6100 ADCCP Programming Manual

Link States and Transitions
ADCCP/X21 Protocol Module
069225 Tandem Computers Incorporated 3–13
DCE Signals Incoming Call. If you have issued a request to accept incoming calls and the
DCE signals an incoming call, ADCCP/X21 signals “call accepted” to the DCE. After
the DCE signals “ready for data”, ADCCP/X21 considers the call established and
completes your request.
Netinfo Sequence. Depending on the facilities offered by the X.21 network, ADCCP/X21
receives various kinds of information from the DCE during connection establishment.
If requested during connection establishment, ADCCP/X21 passes the following
information to the host:
Call progress signals If ADCCP/X21 initiates connection establishment, the DCE
may signal one or more call progress signals (CPS) to
indicate the status of the call request. Depending on its
meaning, a call progress signal may cause ADCCP/X21 to
clear the link.
Called address In some networks, when ADCCP/X21 initiates connection
establishment, the DCE returns the address of the called line
after any call progress signals. ADCCP/X21 passes the
called address back to the host for verification.
Calling address If ADCCP/X21 is waiting for a connection request, the DCE
may transfer the address of the calling line after signaling an
incoming call. ADCCP/X21, in turn, passes the address to
the host for identification.
Connected State
Call-establishment procedures complete successfully when a network connection is
established. The link is then in the connected state and ADCCP/X21 completes your
CONNECT request.
Note While the link is in the connected state, it is important that your application always have a pending passive
DISCONNECT request. If the line is unexpectedly cleared, ADCCP/X21 completes this request with a
DISCONNECT response that contains the reasons for the clearing. Your application can then analyze the
Status and Error Detail codes and initiate an appropriate recovery strategy.
Once the X.21 link is in the connected state, you can set up the ADCCP protocol with
one or more remote stations and transfer frames. During the connected state, the link
is either transmitting an idle sequence or transferring frames. The idle sequence is
transmitted during the idle state, and frames are transferred during the transmit state.
These two states are defined as follows:
Idle In this state, the DTE transmits ones or flags, depending on the SYSGEN
parameter.
Transmit The link enters this state when the ADCCP task has frames in the transmit
queue.