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Method for handling FICON exchange overlap

IP.com Disclosure Number: IPCOM000013974D
Original Publication Date: 2000-Jun-01
Included in the Prior Art Database: 2003-Jun-19

Publishing Venue



Disclosed is a method for handling the exchange overlap which may occur between consecutive operations on the same logical path across a FICON link. An exchange overlap occurs when an FC-4 Information Unit (IU) is received opening a new exchange before the FC-4 IU is received closing the currently open exchange. The method to handle the overlap is for the receiving entity, be it channel or control unit, to respond to the IU opening the new exchange with a Link Busy IU, and then discard all other IUs received on that exchange until the LACK IU in response to the Link Busy IU is received. The sending entity, be it control unit or channel, after sending the LACK IU in response to the Link Busy IU, redrives the IU opening the new exchange, and now the operation proceeds normally. In the examples below, I will use the term Outbound Exchange (OBE) to denote an exchange from the channel and Inbound Exchange (IBE) to denote an exchange from the control unit. An example of an overlap which a control unit would need to handle is: A Status Accept (SAC) IU has been sent closing the existing Outbound Exchange (OBE) and a new Command IU opening a new OBE is sent to the same device, but the Command IU is received first. As Sent by Channel: Channel Device