SIST I-ETS 300 762 E1:2003

• Abstract:

  To describe data communication aspects of a reference videotelephone terminal based on prETS 300 143 and prETS 300 144

• Status: Published
• Publication Date: 30-Nov-2003
• ICS: 33.160.60 - Multimedia systems and teleconferencing equipment
• ICS: 35.180 - IT Terminal and other peripheral equipment
• Technical Committee: SS EIT - Technical Board for the electrotechnical, information technology and telecommunications fields
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-Dec-2003
• Due Date: 01-Dec-2003
• Completion Date: 01-Dec-2003
• Ref Project: ETSI I-ETS 300 762 ed.1 (1997-07) - Terminal Equipment (TE); Videotelephone reference terminal - data communication using in-band signalling principles

SIST I-ETS 300 762 E1:2003 is the Slovenian publication of ETSI I-ETS 300 762 Edition 1 for terminal equipment videotelephone reference terminals. It specifies the data communication part of a reference videotelephone terminal, including how a PC or similar Data Terminal Equipment (DTE) can be attached and how that link uses in-band signalling over ISDN audiovisual framing.

What does SIST I-ETS 300 762 E1:2003 specify?

SIST I-ETS 300 762 E1:2003 specifies the signalling procedures and interfaces for data communication on an audiovisual terminal that uses framed communication as described in ETS 300 144. The data facility is optional, separate from the teleservice itself, and is intended to carry general data between two terminals in either direction.

The standard applies to terminals intended for connection to the pan-European Integrated Services Digital Network (ISDN) at the T reference point, or at coincident S and T reference points, and that support in-band signalling and frame communication. In practice, that means it is relevant when a videotelephone needs to exchange data with an attached PC or similar DTE over the same terminal arrangement used for voice and video.

SIST I-ETS 300 762 E1:2003 is organized into scope, normative references, definitions and abbreviations, general requirements, data communication using the T.120 protocol in the MLP channel, data communication without using T.120, and an informative annex with an implementation example.

Annex	What it covers
Annex A	Example implementation and procedures, including V.24 signal mapping and an open/close procedure

What are the key requirements of SIST I-ETS 300 762 E1:2003?

Physical interface and terminal connection

SIST I-ETS 300 762 E1:2003 requires the signalling link between the videotelephone and the computer to meet ITU-T Recommendation V.24, the electrical interface to meet ITU-T Recommendation V.28, and the physical connector on the videophone to conform to ISO 2110. In practice, this fixes the basic serial connection so equipment from different suppliers can be wired and tested against the same interface expectations.

Two ways to carry data

SIST I-ETS 300 762 E1:2003 describes two optional schemes. One is a fully standardized data-transmission and applications-sharing scheme based on the T.120 protocol, suitable for multipoint operation and software-controlled from the DTEs. The other is a simplified point-to-point scheme for PCs with a common application, controlled by the user from the videophones.

T.120-based operation in the MLP channel

SIST I-ETS 300 762 E1:2003 uses the T.120 protocol stack over the MLP channel for the standardized scheme. The terminal first establishes videophone communication, then exchanges capabilities, opens the data path, initializes T.120, and starts the application. This sequence matters because the data channel only comes up after both ends have agreed on compatible signalling and rates.

The capability exchange includes MLP capability values tied to the baud rate on the V.24 interface. The standard requires the terminal to include only the values it can actually support, and to avoid sending unsupported MLP rates. That protects the link from choosing a data rate the local terminal cannot handle.

Local control messages between videophone and PC

SIST I-ETS 300 762 E1:2003 defines local control primitives for communication between the videophone and the PC. These include acknowledgement, data capability, transmit request, flow control, host poll, videophone poll, and data channel clear messages. In practice, these messages let the two devices negotiate capability, request transmission, regulate buffering, and close the data channel in an orderly way.

Data transfer and conversion inside the videophone

SIST I-ETS 300 762 E1:2003 requires HDLC framing for data transfer and a way to prevent a control message from being confused with data. The videophone also has to perform asynchronous-to-synchronous conversion between the local start-stop link and the synchronous transmission used in the ISDN path. That is important because the PC talks serial asynchronous data, while the network side carries synchronized framed traffic.

Opening, using, and closing the data channel

SIST I-ETS 300 762 E1:2003 lays out a staged procedure for opening, using, and closing the end-to-end data channel. The process covers capability exchange, selection of transmission mode, opening of the MLP channel, T.120 start-up, packet transfer, flow control, and clear-down. For a reader implementing the standard, this is the practical call flow to follow when building or testing the feature.

Simplified non-T.120 data communication

SIST I-ETS 300 762 E1:2003 also defines a non-T.120 option. In this mode, the terminal may open a channel after detecting DTR on the V.24 interface and after user action on the videophone, using capability values in ETS 300 144. The standard states that this format has no inherent error checking, error correction, or retransmission facilities, so implementers must treat it as a simpler transport option rather than a managed data protocol.

What terms does SIST I-ETS 300 762 E1:2003 define?

• H.221 framing - Framing as specified in ETS 300 144, used as the basis for the audiovisual channel structure.
• terminal - An audiovisual terminal together with its attached personal computer or similar device.
• T.120 protocol - The set of protocols defined by ITU-T Recommendations T.122, T.123, T.124, and T.125.
• MLP - A logical data channel defined in ETS 300 144.
• asynchronous - Transmission that uses start-stop characters to control the data flow.
• synchronous - Transmission that does not use start-stop characters to control the data flow.
• DTE - Data Terminal Equipment, the attached computer or similar data source/sink.
• MCU - Multipoint Control Unit, used in multipoint conferencing arrangements.

Who uses SIST I-ETS 300 762 E1:2003?

SIST I-ETS 300 762 E1:2003 is used by manufacturers and software developers of videotelephones, videoconference terminals, and attached PC-based applications that need data sharing over ISDN. It is also relevant to system integrators and test engineers who need to verify the V.24 link, capability exchange, T.120 operation, flow control, and channel clear-down.

It matters to buyers and quality managers when they need to check whether a terminal supports a standardized PC data connection or only the simplified point-to-point option. It is also relevant in multipoint conferencing environments that involve an MCU.

What changed in SIST I-ETS 300 762 E1:2003 from the previous edition?

SIST I-ETS 300 762 E1:2003 is identical to I-ETS 300 762 Edition 1.

Which standards are used with SIST I-ETS 300 762 E1:2003?

SIST I-ETS 300 762 E1:2003 cites the following normative references and companion parts:

Reference	What it contributes
ETS 300 144	Frame structure for the audiovisual channel and in-band signalling syntax
ITU-T Recommendation V.24	DTE/DCE interchange circuit definitions for the serial interface
ITU-T Recommendation V.28	Electrical characteristics of the interface
ISO 2110	25-pole DTE/DCE connector and contact assignments
ITU-T Recommendation T.122	Multipoint communication service definition
ITU-T Recommendation T.123	Protocol stacks for audiographic and audiovisual teleconference applications
ITU-T Recommendation T.124	Generic Conference Control
ITU-T Recommendation T.125	Multipoint communication service protocol specification
CCITT Recommendation Q.922	ISDN data link layer specification for frame mode bearer services
ETS 300 145	Videotelephone systems and terminal equipment operating on one or two 64 kbit/s channels
ITU-T Recommendation H.320	Narrow-band visual telephone systems and terminal equipment
ETS 300 143	In-band signalling procedures for audiovisual terminals using digital channels up to 2 048 kbit/s
ISO/IEC 3309	HDLC frame structure used by the data transfer primitives
ITU-T Recommendation V.14	Start-stop character transmission over synchronous bearer channels

What does the SIST I-ETS 300 762 E1:2003 document contain?

SIST I-ETS 300 762 E1:2003 contains the capability rules, control message set, packet handling, and call-flow sequence needed to move data between the videophone and the attached DTE. It includes tables for capability values, MLP rate coding, control message types, and the rate relationships between the asynchronous interface and the MLP channel.

The document also gives the sequencing for opening, using, and closing the data channel, including how the terminal behaves when messages are acknowledged, repeated, or rejected. Annex A adds an implementation example with V.24 signal mapping and an example open/close procedure, which is useful for design work, interoperability testing, and implementation review.

SLOVENSKI STANDARD
01-december-2003
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Terminal Equipment (TE); Videotelephone reference terminal - data communication
using in-band signalling principles
Ta slovenski standard je istoveten z: I-ETS 300 762 Edition 1
ICS:
33.160.60 9HþSUHGVWDYQLPXOWLPHGLMVNL Multimedia systems and
VLVWHPLLQRSUHPD]D teleconferencing equipment
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35.180 Terminalska in druga IT Terminal and other
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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERIM
EUROPEAN I-ETS 300 762
TELECOMMUNICATION July 1997
STANDARD
Source: ETSI TC-TE Reference: DI/MTA-004114
(Formerly: DI/TE-04114)
ICS: 33.020
Key words: Audio, data, in-band, terminal, video, telephony
Terminal Equipment (TE);
Videotelephone reference terminal - data communication
using in-band signalling principles
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: [e-mail]
Tel.: [phone] - Fax: [phone]
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1997. All rights reserved.

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I-ETS 300 762: July 1997
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.

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I-ETS 300 762: July 1997
Contents
Foreword .5
1 Scope .7
2 Normative references.7
3 Definitions and abbreviations .8
3.1 Definitions .8
3.2 Abbreviations .8
4 General.9
4.1 Overview .9
4.2 Physical link between videotelephone and computer .9
5 Data communication using T.120 protocol in the MLP channel.10
5.1 General description.10
5.2 Selection of transmission mode.12
5.3 Communication between videophone and PC.13
5.3.1 Control primitives.13
5.3.2 Data transfer primitives .15
5.3.3 Interleaving of control and data packets.16
5.4 Opening, using and closing the end-to-end data channel.16
6 Data communication without using T.120 protocol .19
6.1 Format .19
6.2 Asynchronous to synchronous conversion .19
Annex A (informative): An example of implementation and procedures.20
A.1 Mapping of ITU-T Recommendation V.24 signals .20
A.2 Open/close procedure.21
History.22

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I-ETS 300 762: July 1997
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I-ETS 300 762: July 1997
Foreword
This Interim European Telecommunication Standard (I-ETS) has been produced by the Terminal
Equipment (TE) Technical Committee of the European Telecommunications Standards Institute (ETSI).
An ETSI standard may be given I-ETS status either because it is regarded as a provisional solution ahead
of a more advanced standard, or because it is immature and requires a "trial period". The life of an I-ETS
is limited to three years after which it can be converted into an ETS, have it's life extended for a further
two years, be replaced by a new version, or be withdrawn.
Proposed announcement date
Date of adoption: 20 June 1997
Date of latest announcement of this I-ETS (doa): 31 October 1997

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I-ETS 300 762: July 1997
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I-ETS 300 762: July 1997
1 Scope
This Interim European Telecommunication Standard (I-ETS) specifies the necessary signalling
procedures and interfaces of a data port of ITU-T Recommendation V.24 [2] type on an audiovisual
terminal using framed communication as described in ETS 300 144 [1], to which a Personal
Computer (PC) or similar Data Terminal Equipment (DTE) may be attached.
The data communication facility is intended to provide an independent data channel which may be used
for general data communication between two terminals. The data channel may support transmission in
either or both directions.
This I-ETS describes two optional schemes:
- a fully standardized data-transmission and applications-sharing scheme capable of multipoint
operation; this requires conformant software to be present on both DTEs, and the link can be
controlled from the DTEs themselves. The scheme could be extended to higher bit-rates, as may
be required in the future;
- a simplified scheme which does not require standardized software but which gives point-to-point
communication between PCs having a common application; in this case the link must be controlled
by the user from the videophones.
Terminals supporting teleservices using in-band signalling (e.g. telephony 7 kHz or videotelephony) may
offer data communication facilities as described in this specification as an option, but the facility is not a
part of the defined teleservice.
This interim standard is applicable to terminals which are intended for connection to the pan-European
Integrated Services Digital Network (ISDN) as provided by European public telecommunication operators
at the T reference point or coincident S and T reference point, and which support in-band signalling and
frame communication as described in ETS 300 144 [1].
2 Normative references
This I-ETS incorporates by dated and undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references, subsequent amendments to or revisions of any of these publications
apply to this I-ETS only when incorporated in it by amendment or revision. For undated references the
latest edition of the publication referred to applies.
[1] ETS 300 144 (1996): "Integrated Services Digital Network (ISDN): Audiovisual
services; Frame structure for a 64 kbit/s to 1 920 kbit/s channel and associated
syntax for inband signalling".
[2] ITU-T Recommendation V.24 (1994): "List of definitions for interchange circuits
between Data Terminal Equipment (DTE) and Data Circuit-terminating
Equipment (DCE)".
[3] ITU-T Recommendation V.28 (1994): "Electrical characteristics for unbalanced
doubled-current interchange circuits".
[4] ISO 2110 (1989): "Information technology- Data communication - 25-pole
DTE/DCE interface connector and contact number assignments".
[5] ITU-T Recommendation T.122 (1995): "Multipoint communication service for
audiographic and audiovisual conferencing service definition".
[6] ITU-T Recommendation T.123 (1995): "Protocol stacks for audiographic and
audiovisual teleconference applications".
[7] ITU-T Recommendation T.124 (1995): "Generic Conference Control".

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I-ETS 300 762: July 1997
[8] ITU-T Recommendation T.125 (1994): "Multipoint communication service
protocol specification".
[9] CCITT Recommendation Q.922 (1992): "ISDN data link layer specification for
frame mode bearer services".
[10] ETS 300 145 (1996): "Integrated Services Digital Network (ISDN); Audiovisual
services; Videotelephone systems and terminal equipment operating on one or
two 64 kbit/s channels".
[11] ITU-T Recommendation H.320 (1994): "Narrow-band visual telephone systems
and terminal equipment".
[12] ETS 300 143 (1994): "Integrated Services Digital Network (ISDN); Audiovisual
services Inband signalling procedures for audiovisual terminals using digital
channels up to 2 048 kbit/s".
[13] ISO/IEC 3309 (1993): "Information technology - Telecommunications and
information exchange between systems - High-layer Data Link Control (HDLC)
procedures - Frame structure".
[14] ITU-T Recommendation V.14 (1994): "Transmission of start-stop characters
over synchronous bearer channels".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of this I-ETS, the following definitions apply:
H.221 framing: Framing as specified in ETS 300 144 [1].
terminal: Audiovisual terminal and attached personal computer or similar.
T.120 protocol: Protocols conforming to the specifications given in ITU-T Recommendations T.122 [5],
T.123 [6], T.124 [7] and T.125 [8].
MLP: A logical data channel defined in ETS 300 144 [1].
asynchronous: Transmission where start-stop characters are used to control the data flow.
synchronous: Transmission where no start-stop characters are used to control the data flow.
3.2 Abbreviations
For the purposes of this I-ETS, the following abbreviations apply:
API Application Program Interface
DCE Data Communication Equipment
DSR Data Set Ready
DTE Data Terminal Equipment
DTR Data Terminal Ready
FCS Frame Check Sequence, see ITU-T Recommendation Q.922 [9]
HDLC High-layer Data Link Control
ISDN Integrated Services Digital Network
MLP See subclause 3.1
MCU Multipoint Control Unit
PC Personal Computer
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I-ETS 300 762: July 1997
4 General
4.1 Overview
start-stop
ITU-T Recommendations V.14/V.24
DTE videophone ISDN videophone DTE
Figure 1: System configuration
Figure 1 shows the general arrangement to which the provisions of this I-ETS apply: two terminals using
in-band signalling as specified in ETS 300 144 [1] are in communication via the ISDN. To each terminal a
DTE (such as a PC) is attached, the DTEs are able to communicate across the telecommunication link
between the videophones.
This I-ETS describes two schemes:
- a fully standardized data-transmission and applications-sharing scheme (see clause 5), capable of
multipoint operation; this requires conformant software to be present on both DTEs, and the link
can be controlled from the DTEs themselves. The scheme could be extended to higher bitrates, as
may be required in the future (see figure 2);
PC-based
Videophone DTE
videophone
DTE Videophone
ISDN
Multipoint
PC-based
Control
videophone
Unit (MCU)
Figure 2: Multipoint communication using T.120 protocol
- a simplified scheme (see clause 6) which does not require standardized software but which gives
point-to-point communication between PCs having a common application; the link must be
controlled by the user from the videophones.
NOTE: If the videophone is programmed to carry out the procedures of clause 5 then a PC
may be attached and either of these schemes activated.
4.2 Physical link between videotelephone and computer
The signalling link between the videotelephone and the computer shall meet the requirements of ITU-T
Recommendation V.24 [2]. The electrical interface shall meet the requirements of ITU-T
Recommendation V.28 [3]. The physical connector used on the videophone shall conform to the
requirements set out in ISO 2110 [4] (see note).
NOTE: This interface is often referred to as RS-232.

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I-ETS 300 762: July 1997
5 Data communication using T.120 protocol in the MLP channel
In this clause, a PC is used as a typical example of a suitable DTE.
5.1 General description
The T.120 protocol is described in ITU-T Recommendations T.122 [5], T.123 [6], T.124 [7] and T.125 [8].
It provides a wide range of functionalities which can be exercised in multipoint as well as point-to-point
communications. Its implementation involves a small amount of hardware and simple software within the
videophone, dealing with the synchronous multiplexing of the single data stream into the ITU-T
Recommendation H.221 frame structure, and software (several hundred kilobytes in all) which, in the
scheme described here, resides in the PC (see note).
NOTE: Other videotelephone and videoconference terminals, notably those which are
themselves PC based, are able to contain this amount of software without depending
on an attached PC.
In this subclause, the description is made in terms of the intercommunication between two videophones,
as in figure 1; however, the operation is equally valid when the remote end-point is of a different type
(providing that it is conformant with ETS 300 145 [10] or ITU-T Recommendation H.320 [11]), such as a
PC-based videophone/videoconference terminal or a Multipoint Control Unit.
The cable link between the PC and the videophone is that commonly available on PCs, namely a
start-stop link. Since the transmission within the ITU-T Recommendation H.221 frame is synchronous, the
videophone must contain an asynchronous-to-synchronous High-layer Data Link Control (HDLC)
conversion (see figure 3).
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I-ETS 300 762: July 1997
PC
T.120 ISDN
Conformant
Transport
Applications ISDN
Connection
APP Protocols
GCC
T.120
MCS
ISDN
PSTN
X.224
Videophone
Protocol
SCF
Stack
HDLC
LAP F
Asynchronous to
synchronous HDLC
HDLC
conversion
Serial port and control
Serial port and Control
message handler
message handler
V.24 [2]
Standard
Comm. Comm.
Port Port
ITU-T Recommendation V.24 [2]
asynchronous communications link
Maximum data rate determined by serial capability
Local protocol to support call control
Local
Two types of datapacket
control
and bandwidth management
are defined:
1. Local C & I
2. T.120 data
HDLC frames
Figure 3: Data communication using the T.120 protocol stack
Two types of data packet pass across the ITU-T Recommendation V.24 [2] interface:
- applications data for transmission to the remote terminal, contained within the T.120 protocol. In
particular, the PSTN option defined in ITU-T Recommendation T.123 [6] is selected for the lower
layers;
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I-ETS 300 762: July 1997
- local control and indication messages supporting bandwidth control and flow management defined
in subclause 5.3. The establishment of communication between the PCs takes place in several
stages:
a) communication between the videophones, according to ETS 300 145 [10];
b) the exchange of capability information between the terminals, taking into account the
communicating applications which it is intended to run in the PCs;
c) opening of the data path between the PCs;
d) initialisation of the T.120 protocol;
e) start-up of the application itself.
5.2 Selection of transmission mode
The establishment of communication between the videotelephones shall follow the procedures set out in
ETS 300 145 [10].
Prior to invoking an application which involves communication between the PCs, a capability exchange
according to ETS 300 143 [12] and including appropriate data capability values (see below) must take
place between the videotelephones. This may occur within the initialisation sequence if the PC is already
active, or by means of a new capability exchange if the PC is activated later in the videotelephony session.
A terminal which is able to communicate using the T.120 protocol (see ITU-T Recommendations T.122 [5]
to T.125 [8]) shall, according to the specific rates it can operate at, include in its capability set the values
shown in table 1; these are defined in ETS 300 144 [1], tables 8, 10 and 12.
Table 1: Capability values required for communication using T.120 protocol
Baud rate on Mandatory Optional
ITU-T Recommendation capability capability
V.24 [2] interface values values
9 600 Baud MLP-6.4k, T.120-cap none
19 200 Baud MLP-6.4k, MLP-14.4k, T.120-cap MLP-8k, MLP-16k
38 400 Baud MLP-6.4k, MLP-14.4k, MLP-32k, MLP-8k, MLP-16k, MLP-22.4k,
T.120-cap MLP-24k, MLP-30.4k
Optional values may be advantageous, in that they extend the range of compatibility with remote terminals
which may be other than videophone + DTE. Other MLP rate values than those in table 1 shall not be
sent, since the terminal is not able to cope with incoming transmissions at higher rates than these.
If the received capability set from the remote end-point includes values conforming to the same rule as in
table 1 and the MCC command has not been received, either terminal may initiate data communication by
mode switching according to ETS 300 143 [12] to open an MLP channel at a mutually acceptable rate.
NOTE: MCC is transmitted by a Multipoint Control Unit to cause a terminal to send exactly the
same audio, data and video bit-rates as it receives.
If MCC has been received, the terminal shall not initiate data transmission but shall await an incoming
MLP command. Subsequent action shall be according to the procedure defined in ETS 300 145 [10].
In general, the ITU-T Recommendation V.24 [2] interface must run at a higher rate than the MLP channel.
The required MLP channel rate to support commonly available rates on the ITU-T Recommendation
V.24 [2] interface are as shown in table 2. The reason for providing a lower synchronous bit-rate is to
prevent t
...