190 Part 3 · Data Communications and Networking "Ethernet" offers so much power and value that it has, over time, knocked out all the other net- work contenders for our market, and full-duplex, switched Ethernet is now effectively a de facto networking standard for entertainment control. You will now find Ethernet even on small shows, carrying lighting control data; connecting video control equipment; linking show con- trol computers to sensing systems; transporting multichannel digital audio and control data throughout entertainment facilities of all sizes; and even connecting components in scenic motion-control systems. Ethernet was developed in the 1970s by Xerox, Intel, and Digital Equipment Corporation (DEC) to enable users at their "workstations," then a radical concept, to transfer files using a nonproprietary network. In 1983, Ethernet was first standardized by the Institute of Electrical and Electronic Engineers (IEEE), and the IEEE 802.3 Ethernet Working Group is still very actively developing extensions and new systems. IEEE 802 is such an unwieldy mouthful, and few people use the formal name anyway, so the network will be referred to in this book as sim- ply Ethernet. Ethernet is responsible only for transporting bits from one place or another, while higher-level protocols (such as TCP, IP, ARP, etc.; see Chapter 19, on page 177) are responsible for packag- ing the data and making sure that the message is delivered reliably and appropriately. Ethernet breaks down into three general layers (from top to bottom): Logical Link Control (LLC), Media Access Control (MAC), and Physical Layer (PHY). LLC and MAC can be thought of as occupying OSI Layer 2, DataLink, and the Ethernet PHY layer of course fits into OSI Layer 1, Physical. OSI Upper Layers Layer 2: Data Link Layer 1: Physical Ethernet Layer Not Ethernet. HTTP, DHCP, ARP, FTP, TCP, UDP, IP, etc. Logical Link Control (LLC) Media Access Control (MAC) Physical (PHY) A brief introduction to the function of each Ethernet layer follows. L OGICAL L INK C ONTROL (LLC) The LLC layer receives data packaged by an upper-level protocol such as IP (and, therefore, TCP or UDP, and everything above those layers) and passes it on to the MAC layer for physi- cal transmission onto the network (of course, data also flows in the other direction upon return).