speed of light and repeaters cause some additional delay. 6.5.5.1 Operation of Collision Detection The Ethernet transmitter operates as a current generator (see Figure 6.28). When the pulse is transmitted, the current of -82 mA is driven to the cable and the pulse amplitude is -2V (25-S2 impedance). The Manchester line code used (see Chapter 4) gives the average current of about -41 mA when the transmission is on. The average voltage of the cable is monitored by an integrator (lowpass filter) and a comparator that compares average voltage in the cable with the threshold level, which is set to approximately 1.5V, as shown in Figure 6.28. If two transmitters are active at the same time, each generates -41 mA on average and, with no attenuation taken into account, the average voltage is -2V with two active stations at a time. When three stations are active, the average voltage is -3V. If the average voltage goes below - -1.5V, the output of the comparator changes state and the collision is detected (multiple stations are transmitting at the same time). The principle just described is specified in the CDMA/CD standard (IEEE 802-3/ISO 8802-3). However, actual implementations may perform the collision detection in a different way. They may read signals back from Data Communications Operation principle of the transmitter: 50 ohms 50 ohms Current source Current of the transmitter: Collision detection circuitry: -41 mA -82 mA Collision detection signal Voltage with one transmitter: Comparator -1.025 V CD threshold CD threshold -2.050 V -1.492...-1.629 V Figure 6.28 Collision detection in Ethernet. the cable and if they are different from the original ones, collision is detected. They may measure the timing jitter of the pulse edges and detect collision if the edge locations in time do not occur at regular time instants. It is up to the manufacturer of the LAN cards to design the implementation as long as it is equal to or better than the one defined in the standard. 6.5.5.2 Contention Algorithm of CDMA/CD Any station that