Topology of the computer network - a way of association of units in a network. Unit of the computer network - a computer, the terminal or other device connected to the computer network. To each unit the unique address allowing other units of a network to contact by it under data links is appropriated. The most widespread kinds of topology are: bus, ring, tree and star.

The concept of a network topology in the form of a star has come from area of greater COMPUTERS in which the head server receives and processes all data from peripheral devices as active unit of data processing. This principle is applied in systems of data transmission. The information between two peripheral workplaces passes through the central unit of the computer network.

Fig.1 Star topology

Throughput of a network is defined by computing capacity of unit and guaranteed for each workstation. Collisions (collisions) of data does not arise. Cable connection simple enough as each workstation is knotted with. Expenses for a lining of cables high, especially when the central unit is geographically located not in the center of topology. At expansion of computer networks the executed cable communications cannot be used earlier: to a new workplace it is necessary to lay a separate cable from the center of a network.

The topology in the form of a star is most high-speed of all computer network topologies, as data transmission between workstations passes through the central unit (at its good productivity) on the separate lines used only by these workstations. Frequency of inquiries of transfer of the information from one station to another low in comparison with maybe in other kind of topology.

Productivity of the computer network first of all depends on capacity of the central file server. It can be a bottleneck of the computer network. In case of failure of the central unit work of all network is broken. The central unit of management - a file server realizes the optimum mechanism of protection against not authorized access to the information. All computer network can cope from its center.

At ring topology of a network workstations are connected one with another on a circle, i.e. a workstation 1 with a workstation 2, the workstation 3 with a workstation 4, etc. Last workstation is connected with the first. Communication becomes isolated in a ring.

Fig.2 Ring topology

The lining of cables from one workstation up to another can be enough complex and expensive, especially if a geographical arrangement of workstations far from the form of a ring (for example, in a line). Messages circulate on a regular basis on a circle. The workstation sends to the certain final address the information, preliminary having received from a ring inquiry. Transfer of messages is very effective as the majority of messages can be sent "in road" on cable system one behind another. Very simply it is possible to make ring inquiry about all stations.

Duration of transfer of the information increases proportionally to quantity of the workstations entering into the computer network.

The basic problem at ring topology consists that each workstation should participate actively in transfer of the information, and in case of failure even all network will be paralysed by one of them. Malfunctions in cable connections are localized easily.

Connection of a new workstation demands briefly urgent deenergizing of a network as during installation the ring should be opened. Restrictions on extent of the computer network do not exist, as it, finally, is defined exclusively by distance between two workstations. The special form of ring topology is the logic ring network. Physically it is mounted as connection of star topology.

Separate stars join by means of switches. Depending on number of workstations and lengths of a cable between workstations apply active or passive concentrators. Active concentrators in addition contain the amplifier for connection from 4 up to 16 workstations.

Management of a separate workstation in a logic ring network occurs the same as and in a usual ring network. Each workstation the address corresponding it on which management (from the senior to younger and from the most younger to the senior) is transferred is appropriated. Break of connection occurs only for nearest unit of the computer network so only in rare cases work of all network can be broken.

At bus the environment of the information transfer is represented to topology in the form of the communication way, accessible day of all workstations to which all of them should be connected. All workstations can directly come into contact to any workstation which is available.

Fig.3 Bus topology

Workstations at any time, without interruption of work of all computer network, can be connected to it or are disconnected. Functioning of the computer network does not depend on a condition of a separate workstation.

In a standard situation for bus networks Ethernet often use a thin coaxial cable. Switching-off and especially connection to such network demand break of the trunk that causes infringement of a circulating stream of the information and lag of system. New technologies offer passive plug boxes through which it is possible to disconnect and-or connect workstations during work of the computer network. Owing to that workstations can be connected without interruption of network processes and the communication environment, very easily to listen to the information, i.e. to branch off the information from the communication environment.

In a LAN from a straight line transfer of the information always can exist only one station transferring the information. The time method of division according to which for each connected workstation during the certain moments of time the exclusive right to use of the data link is given is in most cases applied to prevention of collisions. Therefore requirements to throughput of the network at the raised loading raise, for example, at input of new workstations.

Workstations join the bus by means of devices TAP (Terminal Access Point). TAP represents special type of connection to a coaxial cable. The probe of the needle form takes root through an external environment of an external conductor to an internal conductor and joins it. In a LAN with the modulated broadband transfer of the information various workstations receive, as required, frequency on which these workstations can send and receive the information. Sent data are modulated on corresponding bearing frequencies, i.e. between Wednesday of transfer of the information and workstations there are accordingly modems for modulation and demodulations. The technics of broadband messages allows to transport simultaneously in the communication environment enough great volume of the information.

For the further development of discrete transportation of data does not play a role, what initial information is submitted to the modem (analog or digital) as it all the same in the further will be transformed.

Alongside with known topology of computer networks "ring", "star" and "bus", in practice it is applied and combined, on an example the structure is tree. It is formed basically in the form of combinations of the above-named topology of computer networks. The basis of a tree of the computer network settles down in a point in which lines of communication of the information gather. Computer networks with treelike structure are applied there where direct application of base network structures in the pure state is impossible.

Network amplifiers and-or switches apply to connection of the big number of workstations accordingly adapter to payments. The switch possessing simultaneously and functions of the amplifier. In practice their two versions providing connection accordingly of 8, 16, 24 or 48 lines apply.