3.5 General information about communication networks

Communication networks - a set of technical means for the transmission and distribution of messages. Depending on whether special switching devices are present or absent in the network, there are distinguished switched and non-switched networks. The rules for building networks depend on the method of distribution and the type of messages transmitted.
Among non-switched networks, the most common methods of networking are: “common bus” (Figure 3.5, a), “ring” (Figure 3.5, b), fully connected network (“every with each”) (Figure 3.5, c). Such configurations are most typical for computer networks.

Each of the methods of networking has its advantages and disadvantages. Thus, in the structures of a common bus and a ring, all network participants use a common signal propagation environment and have unique features that are characteristic only of a given subscriber and are called an address. This address is surely available in the transmitted message, and at this address the receiving party judges that the message is intended for it or for another member of the network.

Fig. 3.5 Non-switched networks:
a) a common tire;
b) ring; c) fully connected network

The advantage of such networks is the simplicity of the organization. The disadvantages of such structures are as follows. When the link is broken anywhere, communication becomes impossible for a whole group of users. In addition, in such networks at any given time only one pair of network participants can transmit a message.

Networking on a “one-to-one” basis requires a significantly larger number of connecting lines. But the network is distinguished by the best efficiency: at any time, any pair of subscribers can be connected. In general, such a network is more reliable: the failure of one line will cause a failure of communication of only one pair of subscribers. The rest of the network will continue to work in the same conditions.

For these reasons, the above structure of the organization of networks most effectively work only with a small number of subscribers. With an increase in the number of subscribers, the complexity of organizing such networks increases, or the time available to each of the subscribers to use common resources decreases, or as the number of subscribers grows, the number and length of lines connecting them rapidly increase.

With an increase in the number of network participants, switched networks are the most effective. In such networks, subscribers are divided into groups, and in each group, each of the subscribers is connected to a special switching node with communication lines called subscriber lines. In switching nodes, flows from individual subscribers are combined and transmitted to other switching nodes via communication lines, called connecting lines, and capable of carrying large amounts of information than subscriber lines. The total length of the required communication lines in such networks is reduced.

With the introduction of a special device - a switching node - the number of necessary lines for connecting subscribers and their total length can be reduced. At the same time, the network retains high efficiency and sufficiently high reliability associated with disruptions in the operation of communication lines: when a subscriber line breaks, only one user is denied communication services. But in such structures, high responsibility falls on the switching nodes: disruptions in its operation can lead to a breakdown in communication of the entire network.

The simplest switched network has one switching node. Such a network structure is called radial, or “star” (Figure 3.6, a). With an increase in the number of network users, the radial-node structure is more effective (Figure 3.6, b).

Fig. 3.6 Switched networks:
a) radial; b) radial node

In a switched network, to ensure the transmission of messages intended for a specific user, the end devices of subscribers are pre-connected by means of switching nodes and connecting lines. An electrical circuit (channel) consisting of several sections is called a connecting path.

The process of selecting electrical circuits and combining them into a connecting path is called circuit switching. A network that provides circuit switching is called a circuit switched network. After establishing a connection in such a network, information from the source to the recipient arrives in real time, taking into account only the physical delays of signal propagation through the circuit. This is the advantage of such networks. The disadvantage of this network mode is as follows. While the network share (switching nodes and connecting lines) is occupied by one pair of network users, other subscribers cannot use the network during this time interval, even if no information is transmitted via it.

In communication networks, other modes of operation are possible. The transmission of documentary messages can be performed not only after the establishment of the entire connecting path (“from end to end”), but in stages, from one switching node to another. At each subsequent node, the received message is queued and sent to the next node as the line is released. This organization of information delivery is called message switching, and the network that provides message switching is called a message switched network. The “downtime” of connecting lines in such a network is shorter, and in general such a network can transmit a greater amount of information.

A variation of a message-switched network is a packet-switched network. In such a network, the sent messages are divided into blocks (packets) of a fixed size. On the network, each such packet is transmitted as an independent message. At the receiving location, the original message is restored from the set of received packets. The effectiveness of this mode of network operation is even higher. In practice, the most commonly used methods are circuit-switched and packet-switched.

According to hierarchical features (the scale of the coverage of the territory and the number of participants) the networks are divided into global (world) and regional (national, zonal or local). Examples of global networks are Internet computer networks, GSM cellular networks, etc. Regional networks serve the territory of the respective region. Computer networks on this basis are classified into global networks and local networks.

According to the functional characteristics, the communication networks are divided into transmission networks (backbone networks), distribution networks (switching systems) and control networks.

By type of transmitted messages, networks are divided into: telephone networks, telegraph networks, radio and television broadcasting networks, cellular networks, discrete messages, newspapers, etc.

The telephone network is one of the most extensive networks and is built on a radial-nodal principle. The end devices of the telephone network are telephone sets and fax modems.

The telegraph network is also built on a radial-node principle, taking into account the administrative division of the country. The end devices of the telegraph network are the telegraph offices of communication departments or other users.

Cellular networks are also built on a radial-node principle, taking into account the characteristics of radio wave propagation.

Discrete message networks have a similar structure and are one of the most dynamically developing participants in the information transfer process.

Newspaper news networks provide facsimile transmission of newspaper information.

The most important mass messaging networks are broadcast networks. Broadcasting is the process of simultaneously transmitting messages of a general nature to a wide range of subscribers using technical communications.

A broadcast program is a sequential in time transmission of various messages. Broadcasting technology includes both the preparation of broadcast programs, and bringing these programs to subscribers. The main requirements for broadcasting networks are the high quality of the transmitted programs, reliability and cost-effectiveness in covering broadcasting of the entire population of the country.

Broadcasting and television broadcasting networks are built on a radial node principle. Distribution of programs in the networks of radio and television broadcasting is carried out via communication channels, branching is performed on special nodes. According to the method of bringing broadcast programs to subscribers, there is a distinction between broadcasting (including terrestrial television) using transmitting radio and television stations and wired broadcasting (including cable television).

The zone of reliable reception of a television signal is limited to the limits of direct visibility between the transmitting antenna of the telecentre and the receiving antenna of the subscriber. The radius of this zone increases with increasing height of the antenna. Typical radio transmitting stations with supports for antennas with a height of 200 ... 300 meters provide a reception area with a radius of 60 ... 100 kilometers.

A modern type of on-air television is satellite television with direct reception to installations located at subscribers (direct television - NTV).