The text phone protocols listed in Table 3-1 can be broken down into carrier based and carrierless groupings. Carrier-based protocols implement a constant carrier signal even when text data is not being sent or received. Carrierless protocols either do not use a carrier or only have a carrier present when information is being communicated.
The presence of a carrier signal is common with data modem protocols, as detailed in the "Modulation" section of Chapter 1, "How Modems Work." Because many text protocols use a data modem modulation standard, these implementations must be carrier based. Other text phone protocols, with roots going back to the original teletype devices that predate modems, use a carrierless design. Figure 3-3 exemplifies a carrier based and a carrierless text protocol.
Figure 3-3 Carrier Based and Carrierless Modulation
Carrier Based Text Protocol
Carrierless Text Protocol
There is an inherent benefit to the carrierless text phone protocols. Because a carrier is not always present, mixing speech and text signaling on the same line is simple. When text telephone data is not being sent then the user can speak if necessary while possibly still receiving responses on the text telephone. This type of scenario is common with deaf people who sometimes like to speak instead of type.
In order to mix both text and speech with carrier-based text protocols, the carrier must be interrupted. This is usually done through picking up the handset or hitting a key on the text device. At this point, voice communication can occur but if the text conversation is resumed then the carrier must be re-connected. Compared to a carrierless design, carrier-based text protocols have an additional step when transitioning between text and voice.
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