2.3. Identifying the OSI Layers at Which Various Components Operate

Some of your most important responsibilities as a network designer and administrator are to control traffic, keep communication speeds to a maximum, and maintain as much available bandwidth as possible. In order to accomplish these tasks, you must understand the differences in the devices that you can use to construct a network and connect devices. The capabilities of various network devices are dependent in large part on the layer of the OSI model at which they operate. Typically the higher a device can operate in the OSI model, the more intelligence and filtering capabilities it has for the network.

2.3.1. Critical Information

You should understand the basic workings of each device, and you should know the layer of the OSI model at which it operates. In this section, we discuss each of the most common devices and the layer of the OSI model at which they operate.

2.3.1.1. Hubs

A hub is a component that operates at the Physical layer. It connects devices but does not provide any intelligence or filtering. Most hubs have a power supply and can therefore also provide amplification of signal, but they do not change the signal in any way except to make it stronger.

2.3.1.2. Switches

A switch is a component that operates at the Data Link layer. As discussed in Chapter 1, switches provide filtering and network segmentation by automatically building a table of MAC addresses and the corresponding port on the switch. After a switch has built its table, it has the effect of creating a permanent virtual circuit (PVC) for each device connected through the switch. By using Layer 2 MAC addresses, switches help you control traffic and make the most of the band-width in your network while reducing or eliminating collisions.

2.3.1.3. Bridges

A bridge is a component that operates at the Data Link layer. Bridges can also segment traffic, but they do it using software rather than the faster circuitry that is used by switches. You can also use bridges to connect dissimilar networks or connect similar networks through a dissim-ilar network. For example, you can use a bridge to connect two Ethernet networks through a Token Ring network. Bridges accomplish this filtering and connecting using a minor level of intelligence based on Layer 2 MAC addresses.

2.3.1.4. Routers

A router is a component that operates at the Network layer. Routers are primarily responsible for two things. They determine whether each packet belongs on their network; then they either deliver the packet if it does belong or they consult their tables to determine where to send the packet. Routers typically communicate with other routers to update their tables so that they can pass packets in any efficient manner. Routers operate at a higher level of intelligence using the Layer 3 IP addresses configured on the network.

2.3.1.5. Network Interface Card (NIC)

A network interface card (NIC) is a component that operates at the Data Link layer and the Physical layer. A NIC is like a little computer within itself. It examines every packet that comes down the wire and determines whether that packet is addressed to it. If it is, then it passes the packet up the OSI model and into the ports of the computer. If it is not, then the NIC ignores the packet. At the same time the NIC converts parallel data coming out of the computer into serial data that can be passed down the network cable.

2.3.1.6. Wireless Access Points

A wireless access point (WAP) is a component that operates at the Physical layer. It connects a device that uses a wireless NIC to a wired network and the resources that the network provides. Wireless access points give you greater flexibility of network design, but you should also consider the inherent security risks.

2.3.2. Exam Essentials

Know the layer of the OSI model at which each network component operates. Hubs and WAPs operate at the Physical layer of the OSI model. Switches and bridges operate at the Data Link layer. Routers operate at the Network layer. NICs operate at the Physical and Data Link layers.

Know the level of intelligence that each of the network components use at their level in the model. You should know the level of intelligence or addressing schemes used by each of the most common network components. In addition, you should know about any filtering, segmentation, or routing that the components can provide for the network based on their respective levels of intelligence.