SIST ETS 300 145 E1:2003

Integrated Services Digital Network (ISDN); Audiovisual services Videotelephone systems and terminal equipment operating on one or two 64 kbit/s channels

To specify system aspects of a narrowband visual telephone system.

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

General Information

• Status: Published
• Publication Date: 30-Nov-2003
• ICS: 33.080 - Integrated Services Digital Network (ISDN)
• 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 ETS 300 145 ed.1 (1994-05) - Integrated Services Digital Network (ISDN); Audiovisual services Videotelephone systems and terminal equipment operating on one or two 64 kbit/s channels

Standards Content (Sample)

SLOVENSKI STANDARD
SIST ETS 300 145 E1:2003
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 1
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
SIST ETS 300 145 E1:2003 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ETS 300 145 E1:2003

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SIST ETS 300 145 E1:2003
EUROPEAN ETS 300 145
TELECOMMUNICATION May 1994
STANDARD
Source: ETSI TC-TE Reference: DE/TE-04118
ICS: 33.080
ISDN, audiovisual services
Key words:
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
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 - [e-mail]
X.400: Internet:
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 1994. All rights reserved.
New presentation - see History box

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ETS 300 145: May 1994
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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Contents
Foreword .5
1 Scope .7
2 Normative references.7
3 Definitions.8
4 Symbols and abbreviations .9
5 System description.9
5.1 Block diagram and identification of elements (informative) .9
5.2 Videotelephone terminal types.10
5.3 Signal structure and procedures.11
5.3.1 Frame structure.11
5.3.2 Control and Indication (C&I) .11
5.3.3 Inband signalling procedures .13
5.3.4 Communication modes of videotelephones .13
6 Terminal procedures .14
6.1 Call establishment and mode initialisation.14
6.1.1 Phase A - Call set-up for the initial channel .14
6.1.2 Phase B - mode initialisation .15
6.1.3 Phase CA - call set-up for the additional channel .16
6.1.4 Phase CB - mode initialisation on the additional channel .16
6.2 Phase C - videotelephone communication .16
6.2.1 Mode switching.16
6.2.2 Capability change.17
6.2.3 Loopbacks for maintenance purposes (optional) .17
6.2.4 Terminal arrangements.18
6.2.4.1 Video.18
6.2.4.2 Exchange of video by mutual agreement (optional) .18
6.2.4.3 Delay compensation (optional) .18
6.3 Changes of communication involving D-channel and in-band signalling.18
6.3.1 Supplementary services (optional) .18
6.3.1.1 HOLD.19
6.3.1.1.1 Invocation procedure .19
6.3.1.1.2 Retrieval after invocation of HOLD .19
6.3.1.2 Terminal Portability.19
6.3.1.2.1 Invocation procedure .19
6.3.1.2.2 Resumption after invocation of
Terminal Portability .19
6.3.1.3 Explicit Call Transfer, invocation procedure .20
6.3.2 Change of connection arrangements .20
6.3.2.1 Expanding a 1B call to a 2B call .20
6.3.2.2 Reducing a 2B call to a 1B call.21
6.4 Phase D/E - call clearing.21
6.4.1 Clearing terminal .21
6.4.2 Cleared terminal.22
7 Optional enhancements .22
7.1 Data ports .22
7.2 Encryption .22
8 Fault indication .22

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Annex A (informative): Bibliography . 23
History. 24

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Foreword
This European Telecommunication Standard (ETS) has been produced by the Terminal Equipment (TE)
Technical Committee of the European Telecommunications Standards Institute (ETSI).
The attention of the user of this ETS is drawn to the possibility that compliance may require the use of
technology covered by patent or similar rights.
ETSI is not responsible for identifying patent or similar rights or for granting licenses which may be
required and, as a result, ETSI is not in a position to give authoritative or comprehensive information
concerning the existence, validity or scope of patent or similar rights in connection with this ETS.

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1 Scope
This ETS 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 to ETS 300 264 [9].
Conformance to this ETS allows interworking with terminals supporting telephony 3,1 kHz teleservice to
ETS 300 111 [3], telephony 7 kHz teleservice to ETS 300 263 [8], terminals to CCITT Recommendation
G.725 [13], and Public Switched Telephone Network (PSTN) telephony. Interworking with other audio-
visual terminals can take place if their operation on one or two B-channels is according to this ETS.
This ETS is applicable to terminal equipment and other equipment supporting the videotelephony service,
such as multipoint conference units.
Interworking with terminals not connected to an ISDN may be possible if one or two 64 kbit/s (unrestricted
or restricted) digital channels can be established between the two terminals.
The inband 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
[5].
NOTE: Interworking with terminals to CCITT Recommendation G.725 [13] may be restricted to
mode 0.
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 300 012 (1991): "Integrated Services Digital Network (ISDN); Basic user-
network interface, Layer 1 specification and test principles".
[2] ETS 300 102-1: "Integrated Services Digital Network (ISDN); User-network
interface, layer 3, Specifications for basic call control".
[3] ETS 300 111 (1992): "Integrated Services Digital Network (ISDN); Telephony
3,1 kHz teleservice, Service description".
[4] 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".
[5] ITU-T Recommendation H.261 (1993): "Video codec for audiovisual services at
p x 64 Kbit/s".
[6] 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".
[7] ETS 300 144: "Integrated Services Digital Network (ISDN): Audiovisual services;
Frame structure for a 64 to 1 920 kbit/s channel and associated syntax for
inband signalling".
[8] ETS 300 263: "Integrated Services Digital Network (ISDN): Telephony 7 kHz
teleservice, Service description".
[9] ETS 300 264: "Integrated Services Digital Network (ISDN): Videotelephony
teleservice; Service description".

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[10] ETS 300 267: "Integrated Services Digital Network (ISDN); Telephony 7 kHz
and videotelephony teleservices, Digital Subscriber Signalling System No. one
(DSS1)".
[11] CCITT Recommendation G.711 (1988): "Pulse code modulation (PCM) of voice
frequencies".
[12] CCITT Recommendation G.722 (1988): "7 kHz audio-coding within 64 kbit/s".
[13] CCITT Recommendation G.725 (1988): "System aspects for the use of the 7
kHz audio codec within 64 kbit/s".
[14] CCITT Recommendation G.728 (1992): "Coding of speech at 16 kbit/s using
low-delay code-excited linear prediction".
[15] CCITT Recommendation H.100 (1984): "Visual telephone systems".
[16] CCITT Recommendation H.233 (1992): "Confidentiality system for audiovisual
services".
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 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.

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4 Symbols and abbreviations
For the purposes of this ETS, the following symbols and abbreviations apply.
BAS Bit-rate Allocation Signal
C&I Control and Indication
CCITT The International Telegraph and Telephone Consultative Committee
CONNECT D-channel message (as in ETS 300 102-1 [2])
ECS Encryption Control Signal
ETS European Telecommunication Standard
ETSI European Telecommunications Standards Institute
FAS Frame Alignment Signal
H-MLP High Speed Multi Layer Protocol
HOLD Supplementary service
HSD High Speed Data
ISDN Integrated Services Digital Network
ITU-TS International Telecommunications - Telecommunications Standardization Sector
LSD Low Speed Data
MCU Multipoint Control Unit
MLP Multi Layer Protocol
MSB Most Significant Bit
PSTN Public Switched Telephone Network
SC Service Channel
TEA Terminal Equipment Alarm
TERM1, TERM2,
TERM3 Terminals
5 System description
For a videotelephone as described in this ETS, at most, 2 B-channels are available at a time, because it
has to be connected to an ISDN basic access conforming to ETS 300 012 [1], ETS 300 102-1 [2] and ETS
300 125 [4].
5.1 Block diagram and identification of elements (informative)
A generic videotelephony system is shown in figure 1. It consists of the videotelephone terminal, network,
Multipoint Control Unit (MCU) and other system operation entities.
A configuration of a complete videotelephone consisting of several functional units is also shown in figure
1.
- Video I/O equipment includes camera(s) and 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.
- Telematic equipment includes visual aids such as an electronic blackboard and a still picture
transceiver to enhance basic videotelephone communication as defined in CCITT T.120 series of
Recommendations: these are optional.
- The system control unit carries out such functions as network access through end-to-network
signalling, end-to-end C&I to establish a suitable mode of operation and signalling for proper
operation of the terminal through end-to-end and end-to-network signalling.
- The video codec carries out redundancy reduction coding and decoding for video signals: the
algorithm is defined in ITU-T Recommendation H.261 [5].

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Video I/O
Video Codec
Equipment
Audio I/O
Audio Codec Delay
E quipm ent
Telematic Equipment
End-to-end Signalling
System MCU
Control
User-to-network Signalling
Figure 1: Visual telephone system
- The audio codec carries out redundancy reduction coding and decoding for audio signals: the
algorithms are defined in CCITT Recommendations G.711 [11], G.722 [12] and G.728 [14]. The
audio characteristics are defined in I-ETS 300 302 Parts 1 to 6.
NOTE 1: Parts 2 to 6 of this prI-ETS are still under study.
- Delay may be introduced into the audio path equal to the video codec delay, to maintain lip
synchronisation.
NOTE 2: The effect of lip synchronisation on the overall quality of a videotelephony
communication is under study.
- The Mux/dmux unit multiplexes transmitting 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 [1], ETS 300 102-
1 [2] and ETS 300 125 [4].
- 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 [2], ETS 300 125 [4], ETS 300 267 [10],
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).
For 2B categories, the terminal shall be able to operate in 1B and 2B modes. The type of remote terminal
is identified through the capability exchange defined in ETS 300 143 [6].
MUX / DM UX
Network Interface
Network

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Table 1: Videotelephone terminal type
1B 2B
Type: Xa Xb1 Xb2/3 Xb4 Xb5
Mode transfer audio G.728 G.728 G.728
rate coding G.722 G.722
G.711 G.711 G.711 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. They are mandatory where so indicated.
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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Table 2: C&I signals for videotelephone
C&I signal Source Sink Sync. Transmission Code word Mandat-
with channel definition ory (see
picture NOTES)
C/I RX TX
(1) (1)
Picture format I decoder coder no BAS ETS 300 144 M
Picture format C coder decoder yes embedded in ITU-T MM
video Recom-
mendation
H.261
Video minimum decodable I decoder coder no BAS ETS 300 144 M
picture interval
freeze picture request C coder or decoder no BAS ETS 300 144 M
control, VCF MCU
fast update request C decoder coder no BAS ETS 300 144 M M
control, VCU or MCU (3)
freeze picture release C coder decoder yes embedded in ITU-T M
control video Recom-
mendation
H.261
MCU related C terminal terminal no MLP CCITT T.120
message or MCU or MCU series
MCU multipoint command C MCU, terminal no BAS ETS 300 144 M
conference/cancel, etc.
MCC/cancel-MCC
multipoint command C MCU terminal no BAS ETS 300 144 M
symmetrical data (4)
transmission, MCS,
and MCN
MIZ and MIS C MCU terminal no BAS ETS 300 144 M
audio loop request C terminal terminal no BAS ETS 300 144
control, LCA (1)
Mainte- video loop request C terminal terminal no BAS ETS 300 144
nance control, LCV (1)
digital loop request C terminal terminal no BAS ETS 300 144 M
control, LCD (1) (2)
loop off request, LCO C terminal terminal no BAS ETS 300 144 M
(1) (2)
Confe- split-screen indication I sending receiving yes embedded in ITU-T M
rence terminal terminal video Recom- (5)
mendation
H.261

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Table 2: C&I signals for videotelephone (concluded)
C&I signal Source Sink Sync. Transmission Code word Mandat-
with channel definition ory (see
picture NOTES)
C/I RX TX
(1) (1)
document camera I sending receiving yes embedded in ITU-T M
indication terminal terminal video Recom- (6)
mendation
H.261
Termi- audio active/muted I sending receiving no BAS ETS 300 144 M M
nal indication, AIA/AIM terminal terminal (7) (7)
video active I sending receiving no BAS ETS 300 144 M M
/suppressed terminal terminal (7) (7)
indication VIA, VIA2,
VIA3, VIS
video ready to I sending receiving no BAS ETS 300 144 M M
activate indication, terminal terminal (8) (8)
VIR
NOTE 1: RX - mandatory (or conditionally mandatory - see following NOTES) to recognise and act upon:
TX - 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 [15].
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.
5.3.3 Inband signalling procedures
The terminal shall conform to ETS 300 143 [6] for point-to-point communications.
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 to this ETS shall be capable of operation in mode A
O 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 [11], A or μ-law.
0
Mode 0F: 56 kbit/s audio to CCITT Recommendation G.711 [11], A or μ-law:
6,4 kbit/s video to ITU-T Recommendation H.261 [5] may, or may not, be
activated.
Mode a 16 kbit/s audio to CCITT Recommendation G.728 [14] and 46,4 kbit/s video to
1:
ITU-T Recommendation H.261 [5].
Mode b : 56 kbit/s audio to CCITT Recommendation G.711 [11] A or μ- law and 68,8
1
kbit/s video to ITU-T Recommendation H.261 [5].
Mode b : 48 or 56 kbit/s audio to CCITT Recommendation G.722 [12] and 76,8 kbit/s or
2
68,8 kbit/s video to ITU-T Recommendation H.261 [5].

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Mode b : 16 kbit/s audio to CCITT Recommendation G.728 [14] and 108,8 kbit/s video to
3
ITU-T Recommendation H.261 [5].
Table 3: Communication modes of videotelephone
Videotelephone Channel rate ISDN channel Audio coding Video to ITU-T
mode Recommendation
H.261
a0 64 kbit/s B G.711 optional, on/off
a1 G.728
b1 G.711 ON
b2 128 kbit/s 2B (NOTE) G.722
b3 G.728
NOTE: In the case of two B-channels, the channels shall be synchronised according to
ETS 300 143 [6].
6 Terminal procedures
The provision of the communication is made in 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, inband 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);
- phase D/E: call clearing, inband and D-channel signalling (subclause 6.4).
6.1 Call establishment and mode initialisation
To establish intercommunication between various audiovisual terminals, in-band procedures according to
ETS 300 143 [6] and D-channel procedures according to ETS 300 267 [10], Clause 7 shall be carried out.
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
[10], Clause 7.
The called terminal shall send an ALERTING message according to ETS 300 267 [10], 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 the
initialize of timer T1=0 in the phase B procedure which follows.

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If the called terminal is not a videotelephone, then the D-channel procedures described in
ETS 300 267 [10], Clause 7 shall apply for the cases given below:
- fallback not allowed;
- fallback allowed to a 3,1 kHz telephone;
- fallback allowed to a 7 kHz telephone;
- fallback allowed to a PSTN telephone.
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, in the 2B case, multiframe alignment has been gained, terminal capabilities are
exchanged. During this phase, a detection of A/μ-law shall be performed. Multiframe alignment shall be
performed, if a 2B transfer rate capability has been declared.
If frame alignment cannot be gained, an indication shall be given to the user, to the effect that the call
results in an audio communication.
If the restricted capability BAS code has been received at the local terminal, and if the local terminal is
unable to work in a restricted mode, the call shall be released according to phase D/E and an indication
shall be given to the human user to inhibit the re-establishing of a call 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
capability exchange.
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: Appropriate mode (default) 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) 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.
To ensure that the picture builds up quickly when the video transmission first begins, the encoder shall
transmit in INTRA mode to ITU-T Recommendation H.261 [5].
The video transmission can start at any time after the completion of the initial capability exchange. During
the establishment of any additional channels, the video data may be introduced, using a data rate up to
the maximum common transfer rate indicated in the capability sets.
If the remote terminal's decoder is unable to decode the vide
...