In the early 1980s, the ISO began to publish a series of Open Systems Interconnect (OSI) documents that defined standards for data communications. One of these described the OSI layered model for data communications. A simpler model was used by the Internet Engineering Task Force (IETF), whose committees produced the TCP/IP communications standards. Figure 1.2 shows the two layered models.
LAN technology operates at Layers 1 and 2. The job of Layer 1, the physical layer, is to transmit 0s and 1s across a medium. As indicated in