Networking

LAN: LOCAL AREA NETWORK
A LAN is basically interconnection of several computers by communication lines within a geographical limit. It may be described as a group of computers and other systems located reasonably close to one another, connected via communication links to share computing resources such as printers, storage devices etc. Also it can be defined as LAN is a network of computers connected by specific type of transmission media (cables etc.) and network interface cards and control by any of a number of network operating systems that support all necessary communication protocols & standards. LAN today acts as a problem solver. To allow communication, information sharing, collaboration among different users, LAN can be used. LAN is typically used in a small area, say, a building or an office complex. The need for LANs really cropped when PCs appeared on the computing scene. The hard disk drives and printers were expensive and most organizations could not or did not want to afford a large hard disk or a printer for every PC. The disk server was the first real example of resource-sharing. It allowed a few PC users access to a single common hard disk drive. The operating system used on the first disk server was CP/M. CP/M was the force-runner to DOS, the most popular operating system today. The software used with the disk served divided the hard disk into volumes. Each volume acted like a private hard disk drive (logical drive) for a specified user. The software also created a public volume that could be accessed by all users. Today, disk servers have been replaced by File Servers. On a file server, the need to know which volume stores which file does not exist. Further, a file server also allows two different users to share the same file. Unauthorized access can, however, be prevented very effectively using the different security features that a file server provides. A point to be noted before we discuss anything more is that there are many different versions of LAN being sold in the market by many different vendors. Most of the LANs support live or more terminals. There are LANs which support even one hundred terminals. The figure shows several terminals attached to the file server. In many networks these terminals, can be either dumb or intelligent. A dumb terminal is nothing but a keyboard and monitor and a small interface unit which links it to the host computer. The used siding at the dumb terminal can access the host, but since the terminal does not have a hard disk or a floppy disk drive he has to store all his data on the file server itself. In fact, the dumb terminal does not have a hard disk or a floppy disk drive he has to store all his data on the file server itself. In fact, the dumb terminal does not have any processing power. An intelligent terminal however is just like a PC (in fact a PC, in most cases), with its own processor, memory and hard-disk drive. It can be used to work with the files stored in the host or it can even work as a stand-alone unit. In a LAN however, every terminal is intelligent.
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Network Basics
When dealing with networks one has to understand the different types and how they are used along with the advantages and disadvantages of each. A mesh network allows for redundancy in that if one node goes down all terminals are able to still communicate with other terminals and networks by taking several possible paths. The mesh network will be more complex and time consuming to install, but is also the more reliable than other topologies. If each terminal having reliable connectivity is mission critical, the knowledge is available and cost is not really a factor then a mesh would be the preferred network. Bus topology is probably the least preferred topology to implement. If one node is removed or non-operational on the bus then all terminals will not be able to utilize the network. According to Annabel Dodd (2005) “Prior to hubs, each device in a LAN was wired to another device in a “bus” arrangement.”(p. 26) Circumstances that may play a factor in this use of this particular topology would be cost savings in regards to cable and the importance of that particular network to the department's mission, as well as the network’s size as well as security. Required speed, number of attachments, cable length, and use play a factor in determining the use of the bus topology. Bates & Gregory (2000) tell us that bus speed is limited to “10Mbits/s with effective throughput of 3.3-4 Mbits/s” and that attachments cannot exceed “1024 addressable nodes” and are “limited to cable lengths of 1500 meters.” They continue on to say that “collisions when the network gets 40% busy, resulting in fewer throughputs.” It seems that the bus topology is also the least secure in respect to the fact that all transmissions are a broadcast to the entire network. The star topology is network architecture. Circumstances to install this type of network would be similar to those of the bus topology. The star topology would be primarily used with hubs and switches. This topology allows for all terminals to continually communicate on the network when a terminal is dropped or goes down. This type of topology is important when connectivity of each terminal in the network is important and is mission critical to operations. Combinations of all the various topologies will be seen in MAN, WAN, and even some larger LANs. An example would be a college campus in which each department uses a star topology internally, but meshes with all the other outlying departments. So in the case of a college campus all departments would still be able to access records from other departments on campus when one or more departments lose connectivity.
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