ETSI TR 103 708 V1.1.1 (2022-08)

Human Factors (HF); Real-Time Text (RTT) in Multiparty Conference Calling

DTR/HF-00103708

General Information

• Status: Not Published
• Technical Committee: HF - Human Factors
• Current Stage: 12 - Completion
• Due Date: 15-Aug-2022
• Completion Date: 11-Aug-2022

Standards Content (Sample)

TECHNICAL REPORT
Human Factors (HF);
Real-Time Text (RTT) in Multiparty Conference Calling

2 ETSI TR 103 708 V1.1.1 (2022-08)

Reference
DTR/HF-00103708
Keywords
accessibility, HF, ICT
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis Cedex - FRANCE

Tel.: [phone]  Fax: [phone]

Siret N° 348 623 562 00017 - APE 7112B
Association à but non lucratif enregistrée à la
Sous-Préfecture de Grasse (06) N° w061004871

Important notice
The present document can be downloaded from:
http://www.etsi.org/standards-search
The present document may be made available in electronic versions and/or in print. The content of any electronic and/or
print versions of the present document shall not be modified without the prior written authorization of ETSI. In case of any
existing or perceived difference in contents between such versions and/or in print, the prevailing version of an ETSI
deliverable is the one made publicly available in PDF format at www.etsi.org/deliver.
Users of the present document should be aware that the document may be subject to revision or change of status.
Information on the current status of this and other ETSI documents is available at
https://portal.etsi.org/TB/ETSIDeliverableStatus.aspx
If you find errors in the present document, please send your comment to one of the following services:
https://portal.etsi.org/People/CommiteeSupportStaff.aspx
If you find a security vulnerability in the present document, please report it through our
Coordinated Vulnerability Disclosure Program:
https://www.etsi.org/standards/coordinated-vulnerability-disclosure
Notice of disclaimer & limitation of liability
The information provided in the present deliverable is directed solely to professionals who have the appropriate degree of
experience to understand and interpret its content in accordance with generally accepted engineering or
other professional standard and applicable regulations.
No recommendation as to products and services or vendors is made or should be implied.
No representation or warranty is made that this deliverable is technically accurate or sufficient or conforms to any law
rule and/or regulation and further, no representation or warranty is made of merchantability or fitness
and/or governmental
for any particular purpose or against infringement of intellectual property rights.
In no event shall ETSI be held liable for loss of profits or any other incidental or consequential damages.

Any software contained in this deliverable is provided "AS IS" with no warranties, express or implied, including but not
limited to, the warranties of merchantability, fitness for a particular purpose and non-infringement of intellectual property
rights and ETSI shall not be held liable in any event for any damages whatsoever (including, without limitation, damages
for loss of profits, business interruption, loss of information, or any other pecuniary loss) arising out of or related to the use
of or inability to use the software.
Copyright Notification
No part may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm except as authorized by written permission of ETSI.
The content of the PDF version shall not be modified without the written authorization of ETSI.
The copyright and the foregoing restriction extend to reproduction in all media.

© ETSI 2022.
All rights reserved.
ETSI
3 ETSI TR 103 708 V1.1.1 (2022-08)

Contents
Intellectual Property Rights . 6
Foreword . 6
Modal verbs terminology . 6
Executive summary . 6
Introduction . 7
1 Scope . 8
2 References . 8
2.1 Normative references . 8
2.2 Informative references . 8
3 Definition of terms, symbols and abbreviations . 10
3.1 Terms . 10
3.2 Symbols . 10
3.3 Abbreviations . 11
4 Real-Time Text (RTT) . 11
4.1 What is RTT? . 11
4.2 RTT is for conversation not messaging . 11
5 Multiparty calls . 12
6 User interface elements and use cases . 12
6.1 Background . 12
6.2 Types of media . 12
6.2.1 Real-Time Text (RTT) . 12
6.2.2 Audio . 13
6.2.3 Video . 13
6.2.4 Total Conversation . 13
6.2.5 Messaging . 13
6.2.6 Documents, drawings and stored or streamed media . 13
6.3 Use of media and user interface elements . 14
6.3.1 Potential user interface components . 14
6.3.2 User generated media presentation . 14
6.3.3 Active and waiting user activity indicatio n . 14
6.3.4 User control of presentation and input . 14
6.4 Use cases . 15
6.4.1 Use case variations. 15
6.4.2 Call using RTT within a small group of Deaf persons . 15
6.4.3 Deaf person calling emergency service and using RTT . 15
6.4.4 Hard-of-hearing user talking with hearing friends . 15
6.4.5 Deaf user participating in conference getting transcription support . 16
6.4.6 Deaf user participating in conference contributing by text-to-speech . 16
6.4.7 Deaf-Blind user participating in remote meeting . 16
6.4.8 Person in a critical situation making an emergency call by RTT . 16
6.4.9 Person in remote group meeting in occasional noise . 17
6.4.10 Relay service using multiparty technology . 17
6.4.11 Using an RTT relay service to connect to a voice conference call . 17
7 RTT text creation and presentation . 17
7.1 Text creation and transmission . 17
7.1.1 Variation in text creation means . 17
7.1.2 Verification of produced text . 18
7.1.3 Erasure . 18
7.1.4 Input language and character set . 18
7.1.5 Sending of emoji . 18
7.1.6 Graphic rendition . 18
ETSI
4 ETSI TR 103 708 V1.1.1 (2022-08)

7.1.7 Time from text entry to presentation . 19
7.1.8 Support for user to send text when the situation allows . 19
7.1.9 Reliability versus rapidity in transport of text. 19
7.2 RTT text presentation . 19
7.2.1 RTT text presentation basics . 19
7.2.2 Visual presentation variants . 20
7.2.2.1 Introduction . 20
7.2.2.2 One column view . 20
7.2.2.3 One column per contributing user . 20
7.2.2.4 Text beneath a video image of the corresponding participant . 21
7.2.3 RTT text presentation by screen reader with spoken output . 21
7.2.4 RTT presentation by screen reader software with braille output . 21
7.2.4.1 Notes on braille device user interfaces . 21
7.2.4.1.1 Braille Devices . 21
7.2.4.1.2 Braille Keyboards . 21
7.2.4.1.3 Braille Displays . 21
7.2.4.1.4 Braille Matrix Displays . 22
7.2.4.2 Anatomy of a Braille Display. 23
7.2.4.3 Example RTT Implementations with Braille Displays . 24
7.2.4.3.1 The purpose of the following examples . 24
7.2.4.3.2 Example: RTT using Reserved Status Cells . 24
7.2.4.3.3 Example: RTT without Status Cells on Portable Displays . 24
7.2.4.3.4 Example: RTT on a Braille Matrix Display. 24
8 Control of RTT in multiparty calls . 25
8.1 General aspects of control of RTT . 25
8.2 Control and indications of the visible user interface components . 25
8.2.1 Control of the screen space . 25
8.2.2 Scrolling in RTT presentation . 26
8.2.3 Searching and selecting the RTT contents . 26
8.2.4 Varying the RTT presentation . 26
8.2.5 Notifications and status information about participants and media . 26
8.2.6 Selection between meeting views . 26
8.2.7 Notifications by audio and visual means . 27
8.3 Control via assistive technologies . 27
9 Control of calls with RTT . 27
10 Transport of RTT with multiparty contents in various technical platforms . 27
10.1 General transport considerations . 27
10.2 Multiparty RTT transport in centralized SIP conferences . 28
10.3 Multiparty RTT transport in WebRTC . 28
10.4 Multiparty RTT transport in PSTN . 28
10.5 Multiparty RTT transport in PEMEA . 29
11 Interaction with services . 29
11.1 Services of specific importance for RTT users . 29
11.2 Continuous real-time conversational services . 29
11.3 Conference services . 29
11.4 Emergency services . 29
11.5 Relay services . 30
11.5.1 The purpose of Relay Services . 30
11.5.2 Functionality and connection alternatives . 30
11.5.3 Delay caused by translation or interpretation . 30
11.5.4 Queueing situations. 30
11.5.5 Language and translation . 31
12 Relations to specifications from other groups . 31
12.1 General about support from other standardisation bodies . 31
12.2 Relations to 3GPP specifications . 31
12.3 Relations to GSMA specifications . 32
12.3.1 Introduction. 32
12.3.2 Analysis of GSMA NG.114, IR.92 and IR.94 . 33
ETSI
5 ETSI TR 103 708 V1.1.1 (2022-08)

12.3.3 Other GSMA profiles for conversational voice, video and RTT . 33
12.4 Relations to emergency service specifications . 34
13 Proposed information in EN 301 549 . 34
13.1 The need to update EN 301 549 to better address RTT . 34
13.2 Changes to informative references in clause 2.2 . 34
13.3 Changes to definitions of terms in clause 3.1 . 35
13.4 Changes to existing requirements in clause 6 . 35
13.4.1 Continuous real-time conversation . 35
13.4.2 RTT communication . 35
13.4.3 Concurrent voice and text . 36
13.4.4 Distinguishable display . 36
13.4.5 Programmatically determinable send and receive origination . 36
13.4.6 User identification . 37
13.4.7 Visual indication of Audio with RTT . 37
13.4.8 Additional clauses about RTT presentation . 37
13.4.9 Interoperability . 38
13.4.10 RTT responsiveness . 39
13.4.11 Further requirements on RTT . 39
13.5 Identification of actively speaking users . 40
13.6 Identifying who wishes to communicate next . 41
13.7 Awareness versus distraction . 41
13.8 Programmatic determinability of the new notifications . 41
13.9 Changes to requirements on services in clause 13 . 41
13.9.1 Access to relay services . 41
13.9.2 Access to emergency services . 42
Annex A (informative): EN 301 549 extracts with proposed changes . 43
A.1 Introduction . 43
A.2 The EN 301 549 revisions and additions . 43
Annex B (informative): Bibliography . 51
History . 52

ETSI
6 ETSI TR 103 708 V1.1.1 (2022-08)

Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
ETSI Web server (https://ipr.etsi.org/).
Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not
referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
DECT™, PLUGTESTS™, UMTS™ and the ETSI logo are trademarks of ETSI registered for the benefit of its

Members. 3GPP™ and LTE™ are trademarks of ETSI registered for the benefit of its Members and of the 3GPP
Organizational Partners. oneM2M™ logo is a trademark of ETSI registered for the benefit of its Members and of the ®
oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association.
Foreword
This Technical Report (TR) has been produced by ETSI Technical Committee Human Factors (HF).
Modal verbs terminology
In the present document "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be
interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Executive summary
Real-Time Text (RTT) is text communication sent as it is created without any specific sending action by the user. It is
required in EN 301 549 [i.13], including its use in multiparty calls, but has lacked details on user interface requirements
and how to accomplish the multiparty function.
The present document explains the use and characteristics of RTT including its multiparty aspects. It also provides an
analysis of documents from other standardisation bodies, and briefly proposes modifications to them to make lower
layers of RTT implementation support multiparty calling and to ensure that emergency services are able to support RTT
with multiparty functionality. Finally, detailed change proposals for EN 301 549 [i.13] version 3.2.1 are included in the
final clause of the present document and in Annex A.
ETSI
7 ETSI TR 103 708 V1.1.1 (2022-08)

Introduction
The present document was developed to support future upgrading of those parts of the EN 301 549 "Accessibility
requirements for ICT products and services" [i.13] standard that relate to the accessibility of Real-Time Text (RTT)
when used in multiparty/conference applications. Multiparty usage is of particular importance in ensuring that
communication with Emergency Services will meet the accessibility needs of persons for whom voice communication
is undesirable or impossible. The inclusion of multiparty requirements will also ensure that EN 301 549 [i.13] remains
relevant to the design of interoperable RTT and Total Conversation services.

ETSI
8 ETSI TR 103 708 V1.1.1 (2022-08)

1 Scope
The present document establishes user interface guidelines for RTT conference call interfaces and identifies technical
support needed to implement the guidelines. The present document:
• introduces RTT;
• addresses the types of media and user interface elements that can be associated with RTT;
• describes some RTT use cases, highlights issues particularly relevant in multiparty scenarios;
• explains technical support issues related to text creation and presentation, and the standards and related
services associated with RTT transport and control.
The final and crucial part of the present document proposes some potential changes and additions to EN 301 549 [i.13]
to ensure that it covers the additional accessibility issues that arise when considering multiparty scenarios.
2 References
2.1 Normative references
Normative references are not applicable in the present document.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] ETSI ES 202 975: "Human Factors (HF); Requirements for relay services".
[i.2] ETSI TS 101 470: "Emergency Communications (EMTEL); Total Conversation access to
Emergency Services".
[i.3] ETSI TR 103 201: "Emergency Communications (EMTEL); Total Conversation for emergency
communications; implementation guidelines".
[i.4] ETSI TS 103 478: "Emergency Communications (EMTEL); Pan-European Mobile Emergency
Application".
[i.5] ETSI TS 103 479: "Emergency Communications (EMTEL); Core elements for network
independent access to emergency services".
[i.6] ETSI TS 123 167:"Universal Mobile Telecommunications System (UMTS); LTE; IP Multimedia
Subsystem (IMS) emergency sessions (3GPP TS 23.167)".
[i.7] ETSI TS 123 226: "Digital cellular telecommunications system (Phase 2+) (GSM); Universal
Mobile Telecommunications System (UMTS); LTE; Global text telephony (GTT); Stage 2 (3GPP
TS 23.226)".
[i.8] ETSI TS 124 147: "Digital cellular telecommunications system (Phase 2+) (GSM); Universal
Mobile Telecommunications System (UMTS); LTE; Conferencing using the IP Multimedia (IM)
Core Network (CN) subsystem; Stage 3 (3GPP TS 24.147)".
ETSI
9 ETSI TR 103 708 V1.1.1 (2022-08)

[i.9] ETSI TS 124 229: "Digital cellular telecommunications system (Phase 2+) (GSM); Universal
Mobile Telecommunications System (UMTS); LTE; 5G; IP multimedia call control protocol based
on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 (3GPP
TS 24.229)".
[i.10] ETSI TS 124 371: "Universal Mobile Telecommunications System (UMTS); LTE; Web
Real-Time Communications (WebRTC) access to the IP Multimedia (IM) Core Network (CN)
subsystem (IMS); Stage 3; Protocol specification (3GPP TS 24.371)".
[i.11] ETSI TS 126 114: "Universal Mobile Telecommunications System (UMTS); LTE; 5G; IP
Multimedia Subsystem (IMS); Multimedia telephony; Media handling and interaction (3GPP
TS 26.114)".
[i.12] ETSI TS 126 236: "Universal Mobile Telecommunications System (UMTS); LTE; Packet
switched conversational multimedia applications; Transport protocols (3GPP TS 26.236)".
[i.13] EN 301 549 (V3.2.1): "Accessibility requirements for ICT products and services" (jointly
produced by ETSI/CEN/CENELEC).
[i.14] GSMA PRD IR.92: "IMS Profile for Voice and SMS".
[i.15] GSMA PRD IR.94: "IMS Profile for Conversational Video Service".
[i.16] GSMA PRD IR.51: "IMS Profile for Voice, Video and SMS over untrusted Wi-Fi access".
[i.17] GSMA NG.106: "IMS profile for Video, Voice and SMS over trusted Wi-Fi access".
[i.18] GSMA NG.114: "IMS Profile for Voice, Video and Messaging over 5GS".
[i.19] GSMA NG.115: "IMS Profile for Voice, Video and Messaging over Untrusted WLAN Connected
to 5GC".
[i.20] IETF BCP 47: "Tags for identifying languages", M. Davis, A. Phillips, September 2009.
[i.21] IETF RFC 3261: "SIP: Session Initiation Protocol", J. Rosenberg et.al., 2005.
[i.22] IETF RFC 3550: "RTP: A Transport Protocol for Real-Time Applications", H. Schulzrinne et.al.,
2003.
[i.23] IETF RFC 4103: "RTP Payload for Text Conversation", G. Hellstrom, P. Jones, 2005.
[i.24] IETF RFC 5194: "Framework for Real-Time Text over IP Using the Session Initiation Protocol
(SIP)", A. Van der Wijk, G. Gybels, 2008.
[i.25] IETF RFC 6497: "BCP 47 Extension T - Transformed Content", M. Davis, A. Phillips, February
2012.
[i.26] IETF RFC 8373: "Negotiating Human Language in Real-Time Communications", Gellens R.,
2018.
[i.27] IETF RFC 8825: "Overview: Real-Time Protocols for Browser-Based Applications", 2021.
[i.28] IETF RFC 8865: "T.140 Real-Time Text Conversation over WebRTC Data Channels", Holmberg
C. and G. Hellström, 2021.
[i.29] IETF RFC 8866: "SDP Session Description Protocol", A. Began et al., 2021.
[i.30] IETF RFC 9071: "RTP-Mixer Formatting of Multiparty Real-Time Text", DOI
10.17487/RFC9071, Hellström, G., (July 2021).
[i.31] IETF RFC 9248: "Interoperability Profile for Relay User Equipment", DOI 10.17487/RFC9248,
Rosen, B., June 2022.
[i.32] ISO/IEC 10646:2020: "Information technology - Universal coded character set (UCS)".
ETSI
10 ETSI TR 103 708 V1.1.1 (2022-08)

[i.33] Recommendation ITU-T F.700 (2000):"Framework Recommendation for multimedia services".
NOTE: Available at https://www.itu.int/rec/T-REC-F.700-200011-I/en.
[i.34] Recommendation ITU-T T.140 (1988): "Protocol for multimedia application text conversation".
[i.35] Recommendation ITU-T V.18 (2000): "Operational and interworking requirements for DCEs
operating in the text telephone mode".
[i.36] ETSI TS 103 871: "Emergency Communications (EMTEL); PEMEA Real-Time Text (RTT)
Extension".
[i.37] W3C Recommendation 05 June 2018: "Web Content Accessibility Guidelines (WCAG) 2.1".
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
communications assistant (CA): person providing alternative communication for a two-party conversation, multiparty
meeting, or broadcast
NOTE: Communication assistants include captioners/transcribers, sign language interpreters, spoken language
interpreters, relay service operators, call handlers, telephone operators, etc.
conference floor control: functionality for conference administrators and attendees to manage a formal or informal
communication queue or other communication resources
NOTE: Examples of conference floor control include a "hand-raising" feature and a "request to share screen"
feature.
continuous real-time conversation: type of organization in conversation and discourse where one participant's
contribution is made available to other participants while it is being made
programmatically determinable: able to be read by software from developer-supplied data in a way that other
software, including assistive technologies, can extract and present this information to users in different modalities
NOTE: WCAG 2.1 [i.37] uses "determined" where this definition uses "able to be read" (to avoid ambiguity with
the word "determined").
Real-Time Text (RTT): form of a text conversation in point to point situations or in multipoint conferencing where the
text being entered is sent in such a way that the communication is perceived by the user as being continuous
relay service: electronic communications service that enables users of different modes of communication (e.g. text,
sign or speech) to interact by providing conversion between different modes of communication, usually through a
communications assistant
Total Conversation service: audiovisual conversation service providing bidirectional symmetric real-time transfer of
motion video, text and voice between users in two or more locations (from ETSI TS 101 470 [i.2])
WebSocket: computer communications protocol, providing interaction between a web browser (or other client
application) and a web server facilitating real-time data transfer from and to the server
3.2 Symbols
Void.
ETSI
11 ETSI TR 103 708 V1.1.1 (2022-08)

3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
3GPP 3G (mobile) Partnership Project
th
4G 4 Generation (mobile networks)
th
5G 5 Generation Mobile Networks
API Application Programming Interface
BCP Best Current Practice
CN Core Network
EMTEL Emergency Telecommunications
GSMA GSM Association
HF Human Factors
ICT Information and Communication Technology
IETF Internet Engineering Task Force
IM Instant Messaging
IMS IP Multimedia Subsystem
IP Internet Protocol
IR International Roaming Expert Group
ITU-T International Telecommunication Union - Telecommunication standardization sector
LTE 3GPP Long Term Evolution (4G)
NG Next Generation
PEMEA Pan-European Mobile Emergency Application
PSTN Public Switched Telephone Network
QoS Quality of Service
RFC Request For Comment
RTP Realtime Transport Protocol
RTT Real-Time Text
SIP Session Initiation Protocol
SMS Short Message Service
TC Technical Committee
UE User Equipment
UI User Interface
VoIP Voice Over IP
WCAG Web Content Accessibility Guidelines
WebRTC Web Real Time Communication
WLAN Wireless Local Access Network
4 Real-Time Text (RTT)
4.1 What is RTT?
RTT, or Real-Time Text, sends text characters input by a user shortly after they are entered. This contrasts with most
text chat services where the user has toperform a confirmation step before the composed message is sent, usually by
tapping a "Send" button. At the receiving end of an RTT conversation, the recipient sees the words from other
participants as the characters are entered. The immediacy of the sending and receiving allows a dialogue that has a flow
closely approximating that of a spoken dialogue.
Persons who use RTT use it for conversations in the same way as persons who can talk and hear use voice
communications (or Voice and RTT together).
4.2 RTT is for conversation not messaging
Users may choose to communicate using a stand-alone messaging service or a chat feature in a conferencing system.
They are making an active choice to use this less immediate, asynchronous form of communication.
ETSI
12 ETSI TR 103 708 V1.1.1 (2022-08)

The provision of a messaging/chat capability cannot be seen as providing an effective means for persons who cannot
effectively make spoken contributions to effectively participate in a real-time person-to-person conversation or a
conference call.
5 Multiparty calls
On a multiparty call (where most participants are using speech), including conference calls/meetings, the participants
should not ideally talk at the same time, but in practice it occurs. If two or more participants speak at the same time it
can rapidly lead to the multiparty call becoming ineffective for most of the participants. Multiple participants
simultaneously using RTT could also have a similar effect, but the fact that text stays on the screen and the fact that text
from different sources is presented with separation makes it feasible that some simultaneous text communication from
different sources can be manageable. To what degree this is possible depends on the situation and how the conference is
managed.
An important factor that leads to simultaneous contributions is a lack of awareness of when other participants have
stopped contributing and when it is a suitable time for them to contribute. With voice communication, it is possible for
most participants to hear when other participants are contributing, but they may not be able to identify who is speaking.
When several RTT users are in a multiparty call they need to be made aware when other RTT users are contributing.
They also want to identify the other RTT users when they contribute. These are issues that are addressed in the present
document. When contributions are being made both by voice and by RTT, the awareness of when someone is
contributing becomes a much more complicated issue as contributions may be being made using a means that the user
cannot, or is not able to, continuously monitor. This is even more true when video is used for information, e.g. by using
sign language. User interface techniques that support users in having maximum awareness of when another person is
actively contributing is a significant part of the scope of the present document.
6 User interface elements and use cases
6.1 Background
Availability of RTT is essential or desirable in several different situations in calls with two or more participants. This
clause lists several valid use cases, with different combinations of media, different use of RTT, different characteristics
of the devices for generation and presentation of RTT, and different means for management of the expressions in the
different media in the call. The purpose is to be a base for checks that the specified characteristics of the user interfaces
fulfil the user needs in these use cases.
6.2 Types of media
6.2.1 Real-Time Text (RTT)
The important characteristic of RTT is that text is transmitted at the same rate as it is produced, so that the receiver can
follow the senders' thoughts as soon as they are turned into words. There will be no excess waiting time for completely
expressed messages, similar to spoken communication in audio and signed communication in video. All three real-time
continuous exchanges enable rapid interaction in conversation. A commonly used coding and presentation standard for
RTT clarifying the characteristics of RTT is Recommendation ITU-T T.140 [i.34].
RTT may be used so that all call participants are enabled to create and send RTT to the other participants, and the text
being presented in readable chunks growing in real time with indication of source. The presentation should provide an
approximate view of the relative timing of text from different parties.
RTT may also be used by one or more participants in a call, where other participants communicate by speech or
signing. In such situations, a human-transcribed or automatic translation may be included between the users of the
different media in both directions.
ETSI
13 ETSI TR 103 708 V1.1.1 (2022-08)

A third way to include RTT in a call is to include translation from other media only for creation of RTT, while all
participants have means for presentation of multiparty RTT, and those who need or prefer to express themselves in RTT
are able to send RTT.
6.2.2 Audio
Audio is often used in calls with two or more participants to convey speech between the parties. The most common
configuration is that audio presented to each participant is mixed from all other participants, except from those whose
audio is muted. Transmission from each party can usually
...