Classification of networks.

By territorial distribution

PAN (Personal Area Network) - a personal network designed for interaction various devices belonging to the same owner.

LAN (Local Area Network) - local networks having a closed infrastructure before reaching service providers. The term “LAN” can describe both a small office network and a network at the level of a large factory covering several hundred hectares. Foreign sources even give a close estimate of about six miles (10 km) in radius. Local networks are closed networks, access to them is permitted only limited circle users for whom work in such a network is directly related to their professional activities.

CAN (Campus Area Network) - unites local networks of nearby buildings.

MAN (Metropolitan Area Network) - urban networks between institutions within one or several cities, connecting many local area networks.

WAN (Wide Area Network) is a global network covering large geographic regions, including both local networks and other telecommunication networks and devices. An example of a WAN is a packet-switching network (Frame relay), through which various computer networks can “talk” to each other. Global networks are open and focused on serving any users.

The term "enterprise network" is also used in the literature to refer to the combination of several networks, each of which can be built on different technical, software and information principles.

By type of functional interaction

Client-server, Mixed network, Peer-to-peer network, Multi-peer network

By type of network topology

Tire, Ring, Double Ring, Star, Honeycomb, Lattice, Tree, Fat Tree

By type of transmission medium

Wired (telephone wire, coaxial cable, twisted pair, fiber optic cable)

Wireless (transmitting information via radio waves in a certain frequency range)

By functional purpose

Storage Networks, Server Farms, Process Control Networks, SOHO Networks, House Networks

By transmission speed

low-speed (up to 10 Mbit/s), medium-speed (up to 100 Mbit/s), high-speed (over 100 Mbit/s);

If necessary to maintain a constant connection

Packet network such as Fidonet and UUCP, Online network such as Internet and GSM

Circuit switched networks

One of the most important issues in computer networks is the issue of switching. The concept of switching includes:

1. route distribution mechanism for data transmission

2. synchronous use communication channel

We will talk about one of the ways to solve the switching problem, namely about circuit-switched networks. But it should be noted that this is not the only way solving the problem in computer networks. But let's move closer to the essence of the issue. Circuit switched networks form a common and unbreakable physical section (channel) of communication between the end nodes, through which data passes at the same speed. It should be noted that the same speed is achieved due to the absence of a “stop” in certain sections, since the route is known in advance.

Establishing a connection to circuit switched networks always starts first, because you can’t get a route to the desired goal without connecting. And after the connection is established, you can safely transfer the necessary data. Let's take a look at the benefits of circuit switched networks:

1. data transfer speed is always the same

2. there is no delay at the nodes during data transmission, which is important for various On-line events (conferences, communication, video broadcasts)

Well, now I need to say a few words about the shortcomings:

1. It is not always possible to establish a connection, i.e. sometimes the network may be busy

2. We cannot immediately transfer data without first establishing a connection, i.e. time is wasted

3. not very efficient use of physical communication channels

Let me explain about the last minus: when creating a physical communication channel, we completely occupy the entire line, leaving no opportunity for others to connect to it.

In turn, circuit-switched networks are divided into 2 types, using different technological approaches:

1. Frequency Division Multiplexing (FDM) based circuit switching

The scheme of work is as follows:

1. each user transmits a signal to the switch inputs

2. All signals with the help of a switch fill the ΔF bands using the method of frequency modulation of the signal

2. Circuit switching based on time division multiplexing (TDM)

Principle circuit switching based on time multiplexing is quite simple. It is based on time division, i.e. Each communication channel is serviced in turn, and the period of time for sending a signal to the subscriber is strictly defined.

3.Packet switching
This switching technique was specifically designed for efficient transmission of computer traffic. The first steps towards creation computer networks based on circuit switching techniques have shown that this type of switching does not allow achieving high overall network throughput. Typical network applications generate traffic very sporadically, with high levels of data rate burstiness. For example, when accessing a remote file server, the user first views the contents of that server's directory, which results in the transfer of a small amount of data. It then opens the required file in text editor, and this operation can create quite a lot of data exchange, especially if the file contains large graphical inclusions. After displaying a few pages of a file, the user works with them locally for a while, which requires no network transfer at all, and then returns modified copies of the pages to the server - again creating intensive network transfer.

The traffic ripple factor of an individual network user, equal to the ratio of the average intensity of data exchange to the maximum possible, can reach 1:50 or even 1:100. If for the described session we organize channel switching between the user’s computer and the server, then most of the time the channel will be idle. At the same time, the switching capabilities of the network will be assigned to this pair of subscribers and will not be available to other network users.

When packet switching occurs, all user-transmitted messages are broken down at the source node into relatively small pieces called packets. Let us recall that a message is a logically completed piece of data - a request to transfer a file, a response to this request containing the entire file, etc. Messages can be of any length, from a few bytes to many megabytes. On the contrary, packets can usually also have a variable length, but within narrow limits, for example from 46 to 1500 bytes. Each packet is provided with a header that specifies the address information needed to deliver the packet to the destination node, as well as the packet number that will be used by the destination node to assemble the message (Figure 3). Packets are transported over the network as independent information blocks. Network switches receive packets from end nodes and, based on address information, transmit them to each other, and ultimately to the destination node.

Packet network switches differ from circuit switches in that they have internal buffer memory for temporary storage of packets if the output port of the switch is busy transmitting another packet at the time the packet is received (Fig. 3). In this case, the packet remains for some time in the packet queue in the buffer memory of the output port, and when its turn reaches it, it is transferred to the next switch. This data transmission scheme allows you to smooth out traffic pulsation on backbone links between switches and thereby use them most effectively to increase the capacity of the network as a whole.

Indeed, for a pair of subscribers, the most effective would be to provide them with sole use of a switched communication channel, as is done in circuit-switched networks. In this case, the interaction time of this pair of subscribers would be minimal, since data would be transmitted from one subscriber to another without delay. Subscribers are not interested in channel downtime during transmission pauses; it is important for them to quickly solve their problem. A packet-switched network slows down the process of interaction between a particular pair of subscribers, since their packets can wait in the switches while other packets that arrived at the switch earlier are transmitted along the backbone links.

However, the total amount of computer data transmitted by the network per unit time using the packet switching technique will be higher than using the circuit switching technique. This happens because the ripples of individual subscribers, in accordance with the law of large numbers, are distributed in time so that their peaks do not coincide. Therefore, switches are constantly and fairly evenly loaded with work if the number of subscribers they serve is really large. In Fig. Figure 4 shows that the traffic coming from end nodes to switches is distributed very unevenly over time. However, higher-level switches in the hierarchy that service connections between lower-level switches are more evenly loaded, and packet flow on the trunk links connecting upper-level switches is at near-maximum utilization. Buffering smoothes out ripples, so the ripple factor on trunk channels is much lower than on subscriber access channels - it can be equal to 1:10 or even 1:2.

The higher efficiency of packet-switched networks compared to circuit-switched networks (with equal communication channel capacity) was proven in the 60s both experimentally and using simulation modeling. An analogy with multiprogramming is appropriate here. operating systems. Each individual program in such a system takes longer to execute than in a single-program system, where the program is allocated all the processor time until its execution completes. However, the total number of programs executed per unit of time is greater in a multi-program system than in a single-program system.
A packet-switched network slows down the process of interaction between a specific pair of subscribers, but increases the throughput of the network as a whole.

Delays at the transmission source:

· time to transfer headers;

· delays caused by the intervals between the transmission of each next packet.

Delays in each switch:

· packet buffering time;

switching time, which consists of:

o waiting time for a packet in the queue (variable value);

o the time it takes for a packet to move to the output port.

Advantages of Packet Switching

1. High overall network throughput when transmitting bursty traffic.

2. The ability to dynamically redistribute the capacity of physical communication channels between subscribers in accordance with the real needs of their traffic.

Disadvantages of Packet Switching

1. Uncertainty in the data transfer rate between network subscribers, due to the fact that delays in the buffer queues of network switches depend on the overall network load.

2. Variable delay of data packets, which can be quite long during moments of instantaneous network congestion.

3. Possible data loss due to buffer overflow.
Currently, methods are being actively developed and implemented to overcome these shortcomings, which are especially acute for delay-sensitive traffic that requires a constant transmission speed. Such methods are called Quality of Service (QoS) methods.

Packet switched networks, which implement quality of service methods, allow the simultaneous transmission of various types of traffic, including such important ones as telephone and computer traffic. Therefore, packet switching methods today are considered the most promising for building a converged network that will provide comprehensive high-quality services for subscribers of any type. However, circuit switching methods cannot be discounted. Today they not only work successfully in traditional telephone networks, but are also widely used to form high-speed permanent connections in the so-called primary (backbone) networks of SDH and DWDM technologies, which are used to create backbone physical channels between telephone or computer network switches. In the future, it is quite possible that new switching technologies will emerge, in one form or another combining the principles of packet and channel switching.

4.VPN Virtual Private Network- virtual private network) is a generalized name for technologies that allow one or more network connections(logical network) on top of another network (such as the Internet). Despite the fact that communications are carried out over networks with a lower unknown level of trust (for example, over public networks), the level of trust in the constructed logical network does not depend on the level of trust in core networks thanks to the use of cryptography tools (encryption, authentication, infrastructure public keys, means to protect against repetitions and changes in messages transmitted over a logical network).

Depending on the protocols used and the purpose, a VPN can provide connection of three types: node-node,node-network And network-network. Typically, VPNs are deployed at levels no higher than the network level, since the use of cryptography at these levels allows transport protocols (such as TCP, UDP) to be used unchanged.

Users Microsoft Windows the term VPN denotes one of the implementations virtual network- PPTP, which is often used Not to create private networks.

Most often, to create a virtual network, the PPP protocol is encapsulated in some other protocol - IP (this method is used by the implementation of PPTP - Point-to-Point Tunneling Protocol) or Ethernet (PPPoE) (although they also have differences). VPN technology in Lately used not only to create private networks themselves, but also by some “last mile” providers in the post-Soviet space to provide Internet access.

With the proper level of implementation and the use of special software, a VPN network can provide a high level of encryption of transmitted information. At correct setting VPN technology of all components ensures anonymity on the Internet.

A VPN consists of two parts: an “internal” (controlled) network, of which there may be several, and an “external” network through which an encapsulated connection passes (usually the Internet). It is also possible to connect a separate computer to a virtual network. The connection of a remote user to the VPN is made through an access server, which is connected to both the internal and external (public) network. When a remote user connects (or when establishing a connection to another secure network), the access server requires an identification process, and then an authentication process. After successful completion of both processes, the remote user ( remote network) is endowed with authority to work on the network, that is, an authorization process occurs. VPN solutions can be classified according to several main parameters:

[edit]According to the degree of security of the environment used

Protected

The most common version of virtual private networks. With its help, it is possible to create a reliable and secure network based on an unreliable network, usually the Internet. Examples of secure VPNs are: IPSec, OpenVPN and PPTP.

Trusted

They are used in cases where the transmission medium can be considered reliable and it is only necessary to solve the problem of creating a virtual subnet within larger network. Security issues become irrelevant. Examples of such VPN solutions are: Multi-protocol label switching (MPLS) and L2TP (Layer 2 Tunnelling Protocol) (more precisely, these protocols shift the task of ensuring security to others, for example, L2TP is usually used in conjunction with IPSec).

[edit]By method of implementation

In the form of special software and hardware

The implementation of a VPN network is carried out using a special set of software and hardware. This implementation provides high performance and, as a rule, a high degree of security.

As a software solution

Use Personal Computer with special software, providing VPN functionality.

Integrated Solution

VPN functionality is provided by a complex that also solves the problems of filtering network traffic, organizing firewall and ensuring quality of service.

[edit]As intended

They are used to unite several distributed branches of one organization into a single secure network, exchanging data via open communication channels.

Remote Access VPN

Used to create a secure channel between a segment corporate network(central office or branch) and a single user who, working at home, connects to corporate resources with home computer, corporate laptop, smartphone or internet kiosk.

Used for networks to which “external” users (for example, customers or clients) connect. The level of trust in them is much lower than in company employees, so it is necessary to provide special “lines” of protection that prevent or limit the latter’s access to particularly valuable, confidential information.

It is used to provide access to the Internet by providers, usually when several users connect via one physical channel.

Client/Server VPN

It provides protection for transmitted data between two nodes (not networks) of a corporate network. The peculiarity of this option is that the VPN is built between nodes located, as a rule, in the same network segment, for example, between workstation and the server. This need very often arises in cases where it is necessary to create several logical networks on one physical network. For example, when it is necessary to divide traffic between the financial department and the human resources department accessing servers located in the same physical segment. This option is similar to VLAN technology, but instead of separating traffic, it is encrypted.

[edit]By protocol type

There are implementations of virtual private networks for TCP/IP, IPX and AppleTalk. But today there is a tendency towards a general transition to the TCP/IP protocol, and the vast majority of VPN solutions support it. Addressing in it is most often selected in accordance with the RFC5735 standard, from the range of TCP/IP Private Networks

[edit]By level network protocol

By network protocol layer based on comparison with the layers of the ISO/OSI reference network model.

5. Reference model OSI, sometimes called the OSI stack, is a 7-layer network hierarchy (Figure 1) developed by the International Standardization Organization (ISO). This model contains essentially 2 various models:

· a horizontal model based on protocols, providing a mechanism for interaction between programs and processes on different machines

· vertical model based on services provided by adjacent layers to each other on the same machine

IN horizontal model the two programs require a common protocol to exchange data. In a vertical one, neighboring levels exchange data using API interfaces.

Related information.

The distance limits for radio channels are given by suppliers on the assumption that there is no physical interference within the first Fresnel zone. An absolute limitation on the communication range of radio relay channels is imposed by the curvature of the earth, see Fig. 7.15. For frequencies above 100 MHz, waves propagate in a straight line (Fig. 7.15.A) and, therefore, can be focused. For high frequencies (HF) and UHF, the earth absorbs waves, but HF is characterized by reflection from the ionosphere (Fig. 7.15B) - this greatly expands the broadcast area (sometimes several successive reflections occur), but this effect is unstable and strongly depends on the state of the ionosphere.

Rice. 7.15.

When building long radio relay channels, repeaters have to be installed. If the antennas are placed on towers 100 m high, the distance between repeaters can be 80-100 km. The cost of an antenna complex is usually proportional to the cube of the antenna diameter.

The radiation pattern of a directional antenna is shown in Fig. 7.16 (the arrow marks the main direction of radiation). This diagram should be taken into account when choosing an antenna installation location, especially when using high radiation power. Otherwise, one of the radiation lobes may fall on places of permanent residence of people (for example, housing). Considering these circumstances, it is advisable to entrust the design of this kind of channels to professionals.

Rice. 7.16.

On October 4, 1957, the first artificial earth satellite was launched in the USSR, in 1961 Yu. A. Gagarin flew into space, and soon the first telecommunications satellite "Molniya" was launched into orbit - this is how the space era of communications began. The first satellite channel for the Internet in the Russian Federation (Moscow-Hamburg) used the geostationary satellite "Raduga" (1993). The standard INTELSAT antenna has a diameter of 30 m and a beam angle of 0.01 0 . Satellite channels use frequency ranges listed in table 7.6.

Table 7.6. Frequency bands used for satellite telecommunications

Range	Downlink [GHz]	Uplink (Uplink)[GHz]	Sources of interference
WITH	3,7-4,2	5,925-6,425	Ground interference
Ku	11,7-12,2	14,0-14,5	Rain
Ka	17,7-21,7	27,5-30,5	Rain

The transmission is always carried out at a higher frequency than the signal received from the satellite.

The range is not yet “populated” too densely; in addition, for this range the satellites can be 1 degree apart from each other. Sensitivity to rain interference can be circumvented by using two ground receiving stations sufficiently apart long distance(the size of hurricanes is limited). A satellite may have many antennas aimed at different regions of the earth's surface. The size of the “exposure” spot of such an antenna on the ground can be several hundred kilometers in size. A typical satellite has 12-20 transponders (receivers), each of which has a band of 36-50 MHz, which allows the formation of a data stream of 50 Mbit/s. Two transponders can use different signal polarizations while operating at the same frequency. Such throughput sufficient to receive 1600 high-quality telephone channels (32 kbit/s). Modern satellites use narrow aperture transmission technology VSAT(Very Small Aperture Terminals). The diameter of the “exposure” spot on the earth’s surface for these antennas is approximately 250 km. Ground terminals use antennas with a diameter of 1 meter and output power about 1 W. At the same time, the channel to the satellite has a throughput of 19.2 Kbit/s, and from the satellite - more than 512 Kbit/s. Such terminals cannot directly communicate with each other via a telecommunications satellite. To solve this problem, intermediate ground antennas with high gain are used, which significantly increases the delay (and increases the cost of the system), see Fig. 7.17.

Rice. 7.17.

Geostationary satellites hovering above the equator at an altitude of about 36,000 km are used to create permanent telecommunications channels.

Theoretically, three such satellites could provide communications to almost the entire inhabited surface of the Earth (see Fig. 7.18).

Rice. 7.18.

In reality, the geostationary orbit is overcrowded with satellites of various purposes and nationalities. Usually satellites are marked with the geographic longitude of the places over which they hang. At the current level of technology development, it is unwise to place satellites closer than 2 0 . Thus, today it is impossible to deploy more than 360/2=180 geostationary satellites.

A system of geostationary satellites looks like a necklace strung into an orbit invisible to the eye. One angular degree for such an orbit corresponds to ~600 km. This may seem like a huge distance. The density of satellites in orbit is uneven - there are many of them at the longitude of Europe and the USA, but few over the Pacific Ocean, they are simply not needed there. Satellites do not last forever, their lifetime usually does not exceed 10 years, they fail mainly not due to equipment failures, but due to a lack of fuel to stabilize their position in orbit. After failure, satellites remain in place, turning into space debris. There are already many such satellites, and over time there will be even more of them. Of course, we can assume that the accuracy of launching into orbit will become higher over time and people will learn to launch them with an accuracy of 100 m. This will make it possible to place 500-1000 satellites in one “niche” (which today seems almost incredible, because you need to leave space for them maneuvers). Humanity could thus create something similar to an artificial ring of Saturn, consisting entirely of dead telecommunications satellites. It is unlikely that things will come to this, since a way will be found to remove or restore inoperative satellites, although this will inevitably significantly increase the cost of the services of such communication systems.

Fortunately, satellites using different frequency bands do not compete with each other. For this reason, several satellites with different operating frequencies can be located in the same position in orbit. In practice, a geostationary satellite does not stand still, but moves along a trajectory that (when observed from the Earth) looks like a figure 8. The angular size of this figure eight must fit into the working aperture of the antenna, otherwise the antenna must have a servo drive that provides automatic tracking of the satellite . Due to energy problems, the telecommunications satellite cannot provide a high signal level. For this reason, the ground antenna must have a large diameter, and receiving equipment- low noise level. This is especially important for northern regions, where the angular position of the satellite above the horizon is low (a real problem for latitudes greater than 70 0), and the signal passes through a rather thick layer of the atmosphere and is noticeably attenuated. Satellite links can be cost-effective for areas more than 400-500 km apart (assuming no other means exist). Right choice satellite (its longitude) can significantly reduce the cost of the channel.

The number of positions for placing geostationary satellites is limited. Recently, it is planned to use so-called low-flying satellites for telecommunications ( <1000 км; период обращения ~1 час ). These satellites move in elliptical orbits, and each of them individually cannot guarantee a stationary channel, but together this system provides the entire range of services (each of the satellites operates in the “store and transmit” mode). Due to the low altitude, ground stations in this case may have small antennas and low cost.

There are several ways to operate a collection of ground terminals with a satellite. In this case it can be used multiplexing by frequency (FDM), by time (TDM), CDMA (Code Division Multiple Access), ALOHA or query method.

The request scheme assumes that ground stations form logical ring, along which the marker moves. The ground station can begin transmitting to the satellite only after receiving this marker.

Simple system ALOHA(developed by Norman Abramson's group at the University of Hawaii in the 70s) allows each station to begin transmitting whenever it wants. Such a scheme inevitably leads to collisions of attempts. This is partly due to the fact that the transmitting side learns about the collision only after ~270 ms. If the last bit of a packet from one station matches the first bit of another station, both packets will be lost and will have to be resent. After the collision, the station waits some pseudo-random time and re-attempts transmission again. This access algorithm ensures channel utilization efficiency of 18%, which is completely unacceptable for such expensive channels as satellite ones. For this reason, the domain version of the ALOHA system, which doubles the efficiency (proposed in 1972 by Roberts), is more often used. The time scale is divided into discrete intervals corresponding to the transmission time of one frame.

In this method, the machine cannot send a frame whenever it wants. One ground station (reference) periodically sends a special signal that is used by all participants for synchronization. If the length of the time domain is , then the domain number begins at the time instant relative to the signal mentioned above. Since the clocks of different stations operate differently, periodic resynchronization is necessary. Another problem is the spread of signal propagation time for different stations. The channel utilization factor for a given access algorithm turns out to be equal to (where is the base of the natural logarithm). Not a huge number, but still twice as high as the regular ALOHA algorithm.

Frequency multiplexing method (FDM) is the oldest and most commonly used. A typical 36 Mbps transponder can be used to receive 500 64 kbps PCM (Pulse Code Modulation) channels, each operating at a unique frequency. To eliminate interference, adjacent channels must be separated in frequency at a sufficient distance from each other. In addition, it is necessary to control the level of the transmitted signal, since if the output power is too high, interference interference may occur in the adjacent channel. If the number of stations is small and constant, frequency channels can be allocated permanently. But with a variable number of terminals or noticeable fluctuations in loading, you have to switch to dynamic resource allocation.

One of the mechanisms of such distribution is called SPADE, it was used in the first versions of INTELSAT-based communication systems. Each SPADE system transponder contains 794 simplex PCM channels of 64 kbit/s and one signal channel with a bandwidth of 128 kbit/s. PCM channels are used in pairs to provide full duplex communication. At the same time, the upstream and downstream channels have a bandwidth of 50 Mbit/s. The signal channel is divided into 50 domains of 1 ms (128 bits). Each domain belongs to one of the ground stations, the number of which does not exceed 50. When the station is ready to transmit, it randomly selects an unused channel and records the number of this channel in its next 128-bit domain. If two or more stations try to occupy the same channel, a collision will occur and they will be forced to try again later.

The time multiplexing method is similar to FDM and is quite widely used in practice. Synchronization for domains is also necessary here. This is done, as in the ALOHA domain system, using a reference station. Domain assignment to ground stations can be done centrally or decentralized. Consider the system ACTS(Advanced Communication Technology Satellite). The system has 4 independent channels (TDM) of 110 Mbit/s (two upstream and two downstream). Each of the channels is structured in the form of 1-ms frames, which have 1728 time domains. All temporary domains carry a 64-bit data field, which makes it possible to implement a voice channel with a 64 Kbps bandwidth. Managing time domains in order to minimize the time required to move the satellite's radiation vector requires knowledge of the geographic location of ground stations. Temporary domains are managed by one of the ground stations ( MCS- Master Control Station). The operation of the ACTS system is a three-step process. Each step takes 1 ms. In the first step, the satellite receives the frame and stores it in a 1728-cell buffer. On the second, the on-board computer copies each input record to the output buffer (possibly for a different antenna). Finally, the output recording is transmitted to the ground station.

At the initial moment, each ground station is assigned one time domain. To obtain an additional domain, for example, to organize another telephone channel, the station sends an MCS request. For these purposes, a special control channel with a capacity of 13 requests per second is allocated. There are also dynamic methods for resource allocation in TDM (Crouser, Binder and Roberts methods).

The CDMA (Code Division Multiple Access) method is completely decentralized. Like other methods, it is not without its drawbacks. First, CDMA channel capacity in the presence of noise and lack of coordination between stations is usually lower than in the case of TDM. Secondly, the system requires fast and expensive equipment.

Wireless network technology is developing quite quickly. These networks are primarily suitable for mobile devices. The most promising project seems to be IEEE 802.11, which should play the same integrating role for radio networks as 802.3 for Ethernet networks and 802.5 for Token Ring. The 802.11 protocol uses the same access and collision suppression algorithm as 802.3, but here it uses radio waves instead of a connecting cable (Fig. 7.19.). The modems used here can also operate in the infrared range, which can be attractive if all the machines are located in a common room.

Rice. 7.19.

The 802.11 standard assumes operation at a frequency of 2.4-2.4835 GHz using 4FSK/2FSK modulation

FEDERAL COMMUNICATIONS AGENCY

State educational budgetary institution

higher professional education

Moscow Technical University of Communications and Informatics

Department of Communication Networks and Switching Systems

Guidelines

and control tasks

by discipline

SWITCHING SYSTEMS

for 4th year part-time students

(direction 210700, profile - SS)

Moscow 2014

UMD plan for the 2014/2015 academic year.

Guidelines and controls

by discipline

SWITCHING SYSTEMS

Compiled by: Stepanova I.V., professor

The publication is stereotypical. Approved at a department meeting

Communication networks and switching systems

Reviewer Malikova E.E., associate professor

GENERAL GUIDELINES FOR THE COURSE

The discipline “Switching Systems”, part two, is studied in the second semester of the fourth year by students of the correspondence faculty of specialty 210406 ​​and is a continuation and further deepening of a similar discipline studied by students in the previous semester.

This part of the course discusses the principles of exchange of control information and interaction between switching systems, the basics of designing digital switching systems (DSS).

The course includes lectures, a course project and laboratory work. An exam is passed and a course project is defended. Independent work on mastering the course consists of studying the textbook material and teaching aids recommended in the guidelines, and completing the course project.

If a student encounters difficulties while studying the recommended literature, then you can contact the Department of Communication Networks and Switching Systems to obtain the necessary advice. To do this, the letter must indicate the title of the book, the year of publication and the pages where unclear material is presented. The course should be studied sequentially, topic by topic, as recommended in the guidelines. When studying this way, you should move on to the next section of the course after you answer all the control questions that are questions on the exam papers and solve the recommended problems.

The distribution of time in student hours for studying the discipline “Switching Systems”, part 2, is shown in Table 1.

BIBLIOGRAPHY

Main

1. Goldstein B.S. Switching systems. – SPb.:BHV – St. Petersburg, 2003. – 318 p.: ill.

2. Lagutin V. S., Popova A. G., Stepanova I. V. Digital channel switching systems in telecommunication networks. – M., 2008. - 214 p.

Additional

3.Lagutin V.S., Popova A.G., Stepanova I.V. Telephony user subsystem for signaling over a common channel. – M. “Radio and Communications”, 1998.–58 p.

4. Lagutin V.S., Popova A.G., Stepanova I.V. The evolution of intelligent services in converged networks. – M., 2008. – 120s.

LIST OF LABORATORY WORKS

1. Signaling 2ВСК and R 1.5, scenario of signal exchange between two automatic telephone exchanges.

2.Management of subscriber data on a digital PBX. Analysis of emergency messages of digital automatic telephone exchange.

METHODOLOGICAL INSTRUCTIONS FOR COURSE SECTIONS

Features of building digital circuit switching systems

It is necessary to study the features of constructing circuit switching systems using the example of a digital PBX of the EWSD type. Consider the characteristics and functions of digital subscriber access units DLU, the implementation of remote subscriber access. Review the characteristics and functions of the LTG line group. Study the construction of a switching field and the typical process of establishing a connection.

The digital switching system EWSD (Digital Electronic Switching System) was developed by Siemens as a universal circuit switching system for public telephone networks. The switching field capacity of the EWSD system is 25200 Erlang. The number of serviced calls in CHNN can reach 1 million calls. The EWSD system, when used as a PBX, allows you to connect up to 250 thousand subscriber lines. A communication center based on this system allows switching up to 60 thousand connecting lines. Containerized telephone exchanges allow connecting from several hundred to 6000 remote subscribers. Switching centers are produced for cellular communication networks and for organizing international communications. There is ample opportunity to organize second choice paths: up to seven direct choice paths plus one last choice path. Up to 127 tariff zones can be allocated. During one day, the tariff can change up to eight times. Generating equipment provides a high degree of stability of the generated frequency sequences:

in plesiochronous mode – 1 10 -9, in synchronous mode –1 10 -11.

The EWSD system is designed to use -60V or -48V power supplies. Temperature changes are allowed in the range of 5-40 ° C with a humidity of 10-80%.

EWSD hardware is divided into five main subsystems (see Fig. 1): digital subscriber unit (DLU); linear group (LTG); switching field (SN); common channel network control (CCNC); coordination processor (CP). Each subsystem has at least one microprocessor, designated GP. Signaling systems R1.5 (foreign version R2) are used, via common signaling channel No. 7 SS7 and EDSS1. Digital subscriber units DLU serve: analog subscriber lines; subscriber lines of users of digital networks with integration of services (ISDN); analogue institutional substations (PBX); digital PBX. DLU blocks provide the ability to switch on analog and digital telephone sets and multifunctional ISDN terminals. ISDN users are provided with channels (2B+D), where B = 64 kbit/s - standard channel of PCM30/32 equipment, D-channel signaling transmission at a speed of 16 kbit/s. To transmit information between EWSD and other switching systems, primary digital trunk lines (DSL, English PDC) are used - (30V + 1D + synchronization) at a transmission speed of 2048 kbit/s (or at a speed of 1544 kbit/s in the USA).

Fig.1. Block diagram of the EWSD switching system

Local or remote DLU operating mode can be used. Remote DLU units are installed in places where subscribers are concentrated. At the same time, the length of subscriber lines is reduced, and traffic on digital connecting lines is concentrated, which leads to a reduction in the costs of organizing a distribution network and improves the quality of transmission.

In relation to subscriber lines, a loop resistance of up to 2 kOhm and an insulation resistance of up to 20 kOhm are considered acceptable. The switching system can accept dialing pulses from a rotary dialer arriving at a speed of 5-22 pulses/sec. Frequency dialing signals are received in accordance with CCITT Recommendation REC.Q.23.

A high level of reliability is ensured by: connecting each DLU to two LTGs; duplication of all DLU units with load sharing; continuously performed self-monitoring tests. To transmit control information between DLUs and LTG line groups, common channel signaling (CCS) is used on time channel number 16.

The main elements of DLU are (Fig. 2):

subscriber line modules (SLM) of the SLMA type for connecting analog subscriber lines and the SLMD type for connecting ISDN subscriber lines;

two digital interfaces (DIUD) for connecting digital transmission systems (PDC) to line groups;

two control units (DLUC) that control internal DLU sequences, distributing or concentrating signal flows to and from subscriber sets. To ensure reliability and increase throughput, the DLU contains two DLUC controllers. They work independently of each other in a task-sharing mode. If the first DLUC fails, the second can take over control of all tasks;

two control networks for transmitting control information between subscriber line modules and control devices;

test unit (TU) for testing telephones, subscriber lines and trunk lines.

The characteristics of DLU change when moving from one modification to another. For example, the DLUB option provides for the use of analogue and digital subscriber kit modules with 16 kits in each module. A single DLUB subscriber unit can connect up to 880 analogue subscriber lines, and it connects to LTG using 60 PCM channels (4096 Kbps). In this case, losses due to a lack of channels should be practically zero. To meet this condition, the throughput of one DLUB should not exceed 100 Erl. If it turns out that the average load per module is more than 100 Erl, then the number of subscriber lines included in one DLUB should be reduced. Up to 6 DLUBs can be combined into a Remote Control Unit (RCU).

Table 1 presents the technical characteristics of the digital subscriber unit of a more modern modification of DLUG.

Table 1. Technical characteristics of the DLUG digital subscriber unit

Using separate lines, coin-operated payphones, analogue institutional-industrial automatic telephone exchanges РВХ (Private Automatic Branch Exchange) and digital РВХ of small and medium capacity can be connected.

We list some of the most important functions of the SLMA subscriber kit module for connecting analog subscriber lines:

line monitoring to detect new calls;

DC power supply with adjustable current values;

analog-to-digital and digital-to-analog converters;

symmetrical connection of ringing signals;

monitoring of loop short circuits and short circuits to ground;

receiving pulses for ten-day dialing and frequency dialing;

changing the polarity of the power supply (reversing the polarity of wires for payphones);

connection of the linear side and the subscriber set side to the multi-position test switch, overvoltage protection;

DC decoupling of speech signals;

converting a two-wire communication line into a four-wire line.

Function blocks equipped with their own microprocessors are accessed via the DLU control network. Blocks are polled cyclically for readiness to transmit messages, and they are directly accessed for transmitting commands and data. DLUC also carries out testing and monitoring programs to identify errors.

The following DLU bus systems exist: control buses; buses 4096 kbit/s; collision detection tires; buses for transmitting ringing signals and tariff impulses. Signals transmitted along the buses are synchronized by clock pulses. The control buses transmit control information at a transmission rate of 187.5 kbit/s; with an effective data rate of approximately 136 kbit/s.

4096 kbit/s buses transmit speech/data to and from SLM subscriber line modules. Each bus has 64 channels in both directions.

Each channel operates at a transmission rate of 64 kbit/s (64 x 64 kbit/s = 4096 kbit/s). The assignment of 4096 kbit/s bus channels to PDC channels is fixed and determined through the DIUD (see Fig. 3). DLU connection to line groups of type B, F or G (types LTGB, LTGF or LTGG, respectively) is carried out via 2048 kbit/s multiplex lines. The DLU can connect to two LTGBs, two LTGFs (B), or two LTGGs.

Line/Trunk Groupe (LTG) forms the interface between the digital environment of the node and the digital switching field SN (Fig. 4). LTGs perform decentralized control functions and relieve the CP coordination processor from routine work. Connections between the LTG and the redundant switching field are made via a secondary digital link (SDC). The SDC transmission speed from the LTG to the SN field and in the reverse direction is 8192 kbit/s (abbreviated as 8 Mbit/s).

Fig.3. Multiplexing, demultiplexing and

transfer of control information to DLUC

Fig.4. Various options for accessing LTG

Each of these 8 Mbit/s multiplex systems has 127 time slots at 64 kbit/s each to carry payload information, and one time slot at 64 kbit/s is used for message transmission. The LTG transmits and receives voice information through both sides of the switching field (SN0 and SN1), assigning voice information from the active block of the switching field to the corresponding subscriber. The other side of the SN field is considered inactive. If a failure occurs, the transmission and reception of user information immediately begins through it. The LTG power supply voltage is +5V.

LTG implements the following call processing functions:

reception and interpretation of signals arriving through connecting and
subscriber lines;

transmission of signaling information;

transmission of acoustic tones;

transmission and reception of messages to/from the coordination processor (CP);

transmitting reports to group processors (GP) and receiving reports from
group processors of other LTGs (see Fig. 1);

transmission and reception of requests to/from the signaling network controller over a common channel (CCNC);

control of alarms entering the DLU;

coordination of states on lines with states of a standard 8 Mbit/s interface with a duplicated switching field SN;

establishing connections to transmit user information.

Several types of LTG are used to implement different line types and signaling methods. They differ in the implementation of hardware blocks and specific application programs in the group processor (CP). LTG blocks have a large number of modifications, differing in use and capabilities. For example, the LTG block of function B is used to connect: up to 4 primary digital communication lines of the PCM30 type (PCM30/32) with transmission rates of 2048 kbit/s; up to 2 digital communication lines with a transfer rate of 4096 kbit/s for local DLU access.

The LTG function C block is used to connect up to 4 primary digital communication lines with speeds of 2048 kbit/s.

Depending on the purpose of the LTG (B or C), there are differences in the functional design of the LTG, for example, in the group processor software. The exception is modern LTGN modules, which are universal, and in order to change their functional purpose, it is necessary to “recreate” them programmatically with a different load (see Table 2 and Fig. 4).

Table 2. Line Group N (LTGN) Specifications

As shown in Fig. 5, in addition to the standard 2 Mbit/s interfaces (RSMZ0), the EWSD system provides an external system interface with a higher transmission rate (155 Mbit/s) with STM-1 type multiplexers of the SDH synchronous digital hierarchy network on fiber optic lines communications. An N-type termination multiplexer (synchronous dual termination multiplexer, SMT1D-N) installed on the LTGM cabinet is used.

The SMT1D-N multiplexer can be presented in the form of a basic configuration with 1xSTM1 interface (60xРSMЗ0) or in the form of a full configuration with 2xSTM1 interfaces (120хРSMЗ0).

Fig.5. Connecting SMT1 D-N to the network

Switching field SN EWSD switching systems connect the LTG, CP and CCNC subsystems to each other. Its main task is to establish connections between LTG groups. Each connection is simultaneously established through both halves (planes) of the switching field SN0 and SN1, so that if one side of the field fails, there is always a backup connection. In EWSD type switching systems, two types of switching field can be used: SN and SN(B). The switching field type SN(B) is a new development and is characterized by smaller dimensions, higher availability, and reduced power consumption. There are various options for organizing SN and SN(B):

switching field for 504 line groups (SN:504 LTG);

switching field for 1260 line groups (SN: 1260 LTG);

switching field for 252 line groups (SN:252 LTG);

switching field for 63 line groups (SN:63 LTG).

The main functions of the switching field are:

circuit switching; message switching; switching to reserve.

The switching field switches channels and connections at a transmission rate of 64 kbit/s (see Fig. 6). Each connection requires two connecting paths (for example, caller to callee and callee to caller). The coordination processor searches for free paths through the switching field based on information about the occupancy of connecting paths currently stored in the storage device. Switching of connecting paths is carried out by control devices of the switching group.

Each switch field has its own control unit, consisting of a switch group control unit (SGC) and an interface module between the SGCs and a message buffer unit MBU:SGC. With a minimum stage capacity of 63 LTG, one SGC of the switch group is involved in the switching of the connecting path, however, with stage capacities of 504, 252 or 126 LTG, two or three SGCs are used. This depends on whether the subscribers are connected to the same TS group or not. Commands for establishing a connection are issued to each participating GP of the switching group by the CP processor.

In addition to connections specified by subscribers by dialing a number, the switching field switches connections between line groups and the CP coordination processor. These connections are used to exchange control information and are called semi-permanent dial-up connections. Thanks to these connections, messages are exchanged between line groups without consuming resources of the coordination processor unit. Nailed-up connections and connections for signaling over a common channel are also established on the principle of semi-permanent connections.

The switching field in the EWSD system is characterized by complete accessibility. This means that every 8-bit codeword transmitted on a backbone entering the switching field can be transmitted at any other time slot on a backbone emanating from the switching field. All highways with a transmission speed of 8192 kbit/s have 128 channels with a transmission capacity of 64 kbit/s each (128x64 = 8192 kbit/s). Switching field stages with capacities SN:504 LTG, SN:252 LTG, SN:126 LTG have the following structure:

one time switching stage incoming (TSI);

three stages of spatial switching (SSM);

one time switching stage outgoing (TSO).

The small and medium stations (SN:63LTG) include:

one time switching input (TSI) stage;

one spatial switching (SS) stage;

one outgoing time switching stage (TSO).

Fig.6. Example of connection establishment in the switching field SN

Coordination processor 113 (CP113 or CP113C) is a multiprocessor, the capacity of which increases in stages. In the CP113C multiprocessor, two or more identical processors operate in parallel with load sharing. The main functional blocks of the multiprocessor are: the main processor (MAP) for call processing, operation and maintenance; a call processing processor (CAP), designed to process calls; shared storage (CMY); input/output controller (IOC); input/output processor (IOP). Each VAP, CAP and IOP processor contains one program execution unit (PEX). Depending on whether they are to be implemented as VAP processors, CAP processors or I0C controllers, specific hardware functions are activated.

Let us list the main technical data of VAR, CAP and IOC. Processor type - MC68040, clock frequency -25 MHz, address width 32 bits and data width 32 bits, word width - 32 data bits. Local memory data: expansion - maximum 64 MB (based on 16M bit DRAM); expansion stage 16 MB. Flash EPROM data: 4 MB expansion. The CP coordination processor performs the following functions: call processing (analysis of number digits, routing control, service area selection, path selection in the switching field, call cost accounting, traffic data management, network management); operation and maintenance - input to and output from external storage devices (EM), communication with the operation and maintenance terminal (OMT), communication with the data transfer processor (DCP). 13

The SYP panel (see Fig. 1) displays external alarms, for example, information about a fire. External memory EM is used to store programs and data that do not need to be permanently stored in the CP, the entire system of application programs for automatic recovery of data on the tariffing of telephone calls and traffic changes.

The software is focused on performing specific tasks corresponding to the EWSD subsystems. The operating system (OS) consists of programs that are close to the hardware and are usually the same for all switching systems.

The maximum call processing capacity of the SR is over 2,700,000 calls per peak hour. Characteristics of the CP system EWSD: storage capacity - up to 64 MB; addressing capacity - up to 4 GB; magnetic tape - up to 4 devices, 80 MB each; magnetic disk - up to 4 devices, 337 MB each.

The job of the Message Buffer (MB) is to control the exchange of messages:

between coordination processor CP113, and LTG groups;

between CP113 and switching group controllers SGCB) switching field;

between LTG groups;

between LTGs and the signaling network controller via a common CCNC channel.

The following types of information can be transmitted via MV:

messages are sent from DLU, LTG and SN to the coordination processor CP113;

reports are sent from one LTG to another (reports are routed through CP113, but are not processed by it);

instructions are sent from CCNC to LTG and from LTG to CCNC, they are routed through the CP113, but are not processed by it;

commands are sent from CP113 to LTG and SN. The MV converts the information for transmission via the secondary digital stream (SDC) and sends it to the LTG and SGC.

Depending on the capacity stage, a duplicate MB device can contain up to four message buffer groups (MBGs). This feature is implemented in a network node with redundancy, that is, MB0 includes groups MBG00...MBG03, and MB1 includes groups MBG10...MBG13.

EWSD switching systems with signaling over a common channel on system No. 7 are equipped control device of the signaling network via a common CCNC channel. Up to 254 signaling links can be connected to the CCNC device via analogue or digital communication lines.

The CCNC device is connected to the switching field via compressed lines with a transmission speed of 8 Mbit/s. Between the CCNC and each switching field plane, there are 254 channels for each transmission direction (254 channel pairs).

The channels carry signaling data across both SN planes to and from line groups at 64 kbit/s. Analog signal paths are connected to the CCNC via modems. The CCNC consists of: a maximum of 32 groups with 8 signal path terminals each (32 SILT groups); one redundant common channel processor (CCNP).

Control questions

1.In which block is analog-to-digital conversion performed?

2. How many analogue subscriber lines can be included in DLUB? What capacity is this block designed for?

3. At what speed is information transmitted between DLU and LTG, between LTG and SN?

4. List the main functions of the switching field. At what speed is the connection between subscribers implemented.

5. List the options for organizing the switching field of the EWSD system.

6. List the main stages of switching with the switching field.

7.Consider the passage of the conversation path through the switching field of the EWSD switching system.

8. What call processing functions are implemented in LTG blocks?

9. What functions does the MV side implement?

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Circuit-switched networks have several important common properties, regardless of the type of multiplexing they use.

Networks with dynamic switching require a preliminary procedure for establishing a connection between subscribers. To do this, the address of the called subscriber is transmitted to the network, which passes through the switches and configures them for subsequent data transmission. The connection request is routed from one switch to another and eventually reaches the called party. The network may refuse to establish a connection if the capacity of the required output channel is already exhausted. For an FDM switch, the capacity of the output channel is equal to the number of frequency bands of this channel, and for a TDM switch - the number of time slots into which the channel's operating cycle is divided. The network also refuses the connection if the requested subscriber has already established a connection with someone else. In the first case they say that the switch is busy, and in the second - the subscriber. The possibility of connection failure is a disadvantage of the circuit switching method.

If the connection can be established, then it is allocated a fixed frequency band in FDM networks or a fixed bandwidth in TDM networks. These values ​​remain unchanged throughout the connection period. Guaranteed network throughput once a connection is established is an important property required for applications such as voice, video or real-time facility control. However, circuit-switched networks cannot dynamically change the channel capacity at the request of a subscriber, which makes them ineffective in conditions of bursty traffic.

The disadvantage of circuit-switched networks is the inability to use user equipment operating at different speeds. The individual parts of a composite circuit operate at the same speed because circuit-switched networks do not buffer user data.

Circuit-switched networks are well suited for switching constant-rate data streams, where the unit of switching is not a single byte or data packet, but a long-term synchronous data stream between two subscribers. For such flows, circuit-switched networks add a minimum of overhead to route data through the network, using the time position of each bit of the flow as its destination address in the network switches.

Providing duplex operation based on FDM, TDM and WDM technologies

Depending on the direction of possible data transmission, methods of data transmission over a communication line are divided into the following types:

o simplex - transmission is carried out over the communication line in only one direction;

o half-duplex - transmission is carried out in both directions, but alternately in time. An example of such transmission is Ethernet technology;

o duplex - transmission is carried out simultaneously in two directions.

Duplex mode is the most versatile and productive way of channel operation. The simplest option for organizing a duplex mode is to use two independent physical channels (two pairs of conductors or two optical fibers) in a cable, each of which operates in simplex mode, that is, transmits data in one direction. It is this idea that underlies the implementation of the duplex operating mode in many network technologies, such as Fast Ethernet or ATM.

Sometimes such a simple solution is not available or effective. Most often, this happens in cases where there is only one physical channel for duplex data exchange, and organizing a second one is associated with high costs. For example, when exchanging data using modems through the telephone network, the user has only one physical communication channel with the PBX - a two-wire line, and it is hardly advisable to purchase a second one. In such cases, the duplex operating mode is organized on the basis of dividing the channel into two logical subchannels using FDM or TDM technology.

Modems use FDM technology to organize duplex operation on a two-wire line. Frequency modulation modems operate at four frequencies: two frequencies for encoding ones and zeros in one direction, and the remaining two frequencies for transmitting data in the opposite direction.

With digital coding, duplex mode on a two-wire line is organized using TDM technology. Some time slots are used to transmit data in one direction, and some are used to transmit data in the other direction. Typically, time slots in opposite directions alternate, which is why this method is sometimes called “ping-pong” transmission. TDM line division is typical, for example, for integrated services digital networks (ISDN) at subscriber two-wire ends.

In fiber optic cables, when one optical fiber is used to organize a duplex mode of operation, data is transmitted in one direction using a light beam of one wavelength, and in the opposite direction using a different wavelength. This technique belongs to the FDM method, but for optical cables it is called wavelength division multiplexing (WDM). WDM is also used to increase the speed of data transmission in one direction, usually using from 2 to 16 channels.

Packet switching

Packet Switching Principles

Packet switching is a subscriber switching technique that was specifically designed for the efficient transmission of computer traffic. Experiments to create the first computer networks based on circuit switching technology showed that this type of switching does not allow achieving high overall network throughput. The crux of the problem lies in the bursty nature of traffic that typical network applications generate. For example, when accessing a remote file server, the user first views the contents of that server's directory, which results in the transfer of a small amount of data. He then opens the desired file in a text editor, an operation that can create quite a lot of data exchange, especially if the file contains large graphics. After displaying a few pages of a file, the user works with them locally for a while, which requires no network transfer at all, and then returns modified copies of the pages to the server - again creating intensive network transfer.

The traffic ripple factor of an individual network user, equal to the ratio of the average intensity of data exchange to the maximum possible, can be 1:50 or 1:100. If for the described session we organize channel switching between the user’s computer and the server, then most of the time the channel will be idle. At the same time, the switching capabilities of the network will be used - part of the time slots or frequency bands of the switches will be occupied and unavailable to other network users.

When packet switching occurs, all messages transmitted by a network user are broken up at the source node into relatively small parts called packets. Let us recall that a message is a logically completed piece of data - a request to transfer a file, a response to this request containing the entire file, etc. Messages can have an arbitrary length, from several bytes to many megabytes. On the contrary, packets can usually also have a variable length, but within narrow limits, for example from 46 to 1500 bytes. Each packet is provided with a header that specifies the address information needed to deliver the packet to the destination node, as well as the packet number that will be used by the destination node to assemble the message (Figure 2.29). Packets are transported in the network as independent information blocks. Network switches receive packets from end nodes and, based on address information, transmit them to each other, and ultimately to the destination node.

Rice. 2.29. Splitting a message into packets

Packet network switches differ from circuit switches in that they have internal buffer memory for temporary storage of packets if the output port of the switch is busy transmitting another packet at the time the packet is received (Fig. 2.30). In this case, the packet remains for some time in the packet queue in the buffer memory of the output port, and when its turn reaches it, it is transferred to the next switch. This data transmission scheme allows you to smooth out traffic ripples on the backbone links between switches and thereby use them in the most effective way to increase the throughput of the network as a whole.

Rice. 2.30. Smoothing Burst Traffic in a Packet Switched Network

Indeed, for a pair of subscribers, the most effective would be to provide them with sole use of a switched communication channel, as is done in circuit-switched networks. With this method, the interaction time of this pair of subscribers would be minimal, since data would be transmitted from one subscriber to another without delay. Subscribers are not interested in channel downtime during transmission pauses; it is important for them to quickly solve their own problem. A packet-switched network slows down the process of interaction between a particular pair of subscribers, since their packets can wait in the switches while other packets that arrived at the switch earlier are transmitted along the backbone links.

However, the total amount of computer data transmitted by the network per unit time using the packet switching technique will be higher than using the circuit switching technique. This happens because the pulsations of individual subscribers, in accordance with the law of large numbers, are distributed over time. Therefore, switches are constantly and fairly evenly loaded with work if the number of subscribers they serve is really large. In Fig. Figure 2.30 shows that the traffic coming from end nodes to switches is very unevenly distributed over time. However, higher-level switches in the hierarchy that service connections between lower-level switches are more evenly loaded, and packet flow on the trunk links connecting upper-level switches is at near-maximum utilization.

The higher efficiency of packet-switched networks compared to circuit-switched networks (with equal communication channel capacity) was proven in the 60s both experimentally and using simulation modeling. An analogy with multiprogram operating systems is appropriate here. Each individual program in such a system takes longer to execute than in a single-program system, where the program is allocated all the processor time until it completes its execution. However, the total number of programs executed per unit of time is greater in a multi-program system than in a single-program system.

Wide Area Communications Based on Circuit Switched Networks

Leased lines represent the most reliable means of connecting local networks through global communication channels, since the entire capacity of such a line is always at the disposal of interacting networks. However, this is also the most expensive type of global connections - if there are N remote local networks that intensively exchange data with each other, you need to have Nx(N-l)/2 leased lines. To reduce the cost of global transport, dynamically switched channels are used, the cost of which is divided among many subscribers of these channels.

Telephone network services are the cheapest, since their switches are paid for by a large number of subscribers using telephone services, and not just by subscribers who combine their local networks.

Telephone networks are divided into analog and digital depending on the method of multiplexing subscriber and trunk channels. More precisely, digital are networks in which information is presented at subscriber ends in digital form and in which digital multiplexing and switching methods are used, and analog are networks that receive data from subscribers in analog form, that is, from classic analog telephones, and Multiplexing and switching are carried out using both analogue and digital methods. In recent years, there has been a fairly intensive process of replacing telephone network switches with digital switches that operate on the basis of TDM technology. However, such a network will still remain an analog telephone network, even if all switches operate using TDM technology, processing data in digital form, if its subscriber ends remain analog, and analog-to-digital conversion is performed on the PBX network closest to the subscriber. The new V.90 modem technology was able to take advantage of the fact that there are a large number of networks in which the majority of switches are digital.

Telephone networks with digital subscriber terminations include the so-called Switched 56 services (56 Kbit/s switched channels) and digital networks with integrated ISDN services (Intergrated Services Digital Network). Switched 56 services have appeared in a number of Western countries as a result of providing end subscribers with digital termination compatible with T1 line standards. This technology has not become an international standard, and today it is replaced by ISDN technology, which has such a status.

ISDN networks are designed not only to transmit voice, but also computer data, including through packet switching, due to which they are called networks with integrated services. However, the main mode of operation of ISDN networks remains circuit switching, and the packet switching service has a speed that is too low by modern standards - usually up to 9600 bps. Therefore, ISDN technology will be discussed in this section on circuit-switched networks. The new generation of integrated services networks, called B-ISDN (from broadband), is based entirely on packet switching technology (more precisely, ATM technology cells), so this technology will be discussed in the section on packet switching networks.