A proposal for the integration of PSTN and IP textphone communications
Michael J Day
Tel: +44 20 8546 1601
This contribution proposes a general methodology for the integration of PSTN and future IP textphones. The principle that forms the basis for this proposal is that while the legacy textphone should be supported the future is with IP. The main objective is to allow textphone users to benefit from new technologies and mainstream products by facilitating their migration to IP based textphones while maintaining a means by which they can connect to existing legacy textphones.
Through the use of local PSTN/IP gateways the proposal allows calls to be made from a PSTN textphone to a PSTN textphone, from an IP textphone to an IP textphone and between the PSTN textphones and IP Textphones. The gateways could also support for locally available textphone protocols and provide interoperability between incompatible textphone protocols.
3.2 From an Internet, Mobile or Wireless Terminal 4
3.3 From a PC 5
3.4 Connecting a Relay 5
4 Conclusion 5
There has been a lot of discussion about standards and protocols but not much about how they could work together, the migration or interoperation and possibly more important convergence i.e. the ability to use other terminals.
In the US the discussion focuses on transmitting Baudot across a network. While this is very understandable given their regulatory situation it does have a flaw as a method for the future of textphone communications because it assumes that all textphones communicate using compatible protocols.
This document describes a gateway based solution for the future of textphone communications. This is a high level description of a solution that both allows textphone users to benefit from new technologies while maintaining and improving the facilities available to legacy textphones.
The heart of the solution is The Internet and Internet Protocols (IP). In order to help understanding it is necessary to clarify some of the terms used in the diagram attached and explain how they relate to the customer’s equipment.
Textphones, TTYs and Telephones – domestic analogue terminals connected to a telephone line; this would include PCs with modems that are being used as textphones.
Internet terminals – any terminal connected to The Internet, this would include PCs that are directly connected using ADSL and those using a dial-up connection via ISDN or the telephone network (PSTN).
Mobile terminals – GSM and GPRS terminals connected using a data channel.
The Local Textphone Network (LTN) could be provided on a country or state basis and designed to meet the needs of the textphone users within its catchment area.
The composite parts are:
2.1Gateway In and Gateway Out
This is where the Telephone Network (PSTN) interconnects to the Local Textphone Intranet. It provides both PSTN textphone and PSTN voice connections. Ideally this would be ITU-T V.18 based however because this is a local access point it may only be required to support those protocols available locally. The gateway function provides connectivity with The Internet for the PSTN textphones and voice telephones removing the need for any changes to be made to the terminals, it would also provide the ability for incompatible textphones to communicate.
2.2The Local Textphone Intranet
The Local Textphone Intranet (LTI) is the backbone of the textphone communications using internationally supported protocols for call set up, and transmission of text and voice. These protocols would be used for communications between LTNs and from LTNs to Internet connected terminals.
Providing the LTI as a segment of The Internet means that textphone conversations are provided with the required level of speed to facilitate textphone communication that must of course include a voice element. LTIs could be interconnected independent of The Internet in order to maintain the required speed and provide a level of resource sharing that would ensure service availability while providing a cost effective solution. Such a service could be access to a language not support by the local relay, in this situation a suitable relay could be accessed via The Internet.
With the dynamic nature of The Internet comes the need for some form of directory server, the basic function being to lookup telephone numbers and translate them to IP addresses when available. This could be a simple lookup directory like that used for Microsoft NetMeeting or the type described within the SIP recommendation.
The standard voice to text and text to voice function but accessible from the LTN with access being requested by a gateway or Internet terminal when required.
Access to standard emergency services.
With the LTN being the focus for textphone communications comes the ability to provide services such a network based text/voice answering machine, the number of the last person to call the user, etc.
For a gateway function to work textphone calls from the telephone network need to be routed through the gateway. The easiest way of achieving this is to prefix the dialled telephone number with an access code, this could be automatically added by the textphone. However this is not the only way it could work, a telephone line could be set-up to automatically connect to the gateway when the phone goes off hook, or automatically route a call to a textphone user via the LTN gateway.
3The Customer’s Perspective
Below is a brief description of how a call would be made:
If they are not connected to The Internet then the call is routed via a LTN Gateway
3.3From a PC
If the user is using a PC then there are two possible ways to make a call. Either to use it as a PSTN textphone as above or to connect to The Internet via their Internet Service Provider (ISP) and then make a call in the same way as the Internet Terminal above.
3.4Connecting a Relay
If a relay service is required then the Gateway or textphone application running on the Internet Terminal would connect to the Relay Service.
With the move to higher data rates available on The Internet there comes the ability to have simultaneous text and voice in both directions and it would also allow control functions that could provide handshaking between the terminals. Two possible scenarios for this come to mind:
If a user is more comfortable with the characters appearing a word at a time, or they need to slow down the text because the sender is typing too fast they could set their terminal to control the flow. This would of course delay their response to any conversation and the sender could wonder what was going on. However if the two terminals could communicate then the sender would see a message saying 'The other person is reading' until the recipient has finished the whole sentence.
If one user wanted to use voice then the 'Voice' button would send a request to the other terminal which could be set to reply automatically dependent on the terminal's ability, speaker or microphone plugged in, or the user's ability.
The gateway approach has a number of further benefits:
It enables access for legacy textphones to new protocols and terminals without modification to either textphone or terminal.
Possibly the biggest benefit is that only the gateways need to know about textphone protocols. Should a new protocol be introduced within a country then only the LTNs in that country would need to be changed. The same is true for the network, any changes would be tested between LTNs and would not require testing between textphones. Ideally all gateways would allow the same protocols to be used from and to the PSTN but it need not be the case local proprietary protocols could be supported.
Incompatible terminals can talk to each other such as a textphone to GPRS terminal. All that is needed is for the terminals to be able to talk to the gateway.
Internet to Internet terminals can communicate without touching the telephone network, e.g. a mobile terminal to an internet terminal would not require a gateway function or changes to the network infrastructure.
New technologies would just have to be able to communicate with the gateways to be accessible to textphone users. No more testing against every available textphone.
Access to services such as relay and emergency from Internet connected terminals would not need to go via the telephone network.
These are potential benefits the initial benefit is a dramatic increase in the number of terminals that can support real-time text communications.