It is interesting to note that SIP-based communications for the disabled are
also compatible with the legacy text phones used on the PSTN side of VoIP
gateways. Figure 17.7 shows an example of a text phone designed for use on
the PSTN.
The text phone shown in Figure 17.7 is also an interesting milestone for
revealing the full richness of Internet communications for the disabled as dis-
cussed in this chapter.
RFC 3351 provides solid arguments for the use of SIP for rich communica-
tion scenarios and information about early endpoint implementations. As
with Internet communications in general, we notice a keen competition
between endpoint applications and network-based services along the lines of
traditional PSTN-based relay services. Network-based transcoding services
will be illustrated in the next section.Call Control Models for Transcoding Services
Given the existing PSTN-based relay services and the familiarity of impaired
users with relay services, it makes sense to maintain such services in the tran-
sition period during which communications are migrating to the Internet.
Using SIP call control to support network-based transcoding services [7] is
trivial, as can be seen from the example shown in Figure 17.8.Figure 17.7 Legacy text phone for use on the PSTN296 Chapter 17