General properties of channel switches. By type of network topology

The limit distances for the radio channels are given by suppliers under the assumption that within the first zone of Fresnel any physical interference. The absolute restriction on the range of radio-relay channels imposes the curvature of the Earth, see Fig. 7.15. For frequencies above 100 MHz, the waves apply straightforwardly (Fig. 7.15.A) and, therefore, can focus. For high frequencies (RF) and UHF, the Earth absorbs the waves, but for HF, it is characterized by reflection from the ionosphere (Fig. 7.15b) - this strongly expands the broadcast zone (sometimes several consecutive reflections are carried out), but this effect is unstable and strongly depends on the state of the ionosphere.

Fig. 7.15.

When building long radio relay channels, you have to install repeaters. If the antennas are placed on the towers with a height of 100 m of distances between repeaters can be 80-100 km. The cost of an antenna complex is usually proportional to Cuba diameter antenna.

The radiation diagram of the directional antenna is shown in Fig. 7.16 (the arrow marked the main direction of radiation). This diagram should be considered when choosing an antenna installation site, especially when using high radiation power. Otherwise, one of the petals of radiation can come to the places of permanent residence of people (for example, housing). Given these circumstances, the design of this kind of channels is advisable to instruct professionals.

Fig. 7.16.

On October 4, 1957, the first artificial satellite of the Earth was launched in the USSR, in 1961, in 1961, Yu. A. Gagarin flew into space, and soon the first telecommunications satellite "Lightning" was launched in orbit - the space era of communications began. The first satellite channel for the Internet (Moscow-Hamburg) used the geostationary satellite "Raduga" (1993). The standard Intelsat antenna has a diameter of 30 m and an angle of radiation 0.01 0. Satellite channels use the frequency bands listed in Table 7.6.

Table 7.6. Frequency ranges used for satellite telecommunications

Range	DOWNLINK [GHz]	Ascending Channel (Uplink) [GHz]	Sources of noise
FROM	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

Transmission is always conducted at a higher frequency than the reception of the signal from the satellite.

The range is still "populated" is not too tight, in addition, for this range, satellites can stand apart from each other by 1 degree. Sensitivity to interference from rain can be uploaded to the use of two ground receiving stations separated by a sufficiently large distance (the size of the hurricanes is limited). The satellite can have a lot of antennas aimed at different regions of the earth's surface. The size of the illumination spots of such an antenna on the ground may have a size of several hundred kilometers. An ordinary satellite possesses 12-20 transponders (transceivers), each of which has a strip of 36-50 MHz, which allows you to form a data stream of 50 Mbps. Two transponders can use different signal polarization by operating at the same frequency. Such. bandwidth It is sufficient to obtain 1600 high-quality telephone channels (32QBIT / C). Modern satellites use narrow-perpetrate transfer technology Vsat. (Very Small Aperture Terminals). The diameter of the spots of the "illumination" on the earth's surface for these antennas is approximately 250 km. Ground terminals use antennas with diameter 1 meter and output power of about 1 W. In this case, the channel to the satellite has throughput 19.2 Kbps, and from a satellite - more than 512 kbps. Immediately such terminals cannot work with each other through the telecommunications satellite. To solve this problem, intermediate terrestrial antennas are used with greater gain, which significantly increases the delay (and increases the cost of the system), see Fig. 7.17.

Fig. 7.17.

To create permanent channels of telecommunications, geostationary satellites are served on the equator at an altitude of about 36,000 km.

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

Fig. 7.18.

Really, the geostationary orbit is filled with satellites for various purposes and nationality. Usually satellites are marked with geographic longitude of the places over which they hang. With the existing level of technology development, it is unwise to place satellites closer than 2 0. Thus, today it is impossible to place more than 360/2 \u003d 180 geostationary satellites.

The geostationary satellite system looks like a necklace striking an invisible orbit. One angular degree for such an orbit corresponds to ~ 600 km. It may seem that this is a huge distance. The density of satellites in an orbite is uneven - there are many of them on the longitude of Europe and the USA, and there are few above the quiet ocean, there are simply not needed there. Satellites are not eternal, the time of their lives is usually no exceeding 10 years, they fail the mainly due to equipment failures, and due to the lack of fuel to stabilize their position in orbit. After the failure of satellites remain in their places, turning into space trash. There are already many such satellites now, over time they will become even more. Of course, it can be assumed that the accuracy of the orbit's output will eventually become higher and people will learn to withdraw them with an accuracy of 100 m. This will allow you to post 500-1000 satellites in one "niche" (which today seems almost incredible, because you need to leave space for them maneuvers). Humanity can thus create something similar to the Saturn artificial ring, consisting entirely of dead telecommunications satellites. Prior to this, it is unlikely to reach, as a way to remove or restore non-working satellites will be found, although inevitability will significantly detold the services of such communication systems.

Fortunately, satellites using different frequency bands do not compete with each other. For this reason, in the same position in orbit, there may be several satellites with different operating frequencies. In practice, the geostationary satellite does not stand still, but performs the movement along the trajectory (when observing the Earth), the view of the figure 8. The angular size of this eight must be laid into the antenna working aperture, otherwise the antenna must have a servo that provides automatic tracking of the satellite . Due to energy problems, the telecommunications satellite cannot provide a high level of signal. For this reason, the ground antenna must have a large diameter, and the receiving equipment is a low noise level. This is especially important for the northern regions in which the angular position of the satellite above the horizon is low (the present problem is more than 70 0), and the signal passes a rather thick layer of the atmosphere and is noticeably weakened. Satellite channels can be profitable for areas that are located from more than 400-500 km (provided that other means does not exist). Right choice The satellite (its longitude) may noticeably reduce the cost of the channel.

The number of positions for the placement of geostationary satellites is limited. Recently, for telecommunications it is planned to use so-called low-fat satellites ( <1000 км; период обращения ~1 час ). These satellites are moving through elliptical orbits, and each of them cannot guarantee a stationary channel separately, but in the aggregate this system provides the entire range of services (each of the satellites works in "Remember and transfer" mode). Due to the low height of the flight, ground stations in this case may have small antennas and low cost.

There are several ways to work the totality of ground terminals with a satellite. It can be used multiplexing By frequency (FDM), time (TDM), CDMA (Code Division Multiple Access), Aloha or query method.

Query scheme assumes that ground stations form logical ring, along which the marker moves. The ground station can start the transmission to the satellite, only receiving this marker.

Simple system Aloha. (Developed by a group of Norman Abramson from Hawaiian University in the 70s) allows each station to start transmission when it wants it. Such a scheme with inevitability leads to collisions attempts. This is a partly due to the fact that the transmitting party learns about a collision only after ~ 270 ms. The last bit of the package of one station coincides with the first bit of another station, will be lost both packages and they will have to be sent again. After a collision, the station expects some pseudo-random time and makes a re-attempt to transfer again. Such an access algorithm ensures the efficiency of using a channel at 18%, which is completely unacceptable for such expensive channels as satellite. For this reason, the domain version of the ALOHA system is more often used, which doubles efficiency (proposed in 1972 by Roberts). The timeline is divided into discrete intervals corresponding to the transmission time of one frame.

In this method, the machine cannot send a frame when he wants. One ground station (reference) periodically sends a special signal that is used by all participants for synchronization. If the length of the temporary domain is equal, then the domain with the number begins at the time of time relative to the above signal. Since the clock of different stations operates on -ny, periodic resynimization is needed. Another problem is the scatter of the signal propagation time for different stations. The channel utilization factor for this access algorithm turns out to be equal to (where the basis of the natural logarithm). Not too big digit, but still two times higher than for the conventional Aloha algorithm.

Multiplexing method (FDM.) The oldest and most commonly used. A typical transponder with a strip of 36 Mbps can be applied to obtain 500 64kbbit / with IRM channels (pulse-code modulation), each of which works with its own unique frequency. To exclude interference, adjacent channels must be in frequency at a sufficient distance from each other. In addition, it is necessary to control the level of the transmitted signal, since with too large output power, interference interference may occur in the adjacent channel. If the number of stations is small and constantly, frequency channels can be distributed stationary. But with a variable number of terminals or with a noticeable fluctuation, the download has to go to the dynamic distribution of resources.

One of the mechanisms of such distribution is called SPADE.It was used in the first versions of the communication system based on Intelsat. Each SPADE transponder contains 794 simplex ICM channels of 64-kbps and one signal channel with a 128 Kbps band. IRM channels are used in pairs to ensure full-duplex communication. At the same time, the ascending and downstream channels have a strip of 50 Mbps. 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 for transmission, it randomly selects the unused channel and records the number of this channel in the next 128-bit domain. If the same channel tries to take two or more stations, there is a collision, and they will be forced to repeat the attempt later.

The time multiplexing method is similar to FDM and is quite widely used in practice. It also requires synchronization for domains. This is done, as in the domain system ALOHA, using the reference station. Assignment of domains by land stations can be met centrally or decentralized. Consider the system ACTS. ADVANCED COMMUNICATION TECHNOLOGY SATELLITE). The system has 4 independent channels (TDM) 110 Mbps (two ascending and two descending). Each of the channels is structured in the form of 1-Milicecond frames that have 1728 temporary domains. All temporary domains carry a 64-bit data field, which allows you to implement a voice channel with a 64 Kbps band. Management of temporary domains in order to minimize time on the movement of the vector of the satellite radiation involves the knowledge of the geographical position of ground stations. Management of temporary domains is carried out by one of the ground stations ( MCS. - Master Control Station). The operation of the ACTS system is a three-step process. Each of the steps takes 1 ms. In the first step, the satellite receives a frame and remembers it in a 1728-year-old buffer. On the second - the onboard computer copies each input entry in the output buffer (maybe for another antenna). And finally, the output record is transmitted to the ground station.

At the initial moment of each ground station is put into line with one time domain. To obtain an additional domain, for example, to organize another telephone channel, the station sends the MCS request. For these purposes, a special control channel of 13 queries in secrets is allocated. There are also dynamic methods for distributing resources in TDM (Crowzer methods, Binder [Binder] and Roberts).

CDMA method (Code Division Multiple Access) is fully decentralized. Like other methods, it is not devoid of flaws. First, the CDMA channel capacity in the presence of noise and the absence of coordination between stations is usually lower than in the case of TDM. Secondly, the system requires high-speed and expensive equipment.

Wireless network technology develops quite quickly. These networks are convenient primarily for mobile means. The most promising is the IEEE 802.11 project, which should play for radio networks as an integrating role as 802.3 for Ethernet and 802.5 networks for Token Ring. In the 802.11 protocol, the same access and collision suppression algorithm is used, as in 802.3, but here instead of a connecting cable, radio waves are used (Fig. 7.19.). The modems used here can work in the infrared range, which is attractive if all machines are accommodated in a common room.

Fig. 7.19.

Standard 802.11 involves work at a frequency of 2.4-2.4835 GHz when using 4FSK / 2FSK modulation

Classification of networks.

By territorial prevalence

Pan (Personal Area Network) is a personal network intended for the interaction of various devices belonging to one owner.

LAN (Local Area Network) is local networks that have a closed infrastructure before servicing service providers. The term "LAN" can also describe a small office network, and a network of the level of a large plant occupying several hundred hectares. Foreign sources give even a close estimate - about six miles (10 km) in a radius. Local networks are closed networks, access to them is allowed only by a limited circle of users, for which work in such a network is directly related to their professional activities.

CAN (Campus Area Network is a campus network) - combines local networks of closely located buildings.

MAN (Metropolitan Area Network) - city networks between institutions within one or more cities connecting many local computing networks.

WAN (Wide Area Network) is a global network covering large geographic regions, including both local networks and other telecommunication networks and devices. Example WAN - Package Switching Networks (Frame Relay), through which various computer networks can "talk". Global networks are open and focused on servicing any users.

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

By type of functional interaction

Client Server, Mixed Network, Peel Network, Multigl Networks

By type of network topology

Tire, ring, double ring, star, cellular, grid, tree, fat tree

By type of transmission medium

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

Wireless (transmission of radio wave information in a certain frequency range)

By functional purpose

Data storage networks, server farms, process control network, SOHO network, home network

By speed gear

low-speed (up to 10 Mbps), medium-speed (up to 100 Mbps), high-speed (over 100 Mbps);

By the need to maintain a permanent connection

Batch network, such as Fidonet and UUCP, online network, such as Internet and GSM

Channel Switching Networks

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

1. Route distribution mechanism during data transmission

2. Synchronous communication channel

We will talk about one of the ways to solve the switching task, namely, about network switching networks. But it should be noted that this is not the only way to solve the challenge in computer networks. But we turn closer to the essence of the question. Channel Switching Networks They form a common and in-extended physical section (channel) of communication through which data with the same speed pass between the final nodes. It should be noted that the same speed is achieved due to the lack of a "stop" in some sections, as the route is known in advance.

Installing Communication B. channel Switching Networks It always begins first, because it is impossible to pave the route to the desired goal without connecting. And after installing the connection, you can safely transmit the necessary data. Let's take a look at the benefits of channel switches:

1. Speed \u200b\u200bduring data transmission is always the same

2. No delay on nodes when transferring data, which is important when different on-line events (conference, communication, video broadcast)

Well, now I have to say a few words about the flaws:

1. You can not always establish a connection, i.e. Sometimes the network can be busy

2. We cannot immediately transfer data without prior set of communication, i.e. Time is lost

3. Not very effective use of physical communication channels

I will explain about the last minus: when creating a physical communication channel, we fully occupy all the line, without leaving the opportunity to connect to it.

In turn, the network switched channels are divided into 2 types using different technological approach:

1. Switching channels based on frequency multiplexing (FDM)

The scheme of work is as follows:

1. At the switch inputs each user transmits a signal

2. All signals with the switch are filled with Δf bands by the frequency modulation of the signal

2. Commuting channels based on temporary multiplexing (TDM)

Principle switching channels On the basis of temporarily multiplexing is quite simple. It is based on temporary separation, i.e. Alternately, the maintenance of each of the communication channels is occurring, and the period of time, to send the signal to the subscriber, is strictly defined.

3. Communication packages
This switching technique was specifically designed to effectively transmit computer traffic. The first steps on the way of creating computer networks based on the channel switching techniques showed that this type of switching does not allow to achieve a high total network bandwidth. Typical network applications generate traffic very uneven, with a high level of pulsation rate of data transmission. For example, when accessing a remote file server, the user first browsing the contents of the directory of this server, which generates the transfer of a small amount of data. Then it opens the required file in a text editor, and this operation can create a fairly intensive data exchange, especially if the file contains bulk graphic inclusions. After displaying multiple page files, the user works for some time with them locally, which does not require data on the network at all, and then returns modified copies of pages to the server - and this again generates intensive data transmission over the network.

The ripple rate of the individual network of the network, equal to the ratio of the average data exchange intensity to the maximum possible, can reach 1:50 or even 1: 100. If for the described session to organize the channel switch between the user and the server and the server, then most of the time the channel will be idle. At the same time, the network switching capabilities will be assigned to this pair of subscribers and will be unavailable to other network users.

When switching packets, all user transmitted messages are broken in the source node for relatively small parts, called packages. Recall that the message is called a logically completed data portion - a request to transfer a file, the answer to this request containing the entire file, etc. Messages may have arbitrary length, from several bytes to many megabytes. On the contrary, packets usually can also have a variable length, but in narrow limits, for example from 46 to 1500 bytes. Each packet is supplied with the title, which indicates the address information required to deliver the package to the destination node, as well as the packet number that will be used by the destination node for the message assembly (Fig. 3). Packages are transported over the network as independent information blocks. The network switches take packets from end-nodes and on the basis of address information transmit them to each other, and ultimately - the destination node.

The batch network switches differ from channel switches by the fact that they have internal buffer memory for temporary storage packets if the output port of the switch at the time of accepting the package is engaged in the transmission of another package (Fig. 3). In this case, the package is some time in the packet queue in the output port buffer memory, and when the queue comes to it, it is transmitted to the next switch. Such a data transmission scheme allows you to smooth out the ripple of traffic on the main links between switches and thereby use them to increase network bandwidth as a whole.

Indeed, for a couple of subscribers, it would be the most effective to provide them with the sole use of the uncovered communication channel, as is done in channel switches. In this case, the interaction time of this pair of subscribers would be minimal, since data without delays would be transmitted from one subscriber to another. The downtime during the pause of the transfer of subscribers is not interested, it is important for them to solve their task faster. The packet switching network slows down the process of interaction between the specific pair of subscribers, as their packages can be expected in switches until other packages come to the switch earlier are transmitted.

However, the total amount of computer data transmitted per unit of time with packet switching technique will be higher than with channel switching technique. This is because the pulsations of individual subscribers in accordance with the law of large numbers are distributed in time so that their peaks do not coincide. Therefore, the switches are constantly and quite evenly loaded by work, if the number of subscribers served by them is really great. In fig. 4 It is shown that traffic coming from end nodes to switches is very unevenly distributed. However, the switches of a higher level of hierarchy that serve connections between the low-level switches are loaded more evenly, and the stream of packets in the main channels connecting the top-level switches has an almost maximum utilization factor. Bufferization smoothes ripples, so the ripple coefficient on the trunk channels is much lower than on subscriber access channels - it can be equal to 1:10 or even 1: 2.

Higher efficiency of packet switching networks compared to channel switching networks (with an equal bandwidth of communication channels) was proven in the 60s both experimentally and using simulation modeling. Here is appropriate an analogy with multiprogram operating systems. Each individual program in such a system is performed longer than in a single-program system when the program allocates all processor time until its execution is completed. However, the total number of programs performed per unit of time in the multiprogram system is greater than in single-strware.
The packet switching network slows down the process of interaction between the specific pair of subscribers, but increases the bandwidth of the network as a whole.

Delays in the transmission source:

· Time to transfer headlines;

· Delays caused by intervals between the transfer of each next package.

Delays in each switcher:

· Pack buffering time;

· Switching time, which consists of:

o waiting time in the queue (variable value);

o Time to move the package in the output port.

Dignities of switching packages

1. High overall network bandwidth when transmitting pulsating traffic.

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

Disadvantages of switching packages

1. The uncertainty of data transfer rates between network subscribers due to the fact that delays in the network switches of network switches depend on the total network load.

2. The variable value of the delay of data packets, which can be sufficiently long in the moments of instantaneous network overloads.

3. Possible data loss due to buffer overflows.
Currently, methods are currently being developed and implemented to overcome these disadvantages that are particularly acute for sensitive traffic delays requiring a constant transmission rate. Such methods are called maintenance quality assurance methods (QOS).

Package Switching Networks, which implemented service quality assurance methods, allow you to simultaneously transmit different types of traffic, including such important as telephone and computer. Therefore, packet switching methods today are considered the most promising to build a convergent network, which will provide comprehensive quality services for subscribers of any type. Nevertheless, it is impossible to discount the channels and methods of channel switching. Today, they are not only successfully working in traditional telephone networks, but also widely used to form high-speed permanent connections in the so-called primary (reference) SDH and DWDM technologies, which are used to create trunk physical channels between telephone or computer network switches. In the future, the emergence of new switching technologies is quite possible, in one form or otherwise combining the principles of switching packets and channels.

4.VPN (eng. Virtual Private Network. - Virtual private network) - a generalized name of technologies that allow one or more network connections (logical network) over another network (for example, Internet). Despite the fact that communication is carried out on networks with a smaller unknown level of trust (for example, according to public networks), the level of confidence in the built logical network does not depend on the level of confidence in the basic networks through the use of cryptography (encryption, authentication, open key infrastructure, funds To protect against repeats and changes transmitted by logical network messages).

Depending on the applicable protocols and appointments, VPN can provide compounds of three species: node-knot,net network and network. Typically, VPNs are deployed at no higher-network levels, since the use of cryptography at these levels allows you to use transport protocols in constant form (such as CTCP, UDP).

Microsoft Windows users denote the thermal VPN one of the implementations of the virtual network - PPTP, and the often used not To create private networks.

Most often to create a virtual network, encapsulation of the PPP protocol on some other IP protocol is used (this method uses the implementation of PPTP - Point-to-Point Tunneling Protocol) or Ethernet (PPPOE) (although they have differences). VPN technology has recently been used not only to create private networks, but also certain "last miles" in the post-Soviet space to provide access to the Internet.

With the proper level of implementation and use of special software, the VPN network can provide a high level of encryption of the transmitted information. With the proper configuration of all components, the VPN technology provides anonymity on the network.

The VPN consists of two parts: "Internal" (controlled) network, which can be somewhat, and "external" network, which passes the encapsulated connection (commonly used Internet). It is also possible to connect to a virtual network of a separate computer. Connecting a remote user to VPN is made by means of an access server that is connected to both the internal and external (publicly accessible) network. When you connect a remote user (or when you install a connection with another protected network), the access server requires the passage of the identification process, and then the authentication process. After the successful passage of both processes, the remote user (remote network) is endowed with the authority to work on the network, that is, the authorization process occurs. Classify VPN solutions can be classified by several basic parameters:

[edit] According to the degree of security of the medium

Protected

The most common variant of virtual private networks. With it, it is possible to create a reliable and protected network based on an unreliable network, as a rule, the Internet. An example of protected VPNs are: IPsec, OpenVPN and PPTP.

Trust

Used in cases where the transmitting medium can be considered reliable and it is necessary to solve only the task of creating a virtual subnet within a larger network. Safety problems become irrelevant. Examples of similar VPN solutions are: Multi-Protocol Label Switching (MPLS) and L2TP (Layer 2 Tunnelling Protocol) (more precisely, these protocols shift the security task to other, such as L2TP, as a rule, is used paired with IPSec).

[edit] By way of implementation

In the form of special software and hardware

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

In the form of a software solution

Use a personal computer with special software providing VPN functionality.

Integrated solution

The VPN functionality provides a complex that also solves the tasks of filtering network traffic, network screen organization and maintenance quality.

[edit] By appointment

Used to combine into a single protected network of several distributed branches of one organization, exchanging data on open communication channels.

Remote Access VPN.

Used to create a protected channel between the corporate network segment (by a central office or branch) and a single user who, working at home, connects to corporate resources from a home computer, corporate laptop, smartphone or Internet kiosk.

Use for networks to which "external" users are connected (for example, customers or customers). The level of confidence in them is much lower than to the company's employees, therefore it is required to provide special "frontiers" of protection, preventing or restricting the latest to particularly valuable, confidential information.

Used to provide access to the Internet providers, usually if several users are connected to one physical channel.

Client / Server VPN

It provides protection for the transmitted data between two nodes (non-Networks) of the corporate network. The peculiarity of this option is that the VPN is built between nodes that are usually in one network segment, for example, between the workstation and the server. Such a need is very often arising in cases where several logical networks must be created in one physical network. For example, when the traffic between the Financial Department and the personnel department contacting servers in one physical segment should be divided. This option is similar to VLAN technology, but instead of separating traffic, its encryption is used.

[edit] by type protocol

There are implementations of virtual private networks under TCP / IP, IPX and AppleTalk. But today there is a tendency to a universal transition to the TCP / IP protocol, and the absolute majority of VPN solutions supported it. Addressing in it is most often selected in accordance with the RFC5735 standard, from the TCP / IP Private Network Range

[edit] Upon Network Protocol

By the level of the network protocol based on the comparison with the levels of the reference network model ISO / OSI.

5. The OSI reference model, sometimes called the OSI stack is a 7-level network hierarchy (Fig. 1) developed by the International Standardization Organization - ISO). This model contains essentially 2 different models:

· Horizontal model based on protocols that ensures the mechanism of interaction of programs and processes on various machines

· Vertical model based on services provided by adjacent levels to each other on one machine

In the horizontal model, two programs require a general protocol for data exchange. Vertical - adjacent levels are exchanged by data using API interfaces.

Similar information.

Channel Switching Networks have several important common properties no matter what type of multiplexing in them is used.

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 runs through switches and configures them to subsequent data transmission. The connection to establish the connection is routed from one switch to another and eventually reaches the called subscriber. The network may refuse to establish a connection if the capacitance of the desired output channel is already exhausted. For the FDM switch, the capacitance of the output channel is equal to the number of frequency bands of this channel, and for the TDM switch - the number of time slots to which the channel operation cycle is divided. The network refuses to connect as well 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 ability to failure in the compound is a lack of channel switching method.

If the connection can be set, it allocates a fixed frequency band in FDM networks or fixed bandwidth in TDM networks. These values \u200b\u200bremain unchanged during the entire period of the connection. The guaranteed network bandwidth after establishing the connection is an important property necessary for applications such as voice, image or object management in real time. However, dynamically change the bandwidth of the channel at the request of the channel switching subscriber can not, which makes them ineffective under the conditions of pulsating traffic.

The disadvantage of channel switching networks is the impossibility of using user equipment operating at different speeds. Separate parts of the composite channel work at the same speed, as the network switched networks do not buffer user data.

Channel Switching Networks are well adapted for switching a constant speed data streams when the switching unit is not a separate byte or data packet, but a long-term synchronous data flow between two subscribers. For such streams, the channel switches are added to a minimum of service information to routing data via the network using the time position of each flow bit as its destination address in the network switches.

Providing duplex mode of work based on FDM, TDM and WDM technologies

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

o Simplex - transmission is carried out on the communication line in only one direction;

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

o Duplex - the transfer is carried out simultaneously in two directions.

Duplex mode is the most universal and productive way to operate the channel. The easiest way to organize a duplex mode is to use two independent physical channels (two pairs of conductors or two light guides) in the cable, each of which works in simplex mode, that is, transfers the data in one direction. It is such an idea that underlies the implementation of a duplex mode of operation in many network technologies, such as FAST Ethernet or ATM.

Sometimes such a simple solution is inaccessible or ineffective. Most often, this is happening in cases where there is only one physical channel for duplex data, and the organization of the second is linked to high costs. For example, when exchanging data using modems via a telephone network, a user has only one physical communication channel with a PBX - a two-wire line, and it is hardly advisable to acquire the second. In such cases, the duplex mode of operation is organized on the basis of a channel separation into two logical subchannel using FDM or TDM technique.

Modems To organize a duplex mode of operation on a two-wire line, use FDM technique. Modems using frequency modulation are operated on four frequencies: two frequencies - for encoding units and zeros in one direction, and the remaining two frequencies - to transmit data in the opposite direction.

With digital encoding, duplex mode on a two-wire line is organized using TDM technology. Part of the time slots is used to transmit data in one direction, and part for transmission in another direction. Typically, the time slots of opposite directions alternate, because of which such a method is sometimes called "ping-pong" transmission. TDM-split line is characteristic, for example, for digital networks with the integration of services (ISDN) on subscriber two-wire terminations.

In fiber optic cables, using one optical fiber to organize a duplex operation mode, data transmission is used in one direction using a light beam of one wavelength, and in the opposite - another wavelength. This technique belongs to the method of FDM, however, for optical cables, it has received a division of the wave division multiplexing, WDM. WDM is used to increase the data rate in one direction, typically using 2 to 16 channels.

Switching packs

Package switching principles

Package switching is a subscriber switching technique that has been specifically designed to effectively transmit computer traffic. Experiments on the creation of the first computer networks based on channel switching techniques showed that this type of switching does not allow to achieve a high total bandwidth of the network. The essence of the problem is the pulsating nature of the traffic that generate typical network applications. For example, when accessing a remote file server, the user first browsing the contents of the directory of this server, which generates the transfer of a small amount of data. Then it opens the required file in a text editor, and this operation can create a fairly intensive data exchange, especially if the file contains bulk graphic inclusions. After displaying multiple page files, the user works for some time with them locally, which does not require data on the network at all, and then returns modified copies of pages to the server - and this again generates intensive data transmission over the network.

The ripple rate of the individual network of the network, 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 to organize the channel switch between the user and the server and the server, then most of the time the channel will be idle. At the same time, the network switching capabilities will be used - part of the time-slots or frequency bands of switches will be occupied and is not available to other network users.

When switching packets, all user-transmitted messages are broken in the source node for relatively small parts, called packages. Recall that the message is called a logically completed data portion - a file transfer request, the answer to this query containing the entire file, etc. Messages may have arbitrary length, from several bytes to many megabytes. On the contrary, packets usually can also have a variable length, but in narrow limits, for example from 46 to 1500 bytes. Each packet is supplied with the title, which indicates the address information required to deliver the destination node package, as well as the package number that will be used by the destination node for the message assembly (Fig. 2.29). Packages are transported in the network as independent information blocks. The network switches take packets from end-nodes and on the basis of address information transmit them to each other, and ultimately - the destination node.

Fig. 2.29. Breakage of the message to packages

Batch network switches are different from channel switches in that they have internal buffer memory for temporary storage packages if the output switch port at the time of accepting the package is engaged in the transmission of another package (Fig. 2.30). In this case, the package is some time in the packet queue in the output port buffer memory, and when the queue comes to it, it is transmitted to the next switch. Such a data transmission scheme allows you to smooth traffic ripples on the main links between switches and thereby use them most efficiently to increase network bandwidth as a whole.

Fig. 2.30. Smoothing of traffic ripples on a network switched network

Indeed, for a couple of subscribers, it would be the most effective to provide them with the sole use of the uncovered communication channel, as is done in channel switches. In this case, the time of the interaction of this pair of subscribers would be minimal, since data without delay would be transmitted from one subscriber to another. The downtime during the pause of the transfer of subscribers is not interested, it is important for them faster to solve their own task. The packet switching network slows down the process of interaction between the specific pair of subscribers, as their packages can be expected in switches until other packages come to the switch earlier are transmitted.

Nevertheless, the total amount of computer data transmitted per unit of time with packet switching technique will be higher than with channel switching techniques. This is because the pulsations of individual subscribers in accordance with the law of large numbers are distributed over time. Therefore, the switches are constantly and quite evenly loaded by work, if the number of subscribers served by them is really great. In fig. 2.30 It is shown that traffic coming from end nodes to switches is very unevenly distributed over time. However, the switches of a higher level of hierarchy that serve connections between the low-level switches are loaded more evenly, and the stream of packets in the main channels connecting the top-level switches has an almost maximum utilization factor.

Higher efficiency of packet switching networks compared to channel switching networks (with an equal bandwidth of communication channels) was proven in the 60s both experimentally and using simulation modeling. Here is appropriate an analogy with multiprogram operating systems. Each separate program in such a system is performed longer than in a single-strware system, when the program allocates all processor time until it completes its execution. However, the total number of programs performed per unit of time in the multiprogram system is greater than in single-strware.

Federal Agency Communication

State Educational Budget Institution

higher professional education

Moscow Technical University of Communications and Informatics

Department of Communication Networks and Switching Systems

Methodical instructions

and control tasks

by discipline

Switching systems

for students of the correspondence form of training 4 courses

(Direction 210700, profile - ss)

Moscow 2014.

LDD plan for 2014/2015 ac.

Methodical instructions and control

by discipline

Switching systems

Compiler: Stepanova I.V., Professor

The edition is stereotypical. Approved at the meeting of the Department

Communication networks and switching systems

Reviewer Malikova E.E., Associate Professor

General course guidelines

The discipline of the "switching system" part of the second is studied on the second semester of the fourth year by students of the correspondence faculty of the specialty 210406 \u200b\u200band is a continuation and further deepening of the similar discipline studied by students on the previous semester.

In this part of the course, the principles of the exchange of information management and interaction between switching systems are considered, the basics of designing digital switching systems (CSK).

Lectures are read at the rate, a course project and laboratory work are performed. The exam is surrendered and the course project is protected. Independent work on the development of the course is to work out the material of the textbook and the textbooks recommended in the methodological instructions, and in the implementation of the course project.

If a student has difficulty studying recommended literature, then you can contact the Department of Communication Networks and Switching Systems in order to obtain the necessary advice. To do this, in the letter it is necessary to specify the name of the book, the year of publication and the page, where the unclear material is settled. The course should be studied successively, the topic is per theme, as recommended in the methodological instructions. With this study, the next section of the course should move after you answer all the test questions that are questions of exam tickets, and decide the recommended tasks.

The distribution of time in the student's hours to study the discipline of the "switching system", part 2, given in Table 1.

BIBLIOGRAPHY

Basic

1.Goldstein B.S. Switching systems. - SPb.: BHV - St. Petersburg, 2003. - 318 C.: IL.

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

Additional

3.Obutin V.S., Popova A.G., Stepanova I.V. Telephony user subsystem for signaling over a common channel. - M. Radio and Communication, 1998.-58 p.

4. Lagutin VS, Popova A.G., Stepanova I.V. Evolution of intellectual services in convergent networks. - M., 2008. - 120s.

List of laboratory work

1. Alarm 2VSK and R 1.5, signal exchange scenario between two PBXs.

2. Subscriber data on digital PBX. Analysis of emergency messages digital PBX.

Methodical instructions for courses

Features of constructing digital channel switching systems

The features of constructing channel switching systems should be studied using the example of digital PBX type EWSD. Consider the characteristics and functions of digital DLU subscriber units, the implementation of remote subscriber access. Consider the characteristics and functions of the LTG linear group. Examine the construction of the switching field and the typical process of establishing the connection.

The Digital ELECTRONIC Switching System (Digital Electronic Switching System) is developed by Siemens as a universal channel switching system for public telephone networks. The EWSD switching field bandwidth is 25200 Erlang. The number of serviced calls to the CNN can reach 1 million calls. The EWSD system when used as a PBX allows you to connect up to 250 thousand subscriber lines. The bond node on the basis of this system allows you to switch up to 60 thousand connecting lines. Telephone stations in container design allow you to connect from several hundred to 6,000 remote subscribers. Commutation centers are available for cellular communication networks and for the organization of international communication. There are wide opportunities for organizing second selection paths: up to seven ways of direct choice plus one path of the last choice. Up to 127 tariff zones may be allocated. Within one day, the tariff may vary up to eight times. The generator equipment provides a high degree of stability of frequency sequences produced:

in PlesiOhron mode - 1 10 -9, in synchronous mode -1 10 -11.

The EWSD system is designed to use power sources -60B or -48B. It is allowed to change the temperature in the range of 5-40 ° C with a humidity of 10-80%.

EWSD hardware are divided into five main subsystems (see Fig. 1): Digital Subscriber Block (DLU); Linear Group (LTG); Switching field (SN); control device of the alarm network on a shared channel (CCNC); Coordination processor (CP). Each subsystem has at least one microprocessor denoted by GP. Alarm systems R1.5 (foreign version R2) are used, according to the total alarm channel No. 7 SS7 and EDSS1. Digital subscriber blocks DLU Service: Analog Subscriber Lines; subscriber lines of users of digital networks with services integration (ISDN); Analog institutional substations (UPATS); Digital UPATS. DLU blocks provide the ability to enable analog and digital telephones, Multifunctional ISDN terminals. ISDN users provide channels (2b + d), where B \u003d 64 kbit / s is the standard ICM30 / 32 hardware channel, the alarm transmission d-channel with a speed of 16 kbps. To transmit information between EWSD and other switching systems, primary digital connecting lines are used (CSL, English) - (30V + 1D + synchronization) at a transmission rate of 2048 kbps (or at 1544 kbps in the USA).

Fig.1. EWSD switching system circuit

Local or remote DLU operation can be used. Remote DL blocks are installed in the concentrations of subscribers. At the same time, the length of subscriber lines is reduced, and traffic on digital connecting lines is concentrated, which leads to a decrease in the cost of organizing the distribution network and improves the quality of the transfer.

With regard to the subscriber lines, the resistance of the loop to 2 kΩ and the insulation resistance is considered to be permissible - up to 20 com. The switching system can perceive the pulses of the dialing of the number from the disk dialer coming at a speed of 5-22 pulses / s. Receiving frequency dialing signals The number is carried out in accordance with the recommendation of the SSP REC.Q.23.

The high level of reliability is provided by: connecting each DLU to two LTG; duplication of all DL blocks with load separation; Continuously performed self-control tests. To transmit control information between DLU and LTG linear groups, uses alarm on a shared channel (CCS) via the time channel number 16.

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

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

two digital interfaces (DIUD) to connect digital transmission systems (PDC) to linear groups;

two control devices (DLUC) controlling internal DL sequences distributing or concentrating signal streams going to subscriber sets and from them. To ensure reliability and increasing bandwidth, the DLU contains two DLUC controllers. They work independently from each other in the mode of separation of tasks. If the first DLUC failures, the second may assume all tasks;

two control networks to transfer control information between subscriber lines and control devices;

test unit (TU) for testing phones, subscriber and connecting lines.

DLU characteristics are changed when moving from one modification to another. For example, the DLUB option provides for the use of analog and digital subscriber sets with 16 sets in each module. Up to 880 analog subscriber lines can be connected to a separate subscriber unit DLUB, and it connects to LTG using 60 ICM channels (4096 kbps). At the same time, losses due to lack of channels should be almost equal to zero. To perform this condition, the bandwidth of one dlub should not exceed 100 Earl. If it turns out that the average load on one module is greater than 100 Earl, then the number of subscriber lines included in one Dlub should be reduced. Up to 6 DLUB blocks can be combined into a remote control unit (RCU).

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

Table 1.Technical characteristics of the DLUG digital subscriber unit

With the help of individual lines, mint payphones can be connected, analogue-industrial automatic telephone stations RVH (Private Automatic Branch Exchange) and digital moral and medium tanks.

We list part of the most important functions of the SLMA subscriber sets module to connect analog subscriber lines:

control of lines to detect new calls;

powered by a constant voltage with adjustable current values;

analog-digital and digital analog converters;

symmetric connection of the call signals;

control of short-circuits of the loop and short circuits to the ground;

receiving pulses of a decade dialing of the number and with frequency set;

change of polarity of the power supply (caster reversing for payphones);

connecting the line side and side of the subscriber kit to the multi-position test switch, overvoltage protection;

distant current speech signals;

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

Appeal to functional blocks equipped with its own microprocessors is carried out through the DLU control network. The blocks are interviewed cyclically for the readiness of the transmission of messages, direct access to commands for transmitting commands and data is carried out. DLUC also performs test programs and observations in order to recognize errors.

There are the following DLU tire systems: control tires; Tires 4096 kbps; collision detection tires; Tires of transmission of call signals and tariff impulses. Signals transmitted over the tires are synchronized with clock pulses. In the control tires, control information is transmitted at a transfer rate of 187.5 kbps; Moreover, the effective data transfer rate is approximately 136 kbps.

On the bus 4096 kbps, it is sent by speech / data to SLM subscriber lines modules and back. Each tire has in both directions of 64 channels.

Each channel functions with a transfer rate of 64 kbps (64 x 64 kbps \u003d 4096 kbps). Assigning tire channels 4096 kbps / with RDC channels is fixed and is determined via DIUD (see Fig.3). Connecting DLU to linear groups of type B, F, or G (respectively, LTGB, LTGF or LTGG types) is carried out according to multiplex lines 2048 Kbps. DLU can be connected to two LTGB, two LTGF (B) or two LTGG.

Linear Group Line / Trunk Groupe (LTG)forms an interface between a node digital medium and a digital switching field Sn (Fig. 4). LTG groups perform the functions of decentralized management and exempt the CP coordination processor from routine work. Connections between the LTG and the duplicate switching field are carried out on the secondary digital communication line (SDC). The SDC transmission rate in the LTG direction to the Ltg group to the Sn field and in the opposite direction is 8192 kbps (abbreviated 8 Mbps).

Fig.3. Multiplexing, demultiplexing and

transmission of control information in DLUC

Fig.4. Different access options to LTG

Each of these multiplex systems 8 Mbps has 127 time intervals with a speed of 64 kbps in each for the transfer of useful information, and one time interval with a speed of 64 kbps is used to transmit messages. The LTG group transmits and accepts voice information through both sides of the switch field (SN0 and SN1), performing the appropriate voice information to the appropriate subscriber from the active block of the switching field. The other side of the SN field is considered inactive. If the failure occurs, the transmission and reception of custom information immediately begin. The LTG power supply voltage is + 5V.

The following call processing functions are implemented in LTG:

receiving and interpretation of signals entering the connecting and
subscriber lines;

transmission of signaling information;

transmission of acoustic tonal signals;

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

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

transfer and receiving requests to / from a network signal controller over a common channel (CCNC);

control of the alarm coming into DLU;

coordination of states on lines with states of the standard interface 8 Mbps with a duplicate SN switching field;

setting connections to send user information.

To implement various types of lines and signaling methods, several types of LTG are used. They are distinguished by the implementation of hardware blocks and specific application programs in the group processor (CP). LTG blocks have a large number of modifications that are characterized by the use and capabilities. For example, the LTG function is used to connect: up to 4 primary digital PCM30 type communication lines (ICM30 / 32) with 2048 kbps transmission rates; Up to 2 digital communication lines with a transmission speed of 4096 Kbps for local DLU access.

LTG Function Block is used to connect up to 4 primary digital communication lines with 2048 Kbps speeds.

Depending on the purpose of LTG (B or C) there are differences in the LTG functional execution, for example, in the group processor software. The exceptions are modern LTGN modules that are universal, and in order to change their functional purpose, it is necessary to "re-create" them software with another loading (see Table 2 and Fig.4).

Table 2. Specifications of the Linear Group N (LTGN)

As shown in Fig.5, in addition to standard interfaces 2 Mbit / s (RSMZ0), the EWSD system provides an external system interface with a higher transmission rate (155 Mbps) with multiplexers of the SDH synchronous digital hierarchy network on fiber optic lines. Communication. The terminal multiplexer of type N is used (synchronous double terminal multiplexer, SMT1D-N) installed on the LTGM host.

The SMT1D-N multiplexer can be represented as a base configuration with 1xstm1 interface (60HRSM0) or in the form of a complete configuration with 2xstm1 interfaces (120HRSM0).

Fig.5. Enabling SMT1 D-N network

Switching field SN. EWSD switching systems connect to each other LTG, CP and CCNC subsystem. The main task is to establish connections between LTG groups. Each connection is simultaneously installed through both half (plane) of the SN0 and SN1 switch field, so if one of the sides of the field is always a backup connection. Two types of switching fields can be used in switching systems: SN and SN (B). SN (B) type switching field is a new development and is smaller than the size, higher availability, reduced power consumption. There are various options for the organization SN and SN (B):

switching field on 504 linear groups (SN: 504 LTG);

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

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

switching field on 63 Linear groups (SN: 63 LTG).

The main functions of the switching field are:

switching channels; Switching messages; Switch to reserve.

The switching field switches the channels and connections with a transfer rate of 64 kbps (see Fig. 6). For each connection, two connecting paths are needed (for example, from the caller to the called and from the called subscriber to the caller). The coordination processor searches for free paths through a switching field based on the currently stored in the storage device information about the employment of connecting paths. Commuting connecting paths is carried out by control devices of the switching group.

Each switching field has its own control device consisting of a switching group control device (SGC) and an interface module between SGC and a MBU message buffer unit: SGC. With a minimum capacity of the stage 63 LTG in switching the connecting path, one SGC of the switching group is involved, but two or three SGC are used at the boosters from 504, 252 or 126 or 126 LTG. It depends on whether subscribers are connected with the same TS temporary switching group or not. Commands to establish a compound are set to each GP switch-based GP processor of the CP processor.

In addition to the connections specified by subscribers by dialing the number, the switching field commutes connections between linear groups and the CP coordination processor. These compounds are used to exchange control information and are called semi-permanent switched compounds. Thanks to these connections, messaging is exchanged between linear groups without the costs of the coordination processor unit. Uncommunicable (Nailed-Up) Connections and connections for signaling across a common channel are also set on the principle of semi-permanent compounds.

Switching field in the EWSD system is characterized by full availability. This means that each 8-bit code word transmitted by the highway included in the switching field can be transmitted in any other time interval on the highway emanating from the switching field. In all highways with a transfer rate of 8192 kbps, there are 128 channels with a transmission bandwidth 64 kbps each (128x64 \u003d 8192 Kbps). Switching steps SN: 504 LTG, SN: 252 LTG, SN: 126 LTG have the following structure:

one stage of temporary switching, incoming (TSI);

three spatial switching steps (SSM);

one stage of temporary switching, outgoing (TSO).

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

one incoming stage of temporal switching (TSI);

one step of spatial switching (SS);

one outgoing time switching step (TSO).

Fig.6. An example of establishing a connection in the SN switch field

Coordination processor 113 (CP113 or CP113C) It is a multiprocessor, the capacity of which is increasing in steps. In the CP113S multiprocessor, two or more identical processors operate in parallel with the load separation. The main functional blocks of the multiprocessor are: the main processor (VAR) for processing calls, operation and maintenance; Call Processing Processor (CAP) for Call Processing; General storage device (CMY); I / O controller (IOC); I / O processor (IR). Each VAR processor, CAP and IOP contains one program execution module (REH). Depending on whether they should be implemented as Var processors, CAP processors or I0C controllers are activated by specific hardware functions.

We list the main technical data Var, CAP and IOC. Processor Type - MC68040, Top Frequency -25 MHz, Bit Challenge 32 Bit and Data Bitness 32 Bitting, Data Bigness - 32 Data Bit. Local Memory Information: Extension - Maximum 64 MB (based on DRAM 16M BIT); Stage expansion 16 MB. EPROM Flash Memory Information: Expand 4 MB. The CP coordination processor performs the following functions: call processing (number of number numbers, routing control, selection of the service area, select the path in the switch field, accounting for the cost of talking, traffic management, network management); Operation and maintenance - entering into external storage devices (EM) and output from them, communication with the operation and maintenance terminal (OMT), communication with the data transfer processor (DCP). 13

On the SYP panel (see Fig. 1), external alarm is displayed, for example, information about the fire. The external memory is used to store programs and data that should not be constantly stored in the CP, the entire system of application programs for automatically restoring telephone conversation data and traffic change.

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

Maximum CP performance of call processing is over 2700,000 calls per hour of the highest load. CP characteristics of the EWSD system: 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.

MESSAGER BUFFER MESSABLE (MV) task is messaging management:

between the CP113 coordination processor, and LTG groups;

between CP113 and controllers of switching groups SGCB) switching field;

between LTG groups;

between LTG groups and the alarm network controller over the shared CCNC channel.

The following types of information can be transferred via MV:

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

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

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

teams are sent from CP113 to LTG and SN. MW converts information for transmission through the secondary digital stream (SDC) and sends it to LTG and SGC.

Depending on the stage of the tank, the duplicate MV device may contain up to four groups of messages buffer (MBG). This feature is implemented in a network node with redundancy, that is, the MBG00 ... MBG03 groups are part of the MB0, and the MBG10 ... MBG13 groups are part of the MB1.

EWSD switching systems with a signaling on a common channel on system number 7 are equipped the control device of the alarm network on the shared channel CCHNS. Up to 254 signaling links can be connected to the CCNC device through analog or digital communication lines.

The CCNC device connects to the switching field by compacted lines having 8 Mbps transmission speed. Between CCNC and each plane of the switching field there are 254 channels for each direction of transmission (254 pairs of channels).

The channels are transmitted through the channels through both SN planes to linear groups and from them with a speed of 64 kbps. Analog signaling paths are connected to CCNC through modems. CCNC consists: from the maximum 32 groups with 8 terminal devices of the signal paths each (32 SILT groups); One duplicate processor of the signaling system across the shared channel (CCNP).

Control questions

1. What unit performs analog-digital conversion?

2. How many analog subscriber lines can be maximally included in Dlub? What bandwidth is this block?

3. What speed is the information between DLU and LTG, between LTG and SN?

4. List the basic functions of the switch field. At what speed the connection between subscribers is implemented.

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

6. List the main switching steps with a switch field.

7.Reat the passage of the conversation through the Switching Field of the EWSD switching system.

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

9. What functions implements side by MV?

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In network switching networks between calling and called terminal installations, through the entire transmission time there is a through connection (Fig. 3.3).

Fig. 3.3. Channel Switching Severe

The connecting path consists of a number of areas that, during the establishment of the connection, are turned on successively with each other. It is "transparent" with the codes used in the terminal installations during data transmission, and control methods. The distribution time of the data signal on the connecting path constantly.

In the communication session, three phases are distinguished: establishing a connection, data transfer and disconnection (see Fig. 3.1 a). The connection to establish the connection controls the caller

the terminal setting that sends a call signal to its switching unit receives a response signal from the node (invitation to the number of numbers) and follows the address information (number set signs) to the node. The switching unit processes this information, takes one of the channels in the beam leading to the next switching node, and transmits the last set signs required to further establish the connection. Thus, gradually, the connecting path is formed up to the called terminal installation. After completing this process from the network, the calling and called end settings receive signals that are notified that the connection is enabled and ready to transmit data.

From this point on, the data transfer is determined by the terminal installation. In the terminal installation (automatically or with the participation of the Subscriber), a decision is made on measures that must be taken to detect and correct the transfer errors. Measures may be different depending on certain working conditions.

The disconnection can be started by any of the two-related terminal settings using the abnormal signal. Upon this signal, all switching nodes involved in the formation of the connecting path are disconnected.

Among the transmission networks with switching channels, two types are distinguished: synchronous and asynchronous networks.

3.3.1. Asynchronous channel switches

3.3.1.1. Distinctive features of asynchronous networks

In asynchronous networks, the overall synchronization of the elements is missing and uniform "tacts" are not specified for the network. Separate ADFs and switching devices have independent, independent clock generators.

In fig. 3.4 schematically shows the structure of such a network with terminal installations, multichannel equipment and switching nodes. Subscriber lines and multi-channel system channels are used to communicate with switching units. Switching nodes are interconnected by canal beams. Before nodes, bundles are split into separate channels.

The splitting admits a certain freedom to organize the network. For example, when transmitting communication lines, a system of both frequency and temporal channel separation can be used (see section 1.4.2), the equipment of both the spatial and time switching of the channels can be installed in the network nodes (see volume 1, section. 6.1.3, as well as). Such freedom in choosing

Fig. 3.4. Asynchronous Channel Switching Network

The channel-forming and switching equipment is necessary, in particular, when organizing telegraph communications and data transmission on a common network, when the telegraph network has already existed equipment, such as a tone telegraph system (CM, Section 1.4.2.2), must be used primarily. Then, as the technical and economic opportunities, the specified equipment will gradually be complemented or replaced by more perfect, based on new construction techniques.

As shown in Fig. 3.4, the connecting path between the calling and called terminal settings consists of several sections, which are consistently included in each other. Since each transmission path section and each switching node contributes its share in the overall distortion of the transmitted data signal, then the transmission and switching must be carried out with perhaps smaller distortion.

The requirement of a minimum of distortion is important primarily for unlard signals, which are not fundamentally corrected. Iceoral data signals, on the contrary, can be adjusted on each section of the transmission path and in each switch node. In temporary separation systems having synchronous channels or channels with the formation of iconic cycles (see section 1.4.2.3), the correction is carried out automatically. In frequency separation systems that allow transmission with varying speed, that is, they are "transparent" (see 1.4.2.2) for correction, additional devices must be installed. However, due to the high costs, this is usually refused, as a result of which, in such cases, the transfer and switching should also be carried out with possibly smaller distortion.

3.3.1.2. Transmission systems with VRK in asynchronous network switches

In the asynchronous network switched channel, each transmission system with temporary separation (VRK) has its own synchronism, not dependent on the synchronism of other systems. As a result, the clock frequencies of systems with VRK are different, i.e. the connecting path between subscribers consists of sections with not exactly the same transmission rates.

In systems with a temporary separation of synchronous channels (see Section 1.4.2.3), in which each bit from the OOD is placed in accordance with one bit in the group stream, due to the difference in transmission speeds, there may be a phenomenon of signal slips with bit or adding signals. unnecessary. This means that one of the bits is not transmitted further, since the next system has a too low transmission rate, or, on the contrary, any of the bits turns out to be transmitted again, since the next system has too high speed (Fig. 3.5).

Fig. 3.5. Slipping of bits in an asynchronous network switched

Therefore, in systems with VRK, working in asynchronous network switching networks, it is necessary to apply special methods for aligning velocities at which due to the exclusion or addition of matching ("empty") bits in each individual data channel achieved coordination with the transfer rate through the channels of the connecting path. In other words, systems with temporary separation, having channels with speed coordination - Staffing channels (see Section 1.4.2.3).

With the phenomenon of bits, it is also considered to be considered in the case of the application of temporary separation systems having

channels with the formation of iconic cycles (see Section 1.4.2.3). Such systems must identify the iconic cycles and eliminate the differences in the speeds between the data channels by shortening or lengthening the stop element.

In temporary separation systems with "transparent" channels (see Section 1.4.2.3), converting the SIM signals into the transmitted sequence of bits by positioning and temporary coding, the problem of slipping bits does not occur. Indeed, in this case, the signal after each section of the transmission is characterized, in principle, the unnecessary time relations and the same is transmitted further. Of course, the distortions arising from the multiple coding will not be too large, the error is inevitable when encoding should remain at a fairly low level.

3.3.1.3. Time switching equipment of channels in asynchronous networks

If systems are connected to the switchboat nodes of an asynchronous network, which have stuffing channels or channels with the formation of iconic cycles, then in sequential time switching devices on bits (see volume 1, section 6.1.3.2) allowed distortions of data signals that constitute no more than half Single interval.

When using temporary separation systems with "transparent" channels or frequency separation systems of distortion channels arising in the process of consistent bits, should be very small, as they are included in the total distortion. Although in the case of isochronous data signals between the switching equipment and the multichannel transmission system, it would be possible to establish a corrector, it would be necessary to implement the specified in the section. 3.3.1.2. The coordination of speeds and would have to reconcile with these costs.

In the presence of stafinting channels and channels, the combat of bit-made cycles may apply, which provides higher performance (see Section 2. 1.1.1, Example 3, Table 2.1).

3.3.1.4. The structure of the asynchronous network switched

The structure of the asynchronous network switched network is shown in Fig. 3.6, where the lower level of the network is depicted, part of the network from subscribers to the switching unit. Subscriber joints form the border between the ADD and the data network. In the locations of subscribers are also connected devices

(PP) that ensure the pairing of the ODD with the network (see Section 2.2.2). In cases where the ODO does not control directly through the data circuits of the joint by the process of establishing and disconnecting connections, instead of PP, the output devices (VP) are installed, containing the elements necessary for such control (see Section 2.2.1).

Fig. 3.6. The structure of the asynchronous network switched channels:

1 - subscriber joints; 2 - Connection devices or calling devices; 3 - subscriber lines; 4 - multiplexers; 5 - hubs; 6 - connecting lines; 7 - Switching unit

Through subscriber lines of PP and HP are associated with multiplexers or hubs, which are usually placed in the same place where the equipment of the telephone network switching station. With the help of a multiplexer, a canal beam is formed, the number of which is equal to the number of subscriber lines. The hub, on the contrary, collects and compacts the load of subscriber lines, so there must be fewer channels in the beam than subscriber lines (see Section 2.1.1.2).

Switching nodes of the data network sets are installed at the location of the central switching stations of the telephone network, and at the high density of subscribers - and in the places of the main switching stations of this network. Switching nodes of the top level of the data network are related to a branched line of lines.

3.3.1.5. Synchronization of data terminal equipment

According to ICTT recommendations regarding subscriber dataset equipment junctions when connecting to a network of data of synchronous terminal equipment (see Section 1.1.3), the network should provide for each ODD clock sync signal and mutual synchronism on the elements between the transmitting and receiving ADD. In asynchronous network switching networks, where the internal network clock synchronization is missing, this requirement is performed by installing in PP or VP of those subscribers who have synchronous OOD synchronous clock generators. These generators form the transmission clock signals and after establishing the connection are isolated from the data clock sync signals received from the opposite side. The synchronism achieved in this way is individual for each compound and is saved only at that time until this compound exists.

3.3.1.6. Independence of the transmission from the sequence of bits in asynchronous networks

Transmission between synchronous terminal installations should not depend on the type of transmitted bit sequence. In asynchronous networks, the required independence can be provided with the help of scramblers (see Section 2.2.1.1, 2.2.2.2). According to this method, the signals coming from the ODD in the data transfer phase are scrambled (their bits are mixed) in PP or VP on the transmitting side. In the PP or VP on the receiving side, the signals are restored in their original form with the help of descrambler.

Before the start of the transmission of PP or VP, includes a scrambler and after the expiration of the time that needs to be descrambler on the opposite side to enter synchronism, it applies to the ODO signal allowing the transmission. From this point on, the scrambler ensures the change of symbols in the switch sent to the switching unit, even if the OOD gives the long sequence of identical characters. This prevents the possibility of accidental separation against the desire of subscribers, since the long sequence of zeros, which could be accepted for the aback signal, does not appear.

If you really need to disconnect the connection, then the PP or VP, controlled through the joint from the OOD, turn off the scrambler and sent a long sequence of zeros into the communication line. If within a certain time interval, the switching unit received only the characters "0", in a row following each other, then it displays the connection.

Transmission can be made independent of the symbol sequence (bits) and in another way: to the sequence of bits issued by the ODA, according to a specific rule using PP or VP to enter additional bits. However, this method leads to an increase in the transmission rate (see section 3.3.2.5) and therefore, in asynchronous networks with switches, the channels limits the freedom in choosing the type of ADF.