To establish a connection, your system and the remote device exchange information that includes the exchange identifiers (XIDs).
Systems Network Architecture (SNA) hosts that use parallel connections require the XIDs. You define the XIDs in the controller description. Typically, during the connection process your system conducts an initial poll with NULL XIDs to determine whether the remote device is active or not. If the remote device does not respond to the poll within the specified time period, your system can send another XID to the device. If the remote device responds to the poll, your system and the remote device exchange XIDs and establish the link.
The controller description for SNA hosts names the XID value as the local exchange identifier. The Advanced Program-to-Program Communication (APPC) controller description names the XID value as the exchange identifier.