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FICON Channel Extension Guidelines
B-76 Sun Confidential: Internal Only 96257
Revision A
■ Definition of Terms
The following terms are used in this appendix:
• Front-end
– any equipment between a host and a VTSS
• Back-end
– any equipment between a VTSS and RTD
• Channel extension
– a configuration of equipment that exceeds the maximum distance
allowed by native FICON protocol, implemented by adding a pair of channel extenders.
• Channel extender
– a piece of equipment that can lengthen the maximum distance al-
lowed between two pieces of FICON-capable equipment. Channel extenders are used in
pairs, usually with a WAN network between them. Some channel extenders have
FICON director/switch capabilities, and hence are also labeled as FICON switches.
• FICON director or FICON switch
– a piece of equipment that is capable of acting like an
electronic ‘patch panel’. Directors are used to reduce the number of cables required to
achieve connectivity between multiple pieces of equipment. Note that some vendors sell
FICON directors/switches that also can function as channel extenders (when appropriate
cards have been added).
• Cascading switches
– a hardware configuration which includes at least one FICON direc-
tor/switch connected to another FICON director/switch. In the IBM native FICON proto-
col, cascading can involve no more than two switches; however, most switch vendors al-
low more than two switches in a cascaded configuration.
•ISL
– inter-switch sink; a link between two switches. ISLs can be channel-extended.
•Direct attach
– any connection between two pieces of equipment that does not go
through a FICON director/switch. The connection could, however, still include channel
extenders (which are invisible to the FICON protocol).
•RTD
– Real Tape Drive; a physical tape drive linked to a VTSS box, as opposed to a host.
Note that if a FICON director/switch is used between the tape drive and VTSS, the drive
could function as a RTD at one point in time, and as a conventional tape drive at another
time; this would require varying the drive offline from VTCS and online to MVS.
• Conventional tape drive
– a tape drive linked to a host, as opposed to a VTSS box. Note
that if a FICON director/switch is used between the tape drive and VTSS, the drive could
function as a conventional tape drive at one point in time, and as a RTD at another time;
this would require varying the drive offline from MVS and online to VTCS.
•Cluster
– a pair of VTSS boxes connected by one or more CLINKs. Depending on the di-
rection of the CLINKs, a cluster can be uni-directional or bi-directional. Clustering is used
to provide hardware fallback in case one of the VTSS boxes becomes inoperative. VTVs
can be replicated between the two VTSS boxes in normal mode (over the CLINKs), al-
lowing one box to take over from the other in case of an outage.
•CLINK
– Cluster LINK; a connection between two VTSS boxes in a cluster. Each CLINK
allows data to flow in only one direction. For bi-directional clustering, at least one CLINK
in each direction must be used. A CLINK connection between two VTSS boxes can in-
clude FICON directors/switches and channel extenders.
•VTD
– Virtual Tape Drive; a virtual (as opposed to physical) tape drive that exists within a
VTSS, as defined by the VTCS (Virtual Tape Control System) host software. A VTD is a
transport in a VTSS that emulates a physical 3490E tape drive to a MVS system. Data
that are ‘written’ to a VTD actually are written to the disk buffer (VTSS). A VTSS has 64
VTDs that perform virtual mounts of VTVs.