■■ Impaired users require a fast-response real-time text application,
whereby, just as with voice, users can follow the thoughts of the other
party as they are expressed in typing, and interrupt the other party.
This application is called Text over IP (ToIP) or text conversation, and
requires short delay times, less stringent than, but similar to, voice.
ToIP is another media that can be part of interactive communication ses-
sions using SIP for signaling, besides IM, voice, video, and other forms. ToIP is
a relatively simple application, and it is recommended that it be part of all SIP
telephony devices [4]. ToIP is also carried in RTP packets, as described in [5].
The MIME types are called:
■■ text/t140for simple ToIP in an IP-IP environment
■■ audio/t140for use with IP-PSTN gateways, so as to interleave voice
and text using the same gateway port to save gateway ports.
The payload of the RTP packets consists of short blocks of text that are
defined for backward-compatibility using the ITU-T recommendation T.140
for text transmission.
An example for the RTP packet carrying the text/redand text/t140
payloads with one redundant block is shown in Figure 17.3.
Figure 17.2 Usability as a function of frame rate and image resolution for different video
QoS, as required for sign language and lip reading
Temporal resolution, fps20
12
8Good usabilitySQ CIF
112 x 96QCIF
176 x 144CIF
352 x 288Usable with some
restrictionsVery limited
usabilityNo practical
usabilityVideo image resolutionAccessibility for the Disabled 291