a channel control unit connection one for each direction. So while a Fibre Channel exchange is capable of carrying a command and response on a single exchange, and all other FC-4 protocols work that way, the response to a FICON IU is always on a different exchange from the IU to which it is a response. The two exchanges that implement a connection are called an exchange pair (Note that the concept of the two exchanges being related exists only at the FC-4 layer). While other FC-4s have a single "data structure type" code that characterizes their IUs in Fibre Channel frame headers, FICON has two. One is for IUs from channel to control unit; the other for control unit to channel. Except for some initialization dialogue that requires stronger synchronization, FICON uses Fibre Channel class of service 3 (Datagram). Thus, at the Fibre Channel physical (FC-2 and below) level, the communication is connectionless, frames and sequences may arrive out of order, and there is no acknowledgement of arrival. But all of that exists at the FC-4 level. Additional CRC The integrity of customer data carried within one or more IUs is protected by a running 32-bit cyclic redundancy check (CRC) contained in the last frame of an IU classified as an ending IU within each data transfer. This is in addition to the standard Fibre Channel CRC used to verify the integrity of each individual FC frame. As such, the FICON CRC has the capability of detecting missing or out of sequence frames/IUs. Cables FICON may employ Fibre Channel fiber optic cables with either short wavelength (multi-mode; 62.5 or 50 micrometer core) or long wavelength (single mode; 9 micrometer core). Long wavelength is used in the majority of applications owing to its superior optical power budget and bandwidth. FICON cannot use Copper Fibre Channel cables. Usage FICON is used exclusively with computers based on the IBM z/Architecture (current descendant of System/360, System/370, etc.), commonly called mainframes. FICON and its predecessors are the only protocols sufficient to communicate with traditional mainframe peripheral devices, especially for z/OS. However, most mainframe operating systems also support FCP (SCSI command set over Fibre Channel). FICON is technically quite similar to more popular storage protocols, such as FCP. Both FICON and FCP share levels 1–3 of Fibre Channel specifications. Some administrators argue that FICON offers somewhat easier troubleshooting in mainframe environments, but the inherent technical