Text over IP
Real-time text is streaming text that is transmitted as it is produced, allowing text to be used conversationally. Real-time text is defined in ITU-T Multimedia Recommendation F.700 18.104.22.168 . Real-time text is designed for conversational use where people interactively converse with each other. To achieve this, particular user requirements have been specified for the delay of each character and the character loss rate (see F.700 Annex A.3).
Real-time Text over IP can be used:
- in conjunction with voice and/or video in a multimedia communication or on its own, on fixed or mobile accesses,
- by people who want a fast and really interactive means of conversing,
- in noisy environments where it may be hard to hear,
- in environments where other people are nearby but where communications privacy is required,
- to transfer information e.g. numbers, addresses etc., where exactness is necessary,
- by people with hearing loss or speech impairments to communicate with non-disabled and deaf or hard of hearing or speech impaired people.
- to provide real time captioning of a voice conversation for people with a hearing loss. See also captioned telephony in Telecommunications Relay Service
- to provide all voice callers with a convenient means to accurately pass numbers, addresses and other detailed information in text.
Features of Text over IP
ToIP is designed around the ITU-T T.140 real-time text presentation layer protocol (defined for H.32x multimedia services). T.140 allows real-time editing of text e.g. by using 'backspace' and retyping. T.140 is based on the ISO 10646-1 character set that is used by most IP text specifications and uses the UTF-8 format.
RFC 4103 supports an optional forward error correction scheme based on redundant transmission (using RFC 2198). This results in a very low end-to-end packet loss across IP transmission links that have moderately high packet loss. To improve efficiency, text can be buffered for 0.3 – 0.5 seconds before it is sent whilst still meeting the delay requirements.
RTP is usually transported over the User Datagram Protocol (UDP). However, because 2.5G mobile/cellular networks supported the Transmission Control Protocol (TCP) but did not consistently support UDP, some implementations of ToIP over mobile/cellular networks use TCP internally. 3G mobile/cellular networks can support UDP.
The protocol stack for a ToIP medium is:
Very fast typing (30 characters/sec) results in a 2 kbit/s traffic load (including overheads for RFC4103 with the maximum level of redundancy, RTP, UDP and IP).
- SIP is used without any alteration.
- Real-time text encoding is identified by using the SDP media definition 'm=text'.
- The 3GPP IMS defines the features of SDP that ToIP uses in 3GPP TS 26.114 v7.4.0 A5
See IETF RFC 5194 “Framework for real-time text over IP using the Session Initiation Protocol (SIP)” and IETF RFC 4504 “SIP Telephony Device Requirements and Configuration” Section 2.9 for more information.
Text over IP developments
Next Generation Networking (NGNs) is a concept developed by telecommunication service providers and their suppliers. It aims to create a true multi-service network based in IP technology.
ToIP has been specified for inclusion in the 3GPP IP Multimedia Subsystem (IMS) (in 3GPP TS 26.114 v2.0.0 “IMS, Multimedia Telephony, Media handling and interaction”). IMS is being used to implement NGNs in many fixed and mobile networks.
Support of ToIP is being considered in multimedia Emergency Public-safety answering point (PSAPs) in Europe and USA. The ECRIT IETF working group defines ToIP as one form of access to Emergency Services.
ToIP can provide a 'low impact' solution to meeting national regulatory requirements to provide 'equivalent service' to the telephone service for people who have hearing or speech impairments.
A typical terminal on a fixed line access is a home computer that supports multimedia communications - Voice and Video and real-time Text over IP. See External links for information about ToIP equipment and software.
Use of Text over IP by Deaf and hard of hearing individuals and speech impaired people
Telecommunications device for the deaf (TDD) or TTYs (also called Textphone or minicoms) were designed to transport real-time text over the PSTN. TTY/TDDs use a range of modem technologies.
Text-over-IP has been designed as a replacement for TTY/TDDs when using the IP-based networks but also to be of use to mainstream voice call users. It has less service restrictions compared with TTY/TDDs, is designed to be used as a mainstream service and can be used on standard computers or mobile terminals. Proper alerting systems for incoming calls need to be included as well as user interfaces, both hardware and software, that meet the needs of Deaf people, and people with hearing or speech impairments. This can best be achieved with input from end-users in the development stages.
Interworking between TTY/TDDs and ToIP has been implemented using gateways by T-Meeting, Omnitor, Trace R& D, RNID, Center, Voiceriver, and AnnieS. RFC 5194 “Framework for real-time text over IP using the Session Initiation Protocol (SIP)” provides an overview of interworking issues. Work is being proposed in the IETF SIPPING work group on more detailed interworking based on a range of call scenarios.
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Some links to products that support ToIP include:
- http://www.omnitor.se/index_eng.html Omnitor Allan eC Total Conversation PC software with video, real-time text and voice
- http://www.annies.nl/ AnnieS ToIP software for 2.5G GSM and 3G mobile terminals (in Dutch).
- http://www.ictrnid.org.uk/talkbytext.html RNID’s TalkByText software for mobile, web and PC based accesses
- http://www.aupix.com Aupix AP-100 and AP-200 videophone with video, real-time text and voice
- http://www.ives.fr/ develops a web softphone with video, real-time text and voice including T.140 support based on eConf technology.
- http://sourceforge.net/projects/tipcon1/ Trace/Omnitor SIPCon1 open-source PC software with video, real-time text and voice
- http://asterisk.org Asterisk open source IP multimedia exchange version 1.8.0 supports T.140 natively
- http://www.nextalk.com IP network based PBX and call center systems using ToIP to telephony gateways for TTY interoperability over PSTN.
- http://www.packetizer.com/ipmc/diagnostics/cercalc.html Character Error Calculator for ToIP
- http://www.accessaphone.com/iptty/ Tenacity's ipTTY which is a SIP soft phone that supports TTY over IP.
- http://www.textspeak.com/ Text to Speech TTS in 24 languages over Ethernet, WiFi and TCP/IP
Most of these have been successfully tested at an ETSI Interop in April 2007. Some ToIP-TTY Gateways have also been developed e.g. by Omnitor, AnnieS and RNID that also support ToIP.
The Real-time Text Task Force (R3TF) is promoting real-time text based on ToIP and XMPP. See: http://www.realtimetext.org/ .
Some links to other real-time text protocols:
- http://realjabber.org - XMPP-based real-time text protocol based on XEP-0301