Grid topology. Network topologies: basic characteristics. Local networks concept and types

The topology of a computer network refers to the way in which its individual components (computers, servers, printers, etc.) are connected. There are three main topologies:

• star topology;
• ring type topology;
• common bus type topology.

When using a topology like star information between network clients is transmitted through a single central node. A server or a special device can act as a central node - hub(Hub).

The advantages of this topology are as follows:

1. High network performance, since the overall network performance depends only on the performance of the central node.
2. There is no collision of transmitted data, since data between the workstation and the server is transmitted over a separate channel without affecting other computers.

However, in addition to the advantages, this topology also has disadvantages:

1. Low reliability, since the reliability of the entire network is determined by the reliability of the central node. If the central computer fails, the entire network will stop working.
2. High costs for connecting computers, since a separate line must be installed for each new subscriber.

With a topology like ring all computers are connected to a line closed in a ring. Signals are transmitted along the ring in one direction and pass through each computer.

The transmission of information in such a network occurs as follows. A token (special signal) is transmitted sequentially, from one computer to another, until it is received by the one that needs to transfer the data. Once the computer receives the token, it creates what's called a "packet" in which it places the recipient's address and data, and then sends the packet around the ring. The data passes through each computer until it reaches the one whose address matches the recipient's address.

After this, the receiving computer sends confirmation to the information source that the data has been received. Having received confirmation, the sending computer creates a new token and returns it to the network.

The advantages of the ring topology are as follows:

1. Forwarding messages is very efficient because... You can send several messages one after another in a ring. Those. a computer, having sent the first message, can send the next message after it, without waiting for the first one to reach the recipient.
2. The length of the network can be significant. Those. computers can connect to each other over considerable distances, without the use of special signal amplifiers.

The disadvantages of this topology include:

1. Low network reliability, since the failure of any computer entails the failure of the entire system.
2. To connect a new client, you must disable the network.
3. With a large number of clients, the speed of the network slows down, since all information passes through each computer, and their capabilities are limited.
4. The overall performance of the network is determined by the performance of the slowest computer.

With a topology like common bus all clients are connected to a common data transmission channel. At the same time, they can directly come into contact with any computer on the network.

The transmission of information in this network occurs as follows. Data in the form of electrical signals is transmitted to all computers on the network. However, the information is received only by the computer whose address matches the recipient's address. Moreover, at any given time, only one computer can transmit data.

Advantages of the common bus topology:

1. All information is online and accessible to every computer.
2. Workstations can be connected independently of each other. Those. When a new subscriber connects, there is no need to stop the transmission of information on the network.
3. Building networks based on a common bus topology is cheaper, since there are no costs for laying additional lines when connecting a new client.
4. The network is highly reliable because The performance of the network does not depend on the performance of individual computers.

The disadvantages of a common bus topology include:

1. Low data transfer speed, because all information circulates through one channel (bus).
2. Network performance depends on the number of connected computers. The more computers are connected to the network, the slower the transfer of information from one computer to another.
3. Networks built on the basis of this topology are characterized by low security, since information on each computer can be accessed from any other computer.

The most common type of network topology common bus is an Ethernet standard network with an information transfer speed of 10 - 100 Mbit/s.

We looked at the main LAN topologies. However, in practice, when creating an organization's LAN, a combination of several topologies can be used simultaneously. For example, computers in one department can be connected according to a star scheme, and in another department using a common bus scheme, and a communication line is laid between these departments.

Updated – 2017-02-16

Types of local area network topology. This question may seem uninteresting and boring to some, but for general development, at least briefly, it will not hurt. Maybe somewhere you can even show off your knowledge of the local network, and they will start looking at you with respect. Or maybe your life will take a turn in such a way that you will even have to face this issue closely.

This is exactly what happened to me - what I was most afraid of was what I had to work with. And it turned out that all my fears were just from lack of knowledge, but now I even really like working on local networks and crimping cables myself. I will write briefly and clearly so as not to bore you with details that may not really be useful to you.

You can read about the advantages of local networks in these articles:

The physical connection diagram of computers is called network topology .

Exists three main typesnetwork topologies. Network Topology Types- what it is? Which network type to choose so that it is both cheap and reliable.

1. Ring network topology . With this type of network topology, the ends of the cables are connected to each other, i.e. form a ring. Each workstation is connected to two neighboring ones. 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.

The advantage of such a network is its fairly high reliability. The more computers there are in the ring, the longer the network takes to respond to requests. But the biggest drawback is that if at least one device fails, the entire network refuses to function. And the cost of such a network is high due to the cost of cables, network adapters and other equipment.

2. Linear network topology or common bus . In a linear topology, all network elements are connected one after another using a single cable.

The ends of the segments must be terminated with special resistances called terminators .

When creating such a network, no additional equipment is used - only a cable. All connected devices on such a network “listen” and accept only those packets of information that are intended only for them, and the rest are ignored.

The advantages of such a network are ease of organization and low cost. But a significant drawback is low resistance to damage. Any damage to the cable entails the failure of the entire network. Moreover, troubleshooting is very difficult.

3. Star topology is dominant in modern local networks. It is the most functional and stable. Each computer on the network is connected to a special device called a hub or switch. When creating this topology, each device gains access to the network independently of each other, and if one connecting cable breaks, only one of the network elements stops working, which greatly simplifies troubleshooting.

In addition, such a network allows you to connect new devices without problems and changes in connecting old devices. You can expand and connect several networks into one network. It is enough to connect a cable from one switch to another switch.

Such networks are quite flexible, easily expandable and relatively inexpensive. So we looked at types of network topologies. Next time I will tell you about network devices.

Now you can choose the type of connection for your home computers and create.

Network topology (from the Greek τόπος, - place) - a way of describing the network configuration, a diagram of the location and connection of network devices.
(Wikiredia)

Topology - this is a diagram of connecting computers or network nodes to each other by communication channels.
The network topology can be

• physical - describes the actual location and connections between network nodes.
• logical - describes the signal flow within the physical topology.
• informational - describes the direction of information flows transmitted over the network.
• exchange management is the principle of transferring the right to use the network.

There are many ways to connect network devices. The following topologies are distinguished:

• fully connected
• cellular
• common bus
• star
• ring
• snowflake

Let's look at each of them in more detail.

1) Fully connectedtopology- a computer network topology in which each workstation is connected to all the others. This option is cumbersome and ineffective, despite its logical simplicity. An independent line must be allocated for each pair; each computer must have as many communication ports as there are computers on the network. For these reasons the network

can only have relatively small final dimensions. Most often, this topology is used in multi-machine systems or global networks with a small number of workstations.

Access technology in networks of this topology is implemented by the token passing method. A marker is a package equipped with a special sequence of bits (it can be compared to a letter envelope). It is transmitted sequentially along the ring from computer to computer in one direction. Each node relays the transmitted token. The computer can transmit its data if it receives an empty token. The token with the packet is transmitted until the computer to which the packet is destined is found. In this computer, the data is received, but the token moves on and returns to the sender.
After the computer that sent the packet verifies that the packet has been delivered to the recipient, the token is released.

Disadvantage: g cumbersome and ineffective option, i.e. To . Each computer should have a large number communication ports

2) Mesh topology - a basic mesh computer network topology in which each workstation on the network is connected to several other 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 to connect to other computers. A cable break will not result in loss of connection between two computers.

Obtained from a fully connected one by removing some possible connections. This topology allows the connection of a large number of computers and is typically typical for large networks.

3) Common bus, is a common cable (called a bus or backbone) to which all workstations are connected. There are terminators at the ends of the cable to prevent signal reflection.

Advantages:

Flaws:

• Network problems, such as cable breaks and terminator failure, completely block the operation of the entire network;
• Difficult fault localization;
• As new workstations are added, network performance decreases.

A bus topology is a topology in which all LAN devices are connected to a linear network data transmission medium. This linear medium is often called a channel, bus, or trace. Each device, such as a workstation or server, is independently connected to a common bus cable using a special connector. The bus cable must have a termination resistor, or terminator, at the end, which absorbs the electrical signal, preventing it from being reflected and moving in the opposite direction along the bus.

4) Star - the basic topology of a computer network in which all computers on the network are connected to a central node (usually a switch), forming a physical segment of the network. Such a network segment can function either separately or as part of a complex network topology (usually a “tree”). All information exchange takes place exclusively through the central computer, which is subject to a very large load in this way, so it cannot do anything else except the network. As a rule, it is the central computer that is the most powerful, and it is on it that all functions for managing the exchange are assigned. In principle, no conflicts are possible in a network with a star topology, because management is completely centralized.

The access method is implemented using Arcnet technology. This access method also uses a token to transmit data. The token is passed from computer to computer in ascending address order. As in a ring topology, each computer regenerates a token.

Comparison with other topologies.

Advantages:

• 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:

• failure of the central hub will result in the inoperability of the network (or network segment) as a whole;
• laying a network 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.

5) Ring - this is the 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 every line of communication, as in the case stars , only one transmitter and one receiver are working. This allows you to avoid using external terminators.

The work in a ring network 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 ring 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.

Computers in a ring 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 a bus topology, the maximum number of subscribers in a ring can be quite large (1000 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) .

In a ring, unlike other topologies (star, bus), the concurrent method of sending data is not used; the computer on the network receives data from the previous one in the list of recipients and redirects it further if it is not addressed to it. The mailing list is generated by a computer, which is the token generator. The network module generates a token signal (usually about 2-10 bytes to avoid attenuation) and transmits it to the next system (sometimes in ascending order of the MAC address). The next system, having received the signal, does not analyze it, but simply transmits it further. This is the so-called zero cycle.

The subsequent operating algorithm is as follows - the GRE data packet transmitted by the sender to the recipient begins to follow the path laid by the marker. The packet is transmitted until it reaches the recipient.

Comparison with other topologies.

Advantages:

• Easy to install;
• Almost complete absence of additional equipment;
• Possibility of stable operation without a significant drop in data transfer speed under heavy network load, since the use of a marker eliminates the possibility of collisions.

Flaws:

• The failure of one workstation and other problems (cable break) affect the performance of the entire network;
• Complexity of configuration and setup;
• Difficulty in troubleshooting.
• The need to have two network cards on each workstation.

6) Cnezhinka ( Hierarchical Star or tree topology) -star-type topology, but several concentrates are used, hierarchically interconnected by star-type connections.A snowflake topology requires less cable length than a star, but more elements.

Most common way of connections both in local networks and the site lyceum1.perm.ru

Few people are familiar with the term network topologies, but the average computer user still has the concept of a local network. So, network topologies are tools that determine the operation of created computer networks that allow 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 of data transfer.

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 the common network through a T-shaped connector in contact with a common mono line channel.

Electronic data arrives at the terminators, and they arrive simultaneously at all network nodes, but only the computer to which the message was intended can accept electronic documents for review. 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 is connected 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.

Under topology(layout, configuration, structure) of a computer network usually refers to the physical location of computers on the network relative to each other 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, since each communication session can be carried out along its own path.

The topology determines the requirements for equipment, the type of cable used, the permissible and most convenient methods of managing the exchange, reliability of operation, and possibilities for network expansion. And although a network user rarely has to choose a topology, it is necessary to know about the features of the main topologies, their advantages and disadvantages.

Factors affecting the physical performance of the network and directly related to the concept of topology:

1) Serviceability of computers (subscribers) connected to the network. In some cases, a subscriber breakdown can block the operation of the entire network. Sometimes a subscriber malfunction does not affect the operation of the network as a whole and does not prevent other subscribers from exchanging information.

2) Serviceability of network equipment, that is, technical equipment directly connected to the network (adapters, transceivers, connectors, etc.). The failure of one subscriber's network equipment can affect the entire network, but can disrupt communication with only one subscriber.

3) Network cable integrity. If a network cable breaks (for example, due to mechanical impacts), the exchange of information throughout the entire network or in one of its parts may be disrupted. For electrical cables, a short circuit in the cable is equally critical.

4) Cable length limitation, associated with the attenuation of the signal propagating through it. As you know, in any medium, when a signal propagates, it weakens (attenuates). And the greater the distance the signal travels, the more it attenuates (Fig. 1.8). It is necessary to ensure that the length of the network cable does not exceed the maximum length Lpr, beyond which the attenuation becomes unacceptable (the receiving subscriber does not recognize the weakened signal).

Rice. 1.8. Signal attenuation when propagating through a network

There are three basic network topologies:

Bus (bus) - all computers are connected in parallel to one communication line. Information from each computer is simultaneously transmitted to all other computers (Fig. 1.5).

Rice. 1.5. Network topology bus

Bus topology(or, as it is also called, a common bus) by its very structure assumes the identity of the network equipment of computers, as well as the equality of all subscribers in accessing the network. Computers on the bus can transmit information only one at a time, since in this case there is only one communication line. If several computers transmit information simultaneously, it will be distorted as a result of overlap (conflict, collision). The bus always implements the so-called half-duplex exchange mode (in both directions, but in turn, and not simultaneously).

In the bus topology, there is no clearly defined central subscriber through which all information is transmitted, this increases its reliability (after all, if the center fails, the entire system controlled by it 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, the bus requires a minimal amount of connecting cable compared to other topologies.

Since there is no central subscriber, resolving possible conflicts in this case falls on the network equipment of each individual subscriber. In this regard, network equipment in the bus topology is more complex than in other topologies. However, due to the widespread use of networks with a bus topology (primarily the most popular Ethernet network), the cost of network equipment is not too high.

Rice. 1.9. Cable break in a network with bus topology

An important advantage of the bus is that if any of the computers on the network fails, healthy machines will be able to continue communication normally.

It would seem that if the cable breaks, you get two fully functional buses (Fig. 1.9). However, it must be taken into account that 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 matching devices, terminators, shown in Fig. 1.5 and 1.9 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. 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.

A failure of any subscriber's network equipment on the bus can bring down the entire network. In addition, such a failure is quite difficult to localize, since all subscribers are connected in parallel, and it is impossible to understand which of them has failed.

When passing through a network communication line with a bus topology, information signals are weakened and not restored in any way, which imposes strict restrictions on the total length of communication lines. Moreover, each subscriber can receive signals of different levels from the network depending on the distance to the transmitting subscriber. This places additional demands on receiving nodes of network equipment.

If we assume that the signal in the network cable is attenuated to the maximum permissible level over the length Lpr, then the total length of the bus cannot exceed the value of Lpr. In this sense, the bus provides the shortest length compared to other basic topologies.

To increase the length of a network with a bus topology, several segments (parts of the network, each of which represents a bus) are often used, interconnected using special amplifiers and signal restorers - repeaters or repeaters (Fig. 1.10 shows the connection of two segments, the maximum network length in this case it increases to 2 Lpr, since each of the segments can be of length Lpr). However, this increase in network length cannot continue indefinitely. Length restrictions are associated with the finite speed of signal propagation along communication lines.

Rice. 1.10. Connecting bus network segments using a repeater

Star— one central computer is connected to other peripheral computers, each of them using a separate communication line (Fig. 1.6). Information from a peripheral computer is transmitted only to the central computer, and from the central computer - to one or more peripheral ones.

Rice. 1.6. Star network topology

Star- this is the only network topology with a clearly designated center to which all other subscribers are connected. The exchange of information occurs exclusively through the central computer, which bears a heavy load, so, as a rule, 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 all subscribers (as in a bus). Usually the central computer is the most powerful; all functions for managing the exchange are assigned to it. In principle, no conflicts are possible in a network with a star topology, since management is completely centralized.

If we talk about the star’s resistance to computer failures, then the failure of a peripheral computer or its network equipment does not in any way affect the functioning of the rest of the network, but any failure of the central computer makes the network completely inoperable. In this regard, special measures must be taken to increase the reliability of the central computer and its network equipment.

A cable break or short circuit in a star topology disrupts communication with only one computer, and all other computers can continue to work normally.

Unlike a bus, in a star there are only two subscribers on each communication line: a central one and one of the peripheral ones. Most often, two communication lines are used to connect them, each of which transmits information in one direction, that is, on each communication line there is only one receiver and one transmitter. This is the so-called point-to-point transmission. All this significantly simplifies network equipment 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 in a star more easily than in the case of a bus, because each receiver always receives a signal of the same level. The maximum length of a network with a star topology can be twice as long as in a bus (that is, 2 Lpr), since each of the cables connecting the center with a peripheral subscriber can have a length of Lpr.

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. Within these limits, connecting new subscribers is quite simple, but beyond them it is simply impossible. In a star, it is possible to connect another central subscriber instead of a peripheral one (the result is a topology of several interconnected stars).

The star shown in Fig. 1.6, is called an active or true star. There is also a topology called passive star, which is only superficially similar to a star (Fig. 1.11). Currently, it is much more widespread than an active star. Suffice it to say that it is used in the most popular Ethernet network today.

In the center of a network with this topology there is not a computer, but a special device - a concentrator or, as it is also called, a hub, which performs the same function as a repeater, that is, it restores incoming signals and forwards them to all other communication lines .

Rice. 1.11. Passive star topology and its equivalent circuit

It turns out that although the cable layout is similar to a true or active star, in fact we are talking about a bus topology, since information from each computer is simultaneously transmitted to all other computers, and there is no central subscriber. Of course, a passive star is more expensive than a regular bus, since in this case a hub is also required. However, it provides a number of additional features associated with the benefits of a star, in particular, it simplifies network maintenance and repair. That is why recently the passive star is increasingly replacing the true 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 arriving at it, but also controls the exchange, but does not itself participate in the exchange (this is done in the 100VG-AnyLAN network).

The great advantage of a star (both active and passive) is that all connection points are collected in one place. This makes it possible to easily monitor the operation of the network, localize faults by simply disconnecting certain subscribers from the center (which is impossible, for example, in the case of a bus topology), and also limit the access of unauthorized persons to connection points vital for the network. In the case of a star, a peripheral subscriber can be approached by either one cable (which transmits in both directions) or two (each cable transmits in one of two opposite directions), with the latter being much more common.

A common disadvantage for all star topologies (both active and passive) is the significantly higher cable consumption than other topologies. For example, if computers are located in one line (as in Fig. 1.5), then when choosing a star topology you will need several times more cable than when choosing a bus topology. This significantly affects the cost of the network as a whole and significantly complicates cable installation.

Ring— computers are connected in series into a ring. The transmission of information in the ring is always carried out in only one direction. Each computer transmits information only to one computer next in the chain behind it, and receives information only from the previous computer in the chain (Fig. 1.7).

Rice. 1.7. Network topology ring

Ring is a topology in which each computer is connected by communication lines to two others: from one it receives information and transmits it to the other. On each communication line, as in the case of a star, only one transmitter and one receiver operate (point-to-point communication). This allows you to avoid using external terminators.

An important feature of the ring is that each computer relays (restores, amplifies) the signal coming to it, that is, it acts as a repeater. Signal attenuation in the entire ring does not matter, only the attenuation between neighboring computers on the ring matters. If the maximum cable length, limited by attenuation, is Lpr, then the total length of the ring can reach NLpr, where N is the number of computers in the ring. The total size of the network will ultimately be NLpr/2, since the ring will have to be folded in half. In practice, the size of ring networks reaches tens of kilometers (for example, in an FDDI network). The ring is significantly superior to any other topology in this regard.

In a ring topology, there is no clearly defined center; all computers can be identical and have equal rights. However, quite often a special subscriber is allocated in the ring who manages or controls the exchange. It is clear that the presence of such a single control subscriber reduces the reliability of the network, since its failure will immediately paralyze the entire exchange.

Strictly speaking, computers in a ring are not completely equal (unlike, for example, a bus topology). After all, one of them necessarily receives information from the computer transmitting at the moment earlier, and the others - later. It is on this feature of the topology that methods for controlling network exchange, specifically designed for the ring, are based. In such 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 quite simple, although it requires a mandatory shutdown of the entire network for the duration of the connection.

As with a bus, the maximum number of subscribers in a ring can be quite large (up to a thousand or more). The ring topology usually has high resistance to overloads, ensures reliable operation with large flows of information transmitted over the network, since, as a rule, there are no conflicts (unlike a bus), and there is also no central subscriber (unlike a star), which can be overloaded with large flows of information.

Rice. 1.12. Two ring network

The signal in the ring passes sequentially through all computers on the network, so the failure of at least one of them (or its network equipment) disrupts the operation of the network as a whole. This is a significant drawback of the ring.

Likewise, a break or short circuit in any of the ring cables makes the entire network impossible to operate. Of the three topologies considered, the ring is the most vulnerable to cable damage, therefore, in the case of the ring topology, it is usually necessary to lay two (or more) parallel communication lines, one of which is in reserve.

Sometimes a network with a ring topology is based on two parallel ring communication lines that transmit information in opposite directions (Fig. 1.12). The purpose of such a solution is to increase (ideally, double) the speed of information transfer over the network. In addition, if one of the cables is damaged, the network can work with another cable (although the maximum speed will decrease).