A local area network (lan) is a group of computers and associated devices that share a common communications line and typically share the resources of a single processor or server within a small geographic area (for example, within an office building).
Usually, the server has applications and data storage that are shared in common by multiple computer users.
A local area network may serve as few as two or three users (for example, in a home network) or as many as thousands of users (for example, in an fddi network).
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The main local area network technologies are:
ethernet
ethernet is the most widely-installed local area network (LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox and then developed further by Xerox, DEC, and Intel.
An ethernet LAN typically uses coaxial cable or special grades of twisted pair wires.
ethernet is also used in wireless LANs. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps.
Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.
Fast ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards.
gigabit ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second).
10-gigabit ethernet provides up to 10 billion bits per second
token ring
A token ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a binary digit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time.
The token ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar.
The IEEE 802.5 token ring technology provides for data transfer rates of either 4 or 16 megabits per second.
Very briefly, here is how it works:
Empty information frames are continuously circulated on the ring. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.
The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.
When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.
The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.
The token scheme can also be used with bus topology LANs. The standard for the token ring protocol is Institute of electrical and electronics engineers (IEEE) 802.5. The fiber Distributed-Data Interface (FDDI) also uses a token ring protocol.
arcnet
ARCNET is a widely-installed local area network (lan) technology that uses a token-bus scheme for managing line sharing among the workstations and other devices connected on the lan.
The lan server continuously circulates empty message frames on a bus (a line in which every message goes through every device on the line and a device uses only those with its address).
When a device wants to send a message, it inserts a "token" (this can be as simple as setting a token bit to 1) in an empty frame in which it also inserts the message.
When the destination device or lan server reads the message, it resets the token to 0 so that the frame can be reused by any other device.
The scheme is very efficient when traffic increases since all devices are afforded the same opportunity to use the shared network. ARCNET can use coaxial cable or fiber optic lines.
FDDI
FDDI (fiber Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber optic lines in a local area network (lan) that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users. FDDI is frequently used on the backbone for a wide area network (WAN).
An FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles).
FDDI is a product of American National Standards Committee X3-T9 and conforms to the Open Systems Interconnection (OSI) model of functional layering.
It can be used to interconnect lans using other protocols.
FDDI-II is a version of FDDI that adds the capability to add circuit-switched service to the network so that voice signals can also be handled.
Work is underway to connect FDDI networks to the developing Synchronous Optical Network (SONET).
Typically, a suite of application programs can be kept on the lan server. Users who need an application frequently can download it once and then run it from their local hard disk.
Users can order printing and other services as needed through applications run on the lan server. A user can share files with others at the lan server; read and write access is maintained by a lan administrator.
A lan server may also be used as a Web server if safeguards are taken to secure internal applications and data from outside access.