IBM Midrange System DS4000 Series Hardware Manual page 32

FC4 is the application protocol mapping layer. In storage connectivity applications, FCP
protocol is used to encapsulate SCSI data into FC frames.
FC cable types
FC implementations can utilize either single-mode or multi-mode FC cables. The name
multi-mode fiber indicates that multiple modes, or rays of light, can travel through the cable
core simultaneously. The multi-mode fiber cable uses a larger diameter core, which makes it
easier to couple than the single-mode fibre cable. With a throughput of 8 Gbps, the length of
the cable can be up to 150 m.
Single-mode fiber transfers a single ray of light. The core diameter is much smaller than the
core of multi-mode cable. Therefore, coupling is much more demanding and tolerances for
single-mode connectors and splices are very low. However, single-mode fiber cables can be
much longer. The cable length can exceed 50 km.
Multi-mode cabling is much more common, as it is easier to work with and meets the
requirements of most customer scenarios. However, in situations where very long cable
lengths are needed, single-mode cabling will be required.
FC world wide names (WWN)
FC devices are presented with a unique identifier called world wide name (WWN). The
WWNs are somewhat similar to the MAC addresses in Ethernet terms. For example, each FC
HBA has its own WWN, which is hardcoded (or burned-in) during manufacturing. The HBA
will be uniquely identified by the storage subsystem using its WWN.
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IBM Midrange System Storage Hardware Guide