A LAN is a Local Area Network, i.e. a number of computers linked within a local area, usually no more than a few hundred meters, so they can interoperate and share resources.

The hardware connections usually involve a network interface card on each computer, cabling, and sometimes different hubs and routers.

There are several standard ways to build LANs. Here are the most common ways in descending order of popularity:

Ethernet

Ethernet is the most popular scheme for forming a network. Ethernet was invented at Xerox Corporation's Palo Alto Research Center (PARC) in the late 1970s and early 1980s. Ethernet is standardized as IEEE 802.3 CSMA/CD (Carrier Sense Multiple Access with Collision Detection) of the OSI Data Link Layer.

Here is how the scheme works:

The Ethernet scheme is such that a speaker first checks the line to see if anyone else is talking (i.e. the CS in CSMA/CD). If no one else is talking, the speaker places a "preamble" with a time stamp on the line and all other parties are expected to listen. The preamble bounces up and down the entire line until it gets to the addressed party and the addressed party confirms receipt.

Multiple computers may be communicating across the media at any given time (i.e. the MA in CSMA/CD). Any number may speak but they cannot start speaking at the exact same time. Multiple speakers creates a collision that everyone can hear (i.e. the CD in CSMA/CD). This means that each speaker, in order to avoid another collision, will wait a random period of time before trying to start again.

Contrast CSMA/CD with the CSMA/CA used by AppleTalk for Macintosh networks. The CA is Collision Avoidance. Mac stations send out a signal before transmitting. This causes other stations to hesitate before they transmit, thus avoiding collisions.

Here are some Ethernet specifications:

Token Ring

A scheme for forming a network, second in popularity only to Ethernet. Token Ring is standardized as IEEE 802.5 of the OSI Data Link Layer.

Here is how the scheme works:

In Token Ring, each station is connected in a complete circle or ring. Any station can send a packet around the ring but only the station with the single logical token can do so. The token is actually a small data frame. The token is passed downstream around the ring giving all stations an opportunity to speak.

Because there is only one speaker at a time, a Token Ring avoids the collisions of Ethernet. A Token Ring must be in a ring, unlike Ethernet which can form rings, lines, or stars. Token Ring is good for high traffic, but is not as simple, inexpensive, or flexible as Ethernet.

Like Ethernet, Token Ring is a logical implementation of how a computers communicate in a network. The physical implementation of a Token Ring usually does not involve a physical ring but a Token Ring hub, thus forming a hybrid star ring topology.

Every seven seconds a Token Ring network "beacons" around its network to ensure that the ring is continuous. 

Here are some Token Ring specifications:

ARCnet

ARCnet (Attached Resource Computer NETwork) is a scheme for forming a network, much less popular than either Ethernet or Token Ring

Here is how the scheme works:

Stations on an ARCnet use an ARCnet network adapter with DIP switches to set its identifying number.

A token is passed from lower to higher numbered stations. Only the station who currently possesses the token may broadcast onto the network.

Here are some ARCnet specifications:

FDDI

FDDI (Fiber Distributed Data Interface) is scheme for forming a network that is for high speed transmission over great distances. FDDI can form large rings that are good for MANs; these MANs can then be connected to a WAN. 

Here is how the scheme works:

FDDI uses a token-passing method similar to Token Ring and ARCnet.

FDDI gives some stations higher priority over usage of the token.

FDDI utilizes a double ring topology. Data is transmitted over a primary ring. If there is a break in the primary ring, then data is sent to the secondary ring in the opposite direction to work around the break in the primary ring.

Here are some FDDI specifications:

Page Modified: (Hand noted: 2007-09-13 18:35:57Z) (Auto noted: 2007-11-17 06:38:46Z)