ISO/IEC TR 15294:2000

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
• ICS: 35.100.10 - Physical layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Current Stage: 9093 - International Standard confirmed
• Start Date: 26-May-2015
• Completion Date: 12-Feb-2026

Overview

ISO/IEC TR 15294:2000 is an informative Technical Report that documents methods for data flow control at synchronous and asynchronous DTE–DCE interfaces. It provides guidance to designers and implementers on choosing and implementing local flow-control mechanisms between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE). The report focuses on physical‑layer techniques, coordinates material from other standards (notably ITU‑T V.24 and V.43 alignment), and includes tutorial material rather than mandatory prescriptions.

Key topics and requirements

• Scope: guidance for flow control of transmitted and received data, separately for asynchronous and synchronous modes. End‑to‑end flow control and higher‑layer protocols (e.g., HDLC) are out of scope.
• Physical‑layer mechanisms described:
  • V.24 interchange circuits (recommended where applicable):
    • Circuit 106 - Ready for sending (DCE indicates it cannot accept transmitted data).
    • Circuit 133 - Ready for receiving (DTE indicates it cannot accept received data).
    • Data‑rate selector circuits (111, 112) to switch between two signalling rates.
    • Signal timing circuits (114, 128, 113) used for transmitter/receiver clock control.
  • DC3/DC1 character flow control (IRA DC3 = 1/3 to stop, DC1 = 1/1 to resume) - useful in asynchronous streams but unsafe when binary data may be confused with control characters.
  • Clock‑rate adjustments and stopping the clock (synchronous mode): halving the clock rate or halting timing to slow or stop the peer transmitter. Notes and cautions cover DTEs with narrow‑band PLLs or alarms on clock stoppage.
• Implementation guidance and cautions:
  • Buffer sizing guidance for DCEs when using circuit‑106: suggested remaining buffer capacity 2,000–4,000 octets, implying total buffers in the range **4,000–10,000 octets** depending on trigger thresholds.
  • Emphasis that not all DTEs/DCEs will respond to every mechanism - designers should provide multiple compatible methods where feasible.
  • The TR is informative and highlights areas for further study (e.g., binary file transfer ambiguity and duration limits for stopped clocks).

Applications and users

• Practical use cases:
  • Designers and firmware engineers of modems, terminal adapters, multiplexers, and serial interface equipment.
  • System integrators implementing reliable serial links where DCEs perform compression or error correction.
  • Network engineers troubleshooting flow‑control interactions between legacy DTEs and DCEs.
• Benefits:
  • Helps prevent buffer overruns/underruns when compression or error correction changes instantaneous throughput.
  • Guides interoperable designs for mixed asynchronous and synchronous deployments.

Related standards

• ITU‑T Recommendation V.24 (definitions for interchange circuits).
• ITU‑T V.14 / V.42 (asynchronous-to-synchronous conversion references mentioned).
• ISO/IEC TR 15294 is technically aligned with ITU‑T V.43 but is an informative ISO/IEC Technical Report.

Keywords: ISO/IEC TR 15294, data flow control, DTE‑DCE interface, V.24, DC1 DC3, synchronous flow control, asynchronous flow control, modem buffer sizing, serial interface standards.

Standards Content (Sample)

TECHNICAL ISO/IEC
REPORT TR
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 2000
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ii © ISO/IEC 2000 – All rights reserved

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

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

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

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
higher layer
S/A
Output Control
function function
112 A
114 S
A
A
DC3/DC1 DC3/DC1
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.
2 © ISO/IEC 2000 – All rights reserved

b) Use of circuit 112 - D
...