Architectural Layers in the 802.3 Fast Ethernet (100 MB/sec) Model
Here, we see a logical drawing of the Fast Ethernet Data Link Layer sublayers. Data is passed down from the upper layers (such as TCP/IP or Novell Netware) to the LLC sublayer. From there it is passed to the MAC sublayer and then, depending on whether this is a 100BASE-T4 or 100BASE-TX environment, either down the right or left-hand path to the wire. We will intentionally avoid a detailed discussion of exactly what goes on at each of these layers here. Some of the layers' functions, such as 8B6T encoding, Fan-out, and NRZI signaling are labeled, and will be discussed in this essay.
In 10Mbps Ethernet, the data is handed directly from the MAC layer to the PMA (Physical Medium Attachment) sublayer and onto the wire. The Reconciliation, PCS,
and PMD sublayers do not exist in 10Mbps Ethernet.