When a link station (LS) is opened, the token-ring data link control (DLCTOKEN) is initialized at the open LS as a combined station in asynchronous disconnect mode (ADM). As a secondary or combined station, DLCTOKEN is in a receive state waiting for a command frame from the primary or combined station. The following command frames are accepted by the secondary or combined station at this time:
|SABME||Set asynchronous balanced mode extended|
|XID||Exchange station identification|
|UI||Unnumbered information - datagram|
Any other command frame is ignored. Once a SABME is received, the station is ready for normal data transfer and the following frames are also accepted:
|RNR||Receive not ready|
As a primary or combined station, DLCTOKEN can perform ADM XID, ADM TEST exchanges, send datagrams, or connect the remote to the asynchronous balanced mode extended (ABME). XID exchanges allow the primary or combined station to send out its station-specific identification to the secondary or combined station and obtain a response. Once an XID response is received, any attached information field is passed to the user for further action.
TEST exchanges allow the primary or combined station to send out a buffer of information that will be echoed by the secondary or combined station in order to test the integrity of the link.
Initiation of the normal data exchange mode, ABME, causes the primary or combined station to send an SABME to the secondary or combined station. Once sent successfully, the connection is said to be contacted and the user is notified. Information frames can now be sent and received between the linked stations.