FICON

FICON (Fiber Connectivity) is the IBM proprietary name for the ANSI FC-SB-3 "Single-Byte Command Code Sets-3 Mapping Protocol" for Fibre Channel (FC) protocol. It is a FC layer 4 protocol used to map both IBM’s antecedent (either ESCON or parallel) channel-to-control-unit cabling infrastructure and protocol onto standard FC services and infrastructure. The topology is fabric utilizing FC switches or directors. Valid rates include 1, 2, and 4 Gigabit data rates at distances up to 100 km.

Protocol internals

Each FICON channel port is capable of multiple concurrent data exchanges (currently a maximum of 32) in full duplex mode. Information for active exchanges is transferred in Fibre Channel sequences mapped as FICON "Information Units" (IUs) which consist of one to four Fibre Channel frames, only the first of which carries 32 bytes of FICON (FC-SB-3) mapping protocol. Each FICON exchange may transfer one or many such IUs.

FICON channels use five classes of IUs to conduct information transfers between a channel and a control unit. They are: Data, Command, Status, Control, Command and Data, and lastly Link Control. Only a channel port may send Command or Command and Data IUs, while only a control unit port may send Status IUs.

As with prior Z channel protocols, there is a concept of a channel to control unit "connection." In its most primitive form, a connection is associated with a single channel program. In practice, a single channel program may result in the establishment of several sequential connections. This normally occurs during periods where data transfers become dormant while waiting for some type of independent device activity to complete (such as the physical positioning of tape or a disk access arm). In such cases, the connection may be closed by action of the device's contol unit with the signaling of appropriate status. When the control unit is ready to resume such an interrupted connection, it signals the channel to reconnect and resume the interrupted channel program. By closing temporarily dormant connections, channel and control unit facilities may be better utilized to serve other active channel programs and certain protocol timeouts that might otherwise occur may be avoided.

FICON uses two Fibre Channel exchanges for 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.

FICON has no inherent advantage over other popular protocols, for example FCP, other than its compatibility with the z/Architecture. In fact, there are disadvantages inherent in its lesser popularity. It is used in situations where breaking a historical attachment to the z/Architecture would be more costly than deploying an esoteric I/O protocol. There are also numerous mainframe-specific features, such as GDPS, which require FICON (or ESCON).

IBM's System Storage DS8000 product illustrates the contrast. The DS8000 can be attached to a host via a mixture of FICON, FCP, and ESCON ports. The storage within the product is divided into fixed block storage volumes and mainframe-specific CKD/ECKD storage volumes. To access the fixed block volumes, one uses the FCP ports. To access the CKD/ECKD volumes, one uses the FICON and/or ESCON ports.

FICON has essentially replaced ESCON in current deployments because of FICON's technical superiority.

FICON distance limitations have been overcome through the use of channel extenders. These extenders have been utilized to span continents as well as oceans for specific applications without subjecting FICON information transfers to performance sags imposed by FICON protocol delaysfact|date=November 2007 normally associated with such distances.

Devices That Use FICON

Disk storage facilities that can be attached via FICON include
* EMC DMX
* EMC Symmetrix
* Hitachi Data Systems Lightning
* Hitachi Data Systems Universal Storage Platform
* IBM System Storage IBM Enterprise Storage Server (Shark)
* IBM System Storage DS6000
* IBM System Storage DS8000
* Sun StorageTek V2X4f
* 3PAR 'S'-series

Tape storage facilities include:
* IBM Virtual Tape Server (VTS)
* IBM UltraScalable Tape Library
* Sun StorageTek Virtual Storage Manager (VSM)
* Bus-Tech MAS and MDL Virtual Tape Library