As discussed earlier, networks are used to connect communication parties — the senders and the receivers. The communication relationships among these parties are different in different scenarios. In a broadcast network, there is one sender, but there are many receivers. The communication is one-way — that is, it's a simplex transmission. In most data communication sessions, there is one sender and one receiver. In a telephone conference call, there is a group of people who are both senders and receivers. In the following, we will focus on the common case of one-to-one communication.
A network is made of components of nodes and links. Let's explore some properties of the transportation network and the computer network.
nodes
A node is the device where two or more links are terminated (or 'originate'). A node serves to switch from an incoming link to an outgoing link. The nodes are sometimes referred to asswitching or routing hubs.
links
A link is a connection between two nodes. The two nodes so connected become adjacent nodes. A link serves to connect, or to link up, the two nodes when there is a need for data flow between the two nodes. The links are sometimes referred to astransmission links or channels.
The need for network
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Transportation network
Using a transportation network as an example, if there were only isolated villages, each of which was self-sufficient, there would only be a need for small paths within each village. There would be a limited need for roads to connect a village to the outside world. With the development of the economy with specialization and division of labour, some villages grow into cities with more people and business activities. We begin to need roads to carry raw material, goods and people around between cities and villages.
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Computer network
The same is true for computer networks. If there were no demand for data transfer between computers, there would be no computer networks. But we have big computers that need connections to other computers or terminals. Hence, there is a need for a computer network. With more specialization of processing in different computers, the need for a computer network grows. The processing of data is distributed among different computers, and semi-processed data are passed between computers. This is called distributed processing, which has become common nowadays.
Network criteria
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Transportation network
A transportation network has criteria of performance. For example, one key performance parameter of a highway is the time a car takes to travel between the two connected cities. Of course, it depends on the length of the road, the speed limit allowed, and other factors like traffic conditions. A transportation network has other criteria such as reliability and security. A highway is reliable if there is no failure or breakdown, and drivers can use its service under all circumstances, even during extreme weather conditions. A highway is secure if external unauthorized access can be denied and, if there is an accident, it will be quickly identified, isolated, and controlled so as not to affect normal traffic flow.
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Computer network
Similarly, a computer network has criteria of performance, reliability and security. One key performance parameter is the transit time between the two connected computers. This depends on the speed of the transmission medium and other factors like the number of simultaneous users. A computer network has the other criteria of reliability and security too. A computer network is reliable if it is available whenever the users want service. This means there is no breakdown, and even if there is a breakdown, it should be able to recover from the breakdown within the shortest time. Computer security covers a wide range of aspects, including prevention of unauthorized access and data privacy. Security will be discussed in greater detail in Unit 9.
Connection and physical structure
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Transportation network
A transportation network has different types of connection and different types of physical structure. Connecting roads can be point-to-point highways or multi-entry and multi-exit highways. Physical structures can be a mesh-connected (i.e. fully-connected) network with roads linking each pair of cities. This is complicated when there are many cities. Physical structures can take the form of a star, with roads radiating from a central big city to connect to the adjacent cities. They can take the form of big roads with side-ways, or ring-roads around the outer part of the city.
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Computer network
Computer networks similarly have different types of connection. They have point-to-point and multi-point (also called multi-drop) connections. Likewise, they have different types of physical topology. Physical topology can take the form of mesh, star, bus and ring.
Categorization
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Transportation network
Transportation networks can be categorized into within a city (metropolitan), intercity, interstate or international.
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Computer network
Similarly, computer networks can be categorized into local area networks (LAN) for a network within the same office, metropolitan-area networks (MAN) for a network within the same metropolitan area, and wide-area networks (WAN) for a network linking computers spread far apart.
In the following sections, we will discuss about the above basic concepets of network in more detail.
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