Interoperability and Extensibility

Tom-PT Taylor taylor at NORTELNETWORKS.COM
Thu Jun 24 14:40:04 EDT 1999

Modifications to                        	
For text telephone support.

Issued by
Gunnar Hellström
L M Ericsson
Gunnar.hellstrom at

The following additions to the megaco requirements spec are intended to
cover the needs from text telephony, and its gatewaying to IP text

They are now concentrated in the Application-specific requirements section.
Since many of the requirements could as well have been specified under
Termination Class Requirements for an Analogue Gateway, section 11.1.5, I
want to leave it to the authors of the main document to insert the
information where it is appropriate.

<<<<<<In 11.2.4, add information about a text telephone capable gateway >>>>
11.2.4.  Trunking/Access Gateway with text telephone access ports

An access gateway with ports capable of text telephone communication, shall
provide communication between text telephones in the SCN and text
conversation channels in the packet network. 

Text telephone capability of ports is assumed to be possible to combine
with other options for calls as described in the section on "Adaptable NASes".

The port is assumed to adjust for the differences in the supported text
telephone protocols, so that the text media stream can be communicated
T.140 coded in the packet network without further transcoding. 

The protocol must be capable of reporting the type of text telephone
that is connected to the SCN port. The foreseen types are the same as the
ones supported by ITU-T V.18:  DTMF, EDT, Baudot-45, Baudot-50, Bell, V.21,
Minitel and V.18. It should be possible to control which protocols are
supported. The SCN port is assumed to contain ITU-T V.18 functionality.
The protocol must be able to control the following functionality levels of
text telephone support:

a.   Simple text-only support: The call is set into text mode from the
     beginning of the call, in order to conduct a text-only conversation.

b.   Alternating text-voice support: The call may begin in voice mode or
     text mode and, at any moment during the call, change mode on request
     by the SCN user. On the packet side, the two media streams for voice
     and text must be opened, and it must be possible to control the
     feeding of each stream by the protocol.

c.   Simultaneous text and voice support: The call is performed in a mode
  when simultaneous text and voice streams are supported. The call may
  start in voice mode and during the call change state to a 
  text-and-voice call. 

A port may implement only level a, or any level combination including level a.

The protocol must support:

a. A text based alternative to the IVR functionality of the gateway
   when the port is used in text telephone mode.

b. Selection of what national translation table to be used between
   the Unicode based T.140 and the 5-7 bit based text telephone protocols. 

c. Control of the V.18 probe message to be used on incoming calls. 

<<<< End of 11.2.4 addition >>>>>>> 

<<< IN 11.2.6 e, add ",text telephone" after "fax" to read like this: >> 

e.   Adaptable NASes: The protocol must be able to support multiple
     options for an incoming call to allow the NAS to dynamically select
     the proper type of call.  For example, an incoming ISDN call coded
     for "Speech" Bearer Capability could actually be a voice, modem,
     fax, text telephone or 56 kbit/s synchronous call.  The protocol
     should allow the NAS to report back to the MGC the actual type of 
     call once it is detected.
<<<< end of change for 11.2.6 >>>>>


Gunnar Hellström
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Gunnar Hellström

Ericsson Home Systems 

Tel +46 751 100 501
Fax +46 8 556 002 06

e-mail gunnar.hellstrom at
Video +46 8 556 002 05

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