Network interface card (NIC)

Although the OSA card is the only NIC for z/OS, this is a bit of an understatement. The OSA card variants support Ethernet in all of its current implementations. This means that it physically can connect to either a fiber optic cable or a copper (twisted pair) media. When connected to the latter, the ubiquitous RJ-45 is the connection type used.

Note: RJ-45 (8 wire positions) is the older sibling of RJ-11 (6 wire positions). RJ stands for Register Jack and the 11 standard is the North American (and elsewhere) standard for phone equipment--presumably everyone has seen an RJ-11 connection.

RJ-45 is the worldwide standard for copper media-based Ethernet cabling. So whether connecting a simple personal computer to a twisted pair local area network (LAN) or a enterprise scale mainframe, RJ-45 is used.

What exactly is twisted pair? Exactly what it says: wires running from an RJ-45 adapter are twisted as pairs and housed as a unit within a larger cable casing. This cable is referred to as UTP Cat 5, which stands for Unshielded Twisted Pair, Category 5.

The UTP Cat 5 standard tops out at 100 Mbps (megabits per second). To get faster speeds, the OSA cards switch to higher quality cabling, such as 100Base-TX. And rather than staying with copper media, higher speed networks can use fiber optic cables: 1 Gbps (gigabits per second) and 10 Gbps speeds are supported at the time of writing.

OSA card fiber optic connections can be accomplished using one of two interface types: the SC or LC. In addition, each of these interface types can be attached to one of several cable types.

Thus, in order to explain how that RJ-45 adapter attached to a UTP CAT 5 cable is going to be used, we must begin talking about layers. The layer that is concerned with how data signalling and movement is effected over the physical layer is called the data link layer.