Data Bases • Custom Term Papers • Free Term Papers • Free Research Papers • Free Essays • Free Book Reports • Plagiarism? Links • Top 100 Term Paper Sites • Top 25 Essay Sites • Top 50 Essay Sites • Search 97,000 Papers @ DirectEssays.com • Search 101,000 Papers @ ExampleEssays.com • Search 90,000 Papers @ MegaEssays.com Free Essays • Term Paper Sites • Chuck III's Free Essays • Free College Essays • TermPaperSites.com • Free Essays • My Term Papers • Essay World • Planet Papers	Search Lots of Essays Back to Subjects - Computers The OSI Model and The Pony Express The OSI Model and The Pony Express The Open Systems Interconnection (OSI) reference model is essential to the world of computer networking. The model was created in 1977 by the International Standards Committee, in response to a difficulty that was facing computer networkers at the time (Shelly, Cashman, and Serwatka 142). In order to understand the difficulty, one must first realize that computer networks consist of computer hardware, the software that is to be used in conjunction with this hardware, and the medium (such as wiring or cabling) that will interconnect the computing devices that are in the network. The computer networker’s job is to determine which hardware, software, and medium types will create the network that will best suit his client’s needs. Then, the networker must combine these elements into a functional system of interconnected computers (Fortino and Villeneuve 112). It was in attempting this latter task that the computer networker of the late 1970s often found himself in a pickle. The problem was that each vendor of computing equipment had developed his own unique set of products; products that were incompatible with the products of other vendors. This incompatibility made it very difficult for a computer networker to combine the various network components into an operational computer network (Stamper 27). The OSI model provided for a solution to this problem. The model organized those tasks that are essential for computer network operation, into seven groups. These groups were called layers. All manufacturers of computing equipment were recommended to make their products compliant with the OSI model. This meant that each computing product was to perform the functions associated with a specific layer within the model; any method of accomplishing these functions was acceptable. In a network composed of equipment that operated according to OSI guidelines, a separate product would be used to perform each layer of OSI functionality. Thus, all elements of the network would be aware of the specific jobs performed by every other network element; this would allow for compatibility between networking products that were created by different vendors. By designing networks according to OSI guidelines, the networker was able to combine any group of products, made by any number of vendors, into a functioning computer network (Stamper 28). The OSI model was an extremely successful solution to the problem of multi-vendor product integration. In fact, the OSI model was such a perfect solution to the difficulty that the model is still used today (Shelly, Cashman, and Serwatka 142). One may wonder why the OSI model is so successful. The author believes that the OSI model’s manner of network-function organization is the cause of OSI’s great success: the model defines clearly each function that is essential in inter-computer communications, as well as the relationship between each of these functions. The author will further contend that the scope of OSI’s method of communication-functions organization is not limited to inter-computer communications. Rather, the model’s clear method of organizing the elements of a communications system is an efficient tool for analyzing the elements of any method of communication. The author will now demonstrate this point by evaluating the functionality of the Pony Express according to the layers of the Open Systems Interconnection reference model. By prefacing the analysis of the Pony Express with some background information, a better analysis of the Pony Express system can be performed. Therefore, a short description of the Pony Express will be included at this point. The Pony express opened for business in April 1860, and was operational for sixteen months (Oslin). Its purpose was to transport large volumes of mail across the mid-western and Rocky Mountain regions of the United States, areas that were unpopulated at the time (Dicerto 4). Furthermore, the Pony Express pledged to do this in a timely fashion. The Pony Express bore this challenge very successfully; in a period of ten days, it would relay mail between St. Joseph, Missouri, and Sacramento, California. When compared with the regular three-week transport time for mail that was traversing the United States, the Pony Express operated at lightning speed (Dicerto 3). The rapid speed of Pony Express mail delivery was very important in 1860: 1860 was a year marked by the brink of civil war; a year during which efficient cross-country communications was a necessity (Pony Express). Bearing this background information in mind, we can discuss the first layer of OSI functionality and its manifestation in the Pony Express system. This top layer of the OSI reference model is called the Application Layer. The Application Layer requires that a communications system include a device or method that will create a message; this message is that which one wishes to communicate to another. Furthermore, this message must be of a specific type; in computing, this message may be a word-processed document, a sound file, or a graphics file, to name a few examples (Stamper 28). In the Pony Express system of communication, Application Layer protocols state that messages were to be printed matter (Dicerto 6). Material that was acceptable for Pony Express delivery included urgent messages and news-breaking events. Such communications were prevalent during 1860-1861. Furthermore, these matters needed to be communicated to the Western United States in a high-speed manner, due to the great internal conflict that was dividing the United States (Dicerto 7). Examples of Pony Express-acceptable communications include the news of President Lincoln’s election, and the news of the outbreak of the Civil War. Both of these items were communicated to the Western US by means of the Pony Express (Pony Express). The next layer of the OSI Model is called the Presentation Layer (Fortino and Villeneuve 113). The device that performs Presentation Layer tasks must do any necessary message formatting. If the message content is to be translated or encrypted, for example, Presentation Layer devices must make the necessary changes to the message (Stamper 28). Pony Express message formatting requirements stated that no packages were to be delivered by the Pony Express, rather only letters and telegraphs would be delivered (Dicerto 5). The reason for this is that bulky or heavy packages would have put a serious hamper on the Pony Express’ ability to guarantee speedy delivery. The Session Layer of the OSI Model executes the next group of communication- essential functions. Session Layer functions dictate that there must be a point of message transference. At this point, the message will be transferred from the premises of the sender to the premises of the communications system, as well as, to the premises of the receiver from the premises of the communications system. It is the sender’s/ receiver’s job to affect this change of premises (Fortino and Villeneuve 113). The Session Layer requirements of the Pony Express specified that an individual must bring his mail or telegraph to the appropriate local station. At this local postal station, the message would be accepted into the mail system (Dicerto 12). It was removed from the system, in order to be delivered to the appropriate party, at a similar station on the opposite shore of the US. One such station was the Alta telegraph station in Sacramento (Dicerto 36). The Transport Layer of the model can be paired with the Session Layer. The Transport Layer is the communication service’s side of the point of message transference. That is, the Transport Layer protocols state that the communication system must have a method of receiving the message from the sender. In addition, the system, upon receiving the message from the sender, must prepare it to be transported to its correct destination. Similarly, the system must have a method of giving the message to the receiver. This entails preparing the message for removal from the communications-system upon arrival at its destination (Shelly, Cashman, and Serwatka 144). The Pony Express implemented Transport Layer protocols by providing a post office or telegraph office that received letters from the senders. This post or telegraph office sorted the letters so that they could be sent to their appropriate destinations. At the destination post office or telegraph office, the letter was sent out by mailman to be given to the recipient (Dicerto 12). The job of the OSI Network Layer is to route the message along its path, via intermediate destinations, to its final destination (Shelly, Cashman, and Serwatka 145). While routing the message, the Network Layer functions perform Flow Control; that is, they control the speed of message transmission or transport (Stamper 29). Network Layer functions are also responsible for ensuring that the message will not be tampered with along the route (Stamper 29). In order to implement Flow Control and tamper prevention measures, the Network Layer may need to add information regarding these procedures to the message itself. This additional information will be interpreted by the Network Layer devices, and removed from the original message by these devices (Fortino and Villeneuve 114). In the Pony Express system, implementing Network Layer functions entailed routing the mail to St. Joseph, Missouri, by train from points east. In St. Joseph, the mail was given over to a Pony Express horse and rider (Dicerto 10). What did the Pony Express rider do with the mail? There were 190 stations that spanned the 1,966 miles from St. Joseph to Sacramento. The average distance between stations was 25 miles. The Pony Express rider would bring the mail from one station to the next on the route, until the mail had traveled the entire 1,966-mile distance (Dicerto 15). This one route was the only possible way by which the Pony Express letters could travel; this makes the Pony Express system an example of Static Routing (Shelly, Cashman, and Serwatka 145). These in-route stations were of two types: there were relay stations and home stations. At the relay station, a fresh horse would continue along for the next leg of the journey, until the next relay station. The horse that had carried the mail up to that relay station was given a chance to rest and eat. All of the 190 stations served as relay stations (Dicerto 16). Some of these stations also served as home stations. In the home stations, the Pony Express Rider himself would be given a chance to rest and eat, and another Rider went along the next segment of the route (Dicerto 17). Horses and riders were frequently replaced, so as to ensure that the letters would arrive at their destinations in as short a time as possible (Dicerto 17). This effort to minimize message transport time is an example of Network Layer Flow Control. Furthermore, the relay and home stations had to be kept in functioning order, so that the horses and Riders at each station could be best prepared to continue along the route. This maintenance sometimes entailed extraordinary measures; for example, in some stations that were located in the desert, drinking water for the horses and station manager was not accessible, and this water had to be brought from remote locations by pack mule (Dicerto 33). Since these measures were necessary to maintain the speed of the Pony Express, the author believes that they are also aspects of Flow Control. An additional example of Flow Control implementation in the Pony Express system involves the speed at which a rider would perform his duties at the relay stations. From the time of his arrival at a station, until he had changed horses together with the letters and left the station, was approximately two minutes. The new horse was prepared for the Rider by the time of his arrival, so that the only thing that the Rider had to do was strap the mail to the horse, mount the horse, and continue on his journey. (The mailbags were made in a manner so that they could be removed from one animal and placed on another in a minimal amount of time). At home stations, Riders were switched in a speedy fashion, as well (Dicerto 16). Another aspect of Flow Control performed by the Pony Express was the method of keeping track of the times at which the Riders arrived at each station. A time card would travel with the Rider, and the station keeper at each station would indicate the time at which the Rider arrived at that station. This insured that the Riders were transporting the letters at optimum speed. Only the station keepers had access to this time card because of the manner in which it was stored (as will be discussed in the section dealing with the Physical Layer). This limited access to the time card ensured proper in-route protection of the message it contained (Dicerto 24). The letters were locked up in a manner so that they could only be accessed at military forts (as will be further detailed in the Physical Layer section); this is a further example of protection performed by the Pony Express (Oslin). Sacramento was the end of the Pony Express’ route. Letters that were to continue on to San Francisco were loaded onto a steamboat for this final leg of the journey (Dicerto 27). The next layer within the OSI Model is entitled the Data Link Layer. It is the job of the Data Link layer to make sure that the contents of the message do not become corrupted while the message is transported from one intermediate destination, along the route, to the next (Stamper 29). In the Pony Express, Data Link Layer tasks were the responsibility of the Rider. The Rider brought the letters from one station to the next, traveling 75 miles per day. During this letter transport, the rider made sure that the letters were not lost or destroyed along the way (Dicerto 18). However, the extent to which a Rider looked after the letters might be only as great as the rider’s own personal integrity and level of caring for the success of the Pony Express as a system. Therefore, measures were taken to ensure the integrity of the Rider himself. The Rider was made to swear to live a moral lifestyle for the duration of his employment with the Pony Express. The Rider was also made to swear to do his job to the best of his ability (Dicerto 25). The final layer within the model, is the Physical Layer (Stamper 30). The Physical Layer requires that there be a physical medium that the message can traverse. This physical medium must connect the sender to the receiver so that the message can be transported from its origin to its destination. In the Pony Express, the letters were physically transported in a sack called a mochila. The mochila was placed upon the saddle and held in place by the rider’s sitting on it. It had four pockets called cantinas. Two would hang over the left side of the animal, and two would hand over the right side. Two cantinas would be behind the Rider’s legs, and two in front of them. Three of the cantinas contained letters, and the fourth contained a time card upon which each station manager indicated the Rider’s arrival time at his station (Dicerto 33). All of the cantinas were kept locked. The station managers had the key for the one that contained the time card, and the other three (that contained the mail) could only be opened at a military fort (Dicerto 39). The Rider sat on the mochila, which was placed upon a saddle, which was strapped to the back of a horse. The horse carried the mochila with the letters and the Rider. The author has categorized and enumerated the elements of the Pony Express mail system. In doing this, a clear understanding of each factor in the Pony Express, its necessity, and its relation to the other factors, has been developed. However, it is only by using the OSI Model as the basis for categorization, that this clarity has been developed. Thus, the author has illustrated the OSI Model’s great usefulness as a tool for analyzing communication systems. It is now clear that the Open System Interconnection reference model is quite a powerful tool; this is the secret of its success in the field of computer networking, and this is also the key to its use in other areas of communication as well. Derfler, Frank J., Jr., and Les Freed. How Networks Work. California: Macmillian Dicerto, Joseph J. The Pony Express: Hoofbeats In The Wilderness. New York: Franklin Fortino, Andres, and Arnold Villeneuve. Networking Technologies: A Complete Guide To Passing The Novell CNE Exam. New York: The McGraw-Hill Companies, Introduction To Networking. Diskette. 1992. Networking Technologies. Diskette. 1992. Oslin, George P. “Pony Express.” Encyclopedia Americana. 1999 ed. “Pony Express.” Microsoft Encarta 98 Encyclopedia. CD-ROM. Microsoft, 1997. Settle, Raymond W., and Mary Lund Settle. Saddles and Spurs: The Pony Express Saga. Lincoln: University of Nebraska Press, 1955. Stamper, David A. Business Data Communications. 5th ed. Massachusetts: Addison Shelly, Gary B., Cashman, Thomas J., and Judy A. Serwatka. Business Data Communications: Introductory Concepts and Techniques. 2nd ed. Massachusetts: Course Technology, 1998. Bibliography: Word Count: 2835
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