PC ler arasındaki veri iletişimini sağlayan bir yazılım

Abstract

Bu çalışmada, hiç bir ek donanıma gereksinim olmadan birden fazla kişisel bilgisayarın veri alışverişinde bulunabilmesi üzerine geliştirilen bir ' yazılım sunulmuştur. Bu yazılım, genel olarak, kişisel bilgisayarların birbirleriyle haberleşmesine yönelik olmakla birlikte gözönünde bulundurulması gereken önemli bir nokta da ; bu haberleşme sağlanırken bilgisayarların salt diğer bilgisayarlardan gelen mesajı algılaması ya da cevap vermesi değil, bu işlemleri yaparken kendi ortamındaki mevcut işe devam edebilmesidir. Bu yazılım sayesinde birden fazla kişisel bilgisayar çoklu hizmet özelliğini kullanarak birbirleriyle veri alışverişinde bulunabilecektir. Bu yazılımın geliştirilmesinde amaç olarak küçük işletmelerde en düşük maliyetle bir mini bilgi ağının kurulabilir ligi esas alınmıştır. Bu çalışmadaki yazılımlar, kişisel bilgisayarlarda DOS (Disk Operating System/Disk İşletim Sistemi) ile çalışmaktadırlar. Haberleşme RS-232 seri port vasıtası ile yapılmaktadır, iki makine arasında kullanılan bağlantı elemanları standart, ucuz ve piyasada rahatlıkla bulunabilen malzemelerden (kablo ve konnektörler ) ibarettir. Zaten günümüzde pazarlanan tüm PC lerde en az bir tane RS-232 seri port mevcut olmakla beraber bu sayı artırılabilmektedir. Bu çalışmada bir ana makine ve iki kullanıcı makine esas alınmıştır. Ancak iki olan kullanıcı sayısı yine piyasada çok ucuza satılan RS-232 seri portları vasıtasıyla artırılabilir. Tabiiki kullanıcı sayısı arttıkça sistemin verimliliği düşecektir. Bu sistemin ideal çalışma ortamı bir ana makine ve iki kullanıcı makine olarak tasarlanmıştır.

Many organization already have a substantial number of computers in operation, often located far apart. For example, a company with many factories may have a computer at each location to keep tracck of inventories, monitor productivity, and do the local payroll. Initially, each of these computers may have worked in isolation from the others, but at some point, management may have decided to connect them to be able to extract and correlate information about the entire company. Put in slightly more general form, the issue here is resource sharing, and the goal is to make all programs, data, and equipment available to anyone on the network without regard to the physical location of the resource and the user. In other words, the mere fact that a user happens to be 1000 km away from his data should not prevent him from using the data as though they were local. Load sharing is another aspect of resource sharing. This goal may be summarized by saying that it is an attempt to end the "tyranny of geography". A second goal is to provide high reliability by having alternative sources of supply. For example, all files could be replicated on two or three machines, so if one of them is unavailable (due to a hardware failure), the other copies could be used. In addition, the presence of multiple CPUs means that if one goes down, the others may be able to take over its work, although at reduced performance. For military, banking, air traffic control, and many other applications, the ability to continue operating in the face of hardware problems is of great importance. Another goal is saving money. Small computers have a much better price / performance ratio than large ones. Mainframes are roughly a factor of -ten faster than the fastest single chip microprocessors, but they cost a thousand times more. This imbalance has caused many systems designers to build systems consisting of powerful personal computers, one per user, with data kept on one or more shared file server machines. This goal leads to networks with many computers located in the same building. Such a network is called a LAN (Local Area Network) to constrast it with the farf lung WAN (Wide Area Network), which is also called a long haul network. A closely related point is the ability to increase system performance gradually as the workload grows just by adding more processors. With central mainframes, when the system is full, it must be replaced by a larger one, usually at great expense and with even greater disruption to the users. Yet another goal of setting up a computer network has little to do with technology at all. A computer network can provide a powerful communication medium among widely separated people. Using a network, it is easy for two or more people who live far apart to write a report together. When one author makes a change to the document, which is kept online, the others can see the change immediately, instead of waiting several days for a letter. Such a speedup makes cooperation among far-flung groups of people easy where it previously had been impossible. In the long run, the use of networks to enhance human-to-human communication may prove more important than technical goals such as improved reliability. In the following figure we give a classification of multiple processor systems arranged by physical size. At the top are data flow machines, highly parallel computers with many functional units all working on the same program. Next come the multiprocessors, systems that communicate by exchanging messages. Finally, the connection of two or more networks is called internetworking. VI Replacing a single mainframe by workstations on a LAN does not make many new applications possible, although it may improve the reliability and performance. IN contrast, the availability of a (public) WAN makes many new applications feasible. Some of these hew applications may have important effects on society as a whole. To give an idea about some important uses of computer networks, we will now briefly look at just three examples; access to remote programs, access to remote databases, and value-added communication facilities. A company that has produced a model simulating the world economy may allow its clients to log in over the network and run the program to see how various projected inflation rates, interest rates, and currency fluctuations might affect their businesses. This approach is often preferable to selling the program outright, especially if the model is constantly being adjusted or requires an extremely large mainframe computer to run. Another major area of network use is access to remote databases. It may soon be easy for the average person sitting ta home to make reservations for airplanes, trains, buses, boats, hotels, restaurant, theatre, and so on, anywhere in the world with instant confirmation. Home banking and the automated newspaper also fall in this category. Present newspaper affer a little bit of everything, but electronic ones can be easily vii tailored to each reader's personal taste, for example, everything about computers, the major stories about politics and epidemics. Next step beyond automated newspaper (plus magazines and scientific journals) is the fully automated library. Depending on the cost, size, and weight of the terminal, the printed word may become obsolete. Skeptics should take note of the effect the printing press had on the medieval illuminated manuscript. All these applications use networking for economic reasons; calling up a distant computer via a network is cheaper than calling it directly. The lower rate is possible because a normal telephone call ties up an expensive, dedicated circuit for the duration of the call, whereas access via a network ties up long-distance lines only while data are actually being transmitted. A third category of potential widespread network use is as a communication medium. Computer scientists already take it for granted that they can send electronic mail from their terminals to their colleagues anywhere in the world. In the future, it will be possible for everyone, not just people in the computer business, to send and receive electronic mail. Furthermore, this mail will also be able to contain digitized voice, still picture and possibly even moving television and video images. One can easily imagine children in different countries trying to learn each other's languages by drawing a picture of a child on a shared screen and labeling it girl, jeune fille, or meisje. Electronic bulletin board systems already exist, but these tend to be used by computer experts, are oriented towards technical topics, and are often limited in geographic scope. Future systems will be national or international, be used by millions of nontechnical people, and cover a much broader range of sunjects. Using a bulletin board may be as common aB reading a magazine. It is sometimes said that there is a race going on between trasportation and communication, and whichever one wins will make the other unnecessary. Using a computer network as a sophisticated communication viii system may reduce the amount of traveling done, thus saving energy. Home work may become popular, especially for part-time workers with young children The office and school as we now know them may disappear. Stores may be replaced by electronic mail order catalogs. Cities may disperse, since high quality communication facilities tend to reduce the need for physical proximity. The information revolotuion may change society as much as the industrial revolution did. The above discussion of computational costs neglects the cost of software. While the art of software design has been improving, the improvement is partly counter balanced by the increasing cost of good software engineers. When software can be replicated, however, the cost per unit goes down inversely with the number of replicas. Thus, even though software is a major cost of a new com puter system, the increasing market decreases its unit cost. Each advance in solid state technology decreases cost and increases the performance of computer systems; this leads to an increase in market, thus generating decreased unit software costs, leading, in a feedback loop to furher increases in market. Each new application, however, requires new specialized software which is initially expensive and which requires a user learning curve. Thus, it is difficult to forecast the details of the growth of the computer market and similarly of the data network market. The cost of transmitting data on a communication link from one point to another has also been dropping, but at a much slower rate than computational costs. The cost of a data link increases with the available data rate, but the increase is much less than linear in the data rate. Thus, the cost per transmitted binary symbol decreases with the data rate of the link. It is often economically advantageous for many users to share one high-rate data link rather than having separate data links for each. One result of sharing high-speed communication links is that the cost of sending data from one point to another increases less than linearly with the geogrraphic separation of the points. This occurs because the communication path could include a short link to a shared long-distance, high speed link and then another short link to the destination. Estimating the cost of transmission facilities is XX highly specialized and complex. The cost of a communication link depends on whether one owns the facility or leases it; with leasing, thee cost depends on the current competitive and regulatory situation. The details of communication cost will be ignored in what follows, but there are two overall effects of these costs that are important. First, for wide area networks, the cost of a network is presently dominated by transmission costs. Thus, it is desirable to use the the communication links efficiently, perhaps at added computational costs. The sporadic nature of most data communication, along with the high cost of idle communication links, have led to the development of packet data networks which can increase utilization o the links. Second, for local area networks, the cost of a network iss not dominated by transmission costs. Coaxial cable and even a twisted pair of wires can achieve relatively high-speed communication at modest cost in a small geographic area. The use of such media and the desire to avoid relatively expensive switching have led to a local area network technology in which many nodes share a common high speed communication mediumon a shared multiaccess basis. The software presented in this study has been developped on a data exchange basis available among several personal computers without the need of an additional hardware. Although this software is generally focused on the communication among personal computers, another point which should be considered is that while this communication is provided, th PC should not only perceive or respond to the message from the other PCs but it should continue the present job which it's performing. Due to this software several PCs will be able to exchange using their multitasking facility. The aim of developping this software is to establish a mini network of low cost in the small enterprises. The softwares presented in this study, are being used in personal computers by Disk Operating System (DOS). Communication is provided through serial port RS-232. The tools (such as cable and connector) connecting two machines are standard and cheap equipments easily available in the market. In all personal computers marketed nowadays, there is at least one serial port RS-232, but this number can be increased. Although the concern of this study is one main machine and two user machines, the number of user machines can be increased due to serial port RS-232 which is very cheap in t,he market. But the increase in the number of user machines will decrease the system performance. The ideal functionary environment of this system is planned for one main machine and two user machines. The programs in this study is written by Pascal programming language and basically they run as a sample network logic.