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Text over IP (or ToIP) is a means of providing a real-time text service that operates over IP-based networks. It complements Voice over IP (VoIP) and Video 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 2.1.2.1 . 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. Transport of ToIP uses the same Real-time Transport Protocol (RTP) as VoIP and Video-over-IP. The text is encoded according to IETF RFC 4103 “RTP Payload for Text Conversation”. 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). Control of ToIP sessions has been defined using the standard Session Initiation Protocol (SIP) (RFC 3261) and the Session Description Protocol (SDP) (RFC 4566) protocols. * 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Text over IP」の詳細全文を読む スポンサード リンク
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