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10.Send and Receive FSK Data
This chapter describes the Analog Display Services Interface (ADSI) protocol, two-way frequency
shift keying (FSK), and guidelines for implementing ADSI and two-way FSK support using voice
library functions.
•Overview of ADSI and Two-Way FSK Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
•ADSI Protocol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
•ADSI Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
•One-Way ADSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
•Two-Way ADSI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
•Fixed-Line Short Message Service (SMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
•ADSI and Two-Way FSK Voice Library Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
•Developing ADSI Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
•Modifying Older One-Way ADSI Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
10.1 Overview of ADSI and Two-Way FSK Support
The Analog Display Services Interface (ADSI) is a Telcordia Technologies (formerly Bellcore)
standard that defines a protocol used to transmit data to a display-based, ADSI-compliant
telephone. ADSI enables data to be sent across an analog telephone line, providing asynchronous
data communications with 8 data bits, 1 start and 1 stop bit, and no parity.
For many years, one-way ADSI support was provided through the dx_play() and dx_playf( )
functions. This ADSI support enabled developers to use Intel telecom boards to make ADSI servers
that work with ADSI phones and to support ADSI features such as visual voice mail. This is
referred to as the “older” implementation of one-way ADSI.
Intel has expanded the capabilities of basic ADSI with the introduction of two-way frequency shift
keying (FSK) capabilities. Two-way FSK is a convenient and robust mechanism to exchange small
amounts of data between the telephone and the server using FSK as the transport layer. The two-
way FSK functionality allows products to transmit and receive half-duplex FSK Bell 202 1200 bps
data over the Public Switched Telephone Network (PSTN).
One of the applications of two-way FSK is fixed-line short message service, also called small
message service, or SMS. (This service is also known as text messaging.) This service allows the
server and display-based telephone to exchange short text messages via the PSTN.
As with basic ADSI, the transmission and reception of two-way FSK data is initiated after a call
between the server and the display-based telephone (or CPE) has been established, by one of the
devices sending a special alerting signal (typically a CAS tone). The other device will then