Eliminating Collisions to Allow Full Duplex Ethernet

The original Ethernet specifications used a shared bus, over which only one frame could be sent at any point in time. So, a single device could not be sending a frame and receiving a frame at the same time because it would mean that a collision was occurring. So, devices simply chose not to send a frame while receiving a frame. That logic is called half-duplex logic.

Ethernet switches allow multiple frames to be sent over different ports at the same time. Additionally, if only one device is connected to a switch port, there is never a possibility that a collision could occur. So, LAN switches with only one device cabled to each port of the switch allow the use of full-duplex operation. Full duplex means that an Ethernet card can send and receive concurrently. Consider Figure 3-11, which shows the full-duplex circuitry used with a single PC cabled to a LAN switch.

Figure 3-11 10BASE-T Full-Duplex Operation Using a Switch


Receive Switch NIC

Full duplex allows the full speed—10 Mbps, in this example—to be used in both simultaneously. For this to work, the NIC must disable its loopback circuitry.

So far in this chapter, you have read about the basics of 12 years of Ethernet evolution. summarizes some of the key points as they relate to what is covered in this initial the chapter.

Table 3-4 Summary of Some Basic Ethernet Features

Ethernet Environment


10BASE2, 10BASE5

Single bus cabled serially between devices using coaxial cable. Neither is used much today.

10BASE-T with a Hub

One electrical bus shared among all devices creating a single collision domain, cabled in a star topology using twisted-pair cabling.

10BASE-T with a Switch

One electrical bus per switch port creating multiple collision domains, cabled in a star physical topology but a logical bus topology using twisted-pair cabling.

Half Duplex

Logic that requires a card to only send or receive at a single point in time. Used to avoid collisions.

Full Duplex

Logic that enables concurrent sending and receiving, allowed when one device is attached to a switch port, ensuring that no collisions can occur.

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