INTERNATIONAL TELECOMMUNICATION UNION
ITU-T
V.250
TELECOMMUNICATION
STANDARDIZATION SECTOR
OF ITU
(07/2003)
SERIES V: DATA COMMUNICATION OVER THE
TELEPHONE NETWORK
Control procedures
Serial asynchronous automatic dialling and
control
ITU-T Recommendation V.250
ITU-T V-SERIES RECOMMENDATIONS
DATA COMMUNICATION OVER THE TELEPHONE NETWORK
General V.1–V.9
Interfaces and voiceband modems V.10–V.34
Wideband modems V.35–V.39
Error control V.40–V.49
Transmission quality and maintenance V.50–V.59
Simultaneous transmission of data and other signals V.60–V.99
Interworking with other networks V.100–V.199
Interface layer specifications for data communication V.200–V.249
Control procedures V.250–V.299
Modems on digital circuits V.300–V.399
For further details, please refer to the list of ITU-T Recommendations.
ITU-T Rec. V.250 (07/2003) i
ITU-T Recommendation V.250
Serial asynchronous automatic dialling and control
Summary
This version of ITU-T Rec. V.250 integrates Amendment 1 (07/2001), Amendment 2 (03/2002) and
Amendment 3 (07/2003) with the 05/1999 version of the Recommendation.
This Recommendation defines commands and responses for use by a DTE to control a V-series DCE
using serial data interchange over an asynchronous interface. It contains four elements:
– codifies existing practice in common GSTN-DCE that use the ATtention (AT) command set;
– defines a format for orderly extension of the AT command set;
– provides a set for standardized extensions for common functions to identify the DCE, to
control the DTE-DCE interface, and to control DCE-DCE protocols (signal conversion,
error control and data compression);
– provides a mapping for these commands into V.25 bis frame format for use with DCEs
employing synchronous serial interfaces.
Source
ITU-T Recommendation V.250 was approved on 14 July 2003 by ITU-T Study Group 16
(2001-2004) under the ITU-T Recommendation A.8 procedure.
Keywords
AT Commands, data modems, data transmission, DCE control.
ii ITU-T Rec. V.250 (07/2003)
FOREWORD
The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of
telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of
ITU. 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 Telecommunication Standardization Assembly (WTSA), which meets every four years,
establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on
these topics.
The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
In some areas of information technology which fall within ITU-T's purview, the necessary standards are
prepared on a collaborative basis with ISO and IEC.
NOTE
In this Recommendation, the expression "Administration" is used for conciseness to indicate both a
telecommunication administration and a recognized operating agency.
Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain
mandatory provisions (to ensure e.g., interoperability or applicability) and compliance with the
Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some
other obligatory language such as "must" and the negative equivalents are used to express requirements. The
use of such words does not suggest that compliance with the Recommendation is required of any party.
INTELLECTUAL PROPERTY RIGHTS
ITU draws attention to the possibility that the practice or implementation of this Recommendation may
involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence,
validity 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, ITU had not received notice of intellectual property,
protected by patents, which may be required 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.
ITU 2004
All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the
prior written permission of ITU.
ITU-T Rec. V.250 (07/2003) iii
CONTENTS
Page
1 Introduction and scope.................................................................................................. 1
2 References..................................................................................................................... 1
2.1 Normative references...................................................................................... 1
2.2 Informative references.................................................................................... 2
3 Definitions and abbreviations....................................................................................... 3
3.1 Definitions ...................................................................................................... 3
3.2 Abbreviations ................................................................................................. 4
4 Physical layer................................................................................................................ 4
4.1 Circuits ........................................................................................................... 4
4.2 Character formatting....................................................................................... 5
4.3 Data rates ........................................................................................................ 5
5 Syntax and procedures.................................................................................................. 5
5.1 Alphabet ......................................................................................................... 5
5.2 DTE commands lines ..................................................................................... 6
5.3 Basic Syntax commands................................................................................. 7
5.4 Extended Syntax commands........................................................................... 8
5.5 Issuing commands .......................................................................................... 12
5.6 Executing commands...................................................................................... 12
5.7 DCE responses................................................................................................ 13
5.8 Manufacturer-specific characteristics............................................................. 16
6 Functions ...................................................................................................................... 17
6.1 Generic DCE control ...................................................................................... 18
6.2 DTE-DCE interface commands...................................................................... 25
6.3 Call control ..................................................................................................... 37
6.4 Modulation control commands....................................................................... 48
6.5 Error control commands ................................................................................. 60
6.6 Data compression commands ......................................................................... 68
6.7 DCE testing .................................................................................................... 73
6.8 PCM DCE commands .................................................................................... 85
6.9 V.59 Command (+TMO)................................................................................ 91
Appendix I – Summary of basic and extended format commands .......................................... 93
Appendix II – DCE configuration, dialling, negotiation and reporting, example session....... 95
Appendix III – Encapsulation of V.250 messages in V.25 bis DCE ....................................... 96
III.1 Scope .............................................................................................................. 96
III.2 Encapsulation of V.250 messages .................................................................. 96
III.3 Applicable V.250 commands ......................................................................... 96
III.4 Applicable V.250 responses ........................................................................... 97
