Ring topology network. Network topologies. What is the marker used for?

the subscriber must be significantly more complex than the equipment of peripheral subscribers. In this case, there is no need to talk about equal rights for all subscribers (as in a bus). Usually the central computer is the most powerful; all functions for managing the exchange are assigned to it. In principle, no conflicts are possible in a network with a star topology, since management is completely centralized.

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

Broken cable or short circuit in it, with a star topology, the exchange with only one computer is disrupted, and all other computers can continue to work normally.

Unlike a bus, in a star there are only two subscribers on each communication line: a central one and one of the peripheral ones. Most often, two communication lines are used to connect them, each of which transmits information in one direction, that is, on each communication line there is only one receiver and one transmitter. This is the so-called transfer point-to-point. This makes it all much easier network hardware compared to the bus and eliminates the need to use additional, external terminators.

The problem of signal attenuation in a communication line is also solved in a star more easily than in the case of a bus, because each receiver always receives a signal of the same level. The maximum length of a network with a star topology can be twice as long as in a bus (that is, 2 L inc), since each of the cables connecting the center with a peripheral subscriber can have a length of L inc.

A serious disadvantage of the star topology is the strict limitation on the number of subscribers. Typically, a central subscriber can serve no more than 8-16 peripheral subscribers. Within these limits, connecting new subscribers is quite simple, but beyond them it is simply impossible. In a star, it is possible to connect another central subscriber instead of a peripheral one (the result is a topology of several interconnected stars).

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

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

Rice. 1.11.

It turns out that although the cable layout is similar to a true or active star, in fact we're talking about about bus topology, since information from each computer is simultaneously transmitted to all other computers, and there is no central subscriber. Of course, a passive star is more expensive than a regular bus, since in this case a hub is also required. However, it provides a number of additional features, associated with the advantages of the star, in particular, simplifies the maintenance and repair of the network. That is why in Lately the passive star is increasingly replacing the true star, which is considered an unpromising topology.

It is also possible to distinguish an intermediate type of topology between an active and passive star. In this case, the hub not only relays the signals arriving at it, but also controls the exchange, but does not itself participate in the exchange (this is done in the 100VG-AnyLAN network).

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

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

Ring topology

A ring is a topology in which each computer is connected by communication lines to two others: from one it receives information, and to the other it transmits. On each communication line, as in the case of a star, only one transmitter and one receiver operate (point-to-point communication). This allows you to abandon the use of 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. Full size the network in the limit will be NL pr/2, since the ring will have to be folded in half. In practice sizes ring networks reach tens of kilometers (for example, in the FDDI network). The ring is significantly superior to any other topology in this regard.

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

But it has two design differences:

The network is closed in a ring - thus no terminators are required;
- one of the computers on the network creates a “token” that is passed from computer to computer. Transport protocol, on the basis of which such a network usually operates, is called Token Ring.

What is a marker?

A token is a three-byte frame that is sent from one network node to another. There are two modes of network operation with a marker: normal (network data transfer speed up to 4 Mbit/s) and with fast marker release (data transfer speed up to 16 Mbit/s). Experiments with the introduction of this technology into a 100 megabit network failed, so over time this technology was abandoned in currently it is outdated and is unlikely to ever come across on your life path.

What is the marker used for?

In order not to clog the network with unnecessary transit traffic and to avoid collisions, a marker is introduced. The principle of operation is this: only the computer that received the token can start transmitting data to another host on the network. If the computer that received the token is not transmitting data, then the token goes to next computer. The remaining computers on the network that are in this moment do not have a marker, are listeners. The exception to this rule is networks operating in fast token release mode. In these networks, the computer that starts the transmission immediately generates a free token.

The computer that receives the token and has information to transmit changes one bit in the token and starts starter pack which flies to its destination. Having flown the circle, the marker or the next data packet returns to the sending station. In this case, the sending station can check the information from the returned packet and check whether the packet was delivered to the recipient. After this, the package is destroyed.

Token Ring technology has had its fans and its opponents, however, like any other technology, it has its pros and cons.

Higher reliability of data transmission, because the network is used in a more “organized” manner and no collisions occur;
- low cost of installation, although more cable is required;
- if one network node fails, the remaining nodes continue to function fully (unless the cable is damaged).

This network topology (its diagram is shown in Fig. 4.5) is widely used for building networks SDH using the first two levels of transmission systems SDH(transfer rates of 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 of synchronous multiplexers DIM two pairs of optical linear (aggregate) ports. They make it possible to form an SLT in the form of a double ring structure with counter digital streams (they are shown by arrows in Fig. 4.5).

The ring topology has a number of properties that allow the network to self-heal, i.e., provide protection from some sufficient 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 the formation of circular self-healing (“self-healing”) networks of two types: unidirectional and bidirectional.

The first type of network uses two optical fibers. Each transmitted digital stream is sent along the ring network in both (opposite) directions, and at the receiving point, as in the case of “1+1” protection in the “point-to-point” network topology (see Fig. 4.2), a choice is made one of two received signals (the best in quality, for example, in terms of the lowest error rate). The transmission of digital streams across all main sections of the SLT occurs in one direction (for example, clockwise), and through all backup sections - in the opposite direction. Therefore, such a ring network is called unidirectional with SLT switching or with an assigned reserve. The flow diagram of signals through the main and backup sections of the SLT of the ring network under consideration is shown in Fig. 4.5.

A bidirectional ring network can be formed using two (topology

"dual ring") or four (two "dual rings") optical fibers. In a bidirectional ring network with two fibers, the transmitted DLCs are not duplicated. When such a network operates, digital streams of access points are transmitted along the ring along the shortest path in opposite directions (hence the name “bidirectional ring”). If a failure occurs at any section of the SLT through DIM switched on at the ends of the failed section, the entire digital stream entering this section is switched to reverse direction. This network configuration is also called ring with switching sections or a ring protected by a shared reserve.

An example of a bidirectional ring network with two OBs is shown in Fig. 4.6. It shows signal flow diagrams for one of the options for connecting access points in operating (pre-emergency) mode (Fig. 4.6, A) and in emergency mode in the event of failure of one of the SLT sections 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 diagram, 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 if one section fails, the full functionality of either of these networks can be maintained. However, in most cases, a bidirectional ring network is more economical because it requires less bandwidth. This is explained by the fact that the same optical fibers are used for signals transmitted at different intersecting sections of the ring network (both in the main and in emergency modes of 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 networks of access to the nearest node. Bidirectional network rings are preferable for uniform traffic, for example, for building digital connecting lines between powerful electronic telephone exchanges, or digital switching stations (DSCs).

Bidirectional four-fiber ring network provides more high level fault tolerance than a ring network with two optical fibers, but the cost of building a four-fiber ring network is significantly higher. IN network structures with two double rings, if there is 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 a reconfiguration of the ring network is carried out, similar to that shown in Fig. 4.6, b.

Despite the high cost of a four-fiber ring network, it has recently found increasing use in high-speed networks SDH, as it provides very high reliability.

Above, we considered only the case when a section of the SLT of the ring network was in emergency condition, i.e. optical fiber line cable. However, in such a network the multiplexer may 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 units) is restored by eliminating the damaged multiplexer from the operation scheme. Modern systems management DIM provide a workaround that allows a digital stream to bypass a failed multiplexer at a given point in the ring network.

The water supply network is a set of pipelines through which water is transported to consumers. The main purpose of the water supply network is to supply consumers with water in the required quantity, good quality and with the required pressure. Typically, the plumbing system, along with supplying water for household needs, also provides fire extinguishing needs. Design a water supply network taking into account collaboration pumping stations, water towers and other elements of the water supply system.

Tracing water supply network consists in giving it a certain geometric shape. It depends on: the configuration of the settlement, the location of streets, blocks, public and industrial buildings, the location of the water supply source and many other factors.

N.S. - pumping station

B - water tower

Figure - Diagram of the ring water supply network

The ring network is used in populated areas close in outline to a square or rectangle. In these networks, pipelines form one or more closed circuits - rings. Thanks to ringing, each section receives power from two or more lines, which significantly increases the reliability of the network and creates a number of other advantages. Ring networks ensure uninterrupted water supply even in case of accidents separate areas: when the emergency section is turned off, the water supply to other network lines does not stop. They are less prone to accidents because... they do not experience strong hydraulic shocks. When a pipeline is quickly closed, the water flowing to it rushes into other lines of the network and the effect of the water hammer is reduced. The water in the network does not freeze, because... even with a small amount of water, it circulates through all lines, carrying heat with it. Ring networks are usually slightly longer than dead-end networks, but are made of smaller diameter pipes. The cost of ring networks is slightly higher than dead-end networks. Thanks to high reliability they are widely used in water supply. They fully meet the requirements of fire-fighting water supply. After the calculation of water consumption has been completed settlement, the ring distribution network is traced. For this purpose, pipelines are drawn on the territory of the water supply facility (plan of the village), their ends and beginnings are connected, forming closed loops, and water is supplied to large facilities. Next, nodes and sections are outlined on the ring network. Each section of the network is analyzed and measured. All results are summarized in a table. It should be noted that a feature of ring networks is that water is distributed to water consumers in almost all of its sections, which means that all of them are areas with travel costs. The only exceptions are those areas where it is clearly inappropriate to disassemble the water. These may be areas that supply water to large water consumers (for example, a bathhouse, a hospital, a medical facility, etc.).

Ring topology is a topology in which each computer is connected by communication lines to only two others: from one it only receives information, and to the other it only transmits. On each communication line, as in the case of a star, there is only one transmitter and one receiver. This allows you to avoid using external terminators.
Each computer retransmits (renews) the signal, that is, acts as a repeater, therefore the attenuation of the signal throughout the ring does not matter, only the attenuation between neighboring computers of the ring is important. In this case, there is no clearly defined center; all computers can be the same. However, quite often a special subscriber is allocated in the ring who manages the exchange or controls the exchange. It is clear that the presence of such a control subscriber reduces the reliability of the network, because its failure will immediately paralyze the entire exchange.
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 The number of subscribers in the ring can be quite large (1000 or more). Used as a media on the network twisted pair or fiber optic. Messages circulate in circles.
A workstation can transmit information to another workstation, only after receiving the right to transmit (token), so collisions are excluded. Information is transmitted along a ring from one workstation to another, so if one computer fails, unless special measures are taken, the entire network will fail.
The ring topology is usually the most resistant to overloads; it ensures reliable operation with the largest flows of information transmitted over the network, because, as a rule, there are no conflicts (unlike a bus), and there is no central subscriber (unlike a star) .