 packet, which functions as an alert to a receiver that a packet is arriving. The selection of the 4B/5B coding was based on the need to reduce the signaling level from 200 MHz to a 125-MHz rate, which, when FDDI was standardized, considerably reduced the cost of electronic compo- nents. FDDI uses 4B/5B encoding to operate on 4 bits at a time, convert- ing them into a 5-bit code. Then each bit is encoded using non-return- to-zero-inversion (NRZI) transmission. As a refresher, NRZ codes a 0 as zero and a 1 bit as a plus pulse. Thus, NRZI reverses that coding, result- ing in a pulse of light signaling a binary 0, while the absence of light signifies a binary 1. Fiber Specifications FDDI represents a token-passing dual-ring network. The ANSI PMD standard specifies several optical characteristics that govern FDDI opera- tion. Those optical characteristics include the type of optical fiber used, the wavelength of the optical signal that transmits the data, and the amount of power loss in the cable; the latter is specified in terms of both a power budget expressed in decibels and cable attenuation expressed in decibels per kilometer. Fiber in the LAN TABLE 5.1 4-bit code function 5B code Symbol FDDI 4B/5B Codes Starting Delimiter First symbol 11000 Second symbol 10001 Data symbols 11110 01001 10100 10101 01010 01011 01110 01111 10010 10011 10110 10111 11010 11011 11100 11101 Ending Delimiter 01101 Control indicators Logical 0 (reset) 00111 Logical 1 (set) 11001 Line status symbols Quiet 00000 11111 00100 Chapter Five TABLE 5.1 4-bit code function 5B code Symbol FDDI 4B/5B Codes Invalid-code assignments (Continued) 00001 Void or halt 00010 Void or halt 00011 00101 00110 01000 Void or halt 01100 10000 Void or halt OPTICAL FIBER SUPPORT FDDI can support 62.5/125-, 50/125-, and 100/140-um multimode fiber sizes; 62.5/125 multimode fiber is the preferred medium. FDDI also supports the use of single-mode fiber, which is commonly used for long-distance transmission that enables the technology to extend to a metropolitan area n