|  | 9.4 | LAN Switching
| 9.4.1 | Full-duplex and half-duplex Ethernet operations
In this review of LAN switching, you will see that one way to help reduce network congestion is to whenever possible, use switches instead of hubs. In addition, to increase the efficiency of the switch you may wish to utilize some or all of these switching features: Fast Ethernet (100 Mbps) switch ports and host NIC cards Full-duplex communications Cut-through switching There are two types of Ethernet communications: Half-duplex Full-duplex Half-duplex communications allows for two or more devices to communicate with each other, but only one device at a time. If multiple devices attempt to communicate at the same time, a collision will occur, those devices back-off, and a random algorithm within each NIC determines which device will send first. Hosts that are connected to a hub must operate in half-duplex. This is because the host must be able to detect when a collision occurs to stop transmitting. Full-duplex communications allows two devices to communicate with each other simultaneously. One of the limitations of full-duplex Ethernet is that there must only be one device connected to the switch-port. That device may be a computer, a printer, a router, or another switch. If you have single devices attached to switch ports (no hubs) it is a good idea to have them operate in full-duplex. Because full-duplex doubles the amount of throughput on that link. For example, on a standard Ethernet 10 Mbps link, the throughput would be 20 Mbps, 10 Mbps transmitted plus 10 Mbps received. There are virtually no collisions on a full-duplex connection, because there are only two devices in the collision domain. Configure both ends It is important that both the switch port and the device connected to the switch are using the same mode of communications, either half-duplex or full-duplex. Both the NIC card in the host, (or in the case of a router, the Ethernet interface) and the switch port must both be using the same mode. Answer : How does the computer’s NIC card get configured for full-duplex or half-duplex operation? Question: In older NIC cards it was done manually, either by software which came with the NIC card or with hardware on the NIC card itself. Today, most NIC cards are auto-sensing, and will adapt to the mode the switch is operating in. Switches can be auto-sensing or software configurable, depending upon the vendor and model of the switch. If both devices, the host and the switch, are auto sensing and there is only a single device on that switch port, then the link will most likely be configured to be full-duplex. Configuring a Router If a router is connected to a switch, you may need to configure the router’s Ethernet interface to be either half or full-duplex. This is a good item to check if you have to troubleshoot a problem with an Ethernet interface on a router. Mismatch between two switches Another problem you may run across is when two switches are interconnected, and the link seems to be slower than it should be. The problem might be that there is a mismatch between themodes on the ports that link the two switches together. Where one switch is operating in half-duplex and the other switch is in full-duplex,. You may notice the collision light flashing frequently on the device running in half-duplex. The full-duplex device is sending at-will, not attempting to sense whether the other device is sending frames or not. This will cause a large number of collisions to occur as the switch operating in half-duplex senses no traffic on the link, forwards the frame at the same time the switch operating in full-duplex sends a frame. Note: This same problem may also occur if a host and a switch are operating in different duplex modes, one in half and the other in full.