Under topology(layout, configuration, structure) of a computer network usually refers to the physical arrangement of computers on the network one relative to one and the way they are connected by communication lines. It is important to note that the concept of topology refers primarily to local networks, in which the structure of connections can be easily traced. In global networks, the structure of connections is usually hidden from users and is not very important, because each communication session can be carried out along its own path.
The topology determines the requirements for equipment, the type of cable used, the possible and most convenient methods of managing the exchange, reliability of operation, and possibilities for expanding the network.

There are three main network topologies:

1. Network topology bus(bus), in which all computers are connected in parallel to one communication line and information from each computer is simultaneously transmitted to all other computers (Fig. 1);

2. Star network topology(star), in which other peripheral computers are connected to one central computer, each of them using its own separate communication line (Fig. 2);

3. Network topology ring(ring), in which each computer always transmits information to only one computer next in the chain, and receives information only from the previous computer in the chain, and this chain is closed in a “ring” (Fig. 3).

Rice. 1. Network topology “bus”

Rice. 2. Star network topology

Rice. 3. Network topology “ring”

In practice, combinations of the basic topology are often used, but most networks are focused on these three. Let us now briefly consider the features of the listed network topology.

Bus topology(or, as it is also called, “common bus”), by its very structure, allows for the identity of the network equipment of computers, as well as the equality of all subscribers. With such a connection, computers can only transmit in turns, because there is only one communication line. Otherwise, the transmitted information will be distorted as a result of overlap (conflict, collision). Thus, the bus implements a half-duplex exchange mode (in both directions, but in turn, and not simultaneously).
In the “bus” topology, there is no central subscriber through which all information is transmitted, which increases its reliability (after all, if any center fails, the entire system controlled by this center ceases to function). Adding new subscribers to the bus is quite simple and is usually possible even while the network is running. In most cases, when using a tire, a minimum amount of connecting cable compared to other topology. However, you need to take into account that each computer (except the two outer ones) has two cables, which is not always convenient.
Because resolving possible conflicts in this case falls on the network equipment of each individual subscriber, equipment network adapter With a bus topology, the “bus” is more complicated than with another topology. However, due to the widespread use of networks with a “bus” topology (Ethernet, Arcnet), the cost of network equipment is not too high.
The bus is not afraid of failures of individual computers, because all other computers on the network can continue to exchange normally. It may seem that the bus is not damaged and the cable is broken, since in this case we have two fully functional buses. However, due to the peculiarities of the propagation of electrical signals over long communication lines, it is necessary to provide for the inclusion of special devices at the ends of the bus - terminators, shown in Fig. 1 in the form of rectangles. Without the inclusion of terminators, the signal is reflected from the end of the line and is distorted so that communication over the network becomes impossible. So, if the cable is broken or damaged, the coordination of the communication line is disrupted, and communication stops even between those computers that remain connected to each other. A short circuit at any point on the bus cable disables the entire network. Any failure of network equipment on the bus is very difficult to localize, because all adapters are connected in parallel, and it is not so easy to understand which of them has failed.
When passing through a communication line of a network with a “bus” topology, information signals are weakened and not renewed in any way, which imposes strict restrictions on the total length of communication lines; in addition, each subscriber can receive signals of different levels from the network depending on the distance to the transmitting subscriber. This places additional requirements on receiving nodes of network equipment. To increase the length of a network with a “bus” topology, several segments (each of which is a bus) are often used, connected to each other using special signal updaters - repeaters.
However, such an increase in the length of the network cannot last indefinitely, because there are also limitations associated with the finite speed of signal propagation along communication lines.

Star topology- this is a topology with a clearly designated center to which all other subscribers are connected. All information exchange takes place exclusively through the central computer, which in this way places a lot of burden on heavy load, therefore it cannot do anything else except the network. It is clear that the network equipment of the central subscriber must be significantly more complex than the equipment of peripheral subscribers. In this case, there is no need to talk about equal rights for subscribers. As a rule, it is the central computer that is the most powerful, and it is to it that all functions for managing the exchange are assigned. In principle, no conflicts in a network with a star topology are possible, because management is completely centralized, there is no reason to conflict.
If we talk about the star’s resistance to computer failures, then the failure of a peripheral computer does not in any way affect the functioning of the part of the network that remains, but any failure of the central computer makes the network completely inoperable. Therefore, special measures must be taken to improve the reliability of the central computer and its network equipment. Broke any cable or short circuit in it, with a “star” topology, communication with only one computer is disrupted, and all other computers can continue to work normally.
On the declination from the bus, in the star there are only two subscribers on each communication line: the central one and one of the peripheral ones. Most often, two communication lines are used to connect them, each of which transmits information in only one direction. Thus, there is only one receiver and one transmitter on each communication link. All this significantly simplifies network installation compared to a bus and eliminates the need to use additional external terminators. The problem of signal attenuation in a communication line is also solved more easily in a “star” than in a “bus”, because each receiver always receives a signal of the same level. A serious disadvantage of the star topology is the strict limitation on the number of subscribers. Typically, the central subscriber can serve no more than 8-16 peripheral subscribers. If within these limits it is quite easy to connect new subscribers, then if they are exceeded it is simply impossible. True, sometimes a star provides for the possibility of expansion, that is, connecting another central subscriber instead of one of the peripheral subscribers (the result is a topology of several interconnected stars).
The star shown in Fig. 2, is called an active, or real star. There is also a topology called passive star, which is only superficially similar to a star (Fig. 4). At this time it is much more widespread than the active star. Suffice it to say that it is used in the most popular Ethernet network today.

Rice. 4. Passive star topology

The center of a network with this topology does not contain a computer, but a concentrator, or hub, which performs the same function as a repeater. It renews the signals that are received and forwards them to other communication lines. Although the cabling pattern is similar to a true or active star, we are actually dealing with a bus topology because information from each computer is simultaneously transmitted to all other computers, and there is no central subscriber. Naturally, a passive star is more expensive than a regular bus, because in this case you also need a hub. However, it provides a number of additional features associated with the star benefits. That is why recently the passive star is increasingly replacing the real star, which is considered an unpromising topology.
It is also possible to distinguish an intermediate type of topology between an active and passive star. In this case, the hub not only relays the signals, but also manages the exchange, but does not itself take part in the exchange.
Big star advantage(both active and passive) is that all connection points are collected in one place. This allows you to easily control the operation of the network, localize network faults by simply disconnecting certain subscribers from the center (which is impossible, for example, in the case of a bus), and also limit access of unauthorized persons to connection points vital for the network. In the case of a star, each peripheral subscriber can be approached by either one cable (which transmits in both directions) or two cables (each of them transmits in one direction), with the second situation being more common. A common disadvantage for the entire star topology is that the cable consumption is significantly higher than with other topologies. For example, if computers are located in one line (as in Fig. 1), then when choosing a “star” topology you will need several times more cable than with a “bus” topology. This can significantly affect the cost of the entire network as a whole.

Ring topology is a topology in which each computer is connected by communication lines to only two others: from one it only receives information, and to the other it only transmits. On each communication line, as in the case of a star, there is only one transmitter and one receiver. This allows you to avoid using external terminators. An important feature of the ring is that each computer relays (renews) the signal, that is, acts as a repeater, therefore the attenuation of the signal throughout the ring does not matter, only the attenuation between neighboring computers of the ring is important. In this case, there is no clearly defined center; all computers can be the same. However, quite often a special subscriber is allocated in the sprat who manages the exchange or controls the exchange. It is clear that the presence of such a control subscriber reduces the reliability of the network, because its failure will immediately paralyze the entire exchange.
Strictly speaking, computers in a sprat are not completely equal (unlike, for example, a bus topology). Some of them necessarily receive information from the computer that is transmitting at this moment earlier, while others - later. It is on this feature of the topology that methods for controlling network exchange, specially designed for the “ring,” are based. In these methods, the right to the next transmission (or, as they also say, to take over the network) passes sequentially to the next computer in the circle.
Connecting new subscribers to the “ring” is usually completely painless, although it requires a mandatory shutdown of the entire network for the duration of the connection. As in the case of the “bus” topology, the maximum number of subscribers in a sprat can be quite large (up to a thousand or more). The ring topology is usually the most resistant to overloads; it ensures reliable operation with the largest flows of information transmitted over the network, because, as a rule, there are no conflicts (unlike a bus), and there is no central subscriber (unlike a star) .
Because the signal in the sprat passes through all the computers on the network, the failure of at least one of them (or its network installation) disrupts the operation of the entire network as a whole. Likewise, any break or short circuit in each of the ring cables makes the entire network impossible to operate. The ring is most vulnerable to cable damage, therefore this topology usually involves laying two (or more) parallel communication lines, one of which is in reserve.
At the same time big advantage ring is that relaying signals by each subscriber allows you to significantly increase the size of the entire network as a whole (at times up to several tens of kilometers). The ring is relatively superior to any other topology.

Disadvantage rings (in comparison with a star) can be considered that two cables must be connected to each computer on the network.

Sometimes a ring topology is based on two ring communication lines that transmit information in opposite directions. The purpose of such a solution is to increase (ideally double) the speed of information transfer. In addition, if one of the cables is damaged, the network can work with another cable (although the maximum speed will decrease).
In addition to the three main, basic topologies considered, the network topology is also often used. tree" (tree), which can be considered as a combination of several stars. As in the case of a star, a tree can be active, or real (Fig. 5), and passive (Fig. 6). With an active tree, central computers are located at the centers of combining several communication lines, and with a passive tree, there are concentrators (hubs).

Rice. 5. “Active tree” topology

Rice. 6. “Passive tree” topology. K - concentrators

Combined topologies are also used quite often, for example star-bus, star-ring.

The ambiguity of the concept of topology.

The network topology determines not only the physical location of computers, but, much more important, the nature of the connections between them, the features of signal propagation throughout the network. It is the nature of the connections that determines the degree of fault tolerance of the network, the required complexity of network equipment, the most appropriate method of managing the exchange, the possible types of transmission media (communication channels), the permissible size of the network (the length of communication lines and the number of subscribers), the need for electrical coordination, and much more.
When people think about network topology in the literature, they may have in mind four completely different concepts that relate to different levels network architecture:

1. Physical topology (that is, the layout of computers and cable routing). In this content, for example, a passive star is no different from an active star, which is why it is often called simply a “star.”

2. Logical topology (that is, the structure of connections, the nature of signal propagation through the network). This is probably the most correct definition of topology.

3. Exchange control topology (that is, the principle and sequence of transferring the right to delight the network between individual computers).

4. Information topology (that is, the direction of information flows transmitted over the network).

For example, a network with a physical and logical “bus” topology can, as a management method, use relay transmission of the right to seize the network (that is, be a ring in this content) and simultaneously transmit all information through one dedicated computer (be a star in this content).

The term "network topology" describes the possible configurations of computer networks. The specificity of network technologies is the need for strict coordination of all characteristics of hardware and software network tools for successful data exchange. At the same time, existing hardware is capable of providing different capabilities (speed, reliability, etc.) for data transfer, depending on the way these devices are used. To take into account all these features of equipment operating modes, the concept of “network topology” was introduced. Currently, two types of topologies are used to describe the network configuration: physical and logical.

Physical topologies

Physical topology describes the actually used methods of organizing physical connections of various network equipment (used cables, connectors and methods of connecting network equipment). Physical topologies vary in cost and functionality. Below we provide a description of the three most commonly used physical topologies, indicating their advantages and disadvantages.

Physical Bus

The simplest form of physical bus topology consists of one main cable terminated on both sides with special types of connectors - terminators. When creating such a network, the main cable is laid sequentially from one network device to another. The devices themselves are connected to the main cable using lead cables and T-connectors. An example of such a topology is shown in the figure.

A more complex form of physical bus topology is a "distributed bus" (more commonly called a "tree topology"). In this topology, the main cable, starting from one point, called the “root”, branches in different directions determined by the actual physical location network devices. Unlike the topology described above, in a distributed bus topology the main cable has more than two terminations. Cable branching is carried out using special connectors. An example of such a topology is shown in the figure.

Physical Star

The simplest form of a physical star topology consists of many cables (one for each connected network device) connected to a single, central device. This central device is called a hub. An example of a physical star topology is 10Base-T Ethernet or 100Base-T Ethernet. In such networks, each network device is connected to a hub using a twisted pair cable.

If a simple physical star topology is used, the actual signal paths may not follow the shape of the star. The only characteristic described by a physical star topology is the way network devices are physically connected. An example of the simplest “physical star” topology is shown in the figure.

In a distributed star topology, the way devices are connected can be significantly more complex. In this topology central devices(hubs) are additionally connected to each other.

Physical Star-Wired Ring

In this topology, all network devices are connected to a central hub in the same way as in a physical star topology. But each of the hubs within itself organizes physical connections that ensure the construction of a single physical ring. When using several hubs, the ring in each of the hubs is opened, and the hubs themselves are connected to each other using two cables, organizing the physical closure of the ring.

The physical ring topology is used in IBM Token-Ring networks. An example of the described topology is shown in Fig.

In this topology, all hubs are "smart" devices. If a physical ring break occurs anywhere on the network, the hub automatically detects the break and restores the ring by shorting the corresponding ports within itself. The figure shows an example of such ring restoration (hub A).

The star topology is currently the most popular because it provides the easiest way to connect new devices to the network. In most cases, connecting a new device to a network consists only of laying a piece of cable connecting the connected network device to a hub.

Logical topologies

The logical topology defines the actual paths for signals to travel when transmitting data along the physical topology used. Thus, logical topology describes the paths for transmitting data flows between network devices. It defines the rules for data transmission in the existing transmission medium, guaranteeing the absence of interference affecting the correctness of data transmission.

Since logical topology describes the path and direction of data transmission, it is closely related to the MAC (Media Access Control) layer of the OSI model (sublayer of the data link layer). For each of the existing logical topologies, there are media access control (MAC) methods that allow monitoring and control of the data transfer process. These methods will be discussed along with their corresponding topology.

Currently, there are three basic logical topologies: “logical bus”, “logical ring” and “logical star” (switching). Each of these topologies provides benefits depending on the use cases. When using the physical topology illustrations discussed earlier, always remember that the logical topology determines the direction and mode of transmission, not the wiring of the physical wires and devices.

Logical bus

In a logical bus topology, sequences of data, called frames, are distributed in the form of signals simultaneously in all directions across the existing transmission medium. Each station on the network examines each frame of data to determine who the data is addressed to. When a signal reaches the end of the transmission medium, it is automatically canceled (removed from the transmission medium) by appropriate devices called "terminators". This destruction of the signal at the ends of the transmission medium prevents the signal from being reflected back into the transmission medium. If terminators did not exist, then the reflected signal would overlap the useful one and distort it.

In a logical bus topology, the transmission medium is shared and simultaneously used by all data transmission devices. To prevent interference when multiple stations attempt to transmit data simultaneously, only one station at any time is allowed to transmit data. Thus, there must be a method for determining which station has the right to transmit data at any given time. In accordance with these requirements, methods for controlling access to the transmission medium were created, which we reviewed in the section “Data exchange process”.

The most commonly used method for controlling access to the transmission medium when organizing a logical bus topology is CSMA/CD - “carrier sense multiple access/collision detection method”. This access method is very similar to several people talking in the same room. To avoid disturbing each other, only one person speaks at any given time, while everyone else listens. And anyone can begin to speak only after making sure that there is silence in the room. The network works in exactly the same way. When a station is about to transmit data, it first “listens” (carrier sense) on the data transmission medium in order to detect any station already transmitting data. If any station in at the moment transmits, the station waits for the end of the transmission process. When the transmission medium becomes free, the waiting station begins transmitting its data. If at this moment transmission begins by one or more stations that were also waiting for the transmission medium to become free, then a “collision” occurs. All transmitting stations detect a collision and send a special signal informing all network stations about the occurrence of a collision. After this, all stations are silent for a random period of time before attempting to transmit data again. After this, the work algorithm starts over again.

A network based on a logical bus topology can also use token passing technology to control access to the data transmission medium. When using this control method, each station is assigned a sequence number indicating the priority in data transmission. After the station with the maximum number transmits data, the queue returns to the first station. The sequence numbers assigned to stations may not correspond to the actual sequence of physical connection of stations to the data transmission medium. To control which station currently has the right to transmit data, a control data frame called an "access token" is used. This marker is transmitted from station to station in a sequence corresponding to their serial numbers. The station that receives the token has the right to transmit its data. However, each transmitting station is limited by the time during which it is allowed to transmit data. At the end of this time, the station must pass the token to the next station.

The operation of such a network begins with the first station having an access token transmitting its data and receiving responses to it within a limited period of time (time slot). If a station completes communication before the end of its allotted time, it simply transmits a station token with the next sequence number. Then the process is repeated. This sequential process of token transmission continues continuously, allowing each station to be able to transmit data after a strictly defined period of time.

The “logical bus” topology is based on the use of the “physical bus” and “physical star” topologies. The access control method and types of physical topologies are selected depending on the requirements for the designed network. For example, Ethernet, 10Base-T Ethernet, and ARCnet® each use a logical bus topology. Cables in Ethernet networks (thin coaxial cable) are connected using a physical bus topology, while 10Base-T Ethernet and ARCnet networks are based on a physical star topology. However, Ethernet (physical bus) and 10Base-T Ethernet (physical star) use CSMA/CD as a media access control method, and ARCnet (physical star) uses an access token.

The first picture shows Ethernet network(physical bus, logical bus), and the second illustrates a 10Base-T Ethernet network (physical star, logical bus). In both figures, notice that the signal (shown by the arrows) originates from one (currently transmitting) station and travels in all directions of the existing transmission medium.

Logical ring

In a logical ring topology, data frames are transmitted along a physical ring until they have traversed the entire data transmission medium. The logical ring topology is based on the physical ring topology with a star connection. Each station connected to the physical ring receives data from the previous station and repeats the same signal to the next station. Thus, the data, repeating, flows from one station to another until it reaches the station to which it was addressed. The receiving station copies the data from the transmission medium and adds an attribute to the frame indicating successful receipt of the data. Next, the frame with the “delivery attribute” set continues to travel around the ring until it reaches the station that originally sent this data. The station, having analyzed the “delivery attribute” and made sure that the data transmission was successful, removes its frame from the network. The figure shows the process of data transmission in the form of a “logical ring” in a network based on a “physical ring with a star connection” topology.

The method of controlling access to the transmission medium in such networks is always based on the technology of “access tokens”. However, the sequence of obtaining the right to transmit data (the route the token follows) may not always correspond to the actual sequence of connecting stations to the physical ring. IBM's Token-Ring is an example of a network that uses a "logical ring" topology based on a "physical ring with a star connection."

Logic star (switching)

The logical star topology uses a switching method to limit the signal propagation in the transmission medium to a certain part of it. The mechanism of such a limitation is fundamental in the logical star topology.

In its purest form, switching provides a dedicated data line to each station. When one station transmits a signal to another station connected to the same switch, the switch transmits the signal only over the data transmission medium connecting the two stations. The figure shows how data is transferred between two stations connected to the same switch. With this approach, simultaneous data transmission between several pairs of machines is possible, since data transmitted between any two stations remains “invisible” to other pairs of stations.

Most switching technologies build on existing network standards and add new levels of functionality. For example, the previously discussed 10Base-T network standard (CSMA/CD control method) allows the use of switching.

Some switches are designed to support the ability to use multiple network standards simultaneously. For example, one switch may have ports for connecting stations using both the 10Base-T Ethernet standard and FDDI (Fiber Distributed Data Interface).

Switches have built-in logic that allows them to intelligently manage the process of transferring data between machines. The internal logic of switches is characterized by high performance, since they must provide the ability to simultaneously transmit data from maximum speed between each pair of ports. Thus, the use of switches can significantly increase network performance.

Switching illustrates that the logical topology is determined not only by the method of media access control, but also by many other aspects of electronic interconnection circuits (a switch is quite complex and expensive electronic device). By combining new switching technologies with existing logic interconnects, engineers can create new logic topologies.

Multiple switches can be interconnected using one or more physical topologies. Switches can be used not only to connect individual stations, but also entire groups of stations. Such groups are called “network segments”. Thus, for a variety of reasons, switching can significantly improve the performance of your network.

Connection to the simplest network

Now that we have discussed issues related to the hardware implementation of various network components and understood the differences between logical and physical topologies, let's look at ways to connect equipment in the simplest network. The figure shows some previously discussed network devices connected to a simple computer network.

The network depicted consists of the following components: three computers connected to a single 10Base-T hub using unshielded twisted pair cable. Each computer has 10Base-T Ethernet network cards installed. A laser printer is also connected to one of the computers.

The computer at the bottom center of the picture is the server and controls the entire network. The two remaining computers are workstations. Workstations use a network controlled by a server. One workstation is personal computer type IBM PC, the other - apple computer® Macintosh.

A 10Base-T hub provides a physical connection between all three computers. It also functions as a signal repeater.

Lines between different network components represent the transmission medium: twisted pair. This network uses a physical star topology but is based on a logical bus topology.

A printer on this network is connected directly to the server using that computer's parallel port. This connection is standard for most printers. The server accepts print jobs for documents received from each workstation. Received print jobs are then sent to the printer through the parallel port of the server via the appropriate cable. Although this method is the simplest for allowing multiple stations to print documents on one printer, there are still other ways to connect printers to a network. You can, for example, connect the printer to a special print server or a computer with special software that provides the ability to simultaneously perform the functions of a workstation and a print server. Nowadays, many printers are available with built-in network card Thus, the printer can connect directly to the transmission medium anywhere on the network.

Computer network topology

One of the most important differences between different types networks is their topology.

Under topology usually understand the relative position of network nodes relative to each other. In this case, network nodes include computers, hubs, switches, routers, access points, etc.

Topology is the configuration of physical connections between network nodes. Network characteristics depend on the type of topology installed. In particular, the choice of a particular topology affects:

• on the composition of the necessary network equipment;
• on the capabilities of network equipment;
• on the possibility of network expansion;
• on the way the network is managed.

The following main types of topologies are distinguished: shield, ring, star, mesh topology And lattice. The rest are combinations of basic topologies and are called mixed or hybrid.

Tire. Networks with a bus topology use a linear monochannel (coaxial cable) for data transmission, at the ends of which special plugs are installed - terminators. They are necessary in order to

Rice. 6.1.

to extinguish the signal after passing through the bus. The disadvantages of the bus topology include the following:

• data transmitted via cable is available to all connected computers;
• If a bus fails, the entire network stops functioning.

Ring is a topology in which each computer is connected by communication lines to two others: from one it receives information, and to the other it transmits it and implies the following data transfer mechanism: data is transmitted sequentially from one computer to another until it reaches the recipient computer. The disadvantages of the ring topology are the same as those of the bus topology:

• public availability of data;
• instability to damage to the cable system.

Star- this is the only network topology with a clearly designated center, called a network hub or “hub”, to which all other subscribers are connected. The functionality of the network depends on the status of this hub. In a star topology, there are no direct connections between two computers on the network. Thanks to this, it is possible to solve the problem of public data availability, and also increases the resistance to damage to the cable system.

Rice. 6.2.

Rice. 6.3. Star topology

is a computer network topology in which each network workstation is connected to several workstations on the same network. It is characterized by high fault tolerance, complexity of configuration and excessive cable consumption. Each computer has many possible ways connections with other computers. A broken cable will not result in loss of connection between the two computers.

Rice. 6.4.

Lattice is a topology in which the nodes form a regular multidimensional lattice. In this case, each lattice edge is parallel to its axis and connects two adjacent nodes along this axis. A one-dimensional lattice is a chain connecting two external nodes (having only one neighbor) through a number of internal nodes (which have two neighbors - on the left and on the right). By connecting both external nodes, a ring topology is obtained. Two- and three-dimensional lattices are used in supercomputer architecture.

Networks based on FDDI use a double ring topology, thereby achieving high reliability and performance. A multidimensional lattice connected cyclically in more than one dimension is called a "torus".

(Fig. 6.5) - a topology that prevails in large networks with arbitrary connections between computers. In such networks, it is possible to identify individual randomly connected fragments ( subnets ), having a standard topology, therefore they are called networks with mixed topology.

To connect large number network nodes use network amplifiers and (or) switches. Active hubs are also used - switches that simultaneously have amplifier functions. In practice, two types of active hubs are used, providing the connection of 8 or 16 lines.

Rice. 6.5.

Another type of switching device is a passive hub, which allows you to organize a network branch for three workstations. The low number of connectable nodes means that the passive hub does not require an amplifier. Such concentrators are used in cases where the distance to the workstation does not exceed several tens of meters.

Compared to a bus or ring, a mixed topology is more reliable. The failure of one of the network components in most cases does not affect the overall performance of the network.

The local network topologies discussed above are basic, i.e. basic. Real computer networks are built based on the tasks that a given local network is designed to solve, and on the structure of its information flows. Thus, in practice the topology computer networks is a synthesis of traditional types of topologies.

Main characteristics of modern computer networks

The quality of network operation is characterized by the following properties: performance, reliability, compatibility, manageability, security, extensibility and scalability.

To the main characteristics productivity networks include:

• reaction time – a characteristic that is defined as the time between the occurrence of a request to any network service and the receipt of a response to it;
• throughput – a characteristic that reflects the amount of data transmitted by the network per unit of time;
• transmission delay – the interval between the moment a packet arrives at the input of a network device and the moment it appears at the output of this device.

For reliability assessments networks are used various characteristics, including:

• availability factor, meaning the proportion of time during which the system can be used;
• safety, those. the ability of the system to protect data from unauthorized access;
• fault tolerance – the ability of the system to operate in conditions of failure of some of its elements.

Extensibility means it can be added relatively easily individual elements networks (users, computers, applications, services), increasing the length of network segments and replacing existing equipment with more powerful ones.

Scalability means that the network allows you to increase the number of nodes and the length of connections within a very wide range, while the network performance does not deteriorate.

Transparency – the ability of a network to hide details of its internal structure from the user, thereby simplifying his work on the network.

Controllability network implies the ability to centrally monitor the status of the main elements of the network, identify and resolve problems that arise during network operation, perform performance analysis and plan network development.

Compatibility means that the network is capable of incorporating a wide variety of software and hardware.

The term “topology” has many meanings, one of which is used in computer world to describe networks. What topology is will be discussed further. But, looking ahead a little, in the simplest case this concept can be considered as a description of the configuration (location) of computers connected to the network. In other words, it all comes down to understanding not even the connections themselves, but the geometric shapes that correspond to each type of terminal arrangement.

What is meant by local network topology?

As is already clear, computers combined into unified networks, connect to them not chaotically, but in a strictly defined order. To describe this circuit, the understanding of topology was introduced.

Essentially, what is topology? Map, diagram, chart, map. The descriptive process, as is already clear, is somewhat akin to elementary knowledge of geometry. However, this term cannot be considered only from a purely geometric point of view. Since we are talking not only about connections, but also about the transfer of information, this factor should also be taken into account.

Main types of networks and their topologies

In general, there is no single concept of computer topology. It is generally accepted that there may be several types of topologies that collectively describe a particular network organization. Actually, networks can be completely different.

For example, the simplest form of organizing the connection of several computer terminals into a single whole can be called a local network. There are also intermediate types of networks (city, regional, etc.).

Finally, the biggest ones are global networks, which affect large geographic regions and include all other types of networks, as well as computers and telecommunications equipment.

But what is meant by local network topology, as one of the simplest forms of organizing the connection of several computers with each other, in this case?

Based on the processes and structures described, they are divided into several types:

• physical - a description of the actual structure of the location of computers and network nodes, taking into account the connections between them;
• logical - description of the signal passage through the network;
• informational - description of the movement, direction and redirection of data within the network;
• exchange control - a description of the principle of using or transferring rights to use the network.

Network topology: types

Now a few words about the generally accepted classification of topology types by connections. In the context of what a topology is, it is worth separately noting another type of classification, which describes exclusively the way a computer connects to the network or the principle of its interaction with other terminals or main nodes. In this case, the concepts of fully connected and incompletely connected topologies become relevant.

A fully connected structure (and this is recognized throughout the world) is extremely cumbersome due to the fact that each single terminal included in a single network structure is connected to all the others. The inconvenience in this case is that additional communication equipment must be installed for each computer, and the terminal itself must be equipped with a sufficiently large number of communication ports. And as a rule, such structures, if used, are extremely rare.

An incompletely connected topology in this regard looks much more preferable, since each individual terminal is not connected to all other computers, but receives or transmits information through certain network nodes or accesses directly a central hub or hub. A striking example of this is the star network topology.

Since we are talking about the main methods of combining terminals into a single whole (network), we should dwell on the basic topologies of all the main types, among which the main ones are “bus”, “star” and “ring”, although there are some mixed types.

Bus network topology

This type of networking of terminals is quite popular, although it has very serious disadvantages.

You can see what a “bus” topology is using a simple example. Imagine a cable with several branches on both sides. At the end of each such branch there is a computer terminal. They are not directly connected to each other, but information is received and transmitted through a single highway, at both ends of which special terminators are installed that prevent signal reflection. This is a standard linear network topology.

The advantage of such a connection is that the length of the main line is significantly reduced, and the failure of a single terminal does not have any impact on the operation of the network as a whole. The main disadvantage is that if there is a disruption in the operation of the highway itself, the entire network becomes inoperative. In addition, the “bus” topology is limited in the number of connected workstations and has rather low performance due to the distribution of resources between all terminals in the network. The distribution may be uniform or uneven.

Star topology

The topology of the “star” network is in some sense reminiscent of a “bus”, with the only difference being that all terminals are connected not to a single backbone, but to a central distribution device (hub, hub).

It is through the hub that all computers can communicate with each other. Information is transmitted from the hub to all devices, but is received only by those for which it is intended. The advantages of such a connection include the ability to connect to all network terminals, as well as the connection of new ones. However, as in the case of the “bus,” the failure of the central switching device has consequences for the entire network.

Ring topology

Finally, we have another type of connection - a ring network topology. As is probably already clear from the name, computers are connected sequentially from one to another through intermediate nodes, as a result of which a vicious circle is formed (of course, a circle in this case is a relative concept).

During transmission, information from the starting point passes through all terminals that are in front of the final recipient. But recognition of the final beneficiary is based on token access. That is, only the terminal marked in the information flow receives information. This scheme is practically not used anywhere due to the fact that the failure of one computer automatically entails a disruption in the operation of the entire network.

Mesh and mixed topology

This type of connection can be obtained by removing some connections from the above connections or adding them additionally. In most cases, this scheme is used in large networks.

In this regard, several main derivatives can be defined. The most common are considered to be schemes such as “double ring”, “tree”, “lattice”, “snowflake”, “Clos network”, etc. As can be seen even from the names, all these are variations on the theme of the main types of connections, which are taken as a basis.

There is also a mixed type of topology, which can combine several others (subnetworks), grouped according to some characteristic characteristics.

Conclusion

Now it’s probably clear what topology is. If we make a general conclusion, this concept is a description of how computers are connected on a network and how they interact. How this is done depends solely on the method of combining the terminals into one. And it is impossible to say that today it is possible to single out one universal connection option. In each specific case and depending on the needs, one or another type of connection can be used. But in local networks, if we talk specifically about them, the most common is the “star” scheme, although the “bus” is still used quite widely.

It remains to add that you can also find the concepts of centralization and decentralization, but they are mostly associated not with connections, but with the system for managing network terminals and exercising control over them. Centralization is clearly expressed in star-type connections, but decentralization is also applicable for this type, ensuring the introduction of additional elements in order to increase the reliability of the network when the central switch fails. A fairly effective development in this regard is the “hypercube” scheme, but it is very difficult to develop.

is a way of describing a network configuration, a diagram of the location and connection of network devices. The network topology allows you to see its entire structure, the network devices included in the network, and their connections with each other.

There are several types of topologies: physical, logical, informational and exchange control topology. In this article we will talk about the physical topology of the network, which describes the actual location and connections between the nodes of the local network.

There are several main types of physical network topologies:

1. Bus network topology- a topology in which all computers on the network are connected to one cable, which is shared by all workstations. With this topology, the failure of one machine does not affect the operation of the entire network as a whole. The disadvantage is that if the bus fails or breaks, the operation of the entire network is disrupted.
2. Zvezda network topology— a topology in which all workstations have a direct connection to the server, which is the center of the “star”. With this connection scheme, a request from any network device is sent directly to the server, where it is processed at different speeds, depending on the hardware capabilities of the central machine. Failure of the central machine leads to the shutdown of the entire network. The failure of any other machine does not affect the operation of the network.
3. Ring network topology- a scheme in which all nodes are connected by communication channels into an unbroken ring (not necessarily a circle) through which data is transmitted. The output of one PC is connected to the input of another. Having started the movement from one point, the data ultimately ends up at its beginning. Data in a ring always moves in the same direction. This network topology does not require the installation of additional equipment (server or hub), but if one computer fails, the operation of the entire network stops.
4. Mesh network topology- a topology in which each workstation is connected to all other workstations on the same network. Each computer has many possible ways to connect to other computers. Therefore, a cable break will not result in loss of connection between the two computers. This network topology allows connection large quantity computers and is typical, as a rule, for large networks.
5. At mixed topology Several types of connections between computers are used. It occurs quite rarely in particularly large companies and organizations.

Why do you need to know the types of topologies and all their pros and cons? The composition of the equipment and software. The topology is chosen based on the needs of the enterprise. In addition, knowledge of the network topology allows you to evaluate it weak points, as well as the dependence of the stability of its operation on individual components, more carefully plan subsequent connections of new network equipment and PCs. In case of any failure, lack of connection with any computer on the network, you can always see on the map where this device located, on which floor, in which office or room, what, first of all, you need to pay attention to and where to go first to troubleshoot the problem.

And here we come to one of the key questions that interests all system administrators, namely: how to draw a network diagram with minimal costs time, effort and money? If the network is large and consists of dozens of servers, hundreds of computers and many other network devices (printers, switches, etc.), even an experienced system administrator(not to mention a beginner) it is very difficult to quickly understand all the connections between network equipment. Creating a network topology manually is out of the question here. Fortunately, the modern software market offers special programs for automatically exploring and constructing a network diagram. This allows the system administrator to know where and what equipment is located without having to manually examine the wires.

Thus, even if you are new to the company, and the previous system administrator was not very eager to “hand over” the network to you according to all the rules, programs for drawing network topology will allow you to quickly get into work and start with building a diagram of your network.