The speed of data transfer in the network, the reliability of servicing customer requests, the network’s resistance to equipment failures, and the cost of creating and operating the network are significantly influenced by its topology.

Under computer network topology refers to the way of connecting its individual components (computers, servers, printers, etc.). The following main topologies are distinguished:

· star topology;

· ring type topology;

· common bus type topology;

· tree topology;

· fully connected network.

Let's consider the network topology data.

Star topology. When using a star topology, information between network clients is transmitted through a single central node (Fig. 11). A server or a special device – a hub (Hub) – can act as a central node.

Rice. 11. Star topology

In star topology can be used active And passive hubs. Active concentrators receive and amplify transmitted signals. Passive hubs pass signals through themselves without amplifying them. Passive hubs do not require connection to a power source.

The advantages of the star topology are as follows:

1. High network performance, since the overall network performance depends only on the performance of the central node.

2. 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 node (server or hub) fails, the entire network will stop working.

2. High costs for connecting computers, since a separate line must be installed for each new subscriber.

3. Lack of ability to select different routes to establish communications between subscribers.

This topology is currently the most common.

Ring topology. With a ring topology, all computers are connected to a cable closed in a ring. Signals are transmitted along the ring in one direction and pass through each computer (Fig. 12).

Rice. 12. Ring topology

The transmission of information in this network occurs as follows. Marker(special signal) is transmitted sequentially, from one computer to another, until it is received by the one that wants to transmit the data. Having received the token, the computer creates a so-called packet, which is used to transmit data. The packet contains the recipient's address and data and is then sent around the ring. The packet 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 packet 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.

3. Absence of collisions (see topic No. 3, section 2) and data collisions, since only one computer is transmitting at a time.

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 interrupt 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. Overall network performance is determined by the performance of the slowest computer.

This topology benefits if the organization creates a system of distributed information processing centers located at a considerable distance from each other.

Common bus topology. With a bus topology, all clients are connected to a common data transmission channel (Fig. 13). At the same time, they can directly come into contact with any computer on the network.

Fig. 13. Common bus topology

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

Advantages of the common bus topology:

1. All information is online and accessible to every computer. Those. from any personal computer you can access information that is stored on any other 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 last advantage is determined by the fact that the bus is a passive topology. Those. computers only receive transmitted data, but do not move it from the sender to the recipient. Therefore, if one of the computers fails, it will not affect the operation of the others.

The disadvantages of a common bus topology include:

1. Low data transfer speed, since 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 more loaded the bus is and the slower the transfer of information from one computer to another.

3. Networks built on this topology are characterized by low security, since information on each computer can be accessed from any other computer.

Tree topology. In networks with a tree topology, computers are directly connected to the central nodes of the network - servers (Fig. 14).

Fig. 14. Tree topology

A tree topology is a combination of a star topology and a bus topology. Therefore, it basically has the same advantages and disadvantages that were indicated for these topologies.

Mesh network. In a mesh network, each computer is connected to all other computers by separate lines (Fig. 15).

Fig. 15. Mesh network

Advantages of a mesh network:

1. High reliability, since if any communication channel fails, a workaround will be found to transmit information.

2. High performance, since information is transmitted between computers via separate lines.

Disadvantages of this topology:

1. This topology requires a large number of connecting lines, i.e. the cost of creating such a network is very high.

2. It is difficult to build a network with a large number of computers, since separate lines must be laid from each computer to the others.

The mesh network topology is usually used for small networks with a small number of computers that operate with a full load of communication channels.

For large computer networks (global or regional), a combination of different topologies is usually used for different areas.

LAN models

There are two models of local area networks:

· peer-to-peer network;

· client-server network.

IN peer-to-peer network all computers are equal to each other. In this case, all information in the system is distributed between separate computers. Any user can allow or deny access to their data. In such networks, the same type of operating system (OS) is installed on all computers, which provides potentially equal capabilities to all computers on the network.

Advantages of this model:

1. Ease of implementation. To implement this network, it is sufficient that the computers have network adapters and a cable that will connect them.

2. Low cost of network creation. Since there are no costs associated with purchasing an expensive server, an expensive network operating system, etc.

Disadvantages of the model:

1. Low performance for network requests. A workstation always processes network requests slower than a specialized server computer. In addition, various tasks are always performed on the workstation (typing text, creating drawings, mathematical calculations, etc.), which slow down responses to network requests.

2. Lack of a unified information base, since all information is distributed on separate computers. In this case, you have to contact several computers to obtain the necessary information.

3. Lack of a unified information security system. Every personal computer protects its information through its operating system. However, personal computer operating systems tend to be less secure than network server operating systems. Therefore, it is much easier to “hack” such a network.

4. The dependence of the availability of information in the system on the state of the computer. If a computer is turned off, the information stored on it will not be available to other users.

On a network like client-server there are one or more main computers - servers. In such systems, all basic information is managed by servers.

A client-server network is functionally asymmetrical: it uses two types of computers - some are focused on performing server functions and run specialized server operating systems, while others perform client functions and run regular operating systems. Functional asymmetry is also caused by hardware asymmetry - for dedicated servers, more powerful computers with large amounts of RAM and external memory are used.

The advantages of this model are:

1. High network performance, since the server quickly processes network requests and is not loaded with other tasks.

2. Availability of a unified information base and security system. It is possible to hack a server, but it is much more difficult than a workstation.

3. Easy to manage the entire network. Since network management consists mainly of managing only the server.

Disadvantages of the model:

1. High cost of implementation, since it is necessary to buy an expensive server and a network operating system for the server.

2. Dependence of network speed on the server. If the server is not powerful enough, the network may become very slow.

3. For proper operation of the network, additional maintenance personnel are required, i.e. The organization must have a network administrator position.

Computer network topology

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

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

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

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

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

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

Rice. 6.1.

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

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

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

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

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

Rice. 6.2.

Rice. 6.3. Star topology

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

Rice. 6.4.

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

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

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

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

Rice. 6.5.

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

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

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

Main characteristics of modern computer networks

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

To the main characteristics productivity networks include:

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

For reliability assessments networks use a variety of characteristics, including:

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

Extensibility means the ability to relatively easily add individual network elements (users, computers, applications, services), increase the length of network segments and replace existing equipment with more powerful ones.

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

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

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

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

Local area network topologies

Chapter 1. Basic concepts of network technologies.

When creating a computer data transmission network, when all network computers and other network devices are connected, a computer network topology .

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

Physical topology of the data network

Historically, certain types of physical network topologies have evolved. Let's look at some of the most common topologies.

"Common bus"

Until recently, the common bus was the most common topology for local networks. In this case, computers are connected to one coaxial cable using the “mount OR” circuit. The transmitted information, in this case, is distributed in both directions.

The use of the “common bus” topology reduces the cost of cabling, unifies the connection of various modules, and provides the possibility of almost instantaneous broadcast access to all network stations. The main advantages of this scheme are the low cost and ease of cable distribution throughout the premises. The most serious disadvantage of the common bus is its low reliability: any defect in the cable or any of the numerous connectors completely paralyzes the entire network.

Another disadvantage of the shared bus is its low performance, since with this connection method only one computer at a time can transmit data to the network. Therefore, the communication channel bandwidth is always divided here between all network nodes.

Figure 5. Diagram of connecting computers using the “common bus” scheme.

Star topology

In this case, each computer is connected by a separate cable to a common device called a switch (concentrator, hub) which is located in the center of the network. The functions of a switch include directing information transmitted by a computer to one or all other computers on the network. The main advantage of this topology over a common bus is significantly greater reliability. Any troubles with the cable affect only the computer to which this cable is connected, and only a malfunction of the switch can bring down the entire network. In addition, the switch can play the role of an intelligent filter of information coming from nodes on the network and, if necessary, block transmissions prohibited by the administrator.

Network hub or Hub (jarg from the English hub - activity center) - a network device designed to combine several Ethernet devices into a common network segment. Devices are connected using twisted pair, coaxial cable or optical fiber. The term hub is also applicable to other data transfer technologies: USB, FireWire, etc.

Currently, network hubs are not produced; they have been replaced by network switches, separating each connected device into a separate segment.

Figure 6. Star connection diagram for computers

Ring topology

In information and computer networks with a ring configuration Data is transmitted along a ring from one computer to another, usually in one direction. If the computer recognizes the data as “its own,” then it copies it to its internal buffer. The ring is a very convenient configuration for organizing feedback - the data, having made a full revolution, returns to the source node. Therefore, this node can control the process of delivering data to the recipient. Often this ring property is used to test network connectivity and find a node that is not working correctly. For this purpose, special test messages are sent to the network.

In a network with a ring topology, it is necessary to take special measures so that in the event of a failure or disconnection of any station, the communication channel between the remaining stations is not interrupted.

Since such duplication increases the reliability of the system, this standard is successfully used in trunk communication channels.

This physical topology is successfully implemented in networks created using FDDI technology.

FDDI (Fiber Distributed Data Interface) - standard for data transmission in a local network, up to 200 kilometers long. The standard is based on the protocol Token Bus . It is recommended to use fiber optic cable as the data transmission medium in FDDI, but copper cable can also be used, in which case the abbreviation is used CDDI (Copper Distributed Data Interface). The circuit is used as a topology double ring, while data circulates in the rings in different directions. One ring is considered the main one; information is transmitted through it in the normal state; the second is auxiliary; data is transmitted through it in the event of a break on the first ring. To control the state of the ring, a network token is used, as in Token Ring technology.

Figure 7. Diagram of connecting computers according to the “ring” scheme

Fully connected topology

Fully connected topology corresponds to a network in which each computer on the network is connected to all the others. Despite its logical simplicity, this option turns out to be cumbersome and ineffective. Indeed, each computer on the network must have a large number of communication ports, sufficient to communicate with each of the other computers on the network. A separate electrical communication line must be allocated for each pair of computers. Fully connected topologies are rarely used, since they do not satisfy any of the above requirements. More often, this type of topology is used in multi-machine systems or global networks with a small number of computers.

Figure 8. Diagram of connecting computers according to the “fully connected topology” scheme

Mesh topology

Mesh topology ) is obtained from a fully connected one by removing some possible connections. In a network with a mesh topology, only those computers between which intensive data exchange occurs are directly connected, and for data exchange between computers that are not directly connected, transit transmissions through intermediate nodes are used. The mesh topology allows the connection of a large number of computers and is typically characteristic of global networks.

Figure 9. Diagram of connecting computers according to the “mesh topology” scheme

While small networks typically have a typical topology - star, ring or bus, large networks are characterized by a symbiosis of various topologies. In such networks, it is possible to identify individual randomly connected fragments (subnetworks) that have a standard topology, which is why they are called networks with mixed topology.

Tree topology

This topology is mixed; systems with different topologies interact here. This method of mixed topology is most often used when building a LAN with a small number of network devices, as well as when creating corporate LANs. This topology combines relatively low cost and fairly high performance, especially when using various data transmission media - a combination of copper cable systems, fiber-optic lines, and also using managed switches.

Figure 10. Computer connection diagram using a tree diagram

In “common bus” and “ring” type topologies, communication lines connecting network elements (computers, network devices, etc.) are distributed . When shared, the line resource is divided between network devices, i.e. they are public communication lines.

Besides distributed , exist individual communication lines , when each network element has its own (not always the only) communication line. An example is a network built using a “star” topology, when a switch-type device is located in the center, and each computer is connected by a separate communication line.

The total cost of a network built using distributed communication lines will be much lower, however, the performance of such a network will be lower, because a network with a distributed environment with a large number of nodes will always work slower than a similar network with individual communication lines, since the capacity of the individual line communication goes to one computer, and when used together, it is divided among all computers on the network.

In modern networks, including global ones, only communication lines between end nodes and network switches are individual, and connections between switches (routers) remain distributed, since messages from different end nodes are transmitted over them.

Figure 11. Individual and distributed communication lines in switch-based networks

Logical topology of the data network

In addition to the physical topology of the data network, it is also assumed logical network topology. The logical topology defines the data transmission routes in the network. There are configurations in which the logical topology differs from the physical one. For example, a network with a physical “star” topology may have a logical “bus” topology - it all depends on how the network switch or Internet gateway, router is designed (VLAN, presence of VPN, etc.).

To determine the logical topology of a network, you need to understand how signals are received in it:

in logical bus topologies, each signal is received by all devices;

In logical ring topologies, each device receives only those signals that were sent specifically to it.

In addition, it is important to know how network devices access the information transmission medium.

Topology of 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 different types of networks have.

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, 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.

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, 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

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 to them can occur 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 originated in the early days of computing, when 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 Ethernet technology6.

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 of a 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 to each other. To build local connections between computers, various types of cable systems, network adapters, repeater hubs, bridges, switches and routers are used. To connect local networks to global communications, 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 for connecting to digital channels (TA - terminal adapters for ISDN networks, servicing devices for digital dedicated channels such as CSU/DSU, etc.).

Topic 1.4: Basics of local networks

Topic 1.5: Basic technologies of local networks

Topic 1.6: Basic software and hardware components of a LAN

Local networks

1.4. LAN Basics

1.4.3. Network topologies

All computers on the local network are connected by communication lines. The geometric location of communication lines relative to network nodes and the physical connection of nodes to the network is called physical topology. Depending on the topology, networks are distinguished: bus, ring, star, hierarchical and arbitrary structures.

There are physical and logical topologies. Logical and physical network topologies are independent of each other. Physical topology is the geometry of the network, and logical topology determines the directions of data flows between network nodes and methods of data transmission.

Currently, the following physical topologies are used in local networks:

• physical "bus" (bus);
• physical “star” (star);
• physical “ring” (ring);
• physical "star" and logical "ring" (Token Ring).

Bus topology

Networks with a bus topology use a linear monochannel (coaxial cable) for data transmission, at the ends of which terminating resistors (terminators) are installed. Each computer is connected to a coaxial cable using a T-connector (T - connector). Data from the transmitting network node is transmitted along the bus in both directions, reflected from the terminal terminators. Terminators prevent signals from being reflected, i.e. are used to cancel signals that reach the ends of a data link.

Thus, information arrives at all nodes, but is received only by the node to which it is intended. In a logical bus topology, the data transmission medium is shared and simultaneously by all PCs on the network, and signals from the PCs are distributed simultaneously in all directions along the transmission medium. Since the transmission of signals in the topology, the physical bus is broadcast, i.e. signals propagate simultaneously in all directions, then the logical topology of this local network is a logical bus.

Rice. 1.

This topology is used in local networks with Ethernet architecture (classes 10Base-5 and 10Base-2 for thick and thin coaxial cable, respectively).

Advantages of bus topology networks:

• the failure of one of the nodes does not affect the operation of the network as a whole;
• the network is easy to set up and configure;
• The network is resistant to failures of individual nodes.

Disadvantages of bus topology networks:

• a cable break can affect the operation of the entire network;
• limited cable length and number of workstations;
• connection defects are difficult to identify.

Star topology

In a network built using a star topology, each workstation is connected by a cable (twisted pair) to a hub or hub ( hub). The hub provides a parallel connection between PCs and thus all computers connected to the network can communicate with each other.

Rice. 2.

Data from the network transmitting station is transmitted through the hub along all communication lines to all PCs. Information arrives at all workstations, but is received only by those stations for which it is intended. Since signal transmission in the physical star topology is broadcast, i.e. Since signals from the PC propagate simultaneously in all directions, the logical topology of this local network is a logical bus.

This topology is used in local networks with 10Base-T Ethernet architecture.

Advantages of star topology networks:

• easy to connect a new PC;
• there is the possibility of centralized management;
• The network is resistant to failures of individual PCs and to interruptions in the connection of individual PCs.

Disadvantages of star topology networks:

• hub failure affects the operation of the entire network;
• high cable consumption.

Ring topology

In a network with a ring topology, all nodes are connected by communication channels into a continuous ring (not necessarily a circle) through which data is transmitted. The output of one PC is connected to the input of another PC. Having started the movement from one point, the data ultimately ends up at its beginning. Data in a ring always moves in the same direction.

Rice. 3.

The receiving workstation recognizes and receives only the message addressed to it. A network with a physical ring topology uses token access, which grants a station the right to use the ring in a specific order. The logical topology of this network is a logical ring. This network is very easy to create and configure.

The main disadvantage of ring topology networks is that damage to the communication line in one place or PC failure leads to the inoperability of the entire network.

As a rule, the “ring” topology is not used in its pure form due to its unreliability, therefore, in practice, various modifications of the ring topology are used.

Token Ring Topology

This topology is based on the star physical ring topology. In this topology, all workstations are connected to a central hub (Token Ring) like a physical star topology. A central hub is an intelligent device that, using jumpers, provides a serial connection between the output of one station and the input of another station.

In other words, with the help of a hub, each station is connected to only two other stations (previous and subsequent stations). Thus, workstations are connected by a cable loop through which data packets are transmitted from one station to another and each station relays these sent packets. Each workstation has a transceiver device for this purpose, which allows you to control the passage of data in the network. Physically, such a network is built according to the “star” type of topology.

The hub creates a primary (main) and backup ring. If a break occurs in the main ring, it can be bypassed by using the backup ring, since a four-core cable is used. A failure of a station or a break in the communication line of a workstation will not result in a network failure as in a ring topology, because the hub will disconnect the faulty station and close the data transmission ring.

Rice. 4.

In a Token Ring architecture, a token is passed from node to node along a logical ring created by a central hub. Such token transmission is carried out in a fixed direction (the direction of movement of the token and data packets is represented in the figure by blue arrows). A station holding a token can send data to another station.

To transmit data, workstations must first wait for a free token to arrive. The token contains the address of the station that sent the token, as well as the address of the station to which it is intended. After this, the sender passes the token to the next station in the network so that it can send its data.

One of the network nodes (usually a file server is used for this) creates a token that is sent to the network ring. This node acts as an active monitor that ensures that the marker is not lost or destroyed.

Advantages of Token Ring topology networks:

• the topology provides equal access to all workstations;
• high reliability, since the network is resistant to failures of individual stations and to interruptions in the connection of individual stations.

Disadvantages of Token Ring topology networks: high cable consumption and, accordingly, expensive wiring of communication lines.