thernet environment, we will begin our examination of the use of fiber by focusing on the network diameter of an Ethernet LAN. Network Diameter Constraints Ethernet uses a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access protocol. Under this protocol a station with data to transmit first listens to determine if another station is transmitting or if the network is idle. If the network is idle, the station will transmit a frame; otherwise, the station waits for an idle condition. Once the sta- tion begins transmission, it will listen for a short period of time to determine if a collision occurred. That period of time is referred to as the Ethernet slot time T and is defined as the time duration required for the transmission of 512 bits of data (64 bytes). The selection of 64 bytes represents the minimum length frame for Ethernet and Fast Ethernet. For 10BASE-T, the slot time is 512 bits X 100 ns/bit or 51.2 us. For 100BASE- T, the slot time is one-tenth the Ethernet slot time or 5.12 us. For both Ethernet and Fast Ethernet, a so-called 512 rule defines the network diameter for half-duplex LAN operations. Basically, the 64-byte frame (512 bits) is the smallest packet size that can be transmitted, and the net- work diameter is determined by the time the shortest frames take to travel round trip to the farthest node in the network. If frames do not arrive by this time, the network can experience late collisions. Thus, the importance of the slot time is twofold. It not only provides time for a station to detect collisions but also specifies the amount of time delay between a transmitting station and a receiving station. Because signals propagate down a transmission medium, this, in turn, results in the slot time governing the network diameter. DETERMINING ONE-WAY DELAY To determine the one-way delay D permissible on an Ethernet LAN, let's consider a two-node net- work such as the one shown in Figure 5.8. In this example let's assume that station A transmits a frame to station B, requiring a time 