Local area network

Local area network (LAN), any communication network for  connecting computers within a building or small group of buildings. A LAN may be configured as (1) a bus, a main channel to which nodes or secondary channels are connected in a branching structure, (2) a ring, in which each computer is connected to two neighbouring computers to form a closed circuit, or (3) a star, in which each computer is linked directly to a central computer and only indirectly to one another. Each of these has advantages, though the bus configuration has become the most common.

Even if only two computers are connected, they must follow rules, or protocols, to communicate. For example, one might signal “ready to send” and wait for the other to signal “ready to receive.” When many computers share a network, the protocol might include a rule “talk only when it is your turn” or “do not talk when anyone else is talking.” Protocols must also be designed to handle network errors.

The most common LAN design since the mid-1970s has been the bus-connected Ethernet, originally developed at Xerox PARC. Every computer or other device on an Ethernet has a unique 48-bit address. Any computer that wants to transmit listens for a carrier signal that indicates that a transmission is under way. If it detects none, it starts transmitting, sending the address of the recipient at the start of its transmission. Every system on the network receives each message but ignores those not addressed to it. While a system is transmitting, it also listens, and if it detects a simultaneous transmission, it stops, waits for a random time, and retries. The random time delay before retrying reduces the probability that they will collide again. This scheme is known as carrier sense multiple access with collision detection(CSMA/CD). It works very well until a network is moderately heavily loaded, and then it degrades as collisions become more frequent.

The first Ethernet had a capacity of about 2 megabits (millions of bits) per second (mbps), and today 10- and 100-mbps Ethernet is common, with gigabit-per-second (billions of bits per second; gbps) Ethernet also in use. Ethernet transceivers (transmitter-receivers) for personal computers are inexpensive and easily installed.

A standard for wireless Ethernet, known as Wi-Fi, has become common for small office and home networks. Using frequencies from 2.4 to 5 gigahertz (GHz), such networks can transfer data at rates up to 600 mbps. Early in 2002 another Ethernet-like standard was released. Known as HomePlug, the first version could transmit data at about 8 mbps through a building’s existing electrical power infrastructure. A later version could achieve rates of 1 gbps. Another standard, WiMax, bridges the gap between LANs and wide area networks (WANs).

Local Area Networks (LANs)

In the modern office environment, each worker is equipped with a personal computer with its own processor and multiple disk drives. The computer may be free-standing (very much the exception these days) or it may be connected to a network, minimally to the Internet. In many small operations, like a doctor's office, a single computer may be used—but linked to the Internet. In most typical office situations, the computers of the organization are interconnected to each other as well by way of a local area network (LAN), typically by means of a single dedicated computer known as the "server," short for "file server." The linkage may be by wire or by a special radio frequency. The server used may also provide each "node" in the network with Internet service; and interoffice communications between computers are by e-mail. As the name suggests, such networks are localand shielded from external influences except as these are mediated by the network server, which is itself protected by so-called "firewalls" from unauthorized interference. In larger organizations local networks may be connected to one another. This extended arrangement is then referred to as a wide area network or WAN. Communications between LANs may be over proprietary communications lines (wired, wireless, or a combination) or may use the Internet.  One of the benefits of a LAN is that it may be installed simply and incrementally, upgraded or expanded with little difficulty, and moved or rearranged with little disruption. LANs are also useful because they can transmit data quickly. Use of such networks is becoming ever easier because new employees almost always bring computer skills and Internet experience easily adapted to the local customs.

HISTORY

The advent of personal computers (PCs) changed the type of information sent over office computer networks. Before their rapid spread in the 1970s, employees communicated with mainframe and mini-computers by means of so-called "dumb" terminals. All the processing took place on the main computer that all individuals used simultaneously. When use was heavy, the system's performance slowed. The PCs took over processing tasks at the desk and thus speeded things up substantially. With massive computing power no longer needed, smaller and simpler "file servers" could be substituted. Computerization thus opened up to even quite tiny operations.

LANs developed simultaneously to connect freestanding computers in offices that, until LANs came, exchanged data by passing diskettes around, and in operations using dumb terminals, such terminals first being replaced by PCs and, later, the connection to mainframes severed with the PCs now connected either to each other or to a server; using servers became by far the most common LAN configuration.

Developments in LANs proceeded along two fronts in the 1990s: competing networking software systems developed and changes in wiring took place to provide ever-faster communications speeds. Wireless transmission appeared in the mid-1990s and had become the leading edge of LAN technology by the mid-2000s using a new radio-communications standard known as 802.11, issued by the Institute of Electrical and Electronics Engineers, Inc. With the foundation of Wi-Fi Alliance in 1998 as a certification agency, "Wi-Fi" has come to mean wireless communications. The abbreviation stands for Wireless Fidelity. Wireless LANs are referred to as WLANs and sometimes as LAWNs.

During the 1990s, as well, global networking brought about by the explosive development of the Internet has played an enhancing role—enhancing the intimate local aspects of LANs by giving such networks national, indeed international, access too. LAN technology, in fact, has migrated from businesses to homes. In many residences multiple computers are linked by network connections, some connected by wire and some by radio links.

PHYSICAL COMPONENTS OF LANs

The physical properties of a LAN include network access units (or interfaces) that connect the personal computer to the network. These units are actually interface cards installed on computer motherboards. Their job is to provide a connection, monitor availability of access to the LAN, set or buffer the data transmission speed, ensure against transmission errors and collisions, and assemble data from the LAN into usable form for the computer.

Network cards may communicate with the network either by wire or by radio signal. Wiring remains the most common form in the mid-2000s but may change over time. Where wiring is used, it determines transmission speeds. The first LANs were connected with coaxial cable, the same type used to deliver cable television. These facilities are relatively inexpensive and simple to attach. More importantly, they provide great bandwidth (the system's rate of data transfer), enabling transmission speeds initially up to 20 megabits per second.

Another type of wiring, developed in the 1980s, used ordinary twisted wire pair (commonly used for telephones). The primary advantages of twisted wire pair are low cost and simplicity. The downside is a more limited bandwidth.

A yet more recent development in LAN wiring was optical fiber cable. This type of wiring uses thin strands of glass to transmit pulses of light between terminals. It provides tremendous bandwidth, allowing very high transmission speeds and (because it is optical rather than electronic) it is impervious to electromagnetic interference. Still, splicing it can be difficult and requires a high degree of skill. The primary application of fiber is not between computers, but between LAN buses (terminals) located on different floors. As a result, fiber-distributed data interface is used mainly in building risers. Within individual floors, LAN facilities remain coaxial or twisted wire pair.

Wireless communication is between radio devices which are themselves cards or specialized modems. Advantages are avoidance of wiring costs and hassle; disadvantages are distance limitations and interference. Unless a wireless system is properly configured to use signal encryption, the problem of the "evil twin" appears—a phrase used to label a device that appears to participate in communications because it inadvertently interferes with a poorly configured network.

WIRED LAN TOPOLOGIES

LANs are designed in several different physical arrangements of node computers, known as topologies. These patterns can range from straight lines to a ring. Each terminal on the LAN contends with other terminals for access to the system. When it has secured access, it broadcasts its message to all the terminals at once. The message is picked up by the terminal for which it is intended—or multiples of these. The branching tree topology is an extension of the bus, providing a link between two or more buses.

A third topology, the star network, also works like a bus in terms of contention and broadcast. But in the star, stations are connected to a single, central node (individual computer) that administers access. Several of these nodes may be connected to one another. For example, a bus serving six stations may be connected to another bus serving 10 stations and a third bus connecting 12 stations. The star topology is most often used where the connecting facilities are coaxial or twisted wire pair.

The ring topology connects each station to its own node, and these nodes are connected in a circular fashion. Node 1 is connected to node 2, which is connected to node 3, and so on, and the final node is connected back to node 1. Messages sent over the LAN are regenerated by each node, but retained only by the addressees. Eventually, the message circulates back to the sending node, which removes it from the stream.

TRANSMISSION METHODS USED BY LANs

LANs function because their transmission capacity is greater than any single terminal on the system. As a result, each station terminal can be offered a certain amount of time on the LAN, like a time-sharing arrangement. To economize on this small window of opportunity, stations organize their messages into compact packets that can be quickly distributed. When two messages are sent simultaneously, they could collide on the LAN causing the system to be temporarily disrupted. Busier LANs usually utilize special software that virtually eliminates the problem of collisions by providing orderly, non-contention access.

The transmission methods used on LANs are either baseband or broadband. The baseband medium uses a high-speed digital signal consisting of square wave DC voltage. While it is fast, it can accommodate only one message at a time. As a result, it is suitable for smaller networks where contention is low. It also is very simple to use, requiring no tuning or frequency discretion circuits. This transmission medium may be connected directly to the network access unit and is suitable for use over twisted wire pair facilities.

By contrast, the broadband medium tunes signals to special frequencies, much like cable television. Stations are instructed by signaling information to tune to a specific channel to receive information. The information within each channel on a broadband medium may also be digital, but they are separated from other messages by frequency. As a result, the medium generally requires higher capacity facilities, such as coaxial cable. Suited for busier LANs, broadband systems require the use of tuning devices in the network access unit that can filter out all but the single channel it needs.

THE FILE SERVER

The administrative software of the LAN resides either in a dedicated file server; in a smaller, less busy LAN; or in a personal computer that acts as a file server. In addition to performing as a kind of traffic controller, the file server holds files for shared use in its hard drives, administers applications such as the operating system, and allocates functions.

When a single computer is used as both a workstation and a file server, response times may lag because its processors are forced to perform several duties at once. This system will store certain files on different computers on the LAN. As a result, if one machine is down, the entire system may be crippled. If the system were to crash due to undercapacity, some data may be lost or corrupted.  The addition of a dedicated file server may be costly, but it provides several advantages over a distributed system. In addition to ensuring access even when some machines are down, its only duties are to hold files and provide access.

OTHER LAN EQUIPMENT

LANs are generally limited in size because of the physical properties of the network including distance, impedance, and load. Some equipment, such as repeaters, can extend the range of a LAN. Repeaters have no processing ability, but simply regenerate signals that are weakened by impedance. Other types of LAN equipment with processing ability include gateways, which enable LANs operating dissimilar protocols to pass information by translating it into a simpler code, such as ASCII. A bridge works like a gateway, but instead of using an intermediate code, it translates one protocol directly into another. A router performs essentially the same function as a bridge, except that it administers communications over alternate paths. Gateways, bridges, and routers can act as repeaters, boosting signals over greater distances. They also enable separate LANs located in different buildings to communicate with each other.

The connection of two or more LANs over any distance is referred to as a wide area network (WAN). WANs require the use of special software programs in the operating system to enable dial-up connections that may be performed by telephone lines or radio waves. In some cases, separate LANs located in different cities—and even separate countries—may be linked over the public network.

LAN DIFFICULTIES

LANs are susceptible to many kinds of transmission errors. Electromagnetic interference from motors, power lines, and sources of static, as well as shorts from corrosion, can corrupt data. Software bugs and hardware failures can also introduce errors, as can irregularities in wiring and connections. LANs generally compensate for these errors by working off an uninterruptable power source, such as batteries, and using backup software to recall most recent activity and hold unsaved material. Some systems may be designed for redundancy, such as keeping two file servers and alternate wiring to route around failures.

Security problems can also be an issue with LANs. They can be difficult to manage and access because the data they use is often distributed between many different networked sources. In addition, many times these data are stored on several different workstations and servers. Most companies have specific LAN administrators who deal with these issues and are responsible for the use of LAN software. They also work to backup files and recover lost files.

PURCHASING A LAN

When considering if a LAN is suitable for a business, several things must be considered. The costs involved and the administrative support needed often far exceed reasonable predictions. A complete accounting of potential costs should include such factors as purchase price of equipment, spare parts, and taxes, installation costs, labor and building modifications, and permits. Operating costs include forecasted public network traffic, diagnostics, and routine maintenance. In addition, the buyer should seek a schedule of potential costs associated with upgrades and expansion and engineering studies.

The vendor should agree to a contract expressly detailing the degree of support that will be provided in installing and tuning up the system. In addition, the vendor should provide a maintenance contract that binds the company to make immediate, free repairs when performance of the system exceeds prescribed standards. All of these factors should be addressed in the buyer's request for proposal that is distributed to potential vendors.  LANs can also be purchased for home use. Initially, these kits were expensive and slow and transmitted data via the phone lines in the home. New products have emerged that are faster, more affordable and use wireless technology to allow multiple computers to share printers and perform other LAN functions. This technology allows phone lines, cable connections, and LANs to be used simultaneously and is perfect for a small business owner who works out of his or her home.