Text over IP

Text over IP (or ToIP) is a means of providing a real-time text (RTT) 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:

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 (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 (UDP). However, because 2.5G mobile/cellular networks supported the (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 (SIP) (RFC 3261) and the (SDP) (RFC 4566) protocols.

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.

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