A station identifier is used by VTAM® to identify a physical unit dialing into VTAM on a switched line. The station identifier must be unique for each station in the network.

VTAM uses the station identifier to locate a PU by using:

1. The PU name. If this is a DLUR dial-in connection, PU name is constructed from V0E type F1 in REQACTPU.
2. The adjacent link station name, if the VERALSID=YES operand is specified on the PU definition statement. The ALS name is constructed from the V0E type F7 received in the XID.
3. The CPNAME and network ID specified on the CPNAME and NETID operands of the PU definition statement.
4. The CPNAME specified on the CPNAME operand of a PU definition statement with no network ID.
5. The station identification number for the PU (IDBLK and IDNUM operands on the PU definition statement). The station identification number is a 48-bit number that is used in the XID exchange during the dial procedure.

When locating a PU, to have VTAM use IDBLK and IDNUM before using CPNAME, code SWNORDER=STATNID on the LINE or GROUP definition statement or as a start option. To eliminate the possibility of using CPNAME as a search option for PU connection requests, define SWNORDER=(STATNID,ONLY) on the LINE or GROUP definition statement. DLUR served PUs can also be limited by specifying DLRORDER=(STATNID,ONLY) start option.

For APPN nodes, the TGN operand can also be specified on the PU definition statement. For details, see Implementing an APPN network.

If the peripheral node is a VTAM host defined as a type 2.1 physical unit, use the CPNAME operand; the value of the CPNAME operand must be the same as the value of the SSCPNAME start option in the attaching VTAM type 2.1 peripheral node. The IDBLK and IDNUM operands are ignored.

If the peripheral node is a type 1 or type 2 physical unit, use the IDBLK and IDNUM operands on the PU definition statement. VTAM uses the IDBLK and IDNUM values to build the station identifier.

The value that you code for IDBLK is a 3-digit hexadecimal number. Each device type has its own device addressing scheme; you can find the IDBLK value in the component description manuals for the device. Note that this only applies to physical units; do not code IDBLK for logical units.

Because a subarea physical unit can have multiple connections, a connection identifier (IDNUM) is used to distinguish the multiple PU definition statements that might apply to a subarea physical unit.

As soon as a dial-in line has been activated and placed in answer mode (during VTAM startup or by the VTAM operator), the physical unit can dial that line number and establish communication with VTAM. A physical unit can be defined as call-in only, call-out only, or both.