Accessibility on Legacy Networks and on the Internet
The legacy telephone network, though it was designed and optimized for 3.1
kHz bandwidth speech, is also used for text communications for the hearing-
and/or speech-disabled. Various signaling and display technologies have
been introduced in various countries. Some of them are shown in Table 17.1 [2]
(Asia and Africa are not included).
Table 17.1 shows that it is effectively impossible to conduct international
text telephone communications for the disabled because of the lack of stan-
dards discipline in the legacy telecommunications world, where there are
countless national, regional, and company-specific “standards.” This lack of
standards discipline is also manifest in all other areas of telecommunications.
If the Internet and SIP did not exist, they would have to be invented.Table 17.1 Some Current PSTN Textphone Systems
NAME FEATURES COUNTRIES
DTMF (Dual Tone Touch-tones as characters, Netherlands
Multi-Frequency) simplex, 4 char/sec
EDT (European V.21 modem signaling, Austria, Italy, Germany, Malta,
Deaf Telephone) simplex, 10 char/sec Spain, Switzerland
V.21 Text Telephone V.21 modem signaling, Norway, Sweden, Finland,
duplex, 30 char/sec Denmark, Ireland, UK,
Chech Republic
Minitel V.23 modem signaling, France, Belgium
simplex, 120/7 char/sec
for forward/backward
EIA-825 (“Baudot”) Simplex, 6 char/sec, USA, Canada, Australia,
uppercase only New Zealand
Bell 103 (“ASCII”) Similar to V.21 USA
V.18 V.21 modulation plus ability Not taken off in the market
to adapt to all modulations except in the UK as TextDirect
above. Intended for “harmo-
nization” of the fragmented
world of PSTN text telephony288 Chapter 17