AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 229

Data Communications Support
Data Call Direction
An important aspect of modem pools is that call origination is in one direction
only:
Analog-to-digital, which enables an analog data endpoint to originate a
call to a digital data endpoint
Digital-to-analog, which enables a digital data endpoint to originate a call
to an analog data endpoint
Using a DHG extension to access a modem pool ensures that the intended
incoming service unit (modem or 7500B Data Module) receives the data call
first. Once communication is established between the data endpoints, the
communication is full duplex.
NOTE:
Modem pools should not be used to originate calls in both directions
because it is possible for a modem/7500B Data Module pair to be called at
both ends at the same time. This could result in connecting the wrong data
endpoints or not completing either or both calls.
Modem Pool Dialing
Calls through a modem pool are placed using two-stage dialing. In the first
stage, users enter the modem pool extension or DHG group extension, if
available, for the analog-to-digital or digital-to-analog modem pool being called.
In the second stage, users enter the extension of the analog or digital data
endpoint for inside calls. For outside calls, users enter the dial-out code and
telephone number of the outside analog or digital data station.
This two-stage dialing should be considered before providing modem pools
dialing access to outside lines.
For example, it is possible for a data call (originated from an external data
endpoint) to come into a modem/7500B Data Module pair and, through second
stage dialing, access another external data endpoint.
CAUTION:
Because the communications system originates the second data call in
the two stage dialing process, a call to an external data endpoint in the
second stage would be charged to the communications system. If this
is a concern, separate analog-to-digital and digital-to-analog modem
pools should be created that service incoming data calls but have no
access to outside lines for second stage dialing.
Data Communications Configuration
Overview 5-11