Digital phone isdn. ISDN technology. ISDN Basics

History of the name

The name was proposed by the XI CCITT group in 1981.

Purpose

The main purpose of ISDN is data transmission at speeds of up to 64 kbit/s over a subscriber wire line and the provision of integrated telecommunications services (telephone, fax, etc.). Using telephone wires for this purpose has two advantages: they already exist and can be used to supply power to the terminal equipment.

The choice of the 64 kbit/s standard is determined by the following considerations. With a frequency band of 4 kHz, according to Kotelnikov's theorem, the sampling frequency must be at least 8 kHz. The minimum number of bits to represent the gating results of a voice signal, assuming a logarithmic transformation, is 8. Thus, multiplying these numbers (8 kHz * 8 (bits) = 64) results in an ISDN B-channel bandwidth of 64 kb /With. The basic channel configuration is 2 × B + D = 2 × 64 + 16 = 144 kbit/s. In addition to the B-channels and the auxiliary D-channel, ISDN can offer other channels with higher capacity: the H0 channel with a bandwidth of 384 kbit/s, H11 - 1536 kbit/s and H12 - 1920 kbit/s (real bit rate). For primary channels (1544 and 2048 kbit/s), the D-channel bandwidth can be 64 kbit/s.

Principle of operation

To combine different types of traffic in an ISDN network, TDM technology is used. Time Division Multiplexing, time multiplexing). For each type of data, a separate band is allocated, called elementary channel(or standard channel). This band is guaranteed a fixed, negotiated share of bandwidth. The band is allocated after a signal is given CALL via a separate channel called off-channel signaling channel.

ISDN standards define the basic channel types from which various user interfaces are formed.

In most cases, type channels are used B And D.

Interfaces are formed from these types of channels; the following types are most widespread:

Basic level interface

Basic level interface(English) Basic Rate Interface, BRI ) - provides two B-channels and one D-channel for communication between the subscriber’s equipment and the ISDN station. The base level interface is described by the formula 2B+D. In standard BRI mode, both B-channels (for example, one for data, one for voice) or one of them can be used simultaneously. When channels operate simultaneously, they can provide connection to different subscribers. The maximum data transfer rate for the BRI interface is 128 kb/s. The D channel is used only to transmit control information. In AO/DI (Always On/Dynamic ISDN) mode, the 9.6 kbit/s D-channel band is used as an always-on dedicated X.25 channel, usually connected to the Internet. If necessary, the bandwidth used to access the Internet is expanded by including one or two B-channels. This mode, although standardized (under the name X.31), has not found widespread use. For incoming BRI connections, up to 7 addresses (numbers) are supported, which can be assigned by different ISDN devices sharing one subscriber line. Additionally, a compatibility mode with conventional, analogue subscriber devices is provided - ISDN subscriber equipment, as a rule, allows the connection of such devices and allows them to operate in a transparent manner. An interesting side effect of this “pseudo-analog” mode of operation was the possibility of implementing a symmetrical X2 modem protocol ( English) from US Robotics, which allowed data transmission over an ISDN line in both directions at a speed of 56 kbit/s.

The most common type of alarm is DSS1. Digital Subscriber System No. 1 ), also known as Euro-ISDN. There are two trunk modes of BRI ports relative to the station or telephones - S/TE and NT. S/TE mode - the port emulates the operation of an ISDN telephone, NT mode - emulates the operation of a station. A separate addition is the use of an ISDN phone with additional power in this mode, since not all ports (and HFC cards) provide power via an ISDN loop as standard. inline power). Each of the two modes can be point-to-multipoint. point-to-multi-point, PTMP) aka MSN (eng. Multiple Subscriber Number ), or "point-to-point" (eng. point-to-point, PTP).
In the first mode To search for the destination recipient on the loop, MSN numbers are used, which, as a rule, coincide with the city numbers allocated by the telephony provider. The provider must report the MSNs it transmits. Sometimes the provider uses so-called “technical numbers” - intermediate MSNs.
In the second mode BRI ports can be combined into a trunk - a conditional highway through which transmitted numbers can be used in multi-channel mode.

ISDN technology uses three main types of BRI interface: U, S and T.

U - one twisted pair, laid from the switch to the subscriber, operating in full or half duplex. Only 1 device can be connected to the U-interface, called the network end. Network Termination, NT-1 or NT-2).
S/T interface (S0). Two twisted pairs are used, transmitting and receiving. Can be crimped into both RJ-45 and RJ-11 socket/cable. Up to 8 ISDN devices - telephones, modems, faxes, called TE1 (Terminal Equipment 1) - can be connected to the S/T interface socket using one cable (loop) using the bus principle. Each device listens to requests on the bus and responds to the MSN associated with it. The operating principle is very similar to SCSI.
NT-1, NT-2 - Network Termination, network termination. Converts one U pair into one (NT-1) or two (NT-2) 2-pair S/T interfaces (with separate pairs for reception and transmission). In fact, S and T are the same interfaces, the difference is that the S interface can supply power to TE devices, phones for example, but not the T interface. Most NT-1 and NT-2 converters can do both, which is why the interfaces are most often called S/T.

Primary Interface

(Primary Rate Interface, PRI) - used to connect to broadband highways connecting local and central telephone exchanges or network switches. The primary level interface combines:
for the standard (common in Europe) 30 B channels and one D channel 30B+D. PRI elementary circuits can be used for both data transmission and digitized telephone signal transmission.
for the T1 standard (common in North America and Japan, as well as in DECT technology) 23 B channels and one D channel 23B+D.

Primary Interface(English Primary Rate Interface, PRI) - a standard ISDN network interface that defines the discipline for connecting ISDN stations to broadband highways connecting local and central telephone exchanges or network switches. The primary level interface combines 23 B-channels and one D-channel for the T1 standard (23B + D=24*64=1536) or 30 B-channels for voice or data, one D-channel for signaling and one H-channel for service E1 standard data (30B + D + H=32*64=2048).

ISDN Network Architecture

An ISDN network consists of the following components:

network terminal devices (NT, English) Network Terminal Devices)
line terminal devices (LT, English) Line Terminal Equipment)
terminal adapters (TA) Terminal adapters)
User terminals

Subscriber terminals provide users with access to network services. There are two types of terminals: TE1 (specialized ISDN terminals), TE2 (non-specialized terminals). TE1 provides direct connection to the ISDN network, TE2 requires the use of terminal adapters (TA).

Signalmen ( as a joke) decipher the abbreviation ISDN How I t S till D oes N whatever ( It still doesn't do anything), thereby hinting that out of more than 230 basic ISDN functions, only a very small part of them is actually used ( really in demand by consumers).

Sources

Alexander Filimonov - Building multiservice Ethernet networks, bhv, 2007 ISBN 978-5-9775-0007-4

Literature

Bocker P. ISDN. Digital communications with integration of services. Concepts, methods, systems. Translation with him. M.: Radio and communication, 1991.

ISDN networks and technologies. ISDN networks (Integrated Services Digital Network) belong to a class of networks originally designed to transmit both data and voice. These are networks that provide a digital connection between network end subscribers to provide a wide range of services that users access through a limited number of standard multifunctional interfaces.

ISDN networks use digital technology, which is becoming increasingly widespread because:

The digital devices used in ISDN are manufactured using highly integrated circuits; Compared to analog devices, they are highly reliable and stable in operation and, in addition, are usually cheaper to manufacture and operate;

Digital technology can be used to transmit any information over one channel (acoustic signals, television video data, fax data);

Digital methods overcome many of the transmission limitations

and storage, which are inherent in analog technologies.

In ISDN networks, when transmitting an analog signal, it is converted into a sequence of digital values, and when received, it is converted inversely.

An analog signal appears as a constant change in amplitude over time. For example, when talking on the telephone, which acts as a converter of acoustic signals into electrical signals, mechanical vibrations of air (alternating high and low pressure) are converted into an electrical signal with the same amplitude envelope characteristic. However, the direct transmission of an analog electrical signal over a telephone communication line is associated with a number of disadvantages: signal distortion due to its nonlinearity, which is increased by amplifiers, signal attenuation during transmission through the medium, exposure to noise in the channel, etc.

With ISDN, these disadvantages can be overcome. Here, the analog signal waveform is represented as digital (binary) images, digital values representing the corresponding amplitude values of the sinusoidal oscillation envelope at points, at discrete levels. Digital signals are also subject to attenuation and noise as they pass through the channel, but at the receiving point it is necessary to note only the presence or absence of a binary digital pulse, and not its absolute value, which is important in the case of an analog signal. Therefore, digital signals are received more reliably and can be fully restored before they fall below the threshold due to attenuation.

User equipment is connected to the ISDN network at one of two standard speeds. The first of these is the “base” speed ( BRI- Basic Rate Interface), and the second is “primary” (PRI - Primary Rate Interface). When transmitting information via BRI, three logical subchannels are created in the channel, two of which are called IN -channels are designed to transmit “useful” user information (in particular, voice). Each of IN -channels require a bandwidth of 64 Kbps. The third subchannel, called D -channel, requires the same bandwidth and is used primarily for transmitting service information, which determines the order of processing information transmitted over B -channels. Sometimes D-channel is used to transmit useful information; its bandwidth is 16 Kbit/s. Therefore, the total bandwidth, i.e. The transmission speed corresponding to the BRI interface is 144 Kbps.

The PRI channel has its own specifics in different countries. In the USA, Canada and Japan it consists of two B -channels and one D -channel, each of them has a throughput of 64 Kbps, and the total throughput PRI-channel is equal to 1536 Kbit/s (including service information). In Europe, the PRI channel occupies 1920 Kbps of bandwidth.

The large channel bandwidth required to build ISDM networks is the main obstacle to their spread, especially in countries with poorly developed infrastructure for high-speed communication channels. However, there are mechanisms that make it possible to build such networks using the bandwidth of communication channels more economically. One of these mechanisms allows compaction B -channels used for voice transmission. In this case, a coding technique is implemented (converting acoustic signals into a digital code), called pulse-code modulation (PCM). Currently, voice coding technology has advanced greatly, providing quite acceptable quality of voice communication with much less bandwidth (in one practical case, the voice information transmitted over each of the B-channels is compressed and transmitted at a speed of 6.33 Kbps) .

Conversion of analog signals to digital is carried out using various methods. One of them - pulse code modulation (ICM), proposed in 1938 by A.Kh. Reeves (USA). When using PCM, the conversion process includes three stages: display, quantization and encoding (Fig. 13.3).

Rice. 13.3. Converting an analog signal to an 8-element digital code

First stage based on theory display Nyquist. The basic premise of this theory is that "if an analog signal is displayed at a regular interval with a frequency of at least twice the maximum frequency of the original signal in the channel, then the display will contain sufficient information to reconstruct the original signal." When transmitting acoustic signals (speech), the electrical signals representing them in the telephone channel occupy a frequency band from 300 to 3300 Hz. Therefore, ISDN adopts a display frequency of 8000 times per second. Mappings, each of which is called pulse amplitude modulation signal (IAM), are remembered and then transformed into binary images.

On quantization stage Each IAM signal is assigned a quantized value corresponding to the nearest quantization level. The entire range of changes in the amplitude of IAM signals is divided into 128 or 256 quantization levels. The more quantization levels, the more accurately the amplitude of the IAM signal is represented by the quantized level.

On coding stage Each quantized display is associated with a 7-bit (if the number of quantization levels is 128) or 8-bit (with 256-step quantization) binary code. In Fig. Figure 13.3 shows signals of an 8-element binary code 00101011, corresponding to a quantum signal with a level of 43. When encoding with 7-element codes, the data transmission rate over the channel should be 56 Kbit/s (this is the product of the display frequency and the bit depth of the binary code), and when encoding 8- element codes - 64 Kbit/s.

Modern ISDN also uses another concept for converting analog signals into digital ones, in which it is not the IAM signals themselves that are quantized and then encoded, but only their changes, and the number of quantization levels is assumed to be the same. Obviously, this concept allows signal conversion with greater accuracy.

Example 13.4. How many simultaneous conversations Np can be provided via a multi-channel communication line in a digital communication network if the following are specified:

V ls = 1.536 Mbit/s - total capacity of the communication line;

V from = 8000 display/s - display speed of analog signals when converting them to digital;

p e = 8-bit binary code representing one display on a communication line?

The total capacity of the communication line is determined by the formula

Where n kc- number of channels in a multichannel communication line.

Since all channels can carry on a conversation simultaneously and independently, then N p = N kc .

Hence, From here

According to experts, the development of ISDN networks and technologies is facilitated by the following factors: liberalization and privatization in the field of telecommunications (this leads to the emergence of new competitors and new network products); the convergence of information technology, telecommunications and the entertainment industry (this has a positive impact on the development of cable television, satellite communications and radio access, while the task of ensuring the comprehensive provision of communication services comes first); development of the Internet; the continuous growth of mobile networks (these networks are growing much faster than fixed networks, and there is a redistribution of traffic from fixed networks to mobile networks). The different state of these factors, which act as driving forces for the development of ISDN networks, leads to differences in strategic and tactical approaches to their implementation in different countries.

The sharp increase in the role of ISDN networks is explained by the fact that they provide integrated access to voice and non-voice services, have an established infrastructure, are digital networks based on the use of 64 Kbit/s digital channels, and have sufficient flexibility. The popularity of the ISDN network is increasing because, by definition, it is multi-service (provides communication services, information delivery, as well as additional services), application-oriented. The term "application" refers to a specific area of ISDN application (for example, distance learning), and the term "solution" is used to explain how the application is implemented using ISDN (distance learning is carried out using a video conferencing service).

ISDN technology is developing steadily, and a network based on it has the necessary interfaces with non-ISDN networks. In addition, there is a large selection of terminal equipment for ISDN networks.

ISDN terminal equipment is divided into the following groups: digital telephones, terminal adapters for PCs, video communication equipment.

Basic means of accessing the ISDN network: routers or bridges of local networks, terminal network devices for basic and primary access for fiber-optic lines and copper communication lines, multiplexers (for collecting and transmitting information from remote subscribers), video conferencing systems, mini-PBXs (managerial automatic telephone exchanges).

Digital PBXs with ISDN functions allow you to: more fully use communication channels for data and speech transmission, connect the subscriber to the ISDN network from various devices (telephone, fax, computer), simultaneously transmit speech and data (if the PBX has two-wire digital telephones with advanced functions and a port for connecting a PC), connect bridges or routers for interaction between remote LANs.

ISDN networks and technologies provide users with the following basic services: data transmission at a speed of 64 Kbit/s, digital speech transmission, teletext, fax, video communication. When using each of them, the subscriber can take advantage of such additional services as organizing closed user groups, organizing conference calls, providing the network with his number or refusing to provide it, etc.

Thus, ISDN networks, the main purpose of which was to combine digital telephone network traffic and computer data into one network, are now widely used to solve problems of information transfer in the following areas: telephony, data transmission, LAN interconnection, access to global computer networks. networks, integration of various types of traffic, transmission of delay-sensitive traffic (sound, video).

SDH networks and technologies. Networks of the SDH standard (Synchronous Digital Hierarchy) implement the technology of synchronous fiber-optic networks. These are high-speed digital communication networks that are built on the basis of fiber-optic cable lines or digital radio relay lines. The basis of the infrastructure of modern high-speed telecommunication networks (backbone, regional or urban) are digital lines and network nodes of the SDH standard.

When building SDH networks The following modules are used:

SDH multiplexers are the main functional modules of SDH networks, designed to assemble a high-speed information stream from low-speed streams and disassemble a high-speed stream into low-speed ones;

Switches provide linking of channels assigned to users by semi-permanent cross-connection between them;

Hubs are used to combine similar streams of several remote network nodes in one distributed node;

Regenerators are multiplexing devices with one optical access channel and one or two outputs used to increase the distance between SDH network nodes. SDH networks and technologies are distinguished by a high level of standardization (which allows the use of equipment from different manufacturers in one network), high reliability (centralized network management provides complete monitoring of the status of nodes), the presence of full software control (monitoring and recording of emergency situations, network configuration management is carried out software from a single management console), the ability to quickly provide services on demand, and a relatively simple network development scheme. Thanks to these advantages, SDH technology has become the main technology in the construction of digital transport networks of various sizes.

The topology of the entire SDH network is formed from separate basic topologies such as “ring”, “linear chain”, “star”, “point-to-point”, which are used as network segments. The radial-ring architecture of the SDH network, built on the basis of ring and linear topologies, is more often used.

In Russia, JSC Rostelecom is the most active in using SDH technology. This JSC annually builds 5-6 thousand km of backbone digital lines based on fiber-optic cables (FOCL) and digital radio relay lines. The RASCOM company built in 1994 and operates a high-speed digital fiber optic trunk line of the SDH standard between Moscow and St. Petersburg with a length of 690 km.

ISDN Basics

As mentioned above, ISDN technology has conceptual differences from the principles used in analog telephony. What are these differences? The main distinguishing feature of an ISDN network from a conventional analogue telephone network is that ISDN stations provide switching of digital, rather than analog, flows. It should be noted that recently many analogue telephone exchanges have appeared that use digital switching of analogue signals. Unlike such stations, ISDN switches switch digital streams. Conversion of analogue signals to digital occurs at the level of ISDN terminals (i.e., on end-user equipment), and therefore an ISDN station has the ability to switch homogeneous digital streams, “not knowing” what exactly is being transmitted over the channel at the moment .

The second feature of ISDN is the implementation of the principle of a single distributed telephone exchange. According to this principle, all stations within one ISDN network are logically combined into a single large station and can be considered by subscribers as an integral ISDN complex. Using this principle allows you to optimize the load on communication channels (for example, minimizing connection routes between subscribers), and also provides a number of services not accepted in analog telephony (for example, the introduction of a unified number plan). One cannot ignore such an important feature that distinguishes ISDN from analog networks as almost instantaneous connection establishment. The maximum delay in an ISDN network does not exceed 30 ms for each communication node.

The fourth distinctive feature of the new technology is the ability of ISDN stations to automatically route connections, which is especially important in cases where there are several alternative connection paths between stations and it is necessary to select the most optimal one. There are other types of switched digital lines, for example, Switched 56, which combines two channels and achieves the same throughput as two 64 Kbps ISDN data channels. However, Switched 56 has a number of significant differences from ISDN, in particular in the quantity (and quality) of services provided. Thus, Switched 56 lacks such types of service functions as determining the source identifier of an incoming signal, routing calls, almost instantly setting up a call using a service (16 Kbit/s) channel, etc.

ISDN interfaces: BRI and PRI

One of the main elements of any communication system is communication lines and the principles underlying their operation. As for ISDN, the historical realities of the development and implementation of this technology inevitably led to the simultaneous use of several fundamentally different types of trunk lines, or interfaces, within ISDN. First of all, this is due to the fact that the “change of milestones” in the history of telephony does not occur abruptly, but gradually, as if smoothly “flowing” from one (analog) state to another (digital). The advent of the ISDN era is happening painlessly for users of traditional telephone services, with the gradual displacement of the principles of analogue telephony. That is why a full-fledged digital PBX must support, in addition to specific ISDN interfaces, all types of connecting lines that exist in analog telephony. Initially, all telephone exchanges were analog, and communication between them (as well as between them and subscribers) was carried out through analog trunk lines.

The increase in load on communication lines, associated with the intensification of information flows and the expansion of the range of problems assigned to telephone communications, led to the need to choose: either increase cable capacity through an increase in the number of communication lines (which led to a significant increase in the cost of telephone services), or look for fundamentally new solutions. As a result, digital communication lines appeared - Digital Trunk Interface (DTI), which made it possible to increase the number of channels while maintaining or even reducing the number of connecting wires. The first ISDN stations, which appeared in the mid-seventies, were developed taking into account the ability to work with analog communication lines and DTI, so they did not introduce any dissonance into the telephone services industry. The vast majority of subscribers continued to use regular analog phones, and the ISDN station had to provide support for both digital ISDN terminals and regular analog phones. Further development of digital communication principles led to an increase in the number of ISDN stations, which, in turn, led to the need to create a specific ISDN interface that ensures communication between ISDN stations. At the same time, the physical compatibility of the new ISDN interface with DTI allows subscribers of ISDN stations, along with ISDN terminals, to continue to use analog telephones, modems and faxes.

ISDN networks use two specific types of interfaces: the basic level interface BRI (Basic Rate Interface), which regulates the connection between an ISDN station and a subscriber, and the primary level interface PRI (Primary Rate Interface), which provides communication between ISDN stations.

Logically, BRI is a specially structured digital stream, divided into three channels: two information channels of type B (bearer) with a capacity of 64 Kbps each and one service channel of type D with a capacity of 16 Kbps. That is why BRI has another name - 2B+D.

When using BRI as a link between an ISDN station and a digital telephone, digitized speech signals are transmitted via B-channels, while when organizing remote access to a PC and LAN or access to the Internet, B-channels are used for data exchange. In this case, two independent message streams can be transmitted over one BRI line - according to the number of B-channels. The D-channel, as mentioned above, performs service functions. The main functions include the following: transmission of service information (call signals, call route, numbers of the called and calling subscribers, etc.), simultaneous maintenance of several B-channels, monitoring the occupancy of B-channels, assigning a specific name to each subscriber ( when a given subscriber is included in the database on an ISDN station), displaying the number and name of the calling subscriber on the display screen of an ISDN terminal and much more.

U-cable - a regular two-wire cable used for analog phones

Network Termination (NT) is a small module required to negotiate ISDN client devices.

ST-cable - customer cabling, with the possibility of wiring to sockets.

Physically, BRI is implemented either as a U-interface or as an S/T interface. The U-interface is designed to work with remote users (up to 5 kilometers) and is a twisted pair cable. The functioning of the U-interface is based on the use of full-duplex mode, i.e. transmitting flow over a communication line in both directions simultaneously. Using the S/T interface, wiring is carried out inside a company office or apartment using a two-pair cable; This provides parallel connection of up to eight devices. To coordinate the U- and S/T interfaces, Network Terminator (NT1) network terminal blocks are usually used, because It was originally assumed that all ISDN devices, phones, etc. will only work with the S/T interface, but now devices are being produced that can work directly with the U-interface, since they have a built-in NT1 unit; in this case, there is no need for a stand-alone NT1 unit.

Many people ask the question: isn't it better to use a terminal device with a U-interface? There are several reasons to answer in the negative:

· First, at the S/T junction the area of responsibility of the telecom operator ends and, according to generally accepted European practice, the NT device is installed at the client’s premises by the operator;

· The second, important reason is that by connecting only one terminal device with a U-interface, you limit yourself in other services, now or in the future. For example, for data transfer you will only install a PC card with a U line interface and an a/b port for connecting an analogue telephone/fax. After a month, you discover that using a BRI line is ineffective and you want to install an additional device. But this turns out to be impossible, and your initial investment in a U interface device will be a mistake;

· The third reason is that such a device must perform NT functions and therefore cost more than an S/T terminal device.

Two types of devices can be connected to the S/T interface: terminal adapters (TA) and terminal equipment (TE1). The terminal adapters have a reference point R, through which you can connect to NT1 (and then to ISDN) equipment designed to transmit an analog signal or work with serial exchange and not provide a direct connection to ISDN: modems, fax machines, regular phones, routers .

ISDN stations, into which BRI interfaces flow, are interconnected by broadband highways that support the PRI primary level interface. Logically, PRI is built on the same principle as the BRI interface: a certain number of B-channels and one D-channel. In other words, PRI can be represented as the formula nB+D (23B+D in the USA and Japan, where the T-1 standard applies, and 30B+D in Europe, where the E-1 standard applies). It should be remembered that D-channels in PRI are 64 Kbps.

History of the name

The name was proposed by the XI CCITT group in 1981.

Purpose

Principle of operation

ISDN standards define the basic channel types from which various user interfaces are formed.

In most cases, type channels are used B And D.

Interfaces are formed from these types of channels; the following types are most widespread:

Basic level interface

ISDN technology uses three main types of BRI interface: U, S and T.

U - one twisted pair, laid from the switch to the subscriber, operating in full or half duplex. Only 1 device can be connected to the U-interface, called the network end. Network Termination, NT-1 or NT-2).
S/T interface (S0). Two twisted pairs are used, transmitting and receiving. Can be crimped into both RJ-45 and RJ-11 socket/cable. Up to 8 ISDN devices - telephones, modems, faxes, called TE1 (Terminal Equipment 1) - can be connected to the S/T interface socket using one cable (loop) using the bus principle. Each device listens to requests on the bus and responds to the MSN associated with it. The operating principle is very similar to SCSI.
NT-1, NT-2 - Network Termination, network termination. Converts one U pair into one (NT-1) or two (NT-2) 2-pair S/T interfaces (with separate pairs for reception and transmission). In fact, S and T are the same interfaces, the difference is that the S interface can supply power to TE devices, phones for example, but not the T interface. Most NT-1 and NT-2 converters can do both, which is why the interfaces are most often called S/T.

Primary Interface

ISDN Network Architecture

An ISDN network consists of the following components:

network terminal devices (NT, English) Network Terminal Devices)
line terminal devices (LT, English) Line Terminal Equipment)
terminal adapters (TA) Terminal adapters)
User terminals

Sources

Alexander Filimonov - Building multiservice Ethernet networks, bhv, 2007 ISBN 978-5-9775-0007-4

Literature

Bocker P. ISDN. Digital communications with integration of services. Concepts, methods, systems. Translation with him. M.: Radio and communication, 1991.

39. Technology ISDN (Narrowband integrated service network-( N - ISDN ), broadband integrated service network-( B - ISDN)).

Goals and history of technology creation ISDN

ISDN ( Integrated Services Digital Network- digital networks with integrated services) refer to networks in which the main switching mode is circuit switching and data is processed in digital form. The idea of moving from public telephone networks to fully digital data processing, in which the end subscriber transmits data directly in digital form, has been expressed for a long time. At first it was assumed that subscribers of this network would transmit only voice messages. Such networks are called IDN (Integrated Digital Network - integrated digital network). The term "integrated" referred to the integration of the network's digital information processing with the digital voice transmission of the subscriber. The idea of such a network was expressed back in 1959. Then it was decided that it should provide its subscribers not only with the opportunity to talk among themselves, but also to use other services - primarily the transfer of computer data. In addition, the network had to support various application level services for subscribers - fax, teletex (data transfer between two terminals), videotex (receiving data stored on the network to your terminal), voice mail and a number of others. The prerequisites for the creation of this kind of networks had developed by the mid-70s. By this time, T1 digital channels were already widely used for transmitting data in digital form between automatic telephone exchanges, and the first powerful digital switch of telephone channels 4 ESS was released by the company Western Electric in 1976.

As a result of work carried out to standardize integrated networks in CCITT , in 1980 a standard appeared G .705, which outlined the general ideas for such a network. Specific Network Specifications ISDN appeared in 1984 as a series of recommendations I . ISDN This set of specifications was incomplete and not suitable for building a complete network. In addition, in some cases it allowed for ambiguity of interpretation or was contradictory. As a result, although the equipment I and began to appear around the mid-80s, it was often incompatible, especially if produced in different countries. In 1988 series recommendations ISDN were revised and took on a much more detailed and complete look, although some ambiguities remained. In 1992 and 1993 standards

were once again revised and supplemented. The standardization process for this technology is ongoing. Implementation of ISDN networks ISDN began quite a long time ago - from the late 80s, however, the technical complexity of the user interface, the lack of uniform standards for many vital functions, as well as the need for large capital investments to re-equip telephone PBXs and communication channels led to the fact that the “incubation period” dragged on for many years, and now, when more than ten years have passed, the prevalence of networks ISDN leaves much to be desired. Moreover, in different countries the fate ISDN turned out differently. Before others on a national scale, these networks began operating in countries such as Germany and France. However, even in these countries the share of subscribers ISDN accounts for slightly more than 5% of the total number of telephone network subscribers. In the USA, the process of network implementation ISDN is far behind Europe, so the network industry has only recently noticed the presence of this kind of network. If you judge certain types of global networks by communication equipment for corporate networks, you may get the false impression that technology ISDN appeared somewhere in 1994-1995, since it was during these years that routers with support for the interface began to appear ISDN .

This circumstance simply reflects the fact that it was during these years that the network ISDN has become quite common in the USA, a country whose companies are leaders in the production of network equipment for corporate networks.

- Network architecture

- provides several types of services (Fig. 16.3):

- non-switched means (dedicated digital channels);

- public switched telephone network;

- circuit-switched data network; packet switching data network;

- data network with frame broadcasting (mode

frame relay); ISDN really cover a very wide range of services, including popular services frame relay .

In addition, much attention is paid to network control tools that allow you to route calls to establish a connection with a network subscriber, as well as monitor and manage the network. Network manageability is ensured by the intelligence of switches and end nodes of the network that support a stack of protocols, including special management protocols. ISDN Standards

also describe a number of application level services: fax communication at a speed of 64 kbit/s, telex communication at a speed of 9600 bit/s, videotex at a speed of 9600 bit/s and some others ISDN In practice, not all networks frame relay support all standard services. Service ISDN although it was developed within the network, however, it is implemented, as a rule, using a separate network of frame switches that does not intersect with the network of switches

ISDN. ISDN Basic network speed is the channel speed D.S.

-0, that is 64 kbit/s. This speed is based on the simplest method of voice coding - PCM, although differential coding allows you to transmit voice with the same quality at a speed of 32 or 16 kbit/s. ISDN One of the original ideas underlying ISDN , is the combined use of the principles of circuit switching and packet switching. However, a packet-switched network operating as part of

, performs service functions - signaling protocol messages are transmitted using this network. But the main information, that is, the voice itself, is still transmitted using a circuit-switched network. There is a clear logic in this separation of functions - subscriber call messages form pulsating traffic, so it is more efficient to transmit it over a packet-switched network. ISDN

User Interfaces ISDN One of the basic principles is to provide the user with a standard interface through which the user can request a variety of services from the network. This interface is formed between two types of equipment installed in the user's premises ( Customer PremisesEquipment, CPE): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end NT, however, it is implemented, as a rule, using a separate network of frame switches that does not intersect with the network of switches

, which is a device that terminates the communication channel with the nearest switch

- The user interface is based on three types of channels:

D B - with a data transfer rate of 64 kbit/s;

- H - with a data transfer rate of 384 kbit/s (H0), 1536 kbit/s (H11) or 1920 kbit/s (H12).

Type B channels carry user data (digitized voice, computer data, or a mixture of voice and data) at rates lower than 64 kbit/s. Data separation is done using technology TDM ISDN . In this case, the division of channel B into subchannels must be handled by the user equipment, the network always switches entire type B channels. Type B channels can connect users using circuit switching techniques with each other, and also form so-called semi-permanent (

semipermanent ) connections that are equivalent to leased circuit service connections. D A Type B link can also connect a user to an X.25 network switch. D Type D channel

is an access channel to a packet-switched service network carrying signaling information. Transmission of address information, on the basis of which type B channels are switched in network switches, is the main function of the channel

. ISDN Another function is to support low-speed packet-switched network services for user data. Typically this service is performed by the network at a time when channels such as

free from performing the main function. Type H channels provide users with high-speed data transfer capabilities. They can operate high-speed fax transmission services, video information, and high-quality audio playback. User interface

is a set of channels of a certain type and with certain speeds. ISDN networksupports two types of user interface - initial ( D Basic Rate Interface, BRI) and main (Primary Rate Interface, PRI). BRI Initial interface Data separation is done using technology BRI BRI provides the user with two 64 kbit/s channels for data transmission (type B channels) and one channel with a capacity of 16 kbit/s for transmitting control information (type B channel D . D Frame transmission lasts 250 ms, which provides a data rate of 64 kbit/s for channels B and 16 kbit/s for channel

. In addition to data bits, the frame contains service bits for frame synchronization, as well as ensuring a zero constant component of the electrical signal. BRI interface can support more than just scheme 2

B+D, but also B+D and just D(when the user sends only packetized data to the network).

The initial interface is standardized in recommendations I .430.Main interface PRI PRI Designed for users with increased network bandwidth requirements. Interface D supports either scheme 30 PRI B + D, or scheme 23 B + D

. PRI In both schemes the channel provides a speed of 64 kbit/s. The first option is intended for Europe, the second - for North America and Japan. Due to the great popularity of digital channel speeds of 2.048 Mbit/s in Europe and speeds of 1.544 Mbit/s in other regions, bring the interface standard it was not possible to reach the general version. PRI Interface options available D with fewer type B channels, for example 20 D B+D

. Type B channels can be combined into one logical high-speed channel with a total speed of up to 1920 kbit/s. When a user has multiple interfaces installed they can all have one channel type Type B channels can be combined into one logical high-speed channel with a total speed of up to 1920 kbit/s. When a user has multiple interfaces installed , while the number of channels B in the interface that does not have a channel, can increase to 24 or 31. The main interface can be based on H-channels. However, the total interface throughput should still not exceed 2.048 or 1.544 Mbit/s. 3 interfaces are possible for H0 channels, can increase to 24 or 31. H0+D D for the American version and 5

for European. For channels PRI H is the channel speed 1 an interface is possible consisting of only one H11 channel (1.536 Mbit/s) for the American version or one channel PRI 12 (1.920 Mbps) and one channel for the European version.

Interface Frames ISDN

have a personnel structure ISDN -1 for T1 or E1 channels. Main interface CCITT standardized in recommendations I.431. Connecting user equipment to the network

Connecting user equipment to the networkcarried out in accordance with the connection diagram developed (Fig. 16.4). The equipment is divided into functional groups, and depending on the group there are several reference points ( reference pointson the cable connecting the user equipment to the network ISDN. Actually N.T. 1 is a type device C.S.U. 1 is a type device , which operates at the physical layer and forms a duplex channel with the corresponding device ISDN installed on the territory of the network operator. Reference point): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end U): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end corresponds to the connection point of the device): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end 1 to the network. Device ISDN 1 may be owned by the network operator (although always installed on the user's premises) or may be owned by the user. In Europe it is generally accepted to consider the device): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end 1 part of the network equipment, so user equipment (for example, a router with an interface): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end ) is produced without a built-in device 1. In North America, the device is generally considered

1 UE accessory, so UEs are often supplied with a built-in device BRI NT 1. If the user is connected via the interface , then the digital subscriber end is made according to a 2-wire circuit (like the usual end of an analog telephone network). To organize the duplex mode, the technology of simultaneous issuance of potential code 2 by transmitters is used B 1 Q

with echo suppression and subtraction of your signal from the total. The maximum length of the subscriber end in this case is PRI 5.5 km. When using the interface

The digital subscriber end is carried out according to the T1 or E1 channel circuit, that is, it is 4-wire with a maximum length of about 1800 m.Functional group devices BRI NT 2 (Network Termination 2) D are channel or network level devices that perform the functions of concentrating user interfaces and multiplexing them. Data separation is done using technology For example, this type of equipment includes: office PBX (PBX), switching several interfaces, a router operating in packet switching mode (for example, over a link ), simple multiplexer 1. In North America, the device is generally considered

, which multiplexes several low-speed channels into one type B channel. Connection point for type B equipment NT 2 to NT device1 is called a Type T reference point. The presence of this type of equipment is not mandatory, unlike Devices of the TE1 functional group ( Terminal Equipment 1) S refer to devices that support a user interface ISDN .): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end Such equipment could be a digital telephone or a fax machine. Since the equipment is S 2 may be missing, then reference points and T are combined and denoted as

S/T. Devices of the TE2 functional group (Terminal Equipment 2) Devices of the TE1 functional group ( are devices that do not support the interface. Such a device can be a computer, a router with serial interfaces not related to ISDN ISDN, such as RS-232C, X.21 or V .35. To connect such a device to the networknessesary to use

terminal adapter ( Terminal Adapter, TA). For computers, terminal adapters are produced in the network adapter format - as a built-in card. Physical interface at the point S/T ISDN is a 4-wire line. Since the cable between the TE1 or TA devices and the network end BRI NT 1 or NT 2 is usually short in length, then standard developers We decided not to complicate the equipment, since organizing duplex mode on a 4-wire line is much easier than on a 2-wire line. For interface PRI The bipolar method was chosen as the encoding method AMI

, and a logical unit is encoded by a zero potential, and a logical zero by alternating potentials of opposite polarity. For interface PRI other codes are used, the same as for the T1 and E1 interfaces, that is, respectively B 8 ZS and HDB 3. Physical interface length

ranges from 100 to 1000 m depending on the device connection diagram (Fig. 16.5). The fact is that with a small number of terminals (TE1 or TE2+TA), it is allowed not to use a local office PBX, but to connect up to 8 devices to one device of the type NT 1 (or NT 2 without switching capabilities) according to the mounting OR circuit (the connection resembles connecting stations to a coaxial cable Ethernet): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end , but only in 4-wire version). When connecting one TE device (via terminal resistors R, matching line parameters) to the network termination B 8 ZS and HDB 3. (see Fig. 16.5, A) cable length can reach. When connecting several devices to a passive cable (see Fig. 16.5, b) the maximum cable length is reduced to 100- 200 m. True, if these devices are concentrated at the far end of the cable (the distance between them does not exceed 25- 50 m): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end 1, which provide a star-shaped connection of up to 8 devices, while the cable length increases to B 8 ZS and HDB 3. (Fig. 16.5, G).

Addressing in networks ISDN

ISDN technology was developed as the basis of a worldwide telecommunications network that allows connecting both telephone subscribers and subscribers of other global networks - computer, telex. Therefore, when developing a node addressing scheme ISDN it was necessary, firstly, to make this scheme capacious enough for worldwide addressing, and secondly, compatible with the addressing schemes of other networks, so that subscribers of these networks, in the event of connecting their networks through the network ISDN , could use familiar address formats. Stack developers TCP/IP took the path of introducing their own addressing system, independent of the addressing systems of the connected networks, and the developers of the technology ISDN took a different path - they decided to achieve use in the address ISDN addresses of the connected networks.

Main purpose ISDN - transmission of telephone traffic. Therefore, as a basis for the address ISDN the format of the international telephone number plan described in the standard was taken ITU-T E.163. ISDN However, this format has been expanded to support a larger number of subscribers and to accommodate addresses from other networks, such as X.25. Network addressing standard

received the number E.164. ISDN The E.163 format provides up to 12 decimal digits in the number, and the address format ISDN in the E.164 standard it is extended to 55 decimal digits. On networks differentiate And subscriber number subscriber's address. The subscriber number corresponds to the T point connecting all user equipment to the network. For example, an entire office PBX can be identified by one number ISDN. ISDN ISDN number S consists of 15 decimal digits and is divided, like a telephone number according to the E.163 standard, into the “Country Code” field (from 1 to 3 digits), the “Area Code” field and the “Subscriber Number” field. Address

includes the number plus up to 40 digits of the subaddress. The subaddress is used to number terminal devices behind the user interface, that is, connected to the point ISDN . ISDN For example, if an enterprise has an office PBX, then it can be assigned one number, for example 7-095-640-20-00, and to call a subscriber with subaddress 134, the external subscriber must dial the number 7-095-640-20-00 -134. ISDN , but to indicate that this is an X.121 address, it must be preceded by a prefix field in which the addressing standard code, in this case the X.121 standard, is placed. Network switches ISDN can process this address correctly and establish communication with the desired subscriber of the X.25 network through the network ISDN , either by switching a type B channel with an X.25 switch, or transmitting data over a type B channel D in packet switching mode. The prefix is described by the standard ISO 7498.

ISO standard 7498 defines a fairly complex address format, with the first two fields being the basis of the addressing scheme. Field AFI ( Authority and FormatIdentifier) defines the values of all other address fields and the format of these fields. Field value AFI

- is one of 6 types of subdomains of the global addressing domain: the four types of domains correspond to the four types of public telecommunications networks - packet switched networks, telex networks, public telephone networks and networks

- ISDN;

- the fifth type of domain is a geographic domain, which is assigned to each country (one country can have several geographic domains); The sixth type of domain is an organizational type domain, which includes international organizations, such as the UN or ATM

Forum. Following the AFI field is the IDI (Initial Domain Identifier) - initial domain identifier),and behind it there is an additional fieldDSP (Domain SpecificPart), which may carry additional digits of the subscriber number if the field width is

INI is missing. The following values are defined

- AFI. International packet-switched networks with an address structure in the X.121 standard are 36 if the address is specified only in decimal digits, and 37 if the address consists of arbitrary binary values. In this case the field INI has a format of 14 decimal digits, and the field DSP

- may contain another 24 digits. ISDN International networks with the address structure in the E.164 standard - 44 if the address is specified only in decimal digits, and 45 if the address consists of arbitrary binary values. In this case the field IDI has a format of 14 decimal digits, and the field has a format of 15 decimal digits, and the field

- may contain another 40 digits. International telephone networks PSTN with the address structure in the E.164 standard - 44 if the address is specified only in decimal digits, and 45 if the address consists of arbitrary binary values. In this case the field with the address structure in the E.163 standard - 42 if the address is specified only in decimal digits, and 43 if the address consists of arbitrary binary values. In this case the field has a format of 14 decimal digits, and the field has a format of 12 decimal digits, and the field

- may contain another 26 digits. International geographic domains with address structure in the standard ) - 38 if the address is specified only in decimal digits, and 39 if the address consists of arbitrary binary values. In this case the field International packet-switched networks with an address structure in the X.121 standard are 36 if the address is specified only in decimal digits, and 37 if the address consists of arbitrary binary values. In this case the field format is three decimal digits (country code), and the field has a format of 14 decimal digits, and the field may contain another 35 digits.

- Domain of international organizations. It's a one-byte field with the address structure in the E.164 standard - 44 if the address is specified only in decimal digits, and 45 if the address consists of arbitrary binary values. In this case the field contains the code of the international organization on which the field format depends DSP.

For the first four domains, the subscriber's address is placed directly in the field with the address structure in the E.164 standard - 44 if the address is specified only in decimal digits, and 45 if the address consists of arbitrary binary values. In this case the field . with the address structure in the E.164 standard - 44 if the address is specified only in decimal digits, and 45 if the address consists of arbitrary binary values. In this case the field For the fifth and sixth types of domains DSP.

contains only the country code or the code of the organization that controls the structure and numbering of the part ISDN Another way to call subscribers from other networks is to indicate in the address ISDN two addresses: addresses ISDN edge device, for example, connecting a network ISDN with the X.25 network, and node addresses in the X.25 network. Addresses must be separated by a special separator. Two addresses are used in two steps - network first ISDN establishes a dial-up connection with an edge device attached to the network

, and then passes it the second part of the address so that this device connects to the required subscriber. ISDN

Protocol stack and network structure On an ISDN network D There are two protocol stacks: a channel stack like

and a channel stack of type B (Fig. 16.6). Type D channels D form a fairly traditional packet switching network. The prototype of this network was the X.25 network technology. For channel network I three protocol levels are defined: the physical protocol is defined by the standard.430/431, channel protocol LAP-D defined by standard Q defined by standard .921, and at the network level the protocol can be used.430/431, channel protocol .931, with the help of which a call to a subscriber of a circuit-switched service is routed, or the X.25 protocol - in this case into protocol frames ISDN X.25 packets and switches are embedded

act as X.25 switches. Channel network type D within an ISDN network serves as a packet-switched transport network for the so-called alarm system number 7 ( Signal SystemNumber 7, SS 7). SS system 7 was developed for the purposes of internal monitoring and management of public telephone network switches. This system is also used online ISDN. SS Service 7 refers to the application level of the model OSI 7 network switches communicate only with each other. In Russia, a slightly modified version of this signaling system is used, called general channel signaling No. 7 (OCS 7). The term “common channel” means that signaling messages are transmitted over a dedicated service channel common to all user channels, and only user information is transmitted over the latter, and no other information.

Type B channels form a digital switched network. In terms of the model SS Service on type B channels in network switches ISDN only the physical layer protocol is defined - protocol I .430/431. D Switching of type B channels occurs according to instructions received through the channel defined by standard .

When protocol packets .931 are routed by the switch, then simultaneous switching of the next part of the composite channel from the original subscriber to the final one occurs. LAP-D protocol belongs to the family HDLC , which includes the protocol described in Chapter 7.430/431, channel protocol LLC 2. LAP Protocol - D): TE user terminal equipment (computer with appropriate adapter, router, telephone) and network end possesses all the generic features of this family, but also has some peculiarities. defined by standard Frame address.430/431, channel protocol consists of two bytes - one byte defines the code of the service to which packets embedded in the frame are sent, and the second is used to address one of the terminals, if the user has a network end 1 several terminals are connected. The terminal device can support different services - the protocol connection establishment service .931, X.25 packet switching service, network monitoring service, etc. Protocol

provides two operating modes: connection-oriented (the only operating mode of the protocol LLC ISDN 2) and without establishing a connection. The latter mode is used, for example, for network management and monitoring. defined by standard Protocol Q

.931 carries the address in its packets ISDN called subscriber, on the basis of which the switches are configured to support a composite channel of type B. The procedure for establishing a connection using the protocol

.931 is illustrated in Fig. 16.7. ISDN Using Services BRI for data transfer PRI - 2.048 Mbit/s. In addition, the quality of digital channels is much higher than analogue ones. This means that the percentage of distorted frames will be much lower, and the useful data exchange rate will be significantly higher.

Usually the interface BRI used in communications equipment to connect individual computers or small local networks, and the interface PRI - in routers designed for medium-sized networks.

As for the interconnection of computer networks to support packet-switched services, the capabilities of networks ISDN not too big.

On type B channels, packet switching mode is supported by a permanent or dial-up connection to an X.25 network switch. That is, type B channels in networks ISDN are transit only for access to the “real” X.25 network. Actually, this comes down to the first use case of the network ISDN - only as circuit-switched networks.

Development of technology for broadcasting frames on type B channels - technologies frame relay - led to the fact that the networks frame relay have become an independent type of network with its own infrastructure of channels and switches.

What remains is the packet switching service available over the link D . D Since after the address information is transmitted, the channel.430/431, channel protocol remains free, it can be used to transmit X.25 computer packets, since the protocol ISDN allows you to do this. Most often the network D is used not as a replacement for the X.25 network, but as an extensive access network to it, as less geographically widespread and highly specialized (Fig. 16.8). This service is usually called “access to the X.25 network via a channel type D "

X.25 network access speed over channel type usually does not exceed 9600 bps. ISDN ISDN networks ISDN are not considered by data network developers as a good means for creating a backbone. The main reason is the lack of high-speed packet switching service and the low speeds of the channels provided to end users. For the purpose of connecting mobile and home users, small branches and creating backup network communication channels