Full-duplex Ethernet

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In 10BASE-T and 10BASE-F, the nature of the cable plant is fundamentally different from the older coax environment. In these environments, each device is attached to the hub with two twisted pair or two fibers, one for transmission and one for reception. This means 10BASE-T and 10BASE-F redefine the mechanism for detecting a collision. Because the signals from two devices cannot really collide in the cable plant as they did in the coaxial world, a collision in 10BASE-T and 10BASE-F is defined as “receiving a signal on the receive circuit while transmitting on the transmit circuit.”

Given the existence of separate transmit and receive pairs in 10BASE-T and 10BASE-F, there is one LAN environment in which a collision would have no impact on the network—a LAN that only attaches two devices. Although seemingly farfetched, this is a perfectly valid Ethernet. On such a point-to-point circuit, the concept of collisions is meaningless. Imagine telling customers who have purchased a point-to-point T-1 that they are not permitted to transmit from one end of the circuit if they are in the process of receiving on that circuit!

Full-duplex Ethernet is an Ethernet variant that permits transmissions and receptions to occur simultaneously. Because devices can simultaneously transmit and receive, the total throughput of the network was doubled (compared to the original Ethernet over coaxial cable), so full-duplex Ethernet was initially described as providing 20 Mbps of throughput. However, calling full-duplex Ethernet a 20 Mbps environment can be grossly misleading because the transmission rate is still 10 Mbps. Again, imagine telling customers that a T-1 operates at 3.088 Mbps because they can simultaneously transmit and receive at 1.544 Mbps!

Although full-duplex Ethernet does improve LAN performance, it requires both a point-to-point circuit and a LAN adapter at either end capable of full duplex operation.This means a conventional hub cannot be used in a full-duplex Ethernet environment. First of all, it implements a multi-station collision domain. Second, it does not have a functioning LAN adapter.

However, a full-duplex circuit can be deployed between two computers, between a computer and a LAN switch, between LAN switches, or any combination of Layer 2 or higher devices. The IEEE specification for full-duplex Ethernet was designated IEEE 802.3x, which has now been incorporated into the 802.3 standard. This specification also defines a mechanism for flow control, which enables a hub to stop the transmission of a station in cases where it could be congested, and vice versa.

Modern LAN adapters can all operate in full-duplex mode, automatically detecting whether they are attached to a point-to-point cable segment, and automatically detecting if the other LAN adapter is full-duplex capable. This capability has also extended itself into all of the higher speed versions of Ethernet.