Simplified 6.2.3 RTT Interoperability and rename to 6.2.3 RTT implementations
With the ambition to simplify the clause 6.2.3 about RTT interoperability, the rules established by FCC in CFR 47 Part 67 has been studied and found to be inspiring. The important accessibility requirement is that RTT works to the same extent that voice works. The additions required for RTT to work between services who have interoperability for voice is just a common protocol for initiation and communication of RTT. The service providers have already decided on common call control protocol, addressing, routing, network interfaces and security.
For freedom of choice of technology, the services may agree on a common RTT protocol for the interoperability of RTT between the services. But if they find it complicated or unnecessary to agree on a specific RTT protocol, they shall be obliged to use RFC 4103 updated by RFC9071.
For the conformance testing they shall declare what protocol they have agreed or decided to use. For the purpose of interoperability between services, the end-user devices within each service may use any RTT protocol, but if they use something else than what is agreed between the services, the ICT must provide conversion between what the terminal uses and what is used between the services. So, for the purpose of interoperability between services, the RTT protocol for terminals does not need to be required.
All the other RTT clauses than 6.2.3 are intended to cater for good functionality of RTT between end-user devices within a service as well as between services. Thus 6.2.3 just needs to specify a safe harbour solution for protocol to use for RTT to function at all.
There may be specific details for RFC 4103 and RFC 9071 applications which are needed for specific services, such as bearers for IMS use or security measures. They need to be agreed separately. That should be mentioned in a note but is not considered to be mentioned in a normative clause.
The full effect of this proposal is not yet totally studied. If it is found to threaten the accessibility when compared to 6.2.3 in EN 301 549 v 3.2.1, then we are obliged to improve it or return to that version. If this proposal or an improvement of it is accepted, the informative NOTE about standards is proposed to be moved to 6.2.3 as shown below.
The title is proposed to be changed to RTT implementations because the word "interoperability" has proven to lead to associations with complex technical interoperability tests deep into networks which this is not at all about. It is just measures for easying fulfillment of the accessibility requirement that RTT shall be available as a replacement for voice in all communications where voice is available.
6.2.3 RTT implementations
Where ICT is a service providing two-way voice communication with another service, the ICT shall provide a means for two-way RTT communications between the end-user devices in the services, by declaring and using a protocol for RTT between the services, being either
RFC 4103 updated by RFC 9071, or
a protocol for RTT communications mutually agreed by the service providers.
NOTE 1: Other details than the RTT protocol to use may need to be mutually agreed between the services for the RTT communication.
NOTE 2: See clause 13 for communication with emergency services.
NOTE 3: See clause 13 for communication with relay services.
NOTE 4: The following examples provide information on how RTT is specified in various technologies. The standards and specifications commonly provide both accessibility requirements and technical solutions. The information in this note reflects the situation at the time of authoring the present document and can be used for planning end-to-end functionality both within and between services and devices.
Presentation
ITU-T has published a standard for coding and presentation of RTT, in Recommendation ITU-T T.140 [i.36] .It is used by all technologies below.
General VoIP and Multimedia calling.
IETF, providing standards for Voice Over IP (VOIP) and Multimedia calls over IP based on the Session Initiation Protocol (SIP), has published IETF RFC 4103 [i.13] for session establishment and transport of RTT and its update RFC 9071 for multiparty use [i.rfc9071] .
Mobile Multimedia Telephony
Mobile standards: 3GPP, providing standards for mobile communication systems, including IP Multimedia Sub-System (IMS) has standardised the Multimedia Telephony concept, for calls with voice, video and RTT using the set of protocols specified in 3GPP TS 26.114 (= ETSI TS 126 114) [i.10] to create the services including RTT and Total Conversation services specified in TS 22.173 (=ETSI TS 122 173) [i.11]. Multimedia Telephony is specified to use IETF RFC 4103 [i.13] for session establishment and transport of RTT.
Mobile product profiles: GSMA, providing selected profiles of standards for implementing globally interoperating mobile communications services has in NG.114 [i.ng114] specified how to apply TS 126 114 to implement Voice over LTE (VoLTE) and Video over LTE (ViLTE). RTT is specified in NG.114 Annex B.2 to use IETF RFC 4103 [i.13] for session establishment and transport of RTT..
Web technologies
For communication in Web Technologies, W3C and IETF have created the WebRTC concept [i.rfc8825]. IETF has published RFC 8865 [i.rfc8865] for implementation of RTT in web technologies using the WebRTC Data channel concept.
Circuit switched technologies
For communication in circuit switched PSTN technologies, a number of modem based technologies for RTT-like communication have been in use, collectively specified including procedures for communication between them in Recommendation ITU-T V.18 [i.23]. The functionality of communication between PSTN text telephony according to V.18 and RTT cannot fulfil all RTT requirements in the present document. They all have limitations in for example speed, simultaneity, character set and multiparty presentation. Recommendation ITU-T V.18 [i.23] or an applicable annex thereof can be used for provision of functionally limited communication with RTT.
Emergency communications
ETSI has published standards for IP based emergency communications in TS 103 479 [i.103479], specifying use of RFC 4103 [i.13] for RTT. This standard has a lot in common with corresponding standards for North America specified by the North American Emergency Number Association (NENA).
Emergency apps
ETSI has published standards for emergency apps, in TS 103 478 [i.ts103478], where RTT use is specified in TS 103 871 [i.ts103871], using Recommendation ITU-T T.140 [i.t140] for coding and presentation as all other RTT standards referenced in this clause.
Relay services
ETSI has published a standard for relay services in ES 202 975 [i.es202975], specifying the need for interoperability in call control and media between relay services and interpersonal communications services and devices.
C.6.2.3 RTT Implementations
Type of assessment
Inspection
Pre-conditions
The ICT is a service providing two-way voice communications.
The ICT provides two-way voice communications with another service.
Procedure
Check 1: Verify if the ICT has declared a mutually agreed RTT protocol to be used with the other service.
Check 2: Verify if the ICT has declared that it provides RFC 4103 updated by RFC 9071 for use for RTT exchange with other services.
Result
Not applicable: Pre-conditions 1 or 2 is not met.
Pass: Check 1 or Check 2 is true
Fail: Check 1 and Check 2 are false