Wow! Baudot - the old telex protocol, i haven’t heard that referred to for nearly 20 years! I’m amazed it’s still in use. Back in the early 80s i used to know Baudot so well that i could read telex tape (5-level punched paper tape)…
I didn’t even realise Asterisk had TTY/TDD capability, but i’ve just had a quick search through the voip-info wiki - without becoming very much more enlightened.
The zap channel apparently has TTY/TDD support - using the TDD mode AGI command ( voip-info.org/wiki/view/tdd+mode ). This AGI command appears to be hardly documented at all though.
There’s no indication of what TTY/TDD support does, or why it’s even necessary - as it only works with zap channels (i.e., POTS phone lines) - however, there is some vague implication that it may be possible to connect a TDD to an Asterisk zap channel and send the message via a SIP channel. Although whether this message would be sent as a text message over a SIP channel or as modem tones, i couldn’t guess.
However, looking at the code in tdd.c, it appears to have the ability to decode Baudot. What it does with it then, though, i can’t tell - i’m not a C programmer, but maybe someone else can help with that.
As far as codecs go, the issues would be the same as faxes and modems. I can’t remember what speed the original telexes worked at, although it was quite likely 300 baud, but i’d imagine (hope) that modern ones would work a bit faster than that - probably at least 14.4kbps.
They ought to work ok with the g711 codec, but you can’t expect them to work properly with any of the compressed codecs (e.g., g729, gsm). They would also be susceptible to packet loss - possibly even losing characters. But how much they would be affected by packet loss would depend on a) the speed of the communications device (modem) and b) the quality of the network connection.
Being only a 5-bit character code - Baudot has a much more limited number of possible characters that can be represented than ASCII does. To overcome this limitation, it uses two characters as “shift” codes to nearly double the number of characters that can be encoded. All characters after a figure shift are interpreted as digits or punctuation-type characters and all characters after a letter shift are interpreted as letters.
So, of course, with Baudot, the loss of one single character could render the rest of the message quite unintelligible - if a figure shift or a letter shift character gets lost and the person receiving it doesn’t understand how the letters and the figures correspond. This would make the effect of line quality on intelligibility much more of an issue with TDD than with fax, i guess.
This issue had never occurred to me before, but it’s an extremely interesting and important issue - on a par with the ability of VOIP to deal with calls to emergency numbers, or possibly more so, as it is potentially harder to deal with. It’s crucial that the current movement of telephony to VOIP doesn’t disadvantage deaf people.
It ought to be relatively simple for it to improve matters for deaf people, but there’s obviously a lot of work to be done for that to happen. I’d be happy to make any small contribution to this that it’s within my capabilities to make. Finding people that have even heard of Baudot wouldn’t be that easy in this field, i’d imagine!
I’m amazed to find myself thinking in terms of Baudot over SIP - i thought i’d seen the last of Baudot 20 years ago!