was published under the authority of the Standards Committee and comes into effect on 15 May 2000 BSI 05-2000 ISBN 0 580 34095 3 BS ISO/IEC TR 15294:2000 Amendments issued since publication Amd. No.DateComment
4、s National foreword This British Standard reproduces verbatim ISO/IEC TR 15294:2000 and implements it as the UK national standard. The UK participation in its preparation was entrusted to Technical Committee IST/6, Data communications, which has the responsibility to: aid enquirers to understand the
5、 text; present to the responsible international/European committee any enquiries on the interpretation, or proposals for change, and keep the UK interests informed; monitor related international and European developments and promulgate them in the UK. A list of organizations represented on this comm
6、ittee can be obtained on request to its secretary. Cross-references The British Standards which implement international or European publications referred to in this document may be found in the BSI Standards Catalogue under the section entitled International Standards Correspondence Index, or by usi
7、ng the Find facility of the BSI Standards Electronic Catalogue. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity f
8、rom legal obligations. Summary of pages This document comprises a front cover, an inside front cover, the ISO/IEC TR title page, pages ii to iv, pages 1 to 6, an inside back cover and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued. Licen
9、sed Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI Reference number ISO/IEC TR 15294:2000(E) TECHNICAL REPORT ISO/IEC TR 15294 First edition 2000-04-01 Information technology Methods for data flow control at synchronous and asynchronous DTE-DCE inte
10、rfaces Technologies de linformation Mthodes pour interfaces DTE-DCE synchrones et asynchrones de commande de flux de donnes Licensed Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ISO/IEC TR 15294:2000(E) ii? Licensed Copy: sheffieldun sheffieldun,
11、na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ISO/IEC TR 15294:2000(E) ?iii Foreword ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies that a
12、re members of ISO or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other internatio
13、nal organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3. In the field of information technology, ISO and IEC have established a joint techni
14、cal committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International Standard requires approval by at least 75 % of the national bodies casting a vote. In exceptional circumstances, when a t
15、echnical committee has collected data of a different kind from that which is normally published as an International Standard (“state of the art“, for example), it may decide by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informati
16、ve in nature and does not have to be reviewed until the data it provides are considered to be no longer valid or useful. Attention is drawn to the possibility that some of the elements of this Technical Report may be the subject of patent rights. ISO and IEC shall not be held responsible for identif
17、ying any or all such patent rights. ISO/IEC TR 15294,waspreparedbyJointTechnicalCommitteeISO/IEC JTC 1,Informationtechnology, Subcommittee SC 6,Telecommunications and information exchange between systems. This Technical Report is technically aligned with ITU-T Recommendation V.43, but is not publish
18、ed as identical text. Licensed Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ISO/IEC TR 15294:2000(E) iv? Introduction When using DCEs incorporating data compression and/or error correction, it is essential that the DCE have some method of controll
19、ing the flow of data from the DTE (a similar requirement applies for the associated DTEs, see below). This is because the degree of compression obtained will vary from moment to moment, and the buffers in the DCE may fill up during periods of lower compression, or while correction of transmission er
20、rors is active. Likewise, during periods of high compression, the DTE may become overloaded with the amount of incoming 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
21、DTE designers should ensure that they provide methods suitable for the associated DTE and DCE, respectively, to be used. 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 DTE
22、s and DCEs although no single mechanism will operate with all DTEs and DCEs, and some DTEs may not respond to any of the mechanisms described. Both the synchronous and the asynchronous modes of operation are addressed. NOTE?The guidelines given in this Technical Report may not be exhaustive. License
23、d Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI TECHNICAL REPORT?ISO/IEC TR 15294:2000(E) ?1 Information technology Methods for data flow control at synchronous and asynchronous DTE-DCE interfaces 1?Scope This Technical Report provides guidance for
24、 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 material on these flow control techniques. 2?Reference 1ITU-T Recommendat
25、ion V.24,List of definitions for interchange circuits between data terminal equipment and data circuit-terminating equipment. 3Abbreviation IRA?International reference alphabet 4?Data Flow Control Methods The guidance is provided for ?flow control of transmitted data, ?flow control of received data.
26、 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 Technical Report is accomplished using synchronous DCEs incorporating asynchronous-to-synchronous conversion according to e.g. Recom
27、mendation V.14 or V.42. This 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 involving the two DTEs or the two DCEs concerned are beyond the scope of this Technical Report. At present thi
28、s Technical Report describes only flow control methods that operate at the physical layer of the OSI Reference Model. In the context of this Technical Report, the use of DC1/DC3 characters is also understood as to fall in this category. Methods that involve the DCE implementing layer 2 or higher lay
29、er protocols, for example HDLC, are for further study. 4.1? Methods for flow control of transmitted data 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 i
30、ndication, the DTE 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
31、DTE may resume the transmission of data on circuit 103. Licensed Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ISO/IEC TR 15294:2000(E) 2? Figure 1 depicts the interchange circuits at the DTE-DCE interface and the functionalities inside the DTE and
32、 the DCE, respectively, which may be used, as appropriate, for the flow control of transmitted data. 103 106 112 114 104 A DC3/DC1 Flow control of transmitted data DC3/DC1 detection DTE?DCE S/A A S Control function A DC3/DC1 Mux Data from higher layer Output function Line Legend: A - applicable for
33、the asynchronous mode of operation S - applicable for the synchronous mode of operation Line Figure 1 4.1.1?Asynchronous mode of operation Two standard methods exist: 4.1.1.1?Flow control by use of V.24 interchange circuits a)?Use of circuit 106 - Ready for sending The DCE not-ready condition is ind
34、icated by turning circuit 106 OFF, and cleared by turning circuit 106 ON. This method should be the preferred 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 trans
35、mitting at the end of the present character. Instead, these DTEs will complete the present frame which may be up to some thousand octets long, and only then detect the OFF condition on circuit 106. It is therefore suggested that the remaining buffer capacity inside the DCE be kept large enough to co
36、pe with this condition. A reasonable remaining buffer size suggested may be between 2,000 and 4,000 octets, so that the minimum total buffer size may be between 4,000 octets (DCE not-ready condition is indicated to the DTE when the buffer is half-full) and 10,000 octets (DCE not-ready condition is i
37、ndicated to the DTE when about 80 % of the buffer is full). NOTE?This assumes that a remaining buffer size of about 2,000 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 o
38、f circuit 106 and, upon recognition of the OFF condition on circuit 106, stop the transmission of data as soon as possible. Licensed Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ISO/IEC TR 15294:2000(E) ?3 b)?Use of circuit 112 - Data signalling r
39、ate selector (DCE source) The DCE may, by controlling circuit 112, select one of two data signalling rates in the DTE to coincide with the data signalling rate in use in the DCE. The ON condition on this circuit selects the higher rate, the OFF condition selects the lower rate. 4.1.1.2?Flow control
40、by use of DC3/DC1 characters The DCE not-ready condition is indicated by transmitting a DC3 character (IRA character 1/3), and cleared by transmitting a DC1 character (IRA character 1/1), on circuit 104. This method should not be applied when there is a risk that binary characters that appear in the
41、 data stream may be confused with DC3 and DC1 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 detects in the data transmitted on circuit 104 either is generated by the DCE for the purpose of fl
42、ow control, or is a binary character and part of the data received from the remote DTE, are for further study. 4.1.2?Synchronous mode of operation A variety of methods is conceivable for the synchronous mode of operation. 4.1.2.1?Flow control by changing the rate of the transmitter signal element ti
43、ming (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 clock 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 cl
44、ock 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 condition. NOTE?Some DTEs use narrow-band phase-locked loops in their received clock circuitry and may not be able to respond to this mechani
45、sm. 4.1.2.2?Flow control by stopping the transmitter signal element timing signal (DCE source) This method halts the clock signal on circuit 114, thus causing the DTE to stop sending. Circuit 114 may be held in the OFF condition for a limited period of time. The duration of the OFF condition shall b
46、e 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-out - raise an alarm under this condition. NOTE 2?The recommended
47、maximum permissible duration of an OFF condition on circuit 114 is for further study. 4.1.2.3?Flow control by use of circuit 106 Circuit 106 (Ready for sending) may be turned OFF when the buffers in the DCE are almost full (i.e. to a specified degree). This method of operation follows the definition
48、 of circuit 106 in Recommendation V.24. The considerations concerning the DCE buffer size given in subclause 4.1.1.1 a) apply. If the transmit clock is sourced from the DTE (circuit 113), this may be the only usable method. Licensed Copy: sheffieldun sheffieldun, na, Tue Nov 21 08:58:41 GMT+00:00 20
49、06, Uncontrolled Copy, (c) BSI ISO/IEC TR 15294:2000(E) 4? 4.2? Methods for flow control of received data 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 mode 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, th