Network Scope


Networks can also be classified by their geographic scope and the type
of services provided into Local Area Networks(LANs) and Wide Area
Networks(WANs).


Local Area Networks (LANs)

A Basic Workgroup LAN

A more advanced workgroup LAN


Turban, McLean and Wetherbe (1996) theorized that a local area
network(LAN) is a system for interconnecting two or more communicating
devices within 2.000 feet which supports full connectivity so that
every user device on the network has the potential to communicate with
any other device.

It is recommended for applications requiring high volumes of data and
high transmission speeds. For instance, because a picture consumes so
many bits of information, an organization may require a LAN for video
transmissions and graphics.

LANs are totally controlled, maintained, and operated by end users.
This produces the advantage of allowing user control, but it also means
that the user must know a great deal about telecommunications
applications and networking.

LANs allow organizations to share expensive hardware and software. For
instance, several PCs can share a single printer by being tied together
in a LAN. LANs can promote productivity because users are no longer
dependent on a centralized computer system or on the availability of a
single peripheral device such as a printer. Finally, there are many new
applications such as electronic mail, graphics, video conferencing, and
on-line applications requiring high-capacity networks.

The most common use of LANs is for linking personal computers within a
building or office to share information and expensive peripheral
devices such as laser printers. Another popular application of LANs is in factories, in which they link computers and computer-controlled machines.

LANs are controlled by network operating systems, which control the flow of data between the devices on the network and handle requests for data. These operating systems prevent a node from sending a message until the network is ready to process it correctly and take care of busy situations where a node cannot receive a message because it is not ready.

LANs may be configured in a number of ways. A network topology is the pattern of connections between the devices on a network. There are three representative network topologies: The star, ring, and bus.

LANs use a variety of methods for their internal communication, one of the most common of which is token passing. A token is a bit pattern that circulates between nodes. A node appends the data to the token to transmit data. When the token arrives at the destination node, it adds a notation that the data has been received, and the token continues back to the sending node. The sending node removes the packet, and the token continues circulating.

Most LANs use either twisted-pair telephone wire or coaxial cable. Fiber-optic cable is use rarely for LANs because it is difficult to install and expensive. LANs use two types of data transmission: baseband and broadband. Most LANs are baseband networks. In baseband networks, the entire capacity of the cable is used to transmit a single digitally coded signal. Ethernet is a common baseband network that uses coaxial cable and operates at 10Mbps. In broadband networks, the capacity of the cable is divided into separate frequencies to permit it to carry several signals at the same time.

It is often necessary to link LANs to other networks. This can be accomplished using a bridge or a gateway, both of which are a combination of hardware and software. Abridge links two compatible networks to enable data to pass between them. With a bridge in place, the two LANS operate like one from a user's viewpoint. A gateway links two incompatible networks. Because of the incompatibility, the gateway must convert data formats and transmission sequences before the two networks can be linked.


Wide Area Networks (WANs)




Wide area networks(WANs) are networks that generally span distances greater than one city. They include regional networks such as telephone companies or international networks such as global communications services providers. They usually have very large capacity circuits with many communications processors to use these circuits efficiently.

WANs are used for many different purposes. Some are designed as a communications backbone for a large distributed organization. For example, Texas Instruments has a WAN that helps engineers around the world collaborate. It includes 23 mainframes, 2,000 minicomputers, 31,000 terminals, and 36,000 desktop computers.

Other WANs focus on a particular transaction processing application, such as taking orders, making reservations, or tracking packages. Many WANs are used to transfer and consolidate corporate data, such as daily transaction summaries from branches.

In addition, WANs can link to workstations or terminals through LANs. The LANs perform local data processing and link to the WAN for data needed or provided beyond the local environment. An example of this approach is the long-term direction of Apollo, the United Airlines reservation system. In this system, LANs are gradually being installed at travel agencies to replace dumb terminals linked to minicomputers and mainframes. The LANs maintain local copies of reservation data. New reservations are uploaded through the WAN, and travel data is downloaded to the LANs. With this arrangement, the reservationist at the travel agencies can keep working even if the WAN or one of the central computers is down.

There are important developments occurred in wireless WANS. For example, IBM and Motorola developed the ARDIS wireless network service to reduce the separation between the office and the field and thereby improve the efficiency of field service and sales work. The service permits employees to communicate with the office using handheld terminals that use radio waves to link to a network of 1,000 base stations covering most of the United States.

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