2.1.2 This page presents a simplified view of how the Internet evolved.
The history of computer networking is complex. It has involved many people from all over the world over the past 35 years. Presented here is a simplified view of how the Internet evolved. The processes of invention and commercialization are far more complicated, but it is helpful to look at the fundamental development.
In the 1940s computers were large electromechanical devices that were prone to failure. In 1947 the invention of a semiconductor transistor opened up many possibilities for making smaller, more reliable computers. In the 1950s large institutions began to use mainframe computers, which were run by punched card programs. In the late 1950s the integrated circuit that combined several, and now millions, of transistors on one small piece of semiconductor was invented. In the 1960s mainframes with terminals and integrated circuits were widely used.
In the late 1960s and 1970s smaller computers called minicomputers were created. However, these minicomputers were still very large by modern standards. In 1977 the Apple Computer Company introduced the microcomputer, which was also known as the Mac. In 1981 IBM introduced its first PC. The user-friendly Mac, the open-architecture IBM PC, and the further micro-miniaturization of integrated circuits led to widespread use of personal computers in homes and businesses.
In the mid-1980s PC users began to use modems to share files with other computers. This was referred to as point-to-point, or dial-up communication. This concept was expanded by the use of computers that were the central point of communication in a dial-up connection. These computers were called bulletin boards. Users would connect to the bulletin boards, leave and pick up messages, as well as upload and download files. The drawback to this type of system was that there was very little direct communication and then only with those who knew about the bulletin board. Another limitation was that the bulletin board computer required one modem per connection. If five people connected simultaneously it would require five modems connected to five separate phone lines. As the number of people who wanted to use the system grew, the system was not able to handle the demand. For example, imagine if 500 people wanted to connect at the same time.
From the 1960s to the 1990s the U.S. Department of Defense (DoD) developed large, reliable, wide-area networks (WANs) for military and scientific reasons. This technology was different from the point-to-point communication used in bulletin boards. It allowed multiple computers to be connected together through many different paths. The network itself would determine how to move data from one computer to another. One connection could be used to reach many computers at the same time. The WAN developed by the DoD eventually became the Internet.
2.1.3 Equipment that connects directly to a network segment is referred to as a device. These devices are broken up into two classifications. The first classification is end-user devices. End-user devices include computers, printers, scanners, and other devices that provide services directly to the user. The second classification is network devices. Network devices include all the devices that connect the end-user devices together to allow them to communicate.
End-user devices that provide users with a connection to the network are also referred to as hosts. These devices allow users to share, create, and obtain information. The host devices can exist without a network, but without the network the host capabilities are greatly reduced. NICs are used to physically connect host devices to the network media. They use this connection to send e-mails, print reports, scan pictures, or access databases.
A NIC is a printed circuit board that fits into the expansion slot of a bus on a computer motherboard. It can also be a peripheral device. NICs are sometimes called network adapters. Laptop or notebook computer NICs are usually the size of a PCMCIA card. Each NIC is identified by a unique code called a Media Access Control (MAC) address. This address is used to control data communication for the host on the network. More about the MAC address will be covered later. As the name implies, the NIC controls host access to the network.
There are no standardized symbols for end-user devices in the networking industry. They appear similar to the real devices to allow for quick recognition.
Network devices are used to extend cable connections, concentrate connections, convert data formats, and manage data transfers. Network devices provide extension of cable connections, concentration of connections, conversion of data formats, and management of data transfers. Examples of devices that perform these functions are repeaters, hubs, bridges, switches, and routers. All of the network devices mentioned here are covered in depth later in the course. For now, a brief overview of networking devices will be provided.
A repeater is a network device used to regenerate a signal. Repeaters regenerate analog or digital signals that are distorted by transmission loss due to attenuation. A repeater does not make intelligent decision concerning forwarding packets like a router or bridge.
Hubs concentrate connections. In other words, they take a group of hosts and allow the network to see them as a single unit. This is done passively, without any other effect on the data transmission. Active hubs concentrate hosts and also regenerate signals.
Bridges convert network data formats and perform basic data transmission management. Bridges provide connections between LANs. They also check data to determine if it should cross the bridge. This makes each part of the network more efficient.
Workgroup switches add more intelligence to data transfer management. They can determine if data should remain on a LAN and transfer data only to the connection that needs it. Another difference between a bridge and switch is that a switch does not convert data transmission formats.
Routers have all the capabilities listed above. Routers can regenerate signals, concentrate multiple connections, convert data transmission formats, and manage data transfers. They can also connect to a WAN, which allows them to connect LANs that are separated by great distances. None of the other devices can provide this type of connection.
The Interactive Media Activities will allow students to become more familiar with network devices.
The next page will introduce some common types of network topologies.
This page will introduce students to the most common physical and logical network topologies.
2.1.4 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 how the hosts access the media to send data. The physical topologies that are commonly used are as follows:
• A bus topology uses a single backbone 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 cables to a central point.
• An extended star topology links individual stars together by connecting the hubs or switches.
• A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls 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 broadcast 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 way as will be explained later in the course.
The second logical topology is token passing. In this type of topology, an electronic token is passed sequentially to each host. When a host receives the token, 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 examples of networks that use token passing are Token Ring and Fiber Distributed Data Interface (FDDI). A variation of Token Ring and FDDI is Arcnet. Arcnet is token passing on a bus topology.
The diagram in Figure shows many different topologies connected by network devices. It shows a network of moderate complexity that is typical of a school or a small business. The diagram includes many symbols and networking concepts that will take time to learn.
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