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:

  • Ethernet can be set in any topology.
  • Ethernet has a base rate of 10 Mb/s consisting of packets no more than 1.5 kB.
  • Ethernet can run on wide variety of media, including, UTP (aka CAT 5 or 10BASE-T), thinnet coaxial cable (10BASE-2), thicknet coaxial cable (10BASE-5),a and fiber optic cable (10BASE-F).
  • For other specifications on Ethernet, see my article on Media.

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:

  • A Token Ring is set in a ring topology.
  • A typical Token Ring hub is a MAU (Multistation Access Unit) or SMAU (Smart MAU). This type of hub has ports for multiple stations as well as a RI (Ring In) and a RO (Ring Out) to connect multiple MAU to form a larger ring. The usual limit for Token Rings is 33 MAUs per ring.
  • Token Rings on Type 1 cable or fiber optic media have a maximum of 260 stations and a rate of 16 Mb/s. The maximum distance between MAUs is 100 m (328 feet).
  • Token Rings on Type 3 cable have a maximum of 72 stations and a rate of 4 Mb/s. The maximum distance between MAUs is 45 m (147 feet).

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:

  • ARCnets can be set as a bus, star, or star bus topology.
  • On RG-62 (93 ohms) coaxial cable, ARCnet has limits of 300 m (1,000 ft) bus segments, 8 stations per segment, and 2.5 Mb/s transmission.
  • On UTP, ARCnet has a limit of 120 m (400 feet) bus segments.
  • A max of 255 stations per network. A max of 4 stations per passive hub.
  • A station can be no more than 600 feet from an active hub or 100 feet from a passive hub.
  • The distance between passive and active hubs can be no more than 100 feet. The distance between two active hubs can be no more than 2,000 feet. Two passive hubs cannot be connected.
  • The maximum distance between stations is 20,000 feet.

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:

  • FDDI is set in a ring topology.
  • FDDI is half-duplexed over fiber cable media.
  • FDDI supports both LED- and laser-generated signals.
  • FDDI has a basic signal rate of 80 Mb/s but can get up to 100 Mb/s.
  • An FDDI ring can include 500 nodes connected with 100 km (62 miles) of fiber optic media.


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