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Setting Up a Basic Ethernet LAN on a Linux PC

You can set up a basic Ethernet LAN on a Linux PC. Ethernet is a standard way to move packets of data among two or more computers connected to a single hub, router, or switch. (You can create larger networks by connecting multiple Ethernet segments with gateways.) To set up an Ethernet LAN, you need an Ethernet card for each PC. Linux supports a wide variety of Ethernet cards for the PC.

Ethernet is a good choice for the physical data-transport mechanism for the following reasons:

Ethernet is a proven technology that has been in use since the early 1980s.
Ethernet provides good data-transfer rates - typically, 100 million bits per second (100 Mbps), although Gigabit Ethernet (1,000 Mbps) is now common.
Ethernet hardware is often built into PCs or can be installed at a relatively low cost. (PC Ethernet cards cost about $10 to $20.)
With wireless Ethernet, you can easily connect laptop PCs to your Ethernet LAN without having to run wires all over the place.

How Ethernet Works

What makes Ethernet tick? In essence, it's the same thing that makes any conversation work: listening and taking turns.

In an Ethernet network, all systems in a segment are connected to the same wire. A protocol is used for sending and receiving data because only one data packet can exist on the single wire at any time. An Ethernet LAN uses a data-transmission protocol known as Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) to share the single transmission cable among all the computers. Ethernet cards in the computers follow the CSMA/CD protocol to transmit and receive Ethernet packets.

The way that the CSMA/CD protocol works is similar to the way in which you have a conversation at a party. You listen for a pause (that's sensing the carrier) and talk when no one else is speaking. If you and another person begin talking at the same time, both of you realize the problem (that's collision detection) and pause for a moment; then one of you starts speaking again. As you know from experience, everything works out.

In an Ethernet LAN, each Ethernet card checks the cable for signals; that's the carrier-sense part. If the signal level is low, the Ethernet card sends its packets on the cable; the packet contains information about the sender and the intended recipient. All Ethernet cards on the LAN listen to the signal, and the recipient receives the packet. If two cards send out a packet simultaneously, the signal level in the cable rises above a threshold, and the cards know that a collision has occurred. (Two packets have been sent out at the same time.) Both cards wait for a random amount of time before sending their packets again.

Ethernet sends data in packets (discrete chunks also known as frames). You don't have to hassle much with the innards of Ethernet packets except to note the 6-byte source and destination addresses. Each Ethernet controller has a unique 6-byte (48-bit) address at the physical layer; every packet must have one.

What is Ethernet?

Ethernet was invented in the early 1970s at the Xerox Palo Alto Research Center (PARC) by Robert M. Metcalfe. In the 1980s, Ethernet was standardized by the cooperative effort of three companies: Digital Equipment Corporation (DEC), Intel, and Xerox.

Using the first initials of the company names, that Ethernet standard became known as the DIX standard. Later, the DIX standard was included in the 802-series standards developed by the Institute of Electrical and Electronics Engineers (IEEE). The final Ethernet specification is formally known as IEEE 802.3 CSMA/CD, but people continue to call it Ethernet.

Ethernet Cables and Linux PCs

Any time you hear experts talking about Ethernet, you'll also hear some bewildering terms used for the cables that carry the data. Here's a quick rundown.

The original Ethernet standard used a thick coaxial cable nearly half an inch in diameter. This wiring is called thicknet, thickwire, or just thick Ethernet although the IEEE 802.3 standard calls it 10Base5. That designation means several things: The data-transmission rate is 10 megabits per second (10 Mbps); the transmission is baseband (which simply means that the cable's signal-carrying capacity is devoted to transmitting Ethernet packets only), and the total length of the cable can be no more than 500 meters. Thickwire was expensive, and the cable was rather unwieldy. Unless you're a technology-history buff, you don't have to care one whit about 10Base5 cables.

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