(RTT) Interoperability (from clause 13.4.9 of TR 103 708)
The interoperability clause 6.2.3 specifies how interoperability can be achieved between ICT with RTT functionality. It lists the main session and transport level standards for RTT in the most common call control protocol environments. After publication of version 3.2.1 of EN 301 549 [i.13], there has been progress in standardisation of RTT including multiparty aspects, for the two most common communication environments: SIP and WebRTC. This makes it possible to add details to the list of interoperability methods in clause 6.2.3.
Item a) in the list in clause 6.2.3 covers PSTN text telephony, but no implementation of this provides full RTT functionality. The different kinds of implemented text telephones collected under item a) show different limitations compared to RTT and no one supports multiparty calling natively. Only a few countries still support PSTN text telephones, and more countries are expected to drop legacy support requirements. Because PSTN text telephony is a legacy system that is not RTT, it does not belong in a clause that is describing RTT interoperability mechanisms.
It is proposed to:
- introduce a new heading "6.2.3 Interoperability and interworking";
- retitle the original clause 6.2.3 to become "6.2.3.1 RTT interoperability";
- modify the introduction to read:
Where ICT has RTT functionality (as required by clause 6.2.1.1) it shall support the applicable RTT interoperability mechanisms described below:";
- item a) in the original list to be deleted;
- the original item b) to be split in two cases a) and b), and a new item c) inserted before the original item c) and d), moved to be items d) and e) followed by notes and example;
- "a or b" changed to "a or b or c" in the original option c) (now option d));
- "option c" in the Example will be changed to "option d";
- The new items a), b), and c) are proposed to read:
a) ICT interoperating with other ICT using VOIP with Session Initiation Protocol (SIP) and using RTT that conforms to IETF RFC 4103 [i.13] including the updates for multiparty use in IETF RFC 9071 [i.rfc9071];
b) ICT interoperating with other ICT using the IP Multimedia Sub-System (IMS) to implement VOIP with RTT using the set of protocols specified in ETSI TS 126 114 [i.10], ETSI TS 122 173 [i.11] and ETSI TS 124 229 [i.12] describing how IETF RFC 4103 [i.13] would apply updated by IETF RFC 9071 [i.rfc9071];
c) ICT interoperating with other ICT using WebRTC [i.rfc8825] technology to implement VoIP using IETF RFC 8865 [i.rfc8865] to implement RTT functionality using web technologies."
- add a new clause "6.2.3.2 Legacy fall-back interworking with text telephony" to address those cases where there is a public or corporate policy to provide some form of interworking between PSTN text telephony and RTT. This proposed text for this clause is:
Where there is a public or corporate policy for ICT with RTT functionality to interwork with ICT supporting legacy PSTN text telephony, the ICT shall use Recommendation ITU-T V.18 [i.23] or an applicable annex thereof for text telephony signals at the PSTN interface for the case that there is a requirement to support that type of text telephone in the location of the PSTN user connection.
NOTE 1: The functionality of interworking between PSTN text telephony and RTT cannot fulfil all RTT requirements in the present document.
NOTE 2: No PSTN text telephone standard can properly support multiparty RTT presentation, and even where they are enabled to participate in multiparty RTT calls, PSTN textphones can only properly present text from one call participant at a time."
The results of these proposed changes can be previewed in Annex A of TR 103 708 ( tr_103708v010101p.pdf tr_103708v010101p.docx).
It is quite hard to follow what the final result of the proposed changes are, So for ease of reading, it is provided here, even if this proposal very likely is superseded by other proposals.
6.2.3 Interoperability and interworking
6.2.3.1 RTT interoperability
Where ICT has RTT functionality (as required by clause 6.2.1.1) it shall support the applicable RTT interoperability mechanisms described below:
a) ICT interoperating with other ICT using VOIP with Session Initiation Protocol (SIP) and using RTT that conforms to IETF RFC 4103 [i.13] including the updates for multiparty use in RFC 9071[i.rfc9071]
b) ICT interoperating with other ICT using the IP Multimedia Sub-System (IMS) to implement VOIP with RTT using the set of protocols specified in ETSI TS 126 114 [i.10], ETSI TS 122 173 [i.11] and ETSI TS 124 229 [i.12] describing how IETF RFC 4103 [i.13] would apply updated by RFC 9071 [i.rfc9071];
c) ICT interoperating with other ICT using WebRTC [i.rfc8825] technology to implement VoIP using RFC 8865 [i.rfc8865] to implement RTT functionality using web technologies.
d) ICT interoperating with other ICT using technologies other than a or b or c, above, using a relevant and applicable common specification for RTT exchange that is published and available for the environments in which they will be operating. This common specification shall include a method for indicating loss or corruption of characters.
e) ICT interoperating with other ICT using a standard for RTT that has been introduced for use in any of the above environments, and is supported by all of the other active ICT that support voice and RTT in that environment.
NOTE 1: In practice, new standards are introduced as an alternative codec/protocol that is supported alongside the existing common standard and used when all end-to-end components support it while technology development, combined with other reasons including societal development and cost efficiency, may make others become obsolete.
NOTE 2: Where multiple technologies are used to provide voice communication, multiple interoperability mechanisms may be needed to ensure that all users are able to use RTT.
EXAMPLE: A conferencing system that supports voice communication through an internet connection might provide RTT over an internet connection using a proprietary RTT method (option d). However, regardless of whether the RTT method is proprietary or nonproprietary, if the conferencing system also offers telephony communication it will also need to support option a or b to ensure that RTT is supported over the telephony connection.
6.2.3.2 Legacy fall-back interworking with text telephony
Where there is a public or corporate policy for ICT with RTT functionality to interwork with ICT supporting legacy PSTN text telephony, the ICT shall use Recommendation ITU-T V.18 [i.23] or an applicable annex thereof for text telephony signals at the PSTN interface for the case that there is a requirement to support that type of text telephone in the location of the PSTN user connection.
NOTE 1: The functionality of interworking between PSTN text telephony and RTT cannot fulfill all RTT requirements in the present document.
NOTE 2: No PSTN text telephone standard can properly support multiparty RTT presentation, and even where they are enabled to participate in multiparty RTT calls, PSTN textphones can only properly present text from one call participant at a time.
A proposal for dramatically simplified text for 6.2.3 is available in #215 (closed) .