Interoperability is essential for the Real-Time Text Framework
To ensure that a unified, Real-Time Text framework, that can be used across mainstream platforms and IP networks, is possible. The Real-Time Text must be able to inter-operate seamlessly between multiple Real-Time Text transport formats. This requires one Real-Time Text standard that any of the multiple Real-Time Text transport formats can interconnect with. The R3TF has identified ToIP (RFC4103) as the primary Real-Time Text standard, which is also most suited to offer the interoperability with other Real-Time Text formats.
It is true that ToIP (RFC4103) cannot be implemented on every network as it is only intended for IP transport. Voice is transported over a number of other kinds of networks. In order to reach the Goal, Real-Time Text transports must also be specified for these alternative communication network technologies. The ETSI Guide EG 202 320 provides an overview of existing standards for Real-Time Text in such other networks, and guidelines for interoperability between them. New network technologies will be created with new ways to transport voice. So, new text transports will also be needed in the future. In such cases, the Real-Time Text protocol on such networks must be reliable and able to interconnect with the RFC4103 based solution.
Examples of different Real-Time Text implementations
RFC4103 specifies the use of RTP to carry the text. RTP is usually carried over UDP. Some legacy terminals, e.g. 2G mobile/cellular terminals, have very limited support for UDP for application, so alternative transports may need to be selected to conveniently support Real-Time Text transport on such legacy terminals.
Some private networks may support different Real-Time Text protocols. Some proprietary real-time protocols may be implemented in some networks.
Maintaining interconnectivity with legacy PSTN text phones
Real-Time Text has been supported over PSTN/ISDN networks for many years. Special terminals have been used — called text telephones (or TTYs or TDDs) — that carry the text characters as modem tones. A range of different text telephone standards are used. ITU-T V.18 describes how to interwork between each of them. It should be used as the basis for any interworking with the IP-based Real-Time Text standard.
Interworking can be achieved through IP-PSTN gateways, either dedicated ones or as a function of mainstream PSTN breakout gateways, supporting RFC4103.
In selecting this architecture, a prime consideration has been the requirement to avoid the use of PSTN text telephony technologies on the IP environment because modem (acoustic) transport technologies are not reliable on IP networks in general and are difficult to interface with the range of IP terminal technologies.
Whilst it is expected that the use of legacy PSTN text phones will decline rapidly once a suitable, single IP-based Real-Time Text solution becomes widely available, interworking between the IP and PSTN solutions will remain an important function as long as there are some remaining PSTN text phone users. (In some countries, there are regulatory requirements to support alternatives to real-time voice for people who are Deaf or Hard of Hearing.)
To achieve interoperability, some form of transcoding of the Real-Time Text protocols or their transport protocols is required. Transcoding gateways will be used for RFC4103 interoperability.
Transcoding gateways and the use of RFC4103 ensure interoperability of Real-Time Text