The UI command sends information without any expected acknowledgment. UI is the connectionless frame format therefore the data contained in a UI frame may be lost if a transmission error occurs. It would be the responsibility of a higher protocol layer to initiate a retransmission, for example, if TCP or SPX did not get an acknowledgment at Layer 4.
The XID conveys the types of LLC services supported and the receive window size. Receipt of an XID command causes the destination LLC to respond with an XID response. The XID format consists of an 8 bit Format Identifier followed by a 16 bit parameter field. The IEEE has defined a ‘Basic XID Format’ which is indicated when the Format Identifier is 81 (hex). In this case the parameter field consists of a one-byte class identifier (Type 1 versus Type 2) and a one-byte Receive Window Size value. The receive window size specifies the maximum number of bytes that can be sent to the station without receiving an acknowledgment. If a Basic Format XID is used and the window size is exceeded then the station will respond with a FRMR.
If the attached system software wants to test the LLC link it can cause a Test frame to be sent. A station receiving a Test frame is expected to send the frame back to it’s source at the earliest possible time. If there is test data included in the frame then the data field is returned in the Test Response.
When a station receives an XID command frame it is expected to return a XID response with the same type of information as described for the command. The XID response is sent at the earliest possible time after receipt of the command.
When a station receives a Test command frame it returns a Test response.