The Telecommunications Relay Service reference article from the English Wikipedia on 24-Apr-2004
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Telecommunications Relay Service

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A Telecommunications Relay Service (TRS, Relay Service, IP-Relay) is an operator service used by deaf persons and those with other hearing difficulties, to allow them to place telephone calls.

In the most common use of TRS, typed messages are relayed as voice messages by a TRS operator, and vice-versa. This allows callers unable to use a regular telephone, to be able to place telephone calls to people who use a regular telephone, and vice-versa.

In less common uses of TRS, there is Voice-Carry-Over (VCO), Hearing-Carry-Over (HCO). VCO allows the TRS to use voice for outgoing communications and text for incoming communications. HCO is the opposite of VCO. The HCO method is most commonly used by people who have difficulty speaking, but have no difficulty hearing voice.

Most TRS operators use regular keyboards to transcribe spoken voice as text for relaying. However, some TRS services may use stenotype or stenomask equipment, similiar to those used by court reporters and closed captioning systems.

Calls to a TRS is usually made by TDD (Telecommunications devices for the deaf) unit, via computer modem, or via Internet. Recently, there have been SMS/AIM based relay services provided by deaf-oriented wireless services. A new method for the deaf to make phone calls, is called a Captioned Telephone or CapTel which now uses voice recognition.

Major Telephone-Line-Based Relay Services

Major Online Relay Services