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LAN( Local Area Network)
What is LAN:
It actually stands for local area network. A network is a group of computers and other devices connected together so they can pass information back and forth. The local area network (LAN) is a network which is designed to operate over a small physical area such as an office, factory or a group of buildings. LANs are very widely used in a variety of applications.
LAN Definition:
The personal computers and workstations in the offices are interconnected via LAN to share resources. The resources to be shared can be hardware like a printer or softwares or data. A LAN is a form of local (limited- distance), shared packet network for computer communications. In LAN all the machines are connected to a single cable. The data rates for LAN range from 4 to 16 Mbps with the maximum of 100 Mbps. The term LAN can also refer just to the hardware and software that allows you to connect all the devices together. In this sense, Local Talk is one kind of LAN, Ethernet is another. (AppleTalk is the protocol for Local Talk.) The components used by LANs can be divided into cabling standards, hardware, and protocols. Various LAN protocols are Ethernet, Token Ring: TCP/IP, 5MB, NetBIOS, Fiber Distributed Data Interchange (FDDI) and Asynchronous Transfer Mode (ATM).
Why Use a LAN?
There are two basic reasons for developing a LAN: Information sharing and resource sharing. Information sharing refers to having users access the same data files, exchange information via e-mail, or use the Internet. For example, a single purchase order database might be maintained so all users can access its contents over the LAN.The main benefit of information sharing is improved decision making, which makes it generally more important than resource sharing. Resource sharing refers: To one computer sharing a hardware device (e.g., printer, an Internet connection) or software package with other computers on the network to save costs. For example, suppose we have 30 computers on a LAN, each of which needs access to a word processing package. One option is to purchase 30 copies of the software and install one on each computer. This would use disk space on each computer and require a significant amount of staff time to perform the installation and maintain the software, particularly if the package were updated regularly. An alternative is to install the software on the network for all to use. This would eliminate the need to keep a copy on every computer and would free up disk space. It would also simplify software maintenance because any software upgrades would be installed once on the network server; staff members would no longer have to upgrade all computers. In most cases, not all users would need to access the word processing package simultaneously. Therefore, rather than purchasing a license for each computer in the network, you could instead purchase ten licenses, presuming that only ten users would simultaneously use the software. Of course, the temptation is to purchase only one copy of the software and permit everyone to use it simultaneously. The cost savings would be significant, but this is illegal. Virtually all software licenses require one copy to be purchased for each simultaneous user. Most companies and all government agencies have policies forbidding the violation of software licenses, and many fire employees who knowingly violate them. One approach to controlling the number of copies of a particular software package is to use LAN metering software that prohibits using more copies of a package than there are installed licenses. Many software packages now come in LAN versions that do this automatically, and a number of third-party packages are also available. Types of LAN:
- It is possible to have a centralized or distributed type arbitration mechanism.
- The most popular example of bus topology is Ethernet (IEEE 802.3). It has a decentralized control and it operates at 10 or 100 Mbps.
- Computers on Ethernet can transmit whenever they want. If collision of their packets takes place, then they wait for a random time and retransmit their packets. Ring Topology:
- This is another broadcast topology.
- In a ring each bit propagates around on its own without waiting for the rest of the packet to which it belongs.
- Since it is a broadcast system, some rules are essential for arbitrating the simultaneous access to the ring.
- An example of ring based LAN is IEEE 802.5 (IBM token ring) operating at 4 and 16 Mbps. Static and dynamic broadcast networks:
- The broadcast networks are further classified into two types namely,
- Static networks and
- Dynamic networks.
- This classification is based on how the channel is allocated.
- In static allocation, each machine is allowed to broadcast only in its allotted time slot.
- But static allocation wastes the channel capacity when a machine does not want to transmit in its allotted time slot.
- Hence most of the systems try to allocate the channel dynamically i.e. on demand. LAN Applications and Benefits: LANs are used almost exclusively for data communications over relatively short distances such as within an office, office building or campus environment. LANs allow multiple workstations to share access to multiple host computers, other workstations, printers and other peripherals, and connections to other networks. LANs are also being utilized for imaging
applications, as well. They are also being used for video and voice communications, although currently on a very limited basis. LAN applications include communications between the workstation and host computers, other workstations, and servers. The servers may allow sharing of resources. Resources could be information, data files, e-mail, voice mail, software, hardware (hard disk, printer, fax, etc.) and other networks. LAN benefits include the fact that a high-speed transmission system can be shared among multiple devices in support of large number of active terminals and a large number of active applications in the form of a multi- user, multi-tasking computer network. LAN-connected workstations realize the benefit of decentralized access to very substantial centralized processors, perhaps in the form of mainframe host computer and storage capabilities (information repositories). Additionally, current technology allows multiple LANs to be inter-networked through the use of LAN switches, routers and the like. Disadvantages of LANs include concern for security of files and accounts.
