The Layers at Which Devices Operate

• Identify the OSI layers at which various network components operate including:

  • Hubs

  • Switches

  • Bridges

  • Routers

  • Network interface cards (NICs)

Now that we have examined the OSI network layer in some detail, we can look at how it relates to the network connectivity devices discussed in Chapter 3: hubs, switches, bridges, routers, and network interface cards (NICs). These devices are said to operate at certain layers of the OSI model based on their functions and roles in the network. Because these devices are covered in Chapter 3, this chapter does not describe them in detail. Instead, this chapter contains a brief description of each device, to jog your memory.

Hubs

Hubs act as the connectivity points of the network on systems that use twisted-pair cabling. There are two types of hubs: active and passive. Each performs the same basic function; they both provide a pathway along which the electrical signals that carry the data can travel. The difference between the two types of hubs is that an active hub has power and a passive hub does not. Even an active hub does nothing with a signal except regenerate it. Therefore, it is said to be a physical-layer device. Recall that the physical layer deals with placing signals on the media.

Switches

In Chapter 3 you learned that, like hubs, switches act as the connectivity points of the network on systems that use twisted-pair cable.

You also learned that a switch offers performance benefits over a hub because it forwards data only to the port on which the destination device is connected. This has the benefit of reducing network traffic because data isn't forwarded to all the ports on a switch. The switch does this by examining the MAC address of the devices connected to it. The use of the MAC address as an identifier places the switch at Layer 2 of the OSI model. Therefore, it is a data-link-layer device.

Bridges

Bridges are used to divide a network into smaller areas through a process known as segmentation. Then, by learning which devices are located on which interface, a bridge is able to block or forward traffic between the interfaces. It does this by using the MAC address of the attached devices. The use of the MAC address makes a bridge a Layer 2 (that is, data-link-layer) device.

Routers

Routers are more complex and more functional than either bridges or switches because they are used to connect networks together and then manage the flow of data between the networks. Unlike switches and bridges, routers use software-configured logical network addresses. Because the routing function is implemented at the network layer of the OSI model, routers are referred to as Layer 3 devices.

NICs

A NIC provides the physical connectivity point to the network for a computer system. But although NICs are physical components, they are defined as data-link-layer devices because they are used in physical media access (which is handled at the MAC sublayer) and the logical access of the network media (which is handled at the LLC sublayer).

REVIEW BREAK: Summary of the Layers at Which Devices Operate

Table 4.3 summarizes the devices discussed in the previous sections and the corresponding layers at which they operate.

Table 4.3. The OSI Model Layers At Which Various Devices Operate

Device	OSI Layer at Which the Device Operates
Hub	Physical (Layer 1)
Switch	Data-link (Layer 2)
Bridge	Data-link (Layer 2)
Router	Network (Layer 3)
NIC	Data-link (Layer 2)