The erly development of nerworks was disorganized in may ways. The early 1980s saw tremendous increases in the number and size of networks. As compaies realized the advantages of using networking technology, networks were added or expanded almost as rapidly as new network technologies were introduced.
By the mid-1980s, these companies began to experience problems from the rapid expansion. Just as people who do not speak the same language have difficulty communicating with eachother, it was difficult for networks that used different spesifications and implementations to exchange information. The same problem occurred with the companies that developed private or proprietary networking technologies. Propietary means that one or a small group of companies controls all usage of the technology. Networking technologies strictly following proprietary rules could not comunicate with tecnologies that followed different propietary rules.
To address the problem of network incompatibility, the international organization for standardization ( ISO ) researched networking models like Digital Equipment Corporation net ( DECnet ), system Network Architecture ( SNA ), and TCP/IP in order to find a generally applicable set of rules for all networks. Using this reserch, the ISO created a network model that helps vendors create networks that are compatible with other networks.
The open system interconnection ( OSI ) reference model released in 1984 was the descriptive network model that the ISO created. It provided vendors with a set of standards thet ensured greater compatibility and interoperability among various network technologies produced by companies around the world.
The OSI model has become the primary for network communications. Although there are other models in existence, most network vendors relate their products to the Osi reference model. This is specially true when they want to educate users on the use of their products. It is cinsidered the best tool available for teaching people about sending and receiving data on a network.
Network topology defines the structure of the network. One part of the topology definition is the physical topology, which is the actual layout of the wire or media. The other part is the logical topology, which defines hoe the hosts access the media to send data. The physical topologies that are commonly used are as follows :
A bus topology uses a single backbome cable that is terminated at both ends. All the hosts connect directly to this backbone.
A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.
A star topology connects all cable to a central point.
An extended star topology links individual star together by connecting the hubs or switches.
A hierarchical topology is similar to an extended star. How ever, instead of linking the hubs or switch together, the system is linked to a computer that control the traffic on the topology.
A mesh topology is implemented to provide as much protection as possible from interruption of service. For example, a nuclear power plant might use a mesh topology in the networked control systems. As seen in the graphic, each host has its own connections to all other hosts. Although the internet has multiple paths to any one location, it does not adopt the full mesh topology.
The logical topology of a network determines how the hosts communicate across the medium. The two most common types of logical topologies are broadcat and token passing.
The use of a broadcast topology indicates that each host sends its data to all other hosts on the network medium. There is no order that the stations must follow to use the network. It is first come, first serve. Ethernet works this wy as will be explained later in the course.
The second topology is token passing. In this type of topology, an electronic token is passed sequentially to each host. When a host receives the tokn, that host can send data on the network. If the host has no data to send , it passes the token to the next host and the process repeats itself. Two example of networks that use token passing are token ring and fiber distributed data interface ( FDDI ). A variation of token ring anf FDDI is Arcnet. Arcnet is token passing on a bus topology.
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