Eliminate ACR for FICON
Original Publication Date: 2000-Jun-01
Included in the Prior Art Database: 2003-Jun-18
When using ESCON (or SBCON) protocols, the channel sends an acknowledgement called Accept Command Response (ACR) to confirm that it has received the indication that the first command of a channel program has been accepted for execution. The control unit is not permitted to begin execution of this command until it has received the ACR. In the event of link errors, this provides a window of opportunity to nullify this first command, and allow the channel to perform recovery by merely retransmitting the first command. When using FICON (or SB-2) protocols, particularly on long links, there are significant performance benefits to be obtained by allowing the control unit to begin execution of the command as soon as it accepts it, and thus be able to eliminate the ACR, which reduces the number of Information Units (IU's) transmitted by one for every channel program executed. This invention takes advantage of fibre-channel links that have -12 inherently low bit error rates (10 ), by optimizing the performance of the normal, non-errored transmissions, and allowing inefficiencies in the abnormal error recovery scenarios. A detailed description of this invention as applied to the FICON Architecture is given in AR6874-02. With this invention, the control unit is able to start the execution of the channel program earlier which reduces latency, and one IU is eliminated per channel program which makes additional channel, control unit and fibre-channel resources available for other channel programs that may be executing.