Terminal Equipment (TE); Integrated Services Digital Network (ISDN); Multipoint communications for audiovisual services; Main functionalities and basic requirements for Multipoint Control Units (MCUs)

To define basic protocols and related functional requirements for an MCU which offers multipoint conference facilities to terminal equipment and systems supporting the videotelephony teleservice in the ISDN.

Terminalska oprema (TE) – Digitalno omrežje z integriranimi storitvami (ISDN) – Večtočkovne komunikacije za avdiovizualne storitve – Glavne funkcionalnosti in osnovne zahteve za večtočkovne krmilne enote (MCU)

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ETS 300 483 E1:2003
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SIST ETS 300 483 E1:2003
Terminal Equipment (TE); Integrated Services Digital Network (ISDN); Multipoint
communications for audiovisual services; Main functionalities and basic requirements for
Multipoint Control Units (MCUs)
Ta slovenski standard je istoveten z: ETS 300 483 Edition 1
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
35.180 Terminalska in druga IT Terminal and other
periferna oprema IT peripheral equipment
SIST ETS 300 483 E1:2003 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ETS 300 483 E1:2003

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SIST ETS 300 483 E1:2003
Source: ETSI TC-TE Reference: DE/TE-04032
ICS: 33.020
ISDN, audiovisual, multipoint
Key words:
Terminal Equipment (TE);
Integrated Services Digital Network (ISDN);
Multipoint communications for audiovisual services;
Main functionalities and basic requirements
for Multipoint Control Units (MCUs)
European Telecommunications Standards Institute
ETSI Secretariat
F-06921 Sophia Antipolis CEDEX - FRANCE
Postal address:
650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Office address:
c=fr, a=atlas, p=etsi, s=secretariat - secretariat@etsi.fr
X.400: Internet:
Tel.: +33 92 94 42 00 - Fax: +33 93 65 47 16
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 1996. All rights reserved.

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ETS 300 483: April 1996
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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Foreword .5
1 Scope .7
2 Normative references.8
3 Definitions.9
4 Abbreviations.9
5 Functional requirements for a MCU .10
5.1 Basic functions.10
5.1.1 Network Interface Unit (NIU) .10
5.1.2 Multiplexer/Demultiplexer Unit (MDU).10
5.1.3 Audio Processor Unit (APU).11
5.1.4 Video Processor Unit (VPU).11
5.1.5 Control Processor Unit (CPU) .11
5.2 Optional functions .13
5.2.1 Additional audio processing .13
5.2.2 Additional video processing.13
5.2.3 Additional call processing.13
5.2.4 Management of the SCM .14
5.2.5 Alternative conference management modes.14
5.2.6 Identification of connected terminals.14
5.2.7 Data transmission and/or broadcast functions.14
5.2.8 Identification of token assignment.15
5.2.9 Transmission of additional information.15
5.2.10 Multiple MCU configurations .15
5.2.11 Off-line or on-line conference administration .17
5.2.12 Security measures.17
5.2.13 Maintenance.17
5.2.14 Operation Administration and Maintenance Terminal (OA&MT).17
5.3 Various types of MCU - minimum functional requirements .17
5.3.1 Different classes of MCU .17
5.3.2 Minimum functional requirements for every class of MCU.18
5.4 Technical requirements .19
5.4.1 General requirements.19
5.4.2 Requirements applicable to individual functions .19
6 Special procedures for multipoint communications.20
6.1 Basic procedures .20
6.1.1 Initialisation procedures.20
6.1.2 Further capability exchanges .23
6.1.3 BAS sequencing.23
6.1.4 Mode switching procedures.23
6.1.5 Video bit-rate changing procedure .23
6.1.6 Video switching procedures .24 Automatic switching.24 Controlled switching.24
6.2 Videoconference with multiple MCUs .25
6.2.1 Designation of master MCU .25
6.2.2 Initialisation.25
6.2.3 Common operating mode.25
6.3 Special maintenance procedures .26
6.3.1 Loop control.26
6.3.2 Loop detection.26

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7 Basic in-band signalling requirements for MCUs. 27
Annex A (informative): Requirements applicable to audiovisual terminals. 30
A.1 Functional requirements . 30
A.1.1 Basic multipoint operation . 30
A.1.2 Enhanced multipoint operation. 30
A.1.2.1 In-band signalling procedures. 30
A.1.2.2 Additional keyboard and display . 30
A.2 In-band signalling requirements. 32
Annex B (informative): Bibliography . 34
History. 35

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ETS 300 483: April 1996
This European Telecommunication Standard has been prepared by the Terminal Equipment (TE)
Technical Committee of the European Telecommunications Standards Institute (ETSI).
Transposition dates
Date of adoption of this ETS: 15 March 1996
Date of latest announcement of this ETS (doa): 31 July 1996
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 31 January 1997
Date of withdrawal of any conflicting National Standard (dow): 31 January 1997

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Blank page

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1 Scope
This ETS specifies the functional requirements applicable to a Multipoint Control Unit (MCU), or a set of
interconnected MCUs, which allows several audiovisual terminals to communicate simultaneously using
single or multiple digital channels up to 1 920 kbit/s when interconnected through the pan-European
Integrated Services Digital Network (ISDN).
This ETS specifies the basic in-band signalling procedures to be supported to control the operation of
MCU and audiovisual terminals involved in a simple multipoint communication or a basic video/
audiographic conference as specified in other standards.
This ETS includes an overall description of the basic and optional functionalities of a MCU, and lists the
related requirements.
Annex A (informative) describes the various requirements which apply to audiovisual terminals in order to
ensure their capability to take part in multipoint communications.
A MCU with any interface connected to the ISDN via a Channel Aggregation Unit (CAU) and/or special
interface adapters may be used for multipoint communication when the composite equipment complies
with all requirements specified in this ETS. However, the various related pieces of equipment are outside
the scope of this ETS.
A MCU may establish conferences which include not only audiovisual terminals connected through an
ISDN, but also some audiovisual terminals directly linked through digital leased lines.
A MCU may set up a conference where the connected terminals include not only videoconference and
videotelephony terminals but also Audiographic conference, 7 kHz telephony or 3,1 kHz telephony
terminals. The description of protocols and requirements is limited to the basic multipoint communication
established between various terminals supporting 3,1 kHz telephony, 7 kHz telephony, or the
Videotelephony teleservice. The protocols and requirements necessary to support more sophisticated
services, such as Videoconference, Audiographic conference, or applications based on data transmission
are not considered here.
NOTE 1: The T.120 series of ITU-T Recommendations presently developed by ITU-T SG 8
should be used to support such enhanced functionalities.
Use of the protocol specified in ITU-T Recommendation H.243 to support these
functionalities should be precluded since it would result in incompatibilities between
MCUs and/or terminals.
NOTE 2: In the case of interworking with Public Switched Telephone Network (PSTN) or mobile
telephony terminals, echoes and/or transmission delays can degrade the audio
communication quality to an unacceptable level.
A MCU may be owned either by a public service provider or by a private customer. In the latter case the
equipment is part of a Private Telephone Network.
The D-channel signalling protocol used to set up, modify and clear a multipoint communication is outside
the scope of this ETS.
On line procedure and/or protocol to be used by an end user to require, modify or cancel a conference
reservation is outside the scope of this ETS.
Implementation of the T.120 series of ITU-T Recommendations for the support of the enhanced functions
associated to various options of the videoconference service (e.g. Conference control, multipoint data
communication service) is outside the scope of this ETS.
Conformance to this ETS will be demonstrated by checking that all mandatory functional requirements are
supported during execution of a test suite which is specified in other ETSs.

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2 Normative references
This ETS incorporates by dated or undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications listed hereafter.
For dated references, subsequent amendments to, or revisions of any of these publications apply to this
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: "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 G. 728 (1993): "Coding of speech at 16 kbit/s using
low-delay code excited linear prediction".
[3] CCITT Recommendation G. 711 (1988): "Pulse code modulation (PCM) of voice
[4] CCITT Recommendation G. 722 (1988): "7 kHz audio-coding within 64 kbit/s".
[5] ETS 300 012: "Integrated Services Digital Network (ISDN); Basic user-network
interface; Layer 1 specification and test principles".
[6] ETS 300 011: "Integrated Services Digital Network (ISDN); Primary rate
user-network interface; Layer 1 specification and test principles".
[7] ETS 300 290: "Business TeleCommunications (BTC); 64 kbit/s digital
unrestricted leased line with octet integrity (D64U); Terminal equipment
[8] ETS 300 248: "Business Telecommunications (BT); Open Network Provision
(ONP) technical requirements; 2 048 kbit/s unstructured leased line (D2048U);
Terminal equipment interface".
[9] ETS 300 420: "Business TeleCommunications (BTC); 2 048 kbit/s structured
leased line (D2048S); Terminal equipment interface".
[10] ETS 300 102-1: "Integrated Services Digital Network (ISDN); User-network
interface layer 3; Specifications for basic call control".
[11] ETS 300 267: "Integrated Services Digital Network (ISDN); Telephony 7 kHz
and videotelephony teleservices; Digital Subscriber Signalling System No. one
(DSS1) protocol".
[12] ETS 300 164: "Integrated Services Digital Network (ISDN); Meet-Me
Conference (MMC) supplementary service; Service description".
[13] ETS 300 185: "Integrated Services Digital Network (ISDN); Conference call add-
on (CONF) supplementary service; Digital Subscriber Signalling System No. one
(DSS1) protocol".
[14] ETS 300 143: "Integrated Services Digital Network (ISDN); Audiovisual services;
Inband signalling procedures for audiovisual terminals using digital channels up
to 2 048 kbit/s".
[15] ETS 300 145: "Integrated Services Digital Network (ISDN); Audiovisual services;
Videotelephone systems and terminal equipment operating on one or two
64 kbit/s channels".
[16] ITU-T Recommendation H. 261: "Video codec for audiovisual services at
p x 64 kbit/s".

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3 Definitions
For the purposes of this ETS, the following definitions apply:
Conference control: The capability conferred to one terminal to control operation of the MCU. This
function requires a specially designed terminal. The terminal user may be the actual chairman of the
meeting or the conference controller.
Directly connected terminal or Local terminal: A terminal connected to a given MCU (this wording
should be used only in case of multiple MCU configuration).
Master MCU: The MCU selected to control the system operation in case of multiple MCU configuration.
Multipoint Control Unit (MCU): A functional entity which is used to establish a multipoint communication
between several audiovisual (or audio only) terminals. A MCU may be owned by the customer (and
located in its premises) or may be offered by a service provider/network operator.
MCU Port: The bi-directional access point of the digital link established between a terminal (or another
MCU) and a MCU. It is a logical entity; a port may eventually require several MCU-Network physical
interfaces, but several ports may also share a MCU-Network physical interface.
Primary Port: A MCU Port where the connected terminal can support the SCM.
Secondary Port: A MCU Port where the connected terminal has a lower capability than the SCM.
Selected Communication Mode (SCM): Common operating mode of most terminals during the
conference. It defines symmetrical rates for audio, video and any data path.
token: A virtual token which may be allocated by a controlling entity (MCU or Conference-control terminal)
to confer, to a requesting terminal, authorisation to initiate a special activity (e.g. video broadcast, data
transmission) which otherwise would be in conflict with normal or similar activities of other terminals. Such
a token may be allocated or "passed" successively from one terminal to another.
4 Abbreviations
For the purposes of this ETS, the following abbreviations apply:
APU Audio Processor Unit
BAS Bit-rate Allocation Signal
C&I Control and Indication
cap-set capability set
CAU Channel Aggregation Unit
CLIP Calling Line Identification Presentation
CPU Control Processor Unit
DPU Data Processor Unit
ECS Encryption Control Signal
FAS Frame Alignment Signal
ISDN Integrated Services Digital Network
MBE Multiple Byte Extension (BAS codes)
MCS Multipoint Communication Service
MCU Multipoint Control Unit
MDU Multiplexer/Demultiplexer Unit
MLP Logical data sub-channel which may contain various data protocols (also
referred to as Multi-Layer Protocol in ETS 300 144 [1])
NIU Network Interface Unit
OA&MT Operation, Administration and Maintenance Terminal
ONP Open Network Provision
PIN Personal Identification Number
PSTN Public Switched (analogue) Telephone Network
SBE Single Byte Extension (BAS codes)
SCM Selected Communication Mode
SCM-cap SCM capability set

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VPU Video Processor Unit
NOTE: A complementary list of abbreviations used for C&I (typed in bold characters) can be
found in tables 5 and A.2.
5 Functional requirements for a MCU
5.1 Basic functions
A MCU includes several types of functional entities. An overall description is given by the schematic of
figure 1 (see subclause 5.1.5). The various functional entities are listed and briefly described below. The
two first types shall be provided for every individual port, while the others are common to all ports involved
in a multipoint communication (conference).
NOTE: There is no particular limit set to the maximum number of ports involved in a
conference, however it is practically limited, taking into account cost, set-up difficulty
and audio performance degradation (noise).
A same piece of equipment called a MCU, may simultaneously handle several
independent videoconferences.
5.1.1 Network Interface Unit (NIU)
The Network Interface Unit (NIU), supported by a local call control facility, has two basic tasks:
- the establishment and release of the digital link related to the port;
- the adaptation of the composite signal (audio + video + signalling + etc.) to the ISDN interface.
NOTE: The network access may use an external standard interface adapter.
5.1.2 Multiplexer/Demultiplexer Unit (MDU)
The Multiplexer/Demultiplexer Unit (MDU) performs the following tasks:
- the generation/recovery of in-band frame and multiframe synchronisation signals;
- the buffering and ordering of audio, video, data and in-band signalling signals;
- the transmission/reception of Bit-rate Allocation Signal (BAS) codes;
- the transmission/reception of optional data sub-channel(s).
This unit is also used to split/aggregate several "64 kbit/s channels" which may be necessary for a given
port, when this is required by the selected transfer rate capability.
NOTE: Splitting/aggregation may be achieved in an external piece of equipment.

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5.1.3 Audio Processor Unit (APU)
The Audio Processor Unit (APU) receives audio inputs from all demultiplexers and generates the same
number of audio outputs sent to the multiplexers. The generation of output signals may use one of several
a) audio mixing - the signal sent to each terminal is the linear sum of the audio signals delivered by the
other terminals;
b) audio switching - the signal sent to a conferee is either the largest audio input signal or is directly
selected according to the controlled operating mode. In any case no signal is returned to the
speaker; the switching shall be performed smoothly and without additional noise.
NOTE: In both cases audio signals with a level lower than an adjustable threshold may be
suppressed in order to avoid the addition of useless noises/echoes.
A small time delay (Θ ) may be incorporated into the automatic switching to prevent its
excessively frequent activation by inadvertent noises (coughing, knocking a
microphone, etc.).
5.1.4 Video Processor Unit (VPU)
The Video Processor Unit (VPU) receives video signals from all demultiplexers and generates video
signals sent to the multiplexers. The generation of output signals is generally based on switching
a) the signal broadcast to conferees is the picture of the current speaker (largest audio input signal). In
this case the signal sent to the current speaker is the signal received from the previous speaker;
b) the signal sent to the conferees is selected according to a special operating mode:
- a terminal requires broadcasting its video to all other conferees (one at a time);
- the MCU itself chooses which video shall be broadcast (cf. subclause
NOTE: In case of automatic video switching, another time constant (Θ ) may be used to
prevent useless activation resulting from a short voice burst.
The MCU should be designed in order to preserve the audio/video alignment as
provided by terminals.
5.1.5 Control Processor Unit (CPU)
The Control Processor Unit (CPU) controls set up and release of the various digital links. In the case of
ports connected to an ISDN, it provides for co-ordinated set-up and release of the initial and additional
The CPU exchanges information with connected terminals by sending and receiving the BAS codes
transmitted through the in-band signalling procedures.
After an initial capability exchange sequence, the CPU defines the SCM and controls the various entities
(NIU, MDU, APU, VPU and DPU) according to:
- capabilities declared by the audiovisual terminals;
- MCU configuration and management data.
After exchange of Controls and Indications (C&I), the CPU routes the various internal signals.

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Demultiplexer Ar2
Network Ar
( NIU ) Vt1 ( APU )
Call Control
Demultiplexer Vr2
Network Vr
( NIU ) Vt1 ( VPU )
Call Control
" Dr1
" DrN
( DPU )
( repeted N times )
Demultiplexer Br2
Network Br
( NIU ) VtN ( CPU )
Call Control
to Call Control
from Call Control
A : audio signal    D : data signal
V : video signal    B : BAS codes (or equivalent)
r : receive        t : transmit
Figure 1: Schematic of a Multipoint Control Unit

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5.2 Optional functions
A wide variety of MCUs may be developed according to the various options which could be provided. In
order to identify, in subclause 5.3, the minimum requirements applicable to a "basic" MCU, a
non-exhaustive list of optional functions is presented below.
Some optional functions require related enhanced functions of connected terminals (e.g. terminal used for
Conference control), however terminals having no such enhancements may take part in a
videoconference without misoperation (cf. annex A).
Some optional functions are not independent since one shall be supported to allow implementation of
another one (see "bound options" in table 1, subclause 5.3.2).
When an option described in this ETS is declared by a manufacturer or service provider, the related
protocol procedures shall conform to the requirements specified in clause 6.
5.2.1 Additional audio processing
Additional functions could be:
a) provision of tones and/or announcements during conference set-up;
b) total control of individual audio source and destination.
5.2.2 Additional video processing
In addition to the basic switching functions listed in subclause 5.1.4, a MCU may provide:
a) video mixing: the signal sent to all conferees results from spatially multiplexing the images sent by
the other terminals into a single composite image in a "split screen" format;
b) total control of individual video source and destination;
c) provision of visual information on call and/or terminal states.
5.2.3 Additional call processing
To be able to interwork with audiovisual terminals, which may offer various interfaces and/or transfer rate
capabilities, a MCU may provide:
a) internal splitting/aggregation of the various "64 kbit/s channels" involved in a MCU-terminal link;
b) compatibility with multiple channel terminals;
c) compatibility with mixed single and multiple channel terminals (SM comp capability);
d) compatibility with remote terminals connected to restricted networks;
e) compatibility with 3 kHz, 4 kHz and/or 7 kHz telephony terminals.
Conference set-up mode results from permanent, reserved and on-demand connections. In the latter case
the MCU may offer several alternative procedures:
f) "Meet-me conference" using a MCU ISDN destination number;
g) "Meet-me conference" using a conference ISDN destination number;
h) "Dial out conference" with previous registration of conferee ISDN numbers;
i) "Mixed mode (Dial out and Meet-me) conference".
During the conference, a MCU may also offer additional facilities related to call control such as:
j) addition of a new conferee (add on conference requiring new dialling);
k) exclusion of a terminal (connection is released).

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5.2.4 Management of the SCM
At least one of the following solutions shall be available to determine the Communication Mode and shall
be clearly indicated to user by manufacturer and/or service provider:
a) the SCM is fixed, it results from the MCU definition and manufacture;
b) the SCM is fixed, it results from a previous arrangement with the service provider at the time of
conference booking;
c) the SCM is set up according to a Multi-Layer Protocol (MLP) based procedure;
d) the SCM is selected automatically according to an identified algorithm (e.g.):
- highest common mode of audiovisual terminals;
- based on capabilities of the chair-control terminal;
- based on an adjustable threshold;
- etc.
NOTE: When the SCM is fixed, any terminal with a lower capability may be rejected;
otherwise, it is considered as a secondary terminal (subclause 6.1.1).
5.2.5 Alternative conference management modes
A MCU shall at least provide the unconducted mode, where switching functions are performed
automatically, and result in selection and transmission of the current speaker image (subclause 5.1.4).
In addition, it may also provide conference control procedures.
When the conference control facility is declared, the following functions shall be provided:
a) claim, assignment of chair-control token;
b) withdrawal of chair-control token (return to voice activation mode);
c) indication of

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