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LANs are contained in buildings or on a single site, and include Ethernet, Fast Ethernet, and Gigabit Ethernet. Ethernet has been around since the late 1970s and remains the leading network technology for local-area networks (LANs). Ethernet is based on a standard referred to as "carrier sense, multiple access, with collision detection," or CSMA/CD.

Simply put, an Ethernet workstation can send data packets only when no other packets are traveling on the network - when the network is "quiet." Otherwise, it waits to transmit, as a person might wait for another to speak during a conversation.

If multiple stations sense an opening and start sending at the same time, a "collision" occurs. Each station then waits a random amount of time and tries to send its packet again. After 16 consecutive failed attempts, the original application that sent it must start again. As more people try to use the network, the number of collisions, errors, and subsequent retransmits grows quickly, causing a snowball effect.

Collisions are normal occurrences, but too many can slow the network. When more than half of the total network bandwidth is used, collision rates begin to cause congestion. Files take longer to print, applications take longer to open, and users are forced to wait. At 60% or higher, the network can slow dramatically or even shut down.

Ethernet bandwidth or data-carrying capacity (also called throughput) is 10 megabits per second (mbps). Fast Ethernet and Gigabit Ethernet work the same way - through collision detection - but provide 10 times the bandwidth, at 100 mbps, and 100 times the bandwidth, at 1,000 mbps, respectively.

The diagram above shows the basic network arrangement: a group of computers and a connection to the internet, all connected together with a hub.

The computers can be anything: Macs, Windows, Unix, Linux, SunOS, Palm Pilots. It doesn't matter, provided they can all handle the wiring (the physical connection) and they can speak the same language and can run compatible applications.

While there are other alternatives, the most common implementation is that all the computers are connected together using an Ethernet hub. The hub exchanges Ethernet information among all the connected units. So each connected machine has an Ethernet port and a wire to the hub. There are other configurations. For example in older systems, the computers were often daisy-chained, with a coax cable running from the first to the second to the third. Sometimes a switch is used instead of a hub. A hub simply relays all the traffic coming in on any port to all the other ports: it's a party line. A switch relays the traffic coming on only to the port that is the intended recipient. This improves bandwidth, since each pair of machines has its own private connection, but switches are quite a bit more costly than hubs. Hubs are only around $50-60 and are often built in to other equipment.

The connection from each machine to the hub is usually over what's called "10baseT" (or 100baseT). This term describes a set of wires and connectors that are like ordinary telephone wiring, except that it needs 2 pairs instead of the single pair that a phone needs. Other possibilities for the wiring are coax cable, special interfaces like Intel's Home Network (runs over your actual telephone lines) and Apple's AirPort. 

The language on the Ethernet for the internet is TCP/IP, so we'll assume each machine speaks that language. There are other, proprietary protocols like NETBUI, AppleTalk, NFS, IPX, etc. These can often be transported over TCP (i.e., encapsulated in a TCP/IP packet), or they go over the Ethernet in addition to TCP/IP. Hubs don't care; they just relay whatever they hear. Routers and the Internet care a great deal. They will generally only route TCP/IP traffic, so if you want your Windows network to connect machines over the Internet, you have to be running TCP/IP. 

The interface to the ISP and the public Internet can be anything from a shared modem to a full-time T1 (1.5 MBit/sec) connection or better. Whatever it is, it has to deal with the traffic from all the connected computers to the Internet. If there's only one machine to connect to the ISP, then a simple modem is used, usually connected to the PC's serial port or plugged into a card slot, or just built-in. If there are several machines, then you need some kind of software or hardware that can keep track of traffic destined for each of the machines on the network. Most commonly, this is a router, or a computer running router software.

To make your network, all you have to do is establish a common electrical connection among the machines, make sure they are all speaking the same language, and have a way to connect your net to the Internet.

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