The physical network (also called the physical layer) begins at the network interface card (NIC). The NIC is effectively a method of connecting the internal data bus of a computer to the external media (cables) of the network.

In the case of z/OS there is essentially only one NIC: the Open Systems Adapter (OSA) card. The OSA card is discussed in detail in the topic on hardware connectivity. There are other ways of attaching a network to a z/OS host, but they are seldom used. These other interfaces (parallel channel, ESCON, and Coupling Facility attachments) are discussed in the topic on hardware connectivity.