FDDI rings are commonly setup as dual rings to provide redundancy in the network topology. Most corporate backbones are traditionally FDDI rings. The physical layer is the hardware abstraction of the OSI model. The network interface card, the cabling or fiber optics, and the signal repeaters are all components at the physical layer. Using our example Macintosh host, the layer-2 frame is placed on the wire, layer 1, as a stream of bits.
Bit streams are series of electrical pulses or bursts of light, depending on the physical media.
Bridging the gap Hopefully, the concepts of the OSI Network Reference Model are a bit clearer to you. It's important to keep in mind the concepts presented and the layers each corresponds to. Layer 2 concepts are important when working with network switches. Two distinct networks can be joined with switches by building a bridge. Bridging is a layer-2 network architecture that relies heavily on MAC addressing schemes and cache tables.