Information technology — Methods for data flow control at synchronous and asynchronous DTE-DCE interfaces

This 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 material on these flow control techniques.

Technologies de l'information — Méthodes pour interfaces DTE-DCE synchrones et asynchrones de commande de flux de données

General Information

Status
Published
Publication Date
29-Mar-2000
Current Stage
9093 - International Standard confirmed
Start Date
26-May-2015
Completion Date
26-May-2015
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ISO/IEC TR 15294:2000 - Information technology -- Methods for data flow control at synchronous and asynchronous DTE-DCE interfaces
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TECHNICAL ISO/IEC
REPORT TR
15294
First edition
2000-04-01
Information technology — Methods for data
flow control at synchronous and
asynchronous DTE-DCE interfaces
Technologies de l'information — Méthodes pour interfaces DTE-DCE
synchrones et asynchrones de commande de flux de données
Reference number
ISO/IEC TR 15294:2000(E)
ISO/IEC 2000
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ISO/IEC TR 15294:2000(E)
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ii © ISO/IEC 2000 – All rights reserved
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ISO/IEC TR 15294:2000(E)
Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission)

form the specialized system for worldwide standardization. National bodies that are 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 international 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 technical 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 technical 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 informative 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 identifying any or all such patent rights.

ISO/IEC TR 15294, was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,

Subcommittee SC 6, Telecommunications and information exchange between systems.

This Technical Report is technically aligned with ITU-T Recommendation V.43, but is not published as identical

text.
© ISO/IEC 2000 – All rights reserved iii
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ISO/IEC TR 15294:2000(E)
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 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 errors 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 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 DTEs 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.
iv © ISO/IEC 2000 – All rights reserved
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TECHNICAL REPORT ISO/IEC TR 15294:2000(E)
Information technology — Methods for data flow control at
synchronous and asynchronous DTE-DCE interfaces
1 Scope

This 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 material on these flow control techniques.

2 Reference

[1] ITU-T Recommendation 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.

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. Recommendation 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 this 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 layer 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 indication, 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 DTE may resume the transmission of data on circuit 103.

© ISO/IEC 2000 – All rights reserved 1
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ISO/IEC TR 15294:2000(E)

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 transmitted data.

DTE DCE
Line
Data from
103
higher layer
S/A
106
Output Control
function function
112 A
114 S
DC3/DC1 DC3/DC1
104
Line
DC3/DC1
Mux
detection
Flow control of
transmitted data
Legend:
A - applicable for the asynchronous mode of operation
S - applicable for the synchronous mode of operation
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 indicated 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 transmitting 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 cope 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 indicated 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 of circuit 106 and, upon recognition of the OFF condition on circuit 106,

stop the transmission of data as soon as possible.
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ISO/IEC TR 15294:2000(E)
b) Use of circuit 112 - D
...

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