124 Voice API Programming Guide — June 2005
Send and Receive FSK Data
10.5 Two-Way ADSI
Two-way ADSI includes several enhancements to one-way ADSI, including two-way frequency
shift keying (FSK). The following topics discuss two-way ADSI:
•Transmit to On-Hook CPE
•Two-Way FSK

10.5.1 Transmit to On-Hook CPE

The transmit to on-hook customer premises equipment (CPE) feature allows messages to be sent to
an ADSI phone when the phone is either on-hook or off-hook.
This feature supports the transmission of FSK data burst messages to CPE devices that are kept in
the on-hook state by either the Central Office (CO) or the PBX/KTS. This allows an ADSI/Caller
ID phone to receive and potentially display messages while it is in the on-hook state. For example,
ADSI phones can be configured, accessed, and downloaded with features, outside of regular
business hours while the phone is on-hook, without ringing and without subscriber intervention.
Note: The transmit to on-hook CPE feature works only if the CO supports this feature.

10.5.2 Two-Way FSK

The two-way frequency shift keying (FSK) feature allows users to send and receive character or
binary data at 1200 bits/second between the server and compatible devices, such as certain ADSI
phones with keyboards. The two-way FSK feature supports applications such as off-line e-mail
editing and sending FSK Caller ID data to a customer premises equipment (CPE) device.
FSK (frequency shift keying) is a modulation technique used to transfer data over voice lines. The
basic ADSI capability supports only FSK Transmit (one-way FSK), in which an FSK message is
sent from the server to an ADSI display phone, with the phone in the off-hook state. The phone
(CPE) sends dual tone multi-frequency (DTMF) messages to the server. As DTMF messages are
sent to the server, the effective data rate is very slow, approximately 6 characters per second
maximum. This speed is satisfactory for ACK/NAK signaling but it is not usable for any bulk data
transport in the inbound direction from the CPE.
FSK data reception uses a DSP-based Bell 202/V.23 low speed (1200 baud) modem receiver. A
1200 baud modem does not need to train for data transmission, and therefore is faster than a high-
speed modem for short data bursts.
Two-way FSK for ADSI supports the transmission and the reception of FSK data between the
server and the CPE. The server initiates the reception of data from the CPE by sending a CAS to
tell the CPE to switch to data mode, followed by a message that tells the CPE to switch to
peripheral mode. Once it is in peripheral mode, the CPE can send FSK messages to the server using
the ADSI Data Message Format (ADMF), instead of the slower DTMF-based scheme.
See Section10.8, “Developing ADSI Applications”, on page 12 7 for a more detailed description of
how to use library functions to develop two-way ADSI data transfer applications. For more