Ethernet II’s Version of Ethernet

Ethernet II is the original Ethernet frame type. Ethernet II and 802.3 are very similar: they both use CSMA/CD to determine their operations. Their main difference is the frames used to transmit information between NICs. The bottom part of earlier Figure 2-3 shows the fields in an Ethernet II frame. Here are the two main differences between an Ethernet II and IEEE:

■ Ethernet II does not have any sublayers, while IEEE 802.2/3 have two: LLC and MAC.

■ Ethernet II has a type field instead of a length field (used in 802.3). IEEE 802.2 defines the type for IEEE Ethernet.

If you examine the IEEE 802.3 frame and the Ethernet II frame, you can see that they are very similar. NICs differentiate them by examining the value in the type field for an Ethernet II frame and the value in the
length field in the IEEE 802.3 frame. If the value is greater than 1500, then the frame is an Ethernet II frame. If the value is 1500 or less, the frame is an 802.3 frame.

Both versions of Ethernet can coexist in the same network. However, because of the frame differences between the two types, a NIC running only 802.3 will discard any Ethernet II frames and vice versa.

Ethernet Physical Layer Properties
Many physical layer standards define the physical properties of an Ethernet implantation. One of the most common is IEEE’s 802.3 10Mb. Table 2-9 shows some of the 10Mb standards.

Ethernet supports a bus topology—physical or logical. In a bus topology, every device is connected to the same piece of wire and all devices see every frame. For example, 10Base5 uses one long, thick piece of coaxial cable. NICs tap into this wire using a device called a vampire tap. With 10Base2, the devices are connected together by many pieces of wire using T-taps: one end of the T-tap connects to the NIC and the other two connect to the two Ethernet cables that are part of the bus. With 10BaseT, all devices are connected to a hub, where the hub provides a logical bus topology. All of these 10Mb Ethernet solutions support only half-duplex: they can send or receive. They cannot do both simultaneously.

Ethernet 10Base2 and 10Base5 haven’t been used in years because of the difficulty in troubleshooting network problems. And many 10BaseT networks have been supplanted by higher-speed Ethernet solutions, like Fast Ethernet and Gigabit Ethernet. Fast Ethernet and Ethernet use the same frame types and support the same CSMA/CD operation. However, there are two main differences between the two: Fast Ethernet supports 100 Mbps speeds and the physical layer is implemented differently. Table 2-10 shows the different implementations of Fast Ethernet. Fast Ethernet supports both half- and full-duplex connections.With full-duplex connections, a device can send and receive simultaneously but requires a point-to-point connection that doesn’t involve a hub.

Gigabit Ethernet is defined in IEEE 802.3z. To achieve 1Gbps speeds, IEEE adopted ANSI’s X3T11 Fiber Channel standard for the physical layer implantation. The physical layer is different from Ethernet and Fast Ethernet in that it uses an 8B/10B encoding scheme to code the physical layer information when transmitting it across the wire. Table 2-11 shows the different implementations of 1Gbps. There is also a 10Gbps implementation of Ethernet that only runs across fiber. This standard is currently in the development process.
Table 2-12 compares the different cable types.