The earliest LAN technologies commonly used either thick Ethernet or thin Ethernet infrastructures. It is important to understand some of the limitations of these infrastructures in order to see where LAN switching stands today.

Adding hubs or concentrators into the network offered an improvement on thick and thin Ethernet technology. A hub is a Layer 1 device and is sometimes referred to as an Ethernet concentrator or a multi-port repeater. Introducing hubs into the network allowed greater access to the network for more users. Active hubs also allowed for the extension of networks to greater distances. A hub does this by regenerating the data signal. A hub does not make any decisions when receiving data signals. It simply regenerates and amplifies the data signals that it receives to all connected devices.

Ethernet is fundamentally a shared technology where all users on a given LAN segment compete for the same available bandwidth. This situation is analogous to a number of cars all trying to access a one-lane road at the same time. Because the road has only one lane, only one car can access it at a time. The introduction of hubs into a network resulted in more users competing for the same bandwidth.

Collisions are a by-product of Ethernet networks. If two or more devices try to transmit at the same time a collision occurs. This situation is analogous to two cars merging into a single lane and the resulting collision. Traffic is backed up until the collision can be cleared. When the number of collisions in a network is excessive, sluggish network response times result. This indicates that the network has become too congested or too many users are trying to access the network at the same time.

Layer 2 devices are more intelligent than Layer 1 devices. Layer 2 devices make forwarding decisions based on Media Access Control (MAC) addresses contained within the headers of transmitted data frames.

A bridge is a Layer 2 device used to divide, or segment, a network. A bridge is capable of collecting and selectively passing data frames between two network segments. Bridges do this by learning the MAC address of all devices on each connected segment. Using this information, the bridge builds a bridging table and forwards or blocks traffic based on that table. This results in smaller collision domains and greater network efficiency. Bridges do not restrict broadcast traffic. However, they do provide greater traffic control within a network.

A switch is also a Layer 2 device and may be referred to as a multi-port bridge. A switch has the intelligence to make forwarding decisions based on MAC addresses contained within transmitted data frames. The switch learns the MAC addresses of devices connected to each port and this information is entered into a switching table.

Switches create a virtual circuit between two connected devices that want to communicate. When the virtual circuit is created, a dedicated communication path is established between the two devices. The implementation of a switch on the network provides microsegmentation. In theory this creates a collision free environment between the source and destination, which allows maximum utilization of the available bandwidth. A switch is also able to facilitate multiple, simultaneous virtual circuit connections. This is analogous to a highway being divided into multiple lanes with each car having its own dedicated lane.

The disadvantage of Layer 2 devices is that they forward broadcast frames to all connected devices on the network. When the number of broadcasts in a network is excessive, sluggish network response times result.

A router is a Layer 3 device. The router makes decisions based on groups of network addresses, or classes, as opposed to individual Layer 2 MAC addresses. Routers use routing tables to record the Layer 3 addresses of the networks that are directly connected to the local interfaces and network paths learned from neighboring routers.

The purpose of a router is to do all of the following:

• Examine incoming packets of Layer 3 data
• Choose the best path for them through the network
• Switch them to the proper outgoing port

Routers are not compelled to forward broadcasts. Therefore, routers reduce the size of both the collision domains and the broadcast domains in a network. Routers are the most important traffic regulating devices on large networks. They enable virtually any type of computer to communicate with any other computer anywhere in the world.

LANs typically employ a combination of Layer 1, Layer 2, and Layer 3 devices. Implementation of these devices depends on factors that are specific to the particular needs of the organization.

Interactive Media Activity Drag and Drop: Devices Function at Layers After completing this activity, students will be able to identify the different OSI layers where networking devices function.