nd data-link layers. Figure 5.1 illustrates the relationship of FDDI, Ethernet, and Token Ring LANs to the seven-layer ISO OSI Reference Model. In examining Fig. 5.1, note that FDDI is simi- lar to Ethernet and Token Ring in that it provides a transport facility Fiber in the LAN for higher-level protocols such as TCP/IP. In fact, FDDI was, and in some locations still is, commonly used at the Internet Service Provider (ISP) peering points that provide interconnections between ISPs. The FDDI standard recognized the need to subdivide the physical layer. Under the FDDI standard the physical-medium-dependent (PMD) sublayer defines the details of the fiber-optic cable used, while the physi- cal (PHY) layer specifies encoding/decoding and clocking operations. Figure 5.2 illustrates the FDDI subdivision of the physical layer. Later LAN standards, such as Fast Ethernet and Gigabit Ethernet, can be con- sidered to use the learning curve from FDDI as they also resulted in the subdivision of the physical layer. 4B/5B Coding The actual coding method used by FDDI results in the encoding of 4 bits with the use of a 5-bit pattern. Thus, this encoding technique is referred to as a 4B/5B code. Because 4 bits are encoded into 5, this means there are 16 4-bit patterns. Those patterns, which are listed in Table 5.1, were selected to ensure that a transition is present at least twice for each 5-bit code. Because 5-bit codes are used, the remaining symbols provide special meanings or represent invalid symbols. Concerning special OSI Reference Model Figure 5.1 The relationship Application between the ISO OSI Reference Model Presentation and common LAN standards. Session Transport Network LAN Standards Data Link Ethernet Token Ring Physical Chapter Five Figure 5.2 PHY layer The FDDI standard encoding/decoding subdivides the physi- clocking cal layer. PDM layer Electrons Photons To fiber meanings, the I symbol is used to exchange handshaking between neigh- boring stations, while J and K symbols are used to form the Start Delim- iter for a