SIST ETS 300 145 E2:2003

SLOVENSKI STANDARD
01-december-2003
Digitalno omrežje z integriranimi storitvami (ISDN) – Avdiovizualne storitve –
Videotelefonski sistemi in terminalska oprema, ki delujejo z enim ali dvema
kanaloma s hitrostjo 64 kbit/s
Integrated Services Digital Network (ISDN); Audiovisual services; Videotelephone
systems and terminal equipment operating on one or two 64 kbit/s channels
Ta slovenski standard je istoveten z: ETS 300 145 Edition 2
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN ETS 300 145
TELECOMMUNICATION January 1996
STANDARD Second Edition
Source: ETSI TC-TE Reference: RE/TE-04038
ICS: 33.080
Key words: ISDN, audiovisual services
Integrated Services Digital Network (ISDN);
Audiovisual services;
Videotelephone systems and terminal equipment operating
on one or two 64 kbit/s channels
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: secretariat@etsi.fr
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 300 145: January 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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ETS 300 145: January 1996
Contents
Foreword.5
1 Scope.7
2 Normative references .7
3 Definitions.9
4 Abbreviations .9
5 System description .10
5.1 Block diagram and identification of elements (informative).10
5.2 Videotelephone terminal types .11
5.3 Signal structure and procedures .12
5.3.1 Frame structure .12
5.3.2 Control and Indication (C&I) .12
5.3.3 In-band signalling procedures.14
5.3.4 Communication modes of videotelephones.15
6 Terminal procedures and arrangements related to videotelephony call.15
6.1 Basic call establishment and mode initialisation .15
6.1.1 Phase A - Call set-up for the initial channel.16
6.1.2 Phase B - mode initialisation .16
6.1.2.1 Case of 1B call.17
6.1.2.2 Case of 2B call.17
6.1.3 Phase CA - call set-up for the additional channel.18
6.1.4 Phase CB - mode initialisation on the additional channel.18
6.2 Videotelephony communication - Phase C .18
6.2.1 Mode switching .18
6.2.2 Capability change .19
6.2.3 Loopbacks for maintenance purposes (optional).19
6.2.4 Terminal arrangements .19
6.2.4.1 Video transmission.19
6.2.4.2 Picture definition.20
6.2.4.3 Exchange of video by mutual agreement (optional).20
6.2.4.4 Audio decoding .20
6.2.4.5 Delay compensation (optional).20
6.3 Change of connection arrangements.20
6.3.1 Expanding a 1B call to a 2B call .20
6.3.2 Reducing a 2B call to a 1B call .21
6.4 Call clearing - Phase D/E .21
6.4.1 Clearing terminal.21
6.4.2 Cleared terminal .21
7 Implementation of supplementary services by videotelephony terminals.22
7.1 Calling Line Identification Presentation (CLIP) and Connected Line Identification Presentation
(COLP) .22
7.2 Multiple Subscriber Number (MSN) .22
7.3 Call Completion to Busy Subscriber (CCBS) and Call Completion on No Reply (CCNR) .22
7.4 Call Deflection (CD).22
7.5 Supplementary services resulting in connection interruption .22
7.5.1 Procedure P.23
7.5.2 Terminal Portability (TP).23
7.5.2.1 Invocation procedure.23
7.5.2.2 Resumption after invocation of Terminal Portability.23
7.5.3 Hold .23
7.5.3.1 Invocation procedure.23

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ETS 300 145: January 1996
7.5.3.2 Retrieval after invocation of HOLD .24
7.5.4 Explicit Call Transfer (ECT) .24
7.5.4.1 Procedures for the transferring terminal A.24
7.5.4.2 Procedures for the transferred and transferred-to terminal .25
8 Optional enhancements.25
8.1 Data ports.25
8.2 Encryption .25
9 Fault indication.25
Annex A (informative): Detailed steps and procedures .26
Annex B (informative): Bibliography .33
History.34

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ETS 300 145: January 1996
Foreword
This European Telecommunication Standard (ETS) was produced by the Terminal Equipment (TE) Technical
Committee of the European Telecommunications Standards Institute (ETSI).
Transposition dates
Date of adoption of this ETS: 31 January 1996
Date of latest announcement of this ETS (doa): 30 April 1996
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 31 October 1996
Date of withdrawal of any conflicting National Standard (dow): 31 October 1996

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ETS 300 145: January 1996
Blank page
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ETS 300 145: January 1996
1 Scope
This 2nd edition of ETS 300 145 specifies the system requirements for a Videotelephone using one or two
B-channels, when connected to the S point or coincident S and T reference point of the pan-European Integrated
Services Digital Network (ISDN). These requirements ensure end-to-end compatibility of terminals supporting the
videotelephony teleservice described in ETS 300 264 [1].
This ETS is applicable in whole to Terminal Equipment (TE) supporting the videotelephony teleservice;
subclauses 5.3, 6.1, 6.2 and clause 8 are also applicable to Multipoint Control Units (MCUs) using one or two
B-channels.
The specific requirements which apply to a terminal in order to support various options related to videoconference
and/or data transmission are specified in other documents (e.g. Draft prETS 300 483 [2]).
Conformance to this ETS allows interworking with terminals supporting the telephony 3,1 kHz teleservice described
in ETS 300 111 [3], the telephony 7 kHz teleservice described in ETS 300 263 [4] and Public Switched Telephone
Network (PSTN) telephony terminals. Interworking with other audio-visual terminals can take place if their operation
on one or two B-channels is according to this ETS.
NOTE 1: Terminals in accordance with ITU-T Recommendation H.320 can interwork, if one or two
64 kbit/s (unrestricted or restricted) digital channels can be established between the terminals
(e.g. using a remote Channel Aggregation Unit in case of a 128 kbit/s H.320 single channel
terminal).
NOTE 2: Interworking with terminals according to CCITT Recommendation G.725 [5] is optional; it may be
restricted to mode 0.
NOTE 3: Audio and video signal interfaces are outside the scope of this ETS.
NOTE 4: Interfaces and protocols used for optional data transmission are outside the scope of this ETS.
The in-band signalling procedures are described in ETS 300 143 [6], and the syntax for these procedures is defined
in ETS 300 144 [7]. The video coding algorithm is described in ITU-T Recommendation H.261 [8].
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 are 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
apply.
[1] ETS 300 264: "Integrated Services Digital Network (ISDN); Videotelephony teleservice,
Service description".
[2] Draft prETS 300 483: "Terminal Equipment (TE); Integrated Services Digital Network
(ISDN); Multipoint Communications for Audiovisual Services; Main functionalities and
basic requirements for Multipoint Control Units and audiovisual terminals".
[3] ETS 300 111 (1992): "Integrated Services Digital Network (ISDN); Telephony 3,1 kHz
teleservice, Service description".
[4] ETS 300 263: "Integrated Services Digital Network (ISDN); Telephony 7 kHz teleservice,
Service description".
[5] CCITT Recommendation G.725 (1988): "System aspects for the use of the 7 kHz audio
codec within 64 kbit/s".
[6] ETS 300 143: "Integrated Services Digital Network (ISDN); Audiovisual services; In-band
signalling procedures for audiovisual terminals using digital channels up to 2 048 kbit/s".

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ETS 300 145: January 1996
[7] 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 in-band
signalling".
[8] ITU-T Recommendation H.261: "Video codec for audiovisual services at p x 64 kbit/s".
[9] ETS 300 102-1: "Integrated Services Digital Network (ISDN); User-network interface,
layer 3, Specifications for basic call control".
[10] ETS 300 012 (1991): "Integrated Services Digital Network (ISDN); Basic user-network
interface, Layer 1 specification and test principles".
[11] ETS 300 125 (1991): "Integrated Services Digital Network (ISDN); User-network interface
data link layer specification, Application of CCITT Recommendations Q. 920/I. 440 and
Q.921/I. 441".
[12] ITU-T/T.120 series of Recommendations:
- ITU-T Recommendation T.122: "Multipoint communication service for
audiographics and audiovisual conferencing, service definition";
- ITU-T Recommendation T.123: "Protocol stacks for audiographic and audiovisual
teleconference applications";
- ITU-T Recommendation T.125: "Multipoint communication service protocol
specification".
[13] CCITT Recommendation G.711 (1988): "Pulse code modulation (PCM) of voice
frequencies".
[14] CCITT Recommendation G.722 (1988): "7 kHz audio-coding within 64 kbit/s".
[15] CCITT Recommendation G.728 (1992): "Coding of speech at 16 kbit/s using low-delay
code-excited linear prediction".
[16] I-ETS 300 302, Parts 1 to 4: "Integrated Services Digital Network (ISDN); Videotelephony
teleservice".
[17] ETS 300 267-1: "Integrated Services Digital Network (ISDN); Telephony 7 kHz and
videotelephony teleservices, Digital Subscriber Signalling System No. one (DSS1), Part 1:
Protocol specification".
[18] I-ETS 300 245-5: "Integrated Services Digital Network (ISDN); Technical characteristics of
telephony terminals, Part 5: Wideband (7kHz) handset telephony".
[19] I-ETS 300 245-2: "Integrated Services Digital Network (ISDN); Technical characteristics of
telephony terminals, Part 2 - PCM A-law handset telephony".
[20] ITU-T Recommendation H.233: "Confidentiality system for audiovisual services".

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ETS 300 145: January 1996
3 Definitions
For the purposes of this ETS, the definitions given in clause 3 of both ETS 300 143 [6] and ETS 300 144 [7] apply
along with the following:
additional channel: One of the channels between two users, which is not the I-channel (see ETS 300 144 [7]).
audio mute: Muting the loudspeaker of a terminal.
channel: A unidirectional link between two users.
connection: A bi-directional link between two users: it carries both (unidirectional) channels.
Control and Indication (C&I): End-to-end signalling between terminals, consisting of Control, which causes a state
change in the receiver, and Indication which provides for information as to the state or functioning of the system
(see also ETS 300 144 [7] for additional information and abbreviations).
data: Refer to any of these: Low Speed Data (LSD), High Speed Data (HSD), Multi Layer Protocol (MLP), High
Speed Multi Layer Protocol (H-MLP) as defined in ETS 300 144 [7].
I-channel: The initial or only B-channel as defined in ETS 300 144 [7].
In-band signalling: Signalling via the Bit-rate Allocation Signal (BAS) of the frame structure, as defined in
ETS 300 144 [7].
lip synchronisation: Operation to provide the feeling that the speaking motion of the displayed person is
synchronised with his speech.
man-machine interface: Interface between human user and terminal/system, consisting of a physical section
(electro-acoustic, electro-optic transducer, keys, etc.) and a logical section dealing with functional operation states.
4 Abbreviations
For the purposes of this ETS, the following abbreviations apply:
BAS Bit-rate Allocation Signal
C&I Control and Indication
CLIP Calling Line Identification Presentation
COLP Connected Line Identification Presentation
CONNECT D-channel message (as in ETS 300 102-1 [9])
ECS Encryption Control Signal
ECT Explicit Call Transfer
FAS Frame Alignment Signal
H-MLP High Speed Multi Layer Protocol
HOLD Supplementary service
HSD High Speed Data
ISDN Integrated Services Digital Network
LSD Low Speed Data
MCU Multipoint Control Unit
MLP Logical data sub channel named "MLP"
NOTE: MLP previously referred to the ITU-T Recommendation T.120 [12] Multilayer Protocol, but now
this is just a name for the logical subchannel which may contain T.120 or H.224 protocol, or
Dummy data (see ETS 300 143 [6] and ETS 300 144 [7]).
MSB Most Significant Bit
MSN Multiple Subscriber Number
PSTN Public Switched Telephone Network
SC Service Channel
TEA Terminal Equipment Alarm
TERM A, TERM B Terminals
TERM C Terminal
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ETS 300 145: January 1996
TP Terminal Portability
5 System description
A videotelephony terminal conforming to this ETS shall be connected to an ISDN basic access in accordance with
ETS 300 012 [10], ETS 300 102-1 [9] and ETS 300 125 [11]; then it cannot use more than 2 B-channels.
5.1 Block diagram and identification of elements (informative)
A generic videotelephony system is shown in figure 1. It consists of several (at least two) videotelephony terminals,
the ISDN, and possibly a Multipoint Control Unit (MCU).
A configuration of a complete videotelephony terminal consisting of several functional units is also shown in figure 1.
- Video I/O equipment includes camera(s), monitor(s), and video processing units to provide functions such as
split-screen scheme.
- Audio I/O equipment includes the handset and/or microphone(s), loud-speaker(s) and/or earphone(s), and
audio processing units to provide such functions as acoustic echo cancellation.
- Data I/O adaptor may be used for additional services, such as facsimile, or additional facilities (e.g. far end
camera control): it is optional.
- Telematic equipment includes visual aids such as an electronic blackboard and a still picture transceiver to
enhance the basic videotelephone communication as defined in ITU-T T.120 series of Recommendations
[12]: these are optional.
- The system control unit carries out such functions as:
- network access through user-to-network signalling,
- end-to-end in-band signalling to establish a suitable mode of operation,
- audio and video signal processing and multiplexing,
- data transmission and/or application.
- The video codec carries out redundancy reduction coding and decoding for video signals: the algorithm is as
defined in ITU-T Recommendation H.261 [8].
- The audio codec carries out redundancy reduction coding and decoding for audio signals: the algorithms are
defined in CCITT Recommendations G.711 [13], G.722 [14] and G.728 [15]. The audio characteristics are
defined in I-ETS 300 302, Parts 1 to 4 [16].
NOTE 1: Part 4 of this prI-ETS is still under study.
NOTE 2: Delay may be introduced into the audio path equal to the video codec delay, to maintain lip
synchronisation (see subclause 6.2.4.5).
- The Mux/dmux unit multiplexes transmitted video, audio, data and control signals into a single bit stream and
demultiplexes a received bit stream into constituent multimedia signals, as defined in ETS 300 144 [7].
- The network interface makes the necessary adaptation between the network and the terminal according to
the user-network interface requirements, as defined in ETS 300 012 [10], ETS 300 102-1 [9] and
ETS 300 125 [11].
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ETS 300 145: January 1996
Video I/O
Video Codec
Equipment
Terminal 2
MUX
Audio I/O
Audio Codec Delay
Equipment +
I
N
N
DEMUX
E
T
Data I/O adaptor and/or
T
E ISDN MCU
Telematic equipment
W
R
S F
O
Y
A
End-to-end signalling
S
R
C
T
E K
E
M
Terminal 3
C D-channel signalling
O
N
T
R
O Terminal 1
L
Figure 1: Visual telephone system
- End-to-end signalling is defined in ETS 300 143 [6] and ETS 300 144 [7].
- User-to-network signalling is defined in ETS 300 102-1 [9], ETS 300 125 [11] and ETS 300 267-1 [17],
clause 7.
5.2 Videotelephone terminal types
Table 1 lists terminal types of videotelephone. The terminal type is categorised according to the audio
communication modes and the number of B-channels with which the terminal can communicate (see
subclause 5.3.4).
The terminal shall be able to operate at 1B and 2B transfer rate according to at least one of the operable audio
modes. The type of remote terminal is identified through the capability exchange defined in ETS 300 143 [6].

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ETS 300 145: January 1996
Table 1: Videotelephone terminal type
Transfer rate 1 B 2 B
Xa Xb1 Xb2/3 Xb4 Xb5
TYPE
G.728 G.728 G.728 G.722
Transfer Audio
G.711 G.711 G.722 G.711
MODE
rate coding
G.711 G.711
a0 1B G.711
a1 1B G.728
b1 2B G.711
b2 2B G.722
b3 2B G.728
NOTE: "indicates operable mode".
5.3 Signal structure and procedures
5.3.1 Frame structure
The terminal shall conform to ETS 300 144 [7].
5.3.2 Control and Indication (C&I)
C&I shall be chosen from the general audiovisual set contained in ETS 300 144 [7]. For videotelephone systems,
relevant values are listed in table 2 giving their source, sink, synchronisation with picture, transmission channel and
reference for code word definition.
C&I functions are defined such that, under various circumstances, the audiovisual system operates in a fault-free
manner and also such that sympathetic presentation to users is possible. Some functions shall therefore be
mandatory, others optional.
Table 2 with its notes clarifies the circumstances under which C&I functions are "mandatory":
M denotes mandatory for any equipment;
CM denotes conditionally mandatory if the relevant option is declared for the audiovisual system;
R means not mandatory but recommended.
NOTE: All videotelephone terminals have a video source providing a picture of participants, and some
terminals may have optional additional video sources: the participant-picture source is
designated #1, having the associated symbol VIA. When incoming video is ON (BAS command
(010)[1] or (010)[2]) and VIA, VIA2, VIA3 have not been transmitted, source #1 is assumed.

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ETS 300 145: January 1996
Table 2: C&I signals for videotelephone
Sync.
C&I with
C/I Source Sink Transmission Code word NOTES
(function or name) picture
(see below)
channel definition
RX TX
(1) (1)
Picture format I decoder coder no BAS ETS 300 144 M
[7]
Picture format C coder decoder yes embedded in H.261 [8] M M
video
Minimum decodable I decoder coder no BAS ETS 300 144 M
picture interval [7]
VCF C coder or decoder no BAS ETS 300 144 M
Video
MCU [7]
VCU C decoder coder no BAS ETS 300 144 MCM
or MCU [7] (3)
Freeze picture C coder decoder yes embedded in H.261 [8] M
release control video
MCU C terminal terminal no MLP ITU-T Rec. R R
related messages or MCU or MCU T.120 series (9) (9)
[12]
MCC / cancel-MCC C MCU terminal no BAS ETS 300 144 M
[7]
MCU
MCS / cancel MCS C MCU terminal no BAS ETS 300 144 CM
(or MCN) [7] (4)
MIZ and MIS C MCU terminal no BAS ETS 300 144 M
[7]
LCA C terminal terminal no BAS ETS 300 144
(1) [7]
Mainte-
LCV C terminal terminal no BAS ETS 300 144
(1) [7]
nance
LCD C terminal terminal no BAS ETS 300 144 MCM
(1) [7] (2)
LCO C terminal terminal no BAS ETS 300 144 MCM
(1) [7] (2)
Confe-
split-screen I sending receiving yes embedded in H.261 [8] CM
rence
indication terminal terminal video (5)
(continued)
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ETS 300 145: January 1996
Table 2 (concluded): C&I signals for videotelephone
Sync.
C&I with
C/I Source Sink Transmission Code word NOTES
(function or name) picture
(see below)
channel definition
Document camera I sending receiving yes embedded in H.261 [8] CM
indication terminal terminal video (6)
Termi- AIA / AIM I sending receiving no BAS ETS 300 144 CM CM
nal terminal terminal [7] (7) (7)
VIA, VIA2, VIA3 I sending receiving no BAS ETS 300 144 CM CM
terminal terminal [7] (7) (7)
VIS
VIR I sending receiving no BAS ETS 300 144 CM CM
terminal terminal [7] (8) (8)
Data transmission C sending receiving no BAS + ETS 300 144
control terminal terminal MLP [7] R R
ITU-T / T.120 (10) (10)
series of Rec
[12].
Data
Video compatibility I sending receiving no BAS ETS 300 144 R R
terminal terminal [7]
(Dummy) (11) (11)
NOTE 1: RX - mandatory (or conditionally mandatory) to recognise and act upon.
TX - mandatory (or conditionally mandatory) to send when / if relevant conditions occur.
NOTE 2: Maintenance terminals only.
NOTE 3: Mandatory if the terminal cannot decode high or low video rates - see subclause 6.2.4.1.
NOTE 4: Mandatory for a terminal which can transmit LSD, MLP, HSD or H-MLP. Terminals not
obeying MCS shall not be able to exchange video in a multipoint call when other terminals
are using their data capability.
NOTE 5: Mandatory for a terminal which can transmit a split-screen picture to CCITT
Recommendation H.100.
NOTE 6: Mandatory for a terminal which has a document camera.
NOTE 7: Mandatory for a terminal which has user controls to mute audio or cut the video without
also switching off the audio/video channel within the transmitted frame.
NOTE 8: Mandatory if the terminal itself does not turn video on until it receives this code from the
remote terminal. See subclause 6.2.4.2.
NOTE 9: Recommended to support enhanced functionalities such as Chair-control, etc.
NOTE 10: Recommended for data transmission and/or additional applications.
NOTE 11: Recommended for a simple terminal in order to be able to participate to a multipoint
conference when other terminals are using a data sub channel (e.g. LSD or MLP).
5.3.3 In-band signalling procedures
The terminal shall conform to ETS 300 143 [6] for point-to-point communications.

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ETS 300 145: January 1996
5.3.4 Communication modes of videotelephones
Communication modes of a videotelephone for the ISDN basic access are defined according to their channel
configuration and coding. All videotelephones shall be capable of operation in mode a and one or more of the
modes a1, b1, b2, b3.
The following modes of operation are defined for an ISDN basic interface (see also table 3):
Mode a : Mode 0U: 64 kbit/s audio to CCITT Recommendation G.711 [13], A or μ-law.
Mode 0F: 56 kbit/s audio to CCITT Recommendation G.711 [13], A or μ-law: 6,4 kbit/s
video to ITU-T Recommendation H.261 [8] may, or may not, be activated.
Mode a : 16 kbit/s audio to CCITT Recommendation G.728 [15] and 46,4 kbit/s video to
ITU-T Recommendation H.261 [8].
Mode a : Video only mode (62,4 kbit/s).
v
Mode b : 56 kbit/s audio to CCITT Recommendation G.711 [13] A or μ- law and 68,8 kbit/s video to
ITU-T Recommendation H.261 [8].
Mode b : 48 or 56 kbit/s audio to CCITT Recommendation G.722 [14] and 76,8 kbit/s or 68,8 kbit/s
video to ITU-T Recommendation H.261 [8].
Mode b : 16 kbit/s audio to CCITT Recommendation G.728 [15] and 108,8 kbit/s video to
ITU-T Recommendation H.261 [8].
Mode b : Video only mode (124,8 kbit/s).
v
NOTE: Modes a and b may be used in special applications such as:
v v
- communications between deaf,
- remote observation, etc.
Table 3: Communication modes of videotelephone
Videotelephone mode Channel rate ISDN channel Audio coding Video to H.261 [8]
a0 64 kbit/s B G.711 optional, ON / OFF
a1 G.728
av none
b1 G.711 ON
b2 128 kbit/s 2B (note) G.722
b3 G.728
bv none
NOTE: In the case of two B-channels, the channels shall be synchronised according to
ETS 300 144 [7].
6 Terminal procedures and arrangements related to videotelephony call
6.1 Basic call establishment and mode initialisation
The videotelephony communication is set up according to the following steps:
- phase A: call set-up of initial channel, D-channel signalling (subclause 6.1.1);
- phase B: mode initialisation on initial channel, in-band signalling (subclause 6.1.2);
- phase CA if relevant: call set-up of additional channel (subclause 6.1.3);
- phase CB if relevant: initialisation on additional channel (subclause 6.1.4);
- phase C: videotelephone communication (subclauses 6.2 and 6.3);

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ETS 300 145: January 1996
- phase D/E: call clearing, in-band and D-channel signalling (subclause 6.4).
6.1.1 Phase A - Call set-up for the initial channel
The calling terminal shall request the initial channel using D-channel signalling according to ETS 300 267-1 [17],
clause 7.
The called terminal shall send an ALERTING message according to ETS 300 267-1 [17], clause 7 or another
message appropriate to this call state. When the called user answers, it sends the CONNECT message to the
network.
Before the terminals receive the CONNECT, respectively the CONNECT ACKNOWLEDGE message, the allowed
transmission from each terminal is one of the following:
1) unframed all-ones;
2) Mode-0U;
3) Mode-0F with full capability sets according to ETS 300 143 [6] in the BAS position;
4) Mode-0F with audio commands (000) [18] and optionally (001) [0].
If 1) or 2) is used there shall be a change to 3) or 4) at, or before, receipt of the CONNECT message at the calling
terminal, and at, or before, receipt of the CONNECT ACKNOWLEDGE message at the called terminal.
The moment of receipt of CONNECT or CONNECT ACKNOWLEDGE message defines initialisation of timer T1 for
phase B procedure which follows.
If there is no videotelephone in the called subscriber premises, the D-channel procedures specified in
ETS 300 267-1 [17], clause 7 result as described below:
a) fallback is not allowed: no communication (the call is ignored or rejected),
b) fallback to telephony is allowed and a 7 kHz telephony terminal answers: the final call shall conform to
I-ETS 300 245-5 [18] (Mode 1, 2 or 3 according to circumstances is established after a mode initialisation
procedure),
c) Fallback is allowed and a 3,1 kHz telephony terminal answers: the call shall then conform to I-ETS 300 245-2
[19] (Mode 0U),
d) Interworking occurs and a PSTN telephone answers: the call shall proceed as in c),
NOTE 1: In case b) an indication of fall-back to the "telephony 7 kHz" teleservice is provided to the calling
terminal in the CONNECT message. It is, then, allowed to initiate a mode initialisation
procedure.
NOTE 2: In case c) an indication of fall-back to the "telephony" (or "3,1 kHz audio teleservice") is provided
to the calling terminal in the CONNECT message. It can, then directly enter Mode 0U, omitting
the initialisation procedure which would fail.
NOTE 3: In case d) an indication of interworking is given to the calling terminal in the PROGRESS
indicator. It can, then directly enter Mode 0U, omitting the initialisation procedure which would
fail.
Only terminals able to work in restricted modes shall accept calls from restricted networks.
NOTE 4: Set-up of such calls requires use of specific BC, HLC and LLC (cf. ETR 18).
6.1.2 Phase B - mode initialisation
Using the procedures provided in ETS 300 143 [6], framed PCM audio is transmitted in both directions. After frame
and, if a 2B transfer rate capability has been declared, multiframe alignment has been gained, terminal capabilities
are exchanged, using the procedures provided in ETS 300 143 [6]. During this phase, a detection of A/μ-law shall
be performed.
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NOTE 1: For A/μ law detection see subclause 6.2.4.4.
If frame alignment cannot be gained (T1 expires), an indication shall be given to the calling user, who can then
decide to clear or not clear the resulting audio communication.
If, after connection, the restricted capability BAS code (100) [22] is received by the local terminal, and if which is
unable to support interworking with restricted networks, the call shall be released according to phase D/E and an
indication shall be given to the calling human user to avoid any new call attempt that cannot be successful.
If 2B capability is received at the calling terminal then it may initiate a request for a second B-channel. Alternatively,
this action may be suspended until the user at the calling terminal has given the go-ahead. If the user at either
terminal does not wish the call to proceed to two channels, even though his terminal has this capability, he shall set
the terminal such that only single-channel capability is declared during the initial capability exchange.
NOTE 2: A videotelephony terminal may declare an initial capability lower than its actual one, and change
later to increase picture quality.
When the terminal capabilities have been exchanged using the procedure given in ETS 300 143 [6], the most
appropriate transmission mode shall be determined. This should normally be the highest mode (see table 4), but a
lower compatible mode may be chosen instead. In the case that both terminals have announced the capability to
work on 2 B-channels, the calling terminal may initiate the request for the additional call set-up.
Table 4: Highest default mode for communication between different types of videotelephones using 1B or
2B channel
Xa Xb1 Xb2/3 Xb4 Xb5 Terminal type
a1 a1 a1 a0 a0 Xa
b3 b3 b1 b1 Xb1
b2/b3 (note 1) b1 b2 Xb2/3
b1 b1 Xb4
b2 Xb5
NOTE 1: "b2/b3" indicates that either of b2 or b3 can be transmitted; b2 gives better audio, while b3 gives
better video.
NOTE 2: If the terminals have not both adopted the same mode, an asymmetric communication will
result.
6.1.2.1 Case of 1B call
If the communication is to remain on 1 B-channel only, a mode switch for a suitable 1B mode shall be performed;
this includes changing the transmitter from A- to μ-law, if necessary. It shall be mandatory to send information about
the used law (A or μ), using the appropriate command. The video transmission can start at any time after the
completion of the initial capability exchange (the call has then reached phase C). This may be at 46,4 kbit/s (less
800 bit/s if Encryption Control Signal (ECS) is also on) with audio to CCITT Recommendation G.728 [15], or at
6,4 kbit/s if a 56 kbit/s to CCITT Recommendation G.711 [13] audio mode is in use (mode a et a in table 3). To
0 1
ensure that the picture builds up quickly when the video transmission first begins, the encoder shall transmit in
INTRA mode according to ITU-T Recommendation H.261 [8].
6.1.2.2 Case of 2B call
If an additional channel is going to be established immediately, mode switching to a mode including video may
optionally be deferred until the second channel has been established.

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During the establishment of the additional channel, the video may be switched on and, according to ETS 300 144
[7], it shall occupy all bit positions not allocated to audio or data. This video mode may be regarded as an interim
state.
If the remote terminal decoder is unable to decode the video information at the start of a video transmission, it
misses the INTRA pictures. In this case, it shall issue a VCU command after the transmission rate has changed
(phase CB) so that it can decode the video data (see ETS 300 144 [7]).
6.1.3 Phase CA - call set-up for the additional channel
When this phase applies, the additional call set-up shall be requested by the calling terminal using D-channel
signalling according to ETS 300 267-1 [17], clause 7. A frame conforming to ETS 300 144 [7] is overlaid on the
additional channel as soon as it is connected.
The receipt of CONNECT or CONNECT ACKNOWLEDGE message defines the end of Phase CA.
A terminal having answered an incoming call shall not make a request for a connection back to the calling terminal.
If the additional connection cannot be established, the communication remains in phase C in a 1 B mode or is
cleared by the calling user.
6.1.4 Phase CB - mode initialisation on the additional channel
Frame and multiframe alignment shall be sought and gained. The synchronisation of the two channels is achieved
as specified in ETS 300 144 [7].
A mode switch towards the highest 2B modes, identified during Phase B shall then be performed using the
procedure defined in ETS 300 143 [6].
If the synchronisation cannot be reached, the communication remains in Phase C in a 1B mode, the second call
shall be cleared and an indication shall be given to the local calling human user (who may then decide either to
retain the call in the 1B mode or clear down and make another attempt to obtain a 2B call).
NOTE: If the terminals have not both adopted the same mode, the result may be an asymmetric
communication.
6.2 Videotelephony communication - Phase C
In the case where more than one channel is used, there shall be intermediate phases CA and CB as described
above. Likewise, if additional channels are dropped during the call there shall be intermediate phases (see
subclause 6.4). The provisions of this subclause apply to any channel, initial or additional, for which phase B has
been completed and phase D/E has not yet started.
NOTE: Changes of communication here do not involve D-channel signalling. Changes of
communication also involving D-channel signalling are described in subclause 6.3.
6.2.1 Mode switching
According to action by either user (for example, starting a facsimile machine) a different mode from the initially
selected mode may become more appropriate. Switching to this mode is made according to the procedure of
ETS 300 143 [6].
A switch to a mode which is unframed in the I-channel shall only be done if:
- the remote terminal is put on hold (e.g. prior to call transfer, see subclause 7.5.3); or
- the call is going to be cleared (see subclause 6.4).
This is to avoid an unexpected loss of frame alignment.

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6.2.2 Capability change
The user may change the capability of his terminal during the call (for example, by connecting or switching-on
auxiliary telematic equipment); the terminal shall then perform a capability exchange according to the procedure
defined in ETS 300 143 [6].
6.2.3 Loopbacks for maintenance purposes (optional)
Some loopback functions defined in ETS 300 144 [7] allow verification of various functional aspects of the terminal,
to ensure correct operation of the system and satisfactory quality of the service to the remote party.
a) Loop at terminal-network interface (towards network)
Upon receiving the LCD command as defined in ETS 300 144 [7], loopback shall be activated at the digital
interface of the terminal toward the network side. In case of a multiple B-channel arrangement, loopback is
activated in each connection.
b) Loop at analogue I/O interface (optional)
Upon receiving the LCV or LCA command as defined in ETS 300 144 [7], loopback should, where possible,
be activated at the analogue interface of the video/audio codec towards the video/audio codec.
Figure 2: Loopback
6.2.4 Terminal arrangements
6.2.4.1 Video transmission
Pictures may or may not be visible to both users as soon as initialisation is complete:
- either user may switch to Video On;
- either user may send the C&I BAS code VIR and wait the remote party answer (cf. subclause 6.2.4.3);
- a terminal may send the C&I BAS code VIS to indicate video suppression.
In any case, the terminal screen shall display a picture/message/symbol to identify the "no fault" condition.

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When only the initial channel is present and the audio is at 56 kbit/s, "Video On" results in a video rate of 4,8 kbit/s
(4,0 kbit/s if the Encryption Control Signal (ECS) channel is also on). A terminal which is not capable of decoding
such a low rate video stream shall disable its decoder. When, subsequently, the video rate is increased sufficiently
that decoding becomes possible, then the decoder shall be re-enabled and a command VCU shall be transmitted
(see ETS 300 144 [7]).
When two B-channels are present and the audio is switched off, "Video On" results in a video rate of 124,8 kbit/s
(124,0 kbit/s if the ECS channel is also on). A terminal which is not capable of decoding such a high rate video
stream shall disable its decoder. When, subsequently, the video rate is decreased sufficiently that decoding
becomes possible, then the decoder shall be re-enabled and a command VCU shall be transmitted (see
ETS 300 144 [7]).
6.2.4.2 Picture definition
The communication modes include the possibility to use CIF format as well as QCIF format. The picture format may
differ between the two directions.
6.2.4.3 Exchange of video by mutual agreement (optional)
The symbol VIR, "Video Indicate Ready-to-activate", is defined in ETS 300 144 [7]. Its use is optional, but when
used the procedure shall be as follows.
Terminal A has been set so that video is not transmitted unless, and until, the remote terminal has also indicated
readiness to transmit video. Terminal A shall send the indication VIR when the initial capability exchange has been
completed, but shall not transmit a video signal until it has received either VIR or the video-ON command.
6.2.4.4 Audio decoding
Whenever a terminal is receiving in mode 0U, or is receiving in mode 0F but has not received a command stating A/
μ coding law, it shall monitor the received audio signal (e.g. according to the algorithm specified in the Appendix to
CCITT Recommendation G.725 [5] to identify the coding law. The audio decoder shall be switched accordingly.
6.2.4.5 Delay compensation (optional)
Video codecs, as in ITU-T Recommendation H.261 [8], require some processing delay, while the audio codecs to
CCITT Recommendations G.711 [13], G.722 [14] and G.728 [15] involve much less delay. Lip synchronisation is not
mandatory, but where it is to be maintained, the video processing delay shall be compensated in the audio path.
Since
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