ISDN - Integrated Services Digital Network - is the modern generation of the worldwide telephone network. Because ISDN uses digital technology, it can carry any type of information, including high quality voice transmission and fast and accurate data transfer from user to user.

Firstly, what is ISDN in everyday life?
Essentially, it's just a PHONE, but with much greater capabilities. Moreover, these possibilities are determined not only by the model and “bells and whistles” of your telephone, but also by the telephone network itself, that is, the “stuffing” of the station. The ISDN network retains the most main opportunity telephone - the ability to pick up the handset and dial a number. But after this the fun begins.
A regular telephone is connected to a telephone exchange using a pair of conductors. In this case, only one can be carried out per pair. phone conversation. At the same time, noise, interference, radio, and extraneous voices are heard in the handset - the disadvantages of analog telephone communication, which “collects” all the interference in its path.
ISDN telephony is a completely different matter. The subscriber is assigned a network termination. All subscriber devices are connected to it, and the network end is connected to the station. The communication channel from subscriber to subscriber is completely digital, so you can hear the fly buzzing on the other side of the line. But good audibility is not the most important thing.
The main advantage of the ISDN network is the ability to conduct two independent telephone conversations simultaneously over one telephone pair (or one is talking and the other is on the INTERNET)! You can connect up to eight devices to one network end, and each one can have its own landline number.

What can an ISDN network do for your business?
First of all, it can help in fully installing telephones in your office. You install an ISDN line instead of a regular telephone line. Then you buy an adapter with which you can connect from two to eight ordinary analog devices, for example a telephone, fax, modem (including INTERNET), answering machine. Your office is fully telephoned and has two entrance lines. It has become much easier to reach your company, and this makes your business more competitive! And this is just one of the options for using the wide capabilities provided by the ISDN network.
In ISDN, just like in analogue phones, there are additional types of services, only they are much broader. In addition to various types of redirection due to busyness, no answer, unconditional call transfer, notification and call hold, etc., there are specific services we will tell you about. Don’t forget that all ISDN services are free - this is also a significant advantage over analog phones.
Calling number identification- this is the definition caller numbers, the so-called Caller ID.
Connected number identification- this is the identification of the number that actually answered you. In the city, many people use the “Redirection” service. By installing such a service, you can identify such a person, where on what phone he is actually located.
Payment information at the end of the call(only calls within the city, you are the initiator of the connection) - by subscribing to this service, you will be able to see the actual number of kopecks for your last conversation and the total amount. With this service, you can really plan your money and control your employees in the office so that there is no abuse.

ISDN technology appeared quite a long time ago - almost 20 years ago. The underlying specifications are contained in Recommendations I.122 of the International Telephony and Telegraph Consultative Committee (the present name of this committee is the International Telecommunication Union). Later, recommendations I.430 and I.431 appeared for the physical layer of the ISO model; Q.921/I.441 for the logical link control layer; Q.931/I.451 and DSS1 for network layer and a number of others. Thanks to the efforts of ETSI (European Telecommunications Standards Institute), EuroISDN is becoming the de facto standard in Europe, which is supported by most European telecommunications providers and equipment manufacturers. In Russia, work is also underway to standardize and ensure compatibility of ISDN networks being built in different regions. For this purpose, a pilot zone for testing ISDN technology was created several years ago and is now expanding, including a number of large cities in Russia.
ISDN Core
The basis of ISDN is the digitalized telephone network, i.e. network based on digital telephone channels 64 kbit/s. Therefore, ISDN is essentially a circuit-switched network, but packet-switched data transmission is also possible in it. The ISDN network allows the use of existing copper cables in the subscriber network. Connections from subscriber to subscriber pass along a continuous digital path. ISDN services are based on two standards:
"Basic Rate Interface (BRI) - two B-channels 64 kbit/s and one D-channel 16 kbit/s
"Primary Rate Interface (PRI) - 30 B-channels 64 kbit/s and one D-channel 64 kbit/s
Typically, BRI bandwidth is 144 Kbps. When working with PRI, the entire digital communication backbone (DS1) is fully used, which gives a throughput of 2 Mbit/s. The PRI D channel capacity is 64 Kbps. The high speeds offered by ISDN make it ideal for a wide range of modern communications services, including high-speed data transmission, screen sharing, video conferencing, large files for multimedia, desktop video telephony and Internet access are just some examples. These services are already successfully used by businesses - large and small - and are also used by residential subscribers.
PRA alarm
PRA signaling is mainly used to connect PBXs of enterprises, banks, etc. To do this, you need to have cards that support E-DSS1 Q931 signaling at both ends in the station. In the station where you connect, a PABX will be created with the main GDN-general directory number and your numbering thresholds DDIRANGE-direct dialing-in range. The implementation of some services will require their installation at the PABX reference station. For example, CGLIP “Calling Line Identification” or charging, it can go either to the GDN number of your PABX or to all numbers individually. This is usually asked when your own call recording system as part of the PBX has not been purchased. Local traffic is charged to the GDN number. Many people ask QUESTIONS: What is the difference between R1.5 alarm and PRA? What will we get from this?
- PRA signaling is one of the ISDN accesses; having ISDN subscribers in your station, you will give them access to the ISDN network and receive the full range of ISDN services on the PSTN network.
- These are bidirectional channels that process all types of traffic, be it urban or intercity, etc. Here the problem with the lack of channels has been solved - there is potential (for example, 3 streams with R1.5 or 2 PRA streams with all the delights.) For streams with R1.5, it is necessary to create 4 bundles - 2 city (incoming & outgoing) and 2 intercity (ZSL & SLM). And remember the holidays, critical days are individual for each enterprise when the load is maximum and the lines are overloaded, in this regard, PRA remains a winner....
- Fast D-channel signaling guarantees very short call setup times.
- High quality is ensured by digital transmission of voice and data from subscriber to subscriber. Speech, data, images and video can be encoded by the user terminal and transmitted to digital form error-free over a completely digital network.
ISDN network numbering
Very often, clients at the service center ask the question, what number will I have? For ISDN, the numbering of the existing telephone network is used. In addition to the ISDN subscriber number, it is possible to transmit an ISDN subaddress. The ISDN subaddress is used for qualified addressing internal devices user selected using the ISDN subaddress. To assign subaddresses to ISDN terminals, the user must subscribe to the "Subaddressing" service. If you want to receive multiple numbers on one ISDN line, you must subscribe to additional numbers. The numbers will be provided to you at the service center; in addition to the main number, you can have an additional 7 numbers, but only 2 of them can talk. Payment for communication services will be made to the main number
Alarm system N7
An integral part of the ISDN network is the N7 signaling system. The signaling system used in ISDN is a common channel signaling system. Its principle is as follows. All linear and control signals (called subscriber number, acceptance confirmation given number, information about the subscriber’s availability “busy” or “free”, disconnection with a specific type of failure, etc.), are packaged in special data packets and provided with identifiers of talk channels, outgoing and incoming stations, as well as service information. These data packets (called signal units) are transmitted over a separate signaling channel. Only with this type of signaling can ISDN networks be interconnected. All stations with ISDN ports in the city and region are connected to each other using N7 signaling. Communication with ISDN subscribers of the directions that are placed in the article “List of open directions and countries” is also carried out using N7 signaling. Moreover, in each direction there is an alternative path, which may not always be with N7 signaling. If the calling and called subscribers are ISDN subscribers and the process of establishing a connection between them does not require switching to other signaling systems, then these subscribers can receive the full range of ISDN services. If one of the subscribers is a regular analog subscriber or a transition to another signaling system is required, then many ISDN services become unavailable.

ISDN Basics

As mentioned above, ISDN technology has conceptual differences from the principles used in analog telephony. What are these differences? Basic distinctive feature ISDN networks differ from a conventional analogue telephone network in that ISDN stations provide switching of digital rather than analogue streams. It should be noted that in Lately Many analogue telephone exchanges have appeared using digital switching of analogue signals. Unlike such stations, ISDN switches switch digital streams. Conversion of analog signals into digital ones 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, “without knowing” what exactly is in this moment transmitted over the channel.

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 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 use of several fundamentally different technologies simultaneously within ISDN. various types connecting lines, or interfaces. 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.

Increased load on communication lines associated with intensification information flows and the expansion of the range of problems assigned to telephone communications, led to the need for a choice: either to 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 to 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 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 turned on of this subscriber to the database on the ISDN station), displaying the number and name of the caller on the display screen of the 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; while ensuring parallel connection 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 amount 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.

ISDN (Integrated Services Digital Network) digital networks are widely used as an alternative to connecting via T1/E1 channels. The difference lies mainly in the payment method. A fixed (quite high) fee is charged for a full (or part) of a T1 channel. subscription fee. In ISDN networks, payment is charged only for the connection time.

ISDN technology allows simultaneous transmission of voice and digital data and provides high-speed connection to global networks. This technology was developed in order to meet the integral needs of a small office.

Similar to T1 channels, this technology is based on the use of a 64 Kbit/s digital channel. Analog (voice) data is pre-sampled (sampling) 8000 times per second. Each sample represents 8 bits of information. That is, PCM is used.

B-channel.

The core component of any ISDN line is the 64 Kbps B-channel. It can transmit digitized audio or video data or actual digital data.

D-channel.

Used to transmit service information. These are, for example, connection establishment and connection termination signals. The entire B-channel band is intended only for transmitting useful information.

There are two standard ISDN channel configurations: BRI and PRI/

BRI interface.

This is a logical combination of two B-channels of 64 Kbps and one D-channel with a bandwidth of 16 Kbps. BRI (Basic Rate Interface) – transmission interface with rated speed.

The BRI interface is the optimal configuration for remote users and small offices. Its total throughput is 128 Kbps, and the D-channel is used only for transmitting service information. BRI allows you to connect up to 8 devices (telephone, digital and video).

For D-channel exchange, the SS7 (Signalling System Number 7) protocol is used.

PRI interface.

PRI (Primary Rate Interface) – transmission interface with basic speed. This interface corresponds to the maximum transmission speed on the T1 line. The PRI configuration consists of 23 64 Kbps channels (B-channels) and one 64 Kbps D-channel. Therefore, the user can transmit at a speed of 1.472 Mbps.

In European ISDN lines, the PRI configuration corresponds to 30 B-channels (since E1 contains exactly that many channels for transmitting useful information).

User connection.

Figure 5.5 shows a typical hardware configuration of an ISDN subscriber complex.

The NT1 (Network Terminator 1) device is used to connect a subscriber to a digital channel.

The NT2 (Network Terminator 2) device occupies an intermediate level between NT1 and any terminal equipment. These can be ISDN network routers and digital office PBXs.

The terminal device of the first type TE1 (Terminal Equipment 1) is considered to be user equipment that is able to connect to devices of the NT type. These are, for example, ISDN workstations, a fax machine, ISDN telephones. The second type of terminal devices TE2 (Terminal Equipment 2) includes all equipment that cannot be directly connected to NT2 (analog telephones, PCs, etc.), but requires for this application a special terminal adapter TA (Terminal Adapter).

Set out below.

Briefly answer what ISDN is - it is a multi-channel telephone with high quality speech and additional services (for example, calling number identification). There are 2 types of ISDN interfaces - BRI and PRI. For corporate users The most widely used interface is the PRI interface (other names are E1, E1 stream, ISDN PRI EDSS1, EuroISDN - essentially they are all the same thing). Let's look at some features of ISDN PRI

• The first feature is to imagine a telephone line consisting of 30 channels. This means that 30 people can talk on it at the same time. These 30 conversations can be either incoming or outgoing.
• The second feature is that there can be any number of actual city numbers in a stream - from one (common for everyone) to... how many you agree with your operator.
• If there is only one number in the stream, then everything is more or less simple - there is one number, there are 30 channels, and if 30 people dial this number at the same time, then they will all reach you (how can you receive all these 30 calls and not lose them? Another question is the question of proper configuration of your mini-PBX and knowledge of its capabilities for processing incoming calls. ISDN PRI streams are ideal for Call centers). It is not necessary that all 30 calls must be incoming; outgoing channels can also work, so if 10 of your employees call somewhere, then only 20 channels will remain for incoming calls. If all 30 channels are busy, the 31st caller will hear a busy signal. If at least one channel is free, then you can be reached on it.
• If there is more than one landline number in the ISDN PRI stream, then it works as follows: each of these city numbers is also multi-channel, but as many people can call it AT THE SAME TIME as there are currently free channels (out of a total of 30 channels). The channels may currently be occupied both for entry via other city lines and for exit by your employees.
• If desired, of course, you can limit both the number of channels per input for each number, and the number of channels that can be assigned to the output (so that there are free channels at the entrance, and you could be reached by phone). But in the general case, channel distribution occurs dynamically and depends on the flow load at a given specific point in time. By the way, you can purchase from the operator not a whole stream, but a reduced stream, say for 15 channels (the remaining 15 in it will be closed, but they can be opened in the future). As a rule, connection and subscription fees for such “cut-down” streams are cheaper.
• The third feature is high quality speech in the ISDN channel and additional services, the most popular of which is caller ID, which can be seen on the displays of system phones, or regular phones that support the Caller ID function (to display numbers on regular phones with Caller ID, you also need so that your PBX supports the function of transmitting Caller ID to analog ports)
• The fourth feature is that ISDN PRI is not suitable for connecting regular phone. ISDN PRI connects to a special card in a mini-PBX (or other telecommunications equipment). ISDN PRI lines support almost all modern digital PBXs. The following models support ISDN PRI:

• Panasonic - KX-TDA100/200, KX-TDA600, KX-TDE100, KX-TDE200, KX-TDE600, KX-NCP500, KX-NCP1000
• Samsung - OS7200, OS7400, OS7100, OS500
• LG-Nortel - ipLDK-60, ipLDK-100, ipLDK-300
• Alcatel - OmniPCX Office

For the ISDN BRI interface, all of the above is true, except that the number of channels for this interface is not 30, but only 2, i.e. It is possible to have 2 conversations at the same time on one ISDN BRI line. For ISDN BRI, there are special models of household ISDN phones, which are not widespread in our country, with the exception of certain regions where operators actively provide this service.

All modern digital PBXs, along with ISDN PRI, also support the ISDN BRI interface (see list above), and as a rule, PBX boards contain several BRI ports, which allows you to connect several BRI lines to the PBX. In some cases, it may be easier and cheaper to negotiate with the operator to connect BRI lines rather than a PRI stream.

In the last few years, around the world there has been a noticeable increase in interest in geographically distributed networks that allow the transfer of various types of information. Experts predict further growth of the global market for integrated telecommunications services by approximately 30 percent or more per year.

Modern telecommunications networks use a wide variety of technologies and protocols. Analog communication systems are becoming less and less responsive to the requirements of the times, although due to their availability they are still quite widely used for telephony and low-speed data transmission, in particular via the X.25 protocol. Higher transmission speeds are available for dedicated digital channels communications built on the basis of copper, optical fiber, wireless and satellite communication channels. But their construction and rent are much more expensive. Very promising networks with asynchronous transmission mode (ATM) are being developed, making it possible to transmit any type of traffic with maximum efficiency and scale the bandwidth. Frame relay network services are actually available, including in a number of Russian cities, usually based on leased lines and supporting multipoint topologies. Frame relay networks can be used to transmit various types of traffic, including delay-sensitive ones. In a number of countries, primarily in the United States, the introduction of technologies for high-speed integrated data transmission over networks has begun cable television(CTV) and regular telephone wires(xDSL). Technologies such as SMDS (Synchronous Multimegabit Digital Service - multipoint data transmission based on cell switching) and B-ISDN (Broadband ISDN) are being developed. These technologies are very promising, but are still poorly accessible and expensive.

The number of digital networks with integrated services (ISDN - Integrated Services Digital Network) is growing all over the world. They are based on “mature” technology and are created partly on the basis of equipment and channels of existing telephone networks public network (PSTN).

ATM, frame relay and ISDN networks are beginning to be used in Russia. Moreover, through the efforts of a number of Russian operators, joint ventures and foreign companies building overlay networks based on the latest SDH and ATM technologies, conditions are being created for the implementation of qualitatively new opportunities in the field of telecommunications.

When building geographically distributed networks of companies and connecting to the main local computer network(LAN) of remote branches and mobile users, LAN administrators and information technology specialists are faced with the difficult task of optimally choosing an information transfer standard. This article discusses the practical aspects of using ISDN channels to build integrated information networks and provides examples of solving specific problems.

A brief excursion into history

ISDN technology appeared quite a long time ago - almost 20 years ago. The underlying specifications are contained in Recommendations I.122 of the International Telephony and Telegraph Consultative Committee (the present name of this committee is the International Telecommunication Union). Later, recommendations I.430 and I.431 appeared for the physical layer of the ISO model; Q.921/I.441 for the logical link control layer; Q.931/I.451 and DSS1 for the network layer and a number of others.

During the initial period of ISDN development, a large number of national ISDN types were introduced, developed in the laboratories of large telecommunications companies and often incompatible with each other. In the 80s This technology, for a number of reasons, in particular due to compatibility problems and the high cost of equipment, developed very sluggishly. But in the early 90s. practical interest in it has grown significantly. A significant number of ISDN lines were installed in Germany, the USA, Japan, France, and England. Thus, according to the research company Dataquest, in the USA in 1995 the number of ISDN lines increased by 80% and amounted to 450 thousand. Approximately the same trend exists in Europe, where more than 5 million ISDN lines are currently installed.

Thanks to the efforts of ETSI (European Telecommunications Standards Institute), EuroISDN is becoming the de facto standard in Europe, which is supported by most European telecommunications providers and equipment manufacturers. In Russia, work is also underway to standardize and ensure compatibility of ISDN networks being built in different regions. For this purpose, a pilot zone for testing ISDN technology was created several years ago and is now expanding, including a number of large cities in Russia.

Application areas of ISDN networks

Standard connection of ISDN lines is carried out via BRI (Basic Rate Interface) or PRI (Primary Rate Interface) interfaces. The first of them (some operators call it ISDN2 - based on the number of B-channels) provides two duplex B-channels of 64 Kbps each (in America, Japan and Canada, the transmission speed over the B-channel is 56 Kbps). Moreover, the telephone company almost always uses copper cable from the public switched telephone network (PSTN) as an ISDN BRI line, thereby reducing the final cost of the ISDN line. Each B-channel is assigned a number similar to a telephone number.

When connecting large organizations to provide more high speeds transmission or for simultaneous connection to the central office of several remote branches, a PRI interface is used (sometimes called ISDN30). In Europe, its total capacity is 2.048 Mbit/s, it contains 30 B-channels for information transmission and a special D-channel with a capacity of 64 Kbit/s. In addition, PRI is often used to connect private branch exchanges to the digital telephone network. Many carriers provide PRIs with as many B-channels as the customer requires, such as four or six.

Digital networks with integration of ISDN services can be used to solve a wide class of information transfer tasks in the following areas:

telephony;

data transfer;

merging remote LANs;

access to global computer networks (Internet);

transmission of delay-sensitive traffic (video, audio);

integration of various types of traffic.

The terminal device of the ISDN network can be a digital telephone, a separate computer with an installed ISDN adapter, a file or specialized server, a bridge or LAN router, a terminal adapter with voice interfaces (for connecting a regular analog telephone or fax) or with serial interfaces (for data transfer). ).

ISDN telephony

The first applications of ISDN were implemented in the field of telephony. To do this, ISDN service support is built into digital telephone exchanges, and digital telephone sets (ISDN terminals) are used as end devices. They allow you to exchange voice and text messages, support audio conferences of several subscribers, dial a number almost instantly (within 1 second), provide high quality voice transmission and have a number of additional functions.

In ISDN networks there is a special service called Centrex. With its help, companies that do not have an office PBX can provide users with a wide range of services, such as creating subscriber groups, call forwarding, line identification, multi-subscriber conferences, internal abbreviated numbering, etc.

But ISDN telephone sets are quite expensive, although cheaper devices have now begun to appear. In addition, when switching to ISDN digital lines, the user usually already has analog telephone equipment - telephones, modems, fax machines. And the best option is to use this equipment in conjunction with digital channels and ISDN service.

For this purpose, a specialized device is used - a terminal adapter, called an A/B adapter, which supports the functions of a mini-PBX. An analog telephone (fax, answering machine) is connected to it. Such devices are available with two, four or more analog ports and with one or more BRI ports. You can combine multiple devices to increase the number of supported ports. If you have an A/B adapter, you can get access through the ISDN BRI line to the public telephone network or build your own overlay telephone network inside the ISDN network using analog terminal equipment. These adapters allow you to perform a number of additional functions, which are carried out using a set of special command codes using the telephone buttons:

assigning a telephone number to each analog port;

CallBack - automatic call back by caller number;

On-Hold - switching between several active lines;

Call Forwarding - switching incoming calls to another device;

changing the signal level in each analog channel;

speed dialing;

holding a conference with several participants.

Internet access

With the development of global information Internet networks it increasingly uses multimedia and computer graphics, video and sound broadcasting. To reproduce information, graphical shells running under Windows and allowing access to Web servers, FTP archives and other Internet services are increasingly being used. Many organizations advertise their products and services via the Internet, develop e-commerce systems, and even build corporate networks (intranets) based on Internet technology and the TCP/IP protocol.

ISDN networks can largely solve Internet access problems. There are three options for connecting individual computers and LANs to the Internet. To access the Internet for single users, you can use ISDN BRI adapters, which are installed in a standard PC bus slot (ISA, PCI or PC-Card). Point-to-Point Protocol (PPP) is usually used to communicate with the provider, and PAP and CHAP protocols are used to authenticate users entering the network. In addition, many manufacturers support MultiLink PPP, which allows you to combine two B-channels into one logical channel during operation.

In addition to internal adapters, there are external terminal adapters (TAs) or an external ISDN modem that are designed to convert the serial interface of a PC, conventional bridge/router or other non-ISDN device to the ISDN BRI format. Some telephones also have an analog port to support telephone/fax. In this case, a computer is connected to the asynchronous TA port, which provides access to the Internet, and a regular telephone or fax is connected through the analog port to another B-channel.

To connect a LAN to the Internet, a router is usually used, which allows you to separate internal and external IP networks and performs the functions of a firewall. The routers themselves can be implemented in software on NetWare (IntranetWare), Windows NT or Unix servers. The use of active or passive adapters is also necessary. The active adapter is based on a processor with its own RAM and is focused on running communications software. It allows you to use significantly less CPU resources file server. A passive ISDN adapter is similar to a regular network adapter and uses the server's CPU resources. Typically, active adapters are more expensive than passive ones, but they are also more productive. Another option is to use a hardware router, which is designed as a separate device and has one or more LAN ports and one or more WAN ports.

Some companies are starting to produce devices with built-in intelligence. They allow the software in the device ROM to independently determine the type of ISDN supported by the operator's switch and set other ISDN parameters. The user only needs to enter the number (or numbers) to establish a connection with the provider.

Video conferencing

Many organizations have geographically distant divisions located in other cities or countries. Employees of these branches have to periodically go on business trips to meet with management, colleagues, customers and suppliers. This requires a lot of time and material costs. With the development of ISDN networks, there is a real opportunity to replace travel with video conferencing sessions and, thus, not only save time and money, but also significantly increase the efficiency of decision-making. You can hold meetings of specialists located in different parts of the globe, communicate “face to face” with bank clients, conduct remote presentations and training, view medical images and much more. As a result, work that previously took several days can be completed in a matter of minutes.

Desktop video conferencing applications from many manufacturers allow you to exchange video and audio information while simultaneously displaying graphs and tables. They are equipped with tools for collaborative document editing and file transfer. A number of companies provide multipoint video conferencing that allows multiple subscribers to communicate simultaneously.

Inexpensive PC-based video conferencing hardware includes dedicated video and audio encoding/decoding cards, an ISDN adapter that typically supports BRI, and external video cameras, microphones, or telephones. There are purely software implementations, but they usually have lower performance. The basis for ISDN video conferencing is the ITU-T H.320 standard, which includes a whole set of recommendations for encoding (compression) of audio (G.711, G.722, G.728), video (H.261), and channel multiplexing (H.221) and a number of others.

As noted above, there are bi- and multilateral conferences. The latter require the use of additional equipment, namely the Multipoint Conferencing Unit (MCU). These devices are produced by a small number of manufacturers and are quite expensive.

Merging remote LANs

Today, many organizations with several remote branches require quick access to corporate information resources, such as databases. In addition, companies and banks must provide their remote departments and “mobile” employees in other cities or even countries with high-quality telephone, fax and video conferencing, as well as access to e-mail. Often the channels providing these methods of communication must not be permanent, but switched when information is available for transmission. In this case, the optimal solution - both in terms of functionality and cost - may be the use of ISDN networks. They provide functions such as on-demand communications, on-demand bandwidth (combining multiple B-channels into one logical channel), data compression in the channel, information security, and allow you to implement a wide variety of solutions to the problems of organizing communication with branches.

Thus, to connect remote LANs based on ISDN, you can use permanent channels and on-demand channels. In the first case, there is a permanent connection between offices - without taking into account the volume of transmitted information. In the second case, the physical connection is broken in the absence of packets, but the logical connection remains and information about the remote LAN is stored in the device. When information appears that needs to be transferred to a remote LAN, the device automatically dials the number and establishes a physical connection within 1 second.

Operators can provide ISDN channels with time-based (for the time the channel is used) and fixed fees. The choice of one or another payment system depends on how much time during the day this channel will be used. When using the channel for more than four to five hours a day, it is more profitable to rent a line with a fixed monthly fee.

To combine LANs based on an ISDN network, active or passive ISDN adapters are usually used as access devices, which are installed in a file server, dedicated router or regular workstation. You also need software like NetWare Multiprotocol Router for ISDN, or similar for Windows NT or Unix. Such adapters are manufactured by several companies, for example AVM Computersysteme (Germany), Eicon (Canada) and Teles (Germany).

Another solution is to use hardware bridges or routers, designed as stand-alone devices that connect to the LAN. They come in a variety of capacities - from the simplest to the most powerful modular ones with support for a variety of protocols (up to ATM) - and are produced, in particular, by 3COM, NewBridge/ACC, Bay Networks, Cisco, Gandalf and Shiva.

But a situation may arise when it is necessary to connect two large LANs to provide online access to central databases for a large number of workstations at the same time, or when inefficient software is used in terms of network traffic. If it is not possible to increase the number of BRI channels, the problem is solved using software or hardware that implements the technology remote control. In this case, a remote access server (RAS), for example WinFrame from Citrix, is installed in the central LAN. Two remote LANs are connected using routers on dedicated computers or using special stand-alone bridges/routers. In this case, each remote machine is connected in remote control mode to the corresponding virtual station on the RAS. Over the ISDN line, only screen updates are transmitted to the remote computer, and only control commands from the keyboard or mouse are transmitted back. This makes it possible to more efficiently share an ISDN line among a large number of users.

Another access server solution is offered by Cubix. The ERS/FT II chassis has 2 power supplies operating in load sharing mode, a remote control system, and status indicators for power supplies and fans. Provides “hot” replacement of modules without stopping the entire system. This hardware solution allows you to combine up to 30 specialized PCs. At the same time, server and client modules of the remote control programs ReachOut, PCAnywhere or Carbon Copy run on PCs installed in the Cubix chassis and remote stations. In addition, unified management of this entire complex is carried out from the workplace of the drug administrator. This solution is also effective when regular analog lines or other types of communication channels are used along with ISDN. In addition to the remote access server, highly reliable specialized servers can be built on the basis of the Cubix chassis: WWW, FAX, E-Mail, file and disk subsystems.

When organizing communications between several remote LANs, it is often necessary to ensure increased connection reliability. Many organizations use ISDN circuits as backup for communication lines, such as frame relay or leased physical lines. Many vendors of equipment, particularly bridges or routers with multiple ports, build into it support for automatic switching from the main line to the backup line in the event of failure of the first line. Some companies use switched PSTN channels as backup lines, but this leads to a significant loss in operating speed.

Homework/small office

With the development of home working (telecommuting) and the expansion of the SOHO (small and home office) sector, the speed of access to information is of particular relevance. Attracting interest in this area is due to the fact that it allows one to significantly reduce office space and increase the time that an employee can devote to work, reduce congestion on transport routes and, as a result, environmental pollution from automobile exhaust gases.

Many companies employ mobile users who usually work outside the office. These could be journalists, insurance and sales agents, etc. The mobile category may include persons who use, for communication sessions with a corporate LAN, a laptop computer connected through one of the network stations or their own adapter, an internal PC-card fax modem with access to a dial-up line, cellular network or ISDN channel.

The ability to remotely configure equipment, download software, monitor and collect statistics is currently becoming increasingly important. Almost all manufacturers build these functions into their equipment and thus greatly simplify corporate network management.

In all these cases, ISDN lines can be used. Particularly attractive is the possibility of providing a channel on demand: when the channel is not in use, the payment counter is turned off.

Equipment for SOHO is produced by a large number of manufacturers. Particularly interesting are the solutions from those companies that offer inexpensive stand-alone bridges/routers with an Ethernet port (for connecting small LANs) and an additional analog port or ports (for telephone/fax). In addition, some companies offer equipment with the ability to compress transmitted information.

ISDN in Russia

Until recently, digital networks with integrated services were more often referred to as offerings from foreign providers. Nevertheless, in some Russian cities, ISDN infrastructure has already been created and continues to develop. So far, ISDN service can be obtained mainly only in large cities - Moscow, St. Petersburg, Novgorod, Nizhny Novgorod, Perm. Digital telephone exchanges, which, in principle, can support or support ISDN service, operate in approximately 80 cities of Russia. To coordinate their work and solve compatibility problems, work is underway to create a nationwide Russian public digital network based on common ITU standards and protocols, in particular OKC7 (SS7) general-channel signaling.

In Moscow, the first operator to provide ISDN services was the Russian-British company Comstar, which has been offering connections to ISDN channels since 1994. The Comstar company installed in Moscow a modern telephone switching system with ISDN support "SystemX" from the English company GPT, which complies international standards. The infrastructure of the Comstar digital network is based on high-speed fiber-optic lines, the length of which increases every year and which cover more and more new areas. To improve reliability, SDH fiber channels connecting ISDN hubs are looped. Subscribers are connected to concentrators via both BRI and PRI interfaces (see sidebar "Basic concepts and ISDN standards"). In addition, Comstar is very active in providing access to global networks, in particular the Internet, via 64 or 128 Kbps dial-up channels. New Comstar solutions and new terminal equipment are tested by Step Logic specialists.

In addition to Comstar, several other operators provide ISDN services in Moscow, including Sovintel and Combellga. In the spring of 1997, Telmos, a joint venture between MGTS and Lucent Technology, began providing ISDN services. Telmos owns its own fiber-optic communication network, which uses 5ESS digital telephone exchanges manufactured by AT&T, supporting various network protocols for data and voice transmission, voice mail, virtual PBX(Centrex). The Telmos backbone network is based on SDH technology and, to increase reliability, is implemented in the form of several rings.

By 1996, digital ISDN equipment was installed in more than half of Moscow telephone exchanges. The current trend is that the number of operating concentrators is doubling every year. Moscow is not inferior to the USA in terms of accessibility for an ISDN line user.

Despite the fact that there are problems with laying new highways, modernizing telephone exchanges, financing and compatibility, the total number of modern networks, including ISDN, in Russia is steadily growing. And although today these networks are not available everywhere and only a small circle of clients can actually work with them, we can assume that in a few years the situation will change dramatically.

ISDN Basics

Channel "B" (Bearer) - channel for transmitting voice, data, video with a capacity of 64 Kbps. It is provided "clean", i.e. its entire bandwidth is available for information transmission, and calls, alarms and other system information are transmitted over the D-channel.

Channel "D" (Delta) - service channel for transmitting control signals with a capacity of 16 (BRI) or 64 (PRI) Kbit/s. One “D” type channel serves 2 or 30 (Europe) B-channels and provides the ability to quickly generate and drop calls, as well as transmit information about incoming calls, including the number of the subscriber accessing the network. Some operators and telecommunications equipment manufacturers support the transmission of additional information via the D-channel, for example, organizing an X.25 channel or a data stream with telemetry information at a transmission rate of up to 9.6 Kbps. But this expansion of channel capabilities does not meet the standard.

BRI (Basic Rate Interface) - standard basic interface with a bandwidth of 144 Kbps (EuroISDN); it combines two "B" channels and one "D" channel. Up to eight different ISDN devices can be connected to the BRI interface. In this case, each device is allocated its own individual number (multiple subscriber numbers). A very important feature of ISDN is that to install a BRI socket, the operator usually does not need to install a new telephone pair - a regular PSTN line is used.

The physical layer of the BRI interface, which defines the rules of interaction between end users and the ISDN switch, is a regular twisted pair cable that operates in full-duplex data transmission mode - the so-called U-interface. Inside buildings, a cable of two twisted pairs is used - an S/T interface, which allows you to connect up to eight ISDN terminal devices. Therefore, to connect internal wiring to an external line, an NT1 device is required (one for each BRI interface).

PRI (Primary Rate Interface) - this interface combines several B-channels (for example, in Europe - 30 B-channels with a total bandwidth of 2.048 Mbit/s). Unlike BRI, it only supports one endpoint. But by connecting, for example, a local PBX or a router with ISDN support, you can split the PRI into many BRI interfaces. Currently, to provide PRI services to offices, a subscriber digital line on one (SDSL) or two (HDSL) telephone pairs is widely used.

SS7 (OX7) - Common channel signaling system number 7. It was developed and standardized by the CCITT Committee (ITU) to increase the capabilities of voice and data integration, effective use of computer systems in telephony, fast connection setup and high-quality call routing, use of unified information databases, integration and complete compatibility of various types of communications (telephony, cellular communications, data transfer) regardless of country or region and, as a result, obtaining a qualitatively new level of service. SS7 covers the three lower levels of the seven-level ISO information network model and consists of two subsystems. The Message Transfer Part (MTP) is responsible for transmitting signaling messages, performs error detection and correction functions, and a number of additional functions. UP (User Part) - a higher-level subsystem - is responsible for user support and includes the ISUP (Integrated Services User Part) responsible for ISDN networks, the TUP (Telephone User Part) responsible for telephony, and a number of others.

In Russia, ISDN has been adopted as the basic standard for creating a nationwide system for implementing OKC7, integrated services networks and mobile networks.

Increasing the efficiency of using ISDN

When connecting remote LANs, accessing a corporate LAN, the Internet or interactive services via ISDN channels, a time-based connection is often used. In this case, the greatest interest is in equipment that allows compression of transmitted data and, consequently, reducing the time of line use per unit of transmitted information. Compression of transmitted data provides additional protection, reducing the likelihood of information being decrypted in the event of an unauthorized connection to the line.

The compression ratio strongly depends on the type of data transmitted over the line. Pre-archived information is the worst to compress. Databases and files containing graphic information are compressed well. Compression algorithms for LAN protocol packet headers are also used. The average compression ratio is 4:1. The leaders in data compression are router XpressConnect 5250i and brouter XpressConnect 5242i from Gandalf (Canada). This equipment allows you to obtain a data compression ratio of up to 8:1.

An important means of ensuring efficient use of the line is the establishment of a connection on demand (Connect on demand) - only for the duration of the data transfer session. Upon completion, the physical connection is terminated. The use of on-demand communication channels allows you to access the network or, conversely, interrupt communication depending on specified conditions or events that have occurred in the network.

Many equipment manufacturers support the spoofing function. A large number of service packets are transmitted over networks, which are exchanged between servers, routers, and workstations. Most of these packages contain information that changes rarely. With the spoofing function, service packets are transmitted over the trunk channel only once, and responses to requests are automatically generated at the end nodes, without cluttering the communication line with additional information. However, this function needs careful configuration.

The protocol filtering function allows you to restrict the passage of certain protocols through the trunk line or change the priority. MAC address filtering allows you to restrict access from certain workstations in remote network and thus reduce traffic.

Typically bridges or routers have a table of telephone numbers (ISDN). This allows, for example, to schedule the connection to each office at a specific time or day of the week. This connection setup is suitable for working with few applications. The important thing is that you can completely prohibit or limit outside access to the company’s LAN on weekends or holidays.

An important function is to establish bandwidth on demand (Bandwidth on demand). If the bandwidth of one B-channel is exceeded, a second one is automatically connected. To increase the throughput of the PPP protocol, which is usually used to connect to the Internet, the Multilink PPP (MPPP) standard was developed. It allows you to combine several B-channels and create one logical channel with increased bandwidth.

From analogue to digital networks with integrated services

Traditional PSTNs use analog equipment. They are designed primarily for voice and data transmission at low speeds. Typically, only one device, such as a telephone or modem, can be used on one line at a time.

More modern are PSTN with digital trunk interchanges (DTI - Digital Trunk Interface), which began to be deployed in the 70s. for more efficient use of channels. The switches themselves remained analog for a long time, since digital equipment was expensive.

Later, digital telephone exchanges appeared. To organize internal telephone networks of enterprises, corporate digital PBXs are used, which improve the quality of voice and data transmission and expand the service for subscribers. But they are primarily connected to the PBX via analog lines, so many of the benefits of digital PBXs are lost. Typically, the data transmission speed over such lines, sometimes called voice-frequency (VoF) lines, does not exceed 28.8 Kbps. Recently, so-called PCM modems have appeared that support speeds of up to 56 Kbps in one direction (x2, K56flex standards etc.) over PM lines using complex signal processing algorithms, but such speeds can only be obtained on high-quality lines connected to digital telephone exchanges. The quality of communication and the speed of information transmission over PM channels depend on many factors and are often completely unpredictable.

Digital channels, such as ISDN networks, can, at least partially, improve the current situation. By using them, you can raise telephone service to a completely new qualitative level, significantly increasing the speed of information transfer, its reliability and security.

Some advantages of ISDN networks compared to PSTN

A completely digital network that ensures high reliability of information transmission.

High speed of transmission of integrated information of various nature.

Wide range of telephony functions, high sound quality.

Fast dialing (less than 1 s).

Wide availability and distribution in the world.

Disadvantages of ISDN networks

Problems of compatibility of ISDN equipment from different suppliers.

The difficulty of upgrading central switches and building a new digital infrastructure.

Difficulty ordering a service.

The need for large financial investments.

Valery Volobuev, NETWORK #04/97 Material taken from the site:

ISDN digital switched lines (ISDN networks).

ISDN networks (Integrated Services Digital Network, digital networks with integrated services) were conceived as digital networks that replace telephone networks and provide high-quality and fast transmission of voice and computer data.

ISDN networks provide many services, including dedicated and switched digital data and voice channels, a packet-switched data network (similar to the X.25 network), and Frame Relay network services.

Currently, most of the ISDN network services are not in demand: Frame Relay networks have been allocated to independent networks, ISDN packet-switched networks do not provide the high speed and quality of service guarantees that ATM networks do. Therefore, ISDN networks are basically used in the same way as analogue telephone networks, but only as faster and more reliable.

If PDH and SONET/SDH technologies are used to create dedicated digital lines, then ISDN technology, in addition, allows you to organize dial-up digital line, with a data transfer rate of up to 128 Kbps (2-wire end), or up to 1.544 or 2.048 Mbps (4-wire end, T1/E1 line speed). An ISDN subscriber can establish a connection with another subscriber that resembles a telephone number.

An ISDN address consists of two parts: subscriber numbers(up to 15 decimal digits: country code, city code, subscriber number, which corresponds to the point of connection of the subscriber’s network to the ISDN network) and subscriber addresses(subnumber specific device in the subscriber's network).

To use ISDN network services, special equipment CPE (Customer Premises Equipment) must be installed at the user's premises, which consists of a network termination NT (Network Termination) and terminal equipment TE (Terminal Equipment). It is the NT network end that receives the subscriber's number, and the TE terminal equipment (computer, router or telephone with an ISDN interface) receives the subscriber's address.

The main problem of connecting a remote user to the Internet or to an enterprise network is the “last mile” problem. In the most global network High-speed SONET/SDH lines can be used, but the user typically connected to the network using a conventional analog modem, which limits the data exchange rate to 33.6 - 56 Kbps, regardless of how “fast” the network itself is.

Installing T1/E1 or ISDN equipment with a 4-wire termination at each user's home is too expensive and technically difficult, and ISDN equipment with a 2-wire termination does not provide a sufficiently high access speed (only 128 Kbps). Most users would like to have cheap and fast digital Internet access using a standard 2-wire telephone line and simple device modem type.

The devices listed below solve the “last mile” problem using special modems: SDSL, Rate Adaptive DSL (VDSL), Asymmetric Digital Subscriber Line (ADSL). Among these, ADSL technology is the most widely used.

ADSL technology is designed for high-speed data transmission over a short section of twisted pair cable connecting the subscriber to the nearest telephone exchange.

While conventional analog modems are designed to operate over a telephone network with an arbitrary number of telephone switches between the client and the provider, ADSL modems can only connect directly to the router at the telephone exchange, without going through telephone switches (thanks to this, high data transfer rates can be achieved on a short section of twisted pair cable between users and the router) (Fig. 29).

Rice. 29. ADSL technology.

If telephone exchange provides ADSL services, then the ATC building must have an Internet router (in the figure it is designated as “R”), which is connected to other routers using high-speed channels. ADSL equipment connects directly to the router. The voice channel is allocated by ADSL equipment and sent to the telephone exchange switch.

One of the main advantages of ADSL technology, compared to analog modems, ISDN and T1/E1, is that voice and data transmissions occur in parallel and do not affect each other in any way.

ADSL modems, connected to both ends of a short line between the subscriber and the ATC, form 3 channels: a fast data transfer channel from the network to a computer, a less fast duplex data transfer channel from a computer to the network, and a simple channel telephone communication, which carries regular telephone conversations.

Data transmission in the “network-to-subscriber” channel occurs at a speed from 1.5 to 6 Mbit/s, in the “subscriber-to-network” channel - at a speed from 16 Kbit/s to 1 Mbit/s. In both cases, the specific data transfer rate depends on the length and quality of the line.

The asymmetric nature of the data transfer rate is introduced specifically because remote user The Internet or corporate network usually downloads data from the network to your computer, and in the opposite direction there is either an acknowledgment of data receipt or a data stream of significantly lower speed.

In addition to subscriber terminations of telephone networks, cable television subscriber terminations have recently begun to be used for high-speed access to the Internet.

For these purposes, a special type of modem has already been developed - cable modems. Cable modems use the existing 75-ohm coaxial tv cable for transferring data from a network to a computer at a speed of 30 Mbit/s, and from a computer to a network at a speed of up to 10 Mbit/s. At the same time, the quality of the transmitted signals is very high.

High-speed subscriber terminations create an additional problem for Internet service providers - they need to have very high-speed access channels to the rest of the Internet, since 10 subscribers with 8 Mbit/s traffic create a total traffic of 80 Mbit/s, which can only be transmitted qualitatively using SONET/SDH or ATM technologies.