[VoiceOps] Porting a TTY Line

[Jay Hennigan]

On 9/21/21 10:42, Shawn L via VoiceOps wrote:
> Does anyone have any familiarity with TTY lines?
> 
> A customer has asked us to port one, but I have no idea what, if 
> anything is required on the back end.  From the limited information I 
> can find, it looks like a standard POTS line, but with a TTY terminal on 
> both ends.  But there's also the mention of an operator who acts like a 
> go-between for communication.  I'd like to know more about how it 
> operates and what we have to provide / if we can provide it before going 
> forward.

To the best of my knowledge it's a POTS line. The terminal uses a modem 
to communicate. Early devices were acoustically coupled, half-duplex and 
used refurbished Teletype machines. The Teletypes were big and noisy, 
but the noisy part isn't really a factor in this application. There are 
smaller portable units these days. Nothing special about the line itself 
from a technical standpoint but there may be discounted billing rates in 
some jurisdictions as well as possible directory listing flags that it's 
TTY.

Modem encoding is FSK, slow and forgiving enough that it should work 
over a VoIP connection with G.711 codecs without issue. Some are 
half-duplex meaning that only one side sends at a time. Typically 
hand-over is done with "GA" at the end of a message, like "over" in a 
two-way radio setup. Then the other party turns their modem to transmit 
and starts typing.

The go-between operator is reached by dialing special service code 711, 
and 711 should recognize both regular speech (inbound call voice->TTY) 
and modem (outbound call TTY -> speech). The operator basically reads 
the text to the hearing party and types the message to the 
hearing-impaired party.

Some good info and history in the book, "A Phone of Our Own"
http://gupress.gallaudet.edu/POOO.html

-- 
Jay Hennigan - jay at west.net
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV