Devices used in network topology. Topological model

Topology computer networks

One of critical differences between different types of networks is their topology.

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

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

• on the composition of the required network equipment;
• on the capabilities of network equipment;
• on the possibility of expanding the network;
• on the way to manage the network.

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

Tire... Bus topology networks use a linear mono channel (coaxial cable) for data transmission, at the ends of which special plugs are installed - terminators (terminator). They are necessary in order

Rice. 6.1.

to turn off the signal after passing through the bus. The disadvantages of a bus topology include the following:

• data transmitted over the cable is available to all connected computers;
• in the event of a bus failure, the entire network ceases to function.

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

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

Star Is the only network topology with an explicitly dedicated center, called network hub or a "hub" to which all other subscribers are connected. The functionality of the network depends on the state of that hub. In a star topology, there are no direct connections between two computers on the network. This makes it possible to solve the problem of public data availability, and also increases the resistance to damage to the cabling system.

Rice. 6.2.

Rice. 6.3. Star topology

Is a topology of a computer network in which each workstation on a network is connected to several workstations on the same network. It is characterized by high fault tolerance, configuration complexity and excessive cable consumption. Each computer has many possible ways connections with other computers. 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. Moreover, each edge of the lattice is parallel to its axis and connects two adjacent nodes along this axis. A one-dimensional lattice is a chain connecting two external nodes (having only one neighbor) through a certain number of internal ones (which have two neighbors - left and right). When both external nodes are connected, a ring topology is obtained. 2D and 3D lattices are used in supercomputer architecture.

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

(fig. 6.5) - the topology prevailing in large networks with arbitrary connections between computers. In such networks, individual arbitrarily connected fragments can be distinguished ( subnets ), have a typical topology, therefore they are called mixed topology networks.

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

Rice. 6.5.

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

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

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

The main characteristics of modern computer networks

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

To the main characteristics productivity networks include:

• reaction time - characteristic, which is defined as the time between the occurrence of a request to any network service and receiving an answer 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 any network device and the moment it appears at the output of this device.

For reliability assessments networks are used various characteristics, including:

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

Extensibility means the possibility of relatively easy addition individual elements networks (users, computers, applications, services), increasing the length of network segments and replacing existing equipment with more powerful ones.

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

Transparency - the property of the network to hide the details of its internal device, thereby simplifying its work on the network.

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

Compatibility means that the network can include a wide variety of software and hardware.

Under the topology(layout, configuration, structure) of a computer network is usually understood physical location network computers are one relative to one and the way they are connected by communication lines. It is important to note that the concept of topology refers primarily to local area networks, in which the structure of the connections can be easily traced. V global networks the structure of communications is usually hidden from users and is not very important, because each communication session can be performed in its own way.
The topology determines the requirements for the equipment, the type of cable used, the possible and most convenient methods exchange control, operational reliability, network expansion possibilities.

There are three main network topologies:

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

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

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

Rice. 1. Network topology "bus"

Rice. 2. Star network topology

Rice. 3. Network topology "ring"

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

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

Star topology is a topology with an explicitly dedicated center to which all other subscribers are connected. All information exchange is carried out exclusively through a central computer, which in this way falls very much huge pressure, therefore, it cannot do anything other than the network. It is clear that the network equipment of the central subscriber must be significantly more complex than the equipment of the peripheral subscribers. In this case, there is no need to talk about the equality of subscribers. As a rule, it is the central computer that is the most powerful, and it is on it that all the functions of managing the exchange are entrusted. In principle, no conflicts in a network with a star topology are possible, because the management is completely centralized, there is no conflict why.
If we talk about the resistance of a star to computer failures, then failure peripheral computer does not in any way affect the functioning of the part of the network that remains, but any failure of the central computer makes the network completely inoperative. Therefore, special measures should be taken to improve the reliability of the central computer and its network equipment. Dropped any cable or short circuit in it, in a star topology, it disrupts communication with only one computer, while all other computers can continue to work normally.
On the declination from the bus, in the star on each communication line there are only two subscribers: the central one and one of the peripheral ones. Most often, two communication lines are used to connect them, each of which transmits information in only one direction. Thus, there is only one receiver and one transmitter on each link. All this greatly simplifies the network installation in comparison with the bus and saves the use of additional external terminators from the need. The problem of signal attenuation in the communication line is also solved in the "star" more easily than in the "bus", because each receiver always receives a signal of the same level. A serious disadvantage of the "star" topology is the severe limitation of the number of subscribers. Typically, a central subscriber can serve no more than 8-16 peripheral subscribers. If within these limits it is quite simple to connect new subscribers, then if they are exceeded, it is simply impossible. True, sometimes a star provides for the possibility of building up, that is, connecting another central subscriber instead of one of the peripheral subscribers (as a result, a topology of several interconnected stars comes out).
The star shown in Fig. 2, is called an active, or real star. There is also a topology called a passive star, which is only superficially similar to a star (Fig. 4). At this time, it is much more common than an active star. Suffice it to say that it is used in today's most popular Ethernet network.

Rice. 4. Topology "passive star"

In the center of a network with this topology, there is not a computer, but a hub, or hub, which performs the same function as a repeater. It resumes signals that come in and forwards them to other communication lines. Although the cabling scheme is similar to a real or active star, in fact we are dealing with a bus topology, because information from each computer is simultaneously transmitted to all other computers, and there is no central station. Naturally, a passive star is more expensive than a conventional bus, because in this case, you also need a hub. However, it provides whole line additional opportunities associated with the benefits of the star. That is why in Lately passive star displaces more and more a real star, which is considered unpromising topology.
It is also possible to distinguish an intermediate type of topology between an active and a passive star. In this case, the concentrator not only retransmits the signals, but also controls the exchange, but does not take part in the exchange itself.
Big star advantage(both active and passive) is that all connection points are collected in one place. This allows you to easily monitor the operation of the network, localize network faults by simple shutdown from the center of certain subscribers (which is impossible, for example, in the case of a bus), as well as to restrict access outsiders to vital network connection points. In the case of a star, each peripheral subscriber can be approached by either one cable (through which there is transmission in both directions), or two cables (each of them transmits in one direction), and the second situation is more common. Common disadvantage for the entire star topology, the cable consumption is significantly higher than for the other topology. For example, if computers are located in one line (as in Fig. 1), then when choosing a "star" topology, you will need several times more cable than in the bus topology. This can significantly affect the cost of the entire network as a whole.

Ring topology- this is a topology in which each computer is connected by communication lines with only two others: from one it only receives information, and to the other only transmits. On each communication line, as in the case of a star, only one transmitter and one receiver works. This eliminates the need for external terminators. An important feature of the ring is that each computer repeats (resumes) the signal, that is, acts as a repeater, therefore the signal attenuation in the entire ring does not matter, only the attenuation between neighboring computers in the ring is important. In this case, there is no clearly designated center; all computers can be the same. However, quite often a special subscriber is allocated in the sprat, who manages the exchange or controls the exchange. It is clear that the presence of such a control subscriber reduces the reliability of the network, because its failure immediately paralyzes the entire exchange.
Strictly speaking, computers in a sprat are not completely equal in rights (unlike, for example, a bus topology). Some of them necessarily receive information from the computer, which is transmitting at this moment, earlier, while others - later. It is on this feature of the topology that the methods of controlling the exchange over the network, specially designed for the "ring", are built. In these methods, the right to the next transfer (or, as they say, to capture the network) is sequentially transferred 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 with the bus topology, maximum amount subscribers in a sprat can be quite large (up to a thousand or more). The ring topology is usually the most resistant to congestion, it ensures reliable operation with the largest flows of information transmitted over the network, because, as a rule, there are no conflicts (unlike the bus), and there is also no central subscriber (unlike a star) ...
Because the signal in the sprat passes through all the computers on the network, the failure of at least one of them (or its network connection) disrupts the robot of the entire network as a whole. Likewise, any open or short circuit in each of the cables in the ring makes the entire network impossible. The ring is most vulnerable to cable damage, therefore, in this topology, it is usually provided for the laying of two (or more) parallel communication lines, one of which is in reserve.
In the same time great advantage ring is that the retransmission of signals by each subscriber can significantly increase the size of the entire network as a whole (sometimes up to several tens of kilometers). The ring is relatively superior to any other topology.

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

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

Rice. 5. Topology "active tree"

Rice. 6. Topology "passive tree". K - concentrators

A combined topology is also used quite often, for example, star bus, star ring.

Significance of the concept of topology.

The topology of a network determines not only the physical location of computers, but, which is much more important, the nature of the connections between them, the features of the propagation of signals over the network. It is the nature of the connections that determines the degree of network fault tolerance, the required complexity of the network equipment, the most suitable exchange control method, the types of transmission media (communication channels) are possible, allowable size networks (length of communication lines and the number of subscribers), the need for electrical coordination, and much more.
When the literature recalls the network topology, they can mean four completely different concepts that refer to different levels network architecture:

1. Physical topology(that is, the layout of computers and cabling). In this content, for example, a passive star is no different from an active star, therefore it is often called simply a "star".

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

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

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

For example, a network with a physical and logical topology "bus" can, as a control method, use the handover of the network capture right (that is, be a ring in this content) and simultaneously transmit all information through one dedicated computer (be a star in this content).

The computer network can be divided into two parts. A physical computer network is, first of all, equipment. That is, all the required cables and adapters connected to computers, hubs, switches, printers, and so on. Everything that should work on a shared network.

The second component of a computer network is logical network... This is the principle of connecting a number of computers and the necessary equipment v unified system(the so-called topology of computer networks). This concept is more applicable to local area networks. It is the selected topology of connecting a number of computers that will affect the required equipment, the reliability of the network, the possibility of its expansion, and the cost of work. Now the most widely used types of computer network topologies are "ring", "star", and also "bus". The latter, however, has practically fallen out of use.

"Star", "ring" and "bus" are the basic topologies of computer networks.

"Star"

The topology of computer networks "star" - a structure, the center of which is a switching device. All computers are connected to it with separate lines.

The switching device can be a hub, that is, a HUB, or a switch. This topology is also called "passive star". If the switching device is another computer or server, then the topology can be called an "active star". It is to the switching device that the signal from each computer is received, processed and sent to other connected computers.

This topology has several advantages. The undoubted advantage is that computers do not depend on each other. If one of them breaks down, the network itself remains in working order. Also, you can easily connect to such a network and new computer... When connecting new equipment, the rest of the network elements will continue to work in normal mode... This kind of network topology is easy to troubleshoot. Perhaps one of the main advantages of the "star" is its high performance.

However, with all the advantages, this type of computer networks also have disadvantages. If the central switching device fails, then the entire network will stop working. It has restrictions on the connected workstations. There cannot be more than the available number of ports on the switching device. And the last drawback of the network is its cost. It takes enough a large number of cable to connect each computer.

"Ring"

The topology of computer networks "ring" has no structural center. Here all workstations together with the server are united in a vicious circle. In this system, the signal moves sequentially from right to left in a circle. All computers are repeaters, due to which the marker signal is maintained and transmitted further until it reaches the recipient.

This type of topology also has advantages and disadvantages. The main advantage is that the work of the computer network remains stable even under heavy workload. This type of network is very easy to install and requires minimum quantity additional equipment.

Unlike a star topology, a ring can paralyze the entire system when any connected computer fails. Moreover, it will be much more difficult to identify the malfunction. In spite of easy installation this option network, its setup is quite complicated, it requires certain skills. Another disadvantage of this topology is the need to suspend the entire network to connect new equipment.

"Tire"

The topology of computer networks "bus" is now encountered less and less often. It consists of a single long line to which all computers are connected.

In this system, as in others, data is sent along with the recipient's address. All computers receive the signal, but receive it directly by the addressee. Bus-connected workstations cannot send data packets at the same time. While one of the computers performs this action, the rest are waiting for their turn. Signals move along the line in both directions, but when they reach the end, they are reflected and superimposed on each other, threatening the well-coordinated work of the entire system. Exists special devices- terminators designed to suppress signals. They are installed at the ends of the highway.

The advantages of the "bus" topology include the fact that such a network is installed and configured quickly enough. Plus, it's pretty cheap to install. If one of the computers fails, the network will continue to work as usual. Connecting new equipment can be done in working order. The network will function.

If the central cable is damaged or one of the terminators stops working, this will lead to a stop of the entire network. It is quite difficult to find a fault in such a topology. An increase in the number of workstations reduces network performance and also leads to delays in the transmission of information.

Derived topologies of computer networks

The classification of computer networks by topology is not limited to three basic options. There are also such types of topologies as "line", "double ring", "mesh topology", "tree", "lattice", "close network", "snowflake", "fully connected topology". All of them are derived from the base ones. Let's consider some options.

Inefficient topologies

In a fully mesh topology, all workstations are connected to each other. Such a system is rather cumbersome and ineffective. It is required to allocate a line for each pair of computers. This topology is used only in multi-machine complexes.

Mesh topology is essentially a stripped-down version of fully mesh. Here, too, all computers are connected to each other by separate lines.

Most efficient topologies

The topology of building computer networks called "snowflake" is a stripped-down version of the "star". Here, hubs, interconnected according to the "star" type, act as workstations. This topology option is considered one of the most optimal for large local and wide area networks.

As a rule, in large local and global networks there is great amount subnets built on different types topologies. This type is called mixed. Here you can simultaneously select the "star", and the "tire", and the "ring".

So, in the above article, all the main available topologies of computer networks used in local and global networks, their variations, advantages and disadvantages were considered.

The term topology describes the physical location of computers, cables, and other components on a network.

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

In addition to the term topology, the following are also used to describe the physical layout:

Physical location;

Layout;

Diagram;

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

the composition of the required network equipment;

characteristics of network equipment;

the possibility of expanding the network;

a way to manage the network.

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

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

Basic topologies

• star

  ring

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

Tire.

The bus topology is often referred to as the linerbus. This topology is one of the simplest and most common topologies. It uses a single cable, called a backbone or segment, along which all the computers on the network are connected.

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

The data is transmitted in the form of electrical signals to all computers on the network; however, the information is received by the one whose address corresponds to the recipient's address encrypted in these signals. Moreover, at a time, only one computer can transmit.

Since data is transmitted to the network by only one computer, its performance depends on the number of computers connected to the bus. The more there are, the slower the network works. The bus is a passive topology. This means that computers only "listen" to the data transmitted over the network, but do not move it from the sender to the recipient. Therefore, if one of the computers fails, it will not affect the work of the rest. In this topology, data is propagated throughout the network, from one end of the cable to the other. If you do not take any action, then signals reaching the end of the cable will be reflected and this will prevent other computers from transmitting. Therefore, after the data reaches the destination, the electrical signals must be extinguished. To do this, terminators (also called plugs) are installed at each end of the cable in a bus topology to absorb electrical signals.

Advantages: The absence of additional active equipment (eg repeaters) makes such networks simple and inexpensive.

Linear topology diagram of a local area network

However, the disadvantage of linear topology lies in the limitations on the size of the network, its functionality and scalability.

Ring

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

One of the principles of transferring data over the ring is called transfer token. Its essence is as follows. The token is sequentially transmitted, from one computer to another, until the one that wants to transfer the data receives it. The sending computer changes the token, puts the email address in the data, and sends it around the ring.

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

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

Star

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

Advantage: if the work in one computer breaks down or the cable connecting one computer breaks down, then only this computer will not be able to receive and transmit signals. This will not affect other computers on the network. The overall network speed is limited only by the bandwidth of the hub.

Star topology is dominant in modern local area networks. Such networks are quite flexible, easily expandable, and relatively inexpensive compared to more complex networks, in which the methods of device access to the network are strictly fixed. Thus, the "stars" have supplanted the obsolete and rarely used line and ring topologies. Moreover, they became a transitional link to the last species topologies - switched stars e.

A switch is a multi-port active network device. The switch "remembers" the hardware (or MAC – MediaAccessControl) addresses of the devices connected to it and creates temporary paths from the sender to the recipient, through which data is transmitted. In a typical LAN with a switched topology, there are multiple connections to the switch. Each port and the device that is connected to it has its own bandwidth (baud rate).

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

In conclusion, it should be said that they distinguish between the topology of physical links (the physical structure of the network) and the topology of logical links (the logical structure of the network)

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

Logical links are the paths of information flows through the network, they are formed by the appropriate setting of the communication equipment.

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

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

Wireless networks.

The phrase “wireless environment” can be misleading because it means complete absence wires in the network. In reality, however, wireless components usually interact with a network that uses cable as a transmission medium. Such a network with mixed components is called a hybrid network.

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

local area networks;

extended local area networks;

mobile networks (laptop computers).

Transfer methods:

infrared radiation;

• narrow spectrum radio transmission (single frequency transmission);

  spread spectrum radio transmission.

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

Currently office network Is not just a connection between computers. It is difficult to imagine a modern office without databases in which both the financial statements of the enterprise and information on personnel are stored. In large networks, as a rule, for the security of databases, and to increase the speed of access to them, separate servers are used to store the databases. Also, nowadays it is difficult to imagine a modern office without access to the Internet. Scheme variant wireless network office is shown in the figure

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

What do you need a network for?

How many users will your network have?

How fast will the network expand?

Does this network need Internet access?

Do you need centralized management of network users?

Then draw a rough diagram of the network on paper. Don't forget about the cost of the network.

As we have defined, the topology is the most important factor improving overall network performance. Basic topologies can be used in any combination. It is important to understand that strong and weaknesses each topology affects the desired network performance and is dependent on existing technologies. A balance must be struck between the actual location of the network (for example, in several buildings), the possibilities of using the cable, the ways of its installation, and even its type.

Topology (configuration) is a way of connecting computers to a network. The type of topology determines the cost, security, performance and reliability of workstations, for which the time of access to the file server is important.

The concept of topology is widely used in networking. One of the approaches to classifying LAN topologies is to distinguish two main classes of topologies: broadcast and sequential.

In broadcast topologies, a PC transmits signals that can be picked up by other PCs. These topologies include topologies: common bus, tree, star.

In serial topologies, information is transmitted to only one PC. Examples of such topologies are: arbitrary (arbitrary PC connection), ring, chain.

When choosing an optimal topology, three main goals are pursued:

Providing alternative routing and maximum reliability of data transmission;

Selection of the optimal transmission route for data blocks;

Providing acceptable response times and bandwidth.

When choosing a specific type of network, it is important to consider its topology. The main network topologies are: bus (line) topology, star topology, ring topology and tree topology.

For example, an ArcNet network configuration uses both linear and star topologies. Token Ring networks physically look like a star, but logically their packets travel around the ring. Data transmission in the Ethernet network occurs over the line bus, so that all stations see the signal at the same time.

Topology types

There are five main topologies (Fig. 3.1): common bus (Bus); ring (Ring); star (Star); tree (Tree); cellular (Mesh).

Rice. 3.1. Topology types

Common bus

A shared bus is a type of network topology in which workstations are located along a single piece of cable called a segment. The common bus topology (Fig. 3.2) assumes the use of one cable to which all computers on the network are connected.

In the case of the Shared bus topology, the cable is used by all stations in turn:

Rice. 3.2. Topology Common bus

1. When transmitting data packets, each computer addresses it to a specific LAN computer, transmitting it over network cable in the form of electrical signals.

2. A packet in the form of electrical signals is transmitted over the "bus" in both directions to all computers on the network.

3. However, information is received only by the address that matches the recipient address specified in the packet header. Since only one PC can transmit on the network at a time, LAN performance depends on the number of PCs connected to the bus. The more there are, the more pending data transmission, the lower the network performance. However, it is impossible to indicate a direct dependence of the network bandwidth on the number of PCs, since it is also affected by:

· Characteristics of the PC network hardware;

· The frequency with which PC messages are transmitted;

· The type of running network applications;

· The type of cable and the distance between PCs in the network.

"Bus" is a passive topology. This means that computers only "listen" to the data transmitted over the network, 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 entire network.

4. Data in the form of electrical signals spreads throughout the network from one end of the cable to the other, and, reaching the end of the cable, will be reflected and occupy the "bus", which will not allow other computers to transmit.

5. To prevent the reflection of electrical signals, terminators (T) are installed at each end of the cable, which absorb the signals passed along the "bus",

6. If there is a considerable distance between PCs (for example, 180 m for a thin coaxial cable) in the "bus" segment, the weakening of the electrical signal can be observed, which can lead to distortion or loss of the transmitted data packet. In this case, the original segment should be divided into two, setting between them additional device- repeater (repeater), which amplifies the received signal before sending it further.

Correctly placed repeaters along the length of the network can increase the length of the network served and the distance between neighboring computers. It should be remembered that all ends of the network cable must be connected to something: to a PC, terminator or repeater.

A break in a network cable or disconnection of one of its ends leads to the termination of the functioning of the network. The network "crashes". The PC networks themselves remain fully functional, but cannot interact with each other. If the LAN is server based, where most of software and information resources are stored on the server, then the PC, although they remain operational, but for practical work are of little use.

Bus topology is used in Ethernet networks, but it has rarely been encountered recently.

Common bus topology examples are 10Base-5 (PC connection with thick coaxial cable) and 10Base-2 (PC connection with thin coaxial cable).

Ring

A ring is a LAN topology in which each station is connected to two other stations to form a ring (Figure 3.3). Data is transferred from one workstation to another in one direction (around the ring). Each PC acts as a repeater, relaying messages to the next PC, i.e. data is transferred from one computer to another as if by relay. If a computer receives data intended for another computer, it transfers them further along the ring, otherwise they will not be transferred further. The main problem with a ring topology is that each workstation must actively participate in the transfer of information, and if at least one of them fails, the entire network is paralyzed. Connecting a new workstation requires a short-term network shutdown. the ring must be open during installation. Topology The ring has a highly predictable response time based on the number of workstations.

Rice. 3.3. Topology Ring

Net ring topology rarely used. Instead, the ring topology plays a transport role in the accessor schema. The ring describes a logical route, and the packet is transmitted from one station to another, eventually completing a full circle. In networks Token Ring the cable branch from the central hub is called the MAU (Multiple Access Unit). The MAU has an inner ring that connects all stations connected to it, and is used as an alternate path when the cable of one workstation is cut or disconnected. When the workstation cable is connected to the MAU, it simply forms an extension of the ring: signals go to the workstation and then return back to the inner ring.

Star

A star is a LAN topology (Figure 3.4) in which all workstations are attached to a central site (eg, a hub) that establishes, maintains, and breaks communications between workstations. The advantage of this topology is that a faulty node can be easily eliminated. However, if the central node fails, the entire network fails.

Rice. 3.4. Topology Star

In this case, each computer through a special network adapter connected with a separate cable to the combining device. If necessary, multiple star networks can be linked together to create branched network configurations. At each branch point, special connectors (distributors, repeaters or access devices) must be used.

An example of a star topology is a 10BASE-T twisted pair Ethernet topology, with the center of a Star usually being a Hub.

The star topology provides protection against cable breaks. Damage to the workstation cable will not damage the entire network segment. It also makes it easy to diagnose connectivity problems as each workstation has its own cable segment connected to the hub. For diagnostics, it is enough to find a break in the cable that leads to the idle station. The rest of the network continues to function normally.

However, the star topology also has disadvantages. First, it requires a lot of cable. Secondly, hubs are quite expensive. Third, cable hubs are difficult to maintain with a large amount of cable. However, in most cases, this topology uses an inexpensive cable such as twisted pair... In some cases, you can even use existing telephone cables... In addition, it is beneficial for diagnostics and testing to collect all cable ends in one place.

Comparative characteristics of basic network topologies are presented in table. 3.1.

Table 3.1. Comparative characteristics basic network topologies