Principles of building local area networks. Ring topology

Term network topology refers to a way of connecting computers to a network. You may also hear other names - network structure or network configuration (This is the same). In addition, the concept of topology includes many rules that determine the location of computers, how cables are laid, how to place communication equipment, and much more. To date, several basic topologies have been formed and established. Of these, it can be noted “ tire”, “ring" and " a star”.

Bus topology

Topology tire (or, as it is often called common bus or highway ) assumes the use of a single cable to which all workstations are connected. A common cable is used by all stations in turn. All messages sent by individual workstations are received and listened to by all other computers connected to the network. From this stream, each workstation selects messages addressed only to it.

Advantages of the bus topology:

• ease of customization;
• relative ease of installation and low cost if all workstations are located nearby;
• failure of one or more workstations does not affect the operation of the entire network in any way.

Disadvantages of bus topology:

• bus malfunctions anywhere (cable break, failure of the network connector) lead to the inoperability of the network;
• the complexity of troubleshooting;
• low performance - at a time, only one computer can transmit data to the network; with an increase in the number of workstations, the network performance decreases;
• poor scalability - to add new workstations, it is necessary to replace sections of the existing bus.

It was on the “bus” topology that local networks were built on coaxial cable... In this case, pieces of coaxial cable connected by T-connectors were used as a bus. The bus was laid through all rooms and went to every computer. The side terminal of the T-connector was inserted into a slot on the network card. This is how it looked: Now such networks are hopelessly outdated and have been replaced everywhere with a "star" on twisted pair, but equipment for coaxial cable can still be seen in some enterprises.

Ring topology

Ring Is a local network topology in which workstations are connected in series to each other, forming a closed ring. Data is transferred from one workstation to another in one direction (in a circle). 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 race. If a computer receives data intended for another computer, it transfers them further along the ring, otherwise they will not be transferred further.

Ring topology benefits:

• ease of installation;
• almost complete absence of additional equipment;
• the possibility of stable operation without a significant drop in the data transfer rate with an intensive network load.

However, the “ring” also has significant disadvantages:

• each workstation must actively participate in the transfer of information; in case of failure of at least one of them or a cable break, the operation of the entire network stops;
• connecting a new workstation requires a short-term shutdown of the network, since during the installation of a new PC the ring must be open;
• the complexity of configuring and setting;
• the complexity of troubleshooting.

Ring network topology is rarely used. She found the main application in fiber optic networks Token Ring standard.

Star topology

Star Is a local network topology where each workstation is attached to a central device (switch or router). A central device controls the movement of packets on the network. Each computer through a network card is connected to the switch with a separate cable. Multiple star networks can be linked together if required, resulting in a network configuration with tree-like topology. Tree topology is common in large companies. We will not consider it in detail in this article.

The "star" topology today has become the main one in the construction of local networks. This happened due to its many advantages:

• failure of one workstation or damage to its cable does not affect the operation of the entire network as a whole;
• excellent scalability: to connect a new workstation, it is enough to lay a separate cable from the switch;
• easy troubleshooting and network interruptions;
• high performance;
• ease of setup and administration;
• additional equipment can be easily integrated into the network.

However, like any topology, the star is not without its drawbacks:

• failure of the central switch will result in the inoperability of the entire network;
• additional costs for network equipment - a device to which all computers on the network will be connected (switch);
• the number of workstations is limited by the number of ports in the central switch.

Star Is the most common topology for wired and wireless networks. An example of a star topology is a twisted-pair cable network with a switch as the central unit. These are the networks found in most organizations.

Did you know, what is the falsity of the concept of "physical vacuum"?

Physical vacuum - the concept of relativistic quantum physics, under which they mean the lowest (ground) energy state of the quantized field, which has zero momentum, angular momentum and other quantum numbers. Relativistic theorists call a physical vacuum a space completely devoid of matter, filled with an unmeasurable, and therefore only an imaginary field. Such a state, according to relativists, is not an absolute emptiness, but a space filled with some phantom (virtual) particles. Relativistic quantum field theory asserts that, in accordance with the Heisenberg uncertainty principle, virtual, that is, apparent (to whom?), Particles are constantly born and disappear in the physical vacuum: so-called zero-point field oscillations occur. Virtual particles of the physical vacuum, and therefore, itself, by definition, do not have a frame of reference, since otherwise Einstein's principle of relativity, on which the theory of relativity is based, would be violated (that is, an absolute system of measurement would become possible with reference from particles of a physical vacuum, which, in turn, would unequivocally refute the principle of relativity, on which the SRT is built). Thus, the physical vacuum and its particles are not elements of the physical world, but only elements of the theory of relativity, which exist not in the real world, but only in relativistic formulas, violating the principle of causality (arise and disappear for no reason), the principle of objectivity (virtual particles can be considered, depending on the desire of the theoretician, either existing or not existing), the principle of actual measurability (not observable, do not have their own IRS).

When this or that physicist uses the concept of "physical vacuum", he either does not understand the absurdity of this term, or is disingenuous, being a hidden or explicit adherent of relativistic ideology.

The easiest way to understand the absurdity of this concept is to refer to the origins of its origin. It was born by Paul Dirac in the 1930s, when it became clear that the denial of the ether in its pure form, as the great mathematician, but the mediocre physicist did, was no longer possible. Too many facts contradict this.

To defend relativism, Paul Dirac introduced the aphysical and illogical concept of negative energy, and then the existence of a "sea" of two energies compensating each other in a vacuum - positive and negative, as well as a "sea" of particles compensating each other - virtual (that is, apparent) electrons and positrons in a vacuum.

Casting network has many names: cape, cape, basting, cover, parachute ... Oddly enough, amateurs of casting net fishing and especially specialists in this fishing are quite rare in our country. There are many reasons for this. Historically, for a long time, box net fishing has been a traditional fishing method in mainly southern countries (South America, Asia). Fishermen in those parts have been engaged in such fishing since childhood and the results are amazing.

Tourists are amazed to observe how the native fisherman throws away an incomprehensible bundle, which on the fly unfolds into a large round net, which soon returned from the murky waters with a rich catch. Astonishment gave way to a completely legitimate desire: we want to catch the same! As a result, the casting network rapidly began to conquer non-traditional countries for itself.

The fishing principle is as follows: The net is assembled on the hand in a certain way (so that it easily turns around in flight), then horizontally pounces on the water and covers a section of water corresponding to the diameter of the opened net. After the loaded part of the net sinks to the bottom, the net is pulled out by the cord attached to the base. It is possible to fish in the water column without lowering the tackle to the bottom, but this requires a slightly modified net.

Casting network (American type) example

Casting networks are divided into two large groups: American type and Spanish. The American type is more convenient for casting, more catchy and easier to make with your own hands. The Spanish type has one advantage: in places with underwater terrain inconvenient for fishing, it is less prone to catching on stones, snags, etc.

Casting net is a net fabric in the form of a regular circle, along the edge of which a cord is sewn, equipped with very often planted lead weights. A central braided cord (twisted is in no way suitable) is used to pull the net, thick enough (so as not to cut your hands when quickly pulling out the tackle), usually at least 5–6 mm. Its standard length is 4–4.5 m, but many amateurs, having perfectly mastered the tackle, increase the length of the line by 1.5–2 times. At the end of the cord there is a loop with a diameter of 20-25 cm.

In American nets, the other end of the central cord is attached to numerous slings (veins) stretched to the cargo cord, in Spanish - to the central part of the net. This design difference also determines the different performance of the net after casting.

When pulling out the American-style tackle, the central cord with the help of veins pulls the cargo cord to the center and practically gathers it together in a compact lump, thereby tightening the net and forming a bag with a closed outlet. When pulling out a Spanish-type net, the sinkers converge to the center under the action of the cord pull and its own gravity, closing the exit, and the catch remains in the pockets of the net located along its perimeter.

On the American tackle, in the very center of the net there is a small round hole (5–6 cm in diameter), and the netting along its edge is attached to a plastic or fluoroplastic sleeve. One hole is drilled in the bushing (for the smallest nets) or several (6–8 cm for the largest ones) through which the vein slings slide.

The netting fabric (with sufficiently small meshes, from 9 to 15 mm) is taken from both monofilament and twisted yarn.

Tips for Making an American Style Casting Net

If someone wants to make an American-style casting network with their own hands, he should follow a few rules:

1. Lead weights are planted on the load cord evenly and very often, with a distance between their centers no more than 10–12 cm. The weight of the weights is from 20 to 35 g, depending on the size of the net; their shape is a highly elongated cylinder; globular weights, especially those capable of falling into the mesh of the network, are inapplicable. If you use the sinkers not purchased, but cast yourself, each must be carefully processed, eliminating all the irregularities and casting defects.
2. The lines (slings) are made of fishing line (monofilament, braid makes it difficult to catch) 1 mm or more thick, their length is not much longer than the radius of the tackle. Veins are attached to the cargo cord quite often, at least every 0.5 m, and, accordingly, their number grows with an increase in the size of the tackle. If not one, but several holes are drilled in the plastic sleeve, then through each it is necessary to pass those veins that lead to the corresponding edge of the network, avoiding crossing. Sharp edges at the edges of the holes, any irregularities and burrs are unacceptable.
3. The knot that collects the veins together is made as compact and neat as possible, without line tails sticking out to the side. Since torque is generated during casting, it is best to attach it to the center line through a swivel of sufficient strength. Sometimes a plastic disk 3-4 cm in diameter with holes along the edge (according to the number of veins) is placed in front of the knot, and each vein is passed into its hole.

DIY casting network video:

What size net should I start casting with?

A question that does not have an unambiguous answer. On the one hand, the smaller the radius of the net, the easier it is to cast, and the learning stage is much faster. However, having perfectly mastered the casting of a three-foot net (mainly suitable for fishing live bait), it is quite difficult to retrain for a large tackle. Much also depends on the physical parameters of the angler: the higher his height and the longer his arms, the easier it will be to learn how to cast a large net.

I advise you to first decide: why, in fact, do you need a casting network? For those who like to catch a predator with live bait, a casting net is an irreplaceable auxiliary tool. In the full sense, irreplaceable: never when catching live bait with a fishing rod or any other design, you will not start catching predators as quickly after coming to the reservoir, as if you had a compact and ready-to-use casting net in your backpack. You can start hunting for pike or pike perch faster only by bringing live bait with you, which is not always convenient.

So if you plan to use the casting network only as a little girl - buy an easy-to-learn three-foot shoe, and the problem with live bait will disappear forever. In addition, as a bonus, sometimes (especially in muddy water or when casting at night), you can also hook a large fish. But if it is assumed that the casting net will become the main fishing tool, moreover, for large enough fish, then it is better to start mastering the tackle with a net with a radius of at least 1.7–2 m. Learning to cast, of course, will take longer, but then the transition even to a ten-foot length will not be a problem will be.

Casting technique

The figure shows the casting phases for both American and Spanish net types. The fisherman stands on the shore, but nevertheless, it is best to start training not on a river or lake, but on some lawn or mowed lawn. Naturally, in the field, on a pond, before starting preparation, the net is thoroughly cleaned of mud and other aquatic vegetation left over from the previous casting.

Phases of net casting

The traction cord is collected in rings in the left hand, the tackle is taken with an outstretched hand for the central part (or for the sleeve - for an American-type net), gently shaken so that the net stretches and straightens out. If the load cord has formed a loop somewhere, it must be straightened with your free hand. Then, with the right hand, the upper part of the tackle is intercepted (from a quarter to half of the net, depending on its radius) and is assembled with one or two loops - also in the left hand. Next comes the turn of the cargo cord. He is taken by two points with the same left hand and right, and the hands are spread wide enough so that the remaining free part of the network stretches as much as possible.

If you look at foreign videos, you can see how specialists sometimes, at this stage of preparation for casting, take one sinker in their teeth in order to achieve even greater stretching of the net.

The next stage is the casting itself. It is performed after two or three swinging movements or after one wide swing (while the angler's body turns almost 180 °). The most important thing at this moment is the plane in which the tackle moves. The casting net, unfolding on the fly, should fly along the most gentle trajectory and finally turn into a circle shortly before touching the water. The latter depends on the strength of the throw, the ability to measure which comes exclusively with training.

Another point in which I disagree with foreign instructors: for the most part, they recommend to overwhelm the loop at the end of the line with the left hand before casting. In training, it turns out well, but on a pond, when your hands are wet, the tackle can easily fly into a river or lake along with the line. It is safer to attach the loop to the waist belt.

The described casting technique is not the only one possible. Almost every fisherman, with the acquisition of experience, begins to modernize it, adjusting it to his individual characteristics and to the specific conditions of fishing. For example, you may not collect the pulling cord with rings on your hand, but leave it lying under your feet (provided that the bank is clean enough and the cord does not catch on branches, roots, driftwood, etc.). The preparation time for casting is reduced, which increases the number of throws per fishing and, accordingly, the size of the catch.

Nets of a small radius (up to 1.7 m, for the tallest fishermen - up to 2 m) can be thrown without collecting the upper part of the net in loops. Both hands, raised and spaced as wide as possible, take hold of the cargo cord, the excess cord is collected in loops, 2-3 in each hand, so that the lower edge of the net does not reach the ground 30-40 cm, then the net is thrown, or rather thrown on a pond with a characteristic movement, reminiscent of those with which a wide tablecloth is thrown on a table or a sheet on a bed. I even happened to see how the casting net was thrown together: they caught two short teenage boys, each of whom could hardly have thrown the tackle on their own - they took the net by the load cord, standing on either side of it, stretched widely in the horizontal plane and, simultaneously swinging, was sent into the reservoir.

Seat selection

Places that are too deep, with a fast current, with a snagged or boulder-covered bottom, with abundant underwater vegetation, are definitely not suitable for fishing with a casting net. Steep underwater slopes - the so-called "" - also do not allow you to capture the fish keeping above them. When fishing from the shore, you should avoid places heavily overgrown with trees, bushes and even such annual vegetation as wormwood, weeds, and so on, for at least a couple of meters around the fisherman there should be a clean and level space.

It makes no sense to fish from cliffs, from embankments and bridges that rise more than 2 m above the water surface - the weights of the net, even if it is properly thrown, when it falls from a great height, begin to converge together, and instead of a regular flat circle, the tackle takes the shape of an elongated cone. Fishing in unfamiliar places is always fraught with snags, net damage and line breaks.

Network sampling

As soon as the load cord of the abandoned net touches the bottom, which is determined by the weakening of the traction cord, the tackle begins to be selected with a sharp jerk. This jerk allows, firstly, to quickly bring the sinkers together, closing the exit of the caught fish, and secondly, it raises the net above the bottom, reducing the likelihood of snags.

Fishing tactics

Fishing with a casting net is no less versatile than fishing, and can be used in a wide variety of conditions, on very different reservoirs and for catching fish that differ in habits and lifestyle.

Catching live bait

The easiest way to fish with a casting net is live bait and, in general, small fish. It is enough just to choose the right place and make a successful cast, sometimes only one, and if the little thing walks in dense flocks, then after the first throw three or four dozen fish are sent to the bucket; now you can move on to catching a predator with live bait. Neither a boat nor fishing for live bait is required to get live bait, casts are made from the shore. You just need to look out in the clear water, where gudgeons lie on a sandbank or flocks of perch or roach swim near algae.

Catching big fish

Larger fish are caught almost always blindly, in places of their accumulation. Even when you see a flock of such fish in shallow water, it is not worth approaching them with a casting net, if the fisherman sees the fish, then the fish sees the fisherman, and the flight of the net makes it quickly recoil to the side. During the spring course of fish, it is very convenient to choose places on the river in front of any natural obstacle with a flat bottom and a shallow depth of 0.5 to 1.5 m. Casting is carried out in approximately the same way as fishing with a spinning rod: first, the net covers the nearest to the fisherman areas, then located at an average distance, then the farthest, as far as the length of the pulling cord allows. It should be borne in mind that the fish is not very much afraid of the splash of the casting net that has fallen on the water (this splash is quiet, if the cast is done correctly), the fish does not rush away, but usually rolls slightly downstream. Therefore, the section of the river chosen for fishing should always be fished, moving along the bank downstream.

Spring fishing is done during the day, but as the water clears, the best catches happen at dusk or at night. In summer, when underwater vegetation appears in large quantities of water bodies, the number of places suitable for blindfishing is sharply reduced. It is much more interesting at this time to hunt with a casting net, tracking down single specimens of large fish.

Fishing for tench is very exciting. They are engaged in it in shallow places of the river with a very slow current and a muddy bottom. A string of bubbles rising from the bottom disturbed by the fish serves as a sign confirming that the tench feeds in this place. The boat is not needed, the feeding grounds for tench are usually located near the coast, sometimes, if the river is not wide and the banks are steep enough, literally a meter from the water's edge. If at the fishing site there are too dense thickets of aquatic vegetation, for example water lilies, it is necessary to make several glades in them in advance, 2-3 times larger than the size of the net. Large bream also often give out their feeding places with bubbles. But catching him with a casting network is much more difficult. Bream is more careful, it feeds in deeper holes and most often manages to escape from the net descending on it.

Catch pike it is convenient on hot sunny days, going around shallow bays and channels, framed by thickets of reeds or cattails, on a boat. The boat should have low sides, with a wide bow of which it is convenient to cast. Having spotted a pike, which is usually frozen in half-water not far from the reed wall, the fisherman points at it to the rower, and when the boat approaches a sufficient distance, throws a net over the fish.
Spring fishing for pikes in shallow-water spawning grounds is more catchy, sometimes from the shore, but more often off the beaten track. Here it is necessary to master long-range casting, it is difficult to come close to the spawning pike. Noticing the place where the fish is splashing, the fisherman throws a net on it from the maximum possible distance, and often, together with the roe pike, pulls out a couple of milkmen. Unsuccessful casts are also frequent, when the underwater vegetation, on which the pike spawns, prevents the nets from closing correctly. The spawning of a large (kilogram and above) crucian carp does not last long, one or two in the morning, but if you manage to catch it with a casting net, then the catch will be very pleasing. The place for a throw here is sometimes determined not only by splashes, but also by indirect signs: by the movement of the stems of aquatic plants sticking out above the water, by the so-called "mustache", which is formed on the surface of the water by a shallow swimming large fish, by small fish, in all directions jumping out of the water (the fry do not understand, a peaceful or predatory fish swims up to them).

Spawning of carp is similar to carp, but carp is a more cautious fish and often spawns in shallow waters more distant from the coast, overgrown with aquatic vegetation. Therefore, it is better to approach it by boat, observing the maximum silence.

Casting Network - Casting Videos

This network topology (its diagram is shown in Fig.4.5) is widely used to build networks SDH using the first two levels of transmission systems SDH(transmission rates 155.52 and 622.08 Mbit / s) on the access network. The main feature and advantage of this topology is the ease of providing a "1 + 1" type protection system due to the presence in synchronous multiplexers DIM two pairs of optical line (aggregate) ports. They make it possible to form SLT in the form of a double ring structure with counter digital streams (in Fig. 4.5 they are shown by arrows).

The ring topology has a number of properties that allow the network to heal itself, that is, to provide protection against some of the 226

common types of failures. Therefore, let us dwell on the basic properties of the ring network topology in more detail.

"Intellectual" capabilities DIM allow to form ring self-healing ("self-healing") networks of two types: unidirectional and bidirectional.

The first type of networks uses two optical fibers. Each transmitted digital stream is directed along the ring network in both (opposite) directions, and at the receiving point, as in the case of protection according to the "1 + 1" scheme in the "point-to-point" network topology (see Fig. 4.2), a choice is made one of the two received signals (the best in quality, for example, the lowest error rate). Transmission of digital streams in all main sections of the SLT occurs in one direction (for example, clockwise), and in all reserve sections - in the opposite direction. Therefore, such a ring network is called unidirectional with SLT switching or with a fixed reserve. The signal flow diagram along the main and backup sections of the SLT of the considered ring network is shown in Fig. 4.5.

A bi-directional ring network can be formed using two (topology

"Double ring") or four (two "double rings") optical fibers. In a bi-directional ring network with two fibers, the transmitted DLNs are not duplicated. During the operation of such a network, digital streams of access points are transmitted along the ring by the shortest path in opposite directions (hence the name "bidirectional ring"). In the event of a failure at any section of the SLT by means of DIM, switched on at the ends of the failed section, the entire digital stream entering this section is switched in the opposite direction. This network configuration is also called ring with switching sections or a ring protected by a shared reserve.

An example of a bi-directional ring network with two OBs is shown in Fig. 4.6. It shows the signal flow diagrams for one of the options for connecting access points in the operating (pre-emergency) mode (Fig. 4.6, a) and in emergency mode in case of failure of one of the sections of the SLT of the ring network, which is crossed out with a cross (Fig. 4.6, b). The damaged section of the SLT is excluded from the ring scheme, but the connection between all access points on the network is preserved.

Comparing unidirectional and bidirectional ring networks with two fibers to each other, it should be noted that in case of failure of one section, full operability of any of these networks can be maintained. In most cases, however, a bi-directional ring network is more economical because it requires less bandwidth. This is due to the fact that for signals transmitted at different intersecting sections of the ring network, the same optical fibers are used (both in the main and in emergency operation). At the same time, a unidirectional network ring is easier to implement.

Unidirectional ring networks are more suitable for "centripetal" traffic, in particular for access networks to the nearest node. Bidirectional network rings are preferable for uniform traffic, for example, for building digital connecting lines between powerful electronic automatic telephone exchanges, or digital switching stations (DSC).

A bi-directional four-fiber ring provides a higher level of resiliency than a two-fiber ring, but the cost of building a four-fiber ring is significantly higher. In network structures with two double rings, in the event of a failure in any section of the SLT, an attempt is initially made to switch to another pair of optical fibers within the same (failed) section. But if this fails, then the ring network is reconfigured similar to that shown in Fig. 4.6, b.

Despite the high cost of the four-fiber ring network, it has recently been increasingly used in high-speed networks. SDH, as it provides very high reliability.

Above, only the case was considered when the SLT section of the ring network, that is, the optical fiber of the line cable, was in an emergency state. However, in such a network, the multiplexer can also fail. In this situation, redundancy as such is not used, and the operability of the network as a whole (at the level of linear blocks) is restored by excluding the damaged multiplexer from the operation scheme. Modern control systems DIM provide a workaround that allows the digital stream to bypass the failed multiplexer at a given point in the ring network.

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 a 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. At a considerable distance between PCs (for example, 180 m for a thin coaxial cable) in the "bus" segment, an attenuation 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 by installing an additional device between them - a 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 a LAN is based on a server, where most of the software and information resources are stored on the server, then PCs, although they remain functional, are of little use for practical work.

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

A pure ring topology is 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 Token Ring networks, 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 node (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 is connected with a separate cable to the unifying 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 twisted-pair cable. 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 of basic network topologies