A link session is initialized by issuing a DLC_START_LS command to the fiber distributed data interface (FDDI) device manager. This creates a combined station and sets it to asynchronous disconnect mode (ADM). As a secondary or combined station, data link control (DLC) FDDI is in receive state waiting for a command frame from the primary or combined station.
The command frames currently accepted are:
|SABME||Set asynchronous balanced mode extended|
|UI||Unnumbered information or datagram|
Any other command frame is ignored. Once a SABME is received, the contact sequence is complete and the station is ready for normal data transfer and the following frames are also accepted as valid packet types in this asynchronous balanced mode extended (ABME) mode:
|RNR||Receive not ready|
As a primary or combined station, DLC FDDI can perform ADM XID and ADM TEST exchanges, send datagrams, or connect the remote into 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 an information buffer that is echoed by the secondary or combined station to test the integrity of the link.
Initiation of the normal data exchange mode, ABME, causes the primary or combined station to send a SABME to the secondary or combined station. Once sent successfully, the attachment is said to be contacted and the user is notified. I-frames can now be sent and received between the linked stations.
The user or the remote station can end DLC FDDI in the following ways:
Note: Protocol violations and resource outages can cause abnormal termination.