16.1 Network Infrastructure Concepts
16.1.5 Switched Ethernet

The most modern implementations of Ethernet use switching technology, which replaces the traditional shared hub with a switch. A switch creates a direct internal connection between any two nodes attached to it when they need to communicate, which effectively eliminates collisions. With 10Mbps shared Ethernet, a hub with 24 ports created a collision domain of 24 nodes. All 24 workstations or servers would be competing for a share of the 10Mbps total bandwidth. If you replace the 24-port hub with a 24-port 10Mbps switch, you will provide a full 10 Mbps bandwidth between any two nodes. The size of the collision domain with a switch is always two nodes, regardless of the number of ports.

With a shared Ethernet hub only one data packet could be on the cable at a time. A switch can handle multiple packets simultaneously so multiple workstations can communicate with each other at the same time. Traditional shared Ethernet hubs can also be connected to a switch to reduce the size of collision domains. Ethernet switches are compatible with Ethernet hubs.

The first Ethernet switches were 10Mbps, but since they did not have to contend with collisions, they provided a dedicated 10Mbps between any two nodes that needed to communicate. This was a vast improvement over the shared collision domain approach.

By using switching technology, this bandwidth can be increased to 20 Mbps with full-duplex transmission. Since the receive circuit of the NIC does not have to listen for collisions, the NIC can transmit and receive at the same time. This results in a full 20 Mbps dedicated bandwidth to a workstation or server.

Switched Ethernet has become increasingly popular in recent years and is the dominant LAN technology being implemented today. Ethernet switches and NICs are available in 10Mbps, 100Mbps (fast Ethernet) and 1000Mbps (Gigabit Ethernet) speeds. Since these are switched technologies, these numbers can all be doubled by using full-duplex data transmission if the NICs in the workstations and servers can support it. The Figure shows the use of a switch to connect workstations.