applications to request network services.
The first three layers are sometimes called the lower layers. They deal with the mechanics of how information is sent from one computer to another over a network. Layers 4 through 7 are sometimes called the upper layers. They deal with how application software can relate to the network through application programming interfaces.
The following sections describe each of these layers in greater detail.
The physical layer
The bottom layer of the OSI model is the physical layer. It addresses the physical characteristics of the network, such as the types of cables used to connect devices, the types of connectors used, how long the cables can be, and so on. For example, Ethernet spells out the exact Layer 1 requirements for twisted-pair cables that can be used at various speeds — 100 Mbps, 1 Gbps, 10 Gbps, and even faster. The star, bus, ring, and mesh network topologies described in Book 1, Chapter 2 apply to the physical layer.
Another aspect of the physical layer is the electrical characteristics of the signals used to transmit data over the cables from one network node to another. The physical layer doesn’t define any meaning to those signals other than the basic binary values of 1 and 0. The higher levels of the OSI model must assign meanings to the bits that are transmitted at the physical layer.
One type of physical layer device commonly used in networks is a repeater, which is used to regenerate the signal whenever you need to exceed the cable length allowed by the physical layer standard. In the old days, we used to use physical layer devices called hubs to split an Ethernet segment to multiple devices. Technically, hubs are known as multiport repeaters because the purpose of a hub is to regenerate every packet received on any port on all the hub’s other ports. Repeaters and hubs don’t examine the contents of the packets that they regenerate, though. If they did, they would be working at the data link layer, and not at the physical layer.
The network adapter (also called a network interface card; NIC) installed in each computer on the network is a physical layer device. You can display information about the network adapter (or adapters) installed in a Windows computer by displaying the adapter’s Properties dialog box, as shown in Figure 1-1. To access this dialog box in Windows, open the Control Panel, choose Network and Internet, choose Network and Sharing Center, and then choose Change Adapter Settings. Then right-click the Local Area Connection icon and choose Properties from the menu that appears.
FIGURE 1-1: The Properties dialog box for a network adapter.
While you’re at it, switch Control Panel from Category view to Small Icons view. This step will eliminate a lot of extra navigation trying to get to the settings pages you need. For example, in Small Icons view, you can go directly from Control Panel to Network and Sharing Center without first having to open the Network and Internet link.
The data link layer
The data link layer is the lowest layer at which meaning is assigned to the bits that are transmitted over the network. Data link protocols address things such as the size of each packet of data to be sent, a means of addressing each packet so that it’s delivered to the intended recipient, and a way to ensure that two or more nodes don’t try to transmit data on the network at the same time.
The data link layer also provides basic error detection and correction to ensure that the data sent is the same as the data received. If an uncorrectable error occurs, the data link standard must specify how the node is to be informed of the error so that it can retransmit the data.
At the data link layer, each device on the network has an address: the MAC address. This address is hard-wired into every network device by the manufacturer. MAC addresses are unique; no two network devices made by any manufacturer anywhere in the world can have the same MAC address.
You can see the MAC address for a computer’s network adapter by opening a command window and running the ipconfig /all command, as shown in Figure 1-2. In this example, the MAC address of the network card is A0-8C-FD-D0-E6-2E. (The ipconfig command refers to the MAC address as the physical address.)
FIGURE 1-2: Display the MAC address of a network adapter.
Two types of data link layer devices are commonly used on networks:
Bridge: An intelligent repeater that's aware of the MAC addresses of the nodes on either side of the bridge and can forward packets accordingly.
Switch: An intelligent hub that examines the MAC address of arriving packets to determine which port to forward the packet to.
Another important layer 2 concept is the idea of virtual local area networks (VLANs). VLANs allow you to create separate isolated networks that share devices. For example, you can create separate VLANs for your accounting and sales departments, but use the same switch to connect computers in each VLAN. Computers in the account VLAN won't be able to communicate with computers in the sales VLAN, even though the computers are on the same switch.
The network layer
The network layer handles the task of routing network messages from one computer to another. The two most popular layer 3 protocols are IP (which is usually paired with TCP) and IPX (typically paired with SPX for use with Novell and Windows networks).
Network layer protocols provide two important functions: logical addressing and routing. The following sections describe these functions.
