tailieunhanh - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 11

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 11 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 69 Tuesday May 20 2003 2 53 PM Networking Models 69 is responsible for a specific part of network communication. These layers interact with the layer above and below them only. This interaction very narrowly defines a layer s purpose. The two common network models that use layers are the Open System Interconnection OSI reference model and the TCP IP reference model. The OSI Reference Model The early development of LANs MANs and WANs was chaotic in many ways. The early 1980s saw tremendous increases in the number and size of networks. As companies realized the money they could save and the productivity they could gain by using networking technology they added networks and expanded existing networks almost as rapidly as new network technologies and products were introduced. By the mid-1980s these companies began to experience difficulties from all the implemented expansions. It became more difficult for networks that used different specifications and implementations to communicate with each other. These companies realized that they needed to move away from proprietary networking systems. Proprietary systems are privately developed owned and controlled. In the computer industry proprietary is the opposite of open. Proprietary means that one company or a small group of companies controls all usage of the technology. Open means that free usage of the technology is available to the public. To address the problem of network incompatibility and the inability to communicate with one another the International Organization for Standardization ISO researched different network schemes such as DECnet Systems Network Architecture SNA and TCP IP to find a set of rules. As a result of this research the ISO created a network model that would help vendors create networks that would be compatible and operate with other networks. The process of breaking down complex communications into smaller discrete tasks can be compared to the process of building an automobile. When