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Getting Enough Fibre? FICON Error Detection

This article was originally published in the z/OS Hot Topics Newsletter Issue 11, August 2004.

First failure data capture is the zSeries* fiber connectivity* (FICON) channels approach to capturing error information at the time of a FICON I/O operation failure. It's very difficult to diagnose a Fibre Channel bit error without error data at the time of a failure. Most Fibre Channel errors are indicated at the z/OS* host as an interface control check (IFCC). When the FICON channel detects an IFCC, the FICON purge-path-extended function requests, collects and transfers link error statistical buffer (LESB) information from each Fibre Channel port in the FICON channel-to-CU path to the z/OS host.

Communication between a zSeries FICON channel and a FICON-supported control unit (CU) uses the Fibre Channel standard. This is an ANSI-standard architecture consisting of the Fibre Channel Framing and Signaling Interface (FC-FS). (Note: The ANSI-standard architecture is found at FICON is a higher-level architecture (FC-SB-2) that uses the FC-FS architecture as its basic transport layer.

The typical topology for FICON communications is a switched point-to-point fiber-optical topology. From the FICON-channel optical port (N_Port) there's a twin fiber-optical connection that transmits and receives fibers from the N_Port to a FICON-switch optical port (F_Port). The Fibre Channel serial frame is routed through the switch and exits the FICON switch at another F_Port. Next, there's another twin fiber- optical connection from the FICON switch F_Port to the FICON CU N_Port. The reverse communication (CU-to-channel) goes from the FICON CU N_Port to the switch F_Port to the other switch F_Port and then to the FICON-channel N_Port.

Each of these four Fibre Channel ports, the two F_Ports and the two N_Ports in the switched point-to-point topology, checks the serial frame for a number of error conditions and increments its port LESB counter if it detects an error.

In addition, when an error is detected at either N_Port, the channel or the CU aborts the FICON I/O operation by sending an abort sequence. After the abort sequence, a FICON channel purge-path sequence eliminates any outstanding information units in the CU that are part of the failed FICON I/O operation.

Kenneth Trowell works for IBM’s Channel Development and System Support. Ken can be reached at

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