Ethernet Protocol

As you know, the first two layers of the OSI model deal with the physical structure of the network and the means by which network devices can send information from one device on a network to another. By far, the most popular set of protocols for the Physical and Data Link layers is Ethernet.

Ethernet has been around in various forms since the early 1970s. The current incarnation of Ethernet is defined by the IEEE standard known as 802.3. Various flavors of Ethernet operate at different speeds and use different types of media.

However, all the versions of Ethernet are compatible with each other, so you can mix and match them on the same network by using devices such as bridges, hubs, and switches to link network segments that use different types of media. The actual transmission speed of Ethernet is measured in millions of bits per second, or Mbps.

Ethernet comes in three different speed versions: 10Mbps, known as Standard Ethernet; 100Mbps, known as Fast Ethernet; and 1000Mbps, known as Gigabit Ethernet. Keep in mind, however, that network transmission speed refers to the maximum speed that can be achieved over the network under ideal conditions.

In reality, the actual throughput of an Ethernet network rarely reaches this maximum speed. Ethernet operates at the first two layers of the OSI model — the Physical and the Data Link layers.

However, Ethernet divides the Data Link layer into two separate layers known as the Logical Link Control (LLC) layer and the Medium Access Control (MAC) layer. Figure below shows how the various elements of Ethernet match up to the OSI model.

Figure 6: Ethernet and the OSI model.

The following sections describe Standard Ethernet, Fast Ethernet, and Gigabit Ethernet in more detail.

Standard Ethernet

Standard Ethernet is the original Ethernet. It runs at 10Mbps, which was considered fast in the 1970s but is pretty slow by today’s standards.

Because the cost of Fast Ethernet has dropped dramatically in the past few years, Fast Ethernet has pretty much replaced Standard Ethernet for most new networks. However, plenty of existing Standard Ethernet networks are still in use.

Standard Ethernet comes in four incarnations, depending on the type of cable used to string the network together:

  • 10Base5: The original Ethernet cable was thick (about as thick as your thumb), heavy, and difficult to work with. It is seen today only in museums.

  • 10Base2: This thinner type of coaxial cable (it resembles television cable) became popular in the 1980s and lingered into the early 1990s. Plenty of 10Base2 cable is still in use, but it’s rarely installed in new networks. 10Base2 (like 10Base5) uses a bus topology, so wiring a 10Base2 network involves running cable from one computer to the next until all the computers are connected in a segment.
  • 10BaseT: Unshielded twisted-pair cable (also known as UTP) became popular in the 1990s because it’s easier to install, lighter, more reliable, and offers more flexibility in how networks are designed. 10BaseT networks use a star topology with hubs at the center of each star.

Although the maximum length of 10BaseT cable is only 100 meters, hubs can be chained together to extend networks well beyond the 100-meter limit. 10BaseT cable has four pairs of wires that are twisted together throughout the entire span of the cable. However, 10BaseT uses only two of these wire pairs, so the unused pairs are spares.

  • 10BaseFL: Fiber-optic cables were originally supported at 10Mbps by the 10BaseFL standard. However, because faster fiber-optic versions of Ethernet now exist, 10BaseFL is rarely used.

Fast Ethernet

Fast Ethernet refers to Ethernet that runs at 100Mbps, which is ten times the speed of standard Ethernet. The following are the three varieties of fast Ethernet:

  • 100BaseT4: The 100BaseT4 protocol allows transmission speeds of 100Mbps over the same UTP cable as 10BaseT networks. To do this, it uses all four pairs of wire in the cable. 100BaseT4 simplifies the task of upgrading an existing 10BaseT network to 100Mbps.
  • 100BaseTX: The most commonly used standard for office networks today is 100BaseTX, which transmits at 100Mbps over just two pairs of a higher grade of UTP cable than the cable used by 10BaseT. The higher-grade cable is referred to as Category 5. Most new networks are wired with Category 5 or better cable.
  • 100BaseFX: The fiber-optic version of Ethernet running at 100Mbps is called 10BaseFX. Because fiber-optic cable is expensive and tricky to install, it isn’t used much for individual computers in a network. However, it’s commonly used as a network backbone. For example, a fiber backbone is often used to connect individual workgroup hubs to routers and servers.

Gigabit Ethernet

Gigabit Ethernet is Ethernet running at a whopping 1,000Mbps, which is 100 times faster than the original 10Mbps Ethernet. Gigabit Ethernet is considerably more expensive than Fast Ethernet, so it’s typically used only when the improved performance justifies the extra cost.

For example, you may find Gigabit Ethernet used as the backbone for very large networks or to connect server computers to the network. And in some cases, Gigabit Ethernet is even used for desktop computers that require high-speed network connections.

Gigabit Ethernet comes in two flavors:

  • 1000BaseT: Gigabit Ethernet can run on Category 5 UTP cable, but higher grades such as Category 5e or Category 6 are preferred because they’re more reliable.
  • 1000BaseLX: Several varieties of fiber cable are used with Gigabit Ethernet, but the most popular is called 1000BaseLX.