Basic topologies of local networks. Types of local networks and their structure. Network topology: definition, types, purpose
is a way of describing a network configuration, a diagram of the location and connection of network devices. The network topology allows you to see its entire structure, the network devices included in the network, and their connections with each other.
There are several types of topologies: physical, logical, informational and exchange control topology. In this article we will talk about the physical topology of the network, which describes the actual location and connections between the nodes of the local network.
There are several main types of physical network topologies:
- Bus network topology- a topology in which all computers on the network are connected to one cable, which is shared by all workstations. With this topology, the failure of one machine does not affect the operation of the entire network as a whole. The disadvantage is that if the bus fails or breaks, the operation of the entire network is disrupted.
- Zvezda network topology— a topology in which all workstations have a direct connection to the server, which is the center of the “star”. With this connection scheme, a request from any network device is sent directly to the server, where it is processed at different speeds, depending on the hardware capabilities of the central machine. Failure of the central machine leads to the shutdown of the entire network. The failure of any other machine does not affect the operation of the network.
- Ring network topology- a scheme in which all nodes are connected by communication channels into an unbroken ring (not necessarily a circle) through which data is transmitted. The output of one PC is connected to the input of another. Having started the movement from one point, the data ultimately ends up at its beginning. Data in a ring always moves in the same direction. This network topology does not require the installation of additional equipment (server or hub), but if one computer fails, the operation of the entire network stops.
- Mesh network topology- a topology in which each workstation is connected to all other workstations on the same network. Each computer has many possible ways to connect to other computers. Therefore, a cable break will not result in loss of connection between the two computers. This network topology allows the connection of a large number of computers and is typically typical for large networks.
- At mixed topology Several types of connections between computers are used. It occurs quite rarely in particularly large companies and organizations.
Why do you need to know the types of topologies and all their pros and cons? The composition of the equipment and software depends on the network design. The topology is chosen based on the needs of the enterprise. In addition, knowledge of the network topology allows you to evaluate its weak points, as well as the dependence of the stability of its operation on individual components, and more carefully plan subsequent connections of new network equipment and PCs. In case of any failure, lack of communication with any computer on the network, you can always see on the map where this device is located, on what floor, in what office or room, what, first of all, you need to pay attention to and where to go to first of all to troubleshoot the problem.
And here we come to one of the key questions of interest to all system administrators, namely: how to draw a network diagram with minimal time, effort and money? If the network is large and consists of dozens of servers, hundreds of computers and many other network devices (printers, switches, etc.), even an experienced system administrator (not to mention a novice) will find it very difficult to quickly understand all the connections between network equipment. Creating a network topology manually is out of the question here. Fortunately, the modern software market offers special programs for automatically exploring and constructing a network diagram. This allows the system administrator to know where and what equipment is located without having to manually examine the wires.
Thus, even if you are new to the company, and the previous system administrator was not very eager to “hand over” the network to you according to all the rules, programs for drawing network topology will allow you to quickly get involved in the work and start with building a diagram of your network.
Computer network topology
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 these network topologies.
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 the basis of 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.
Classification of topological network elements
Local networks consist of end devices and intermediate devices connected by a cable system. Let's define some basic concepts.
Network nodes(nodes) - end devices and intermediate devices endowed with network addresses. Network nodes include computers with a network interface that act as workstations, servers, or both; network peripheral devices (printers, plotters, scanners); network telecommunication devices (modem pools, shared modems); routers.
Cable segment- a piece of cable or a chain of pieces of cables electrically (optically) connected to each other, providing a connection between two or more network nodes. Sometimes, in relation to a coaxial cable, this is also the name for a section of cable terminated with connectors, but we will use the broader interpretation given above.
Network segment(or simply a segment) is a collection of network nodes using a common (shared) transmission medium. In relation to Ethernet technology, this is a set of nodes connected to one coaxial cable segment, one hub (repeater), as well as to several cable segments and/or hubs interconnected by repeaters. In relation to Token Ring, this is one ring.
Net(logical) - a set of network nodes that have a unified third-level addressing system of the OSI model. Examples would be IPX network, IP network. Each network has its own address; routers use these addresses to transmit packets between networks. The network can be divided into subnets, but this is a purely organizational division with addressing at the same third level. A network can consist of many segments, and the same segment can be part of several different networks.
Cloud(cloud) - a communication infrastructure with homogeneous external interfaces, the details of the organization of which are not interested. An example of a cloud would be a local long-distance telephone network: anywhere you can connect a telephone and contact any subscriber.
According to the method of using cable segments, they are distinguished:
Point-to-point connections(point-to-p6int connection) - between two (and only two!) nodes. For such connections, symmetrical electrical (twisted pair) and optical cables are mainly used.
Multipoint connections(multi point connection) - more than two nodes are connected to one cable segment. A typical transmission medium is an unbalanced electrical cable (coaxial cable); other cables, including optical ones, can also be used. Connecting devices with cable segments one after another is called daisy chaining. It is possible to connect multiple devices to one piece of cable using the tap method.
Topology
Topology (configuration) is a way of connecting computers into a network. The type of topology determines the cost, security, performance and reliability of workstations, for which the time to access the file server matters.
The concept of topology is widely used in creating networks. One of the approaches to classifying LAN topologies is to distinguish two main classes of topologies : broadcast And sequential.
IN broadcast topologies The PC transmits signals that can be received by other PCs. These topologies include the following topologies: common bus, tree, star.
IN serial topologies information is transmitted to only one PC. Examples of such topologies are: free(random PC connection), ring, chain.
When choosing the optimal topology, there are three main goals:
Providing alternative routing and maximum reliability of data transmission;
Selecting the optimal route for transmitting data blocks;
Providing acceptable response time and required bandwidth.
When choosing a specific network type, it is important to consider its topology. The main network topologies are: bus (linear) topology, star, ring and tree.
For example, an ArcNet network configuration uses both a linear and a star topology. Token Ring networks physically look like a star, but logically their packets are transmitted around the ring. Data transmission in an Ethernet network occurs over a linear bus, so that all stations see the signal at the same time.
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Introduction
1. Concept of network topology
2. Basic network topologies
2.3 Basic ring network topology
3. Other possible network topologies
3.1 Tree network topology
3.2 Combined network topologies
3.3 "Grid" network topology
4. Polysemy of the concept of topology
Conclusion
Bibliography
Introduction
Today it is impossible to imagine human activity without the use of computer networks.
A computer network is a system of distributed information processing, consisting of at least two computers interacting with each other using special means of communication.
Depending on the remoteness of computers and scale, networks are conventionally divided into local and global.
Local networks are networks that have a closed infrastructure before reaching service providers. The term "LAN" can describe both a small office network and a large plant-level network covering several hundred hectares. Local networks are usually deployed within an organization, which is why they are also called corporate networks.
Sometimes networks of an intermediate class are distinguished - a city or regional network, i.e. network within a city, region, etc.
The global network covers large geographic regions, including both local networks and other telecommunications networks and devices. Global networks have almost the same capabilities as local ones. But they expand their scope. The benefits of using global networks are limited primarily by the speed of operation: global networks operate at a lower speed than local ones.
Of the computer networks listed above, we will turn our attention to local networks in order to better understand the architecture of networks and methods of data transmission. And for this you need to know such a thing as network topology.
1. Concept of network topology
Topology is the physical configuration of a network combined with its logical characteristics. Topology is a standard term used to describe the basic layout of a network. By understanding how different topologies are used, you can determine what capabilities different types of networks have.
There are two main types of topologies:
physical
logical
Logical topology describes the rules for interaction of network stations when transmitting data.
The physical topology determines how the storage media are connected.
The term "network topology" describes the physical arrangement of computers, cables, and other network components. The network topology determines its characteristics.
The choice of a particular topology affects:
composition of the necessary network equipment
network equipment characteristics
network expansion possibilities
network management method
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.
2. Basic network topologies
There are three basic topologies on which most networks are built.
star
ring
If computers are connected along a single cable, the topology is called a "bus". When computers are connected to cable segments originating from a single point, or hub, the topology is called a star topology. If the cable to which the computers are connected is closed in a ring, this topology is called a ring.
Although the basic topologies themselves are simple, in reality there are often quite complex combinations that combine the properties of several topologies.
2.1 Bus network topology
In this topology, all computers are connected to each other with one cable (Figure 1).
Figure 1 - Network topology diagram of the "bus" type
In a network with a "bus" topology, computers address data to a specific computer, transmitting it along the cable in the form of electrical signals - hardware MAC addresses. To understand the process of computer interaction via a bus, you need to understand the following concepts:
signal transmission
signal reflection
Terminator
1. Signal transmission
Data in the form of electrical signals is transmitted to all computers on the network; however, only the one whose address matches the recipient address encrypted in these signals receives information. Moreover, at any given time, only one computer can transmit. Since data is transmitted to the network by only one computer, its performance depends on the number of computers connected to the bus. The more there are, i.e. The more computers waiting to transmit 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 (Figure 2). All ends of the network cable must be connected to something, such as a computer or a barrel connector - to increase the cable length. A terminator must be connected to any free - unconnected - end of the cable to prevent electrical signals from being reflected.
Figure 2 - Terminator installation
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 operational, but as long as the segment is broken, they cannot communicate with each other.
This network topology has advantages and disadvantages. The advantages include:
short network setup time
low cost (less cable and network devices required)
ease of setup
Failure of a workstation does not affect the operation of the network
The disadvantages of this topology are as follows.
such networks are difficult to expand (increase the number of computers in the network and the number of segments - individual sections of cable connecting them).
Because the bus is shared, only one of the computers can transmit at a time.
The “bus” is a passive topology - computers only “listen” to the cable and cannot restore signals that are attenuated during transmission over the network.
The reliability of a network with a bus topology is low. When the electrical signal reaches the end of the cable, it (unless special measures are taken) is reflected, disrupting the operation of the entire network segment.
The problems inherent in the bus topology have led to the fact that these networks, so popular ten years ago, are now practically not used.
The bus network topology is known as 10 Mbps Ethernet logical topology.
2.2 Basic star network topology
In a star topology, all computers are connected via cable segments to a central component called a hub (Figure 3).
Signals from the transmitting computer travel through the hub to everyone else.
This topology originated in the early days of computing, when computers were connected to a central, main computer.
Local network topology
Under topology(layout, configuration, structure) of a computer network usually refers to the physical location of computers on the network relative to each other and the way they are connected communication lines. It is important to note that the concept topology relates primarily to local networks, in which the structure of connections can be easily traced. In global networks, the structure of connections is usually hidden from users and is not very important, since each session communication can be made along its own path.
Topology determines equipment requirements, type of cable used, acceptable and most convenient control methods exchange, reliability work, network expansion opportunities. And although to choose topology a network user rarely has to know about the features of the main topologies, their advantages and disadvantages are necessary.
There are three basic topology networks:
· Tire(bus) - all computers are connected in parallel to one communication lines. Information from each computer is simultaneously transmitted to all other computers (Fig. 1.5).
Rice. 1.5. Network topology bus
· Star(star) - one central computer is connected to other peripheral computers, each of them using a separate communication line(Fig. 1.6). Information from a peripheral computer is transmitted only to the central computer, and from the central computer - to one or more peripheral ones.
Rice. 1.6. Star network topology
· Ring(ring) - computers are sequentially combined into a ring. The transmission of information in the ring is always carried out in only one direction. Each computer transmits information only to one computer next in the chain behind it, and receives information only from the previous computer in the chain (Fig. 1.7).
Rice. 1.7. Network topology ring
In practice, other local network topologies, however, most networks are focused on three basic topology.
Before moving on to analyzing the features of basic network topologies, it is necessary to highlight some of the most important factors that influence the physical performance of the network and are directly related to the concept topology.
· Serviceability of computers ( subscribers) connected to the network. In some cases, breakdown subscriber can block the entire network. Sometimes malfunction subscriber does not affect the operation of the network as a whole, does not interfere with others subscribers exchange information.
· Serviceability of network equipment, that is, technical equipment directly connected to the network (adapters, transceivers, connectors, etc.). Failure of one of the network equipment subscribers may affect the entire network, but may disrupt exchange with only one subscriber.
· Network cable integrity. If the network cable breaks (for example, due to mechanical stress), the information exchange in the entire network or in one of its parts. For electrical cables it is equally critical short circuit in cable.
· Limitation of cable length due to attenuation of the signal propagating along it. As is known, in any medium, when a signal propagates, it weakens (attenuates). And the greater the distance the signal travels, the more it attenuates (Fig. 1.8). It is necessary to ensure that the length of the network cable does not exceed the maximum length L pr, beyond which the attenuation becomes unacceptable (receiving subscriber does not recognize a weakened signal).
Rice. 1.8. Signal attenuation when propagating through a network
Bus topology
Topology The bus (or, as it is also called, the common bus) by its very structure presupposes the identity of the network equipment of computers, as well as the equality of all subscribers by network access. Computers on the bus can only transmit information one by one, since communication line in this case the only one. If several computers transmit information at the same time, it will be distorted as a result of overlap ( conflict, collisions). The bus always implements the so-called half duplex (half duplex) exchange(in both directions, but one at a time, not simultaneously).
IN topology tire has no clearly defined central subscriber, through which all information is transmitted, this increases its reliability (after all, if the center fails, the entire system controlled by it ceases to function). Adding new subscribers connecting to the bus is quite simple and is usually possible even while the network is running. In most cases, when using a bus, a minimal amount of connecting cable is required compared to other topologies.
Since the central subscriber missing, possible resolution conflicts in this case falls on the network equipment of each individual subscriber. In this regard, network equipment when topology the tire is more difficult than with others topologies. However, due to the widespread use of networks with topology bus (primarily the most popular Ethernet network), the cost of network equipment is not too high.
Rice. 1.9. Cable break in a network with bus topology
An important advantage of the bus is that if any of the computers on the network fails, healthy machines will be able to continue normally. exchange.
It would seem that if the cable breaks, you get two fully functional buses (Fig. 1.9). However, it must be taken into account that due to the peculiarities of the propagation of electrical signals over long communication lines it is necessary to provide for the inclusion of special matching devices at the ends of the bus, terminators, shown in Fig. 1.5 and 1.9 in the form of rectangles. Without inclusion terminators the signal is reflected from the end lines and is distorted so that communication over the network becomes impossible. If the cable is broken or damaged, coordination is disrupted communication lines, and stops exchange even between those computers that remain connected. More details about coordination will be described in a special section of the course. A short circuit at any point on the bus cable disables the entire network.
Failure of any network equipment subscriber in the bus can bring down the entire network. In addition, such a failure is quite difficult to localize, since everything subscribers are connected in parallel, and it is impossible to understand which one has failed.
When passing through communication lines networks with topology bus information signals are weakened and not restored in any way, which imposes strict restrictions on the total length communication lines. And each subscriber can receive signals of different levels from the network depending on the distance to the transmitting subscriber. This places additional demands on receiving nodes of network equipment.
If we assume that the signal in the network cable is attenuated to the maximum permissible level at a length of L pr, then the total length of the bus cannot exceed the value of L pr. In this sense, the bus provides the shortest length compared to other basic topologies.
To increase the length of the network from topology tire is often used by several segments(parts of a network, each of which is a bus), interconnected using special amplifiers and signal restorers - repeaters or repeaters(Fig. 1.10 shows the connection of two segments; the maximum network length in this case increases to 2 L int, since each of the segments can be L in length). However, this increase in network length cannot continue indefinitely. Length restrictions are related to the finite speed of signal propagation along communication lines.
Rice. 1.10. Connecting bus network segments using a repeater
Star topology
The star is the only one topology networks with a clearly designated center to which all others connect subscribers. Information exchange goes exclusively through the central computer, which bears a heavy load, so, as a rule, it cannot do anything else except the network. It is clear that the network equipment of the central subscriber must be significantly more complex than peripheral equipment subscribers. About the equality of all subscribers(as in a tire) in this case there is no need to talk. Usually the central computer is the most powerful; all functions for managing the exchange are assigned to it. No network conflicts with topology star are in principle impossible, since control is completely centralized.
If we talk about sustainability stars to computer failures, then the failure of a peripheral computer or its network equipment does not in any way affect the functioning of the rest of the network, but any failure of the central computer makes the network completely inoperable. In this regard, special measures must be taken to improve the reliability of the central computer and its network equipment.
Cable break or short circuit when topology star violates exchange with only one computer, and all other computers can continue to work normally.
Unlike a tire, there is a star on each communication lines there are only two subscriber: central and one of the peripheral. Most often, two are used to connect them communication lines, each of which transmits information in one direction, that is, on each communication lines there is only one receiver and one transmitter. This is the so-called transfer point-to-point. All this significantly simplifies network equipment compared to a bus and eliminates the need to use additional, external terminators.
The problem of signal attenuation in communication lines It is also easier to solve in a star than in the case of a bus, because each receiver always receives a signal of the same level. Maximum network length with topology the star can be twice as large as in the bus (that is, 2 L pr), since each of the cables connecting the center to the peripheral subscriber, may have length L ave.
Serious drawback topology the star lies in the strict limitation of the number subscribers. Usually central subscriber can serve no more than 8-16 peripheral subscribers. Within these limits, connecting new subscribers quite simple, but behind them it is simply impossible. In a star, it is permissible to connect another central one instead of a peripheral one subscriber(the result is topology of several stars connected to each other).
The star shown in Fig. 1.6, is called an active or true star. There is also topology, called a passive star, which only looks like a star in appearance (Fig. 1.11). Currently, it is much more widespread than an active star. Suffice it to say that it is used in the most popular Ethernet network today.
In the center of the network with this topology it does not fit a computer, but a special device - a hub or, as it is also called, hub(hub), which performs the same function as repeater, that is, it restores incoming signals and forwards them to all other communication lines.
Rice. 1.11. Passive star topology and its equivalent circuit
It turns out that although the cable layout is similar to a true or active star, in fact we are talking about a bus topology, since information from each computer is simultaneously transmitted to all other computers, and there is no central subscriber does not exist. Of course, a passive star is more expensive than a regular bus, since in this case a hub is also required. However, it provides a number of additional features associated with the benefits of a star, in particular, it simplifies network maintenance and repair. That is why, recently, a passive star is increasingly displacing a true star, which is considered unpromising topology.
An intermediate type can also be distinguished topology between an active and passive star. In this case, the hub not only relays the signals arriving at it, but also controls exchange, however, he himself exchange does not participate (this is done online 100VG-AnyLAN).
The great advantage of a star (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 faults by simply disconnecting certain subscribers(which is impossible, for example, in the case of a bus topology), as well as restrict access of unauthorized persons to connection points vital to the network. To the peripheral to the subscriber in the case of a star, either one cable (which transmits in both directions) or two (each cable transmits in one of two opposite directions) can be suitable, with the latter being much more common.
Common disadvantage for everyone topologies star type (both active and passive) is significantly larger than with others topologies, cable consumption. For example, if computers are located in one line (as in Fig. 1.5), then when choosing topology star will need several times more cable than with topology tire. This significantly affects the cost of the network as a whole and significantly complicates cable installation.
Ring topology
The ring is topology, in which each computer is connected communication lines with two others: from one he receives information and transmits it to the other. On each communication lines, as in the case of a star, only one transmitter and one receiver operate (point-to-point communication). This allows you to avoid using external terminators.
An important feature of the ring is that each computer relays (restores, amplifies) the signal coming to it, that is, it acts as a repeater. Signal attenuation in the entire ring does not matter, only the attenuation between neighboring computers of the ring is important. If the maximum cable length, limited by attenuation, is L pr, then the total length of the ring can reach NL pr, where N is the number of computers in the ring. The total size of the network will ultimately be NL pr/2, since the ring will have to be folded in half. In practice, the size of ring networks reaches tens of kilometers (for example, in a network FDDI). The ring in this regard is significantly superior to any other topology.
Clearly defined center with annular topology no, all computers can be the same and have equal rights. However, quite often a special ring stands out subscriber, which controls exchange or controls it. It is clear that the presence of such a single manager subscriber reduces the reliability of the network, since its failure immediately paralyzes the entire exchange.
Strictly speaking, computers in a ring are not completely equal in rights (unlike, for example, a bus topology). After all, one of them necessarily receives information from the computer transmitting at the moment earlier, and the others - later. It is on this feature topology and management methods are built exchange over the network, specially designed for the ring. In such methods, the right to the next transmission (or, as they also say, to take over the network) passes sequentially to the next computer in the circle. Connecting new subscribers Connecting to a ring is quite simple, although it requires a mandatory shutdown of the entire network for the duration of the connection. As with a tire, the maximum number subscribers in a ring it can be quite large (up to a thousand or more). Ring topology usually has high resistance to overloads, ensures reliable operation with large flows of information transmitted over the network, since there are, as a rule, no conflicts (unlike the bus), and there is also no central subscriber(unlike a star), which can be overloaded with large flows of information.
Rice. 1.12. Two ring network
The signal in the ring passes sequentially through all computers on the network, so the failure of at least one of them (or its network equipment) disrupts the operation of the network as a whole. This is a significant drawback of the ring.
Likewise, a break or short circuit in any of the ring cables makes the entire network impossible to operate. Of the three considered topologies the ring is most vulnerable to cable damage, so in case topology rings usually provide for the laying of two (or more) parallel communication lines, one of which is in reserve.
Sometimes the network with topology the ring is made on the basis of two parallel ring communication lines, transmitting information in opposite directions (Fig. 1.12). The purpose of such a solution is to increase (ideally, double) the speed of information transfer over the network. In addition, if one of the cables is damaged, the network can work with another cable (although the maximum speed will decrease).
Other topologies
In addition to the three basic topologies network is also often used topology tree, which can be considered as a combination of several stars. Moreover, as in the case of a star, the tree can be active or true (Fig. 1.13) and passive (Fig. 1.14). With an active tree at the centers of merging several communication lines there are central computers, and with passive - concentrators ( hubs).
Rice. 1.13. Active tree topology
Rice. 1.14. Passive tree topology. K - concentrators
Quite often combined topology, among which the most common are star-bus (Fig. 1.15) and star-ring (Fig. 1.16).
Rice. 1.15. Example of star-bus topology
Rice. 1.16. Example of a star-ring topology
In star-bus topology a combination of a tire and a passive sprocket is used. Both individual computers and entire bus segments are connected to the hub. In fact, the physical topology a bus that includes all computers on the network. In this topology Several hubs can be used, connected to each other 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. The user can thus 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 dissemination, this topology equivalent to a classic tire.
In the case of star-ring topology it is not the computers themselves that are united in a ring, but special hubs (shown in Fig. 1.16 in the form of rectangles), to which computers in turn are connected using star-shaped double communication lines. In reality, all computers on the network are included in a closed ring, since inside the hubs communication lines form a closed loop (as shown in Fig. 1.16). 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 the dissemination of information, this topology equivalent to a classic ring.
In conclusion, we must also say about the grid topology(mesh), in which computers communicate with each other not just one, but many communication lines, forming a grid (Fig. 1.17).
Rice. 1.17. Grid topology: full (a) and partial (b)
In full grid topology each computer is directly connected to all other computers. In this case, as the number of computers increases, the number of communication lines. In addition, any change in the network configuration requires changes to the network hardware of all computers, so a complete mesh topology has not received widespread use.
Partial mesh topology assumes direct connections only for the most active computers transmitting maximum amounts of information. The remaining computers are connected through intermediate nodes. Grid topology allows you to select a route for delivering information from subscriber To to the subscriber, bypassing faulty areas. On the one hand, this increases the reliability of the network, on the other hand, it requires significant complication of the network equipment, which must select the route.
