A Basic Understanding of Ethernet

By Walter J. Goralski, Cathy Gadecki, Michael Bushong

Over the years, Ethernet, which is technically IEEE 802.3 CSMA/CD LANs (although no one uses that terminology), has become the most commonly used standard for enterprise networks. These networks carry voice, graphics, and video traffic.

Today’s Ethernet networks run considerably faster than the original Ethernet. The most common top speed, for example, is a full thousand times faster than the original Ethernet — 10 gigabits per second (Gbps) versus 10 megabits per second (Mbps).

With so much data being carried, the potential increases for more and more packet collisions in the collision domain. When a collision occurs, both data frames must be re-sent and this cuts down drastically on the throughput of the LAN.

The collision domain is established by the number of hosts that can be transmitting at the same time and cause a collision. If you can have two hosts blasting away at the same time, you have at least two collision domains on the LAN.

The IEEE addressed this issue by defining transparent bridging, which is generally just called bridging, in the IEEE 802.1D-2004 standard. You can bridge two collision domains, each a LAN in itself, together to make one big LAN, just by plugging them into the same transparent bridge device.

However, extending a LAN by transparently bridging frames still creates one collision and broadcast domain. (A broadcast domain is established by all systems that receive a broadcast “send this to everyone” message. Many protocols use broadcasts in IPv4.)

Internetwork Device Collision Domains Broadcast Domains
Repeater One One
Bridge Many One
Router Many Many
Switch Many Depends on VLAN configuration

The use of the Internetwork devices listed is not mutually exclusive. In other words, a router can be used to segment a LAN into two (or more) segments, and each resulting segment can be divided further with bridges.

In an extreme case, each individual system has its own port and the full media bandwidth available. Think of a LAN switch as a device that bridges many collision domains together but that keeps them apart as far as broadcast domains are concerned.