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The DNR of the LAM is configured as a `hot line'. When the DTE goes on-line, the LAM
sends a message to the ISPBX. The ISPBX now initiates the hot line facility.
The ISPBX establishes a transparent connection between the DLC related to the B-channel of
the dialling X.21 party and the DLC related to the B-channel of the internal X.21 destination.
A synchronised frame structure is established in the B-channel. In this frame structure the user
information and interface control signals are transferred from the sending to the receiving
X.21 DTE.
After the call establishment phase, the ISPBX sends a message to the LAM. The two X.21
DTEs (synchronous) now try to synchronise. After synchronisation, the transparent data path
is available.
A call is cleared either manually via the LAM or the DTE or by data guarding. In the latter case
the LAM monitors the transmit and receive lines of the X.21 interface. If there is no activity
on the lines during a specified time period, the LAM clears the call automatically. The data
guarding option and the time limits can be specified in the CV related to the DNR. This can
then be downloaded to the data port.
The parameter setting for the X.21 interface is described in appendix A.
2.5.
GATEWAYS
This section describes the connection of a gateway server to the ISPBX network, but not the
gateway server itself.
An internal data party may wish to be connected with a special type of data network. If the
ISPBX does not support the interface type used within that network then a gateway is
required. The internal party establishes a connection to the gateway server via which it
transfers user-information. The gateway server takes care that the user-information is
delivered to the data network in the correct format.
A gateway server is usually connected to a V.24 or X.21 interface of a LAM as illustrated
below.
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