![]() Collisions do not occur in full-duplex mode because the Rx and Tx lines are completely independent. In full-duplex operation, the Rx line is not monitored, and the Tx line is always considered available. The following figure shows a full-duplex link. Collisions cause the collision error counter to be incremented – and the sending frame to be retransmitted – after a random back-off delay. If a frame is received on the Rx line while a frame is being sent on the transmitting (Tx) line, a collision occurs. If a frame is present on the Rx link, no frames are sent until the Rx line is clear. In a half-duplex environment, the receiving (Rx) line is monitored. The following figure shows a half-duplex link. This results in one side being 100/full, and the other side being 100/half. When auto-negotiation fails on 10/100 links, the most likely cause is that one side of the link has been set to 100/full, and the other side has been set to auto-negotiation. Each interface advertises the speeds and duplex modes at which it can operate, and the best match is selected (higher speed and full duplex are preferred). If auto-negotiation is running on the other side of the link, the two devices decide TOGETHER on the best speed and duplex mode. So if one side of a link is running auto-negotiation, and the other side of the link is not, auto-negotiation CANNOT determine the speed and duplex configuration of the other side. This is a common misconception that often leads to problems.Īuto-negotiation is a protocol, and as with any protocol, it only works if it’s running on both sides of the link. When auto-negotiation is enabled on a port, it does not automatically determine the configuration of the port on the other side of the Ethernet cable and then match it. A conversation on a telephone is full duplex. A conversation on a two-way radio is usually half-duplex – each person must push a button to talk, and, while talking, that person can not listen.Ī full-duplex interface, on the other hand, can send and receive data simultaneously. On a half-duplex interface, data can only be transmitted or received at any given time. 1,000 Mbps Ethernet is also referred to as Gigabit Ethernet.ĭuplex refers to how data flows on the interface. Common Ethernet speeds include 10 Mbps, 100 Mbps, and 1,000 Mbps. Speed is the rate of the interface, usually listed in megabits per second (Mbps). The driver then dynamically configures the interface to the values determined for the link. Auto-negotiation is the feature that allows a port on a switch, router, server, or other device to communicate with the device on the other end of the link to determine the optimal duplex mode and speed for the connection.
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