MCSE Boot Camp Notes :

Ethernet

Ethernet is a very popular local area network architecture based on the CSMA/CD access method. The original Ethernet specification was the basis for the IEEE 802.3 specifications (see Chapter 2, “Networking Standards”). In present usage, the term Ethernet refers to original Ethernet (or Ethernet II, the latest version) as well as the IEEE 802.3 standards. The different varieties of Ethernet networks are commonly referred to as Ethernet topologies. Typically, Ethernet networks use a bus physical topology, although, as mentioned earlier, some varieties of Ethernet such as 10BASE-T use a star physical topology and a bus logical topology. (Microsoft uses the term “star bus” topology to describe 10BASE-T.)

Ethernet networks, depending on the specification, operate at 10 or 100 Mbps using baseband transmission. Each of the IEEE 802.3 specifications (see Chapter 2) prescribes its own cable types.

The next sections in this chapter examine the following Ethernet topologies:

10BASE2
10BASE5
10BASE-T
10BASE-FL
100VG-AnyLAN
100BASE-X

Note that the name of each Ethernet topology begins with a number (10 or 100). That number specifies the transmission speed for the network. For instance, 10BASE5 is designed to operate at 10 Mbps, and 100BASE-X operates at 100 Mbps.

Ethernet networks transmit data in small units called frames. The size of an Ethernet frame can be anywhere between 64 and 1,518 bytes. Eighteen bytes of the total size are taken up by frame overhead, such as the source and destination addresses, protocol information, and error-checking information.

A typical Ethernet II frame has the following sections:

Preamble. A field that Signifies the beginning of the frame
Addresses. Source and destination addresses for the frame
Type. A field that designates the Network layer protocol
Data. The data being transmitted
CRC. Cyclical Redundancy Check for error checking

The origins of Ethernet are commemorated in the initials DIX, a 15-pin connector used to interface Ethernet components. The acronym “DIX” derives from the combination of leading letters of the founding Ethernet vendors: Digital, Intel, and Xerox.

The term Ethernet commonly refers to original Ethernet (which has been updated to Ethernet II) as well as the IEEE 802.3 standards. Ethernet and the 802.3 standards differ in ways significant enough to make standards incompatible in terms of packet formats, however. At the Physical layer, Ethernet and 802.3 are generally compatible in terms of cables, connectors, and electronic devices.

Ethernet generally is used on light-to-medium traffic networks and performs best when a network’s data traffic transmits in short bursts. Ethernet is the most commonly used network standard. It has become especially popular in many university and government installations.

One advantage of the linear bus topology used by most Ethernet networks (this doesn’t apply to star bus networks such as 10BASE-T) is that the required cabling is minimized because each node doesn’t require a separate cable run to the hub. One disadvantage is that a break in the cable or a streaming network adapter card can bring down the entire network. Streaming is more frequently referred to as a broadcast storm. A broadcast storm occurs when a network card fails and the transmitter floods the cable with traffic, like a faucet stuck open. At this point, the network becomes unusable. See Chapter 13, “Troubleshooting,” for more on broadcast storms.