History of the development of subscriber access networks. Subscriber accesses in the ISDN network. Introduction to ISDN

Local access network provides communication between the telephone user and the local PBX. Regular telephone and ISDN subscribers use two wires or a regular local line, but business customers may require an optical fiber or microwave radio link, which have higher capacity. Many different technologies are used in a local access network to connect subscribers to a public telecommunications network. Figure 9.2 illustrates the structure of a local access network and shows the most important technologies in use. Most subscriber connections to the PBX use pairs of two copper wires. Subscriber cables contain many such pairs, which are protected on the outside by a common shield of aluminum foil and a plastic sheath. In urban environments, cables are laid in the ground and can be very large in capacity, including hundreds of pairs. Distribution boards, which are installed outside or inside buildings, are necessary to divide large cables into smaller ones and distribute subscriber pairs in buildings, as shown in Fig. 9.2. In suburban or rural areas, pole-mounted cables are often a more cost-effective solution than underground cables.

Rice. 9.2. Example of a local access network.

Optical communication is used when high (more than 2 Mbit/s) transmission speed is required, or very good quality transfers. Microwave radio is often a more cost-effective solution than optical fiber, especially when there is a need to replace an existing cable with another cable with higher capacity.

Installation of optical or copper cables takes longer because it requires permission from city authorities. Laying cables is very expensive, especially when they have to be buried in the ground.

One of the technologies for implementing subscriber lines is known as wireless radio access(WLL). This technology uses radio waves and does not require the installation of a subscriber cable; it is a fast and cheap way to connect a new subscriber to the public telephone network. With this technology, new operators can provide services in areas where the old operator has cables. Wireless radio access can also be used to replace old pole-mounted local lines in rural areas.

When the capacity of network cables (due to the connection of new subscribers) must be increased, it may be more economical to install hubs for remote subscribers, or subscriber multiplexers to use existing cables more efficiently. We use each of these terms to describe only one of the remote switching unit connectivity options.

Hub can switch local calls among multiple subscribers connected to it. A hub is essentially a part of a telephone exchange that is moved closer to distant subscribers. Digital transmission between the telephone exchange and the hub significantly improves the use of connecting cables, so that sometimes just a two-wire cable in a pair serves dozens of subscribers.

Subscriber multiplexers can connect each subscriber to an individual corridor (channel) in time in the PCM system. Detailed functionality systems depend on the manufacturer, but it can be said that only those subscribers who often pick up the handset economically use (save) the channel to the local telephone exchange.

We have explained the subscriber access alternatives shown in Fig. 9.2, mainly from the point of view of landline telephone service, but they can also be used to provide access to the Internet.

Local telephone exchange. Subscriber lines connect subscribers to local telephone exchanges, which occupy the lowest level in the hierarchy of switching centers. The main tasks of a digital local telephone exchange:

Detect the fact that a subscriber has picked up the phone, analyze the dialed number and determine whether the route is accessible.

Connect the subscriber to the connecting line leading from the PBX to the MTS for long-distance telephone calls.

Connect a subscriber to another subscriber of the same local telephone exchange.

Determine whether the subscriber is free by the dialed number and send a call signal to him.

Provide traffic measurements and collect statistical data about your subscribers.

Ensure the transition from a two-wire subscriber line to a four-wire line in a long-distance network.

Convert analog speech signal to digital signal(in a PCM transmission system).

The size of a local telephone exchange varies from hundreds of subscribers to

tens of thousands of subscribers or even more. A small local telephone exchange, sometimes called remote switching unit(RSU), performs switching and concentration functions in the same way as all local exchanges. The local telephone exchange reduces the transmission line capacity (number of voice channels) required for external communications, usually by a compression factor of 10 or more; that is, the number of local subscribers is approximately 10 times higher than the number of trunk lines (channels) from the local telephone exchange to external exchanges. Figure 9.2 shows just some of the different local exchange subscriber connections and the ways to physically establish them .

Main switchboard(GShP) - a structure that contains power and testing equipment for cutting the ends of incoming cables and conducting wire installation connecting the external and internal circuits of the station.

All subscriber lines are connected to the main switchboard - cross, which is located close to the local telephone exchange, as shown in Figure 9.3. This is a large structure with a huge number of wire connections. Subscriber pairs are connected to the switching field on one side, and the pairs from the local telephone exchange on the other. There is enough space inside the switching field for cross connections. Cables and connectors are usually placed in a logical manner so that the structure of the network of subscriber pairs and the network of connections can be seen. This fixed connection of cables remains the same for long periods of time, but the connections between the sides of the switching field change daily, for example, because the subscriber has moved to another house within range of the same exchange.

Cross connections in GSP usually made with twisted pairs, which allow data transfer rates of up to 2 Mbit/s. Regular subscriber pairs are used only for connections between analog telephones, analog and digital private branch exchanges, CSIO terminals and ADSL. ADSL telephone, and a regular analog telephone use a regular two-wire subscriber line to connect to the main switchboard. Data and voice can be used at the same time, they are separated in the telephone exchange, where the voice signal goes to a conventional analog exchange interface, and the data goes to the Internet, as shown in Fig. 9.3.

Digital telephone exchange may include both analog and digital subscriber interfaces. For digital private branch exchange ( automatic system switching that serves the institution) digital interfaces with a throughput of up to 2 Mbit/s are available.

If the local switch has the ability to work with ISDN, then the interfaces for the primary and main data rates are available to it.

Regular subscriber pairs are used to connect the ISDN with a basic transmission rate (160-kbit/s in two directions) to a network terminal (NT) located on the client's premises.

ISDN interface for primary data rate (2 Mbit/s) is used

for connecting a digital institutional (private) PBX. It requires two pairs of wires, one for each transmission direction, and supports many simultaneous external calls.

In addition to the main switchboard, network operators can use other switchboards to control and maintain transmission networks. The optical switchboard (OSCHP) contains two fields of fiber optic connectors. Optical network cables are connected to one field of connectors, to another field are connected to optical lines terminal devices. Cross connections between two connector fields are created by optical fibers. This allows maintenance personnel, for example, to replace a defective optical cable connection with a spare one.

Digital switchboard(TSCHP) - a cross-connection system to which digital interfaces from the line system and telephone exchange (or other network equipment) are connected. Using the DSP for the primary data transfer rate (2 Mbit/s), the operator can easily change the connections between the input and output sections of the equipment.

Rice. 9.3. Subscriber access network and local digital telephone exchange inputs .

The digital switchboard can be designed as digital equipment cross-connection (DCS), to which many high-speed data transmission systems are connected. The DSP is controlled remotely via the network management interface and the operator can change the cross-connect configuration using the network management system. Using the network management system, it can, for example, determine which 2-Mbit/s interface is connected to a specific 64-kbit/s time channel of another 2-Mbit/s interface.

Control questions:

1. Describe three options for transmitting data over telecommunications networks.

2. Identify the elements of the basic telecommunications network.

3. By what principle is the subscriber (local) access network organized?

4. Give examples of subscriber access networks.

Basic concepts of subscriber access network (SAD)

Basic concepts of subscriber access network

Subscriber access network (SAD)- is a collection technical means between terminal subscriber devices installed at the user’s premises and that switching equipment, the numbering (or addressing) plan of which includes terminals connected to the telecommunications system.

A model illustrating the main options for building a subscriber network is shown in Figure 1.1. This model is valid for both urban telephone networks (UTNs) and rural telephone networks (RTNs). Moreover, for the GTS, the model shown in Figure 1.1 is invariant to the structure of interstation communication. It is identical for:

Unzoned networks consisting of only one telephone exchange;

Regionalized networks, which consist of several regional automatic telephone exchanges (RATS), connected to each other on the principle of “each to each”;

Regionalized networks built with incoming message nodes (INOs) or with outgoing message nodes (UIS) and OMS.

Figure 1.1 - Main options for building a subscriber network

The model shown in Figure 1.1 can be considered universal with regard to the type of switching station. In principle, it is the same for both a manual telephone exchange and the most modern digital information distribution system. Moreover, this model invariant to the type of interactive network, for example, telephone or telegraph.

Main section of AL(Direct service area) - a section of the subscriber line from the linear side of the cross-connector or input-switching device of the local station, hub or other remote module to the distribution cabinet, including areas of inter-cabinet communication. The term “Main cable” corresponds to the main section of the AL. The backbone section is also considered to be a direct supply zone, within which distribution cabinets are not used to build a subscriber network. The direct supply zone occupies the area adjacent to the telephone exchange within a radius of approximately 500 meters.

AL distribution section- section of the subscriber line from the distribution cable cabinet to the subscriber point. This section of the AL - depending on the structure of the access network - corresponds to the terms "Primary distribution cable" and "Secondary distribution cable". And the part of the area occupied by the distribution area is usually called the “Cross-connection area”.

Subscriber wiring- a section of the subscriber line from the distribution box to the power socket of the terminal subscriber telephone device. In English technical literature two terms are used:

- "Subscriber's lead-in" - the section from the distribution box to the subscriber's premises;

- "Subscriber's service line" - the section from the distribution box to the telephone set.

Cross, VKU- equipment for the junction of station and linear sections of subscriber and connecting lines of urban, rural and combined telephone networks. This element of the access network in English technical literature is called "Main distribution frame"; The abbreviation MDF is often used.

Cable distribution cabinet (SR)- terminal cable device designed for installation of cable boxes (with plinths, without electrical protection elements), in which connections are made between the main and distribution cables of subscriber lines of local telephone networks. The term "Cross-connection point" corresponds to the cable distribution cabinet. If the AL passes through two SRs, then in the English-language technical literature - for the second cabinet - the adjective “secondary” is added. In addition, if the ShR is located in a specially equipped room, then it is referred to as “Cabinet”. In the case when the ShR is located near the wall of a building or other similar place, it is called “Sub-cabinet” or “Pillar”. These designations are usually indicated in parentheses after the functional purpose - "Cross-connection point". In the technical literature, several more terms are used that more or less correspond to ShR. The most common word used is "Curb".

Subscriber distribution box (RK)- a terminal cable device designed to connect cable pairs included in the distribution box plinth with single-pair wires of subscriber wiring. Distribution point (DP) is an analogue of the term “Subscriber distribution box”.

Cable drainage(Duct or Cable duct) - a set of underground pipelines and wells (inspection devices) intended for laying, installation and maintenance of communication cables.

Well (inspection device) for cable ducts(Jointing chamber or Jointing manhole) is a device designed for laying cables in cable ducts, installing cables, placing related equipment and maintaining communication cables.

Cable mine(Exchange manhole) - a cable duct structure located in the basement of a telephone exchange, through which cables are introduced into the station building and in which, as a rule, multi-pair linear cables are soldered into station cables with a capacity of 100 pairs.

The concept of a subscriber line

Subscriber line (AL)- a line of the local telephone network connecting the terminal subscriber telephone device with the subscriber kit (SK) of the terminal station, concentrator or other remote module. In English technical literature the term Subscriber line or simply Line is used.

AL functions in the existing telecommunication system:

Ensuring two-way transfer of messages in the area between the user terminal and the subscriber set of the end station;

Exchange of signaling information necessary for establishing and releasing connections;

Support of specified indicators of quality of information transmission and reliability of communication between the terminal and the end station.

The block diagram and joints of subscriber line equipment for GTS and STS are shown in Figure 1.2.

For the AL block diagram (upper part of Figure 1.2), three options are presented for connecting the subscriber terminal to the switching station.

The top branch of this figure shows a promising option for connecting the TA without the use of intermediate crossover equipment. The cable is laid from the cross-connection to the distribution box, where the connection is made using subscriber wiring.

Figure 1.2 - Block diagram and joints of subscriber line equipment for GTS and STS

The middle branch of the figure shows a variant of connecting the TA using a cabinet system, when intermediate equipment is placed between the cross-connection and the distribution box. In our model, the role of such equipment is assigned to the distribution cabinet.

In some cases, AL is organized using overhead communication lines (ACL). In Figure 1.2 this option is shown on the bottom branch. In such a situation, a cable box (CB) and input-output insulators are installed on the pole. At the location of the distribution box, a subscriber station is mounted protective device(AZU), preventing the possible influence of dangerous currents and voltages on the TA. It should be noted that the organization of the AL or its individual sections through the construction of overhead communication lines is not recommended; but in some cases this is the only option for organizing subscriber access.

Basic concepts of multiservice subscriber access network (MSAD)

Basic concepts of MSAD

A multiservice subscriber access network (MSN) is understood as a network that supports the transmission of heterogeneous traffic between end users (systems) and the transport network using a single network architecture, which makes it possible to reduce the variety of equipment types and apply uniform standards.

The architecture and functions of the MSAD must support three types of services provided:

Speech transmission (sound, telephone communication, voice mail, etc.), - data transmission (Internet, fax, file transfer, Email, electronic payments, etc.);

Transmission of video information (video on demand, TV programs, video conferences, etc.).

The concept of development of multiservice access networks includes mainly two directions:

Intensification of the use of existing subscriber lines;

Construction of access networks using new technologies.

MSAD technologies

Technologies used in MSAD can be classified different ways. One of these ways is to divide technologies into two groups in accordance with the transmission medium:

Wired;

Wireless.

1) Wired ones use (in whole or in part) physical circuits. This could be twisted copper pair, coaxial cable, optical fiber, power supply wiring, etc. Among them, we can distinguish a group of technologies that use copper pairs, which are interesting from at least two points of view. Firstly, they provide support for a number of new infocommunication services. Secondly, by using traditional physical circuits, these technologies can reduce the cost of upgrading the access network, even if the effective demand for new services is at a low level.

Technologies based on wired media can be divided into the following groups:

Services provided to subscribers of the public telephone network (PSTN);

Technologies for accessing integrated services digital network (ISDN) services;

Digital subscriber line technologies – xDSL (twisted copper pair – balanced cable);

Local technologies computer networks LAN (twisted pair, coaxial cable and fiber optic cable);

Optical access technologies OAN (fiber optic cable);

Cable television (CTV) network technologies (coaxial and fiber optic cables);

Technologies of multiple access networks (wiring of power supply networks, wiring of radio broadcasting networks);

In this group, it is also necessary to note the technologies of wireless subscriber lines in combination with physical circuits (WLLx). In this case, the transition to two-wire physical circuits is carried out at some point “x”. These technologies are most often used in rural areas.

The classification of technologies in this group is presented in Table 2.1.

2) Wireless - based on radio communications that complement and expand the capabilities of wired communications and allow the implementation of a full range of information services: transmission of telephone messages, data exchange, transmission of video images.

Wired technologies .

Let's take a closer look at the wired technologies shown in Table 2.1.

The public telephone network (PSTN) was created to provide telephony services. Subscribers' access to a limited range of PSTN services is carried out over communication lines based on copper pairs using equipment (telephones, fax machines and modems) operating in accordance with algorithms for establishing telephone connections.

ISDN network (Integrated Services Digital Network) – a digital network with integration of services – a digital communication network with circuit switching. Access to ISDN networks is also carried out via a symmetrical subscriber cable, however, the range of services provided is significantly larger compared to PSTN.

The development of xDSL access reflects the development of signal transmission methods over twisted copper pair. These technologies provide access to a wide range of multimedia services. Various international organizations (ITU, ANSI, ETSI, DAVIC, ATM Forum, ADSL Forum) deal with issues of standardization, as well as promotion of xDSL technologies on the market. These technologies can be divided into subgroups: symmetric and asymmetric xDSL access. The former are used mainly in the corporate sector, the latter are intended

Table 2.1 - Classification of wired technologies

Wired technologies
PSTN	telephone fax modem PD leased line
ISDN	ISDN-BRA ISDN-PRA
LAN technologies	Ethernet family	Ethernet Fast Ethernet Gigabit Ethernet
Token Ring Family	Token Ring HSTR
FDDI Family	FDDI CDDI SDDI Ethernet over VDSL (EoV)
xDSL family technologies	Symmetrical	IDSL HDSL SDSL SHDSL MDSL MSDSL VDSL, etc.
Asymmetrical	ADSL RADSL G.Lite ADSL2 ADSL2+ VDSL, etc.
Optical Access Technologies	Active FTTx networks	FTTH FTTB FTTC FTTCab etc.
Passive xPON networks	APON EPON BPON GPON etc.
Cable TV technologies	DOCSIS 1.0 DOCSIS 1.1 DOCSIS 2.0 Euro-DOCSIS J.112 IPCable-Com Packet-Cable
Multiple access network technologies	– HPNA 1.x – HPNA 2.0 – HPNA 3.0
Based on power supply networks	Home Plug 1.0 specification
Cable based	EFM

chens to provide services primarily to individual users.

The largest volume of services can be provided to the user using optical access networks OAN (Optical Access Networks) - active (FTTH, FTTB. FTTC, FTTCab) or passive PON (Passive Optical Networks). The international consortium FSAN (Full Service Access Network) is engaged in the creation and promotion of the latest access technologies and, in particular, optical technologies.

Multiple access networks (MANs) are designed to organize relatively inexpensive Internet access for individual users living in apartment buildings. The idea of ​​shared access is to use the existing cable infrastructure in homes (twisted copper pair, radio broadcast networks, electrical wiring). A traffic concentrator is installed in a home connected to the Internet. To connect a hub to a transport network services host, you can use different technologies(PON, FWA, satellite, etc.). Thus, multiple access networks are hybrid, combining both the multiple access networks themselves and the networks that provide traffic transportation.

Cable television (CTV) networks were originally intended to organize the transmission of television programs to users via distribution networks based on coaxial cable and were built according to a unidirectional scheme.

In the early 90s, numerous but unsuccessful attempts were made to create and implement technologies for building interactive access networks to multimedia services based on hybrid CATV networks - Hybrid Fiber Coaxial (HFC). Mass deployment of HFC networks began after the advent of the DOCSIS (Data Over Cable Service Interface Specification) standard in 1997.

LAN technologies were developed to provide user access to resources local networks. For user access to services from other resources (Internet, corporate networks etc.) modern LANs are built using hybrid technology and combine the LAN itself and networks that connect the LAN to transport networks.

ISDN subscriber access networks

ISDN Basics

The ISDN network (Integrated Services Digital Network - ISDN) is created, as a rule, on the basis of a telephone digital network and ensures the transfer of information between end devices in digital form. At the same time, subscribers are provided with a wide range of voice and non-voice services (for example, high-quality telephone communications and high-speed data transmission, text transmission, transmission of television and video images, video conferencing, etc.). ISDN services are accessed through a specific set of standardized interfaces.

Currently, there are mainly two types of subscriber access to ISDN network resources that are most widespread:

Basic (Basic Rate Interface - BRI) with a 2B+D structure, where B-64 kbit/s, D=16 kbit/s, the group speed will be 144 kbit/s, if there is a synchronization channel, the transmission speed in the line can be equal 160 kbps or 192 kbps;

Primary (Primary Rate Interface - PRI) with a 30B+D structure, where B = 64 kbit/s, D = 64 kbit/s, while the transmission speed, taking into account synchronization signals, will be 2048 kbit/s.

Basic ISDN access. Broadcast digital information over a two-wire copper pair in the ISDN network is possible at a speed of 160 kbit/s under normal conditions (cable length no more than 8 km with a cross-sectional diameter of 0.6 mm, or no more than 4.2 km with a cross-sectional diameter of 0.4 mm). Copper pair operating in 2B+D mode (144 kbit/s useful information) with synchronization and data support (160 kbit/s general information), is part of the Uk0 interface. On the user side, the copper pair ends with a network termination (NT). The network termination converts the two-wire Uk0 interface (160 kbit/s) into a four-wire S0 interface (192 kbit/s); for the 2B+D case, the network termination is transparent in both directions. The network operator is responsible for the connection from the station only to the network termination, and the subscriber is responsible for the section from NT to the subscriber. The S0 interface is a connecting bus through which ISDN-compatible equipment can connect to the main ISDN station via a standard connector (see Figure 3.1). For a private station, the S0 interface is the point at which the private station connects to the main ISDN station (see Figure 3.2). The length of the S0 bus should not exceed one kilometer.

Primary ISDN access. Similar to the primary access, the primary access B channels are used and switched individually, and the signal

Figure 3.1 - Basic access for an individual user

Figure 3.2 - Basic access for small capacity PBX

nal information (D-channel messages) is transmitted in the D-channel. But unlike basic access, the D-channel here is used only for signaling information, packet-oriented user data must be separated from the signaling information in the enterprise station and transmitted over B-channels. The PCM link operating as the primary access with 30V+D is called the Uk2pm interface or Uk2m interface. The end of the line on the subscriber side is designed as a network end (NT), where the Uk2m interface is transformed into an S2m interface. From NT to the institutional station the distance should not exceed one kilometer.

The corporate station connects to the public ISDN station via an S2pm interface. When using a corporate station, the S0 interface acts as a bus for connecting terminal equipment (see Figure 3.3).

Subscriber signaling DSS1 in ISDN.

The signaling system on the subscriber portion of the ISDN network was named EDSS1 (European Digital Signaling System No. 1). This system signaling applies to both basic and primary

Figure 3.3 - Primary access for medium and large capacity PBXs

access. With the help of EDSS1, a connection is established and disconnected, services are ordered by users, and information is transferred between subscribers.

User-network signaling is located within the three lower levels of the BOS and performs the following functions:

- data layer(physical layer, layer 1) provides network-synchronized transmission of information across channels simultaneously in both directions and regulates simultaneous access of several terminal devices to a shared D-channel;

- D-channel protection level(data link level, level 2) provides error-proof transmission of signaling information for level 3 and transmission of data packets transmitted in the D channel in both directions between the network and the user device;

- D-channel switching level(network layer, layer 3) ensures the establishment and management of connections in the user-network section. The third level ends with the user-network signaling.

Level 1 is considered using the example of basic access (see Figures 3.1, 3.2, 3.3). Level 1 via the S0 and Uk0 interfaces transmits signaling via the D-channel without signaling control.

The protocol used for layer 2 in the D channel when performing the connection establishment procedure is called LAPD (Link Access Procedure on the D channel). ISDN protocol structure or Layer 2 D-channel message format, or signaling packet, or signaling unit (see Figure 3.4).

Flag: Each signal unit begins and ends with a flag, which marks the beginning of the signal unit and its end. The flag is a sequence of bits: 01111110.

								byte 1 Flag
Address (first byte)
Address (second byte)
Control field
Information
FCS	N-2
N-1
								N Flag

Figure 3.4 – Layer 2 D-channel message format

Address - The address field consists of two bytes. It defines the receiver of the control signal unit and the transmitter of the sent unit.

Control field. The control field specifies the type of D-channel message, which can be a command or a response to a command. The control field can consist of one or two bytes, its size depends on the format. There are three types of control field formats: transmission of packet number information (I format), supervisory functions (S format), unnumbered information and control functions (U format).

Information information field - may not be present in the packet (in this case, the packet does not carry third-level information, but is used by the second level, for example, to control the data link), if it is present, it is located behind the control field. The size of the information field can reach 260 bytes.

FCS (field control bits - check combination). Due to the fact that when transmitted over a network, packets can be distorted by noise at the first level, each of them contains a Frame Check Sequence field: it consists of 16 check bits and is used to check errors in the received packet. If a packet is received with an incorrect sequence of check bits, it is discarded.

Layer 3 is responsible for establishing and managing the connection. It prepares messages for transmission by the second level; the prepared information is placed in the information field of the D-channel message. Layer 3 messages are messages sent between user terminals and the station and vice versa. The third layer contains procedures for managing circuit-switched calls, as well as procedures for using ISDN to make packet-switched calls over the D-channel.

xDSL technologies

xDSL Basic Concepts

xDSL(digital subscriber line, digital subscriber line) - a family of technologies that can significantly increase the capacity of the subscriber line of the public telephone network by using effective linear codes and adaptive methods for correcting line distortion based on modern advances in microelectronics and digital signal processing methods.

xDSL technologies appeared in the mid-90s as an alternative to ISDN digital subscriber termination.

In the abbreviation xDSL the symbol "X" is used to denote the first character in the name of a particular technology, and DSL denotes a digital subscriber line DSL (Digital Subscriber Line; there is also another version of the name - Digital Subscriber Loop). xDSL technologies allow you to transfer data at speeds that significantly exceed those that are available even to the best analog and digital modems. These technologies support voice, high-speed data and video, creating significant benefits for both subscribers and providers. Many xDSL technologies allow you to combine high-speed data transmission and voice transmission over the same copper pair. Existing types of xDSL technologies differ mainly in the form of modulation used and the data transfer rate.

xDSL technologies can be divided into:

Symmetrical;

Asymmetrical.

ADSL technology

ADSL(Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) is a modem technology in which the available channel bandwidth is distributed asymmetrically between outgoing and incoming traffic. Since for most users the volume of incoming traffic significantly exceeds the volume of outgoing traffic, the speed of outgoing traffic is much lower.

Data transmission using ADSL technology is carried out through a regular analog telephone line using a subscriber device - an ADSL modem and an access multiplexer (DSL Access Module or Multiplexer, DSLAM), located on the PBX to which the user's telephone line is connected, and the DSLAM is turned on before the equipment of the PBX itself . As a result, a channel appears between them without any inherent limitations of the telephone network. DSLAM multiplexes multiple DSL subscriber lines into one high-speed backbone network. The block diagram of an ADSL connection is shown in Figure 4.1.

Figure 4.1 – Block diagram of ADSL connection

They can also connect to an ATM network via PVC (Permanent Virtual Circuit) links to Internet service providers and other networks.

It is worth noting that two ADSL modems will not be able to connect to each other, unlike regular dial-up modems.

ADSL technology is a variant of DSL in which the available channel bandwidth is distributed asymmetrically between outgoing and incoming traffic - for most users, incoming traffic is significantly more significant than outgoing traffic, so providing more of the bandwidth for it is quite justified (peer-to-peer traffic is an exception to the rule). networks, video calls and email, where the volume and speed of outgoing traffic are important). A regular telephone line uses a frequency band of 0.3...3.4 kHz for voice transmission. In order not to interfere with the use of the telephone network for its intended purpose, in ADSL the lower limit of the frequency range is at 26 kHz. The upper limit, based on the requirements for data transfer speed and the capabilities of the telephone cable, is 1.1 MHz. This bandwidth is divided into two parts: frequencies from 26 kHz to 138 kHz are allocated to the outgoing data stream, and frequencies from 138 kHz to 1.1 MHz are allocated to the incoming data stream. The frequency band from 26 kHz to 1.1 MHz was not chosen by chance. In this range, the attenuation coefficient is almost independent of frequency.

This frequency division allows you to talk on the phone without interrupting the exchange of data on the same line. Of course, situations are possible when either the high-frequency signal of the ADSL modem negatively affects the electronics of a modern phone, or the phone, due to some features of its circuitry, introduces extraneous high-frequency noise into the line or greatly changes its frequency response in the high-frequency region; To combat this, a filter is installed in the telephone network directly in the subscriber’s apartment low frequencies(frequency splitter, English Splitter), which passes only the low-frequency component of the signal to ordinary telephones and eliminates the possible influence of telephones on the line. Such filters do not require additional power, so the speech channel remains operational when turned off. electrical network and in case of ADSL equipment failure.

Transmission to the subscriber is carried out at speeds of up to 8 Mbit/s, although today there are devices that transmit data at speeds of up to 25 Mbit/s (VDSL), but such a speed is not defined in the standard. In ADSL systems, 25% of the total speed is allocated for service information, in contrast to ADSL2, where the number of service bits in a frame can vary from 5.12% to 25%. The maximum line speed depends on a number of factors such as line length, cross-section and resistivity cable. Also, a significant contribution to the increase in speed is made by the fact that for an ADSL line it is recommended to use twisted pair (not TRP), moreover, shielded, and if it is a multi-pair cable, then in compliance with the direction and pitch of the layer.

When using ADSL, data is transmitted over a common twisted pair cable in full duplex form. In order to separate the transmitted and received data stream, there are two methods: frequency division multiplexing (FDM) and echo cancellation (EC).

An ADSL modem is a device built on the basis of a digital signal processor (DSP or DSP), similar to that used in conventional modems (see Figure 4.2).

ADSL standards:

ITU G.992.3 (also known as G.DMT.bis or ADSL2) is an ITU (International Telecommunication Union) standard that extends the core ADSL technology to the following data rates:

1) towards the subscriber - up to 12 Mbit/s (all ADSL2 devices must support speeds up to 8 Mbit/s);

2) in the direction from the subscriber - up to 3.5 Mbit/s (all ADSL2 devices must support speeds up to 800 kbit/s).

Actual speed may vary depending on line quality:

ITU G.992.4 (also known as G.lite.bis) is a standard for technology

Figure 4.2 – Block diagram of the ADSL modem transmitting node

ADSL2 without using a splitter. Speed ​​requirements are 1.536 Mbit/s towards the subscriber and 512 kbit/s in the opposite direction.

ITU G.992.5 (also known as ADSL2+, ADSL2Plus or G.DMT.bis.plus) is an ITU (International Telecommunications Union) standard that extends the capability of core ADSL technology by doubling the number of bits of the incoming signal to the following data rates:

1) towards the subscriber - up to 24 Mbit/s;

2) in the direction from the subscriber - up to 1.4 Mbit/s.

Actual speed may vary depending on line quality and distance from the DSLAM to the customer's home. The standard specifies speeds for twisted pair; when using a line of another type, the speed can be much lower.

ADSL2+ doubles the frequency range in relation to ADSL2 from 1.1 MHz to 2.2 MHz, which entails an increase in the data transfer rate of the incoming stream of the previous ADSL2 standard from 12 Mbit/s to 24 Mbit/s (see Figure 4.3).

One of the most important problems of telecommunication networks continues to be the problem of subscriber access to network services. The relevance of this problem is determined primarily by the rapid development of the Internet, access to which requires a sharp increase in the capacity of subscriber access networks. The main means of the access network, despite the emergence of new, state-of-the-art wireless methods of subscriber access, remains traditional copper subscriber pairs. The reason for this is the natural desire of network operators to protect their investments. Therefore, at present and in the foreseeable future, the strategic direction for increasing the capacity of subscriber access networks will remain asymmetric digital subscriber line ADSL technology, which uses a traditional copper subscriber pair as a transmission medium and at the same time maintains the services already provided in the form of an analog telephone or basic ISDN access. The implementation of this strategic direction in the evolution of subscriber access networks depends on the specific conditions of the existing subscriber access network in each country and is determined by each telecom operator taking into account these specific conditions. It is clear that the diversity of local conditions determines a large number possible ways migration of the existing subscriber access network to ADSL technology.

Telecommunication technologies are constantly improving, quickly adapting to new requirements and conditions. More recently, the main and only means of subscriber access to network services - and primarily to Internet services - was an analog modem. However, the most advanced analog modems are a modem that meets the requirements of ITU-T Recommendation V.34, with a potential transfer rate of up to 33.6 Kbps, as well as a subsequent generation modem that meets the requirements of ITU-T Recommendation V.90, with a potential transfer rate of 56 Kbit/s practically cannot be provided efficient work user on the Internet.

Thus, a sharp increase in the speed of access to network services, and primarily to Internet services, is critically important. One method for solving this problem is to use the xDSL family of high-speed subscriber line technologies. These technologies provide high-capacity subscriber access networks, the main element of which is the twisted copper pair of the local subscriber telephone network. Although each of the xDSL technologies occupies its own niche in the telecommunications network, it is nevertheless undeniable that the technologies of asymmetric digital high-speed subscriber line ADSL and ultra-high-speed digital subscriber line VDSL are of the greatest interest to telecommunications service providers, equipment manufacturers, and users. And this is no coincidence - ADSL technology appeared as a way to provide the user with a wide range of telecommunications services, including, first of all, high-speed access to the Internet. In turn, VDSL technology is capable of providing the user with wide bandwidth, which allows him to access almost any broadband network service both in the near and distant future, but not in a purely copper, but in a mixed, copper-optical access network . Thus, both of these technologies will provide an evolutionary path for the introduction of optical fiber into the subscriber access network, protecting in the most effective way the past investments of local network operators. ADSL can therefore be seen as the most promising member of the xDSL family of technologies, to be succeeded by VDSL technology.

Although the key idea in migrating the way network services are provided using xDSL technologies is to move from the analogue public telephone network first to ADSL and then, as necessary, to VDSL, this does not exclude the use of other intermediate steps for the same purpose. types of xDSL technologies. For example, IDSL and HDSL technologies can be used to increase the capacity of a subscriber line.

From analog modem to ADSL

The most common migration scenario for access to Internet services is by far the transition from a source access network using analogue PSTN modems to a target access network using ADSL modems.

ADSL (Asymmetric Digital Subscriber Line - asymmetric digital subscriber line). This technology is asymmetrical. This asymmetry, combined with the state of "constantly established connection" (when you eliminate the need to type each time phone number and wait for the connection to be established), makes ADSL technology ideal for organizing access to the Internet, access to local networks (LAN), etc. When organizing such connections, users usually receive much more information than they transmit. ADSL technology provides downstream speeds ranging from 1.5 Mbit/s to 8 Mbit/s and upstream speeds from 640 Kbit/s to 1.5 Mbit/s. ADSL technology allows you to maintain traditional service without significant costs and provide additional services, including:

• § preservation of traditional telephone service,
• § high-speed data transfer at speeds of up to 8 Mbit/s to the service user and up to 1.5 Mbit/s from him,
• § high-speed Internet access,
• § transmission of one television channel with high quality, video-on-demand,
• § distance learning.

Compared to cable modem and fiber optic alternatives, ADSL's main advantage is that it uses your existing telephone cable. At the ends of the existing telephone line, frequency splitters are installed (some use a copy of the English splitter) - one at the telephone exchange and one at the subscriber's. A regular analog telephone and an ADSL modem are connected to the subscriber splitter, which, depending on the design, can act as a router or bridge between the subscriber’s local network and the provider’s edge router. At the same time, the operation of the modem does not at all interfere with the use of normal telephone communication, which exists regardless of whether the ADSL line is functioning or not.

Currently, there are two modifications of ADSL technology: the so-called full-scale ADSL, which is simply called ADSL, and the so-called “light” version of ADSL, which is called “ADSL G. Lite”. Both versions of ADSL are currently governed by ITU-T Recommendations G.992.1 and G.992.2, respectively.

The concept of full-scale ADSL was originally born as an attempt at a competitive response from local telephone network operators to cable television (CTV) operators. Almost 7 years have passed since the advent of ADSL technology, but it has not yet received widespread attention. practical application. Already in the process of developing full-scale ADSL and the first experience of its implementation, a number of factors became clear that required correction of the initial concept.

The main of these factors are the following:

• 1. Change in the main target use of ADSL: currently, the main type of broadband subscriber access is no longer the provision of cable TV services, but the organization of broadband Internet access. To solve this new problem, 20% of the maximum capacity of full-scale ADSL is sufficient, which corresponds to a downstream speed (from the network to the subscriber) of 8.192 Mbit/s and an upstream speed (from the subscriber to the network) of 768 Kbit/s.
• 2. The Internet is not ready to provide full-scale ADSL services. The fact is that the ADSL system itself is only part of the broadband access network to network services. Already the first experiences of introducing ADSL into real access networks have shown that today's Internet infrastructure cannot support transmission speeds above 300-400 Kbps. Although the backbone of the Internet access network is usually carried out on an optical cable, however, it is not this network, but other elements of the Internet access network - such as routers, servers and PCs, including the characteristics of Internet traffic, that determine the real throughput of this network. Therefore, the use of full-scale ADSL on an existing network practically does not solve the problem of broadband subscriber access, but simply moves it from the subscriber portion of the network to the backbone network, exacerbating the problems of the network infrastructure. Therefore, the implementation of full-scale ADSL will require a significant increase in the capacity of the Internet backbone, and, consequently, significant additional costs.
• 3. High cost of equipment and services: for widespread deployment of the technology, it is necessary that the cost of an ADSL subscriber line be no more than $500; existing prices significantly exceed this value. Therefore, other xDSL products are actually used, and primarily modifications of HDSL (such as multi-speed MSDSL) with a throughput of 2 Mbit/s over one copper pair.
• 4. The need to modernize the infrastructure of the existing access network: the concept of a full-scale ADSL requires the use of special separation filters - so-called splitters, separating low-frequency signals of an analog telephone or basic BRI ISDN access and high-frequency signals of broadband access both in the PBX premises and in the user's premises.This operation requires significant labor costs, especially in the PBX cross-country, where thousands of subscriber lines terminate.
• 5. The problem of electromagnetic compatibility, which consists in insufficient knowledge of the influence of full-scale ADSL on other high-speed digital transmission systems (including the xDSL type) operating in parallel on the same cable.
• 6. Large power consumption and footprint: existing ADSL modems, in addition to their high cost, also require a lot of space and consume significant power (up to 8 W per ADSL modem in the active state). To make ADSL technology acceptable for placement in a switching office, it is necessary to reduce power consumption and increase port density.
• 7. Asymmetric mode of operation of full-scale ADSL: with a constant bandwidth of the ADSL line, it is an obstacle for some applications that require a symmetric transmission mode, such as video conferencing, as well as for organizing the work of some users who have their own Internet servers. Therefore, adaptive ADSL is needed, capable of operating in both asymmetric and symmetrical mode.
• 8. The hardware and software of the user's premises have been shown to be also bottleneck ADSL systems. Testing has shown, for example, that popular programs -- Web browsers and platforms hardware PCs may limit PC throughput to 600 Kbps. Therefore, to fully utilize high-speed ADSL connections, improvements in client hardware and software user.

The listed problems of full-scale ADSL initiated the emergence of its “light” version, which is the already mentioned ADSL G.Lite. Let us present the most significant features of this technology.

Ability to operate in both asymmetric and symmetric modes: in asymmetric mode with transmission speeds of up to 1536 Kbps in the downstream direction (from the network to the subscriber) and up to 512 Kbps in the upstream direction (from the subscriber to the network); in symmetric mode - up to 256 Kbps in each transmission direction. In both modes, using the DMT code, the transmission speed is automatically adjusted in 32 Kbps steps depending on the line length and interference power.

Simplifying the process of installing and configuring ADSL GLite modems by eliminating the use of separation filters (splitters) in the user's premises, which allows these procedures to be carried out by the user himself. This does not require replacement of internal wiring in the user's premises. However, as test results show, this cannot always be done. An effective measure to protect a broadband data transmission channel from pulse dialing signals and ringing signals is to install special microfilters directly in the telephone socket.

The available ADSL GLite line lengths make it possible to provide high-speed Internet access to the vast majority of home users. It should be noted that many ADSL equipment manufacturers have chosen the concept of ADSL equipment that supports both full-speed ADSL and ADSL G.Lite modes of operation. It is assumed that the appearance of ADSL G.Lite equipment will dramatically activate the market for broadband Internet access devices. There is a high probability that it will occupy the niche of broadband access to network services for home sector users.

The advent of the ADSL intermediate stage in the form of ADSL G.Lite creates the possibility of a smooth transition from existing analog modems to broadband access - first to the Internet using G.Lite, and then to multimedia services using full-scale ADSL.

Migration from an analog modem to any of the ADSL modifications is beneficial to the service provider because calls of increased duration, such as user calls to the Internet, are routed bypassing the public switched telephone network. If the service provider is a traditional local network operator, then this scenario gives it another additional (but no less important) advantage, since it eliminates the need for an expensive upgrade of the switch of the existing telephone network to an ISDN switch, which would be needed to increase the speed of access to Internet services at option of migration from public telephone network services to ISDN network services. Such a significant additional investment in the transition from analogue PSTN to ISDN is explained by the fact that the latter is a network concept with its own very powerful multi-layer protocol stack. Therefore, this upgrade requires significant changes in the hardware and software of the PSTN digital switching station. At the same time, an ADSL modem is simply a high-speed modem that uses standard data network protocols based on the transmission of packets or ATM cells to support it. This significantly reduces the complexity of accessing the Internet and therefore the required investment.

In addition, from the point of view of Internet users, network operators and Internet service providers, it makes more sense to directly migrate from a PSTN modem not to an ISDN modem, but directly to an ADSL modem. With the maximum throughput of narrowband ISDN equal to 128 Kbps (which corresponds to the combination of two B-channels of the main ISDN access), switching to ISDN gives an increase in access speed compared to the PSTN network, potentially a little more than 4 times and also requires significant investments. Therefore, the intermediate stage of transition from PSTN to ISDN as an effective means of accessing the Internet practically loses its meaning. Of course, this does not apply to those regions where there is already widespread ISDN adoption. Here, of course, the determining factor is the protection of the investments made.

Thus, the main incentives for the considered access network migration method are:

• § A huge increase in the speed of access to Internet services.
• § Retain analogue telephone or basic ISDN access (BRI ISDN).
• § Moving Internet traffic from the PSTN network to an IP or ATM network.
• § No need to upgrade the PSTN switch to an ISDN switch.

If the main driver for migrating from an analog modem to an ADSL modem is high-speed Internet access, then the most appropriate way to implement this service would be to implement an ADSL remote terminal, called an ATU-R, in the form of a card personal computer(PC). This reduces the overall complexity of the modem and eliminates internal wiring problems (from the modem to the PC) in the user's premises. However, telephone network operators are usually reluctant to rent out an ADSL modem if it is the internal card of a PC, as they do not want to be responsible for possible damage to the PC. Therefore, ATU-R remote terminals have so far become more widespread in the form of a separate unit called an external ADSL modem. The external ADSL modem is connected to a LAN port (10BaseT) or to a serial port (serial universal bus USB) computer. This design is more complex because it requires additional space and separate power. But such an ADSL modem can be purchased by a local telephone network subscriber and put into operation by a PC user independently. In addition, an external modem can be connected not to a PC, but to a LAN hub or router in cases where the user has several computers.

And this situation is typical for organizations, business centers and residential complexes.

Migration to ADSL if there is TsSPAL access in the network

The previous migration scenario requires a continuous physical copper pair between the local exchange premises and the user premises. This situation is more typical for developing countries with a relatively underdeveloped telecommunications network, which includes Russia. In countries with a developed telecommunications network on the subscriber telephone network, to increase the covered distances, digital subscriber transmission systems (DSTS) are widely used, mainly using the equipment of primary digital transmission systems of plesiochronous hierarchies (E 1). For example, in the USA in the early 90s, approximately 15% of all subscriber lines were serviced using DSC (in the USA they are called Digital Local Carrier - DLC), in the future it is expected that their total capacity will increase to 45% of the total number of subscriber lines. Currently, very reliable subscriber access networks are being built, which use a combined copper-optical transmission medium and secure ring structures using SDH synchronous digital hierarchy equipment.

Modern DSPALs not only multiplex the signals of a certain number of subscribers into a digital stream transmitted over two symmetrical pairs, but can also perform load concentration functions (2:1 or more), which reduces the load on switching stations. In this case, one end terminal of the TsSPAL is located in the PBX premises, and the other is located at an intermediate point between the PBX and the user's premises. Therefore, an individual physical subscriber line exists only between the user’s premises and the remote TsSPAL terminal. Therefore, the ADSL access multiplexor (DSLAM - DSL access multiplexor) and its component - the ADSL ATU-C station terminal - must be located not on the PBX, but at the location where the remote terminal (RDT) is installed. In this case, the following technical solutions are used to organize ADSL systems:

• 1. Remote DSLAM, which is located in a separate container near the RDT container and is designed to serve a large number of users (usually from 60 to 100 ADSL lines). In this case, no special management and maintenance system is required, since a control system for setting up and monitoring the status of ADSL lines of a standard DSLAM installed in the PBX premises is used. Such a DSLAM can work with almost any DSPAL equipment, since it is a stand-alone equipment; DSLAM simply separates the PSTN traffic from the traffic of the ADSL line itself and transmits it to the DSPAL equipment in analog form. At the same time, such a solution is very expensive: since the DSLAM equipment is autonomous, serious installation and installation work is required, organizing power supply for the equipment and much more; therefore, this solution is only appropriate if there are a large number of DSPAL users.
• 2. ADSL line cards built into TsSPAL equipment. In this case, they are used free places in TsSPAL equipment boards placed in an RDT container, with two options possible:
  • § DSPAL equipment is used only for housing and mechanical protection of ADSL cards, and all connections are made using cables, which is typical for traditional DSP;
  • § The ADSL line card is part of the TsSPAL equipment and is simply integrated into the latter. This second method is usually used in the new generation of TsSPAL equipment and eliminates the need for any installation work in the TsSPAL block.
  • § Remote access multiplexor (RAM - remote access multiplexor), which performs the same functions as DSLAM. It differs from DSLAM in that it is integrated into the existing TsSPAL infrastructure and does not require upgrading the existing subscriber access network infrastructure, which is associated with significant costs. The use of RAM is universal because it provides the ability collaboration with any type of TsSPAL equipment. Typically, RAM units are small in size and can fit into existing RDT hardware containers. The main problem with currently known RAMs is their lack of scalability.

From ISDN to ADSL

In the 90s, as a way to more quick access to the Internet, where possible, ISDN lines began to be widely used. Over time, when throughput ISDN will not be sufficient; a natural solution would be to “supplement” the ISDN subscriber line with a high-speed ADSL channel. As with regular analog lines, this method, called "ISDN under ADSL", uses filters to separate ADSL and ISDN signals.

This solution is especially attractive because it poses virtually no problems with meeting narrowband ISDN standards and, therefore, with implementing the ISDN to ADSL migration path. That's why this method evolution will be particularly popular in countries where narrowband ISDN has been widely adopted, with the transition from ISDN to full-scale ADSL likely to dominate.

From HDSL to ADSL

HDSL (High Bit-Rate Digital Subscriber Line) technology is by far the most mature and least expensive of the xDSL technologies. It emerged as an effective alternative to the outdated equipment of primary digital processing centers E! for use on local network trunks, as well as primary access to ISDN (PRA ISDN). Thanks to the widespread use of HDSL in various regions of the world, the procedures for deploying such systems, their operational maintenance and testing are well established; well known also high quality parameters and high reliability of HDSL systems. Therefore, telecom operators and network service providers willingly use HDSL equipment for high-speed Internet access. However, most often the use of HDSL in a subscriber access network requires the use of at least two copper pairs, which is almost not always possible. Using only one pair to organize an HDSL line significantly reduces the overlapped distances. In addition, HDSL equipment does not provide the ability to organize an analog telephone, which requires the use of an additional subscriber pair for this purpose. Thus, there are significant factors motivating the advisability of switching from HDSL to ADSL. With such migration, the throughput of the access network in the downstream direction (i.e., from the network to the subscriber) sharply increases; just one pair is enough and it becomes possible to organize an analog telephone. However, problems may arise with this migration scenario. Thus, the upstream bandwidth of an ADSL access network (i.e., from the subscriber to the network) is typically less than the corresponding HDSL bandwidth.

From IDSL to ADSL

One of the modifications of xDSL technologies is the so-called IDSL technology, which has a more complete abbreviation "ISDN DSL". IDSL (ISDN Digital Subscriber Line - IDSN digital subscriber line). This technology appeared as an adequate response from equipment manufacturers and Internet providers to the problems associated with overloading the switched ISDN network with traffic from Internet users, and the insufficient speed of Internet access for many users using analog modems.

IDSL technology simply involves the formation of a point-to-point digital path with a capacity of 128 Kbps based on the BRI ISDN basic access format by combining two main B-channels of 64 Kbps each; in this case, the auxiliary D-channel provided in the BRI ISDN format is not used, i.e., the IDSL path has a structure of the “128+0” Kbit/s type. IDSL uses standard ISDN digital subscriber line chips (called U-interface). However, unlike the ISDN U-interface, IDSL equipment connects to the Internet not through a PSTN or ISDN switch, but through a router. Therefore, IDSL technology is used only for data transmission and cannot provide PSTN or ISDN voice services.

The most attractive properties of IDSL are the maturity of ISDN technology, the low cost of ISDN U-interface chips, ease of installation and maintenance compared to installation and technical maintenance standard ISDN (since IDSL bypasses the ISDN switching office), as well as the ability to use standard ISDN measurement equipment. In addition, telecom operators and Internet service providers who deploy ISDN are usually very familiar with the latter. Therefore, there are no problems associated with planning and maintaining IDSL lines. The main incentive to migrate from IDSL to ADSL is to provide faster Internet access compared to an analog modem. However, keep in mind that when using IDSL to access the Internet, a second subscriber line is required to access the PSTN. The transition to ADSL technology, which retains the possibility of subscriber access to the switched telephone network (and, if necessary, to the Internet), allows the user to limit himself to only one subscriber line, which is beneficial not only for the latter, but also for the telecom operator.

SDSL (Symmetric Digital Subscriber Line). Just like HDSL technology, SDSL technology provides symmetrical data transmission at speeds corresponding to the speeds of the T 1 / E 1 line, but SDSL technology has two important differences. Firstly, only one twisted pair of wires is used, and secondly, the maximum transmission distance is limited to 3 km. The technology provides the benefits necessary for business representatives: high-speed Internet access, organization of multi-channel telephone communication (VoDSL technology), etc. MSDSL (Multi-speed SDSL) technology, which allows you to change the transmission speed to achieve optimal range and vice versa.

SDSL can be described in the same way as HDSL. True, it allows you to travel a shorter distance than HDSL, but you can save on a second pair. Very often, the user’s office is no more than 3 km from the operator’s point of presence, and then this technology has a clear advantage over HDSL in terms of price/quality of service for its user. The MSDSL option allows, if the cable condition is not very good, to cover the same distance, but at a lower speed; moreover, not all clients need the full 2 ​​Mbit/s and very often 256 or even 128 kbit/s is sufficient.

As another modification of SDSL, HDSL2 equipment is used, which is an improved version of HDSL using a more efficient linear transmission code.

Possibilities of ADSL's own evolution: from Internet access to the provision of a full range of network services

The considered methods of broadband access migration concern the lower, physical level multi-level telecommunications model, since xDSL technologies themselves are essentially technologies of the physical layer. No less interesting are the paths of ADSL's own evolution from access to the Internet to the provision of a full range of network services. By a full range of network services we mean primarily multimedia services and interactive video.

Currently, approximately 85% of the total volume of broadband services is Internet access and only 15% is access to multimedia services and interactive television. Therefore, the first stage of broadband access will in the vast majority of cases be access to the Internet. The strategy for providing broadband services is currently quite fully represented by the developed ITU-T concept of a broadband network with integration of ISDN services, briefly called B-ISDN. Asynchronous transmission method (ATM) was chosen as a key element of the B-ISDN network, which is based on the concept of optimal use of channel bandwidth for transmitting heterogeneous traffic (speech, images and data). Therefore, ATM technology claims to be a universal and flexible transport, which is the basis for building other networks.

ATM, like any revolutionary technology, was created without taking into account the fact that large investments have been made in existing technologies, and no one will abandon old, well-functioning equipment, even if new, more advanced equipment has appeared. Therefore, the ATM method primarily appeared in territorial networks, where the cost of ATM switches compared to the cost of the transport network itself is relatively small. For a LAN, replacing switches and network adapters is almost equivalent to a complete replacement of network equipment, and the transition to ATM can only be caused by very serious reasons. Obviously, the concept of gradual introduction of ATM into the user’s existing network looks more attractive (and, perhaps, more realistic). In principle, ATM allows you to directly transport application-level protocol messages, but is more often used as a transport for protocols of the link and network layers of networks that are not ATM networks (Ethernet, IP, Frame Relay, etc.).

ATM technology is currently recommended by both the ADSL Forum and the ITU-T for the ADSL line equipment itself (i.e. the ATU-C access point modem and the ATU-R remote user premises modem). This is explained primarily by the fact that ATM is the standard for the B-ISDN broadband access network.

At the same time, the vast majority of servers and user equipment on the Internet support TCP/IP and Ethernet protocols. Therefore, when moving to ATM technology, it is necessary to make maximum use of the stack of existing TCP/IP protocols as the main tool for broadband access to the Internet. This applies not only to transport and network layer TCP/IP, but also at the link level. The above primarily applies to the protocol (or rather, to the protocol stack) PPP ("Point to point protocol"), which is a link-level protocol of the TCP/IP protocol stack and regulates the procedures for transmitting information frames over serial communication channels.

The PPP protocol is currently widely used by network providers to access Internet services using analog modems and provides the ability to control so-called AAA functions:

• § Authentication (authentication, i.e. the process of user identification).
• § Authorization (authorization, i.e., access rights to specific services).
• § Accounting (resource accounting, including tariffing of services).

While performing all these functions, the protocol also guarantees the necessary protection of information. Equally important for an Internet provider is the ability to dynamically distribute a limited number of IP addresses among its clients. This function is also supported by the PPP protocol. Thus, it is very important for both the Internet provider and the user to preserve the PPP protocol when accessing the Internet broadband via an ADSL line using the ATM method.

In addition to the considered method of operating an ADSL network using ATM technology, which is briefly called “PPP over ATM”, there are a number of others: “Classical IP over ATM” (“Classical IP and ARP over ATM” or IPOA), the “Emulation” specification developed by the ATM Forum local networks" (LAN emulation or LANE), the new ATM Forum specification "Multiprotocol Over ATM" (or MPOA).

Although the ATM standard is recognized as the most promising universal standard for the transmission of heterogeneous information (speech, video and data), it is not without its drawbacks, the main of which is still the complex and lengthy process of setting up a permanent virtual channel PVC.

Currently, the most popular data transfer protocol, primarily for Internet applications, is the TCP/IP protocol stack. In connection with the advent of ATM technology, the question arises: “Should we completely abandon TCP/IP and adopt only ATM?” Life has shown that the best thing to do is to combine the advantages of these two technologies. Therefore, as a tool for migrating ADSL technology from Internet access to the provision of a full set of network services, the ADSL Forum considers not only the ATM method, but also the TCP/IP standard. This is quite logical and in the interests of both telecom operators and users, given the wide variety of local access network conditions.

From ADSL to VDSL

As user demand for increased capacity increases, pure copper subscriber access networks will increasingly migrate to combined copper-optical networks, collectively known as FITL (Fiber In The Loop). As the optical fiber in this combined network approaches the user's premises on its copper section, VDSL technology may be in demand, which will replace ADSL. VDSL (Very High Bit-Rate Digital Subscriber Line). VDSL technology is the highest speed xDSL technology. In the asymmetric version, it provides a downstream data transfer rate ranging from 13 to 52 Mbit/s, and an upstream data transfer rate ranging from 1.6 to 6.4 Mbit/s, in a symmetric version - in ranging from 13 to 26 Mbit/s, over one twisted pair of telephone wires. VDSL technology can be considered as a cost-effective alternative to laying fiber optical cable to the end user. However, the maximum data transmission distance for this technology ranges from 300 m (at a speed of 52 Mbit/s) to 1.5 km (at a speed of up to 13 Mbit/s). VDSL technology can be used for the same purposes as ADSL; In addition, it can be used to transmit high-definition television (HDTV), video-on-demand, etc. signals.

Our lag in the development of data transmission networks played a positive role - operators did not have time to invest significant funds in equipment for switched narrowband ISDN networks, as well as in the development of subscriber sections of data transmission networks based on HDSL and IDSL equipment.

From the above, it is clear that in Russian conditions the most widespread scenario will be the evolution of wired subscriber access networks from an analog modem to ADSL. Already today, the demand for high-speed Internet access services has grown so much that it makes sense to at least begin to study the economic and technical issues of deploying subscriber access networks based on xDSL technologies.

Thus, each technology from the xDSL technology family successfully solves the problem for which it was developed. Two of them - ADSL and VDSL - allow telephone operators to provide new types of service, and the existing telephone network has real prospects of becoming a full-service network. As for the operators themselves, most likely, over time, only those that can provide the user with the maximum range of services will remain.

Connecting subscribers using fiber optics

Equipment for connecting subscribers using optical cable has become widespread in Europe and the USA. The advantages of such a solution are obvious: high reliability, transmission quality, and throughput, therefore, virtually unlimited speed at the user interface. Unfortunately, this decision It also has disadvantages. Firstly, the time required to lay the cable and get all necessary permits can be quite significant, which reduces the rate of return on investment. Secondly, the use of optical fiber can be economically justified only when connecting a large number of subscribers concentrated in one place, for example in areas of mass development or in office buildings. In areas where the subscriber density is low, only 5-10% of optical cable resources are used, so it is more economical to densify the existing cable network or use radio access.

Nowadays, optical fiber is widely used instead of multi-core telephone cables in the section between a telephone exchange (PBX) and a remote hub, to which, for example, telephones installed in apartments of a multi-storey building or several buildings are connected. Equipment that implements multiplexing/demultiplexing of lines individual connection subscribers, was called Digital Loop Carrier (DLC), which can be translated as “digital concentration system telephone lines" Such systems are produced in the USA, Western Europe, Asia (AFC, SAT, Siemens, etc.). Several enterprises are preparing to release DLC in Russia.

By its architecture, the DLC equipment is a time division multiplexer with various user interfaces and a line interface for direct connection to the optical fiber. This ensures the integration of multiple subscriber lines into one high-speed digital stream arriving at the PBX (network node) via an optical cable.

Kit user interfaces, as a rule, includes an analog subscriber two-wire interface (regular telephone), an analog interface with E&M signaling, a digital interface (V.24 or V.35), an ISDN interface. Station interfaces provide for connection to analog PBXs (via a subscriber two-wire interface or E&M interface), digital PBXs (via the E! interface with V.51 signaling or the EZ interface with V.52 signaling). Naturally, a connection via the ISDN interface and the V.24/V.35 digital interface (for connecting to a data network) is also provided.

Linear interfaces of modern DLC equipment can be divided into several groups:

• § An optical interface is required for direct connection to optical fibers (line speed usually ranges from 34 to 155 Mbit/s). For example, in the NATEKS 1100E system the speed is 49.152 Mbit/s, reception and transmission are carried out separately over two fibers, the wavelength of the laser emitter is 1310 nm.
• § Electrical interface - from E! (2 Mbit/s) to EZ (34 Mbit/s) - allows you to connect to high-speed networks that provide transparent transmission of digital streams (for example, to an SDH network). The electrical interface also allows you to connect equipment via HDSL paths or microwave lines, and on short distances(up to 1 km along E!) connect the system elements directly.

Subscriber access network – this is a set of technical means between terminal subscriber devices installed in the user’s premises and that switching equipment, the numbering (or addressing) plan of which includes terminals connected to the telecommunication system.

5.1. Subscriber access network models

In a modern telecommunications system, not only the role of the access network is changing. In most cases, the territory within the boundaries of which the access network is created also expands. In order to eliminate the differences in the interpretation of the place and role of the access network in modern publications, in Fig. Figure 5.1 shows a model of a promising telecommunications system.

Figure 5.1 – Model of a telecommunication system

The first element of the telecommunications system is a set of terminal and other equipment that is installed in the premises of the subscriber (user). In English technical literature, this element of the telecommunications system corresponds to the term Customer Premises Equipment (CPE).

The second element of the telecommunications system is, in fact, the subscriber access network. The role of the subscriber access network is to ensure interaction between the equipment installed at the subscriber's premises and the transit network. Typically, a switching station is installed at the interface between the subscriber access network and the transit network. The space covered by the subscriber access network lies between the equipment located on the subscriber's premises and this switching station.

The subscriber access network is divided into two sections - the lower plane of Fig. 5.1. Subscriber lines (Loop Network) can be considered as individual means of connecting terminal equipment. As a rule, this fragment of the subscriber access network is a collection of AL. The Transfer Network serves to improve the efficiency of subscriber access facilities. This fragment of the access network is implemented on the basis of transmission systems, and in some cases load concentration devices are also used.

The third element of the telecommunications system is the transit network. Its functions are to establish connections between terminals included in various subscriber access networks, or between a terminal and means of supporting any services. In the model under consideration, the transit network can cover an area either within the same city or village, or between the subscriber access networks of two different countries.

The fourth element of the telecommunications system illustrates the means of access to various telecommunication services. In Fig. 5.1, in the last ellipse, the name is indicated in the original language (Service Nodes), which is translated in three words - nodes that support services. Examples of such a node can be the workplaces of telephone operators and servers in which any information is stored.

Shown in Fig. 5.1 structure should be considered as a promising model of a telecommunication system. To solve terminological problems, let us turn to the model inherent in subscriber access networks of analogue telephone exchanges. Such a model is shown in Fig. 5.2. When considering existing local networks, we, as a rule, will use two terms - “Subscriber network” or “AL network”. The words "Subscriber Access Network" are used when we're talking about about a promising telecommunications system.

Figure 5.2 – Subscriber network model

This model is valid for both GTS and STS. Moreover, for the GTS shown in Fig. 5.2 model is invariant to the structure of interstation communication. It is identical for:

non-zoned networks consisting of only one telephone exchange;

regionalized networks, which consist of several regional automatic telephone exchanges (RATS), connected to each other on the principle of “each to each”;

regionalized networks built with incoming message nodes (INOs) or with outgoing message nodes (UIS) and OMS.

For all elements of the subscriber network, terms in English are indicated in brackets. It should be noted that the term “inter-cabinet communication line” (Link cable) is not yet used in domestic terminology, since such routes are almost never used in GTS and STS.

A model illustrating the main options for constructing a subscriber network is shown in Fig. 5.3. This picture details some parts of the previous model.

Figure 5.3 – Main construction options

subscriber network

In Fig. 5.3 uses a number of notations that are rarely found in domestic technical literature. The cross-connection point is shown as two concentric circles. This symbol is often used in ITU documents. Also typical is the designation of a distribution box (Distribution point) with a black square.

The model shown in Fig. 5.3 can be considered universal with respect to the type of switching station. In principle, it is the same for both a manual telephone exchange and the most modern digital information distribution system. Moreover, this model is invariant to the type of interactive network, such as telephone or telegraph.

On the other hand, a digital switching station can have its own model that will more accurately reflect the specifics of the subscriber access network. This task is quite difficult. The problem is that the process of introducing a digital switching station leads to changes in the structure of the local telephone network. In some cases, this significantly affects the structure of the subscriber network. A typical example of such a situation is the installation of a digital switching station, replacing several old electromechanical stations. The station section of the digital switching station - with this method of modernizing the local telephone network - actually unites all the territories served by previously dismantled electromechanical telephone exchanges. In addition, when implementing a digital switching station, specific (permanent or temporary) solutions may arise when some groups of remote subscribers are connected through the use of concentrators.

Of course, such decisions must be taken into account at the stage of developing the general concept for modernizing the local telephone network. When the appropriate conceptual decisions have been made, you can begin to search optimal options building a subscriber access network. For a hypothetical digital switching station, these options are presented in Fig. 5.4. The last two figures (5.3 and 5.4) have a number of common points.

Figure 5.4 – Subscriber access network model for a digital switching station

Firstly, both structures imply the presence of a so-called “direct power supply zone” - an enclave within which power lines are connected directly to the cross-connect (without connecting cables in distribution cabinets).

Secondly, behind the “direct power zone” there is the next area of ​​the access network, for which in a digital station it is advisable to use remote subscriber modules (hubs or multiplexers), and for an analog PBX - either uncompressed cables or channels formed by transmission systems.

Thirdly, it should be noted that the structure of the subscriber network - regardless of the type of switching station - corresponds to a graph with a tree topology. This is significant from the point of view of communication reliability: the use of digital switching technology not only does not increase the AL availability factor, but, in some cases, reduces it due to the introduction of additional equipment in the area from the ATS cross-country to the user terminal.

To compile a list of terms required further and, especially, to establish correspondence between the concepts adopted in domestic practice and ITU documents, it is advisable to provide the structure of the AL network, presented at the top of Fig. 5.5.

For the AL block diagram (upper part of Fig. 5.5), three options are presented for connecting the subscriber terminal to the switching station.

The top branch of this figure shows a promising option for connecting the TA without the use of intermediate crossover equipment. The cable is laid from the cross-connection to the distribution box, where the telephone is connected via subscriber wiring.

The middle branch of the figure shows a variant of connecting the TA using a cabinet system, when intermediate equipment is placed between the cross-connection and the distribution box. In our model, the role of such equipment is assigned to the distribution cabinet.

In some cases, AL is organized using overhead communication lines (ACL). In Fig. 5.5 this option is shown on the bottom branch. In such a situation, a cable box (CB) and input-output insulators are installed on the pole. At the location of the distribution box, a subscriber protective device (APD) is installed, which prevents the possible influence of dangerous currents and voltages on the unit. It should be noted that the organization of a substation or its individual sections through the construction of overhead lines is not recommended; but in some cases this is the only option for organizing subscriber access.

Figure 5.5 – Block diagram and joints of subscriber line equipment for GTS and STS