de being 100/full and the other side being 100/half. Figure 3-1 shows a half-duplex link. In a half-duplex environment, the receiving (RX) line is monitored. If a frame is present on the RX link, no frames are sent until the RX line is clear. If a frame is received on the RX line while a frame is being sent on the transmitting (TX) line, a collision occurs. Collisions cause the collision error counter to be incremented-and the sending frame to be retransmitted-after a random back- off delay. This may seem counterintuitive in a modern switched environment, but re- member that Ethernet was originally designed to work over a single wire. Switches and twisted pair came along later. Half Duplex Half Duplex Server If receiving a packet on the RX side of Switch the interface, do not transmit Ethernet interface Ethernet interface If a packet is received while one is being transmitted, this is considered a collision Figure 3-1. Half duplex Figure 3-2 shows a full-duplex link. In full-duplex operation, the RX line is not moni- tored, and the TX line is always considered available. Collisions do not occur in full- duplex mode because the RX and TX lines are completely independent. When one side of the link is full duplex and the other side is half duplex, a large number of collisions will occur on the half-duplex side. Because the full-duplex side sends frames without checking the RX line, if it's a busy device, chances are it will be sending When Autonegotiation Fails Full Duplex Full Duplex Server Switch Transmitting is always allowed Ethernet interface Ethernet interface There is no collision detection in full-duplex operation Figure 3-2. Full duplex frames constantly. The other end of the link, being half duplex, will listen to the RX line and will not transmit unless the RX line is available. It will have a hard time getting a chance to transmit and will record a high number of collisions, making the device appear to be slow on the network. The issue may not be obvious, because a half-duplex interface normally shows co