6100 ADCCP Programming Manual

Link States and Transitions
ADCCP/X21 Protocol Module
069225 Tandem Computers Incorporated 3–15
information is included in the Chargeinfo field of the DISCONNECT response
message.
If T7 times out before the DCE delivers the charging information, ADCCP/X21 enters
the quiescent state, signalling “DTE not ready.” Your DISCONNECT request
completes with a successful completion code but contains the value 0 in the
Chargeinfo Length field.
Note For your application to received call charges, you must do the following:
1. Request that your X.21 network provide charging information. In some X.21 networks, call charges
can be requested as a permanent facility, while in some networks, you may need to request
charges in the selection signal sequence of your CONNECT request each time you initiate a call.
2. Configure ADCCP/X21 to accept and save call charges by issuing a SET X.21 CONFIGURATION
request in which the ACCEPT CHARGES parameter is set to 255.
3. When the link is in the connected state, make sure your application always has an outstanding
passive DISCONNECT request, because ADCCP/X21 returns call charges only in DISCONNECT
response messages. If the link is unexpectedly cleared, and there is no passive DISCONNECT
request pending, ADCCP/X21 will save the charging information and enter the quiescent state.
ADCCP/X21 stores this information until your application retrieves it by issuing a DISCONNECT
request. If you issue a new CONNECT request, any stored charging information is lost.
Link States for Leased
Lines
The X.21 link does not require any call-control procedures for a leased circuit line.
After your application has opened the line and issued a START OPERATION request,
ADCCP/X21 signals “DTE ready.”
If the DCE is ready, ADCCP/X21 sets circuit c to ON and begins to transmit an idle
sequence. Your START OPERATION request finishes when the DCE sets circuit i to
ON and the link enters the data transfer state (state 13). The data transfer state for
leased lines is analogous to the connected state for circuit-switched connections; your
application can now set up the ADCCP protocol connections and transfer frames. In
the leased line data transfer state, ADCCP/X21 either transfers frames or transmits an
idle sequence, which are described above in the subsection “Connected State”.
Figure 3-6 shows the line link states and transitions for leased lines.