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All local network topologies. Basic topologies of local networks. Types of local networks and their structure. About the star topology

Topology local networks.

The composition and configuration of network equipment depending on the network topology.

1. Concept of network topology

The general scheme for connecting computers into local networks is called network topology

Topology is the physical configuration of the network combined with its logical characteristics. Topology is a standard term used to describe the basic layout of a network. By understanding how different topologies are used, you can determine what capabilities they have. Various types networks.

There are two main types of topologies:

physical
logical

Logical topology describes the rules for interaction between network stations when transmitting data.

Physical topology defines the method of connecting storage media.

The term "network topology" describes the physical arrangement of computers, cables, and other network components. The topology of physical connections can take on different “geometric” forms, and what is important is not the geometric location of the cable, but only the presence of connections between the nodes (closed/open, presence of a center, etc.).

The network topology determines its characteristics.

The choice of a particular topology affects:

composition of the necessary network equipment
network equipment characteristics
network expansion possibilities
network management method

The network configuration can be either decentralized (when the cable “runs around” each station in the network) or centralized (when each station is physically connected to some central device that distributes frames and packets between stations). An example of a centralized configuration is a star with workstations located at the ends of its arms. A decentralized configuration is similar to a chain of climbers, where everyone has their own position in the chain, and everyone is connected together by one rope. The logical characteristics of a network's topology determine the route a packet takes as it travels across the network.

When selecting a topology, it must be taken into account that it provides reliable and effective work networks, convenient management of network data flows. It is also desirable that the network should be inexpensive in terms of the cost of creation and maintenance, but at the same time there would remain opportunities for its further expansion and, preferably, for the transition to higher-speed communication technologies. This is not an easy task! To solve it, you need to know what network topologies there are.

According to the topology of connections there are:

networks with a “common bus (bus)” topology;
networks with star topology;
networks with a “ring” topology”;
networks with tree topology;
networks with mixed topology

2. Basic network topologies

There are three basic topologies on which most networks are built.

bus
star
ring

A “bus” is a topology in which computers are connected along a single cable.

A "star" is a topology in which computers are connected to cable segments originating from a single point, or hub.

A topology is called “ring” if the cable to which the computers are connected is closed in a ring.

Although the basic topologies themselves are simple, in reality there are often quite complex combinations that combine the properties of several topologies.

2.1 Bus network topology

In this topology, all computers are connected to each other with one cable. Each computer is connected to a common cable, at the ends of which terminators are installed. The signal passes through the network through all computers, reflecting from the end terminators.

Network topology diagram "bus" type

The "bus" topology is generated by a linear structure of connections between nodes. This topology can be implemented in hardware, for example, by installing two network adapters on central computers. In order to prevent signal reflection, terminators that absorb the signal must be installed at the ends of the cable.

In a network with a bus topology, computers address data to a specific computer, transmitting it along the cable in the form of electrical signals - hardware MAC addresses. To understand the process of computer interaction via a bus, you need to understand the following concepts:

signal transmission
signal reflection
Terminator

1. Signal transmission

Data in the form of electrical signals is transmitted to all computers on the network; however, only the one whose address matches the recipient address encrypted in these signals receives information. Moreover, at any given time, only one computer can transmit. Since data is transmitted to the network by only one computer, its performance depends on the number of computers connected to the bus. The more there are, i.e. The more computers waiting to transfer data, the slower the network. However, to derive a direct relationship between throughput network and the number of computers in it is impossible. Because, in addition to the number of computers, network performance is influenced by many factors, including:

characteristics hardware computers on the network
the frequency with which computers transmit data
type of network applications running
network cable type
distance between computers on the network

The bus is a passive topology. This means that computers only “listen” to data transmitted over the network, but do not move it from sender to recipient. Therefore, if one of the computers fails, it will not affect the operation of the others. In active topologies, computers regenerate signals and transmit them across the network.

2. Signal reflection

Data, or electrical signals, travel throughout the network - from one end of the cable to the other. If no special action is taken, the signal reaching the end of the cable will be reflected and will not allow other computers to transmit. Therefore, after the data reaches the destination, the electrical signals must be extinguished.

3. Terminator

To prevent electrical signals from being reflected, plugs (terminators) are installed at each end of the cable to absorb these signals. All ends of the network cable must be connected to something, such as a computer or a barrel connector - to increase the cable length. A terminator must be connected to any free (not connected to anything) end of the cable to prevent electrical signals from being reflected.

Terminator installation

Network integrity can be compromised if a network cable breaks when it is physically severed or one of its ends is disconnected. It is also possible that there are no terminators at one or more ends of the cable, which leads to reflection of electrical signals in the cable and termination of the network. The network "falls". The computers themselves on the network remain fully functional, but as long as the segment is broken, they cannot communicate with each other.

This network topology has advantages and disadvantages.

D advantages bus topologies:

short network setup time
low cost (less cable and network devices required)
ease of setup
Failure of a workstation does not affect network operation

Flaws bus topologies:

such networks are difficult to expand (increase the number of computers in the network and the number of segments - individual sections of cable connecting them).
Because the bus is shared, only one of the computers can transmit at a time.
The “bus” is a passive topology - computers only “listen” to the cable and cannot restore signals that are attenuated during transmission over the network.
The reliability of a network with a bus topology is low. When the electrical signal reaches the end of the cable, it (unless special measures are taken) is reflected, disrupting the operation of the entire network segment.

The problems inherent in the bus topology have led to the fact that these networks are now practically not used.

The bus network topology is known as 10 Mbps Ethernet logical topology.

2.2 Basic star network topology

In a star topology, all computers are connected to a central component called a hub. Each computer is connected to the network using a separate connecting cable. Signals from the transmitting computer travel through the hub to everyone else.

There is always a center in the “star” through which any signal in the network passes. The functions of the central link are performed by special network devices, and signal transmission in them can proceed in different ways: in some cases, the device sends data to all nodes except the sending node, in others, the device analyzes which node the data is intended for and sends it only to it.

This topology arose at the dawn computer technology, when the computers were connected to a central, main computer.

Star network topology diagram

Advantages"star" typologies:

the failure of one workstation does not affect the operation of the entire network as a whole
good network scalability
easy troubleshooting and network breaks
high network performance (subject to proper design)
flexible administration options

Flaws"star" typologies:

failure of the central hub will result in the inoperability of the network (or network segment) as a whole
networking often requires more cable than most other topologies
the finite number of workstations in a network (or network segment) is limited by the number of ports in the central hub.

One of the most common topologies because it is easy to maintain. Mainly used in networks where the carrier is twisted pair cable. UTP category 3 or 5. (Twisted pair cable categories, which are numbered from 1 to 7 and determine the effective frequency range. A cable of a higher category usually contains more pairs of wires and each pair has more turns per unit length).

The star topology is reflected in Fast technologies Ethernet6.

2.3 Basic ring network topology

In a ring topology, computers are connected to a cable that forms a ring. Therefore, the cable simply cannot have a free end to which a terminator must be connected. Signals are transmitted along the ring in one direction and pass through each computer. Unlike the passive bus topology, here each computer acts as a repeater (repeater), amplifying the signals and passing them on to the next computer. Therefore, if one computer fails, the entire network stops functioning.

Ring network diagram

The functioning of a closed ring topology is based on token passing.

A token is a data packet that allows a computer to transmit data to the network.

The token is transmitted sequentially, from one computer to another, until the one that “wants” to transfer the data receives it. A computer wanting to start a transmission "captures" the token, modifies it, puts the recipient's address in the data, and sends it around the ring to the recipient.

The data passes through each computer until it reaches the one whose address matches the recipient address specified in the data. After this, the receiving computer sends a message to the transmitting one, confirming that the data has been received. Having received confirmation, the sending computer creates a new token and returns it to the network.

At first glance, it seems that transferring the marker takes a lot of time, but in fact the marker moves almost at the speed of light. In a ring with a diameter of 200 meters, the marker can circulate at a frequency of 10,000 revolutions per second.

Advantages ring topology:

ease of installation
almost complete absence of additional equipment
the possibility of stable operation without a significant drop in data transfer speed under heavy network load, since the use of a token eliminates the possibility of collisions.

Flaws ring topology:

failure of one workstation and other problems (cable break) affect the performance of the entire network
complexity of configuration and setup
difficulty in troubleshooting

It is most widely used in fiber optic networks. Used in FDDI8, Token ring9 standards.

3. Other possible network topologies

Real computer networks are constantly expanding and modernizing. Therefore, such a network is almost always hybrid, i.e. its topology is a combination of several basic topologies. It is easy to imagine hybrid topologies that are a combination of star and bus, or ring and star.

3.1 Tree network topology

The tree topology can be considered as a union of several “stars”. It is this topology that is most popular today when building local networks.

Tree network topology diagram

In a tree topology, there is a root of the tree from which branches and leaves grow.

A tree can be active or true and passive. 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).

Figure 6 - Active tree network topology diagram

Figure 7 - Passive tree network topology diagram

3.2 Combined network topologies

Combined topologies are quite often used, among them the most common are star-bus and star-ring.

A star-bus topology uses a combination of a bus and a passive star.

Scheme of a combined star-bus network topology

Both individual computers and entire bus segments are connected to the hub. In fact, a physical bus topology is implemented that includes all computers on the network. In this topology, several hubs can be used, interconnected and forming the so-called backbone, support bus. Separate computers or bus segments are connected to each of the hubs. The result is a star-tire tree. Thus, the user can flexibly combine the advantages of bus and star topologies, and also easily change the number of computers connected to the network. From the point of view of information distribution, this topology is equivalent to a classic bus.

In the case of a star-ring topology, it is not the computers themselves that are united into a ring, but special hubs, to which the computers in turn are connected using star-shaped double communication lines.

Scheme of a combined star-ring network topology

In reality, all computers on the network are included in a closed ring, since within the hubs the communication lines form a closed loop (as shown in Figure 9). This topology makes it possible to combine the advantages of star and ring topologies. For example, hubs allow you to collect all network cable connection points in one place. If we talk about information dissemination, this topology is equivalent to a classic ring.

3.3 "Grid" network topology

Finally, mention should be made of a mesh or mesh topology, in which all or many computers and other devices are directly connected to each other (Figure 10).

Figure 10 - Network mesh topology diagram

This topology is extremely reliable - if any channel is broken, data transfer does not stop, since several routes for information delivery are possible. Mesh topologies (most often not complete, but partial) are used where it is necessary to ensure maximum network fault tolerance, for example, when connecting several sections of a large enterprise network or when connecting to the Internet, although, of course, you have to pay for this: cable consumption increases significantly, Network equipment and its configuration become more complicated.

Currently, the vast majority of modern networks use a star topology or a hybrid topology, which is an amalgamation of several stars (for example, a tree topology), and a CSMA/CD (carrier sense multiple access) transmission method. collision detection).

Fragment computer network

A fragment of a computer network includes the main types of communication equipment used today to form local networks and connect them through global connections with each other. To build local connections between computers, they are used different kinds cable systems, network adapters, repeater hubs, bridges, switches and routers. To connect local networks to global connections, special outputs (WAN ports) of bridges and routers are used, as well as data transmission equipment over long lines - modems (when working over analog lines) or devices connecting to digital channels(TA – terminal adapters ISDN networks, servicing devices for digital dedicated channels such as CSU/DSU, etc.).

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, a bus requires a minimal amount of connecting cable compared to other topologies. 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, the network adapter equipment with the “bus” topology is more complicated than with other topologies. 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 at the ends of the bus of special devices - 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 one 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 occurs exclusively through the central computer, which in this way places a very 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. A cut in any cable or a short circuit in it in a star topology disrupts communication with only one computer, 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 take part in the exchange itself.
Big star advantage(both active and passive) is that all connection points are collected in one place. This allows you to easily monitor 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, the great advantage of the ring is that the retransmission of signals by each subscriber makes it possible 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).

Few people are familiar with the term network topologies, but the average user has the concept of a local network computer equipment still there is. So network topologies are tools that determine the work of created computer networks, allowing you to simultaneously operate information through several machines.

Let's take a closer look at the concept of network topologies in this article, and also find out why they are needed, where and how to use them correctly, what types of these tools exist, what positive and negative characteristics they are endowed with.

Network Topologies - Introduction

Local computer networks cannot work without special network devices. Often more than two computers are involved in one network, often five, ten, twenty, there are networks that unite entire corporations. They are connected to each other by some kind of communication line. The interaction of machines connected to the network can be different. It is possible to combine several devices into one by creating several types of networks:

annular;
starry;
tire;
hierarchical;
arbitrary.

Among IT specialists, the creation of such networks is called topologies. This is a physical toolkit that is applicable to creating local networks. In addition, there are also logical topologies.

Physical and logical topologies operate independently and do not overlap. If physical ones are responsible for the geometry of the network, then logical ones are involved in the redistribution of data flows between various nodes of the created network and determine the most effective method data transmission.

Both physical and logical topologies have both advantages and disadvantages, so in modern times they are used equally. Below we will consider the main characteristics of each type of network topology and find out what their fundamental essence is.

Characteristics of bus topology: operating principle

If a linear mono channel is used when transmitting electronic data from one computer to another, this means that the bus topology of the network is involved in the work. It is at the ends of the mono channel that special so-called terminators are installed. Personal computers participating in the network are connected to shared network via a T-shaped connector in contact with the common line mono channel.

Electronic data arrives at terminators, and they arrive simultaneously at all network nodes, but must be accepted for consideration electronic documents Only the computer to which the message was intended can. The main transmission signal is captured by each computer machine involved in the network, therefore, the electronic data transmission medium is a common component of the network.

Bus topology has gained widespread popularity with the advanced capabilities of Ethernet architecture.

The main advantages of the bus topology are as follows:

ease of configuration, clear configuration of the created network;
the network is not interrupted if several computers included in it fail, which means that it is resistant to all kinds of computer problems.

The main disadvantages of the tire typology are:

the length of the network cable to be laid is limited, and the number of computer equipment included in the network is also limited;
the entire network depends on the health of the mono channel; if it suffers, the entire network suffers; it is often very difficult to find a point of failure in a bus network, especially when all its components are isolated.

Characteristics of star topology: operating principle

When creating a star-type network, each individual Personal Computer connects to a so-called hub or concentrator. Due to this, a parallel connection of all computer units included in the network is created. These components are the main connecting links that allow communication between computers included in the network.

This network also uses a common information field, that is, information is sent to all communication nodes, but can only be received by one section for which it was originally sent.

The main advantages of the star network:

easy to set up and connect new computer equipment;
just like a bus network, it is resistant to breakdowns of computers connected to the network;
allows centralized management of all connected units.

The main disadvantages of the star typology:

high consumption of network cable during installation;
The malfunction of one hub or concentrator leads to a failure of the entire electronic data transmission chain.

A star network can also be based on a central hub. It refers to an intelligent tool that connects certain computer units included in the network. The principle of output-input operation makes it possible not to use a common information field for all units, but to specify the transfer of information from one point to another, third, fourth... It turns out that each computer, in addition to hubs, is also connected to a central hub, if a breakdown occurs within the network, then the entire network does not suffer. In the event of a breakdown, the fault point spontaneously disconnects from the network, which allows you to quickly find it and eliminate all operating defects.

Laying such a network requires a large amount of network cable, but the efficiency of its operation is worth it.

The star typology can also be a kind of tree, which is a combination of several stars. Depending on the intertwining, the network’s active state, passive or true state is distinguished. Depending on the state, either hubs with concentrators or central computers are used to create connections between computer units included in the network.

If a central computer is chosen, then you can create a truly reliable and productive network, but not a cheap one. If you use hubs with concentrators, it will cost several times less, but the performance indicator will be significantly lower.

Characteristics of ring topology: operating principle

Ring topology implies the direct connection of all network channels into one unbroken chain. This does not mean that it is a typical circle. The essence of a ring network is that the output of one computer unit and the input of another are used to transmit electronic data. The movement of information occurs in one stream. If there is information at the output, and it is not received at the input, then it is returned to the output again with a subsequent attempt to reach the input. That is, information always moves along the same route from the sender to the recipient and back.

A logical ring tends to close. The main advantage of a ring network is that it is very easy to set up. But it is not reliable against unexpected breakdowns. If there is a defect in the circuit, the data ring is interrupted. Most often in practice, IT specialists implement projects of a modified ring typology.

Combined solutions for creating local computer networks

To ensure network reliability, combinations of basic network topologies are often used in practice. The most commonly used are star-bus or star-ring topologies. What is the result of combining several tools when laying local computer networks? The answer here is clear - ensuring network reliability, resistance to breakdowns and the absence of mandatory compliance with the principle of transmitting information along the chain, which simplifies work when defects occur in the network.

At the same time, both the operating principle of the network itself and the process of its installation are simplified.

Let's sum it up

Now you know the main types of network topologies. The options presented in this article are the most typical and used in the installation of modern local computer networks. But this does not mean that more advanced topologies are not used; these are often developed for specific service objects, for example, scientific or military ones. But for typical civilian applications, the network topologies discussed here are quite sufficient.

Existing topologies have been created for decades, so it makes sense to use them widely.

Introduction

1. Concept of network topology

2. Basic network topologies

2.3 Basic ring network topology

3. Other possible network topologies

3.1 Tree network topology

3.2 Combined network topologies

3.3 "Grid" network topology

4. Polysemy of the concept of topology

Conclusion

Bibliography

Introduction

Today it is impossible to imagine human activity without the use of computer networks.

Computer network is a distributed information processing system consisting of at least two computers interacting with each other using special means communications.

Depending on the remoteness of computers and scale, networks are conventionally divided into local and global.

Local networks are networks that have a closed infrastructure before reaching service providers. The term "LAN" can describe both a small office network and a large plant-level network covering several hundred hectares. Local networks are usually deployed within a certain organization, which is why they are also called corporate networks.

Sometimes networks of an intermediate class are distinguished - a city or regional network, i.e. network within a city, region, etc.

The global network covers large geographic regions, including both local networks and other telecommunications networks and devices. Global networks have almost the same capabilities as local ones. But they expand their scope. The benefits of using global networks are limited primarily by the speed of operation: global networks operate at a lower speed than local ones.

Of the computer networks listed above, we will turn our attention to local networks in order to better understand the architecture of networks and methods of data transmission. And for this you need to know such a thing as network topology.

1. Concept of network topology

Topology is the physical configuration of a network combined with its logical characteristics. Topology is a standard term used to describe the basic layout of a network. By understanding how different topologies are used, you can determine what capabilities different types of networks have.

There are two main types of topologies:

physical

logical

Logical topology describes the rules for interaction of network stations when transmitting data.

The physical topology determines how the storage media are connected.

The term "network topology" describes the physical arrangement of computers, cables, and other network components. The network topology determines its characteristics.

The choice of a particular topology affects:

composition of the necessary network equipment

network equipment characteristics

network expansion possibilities

network management method

When selecting a topology, you need to take into account that it ensures reliable and efficient operation of the network and convenient management of network data flows. It is also desirable that the network should be inexpensive in terms of the cost of creation and maintenance, but at the same time there would remain opportunities for its further expansion and, preferably, for the transition to higher-speed communication technologies. This is not an easy task! To solve it, you need to know what network topologies there are.

2. Basic network topologies

There are three basic topologies on which most networks are built.

star

ring

If computers are connected along a single cable, the topology is called a "bus". When computers are connected to cable segments originating from a single point, or hub, the topology is called a star topology. If the cable to which the computers are connected is closed in a ring, this topology is called a ring.

Although the basic topologies themselves are simple, in reality there are often quite complex combinations that combine the properties of several topologies.

2.1 Bus network topology

In this topology, all computers are connected to each other with one cable (Figure 1).

Figure 1 - Network topology diagram of the "bus" type

In a network with a "bus" topology, computers address data to a specific computer, transmitting it along the cable in the form of electrical signals - hardware MAC addresses. To understand the process of computer interaction via a bus, you need to understand the following concepts:

signal transmission

signal reflection

Terminator

1. Signal transmission

Data in the form of electrical signals is transmitted to all computers on the network; however, only the one whose address matches the recipient address encrypted in these signals receives information. Moreover, at any given time, only one computer can transmit. Since data is transmitted to the network by only one computer, its performance depends on the number of computers connected to the bus. The more there are, i.e. The more computers waiting to transfer data, the slower the network. However, it is impossible to derive a direct relationship between network bandwidth and the number of computers in it. Because, in addition to the number of computers, network performance is influenced by many factors, including:

hardware characteristics of computers on the network

the frequency with which computers transmit data

type of network applications running

network cable type

distance between computers on the network

2. Signal reflection

3. Terminator

To prevent electrical signals from being reflected, plugs (terminators) are installed at each end of the cable to absorb these signals (Figure 2). All ends of the network cable must be connected to something, such as a computer or a barrel connector - to increase the cable length. A terminator must be connected to any free - unconnected - end of the cable to prevent electrical signals from being reflected.

Figure 2 - Terminator installation

This network topology has advantages and disadvantages. The advantages include:

short network setup time

low cost (less cable and network devices required)

ease of setup

Failure of a workstation does not affect network operation

The disadvantages of this topology are as follows.

such networks are difficult to expand (increase the number of computers in the network and the number of segments - individual sections of cable connecting them).

Because the bus is shared, only one of the computers can transmit at a time.

The “bus” is a passive topology - computers only “listen” to the cable and cannot restore signals that are attenuated during transmission over the network.

The reliability of a network with a bus topology is low. When the electrical signal reaches the end of the cable, it (unless special measures are taken) is reflected, disrupting the operation of the entire network segment.

The problems inherent in the bus topology have led to the fact that these networks, so popular ten years ago, are now practically not used.

The bus network topology is known as 10 Mbps Ethernet logical topology.

2.2 Basic star network topology

In a star topology, all computers are connected via cable segments to a central component called a hub (Figure 3).

Signals from the transmitting computer travel through the hub to everyone else.

This topology originated in the early days of computing, when computers were connected to a central, main computer.

The term topology describes the physical arrangement of computers, cables, and other network components.

Topology is a standard term used by professionals to describe the basic layout of a network.

In addition to the term “topology”, the following is also used to describe the physical layout:

Physical location;

Layout;

Diagram;

The network topology determines its characteristics. In particular, the choice of a particular topology affects:

composition of the necessary network equipment;

characteristics of network equipment;

network expansion possibilities;

network management method.

To share resources or perform other network tasks, computers must be connected to each other. For this purpose, in most cases, a cable is used (less commonly, wireless networks - infrared equipment). However, simply connecting your computer to a cable that connects other computers is not enough. Different types of cables, combined with different network cards, network operating systems, and other components, require different computer layouts.

Each network topology imposes a number of conditions. For example, it can dictate not only the type of cable, but also the way it is laid.

Basic topologies

star
ring

If computers are connected along a single cable, the topology is called a bus. When computers are connected to cable segments originating from a single point, or hub, the topology is called a star topology. If the cable to which the computers are connected is closed in a ring, this topology is called a ring.

Tire.

The bus topology is often called a “linear bus”. This topology is one of the simplest and most widespread topologies. It uses a single cable, called a backbone or segment, along which all computers on the network are connected.

In a network with a bus topology, computers address data to a specific computer by transmitting it along a cable in the form of electrical signals.

Data in the form of electrical signals is transmitted to all computers on the network; however, the information is received by the one whose address matches the recipient address encrypted in these signals. Moreover, at any given time, only one computer can transmit.

Since data is transmitted to the network by only one computer, its performance depends on the number of computers connected to the bus. The more there are, the slower the network works. The bus is a passive topology. This means that computers only “listen” to data transmitted over the network, but do not move it from sender to recipient. Therefore, if one of the computers fails, it will not affect the operation of the others. In this topology, data is distributed throughout the network - from one end of the cable to the other. If no action is taken, the signals reaching the end of the cable will be reflected and this will not allow other computers to transmit. Therefore, after the data reaches the destination, the electrical signals must be extinguished. To do this, terminators (also called plugs) are installed at each end of the cable in a network with a bus topology to absorb electrical signals.

Advantages: the absence of additional active equipment (for example repeaters) makes such networks simple and inexpensive.

Linear local network topology diagram

However, the disadvantage of a linear topology is the limitations on network size, functionality and expandability.

Ring

In a ring topology, each workstation is connected to its two closest neighbors. This relationship forms a local network in the form of a loop or ring. Data is transmitted in a circle in one direction, and each station plays the role of a repeater, which receives and responds to packets addressed to it and transmits other packets to the next workstation “down”. In the original ring network, all objects were connected to each other. This connection had to be closed. Unlike the passive bus topology, here each computer acts as a repeater, amplifying the signals and passing them on to the next computer. The advantage of this topology was the predictable response time of the network. The more devices were in the ring, the longer the network took to respond to requests. Its most significant drawback is that if at least one device fails, the entire network refuses to function.

One of the principles of data transmission over a ring is called passing the token. The gist of it is this. The token is transmitted sequentially, from one computer to another, until the one that wants to transfer the data receives it. The sending computer modifies the token, places the email address in the data, and sends it around the ring.

This topology can be improved by connecting all network devices via hub(Hub device connecting other devices). Visually, a “tweaked” ring is no longer physically a ring, but in such a network data is still transmitted in a circle.

In the figure, solid lines indicate physical connections, and dotted lines indicate data transfer directions. Thus, such a network has a logical ring topology, while physically it is a star.

Star

In a star topology, all computers are connected via cable segments to a central component that has a hub. Signals from the transmitting computer travel through the hub to everyone else. In star networks, cabling and network configuration management are centralized. But there is also a disadvantage: since all computers are connected to a central point, cable consumption increases significantly for large networks. In addition, if the central component fails, the entire network will be disrupted.

Advantage: If one computer breaks down or the cable connecting one computer fails, then only that computer will not be able to receive and transmit signals. This will not affect other computers on the network. The overall network speed is limited only by the hub's bandwidth.

The star topology is dominant in modern local networks. Such networks are quite flexible, easily expandable and relatively inexpensive compared to more complex networks in which the methods of device access to the network are strictly fixed. Thus, “stars” have replaced outdated and rarely used linear and ring topologies. Moreover, they became a transitional link to the last type of topology - dialed stars e.

A switch is a multiport active network device. The switch “remembers” the hardware (or MAC–MediaAccessControl) addresses of devices connected to it and creates temporary paths from the sender to the recipient, along which data is transmitted. In a typical local network with a switched topology, there are several connections to a switch. Each port and the device that is connected to it has its own bandwidth (data transfer rate).

Switches can significantly improve network performance. First, they increase the total bandwidth that is available for a given network. For example, an 8-wire switch can have 8 separate connections, supporting speeds of up to 10 Mbit/s each. Accordingly, the throughput of such a device is 80 Mbit/s. First of all, switches increase network performance by reducing the number of devices that can fill the entire bandwidth of a single segment. One such segment contains only two devices: the workstation network device and the switch port. Thus, only two devices can “compete” for a bandwidth of 10 Mbit/s, and not eight (when using an ordinary 8-port hub, which does not provide for such division of bandwidth into segments).

In conclusion, it should be said that there is a distinction between the topology of physical connections (the physical structure of the network) and the topology of logical connections (the logical structure of the network)

Configuration physical connections is determined by the electrical connections of computers and can be represented as a graph, the nodes of which are computers and communications equipment, and the edges correspond to cable segments connecting pairs of nodes.

Logical connections represent the paths of information flows through the network; they are formed by appropriately configuring communication equipment.

In some cases, the physical and logical topologies are the same, and sometimes they are not.

The network shown in the figure is an example of a mismatch between the physical and logical topology. Physically, computers are connected using a common bus topology. Access to the bus occurs not according to a random access algorithm, but by transferring a token (token) in a ring pattern: from computer A to computer B, from computer B to computer C, etc. Here the order of token transfer no longer repeats physical connections, but is determined by the logical configuration of network adapters. There is nothing stopping you from configuring network adapters and their drivers so that the computers form a ring in a different order, for example B, A, C... However, the physical structure does not change.

Wireless network.

The phrase “wireless environment” can be misleading because it means that there are no wires on the network at all. In reality, wireless components typically interact with a network that uses cable as the transmission medium. Such a network with mixed components is called hybrid.

Depending on the technology, wireless networks can be divided into three types:

local area networks;

extended local area networks;

mobile networks (laptop computers).

Transfer methods:

infrared radiation;

radio transmission in a narrow spectrum (single-frequency transmission);
radio transmission in the scattered spectrum.

In addition to these methods of transmitting and receiving data, you can use mobile networks, packet radio connections, cellular networks and microwave data transmission systems.

Nowadays, an office network is not just about connecting computers to each other. It is difficult to imagine a modern office without databases that store both the financial statements of the enterprise and personnel information. In large networks, as a rule, for the security of databases and to increase the speed of access to them, separate servers are used to store databases. Also, now it is difficult to imagine a modern office without access to the Internet. Scheme option wireless network office is shown in the picture

So let's conclude: the future network must be carefully planned. To do this, you should answer the following questions:

Why do you need a network?

How many users will there be on your network?

How quickly will the network expand?

Does this network require Internet access?

Is centralized management of network users necessary?

After this, draw a rough diagram of the network on paper. You should not forget about the cost of the network.

As we have established, topology is the most important factor in improving overall network performance. Basic topologies can be used in any combination. It is important to understand that the strengths and weaknesses of each topology affect the desired network performance and depend on existing technologies. It is necessary to strike a balance between the actual location of the network (for example, in several buildings), the possibilities of using the cable, the path of its installation and even its type.

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