Global communications based on circuit-switched networks. Circuit switched networks
Classification of networks.
By territorial distribution
PAN (Personal Area Network) is a personal network designed for the interaction of various devices belonging to the same owner.
LAN (Local Area Network) - local networks that have a closed infrastructure before reaching service providers. The term “LAN” can describe both a small office network and a network at the level of a large factory covering several hundred hectares. Foreign sources even give a close estimate of about six miles (10 km) in radius. Local networks are closed networks; access to them is permitted only to a limited number of users for whom work in such a network is directly related to their professional activities.
CAN (Campus Area Network) - unites local networks of nearby buildings.
MAN (Metropolitan Area Network) - urban networks between institutions within one or several cities, connecting many local area networks.
WAN (Wide Area Network) is a global network covering large geographic regions, including both local networks and other telecommunication networks and devices. An example of a WAN is a packet-switching network (Frame relay), through which various computer networks can “talk” to each other. Global networks are open and focused on serving any users.
The term "enterprise network" is also used in the literature to refer to the combination of several networks, each of which can be built on different technical, software and information principles.
By type of functional interaction
Client-server, Mixed network, Peer-to-peer network, Multi-peer network
By type of network topology
Tire, Ring, Double Ring, Star, Honeycomb, Lattice, Tree, Fat Tree
By type of transmission medium
Wired (telephone wire, coaxial cable, twisted pair, fiber optic cable)
Wireless (transmitting information via radio waves in a certain frequency range)
By functional purpose
Storage Networks, Server Farms, Process Control Networks, SOHO Networks, House Networks
By transmission speed
low-speed (up to 10 Mbit/s), medium-speed (up to 100 Mbit/s), high-speed (over 100 Mbit/s);
If necessary to maintain a constant connection
Packet network such as Fidonet and UUCP, Online network such as Internet and GSM
Circuit switched networks
One of the most important issues in computer networks is the issue of switching. The concept of switching includes:
1. route distribution mechanism for data transmission
2. synchronous use of the communication channel
We will talk about one of the ways to solve the switching problem, namely about circuit-switched networks. But it should be noted that this is not the only way to solve the problem in computer networks. But let's move closer to the essence of the issue. Circuit switched networks form a common and unbreakable physical section (channel) of communication between the end nodes, through which data passes at the same speed. It should be noted that the same speed is achieved due to the absence of a “stop” in certain sections, since the route is known in advance.
Establishing a connection to circuit switched networks always starts first, because you can’t get a route to the desired goal without connecting. And after the connection is established, you can safely transfer the necessary data. Let's take a look at the benefits of circuit switched networks:
1. data transfer speed is always the same
2. there is no delay at the nodes during data transmission, which is important for various On-line events (conferences, communication, video broadcasts)
Well, now I need to say a few words about the shortcomings:
1. It is not always possible to establish a connection, i.e. sometimes the network may be busy
2. We cannot immediately transfer data without first establishing a connection, i.e. time is wasted
3. not very efficient use of physical communication channels
Let me explain about the last minus: when creating a physical communication channel, we completely occupy the entire line, leaving no opportunity for others to connect to it.
In turn, circuit-switched networks are divided into 2 types, using different technological approaches:
1. Frequency Division Multiplexing (FDM) based circuit switching
The scheme of work is as follows:
1. each user transmits a signal to the switch inputs
2. All signals with the help of a switch fill the ΔF bands using the method of frequency modulation of the signal
2. Circuit switching based on time division multiplexing (TDM)
Principle circuit switching based on time multiplexing is quite simple. It is based on time division, i.e. Each communication channel is serviced in turn, and the period of time for sending a signal to the subscriber is strictly defined.
3.Packet switching
This switching technique was specifically designed for efficient transmission of computer traffic. The first steps towards creating computer networks based on circuit switching technology showed that this type of switching does not allow achieving high overall network throughput. Typical network applications generate traffic very sporadically, with high levels of data rate burstiness. For example, when accessing a remote file server, the user first views the contents of that server's directory, which results in the transfer of a small amount of data. It then opens the required file in text editor, and this operation can create quite a lot of data exchange, especially if the file contains large graphical inclusions. After displaying a few pages of a file, the user works with them locally for a while, which requires no network transfer at all, and then returns modified copies of the pages to the server - again creating intensive network transfer.
The traffic ripple factor of an individual network user, equal to the ratio of the average intensity of data exchange to the maximum possible, can reach 1:50 or even 1:100. If for the described session we organize channel switching between the user’s computer and the server, then most of the time the channel will be idle. At the same time, the switching capabilities of the network will be assigned to this pair of subscribers and will not be available to other network users.
When packet switching occurs, all user-transmitted messages are broken down at the source node into relatively small pieces called packets. Let us recall that a message is a logically completed piece of data - a request to transfer a file, a response to this request containing the entire file, etc. Messages can be of any length, from a few bytes to many megabytes. On the contrary, packets can usually also have a variable length, but within narrow limits, for example from 46 to 1500 bytes. Each packet is provided with a header that specifies the address information needed to deliver the packet to the destination node, as well as the packet number that will be used by the destination node to assemble the message (Figure 3). Packets are transported over the network as independent information blocks. Network switches receive packets from end nodes and, based on address information, transmit them to each other, and ultimately to the destination node.
Packet network switches differ from circuit switches in that they have internal buffer memory for temporary storage of packets if the output port of the switch is busy transmitting another packet at the time the packet is received (Fig. 3). In this case, the packet remains for some time in the packet queue in the buffer memory of the output port, and when its turn reaches it, it is transferred to the next switch. This data transmission scheme allows you to smooth out traffic pulsation on backbone links between switches and thereby use them most effectively to increase the capacity of the network as a whole.
Indeed, for a pair of subscribers, the most effective would be to provide them with sole use of a switched communication channel, as is done in circuit-switched networks. In this case, the interaction time of this pair of subscribers would be minimal, since data would be transmitted from one subscriber to another without delay. Subscribers are not interested in channel downtime during transmission pauses; it is important for them to quickly solve their problem. A packet-switched network slows down the process of interaction between a particular pair of subscribers, since their packets can wait in the switches while other packets that arrived at the switch earlier are transmitted along the backbone links.
However, the total amount of computer data transmitted by the network per unit time using the packet switching technique will be higher than using the circuit switching technique. This happens because the ripples of individual subscribers, in accordance with the law of large numbers, are distributed in time so that their peaks do not coincide. Therefore, switches are constantly and fairly evenly loaded with work if the number of subscribers they serve is really large. In Fig. Figure 4 shows that the traffic coming from end nodes to switches is distributed very unevenly over time. However, higher-level switches in the hierarchy that service connections between lower-level switches are more evenly loaded, and packet flow on the trunk links connecting upper-level switches is at near-maximum utilization. Buffering smoothes out ripples, so the ripple factor on trunk channels is much lower than on subscriber access channels - it can be equal to 1:10 or even 1:2.
The higher efficiency of packet-switched networks compared to circuit-switched networks (with equal communication channel capacity) was proven in the 60s both experimentally and using simulation modeling. An analogy with multiprogram operating systems is appropriate here. Each individual program in such a system takes longer to execute than in a single-program system, where the program is allocated all the processor time until its execution completes. However, the total number of programs executed per unit of time is greater in a multi-program system than in a single-program system.
A packet-switched network slows down the process of interaction between a specific pair of subscribers, but increases the throughput of the network as a whole.
Delays at the transmission source:
· time to transfer headers;
· delays caused by the intervals between the transmission of each next packet.
Delays in each switch:
· packet buffering time;
switching time, which consists of:
o waiting time for a packet in the queue (variable value);
o the time it takes for a packet to move to the output port.
Advantages of Packet Switching
1. High overall network throughput when transmitting bursty traffic.
2. The ability to dynamically redistribute the capacity of physical communication channels between subscribers in accordance with the real needs of their traffic.
Disadvantages of Packet Switching
1. Uncertainty in the data transfer rate between network subscribers, due to the fact that delays in the buffer queues of network switches depend on the overall network load.
2. Variable delay of data packets, which can be quite long during moments of instantaneous network congestion.
3. Possible data loss due to buffer overflow.
Currently, methods are being actively developed and implemented to overcome these shortcomings, which are especially acute for delay-sensitive traffic that requires a constant transmission speed. Such methods are called Quality of Service (QoS) methods.
Packet switched networks, which implement quality of service methods, allow the simultaneous transmission of various types of traffic, including such important ones as telephone and computer traffic. Therefore, packet switching methods today are considered the most promising for building a converged network that will provide comprehensive high-quality services for subscribers of any type. However, circuit switching methods cannot be discounted. Today they not only work successfully in traditional telephone networks, but are also widely used to form high-speed permanent connections in the so-called primary (backbone) networks of SDH and DWDM technologies, which are used to create backbone physical channels between telephone or computer network switches. In the future, it is quite possible that new switching technologies will emerge, in one form or another combining the principles of packet and channel switching.
4.VPN Virtual Private Network- virtual private network) is a generalized name for technologies that allow one or more network connections (logical network) to be provided over another network (for example, the Internet). Despite the fact that communications are carried out over networks with a lower unknown level of trust (for example, over public networks), the level of trust in the constructed logical network does not depend on the level of trust in the underlying networks due to the use of cryptography tools (encryption, authentication, public key infrastructure, to protect against repetitions and changes in messages transmitted over the logical network).
Depending on the protocols used and purpose, VPN can provide three types of connections: node-node,node-network And network-network. Typically, VPNs are deployed at levels no higher than the network level, since the use of cryptography at these levels allows transport protocols (such as TCP, UDP) to be used unchanged.
Microsoft Windows users use the term VPN to refer to one of the virtual network implementations - PPTP, which is often used Not to create private networks.
Most often, to create a virtual network, the PPP protocol is encapsulated in some other protocol - IP (this method is used by the implementation of PPTP - Point-to-Point Tunneling Protocol) or Ethernet (PPPoE) (although they also have differences). VPN technology has recently been used not only to create private networks themselves, but also by some “last mile” providers in the post-Soviet space to provide Internet access.
With the proper level of implementation and the use of special software, a VPN network can provide a high level of encryption of transmitted information. When all components are properly configured, VPN technology ensures anonymity on the Internet.
A VPN consists of two parts: an “internal” (controlled) network, of which there may be several, and an “external” network through which an encapsulated connection passes (usually the Internet). It is also possible to connect a separate computer to a virtual network. The connection of a remote user to the VPN is made through an access server, which is connected to both the internal and external (public) network. When a remote user connects (or when establishing a connection to another secure network), the access server requires an identification process, and then an authentication process. After successfully completing both processes, the remote user (remote network) is granted authority to work on the network, that is, the authorization process occurs. VPN solutions can be classified according to several main parameters:
[edit]According to the degree of security of the environment used
Protected
The most common version of virtual private networks. With its help, it is possible to create a reliable and secure network based on an unreliable network, usually the Internet. Examples of secure VPNs are: IPSec, OpenVPN and PPTP.
Trusted
They are used in cases where the transmission medium can be considered reliable and it is only necessary to solve the problem of creating a virtual subnet within a larger network. Security issues become irrelevant. Examples of such VPN solutions are: Multi-protocol label switching (MPLS) and L2TP (Layer 2 Tunnelling Protocol) (more precisely, these protocols shift the task of ensuring security to others, for example, L2TP is usually used in conjunction with IPSec).
[edit]By method of implementation
In the form of special software and hardware
The implementation of a VPN network is carried out using a special set of software and hardware. This implementation provides high performance and, as a rule, a high degree of security.
As a software solution
They use a personal computer with special software that provides VPN functionality.
Integrated Solution
VPN functionality is provided by a complex that also solves filtering problems network traffic, organizing a firewall and ensuring quality of service.
[edit]As intended
They are used to unite several distributed branches of one organization into a single secure network, exchanging data via open communication channels.
Remote Access VPN
Used to create a secure channel between a corporate network segment (central office or branch) and a single user who, working at home, connects to corporate resources with home computer, corporate laptop, smartphone or internet kiosk.
Used for networks to which “external” users (for example, customers or clients) connect. The level of trust in them is much lower than in company employees, so it is necessary to provide special “lines” of protection that prevent or limit the latter’s access to particularly valuable, confidential information.
It is used to provide access to the Internet by providers, usually when several users connect via one physical channel.
Client/Server VPN
It provides protection for transmitted data between two nodes (not networks) of a corporate network. The peculiarity of this option is that the VPN is built between nodes located, as a rule, in the same network segment, for example, between a workstation and a server. This need very often arises in cases where it is necessary to create several logical networks on one physical network. For example, when it is necessary to divide traffic between the financial department and the human resources department accessing servers located in the same physical segment. This option is similar to VLAN technology, but instead of separating traffic, it is encrypted.
[edit]By protocol type
There are implementations of virtual private networks for TCP/IP, IPX and AppleTalk. But today there is a tendency towards a general transition to the TCP/IP protocol, and the vast majority of VPN solutions support it. Addressing in it is most often selected in accordance with the RFC5735 standard, from the range of TCP/IP Private Networks
[edit]By network protocol level
By network protocol layer based on comparison with the layers of the ISO/OSI reference network model.
5. The OSI reference model, sometimes called the OSI stack, is a 7-layer network hierarchy (Figure 1) developed by the International Standardization Organization (ISO). This model contains essentially 2 different models:
· a horizontal model based on protocols, providing a mechanism for interaction between programs and processes on different machines
· vertical model based on services provided by adjacent layers to each other on the same machine
In the horizontal model, two programs require a common protocol to exchange data. In a vertical one, neighboring levels exchange data using API interfaces.
Related information.
FEDERAL COMMUNICATIONS AGENCY
State educational budgetary institution
higher professional education
Moscow Technical University of Communications and Informatics
Department of Communication Networks and Switching Systems
Guidelines
and control tasks
by discipline
SWITCHING SYSTEMS
for 4th year part-time students
(direction 210700, profile - SS)
Moscow 2014
UMD plan for the 2014/2015 academic year.
Guidelines and controls
by discipline
SWITCHING SYSTEMS
Compiled by: Stepanova I.V., professor
The publication is stereotypical. Approved at a department meeting
Communication networks and switching systems
Reviewer Malikova E.E., associate professor
GENERAL GUIDELINES FOR THE COURSE
The discipline “Switching Systems”, part two, is studied in the second semester of the fourth year by students of the correspondence faculty of specialty 210406 and is a continuation and further deepening of a similar discipline studied by students in the previous semester.
This part of the course discusses the principles of exchange of control information and interaction between switching systems, the basics of designing digital switching systems (DSS).
The course includes lectures, a course project and laboratory work. An exam is passed and a course project is defended. Independent work on mastering the course consists of studying the material in the textbook and teaching aids recommended in methodological guidelines, and in completing a course project.
If a student encounters difficulties while studying the recommended literature, then you can contact the Department of Communication Networks and Switching Systems to obtain the necessary advice. To do this, the letter must indicate the title of the book, the year of publication and the pages where unclear material is presented. The course should be studied sequentially, topic by topic, as recommended in the guidelines. When studying this way, you should move on to the next section of the course after you answer all the control questions that are questions on the exam papers and solve the recommended problems.
The distribution of time in student hours for studying the discipline “Switching Systems”, part 2, is shown in Table 1.
BIBLIOGRAPHY
Main
1. Goldstein B.S. Switching systems. – SPb.:BHV – St. Petersburg, 2003. – 318 p.: ill.
2. Lagutin V. S., Popova A. G., Stepanova I. V. Digital channel switching systems in telecommunication networks. – M., 2008. - 214 p.
Additional
3.Lagutin V.S., Popova A.G., Stepanova I.V. Telephony user subsystem for signaling over a common channel. – M. “Radio and Communications”, 1998.–58 p.
4. Lagutin V.S., Popova A.G., Stepanova I.V. The evolution of intelligent services in converged networks. – M., 2008. – 120s.
LIST OF LABORATORY WORKS
1. Signaling 2ВСК and R 1.5, scenario of signal exchange between two automatic telephone exchanges.
2.Management of subscriber data on a digital PBX. Analysis of emergency messages of digital automatic telephone exchange.
METHODOLOGICAL INSTRUCTIONS FOR COURSE SECTIONS
Features of building digital circuit switching systems
It is necessary to study the features of constructing circuit switching systems using the example of a digital PBX of the EWSD type. Consider the characteristics and functions of digital subscriber access units DLU, the implementation of remote subscriber access. Review the characteristics and functions of the LTG line group. Study the construction of a switching field and the typical process of establishing a connection.
The digital switching system EWSD (Digital Electronic Switching System) was developed by Siemens as a universal circuit switching system for public telephone networks. The switching field capacity of the EWSD system is 25200 Erlang. The number of serviced calls in CHNN can reach 1 million calls. The EWSD system, when used as a PBX, allows you to connect up to 250 thousand subscriber lines. A communication center based on this system allows switching up to 60 thousand connecting lines. Containerized telephone exchanges allow connecting from several hundred to 6000 remote subscribers. Switching centers are produced for cellular communication networks and for organizing international communications. There is ample opportunity to organize second choice paths: up to seven direct choice paths plus one last choice path. Up to 127 tariff zones can be allocated. During one day, the tariff can change up to eight times. Generating equipment provides a high degree of stability of the generated frequency sequences:
in plesiochronous mode – 1 10 -9, in synchronous mode –1 10 -11.
The EWSD system is designed to use -60V or -48V power supplies. Temperature changes are allowed in the range of 5-40 ° C with a humidity of 10-80%.
EWSD hardware is divided into five main subsystems (see Fig. 1): digital subscriber unit (DLU); linear group (LTG); switching field (SN); common channel network control (CCNC); coordination processor (CP). Each subsystem has at least one microprocessor, designated GP. Signaling systems R1.5 (foreign version R2) are used, via common signaling channel No. 7 SS7 and EDSS1. Digital subscriber units DLU serve: analog subscriber lines; subscriber lines of users of digital networks with integration of services (ISDN); analogue institutional substations (PBX); digital PBX. DLU blocks provide the ability to switch on analog and digital telephone sets and multifunctional ISDN terminals. ISDN users are provided with channels (2B+D), where B = 64 kbit/s - standard channel of PCM30/32 equipment, D-channel signaling transmission at a speed of 16 kbit/s. To transmit information between EWSD and other switching systems, primary digital trunk lines (DSL, English PDC) are used - (30V + 1D + synchronization) at a transmission speed of 2048 kbit/s (or at a speed of 1544 kbit/s in the USA).
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Fig.1. Block diagram of the EWSD switching system
Local or remote DLU operating mode can be used. Remote DLU units are installed in places where subscribers are concentrated. At the same time, the length of subscriber lines is reduced, and traffic on digital connecting lines is concentrated, which leads to a reduction in the costs of organizing a distribution network and improves transmission quality.
In relation to subscriber lines, a loop resistance of up to 2 kOhm and an insulation resistance of up to 20 kOhm are considered acceptable. The switching system can accept dialing pulses from a rotary dialer arriving at a speed of 5-22 pulses/sec. Frequency dialing signals are received in accordance with CCITT Recommendation REC.Q.23.
High level reliability is ensured by: connecting each DLU to two LTGs; duplication of all DLU units with load sharing; continuously performed self-monitoring tests. To transmit control information between DLUs and LTG line groups, common channel signaling (CCS) is used on time channel number 16.
The main elements of DLU are (Fig. 2):
subscriber line modules (SLM) of the SLMA type for connecting analog subscriber lines and the SLMD type for connecting ISDN subscriber lines;
two digital interfaces (DIUD) for connecting digital transmission systems (PDC) to line groups;
two control units (DLUC) that control internal DLU sequences, distributing or concentrating signal flows to and from subscriber sets. To ensure reliability and increase throughput, the DLU contains two DLUC controllers. They work independently of each other in a task-sharing mode. If the first DLUC fails, the second can take over control of all tasks;
two control networks for transmitting control information between subscriber line modules and control devices;
test unit (TU) for testing telephones, subscriber lines and trunk lines.
The characteristics of DLU change when moving from one modification to another. For example, the DLUB option provides for the use of analogue and digital subscriber kit modules with 16 kits in each module. A single DLUB subscriber unit can connect up to 880 analogue subscriber lines, and it connects to LTG using 60 PCM channels (4096 Kbps). In this case, losses due to a lack of channels should be practically zero. To meet this condition, the throughput of one DLUB should not exceed 100 Erl. If it turns out that the average load per module is more than 100 Erl, then the number of subscriber lines included in one DLUB should be reduced. Up to 6 DLUBs can be combined into a Remote Control Unit (RCU).
Table 1 presents the technical characteristics of the digital subscriber unit of a more modern modification of DLUG.
Table 1. Technical characteristics of the DLUG digital subscriber unit
Using separate lines, coin-operated payphones, analogue institutional-industrial automatic telephone exchanges РВХ (Private Automatic Branch Exchange) and digital РВХ of small and medium capacity can be connected.
We list some of the most important functions of the SLMA subscriber kit module for connecting analog subscriber lines:
line monitoring to detect new calls;
DC power supply with adjustable current values;
analog-to-digital and digital-to-analog converters;
symmetrical connection of ringing signals;
monitoring of loop short circuits and short circuits to ground;
receiving pulses for ten-day dialing and frequency dialing;
changing the polarity of the power supply (reversing the polarity of wires for payphones);
connection of the linear side and the subscriber set side to the multi-position test switch, overvoltage protection;
decoupling of speech signals by DC;
converting a two-wire communication line into a four-wire line.
Function blocks equipped with their own microprocessors are accessed via the DLU control network. Blocks are polled cyclically for readiness to transmit messages, and they are directly accessed for transmitting commands and data. DLUC also carries out testing and monitoring programs to identify errors.
The following DLU bus systems exist: control buses; buses 4096 kbit/s; collision detection tires; buses for transmitting ringing signals and tariff impulses. Signals transmitted along the buses are synchronized by clock pulses. The control buses transmit control information at a transmission rate of 187.5 kbit/s; with an effective data rate of approximately 136 kbit/s.
4096 kbit/s buses transmit speech/data to and from SLM subscriber line modules. Each bus has 64 channels in both directions.
Each channel operates at a transmission rate of 64 kbit/s (64 x 64 kbit/s = 4096 kbit/s). The assignment of 4096 kbit/s bus channels to PDC channels is fixed and determined through the DIUD (see Fig. 3). DLU connection to line groups of type B, F or G (types LTGB, LTGF or LTGG, respectively) is carried out via 2048 kbit/s multiplex lines. The DLU can connect to two LTGBs, two LTGFs (B), or two LTGGs.
Line/Trunk Groupe (LTG) forms the interface between the digital environment of the node and the digital switching field SN (Fig. 4). LTGs perform decentralized control functions and relieve the CP coordination processor from routine work. Connections between the LTG and the redundant switching field are made via a secondary digital link (SDC). The SDC transmission speed from the LTG to the SN field and in the reverse direction is 8192 kbit/s (abbreviated as 8 Mbit/s).
Fig.3. Multiplexing, demultiplexing and
transfer of control information to DLUC
Fig.4. Various options for accessing LTG
Each of these 8 Mbit/s multiplex systems has 127 time slots at 64 kbit/s each to carry payload information, and one time slot at 64 kbit/s is used for message transmission. The LTG transmits and receives voice information through both sides of the switching field (SN0 and SN1), assigning voice information from the active block of the switching field to the corresponding subscriber. The other side of the SN field is considered inactive. If a failure occurs, the transmission and reception of user information immediately begins through it. The LTG power supply voltage is +5V.
LTG implements the following call processing functions:
reception and interpretation of signals arriving through connecting and
subscriber lines;
transmission of signaling information;
transmission of acoustic tones;
transmission and reception of messages to/from the coordination processor (CP);
transmitting reports to group processors (GP) and receiving reports from
group processors of other LTGs (see Fig. 1);
transmission and reception of requests to/from the signaling network controller over a common channel (CCNC);
control of alarms entering the DLU;
coordination of states on lines with states of a standard 8 Mbit/s interface with a duplicated switching field SN;
establishing connections to transmit user information.
Several types of LTG are used to implement different line types and signaling methods. They differ in the implementation of hardware blocks and specific application programs in the group processor (CP). LTG blocks have a large number of modifications, differing in use and capabilities. For example, the LTG block of function B is used to connect: up to 4 primary digital communication lines of the PCM30 type (PCM30/32) with transmission rates of 2048 kbit/s; up to 2 digital communication lines with a transfer rate of 4096 kbit/s for local DLU access.
The LTG function C block is used to connect up to 4 primary digital communication lines with speeds of 2048 kbit/s.
Depending on the purpose of the LTG (B or C), there are differences in the functional design of the LTG, for example, in the group processor software. The exception is modern LTGN modules, which are universal, and in order to change their functional purpose, it is necessary to “recreate” them programmatically with a different load (see Table 2 and Fig. 4).
Table 2. Line Group N (LTGN) Specifications
As shown in Fig. 5, in addition to the standard 2 Mbit/s interfaces (RSMZ0), the EWSD system provides an external system interface with a higher transmission rate (155 Mbit/s) with STM-1 type multiplexers of the SDH synchronous digital hierarchy network on fiber optic lines communications. An N-type termination multiplexer (synchronous dual termination multiplexer, SMT1D-N) installed on the LTGM cabinet is used.
The SMT1D-N multiplexer can be presented in the form of a basic configuration with 1xSTM1 interface (60xРSMЗ0) or in the form of a full configuration with 2xSTM1 interfaces (120хРSMЗ0).
Fig.5. Connecting SMT1 D-N to the network
Switching field SN EWSD switching systems connect the LTG, CP and CCNC subsystems to each other. Its main task is to establish connections between LTG groups. Each connection is simultaneously established through both halves (planes) of the switching field SN0 and SN1, so that if one side of the field fails, there is always a backup connection. In EWSD type switching systems, two types of switching field can be used: SN and SN(B). The switching field type SN(B) is a new development and is characterized by smaller dimensions, higher availability, and reduced power consumption. There are various options for organizing SN and SN(B):
switching field for 504 line groups (SN:504 LTG);
switching field for 1260 line groups (SN: 1260 LTG);
switching field for 252 line groups (SN:252 LTG);
switching field for 63 line groups (SN:63 LTG).
The main functions of the switching field are:
circuit switching; message switching; switching to reserve.
The switching field switches channels and connections at a transmission rate of 64 kbit/s (see Fig. 6). Each connection requires two connecting paths (for example, caller to callee and callee to caller). The coordination processor searches for free paths through the switching field based on information about the occupancy of connecting paths currently stored in the storage device. Switching of connecting paths is carried out by control devices of the switching group.
Each switch field has its own control unit, consisting of a switch group control unit (SGC) and an interface module between the SGCs and a message buffer unit MBU:SGC. With a minimum stage capacity of 63 LTG, one SGC of the switch group is involved in the switching of the connecting path, however, with stage capacities of 504, 252 or 126 LTG, two or three SGCs are used. This depends on whether the subscribers are connected to the same TS group or not. Commands for establishing a connection are issued to each participating GP of the switching group by the CP processor.
In addition to connections specified by subscribers by dialing a number, the switching field switches connections between line groups and the CP coordination processor. These connections are used to exchange control information and are called semi-permanent dial-up connections. Thanks to these connections, messages are exchanged between line groups without consuming resources of the coordination processor unit. Nailed-up connections and connections for signaling over a common channel are also established on the principle of semi-permanent connections.
The switching field in the EWSD system is characterized by complete accessibility. This means that every 8-bit codeword transmitted on a backbone entering the switching field can be transmitted at any other time slot on a backbone emanating from the switching field. All highways with a transmission speed of 8192 kbit/s have 128 channels with a transmission capacity of 64 kbit/s each (128x64 = 8192 kbit/s). Switching field stages with capacities SN:504 LTG, SN:252 LTG, SN:126 LTG have the following structure:
one time switching stage incoming (TSI);
three stages of spatial switching (SSM);
one time switching stage outgoing (TSO).
The small and medium stations (SN:63LTG) include:
one time switching input (TSI) stage;
one spatial switching (SS) stage;
one outgoing time switching stage (TSO).
Fig.6. Example of connection establishment in the switching field SN
Coordination processor 113 (CP113 or CP113C) is a multiprocessor, the capacity of which increases in stages. In the CP113C multiprocessor, two or more identical processors operate in parallel with load sharing. The main functional blocks of the multiprocessor are: the main processor (MAP) for call processing, operation and maintenance; a call processing processor (CAP), designed to process calls; shared storage (CMY); input/output controller (IOC); input/output processor (IOP). Each VAP, CAP and IOP processor contains one program execution unit (PEX). Depending on whether they are to be implemented as VAP processors, CAP processors or I0C controllers, specific hardware functions are activated.
Let us list the main technical data of VAR, CAP and IOC. Processor type - MC68040, clock frequency-25MHz, address width is 32 bits and data width is 32 bits, word width is 32 data bits. Local memory data: expansion - maximum 64 MB (based on 16M bit DRAM); expansion stage 16 MB. Flash EPROM data: 4 MB expansion. The CP coordination processor performs the following functions: call processing (analysis of number digits, routing control, service area selection, path selection in the switching field, call cost accounting, traffic data management, network management); operation and maintenance - input to and output from external storage devices (EM), communication with the operation and maintenance terminal (OMT), communication with the data transfer processor (DCP). 13
The SYP panel (see Fig. 1) displays external alarms, for example, information about a fire. The external EM memory is used to store programs and data that do not need to be permanently stored in the CP, the entire system of application programs for automatic recovery data on tariffication of telephone calls and traffic changes.
The software is focused on performing specific tasks corresponding to the EWSD subsystems. operating system(OS) consists of programs that are close to hardware and are usually the same for all switching systems.
Maximum performance The call processing capacity is over 2,700,000 calls per busy hour. Characteristics of the CP system EWSD: storage capacity - up to 64 MB; addressing capacity - up to 4 GB; magnetic tape - up to 4 devices, 80 MB each; magnetic disk - up to 4 devices, 337 MB each.
The job of the Message Buffer (MB) is to control the exchange of messages:
between coordination processor CP113, and LTG groups;
between CP113 and switching group controllers SGCB) switching field;
between LTG groups;
between LTGs and the signaling network controller via a common CCNC channel.
The following types of information can be transmitted via MV:
messages are sent from DLU, LTG and SN to the coordination processor CP113;
reports are sent from one LTG to another (reports are routed through CP113, but are not processed by it);
instructions are sent from CCNC to LTG and from LTG to CCNC, they are routed through the CP113, but are not processed by it;
commands are sent from CP113 to LTG and SN. The MV converts the information for transmission via the secondary digital stream (SDC) and sends it to the LTG and SGC.
Depending on the capacity stage, a duplicate MB device can contain up to four message buffer groups (MBGs). This feature is implemented in a network node with redundancy, that is, MB0 includes groups MBG00...MBG03, and MB1 includes groups MBG10...MBG13.
EWSD switching systems with signaling over a common channel on system No. 7 are equipped control device of the signaling network via a common CCNC channel. Up to 254 signaling links can be connected to the CCNC device via analogue or digital communication lines.
The CCNC device is connected to the switching field via compressed lines with a transmission speed of 8 Mbit/s. Between the CCNC and each switching field plane, there are 254 channels for each transmission direction (254 channel pairs).
The channels carry signaling data across both SN planes to and from line groups at 64 kbit/s. Analog signal paths are connected to the CCNC via modems. The CCNC consists of: a maximum of 32 groups with 8 signal path terminals each (32 SILT groups); one redundant common channel processor (CCNP).
Control questions
1.In which block is analog-to-digital conversion performed?
2. How many analogue subscriber lines can be included in DLUB? What capacity is this block designed for?
3. At what speed is information transmitted between DLU and LTG, between LTG and SN?
4. List the main functions of the switching field. At what speed is the connection between subscribers implemented.
5. List the options for organizing the switching field of the EWSD system.
6. List the main stages of switching with the switching field.
7.Consider the passage of the conversation path through the switching field of the EWSD switching system.
8. What call processing functions are implemented in LTG blocks?
9. What functions does the MV side implement?
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2.2 Review of imported switching systems
The following switching systems are most suitable for my graduation project: DX-200 from Telenokia (Finland), SI 2000 from Iskratel (Slovenia), AXE-10 from Ericsson (Sweden), EWSD from Siemens (Germany) , S12 Alkatel from Alkatel (Germany).
Electronic digital switching system DX-200. The DX-200 system has been actively used all over the world for many years and during this time has earned respect for its reliable and high-quality work. The DX-200 system is characterized by time division of channels in the switching field and a digital method of information transmission based on the PCM-30/32 transmission system. Control is carried out according to a recorded program using distributed functional control devices implemented on microprocessors. The system is built on a modular principle, both hardware and software. All functional blocks and software are divided into modules independent from each other. Modules interact using standardized signals.
The DX-200 system can be used as a reference station, transit station, as well as subscriber concentrators. The reference station provides the establishment of terminal connections between telephone sets of local network subscribers, as well as access to zonal, long-distance and international networks. The stations are also designed to operate on regionalized networks with incoming and outgoing message nodes, as well as on networks without node formation. Networks can use 5-, 6- and 7-digit numbering, as well as mixed numbering.
The transit station is designed for switching channels, passing the transit load to the city telephone exchange and ensures the organization of incoming message nodes, outgoing message nodes, incoming long-distance message nodes, custom connecting line nodes, combined nodes combining the above nodes, and institutional network nodes.
The DX-200 system provides interaction with stations existing on networks: ten-step, coordinate, quasi-electronic automatic telephone exchanges, as well as with special information services of a city telephone exchange.
A number of additional services are provided for DX-200 subscribers:
1) abbreviated dialing;
3) repeated call without new dialing;
5) transfer of a call if the called subscriber is busy to another telephone set;
6) transferring the call to an answering machine or telephone operator;
7) determination of the number of the called subscriber.
In the DX-200 system, time-based call cost accounting is carried out for outgoing calls, taking into account the category of subscribers.
The DX-200 system includes two types of automatic telephone exchanges: DX-210 and DX-220. The DX-210 is primarily used as a low-capacity automatic telephone exchange. The main characteristics of the DX-200 system are shown in Table 2.2.
Electronic digital switching system SI 2000. The SI 2000 system is designed to serve telephone networks in suburban and rural areas. The advanced networking concept of SI 2000 is the basic strategy. In contrast to other solutions, this concept provides incomparable economic benefits and flexibility. The communication networks of many countries are still mostly analogue, and immediate digitalization of all transmission paths is practically impossible. Along with standard capabilities, the SI 2000 system also has some specific features that serve to optimize solutions related to the creation of a digital communication network.
All SI 2000 telephone exchanges have integrated analog line sets. This solution is the most cost-effective for existing analogue transmission equipment.
Developing an optimized network targeting suburban and rural areas requires the creation of digital islands. The ability of the SI 2000 to synchronize from a digital network allows for the digitalization of subordinate automatic telephone exchanges and transmission paths. To ensure smooth development of the communication network, the SI 2000 node will perform the overall switching and analog-to-digital conversion. If a main digital local telephone exchange is installed, the SI 2000 synchronization will be performed from it without any additional equipment.
The SI 2000 system provides the following services to subscribers:
2) the subscriber has a control meter;
3) observation;
5) call forwarding;
6) abbreviated dialing (direct call);
7) setting to wait
and many others with all the necessary support for accounting for their cost.
The remote modules in the SI 2000 are optimized according to advanced networking concepts. When there is a need for large capacities, the SI 2000 family of autonomous telephone exchanges is used. The autonomous automatic telephone exchange can be converted into a remote module or, conversely, without any changes in the hardware.
Long distance transmission in rural areas is more expensive than in urban areas. In order to save on transmission equipment, the SI 2000 system integrates, as a mandatory feature, a PCM-30 channel branch device. In one PCM path, the stream can be divided into a maximum of 15 stations. Data communications equipment can input or output over two data streams at 64 kilobits per second.
The main advantages of the SI 2000 system are reliability (less than 0.5 failures per 100 lines per year), simplicity, distribution and modularity, and cost-effectiveness [7].
The main characteristics of the SI 2000 system are shown in Table 2.2.
Electronic automatic switching system AXE-10. The AXE-10 switching system can be used as a reference automatic telephone exchange, as various communication centers (including international ones), as well as central, hub and terminal automatic telephone exchanges of low capacity on rural telephone networks .
Depending on the proposed use, there are:
1) local station AX;
2) transit station;
3) a mobile (mobile) communication station to create a cellular communication network.
The maximum capacity of the AXE-10, used as a local automatic telephone exchange, is 200,000 subscriber lines with an average call duration of 100 seconds and a load per subscriber line of up to 0.1 Erlang.
The AXE-10 type transit station is designed for up to 2048 digital connecting lines and allows transit loads of up to 200 thousand subscriber lines included in local automatic telephone exchanges. The permissible load on one channel of a connecting digital line is set to 0.8 Erlang.
For analog-to-digital conversion, pulse-code modulation is used with an information transfer rate of 2048 kilobits per second.
The exchange of control signals with coordinate automatic telephone exchanges is carried out on the basis of the R2 signaling system using the multi-frequency code “2 out of 6”.
For long-distance communications, a single-frequency signaling system is used predominantly; a signaling system using a common signaling channel No. 7 is also used.
Through the operation and maintenance system, constant and comprehensive monitoring of the procedure and results of establishing connections and control of the incoming load is ensured.
Main services provided to subscribers:
1) abbreviated dialing;
3) providing information during a conversation;
4) call forwarding to a telephone or to an answering machine;
5) automatic conference call;
6) setting to wait if the subscriber is busy with notification;
7) calling a subscriber by order;
8) accompanying call;
9) switching to another device when busy or when the subscriber does not answer;
10) restriction of outgoing communication;
11) identification of the calling subscriber number if there is a request from the calling subscriber;
12) automatic wake-up.
The switching system can be used to plan and develop communication networks in rural areas. In this case, long distances and low telephone density must be taken into account. The AXE-10 system for rural areas is based on the same equipment as for the urban digital network. Additionally, a remote subscriber multiplexer is included in the delivery, allowing you to connect up to 128 subscriber lines. The use of cable digital communication lines or radio communication lines is provided for connecting remote subscriber multiplexers with a reference automatic telephone exchange. Options have been developed for placing equipment in special containers containing the necessary devices for inclusion in the power supply network for immediate commissioning.
Services such as Centerx and data transmission via dedicated channels have been specially developed for subscribers in the institutional sector. Using this service, some switching system subscribers are united into groups with closed numbering and a general call from the telephone network via a dedicated number. In practice, institutional automatic telephone exchanges can be created based on the same switching equipment.
The AXE-10 switching system is designed for use as a central station for a cellular communication network of the NMT-450 type. The development of a special subsystem for enabling mobile telephone communications made it possible to organize the pairing of the AXE-10 system with cellular base stations.
The main characteristics of the AXE-10 system are shown in Table 2.2.
Electronic automatic switching system EWSD. The EWSD system has acquired an excellent reputation in many countries around the world due to its reliability, cost-effectiveness and variety of services provided.
Digital electronic exchange EWSD is used: using a remote digital unit to optimize the subscriber network or to introduce new services in an area, as a local telephone exchange, as a transit telephone exchange, as a city and transit long-distance exchange, as a switching center for mobile objects , as a rural station, as a small capacity station, as a container station, as a switching system, as a center for the operation and maintenance of a group of stations, as a node in a common channel signaling system, in a digital integrated service network, to provide special services.
EWSD provides operators with many advantages, which in turn come from the versatility, flexibility and performance of the switching system. The main characteristic features of EWSD include: integrated supervision, including operation supervision, error indication, error analysis procedures and their diagnosis, implementation in existing networks, route selection, alternative route selection, telephone cost recording, load measurement, database management and others.
All standard alarm systems can be used in EWSD. Signaling transmission is also carried out by standard systems. The station can work both with subscribers with ten-day dialing and with subscribers with tone dialing. All standard methods are used to record cost accounting.
The following types of services can be provided to an analogue subscriber:
1) abbreviated dialing;
2) connection without dialing a number (direct connection);
3) connection without time delay;
4) transfer of an incoming call in the absence of a subscriber to the absent subscribers service;
5) autoinformer with pre-recorded phrases;
7) temporary ban on incoming communications;
8) placing a call on hold (if the called subscriber is busy);
9) providing information during a conversation;
10) conference call;
11) a printed record of the duration and cost of the call;
12) automatic wake-up;
13) special subscriber;
14) call priority
and others.
The following types of services can additionally be provided to subscribers of the integrated service digital network:
1) connecting up to eight terminal devices simultaneously;
2) changing the terminal device, selecting the terminal device;
3) mobility of the terminal device;
4) service indicators;
5) change of service during the call;
6) work with simultaneous use of two services;
7) registration of call cost accounting for individual services;
8) calls paid by the subscriber and others.
The main characteristics of the EWSD system are shown in Table 2.2.
Electronic automatic switching system Alkatel S12. When developing the system, much attention was paid to the problems of efficiency in production and operation. Economical production is ensured by a high degree of equipment unification.
The main functional characteristic of the Alkatel S12 station is a decentralized structure based on fully distributed control of both information processing functions and direct switching processes.
Combined with hardware and software modularity, distributed control provides:
1) high reliability of equipment operation;
2) the ability to build a station in a wide range of capacities;
3) flexibility in the planned expansion of system capacities according to customer requirements;
4) resistance to change system requirements in the future, since new applications will only be associated with the addition of new hardware or software modules to the station without changing the architectural principles and basic hardware and software;
5) simplification of software.
The modular architecture of the station ensures the flexible implementation of new technological solutions and the provision of new services under operating conditions without interruptions in operation. New technological solutions and software versions have been implemented on networks in various countries, bringing Alkatel S12 to a perfect level of compliance with the requirements for functional and technical and operational characteristics, as well as ensuring its further evolutionary transition to a narrowband and broadband digital network of integrated services.
The Alkatel S12 station equipment is intended for use on general and special-purpose networks, covering the range of applications from small remote subscriber units to large city and long-distance stations. The main equipment configuration options are:
1) low-capacity city automatic telephone exchanges (from 256 to 5376 subscriber lines);
2) city automatic telephone exchanges of medium and high capacity (up to 100,000 subscriber lines);
3) transit switching nodes (up to 60,000 connecting lines);
4) remote subscriber concentrators (up to 976 subscriber lines).
The Alkatel S12 station provides subscribers with the following types of communication:
1) automatic internal communication between all station subscribers;
2) automatic incoming and outgoing local communication to subscribers of other stations;
3) transit connection between incoming and outgoing lines;
4) automatic communication within a certain group of subscribers;
5) automatic outgoing communication to help desks;
6) semi-permanent switching.
Alkatel S12 subscribers are provided with the following types of additional telephone services:
1) forwarding an incoming call to another device;
2) call forwarding if the subscriber is busy;
3) forwarding an incoming call to an answering machine or operator;
4) an accompanying call using a password to the device from which the services were ordered;
5) search alarm;
6) setting to wait for the called subscriber to become available (waiting with callback);
7) repeat call without dialing;
8) connection with a subscriber by prior order;
9) conference calls and others.
The main characteristics of the Alkatel S12 system are given in table 2.2.
Table 2.2 - Main characteristics of imported switching systems
| Name of parameters | SI 2000 | AX-10 | EWSD | Akatel S12 | |
| Maximum subscriber capacity, numbers | 10400 | 200000 | 250000 | 120000 | |
| Maximum number of trunks | 3600 | 60000 | 60000 | 85000 | |
| Bandwidth,(Earl). | 2500 | 30000 | 25200 | 30000 | |
| Maximum number of calls to CHNN | 80000 | 1000000 | 1000000 | 1000000 | |
| Minimum number of ports on 1 board | 60 | 16 | 128 | 256 | 16 |
| Power consumption per room, (W). | 0,6..0,9 | 0,7..1,0 | 0,65..0,7 | 0,6..1,2 | 0,7..1,1 |
As can be seen from the above, the parameters of imported switching systems are close to each other, and in this case the cost is decisive. It is precisely by this criterion that I chose the AXE-10 switching system as the best in terms of quality-price ratio.
Rice. 3.3. Relationships between time slots and frames
3.2. Placing logical channels on physical channels
It is known that logical channels are formed using physical channels. The method of placing logical channels on physical ones is called "mapping" - mapping.
Although most logical channels occupy only one time slot, some logical channels can occupy more than 1 TS. In this case, logical channel information is transmitted in the same physical channel time slot in successive TDMA frames.
Because logical channels are short, multiple logical channels can occupy the same physical channel, allowing for more efficient use of time slots.
In Fig. 3.4. shows the case when on one carrier cell the DCCH channel occupies an additional time slot due to high load.
Rice. 3.4. Placing logical channels on physical channels
3.2.1. Carrier "0", time slot "0"
The zero time slot on the zero carrier frequency in a cell is always reserved for signaling. Thus, when the MS has determined that the carrier frequency is a BCCH carrier, it knows where and how to read the information.
When transmitting from BTS to MS (downlink), BCH and CCCH information is transmitted. The only channel through which information is transmitted only in the direction from the MS to the BTS (uplink) is the RACH channel. The RACH channel is always free, so the MS can access the network at any time.
3.2.2. Carrier "0", time slot "1"
Typically, the first (“1”) time slot on carrier frequency zero in a cell is also always reserved for signaling purposes. The only exception is for cells experiencing high or low traffic.
As can be seen from Fig. 3.4, if the traffic in the cell is heavy, then the third physical channel can be occupied for connection establishment purposes using DCCH. This channel can be any time slot, excluding time slots "0" and "1" on carrier "0".
This also happens when the cell load is low. In this case, it is possible to occupy the time slot “0” on carrier “0” to transmit/receive all signaling information: BCH, CCCH and DCCH. Thus, physical channel “1” can be freed for traffic.
Eight SDCCH channels and 4 SACCH channels can share the same physical channel. This means that 8 connections can be established simultaneously on one physical channel.
3.2.3. Carrier "0", time slots two through seven, and all other time slots of other carriers of the same cell
All other intervals, except for the “0” and “1” signaling intervals, are used in the cell for traffic, that is, for voice or data transmission. In this case, the TCH logical channel is used.
Additionally, during a conversation, the MS transmits the results of measurements of signal level, quality, and time delay. The SACCH channel is used for this purpose, occupying one TCH time slot for a time.
3.3. Example of servicing an incoming call to MS
Rice. 3.5 schematically shows the service incoming call to MS and use of various control channels.
Rice. 3.5. Call to MS
The MSC/VLR has information about which LA the MS is located in. The paging signaling message is sent to the BSC that controls the LA.
1. The BSC distributes the calling message to all base stations in the desired LA. Base stations transmit calling messages over the air using the PCH channel.
2. When the MS detects a PCH that identifies it, it requests control channel allocation via the RACH.
3. The BSC uses the AGCH to inform the MS which SDCCHs and SACCHs it can use.
4. SDCCH and SACCH are used for connection establishment. The TCH channel is occupied, and the SDCCH channel is released.
5. MS and BTS switch to the frequency of the TCH channel and the time slot allocated for this channel. If the subscriber answers, the connection is established. During the conversation, the radio connection is monitored by information transmitted and received by the MS on the SACCH channel.
Chapter 4 - GPRS Packet data service over public radio channels
GPRS shares a shared physical air interface resource with existing circuit-switched GSM system resources. The GPRS service can be thought of as overlaid on the GSM network. This allows the same physical environment in cells to be used for both circuit-switched voice and packet-switched data. GPRS resources can be allocated for data transmission dynamically during periods when there is no circuit-switched information transmission session.
GPRS will use the same physical channels, but the efficiency of using them is much greater compared to traditional circuit-switched GSM, since multiple GPRS users can use the same channel. This allows for increased channel utilization. In addition, GPRS uses resources only during the period of data transmission and reception.
4.1 GPRS network architecture
The figure below shows the structure of the GPRS system. Since GPRS is a new GSM service, it uses the existing GSM infrastructure with some modifications. The GPRS system solution was designed to allow GPRS to be quickly implemented on networks at low cost.
To implement GPRS, it is necessary to upgrade the software of elements of existing GSM networks, with the exception of BSC, which requires hardware upgrades (see Fig. 4.1). Two new nodes appear in the GSM network: the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). These two nodes can be physically implemented as one hardware node. Flexible implementation of GPRS is possible, initially it is possible, for example, to implement a centralized GPRS node, which can be a combination of SGSN and GGSN nodes. In the next stage, they can be divided into dedicated SGSNs and GGSNs.
The following describes how the implementation of GPRS affects GSM nodes and what GPRS terminals exist on the network.
Rice. 4.1 GPRS Network Architecture (BSS, CSS and PSS shown)
The interface between the SSGN and the BSC supports the open Gb interface defined in the ETSI standard. This interface allows the operator to work with a multi-vendor configuration.
4.2 Base Station System (BSS)
The GPRS system interacts with the MS over the radio interface by transmitting and receiving radio signals through the BSS system. The BSS controls the transmission and reception of radio signals for all types of messages: voice and data transmitted in circuit switched and packet switched modes. When implementing GPRS, BTS base stations require additional software and additional hardware units.
BSS is used to separate circuit-switched and packet-switched data, since only circuit-switched messages are sent to the MSC. Packets are forwarded to new GPRS packet switching nodes.
Circuit Switching System (CSS)
CSS is a traditional GSM network SS system, which includes the previously discussed nodes (see Chapter 1, section 1.7: “Description of GSM network components”).
When implementing GPRS, it is necessary to upgrade the MSC software, which allows you to perform combined GSM/GPRS procedures, for example, a combined MS connection procedure (Attach): IMSI/GPRS.
The introduction of GPRS does not affect the GMSC, since this center is involved in establishing connections to GSM subscribers from PSTN fixed-line subscribers.
HLR is a database that contains all subscriber data, including data related to GPRS subscriptions. Thus, the HLR stores data for both the circuit switching service and the packet switching service. This information includes, for example, permission/denial of the subscriber to use GPRS services, the Access Point Name (APN) of the Internet Service Provider (ISP), as well as an indication of whether IP addresses are allocated to the MS . This information is stored in the HLR as a PDP context subscription. The HLR can store up to 5 PDP contexts per subscriber. Information stored in the HLR is accessed from the SGSN. When roaming, access to information may occur in an HLR that is not associated with its own SGSN.
For HLR to work in a GPRS network, its software must be upgraded.
4.3.1 Authentication Center (AUC)
AUC does not require any upgrade when working with GPRS. The only new feature from the point of view of AUC in the GPRS network is the new encryption algorithm, which is defined for GPRS as A5.
Short Message Service - Interworking MSC (SMS-IW-MSC) allows MSs with GPRS capabilities to send and receive SMS over GPRS radio channels. SMS-IW-MSC does not change when GPRS is implemented.
4.3.2 Packet Switching System (PSS)
PSS is a new system designed specifically for GPRS. This system is based on Internet Protocols (IP). It includes new packet switching nodes, generally known as GSNs (GPRS Support Nodes). There are currently two types of GPRS nodes: Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). SGSN interfaces connect it to standard GSM network nodes, such as MSC/BSC, and GGSN interfaces connect this node to external packet data networks, such as the Internet or corporate Internet.
4.3.3 GGSN terminals
There are three classes of MS that can work with GPRS.
Class A: Class A MS supports GPRS and other GSM services simultaneously. This means that the MS simultaneously performs the functions of attaching, activating, monitoring, transmitting information, etc. for both voice and packet data transmission. A Class A MS can simultaneously serve a call for voice service and receive packet data.
Class B: A Class B MS simultaneously monitors GSM and GPRS channels, but can receive/transmit information from either circuit-switched or packet-switched services at any given time.
Class C: Class C MS only supports non-concurrent operations, such as attach. If an MS of this class supports both GSM and GPRS services, it can only receive calls from the default or operator-assigned service. Services that are not assigned or selected are unavailable.
4.3.4 Other objects
Billing Gateway (BGw).
BGw facilitates the implementation of GPRS in the network mobile communications by implementing functions that simplify the management of charging for GPRS in the billing system. In particular, the Advanced Processing function is very useful - advanced processing of billing information.
The charging criteria for GPRS services are fundamentally different from those for circuit-switched services. In particular, they are based on the amount of information transmitted/received, not on the time the channels are occupied. A GPRS session can be active for a fairly long period of time, while actual data transmission occurs in short periods of time when free radio resources are available. In this case, the time it takes to occupy radio resources is an insignificant criterion for calculating the charge in comparison with the data volume.
Charging information can be obtained from SGSNs and GGSNs using interfaces other than the MSC interfaces and a new type of CDR report is generated for this information. Some new types of CDR are:
· S-CDRs associated with radio network usage and transmitted from the SGSN.
· G-CDRs associated with the use of external data networks and transmitted from the GGSN.
· CDRs associated with the use of GPRS-based short message service.
During one GPRS session, several S-CDRs and G-CDRs can be generated.
BGw allows you to charge for data services with minimal impact on existing billing systems. BGw can either transform the data into a format that is recognized by the existing billing system, or it can be used to create a new billing application specifically tailored for volume charging. This allows you to implement data services very quickly and charge for using services immediately, in real time.
GPRS support nodes
The GPRS support nodes are SGSN and GGSN, each of which performs specific functions within the GPRS network. These specific individual functions are described below.
Serving GPRS Support Node (SGSN)
The SGSN is located in the GPRS network as shown in Fig. 4.2. This node communicates with BSC, MSC/VLR, SMS-G and HLR. This node connects to the backbone network to communicate with the GGSN and other SGSNs.
Rice. 4.2 SGSN interfaces
SGSN serves all GPRS subscribers physically located within SGSN's geographic service area. The SGSN performs functions in GPRS similar to those performed by the MSC in the GSM network. That is, this node controls the functions of connecting, MS disconnecting, updating location information, etc. GPRS subscribers can be served by any SGSN node in the network depending on their location.
SGSN functions.
As part of the GPRS network, the SGSN node performs the following functions. Mobility Management (MM). The SGSN node implements the functions of the MM protocol in the MS and over network interfaces. MM procedures supported over this interface are IMSI connection for both GPRS and circuit switched calls, routing zone update, combined routing zone and location zone update, paging signaling.
The MM protocol allows the network to support mobile subscribers. MM allows an MS to move from one cell to another, to move from one SGSN routing area to another, to move between SGSN nodes within a GPRS network.
The concept of location area (LA) is not used in GPRS. The analogue of this concept in GPRS is the routing area (RA). An RA consists of one or more cells. In the first implementation, RA was equivalent to LA.
MM allows subscribers to transmit and receive data while moving within their PLMN network, as well as when moving to another PLMN network. The SGSN supports the standard Gs interface in the MSC/VLR direction for MS classes A and B, which allows the following procedures:
- Combined connection/disconnectionGPRS/ IMSI. The “IMSI attach” procedure is carried out via SGSN. This allows you to combine/combine actions and thus save radio resources. These actions depend on the MS class.
- Combined paging. If the MS is registered simultaneously as an IMSI/GPRS terminal (mode I operation), the MSC/VLR performs paging via the SGSN. The network may also coordinate the provision of circuit-switched or packet-switched services. Paging coordination means that the network transmits paging messages for circuit-switched services over the same channels used for packet-switched services, that is, the GPRS paging channel or the GPRS traffic channel.
- Combined location updates(LA location areas or RA routing areas) for GSM circuit switched services and GPRS packet switched services. The MS performs location update functions separately by transmitting information about the new LA to the MSC and the new RA to the SGSN. Through the Gs interface, both nodes: MSC and SGSN can exchange information about updating the subscriber's location, thereby allowing each other to perform the update. This allows you to save on signaling functions over the air interface.
Session Management (SM)
SM procedures include activating a packet data protocol (PDP) context, deactivating that context, and modifying it.
The PDP context is used to establish and release a virtual data link between the terminal connected to the MS and the GGSN.
The SGSN then stores data which includes:
PDP context identifier is an index used to point to a specific PDP context.
PDP type. This is a PDP context type. IPv4 is currently supported.
PDP address. This is the address of the mobile terminal. This is either an IPv4 address if the subscriber specifies it when concluding a contract for the provision of packet data services, or it is an empty set when using the dynamic address assignment mode.
Access Node Name (APN). This is the network identifier of the external network, for example: wap. *****
Defined quality of service (QoS). This is a QoS profile that a subscriber can subscribe to.
The PDP context must be active in the SGSN before any packet data unit (PDU) can be sent to or received from the MS.
When the SGSN receives a PDP context activation request message, it requests the permission control function. This feature limits the number of registrations within a single SGSN and monitors quality within each zone. The SGSN then checks whether the subscriber is allowed access to a specific ISP network or corporate data network.
Ticketing
This feature provides the operator with sufficient information about the subscriber's activities and allows billing based on the volume of information transferred (volume of data transferred, SMS), as well as the duration of the data session (on/registration time, duration of the active state of the PDP context).
GPRS charging capabilities are fully compliant with ETSI specifications for S-CDR (SGSN), G-CDR (GGSN) and SMS CDR.
The CDR contains all required fields and the following optional fields:
S-CDR: MS class mark, RA routing area information, area code, cell ID, SGSN change information during session, diagnostic information, report sequence number, node ID.
G-CDR: dynamic address flag, diagnostic information, report sequence number, node ID.
All CDRs have identifiers so that all CDRs belonging to a single MM session can be sorted and linked to the corresponding PDP sessions, which is important from a billing perspective. This applies to all CDRs from all GPRS nodes.
CDRs in GPRS nodes first fall into a temporary storage buffer, which is stored for about 15 minutes, then they are written to the hard disk. The charging data storage disk capacity is approximately designed to store the equivalent of 72 hours of charging data.
The operator can configure the following parameters:
Destination (eg billing system);
Maximum amount of disk memory for storing CDRs;
Maximum CDR storage time;
Buffering timer random access memory(RAM);
Amount of buffering in random access memory (RAM);
Data extraction method.
GGSN selection
The SGSN selects the GGSN (including the access server) based on packet data protocol (PDP), access node name (APN), and configuration information. It uses the Domain Name Server in the core network to establish the identity of the SGSN serving the requested APN. The SGSN then establishes a tunnel using the GPRS Tunnel Protocol (GTP) to prepare the GGSN for further processing.
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Below is an example of successful delivery of an SMS message over GPRS radio channels:
SMS-C determines that the message needs to be forwarded to the MS. SMS-C forwards this message to SMS-GMSC. SMS-GMSC checks the destination address and requests routing information from the HLR for SMS delivery. The HLR transmits a result message, which may include information about the SGSN that the target MS is currently in range of, information about the MSC, or information about both nodes. If the resulting message does not contain the SGSN, it means that the HLR has information that the MS is out of range of the SGSN and is not reachable through that SGSN. If the resulting message contains an MSC number, SMS message will be delivered in the traditional way via the GSM network. If the resulting message contains an SGSN, the SMS-GMSC will forward the SMS to the SGSN. The SGSN will transmit the SMS to the MS, and send a successful message delivery message to the SMS-C.
4.6 Gateway GPRS Support Node (GGSN)
The GGSN provides an interface towards the external IP network with packet data transmission. The GGSN provides access functions for external devices such as ISP routers and RADIUS servers that provide security functions. From an external IP network perspective, the GGSN acts as a router for the IP addresses of all subscribers served by the GPRS network. Routing of packets to the desired SGSN and protocol translation are also provided by the GGSN node.
4.7 GGSN functions
GGSN performs the following functions as part of the GSPR network:
- Network connectionIP. GGSN supports connections to external IP networks using an access server. The access server uses a RADIUS server to assign dynamic IP addresses.
- Ensuring the security of data transfer via the protocolIP. This feature ensures secure transmission between SGSN and GGSN (Gi interface). This function is necessary when connecting GPRS subscribers through their own corporate network (VPN). It also improves the security of traffic management between GPRS nodes and control systems. IP security features allow you to encrypt all transmitted data. This protects against illegal access and provides guarantees of confidentiality of data packet transmission, data integrity and data source authentication. Security mechanisms are based on filtering, authentication and encryption at the IP level. To provide greater security over the IP core network, this functionality is integrated into the router in both the SGSN and GGSN (as well as gateway devices operating at the edges of the networks). This solution uses an Opv4 IPSEC authentication header using the MD5 algorithm and an encapsulated security payload (ESP) that uses the American Data Cipher Standard Chained Block Cipher (DES-CBC) mode. The system is also ready to introduce new encryption algorithms (for example, asymmetric authentication protocol with public keys, etc.)
- Routing. Routing is a function of the SGSN.
- Session management. The GGSN supports session management procedures (ie activation, deactivation and modification of the PDP context). Session management is described in the section “SGSN Functions. Session management."
- Support for charging function. The GGSN also generates a CDR for each served MS. The CDR contains a time-stamped log file for session management procedures in the case of a time-based charging mode and a file based on the volume of information transferred.
4.8 Logical channels
There are about 10 types of logical channels defined in the GSM system. These channels are used to transmit different types of information. For example, the paging channel PCH is used to transmit the calling message, and the broadcast control channel BCCH transmits system information. A new set of logical channels has been defined for GPRS. Most of them have names similar to and corresponding to the names of the channels in GSM. The presence in the abbreviated name of the logical channel of the letter “P”, meaning “Packet” and standing in front of all other letters, indicates that this is a GPRS channel. For example, the paging channel in GPRS is designated as PPCH - Packet Paging Channel.
A new logical channel of the GPRS system is the PTCCH channel (Packet Timing advance Control Channel). This is the TA time delay notification channel and is required to adjust this parameter. In the GSM system, information related to this parameter is transmitted on the SACCH channel.
To support GPRS, circuit groups can be assigned for packet switched (PS) connections. The channels assigned to GPRS to serve traffic originating from the Circuit Switched Domain (CSD) are referred to as PDCHs. These PDCHs will belong to a packet switched domain (PSD). For PDCH assignment, a multi-slot frame structure and a TCH capable of supporting PS are used.
In a cell, PDCHs will coexist with traffic serving channels for the CS. The packet transmission control unit PCU is responsible for assigning PDCHs.
In PSD, multiple PS connections can share the same PDCH. One PS connection is defined as a temporary block flow (TBF) that is transmitted in both uplink and downlink directions. The MS may simultaneously have two TBFs, one of which is used in the uplink direction and the other in the downlink direction.
When a TBF is assigned, one or more PDCHs are reserved for the MS. PDCHs are located in a set of PDCHs called PSETs and only one PDCH in the same PSET can be used for an MS. Before reserving a channel, the system must ensure that the PSD contains one or more free channels PDCH.
4.9 Assignment of channels in the GPRS system
The PBCCH channel, like the BCCH channel in GSM, is a broadcast control channel and is used only in the packet data information system. If the operator does not assign PBCCH channels in the system, Information system packet data transmission uses the BCCH channel for its purposes.
This channel consists of logical channels used for general control signaling required for packet data transmission.
This paging channel is used in the downlink direction only. It is used to transmit the ringing signal to the MS before transmitting packets. PPCH can be used in a group of paging channels for both packet switched mode and circuit switched mode. The use of the PPCH channel for circuit switched mode is only possible for GPRS class A and B terminals in a network with operating mode I.
PRACH – Packet Random Access Channel, used only in the uplink direction. PRACH is used by the MS to initiate transmission in the uplink direction for data or signaling.
PAGCH – Packet Access Grant Channel is used only in the downlink direction during the connection establishment phase to convey resource assignment information. Sent to MS before packet transmission begins.
PNCH – Packet Notification Channel is used only in the downlink direction. This channel is used to transmit a PTM-M (Point-to-Multipoin – Multicast) notification to the MS group before transmitting the PTM-M packet. To monitor the PNCH channel, the DRX mode must be assigned. DRX services are not specified for GPRS phase 1.
PACCH - Packet Associated Control Channel carries signaling information associated with a specific MS. The signaling information includes, for example, acknowledgments and terminal power output control information. PACCH also carries resource assignment or reassignment messages. This channel shares resources with the PDTCHs assigned to a particular MS. In addition, a paging message may be sent over this channel to the MS in the circuit-switched connection state indicating that the MS is engaging in packet mode.
PTCCH/U - Packet Timing advance Control Channel is used only in the uplink direction. This channel is used to transmit a random access packet to estimate the time delay of one MS in packet mode.
PTCCH/D - Packet Timing advance Control Channel is used only in the downlink direction. This channel is used to transmit time delay value update information for several MSs. One PTCCH/D is shared with several PTCCH/Us.
Data packets are transmitted over this channel. If the system is operating in PTM-M mode, it is temporarily assigned to one MS from the group. If the system operates in multi-slot mode, one MS can use multiple PDTCHs in parallel for one packet transmission session. All traffic channels for packet transmission are bidirectional, with a distinction being made between PDTCH/U for the uplink transmission direction, and PDTCH/D for the downlink transmission direction.
Chapter 5 - Switching System
Introduction
The mobile radio switching system is shown in Fig. 5.1
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5.2. Mobile Switching Center/Visiting Register (MSC/VLR)
5.2.1 MSC functions
MSC is the main node in the GSM system. This node controls all functions for servicing incoming and outgoing calls between MSs. The main functions of this node are:
IN THE DISCIPLINE “DIGITAL SWITCHING SYSTEMS AND THEIR SOFTWARE”
Literature:
1.”Automatic switching”, ed. O.N. Ivanova, 1988
2. M.A. Barkun. “Digital PBX”, 1990
3. G.V.Melik-Shakhnazarova et al. “ATS MT-20/25”, 1988
4. R.A.Avakov et al. “Foreign electronic digital switching systems”, 1988.
5. V.D. Safronov et al. “Foreign electronic digital switching systems”, part 2, 1989
6. A.G. Popova et al. “Foreign automatic switching systems”, 1991
7. V.G.Bosenko “Digital ATSE-200”, 1989
8. A.G. Popova “Digital switching systems with distributed control” parts 1 and 2, 1992
9. O.N. Ivanova “ATSE-200”, 1988
10. M.F.Lutov et al. “Quasielectronic and electronic automatic telephone exchanges”, 1988
11. Alcatel-Bell “System 12 Study Guide”, 1994
Course sections:
- Principles of digital switching.
- Construction of digital switching fields.
- Construction of connecting line interfaces.
- Subscriber access.
- Alarm systems CSK.
- Principles of USC construction.
- USC software.
- Block diagrams and technical characteristics of various CSKs.
The objectives of this course are to introduce students of the MES faculty to the current state and prospects for the development of digital switching systems. Explain the general structure of digital switching systems (DSS), as well as prospects for the implementation of DSS. Give comparative characteristics and parameters of switching systems implemented on telecommunication networks. To familiarize yourself with the principles of temporal and spatial switching of digital channels and their technical implementation in digital switching fields. Give the concepts of subscriber and trunk interfaces. Explain their functions and design features in the CSK. Explain the features of constructing control devices for the CSK, as well as explain the composition and functions of the software. Explain the principles of organizing the operation and maintenance of modern telecommunication systems.
Currently, many foreign-made digital switching systems are purchased; you need to be able to understand them. They don’t have time to publish literature for the course, so the main focus is on the lecture course. Training programs have been developed at the NPP Department on some issues. The textbooks of Ivanova, Barkun, Lutov set out general issues construction of the CSK. The rest of the literature is system-specific
CSK- hybrid PBXs that can be used in any capacity. Digital switching systems were first developed and produced in France around 1975. The first CSK is MT20/25. In Russia, this system was produced by the Ufa Telephone Plant and is currently used only on city telephone networks.
Brief overview of digital switching systems in Russia
Quantum- electronic automatic telephone exchange, produced by the Belgorod Telephone Plant and the Riga VEF Plant. The Kvant-SIS system was developed to organize a reference and information service. The EuroQuant system is designed for city telephone networks, the maximum capacity is 8000 numbers.
All PBXs purchased abroad must be certified for compliance with Russian telephone networks. Certification is carried out by LONIIS.
DX-200- the system was developed and manufactured by the Finnish company NOKIA. It has been supplied to Russia since the early 80s. The first automatic telephone exchanges of the DX-200 system were installed in St. Petersburg. For Russia it was worked out a new version PBX taking into account the construction of Russian networks. Used on GTS and STS (as USP). Quite a lot of such systems have been purchased in Russia. In Novosibirsk there is an automatic telephone exchange11/15 of the DX-200 system with a capacity of 25 thousand numbers
ATSC-90- this is the name of the DX-200, which is assembled in St. Petersburg, components for it are supplied from Finland. ATTS-90 are supplied to the Leningrad region and Karelia
S-12- hybrid PBX with distributed control. This is a 4th generation system. To bring the system to serial production, costs of about 1 billion dollars were required. Therefore, 5 countries took part in the development of the station: Belgium, Germany, Spain, Italy, France. Therefore, the system 12 has different manufacturing plants. For example, system 12 is supplied to Russia from Belgium by Alcatel-Bell, and to Kazakhstan from Germany. In 1991, a joint venture was created in St. Petersburg, which produces cable products for all manufacturing plants of System 12 (in Russia and abroad). In Russia, 3 service centers for the maintenance of system 12 have been created: in Moscow, St. Petersburg, Novosibirsk. In addition, there is a center for studying system 12 in Moscow. The minimum capacity of system 12 is 128 numbers, the maximum is 100,000 numbers in the 5th version, 200,000 numbers in the 7th version. System 12 is certified by LONIIS for use on GTS, AMTS, UAK, STS
EWSD- produced by Siemens, Germany. Certified for use on GTS and ATS. The Ministry of Communications recommended that in all cities along the Trans-Siberian Railway (from Vladivostok to Chelyabinsk) the reconstruction of automatic telephone exchanges based on EWSD with access to the international network. EWSD has maximum capacity up to 250,000 numbers and centralized management. A joint venture “Izhtel” was created in Izhevsk to produce EWSD on the Russian market. The EWSD maintenance service center is located in Novosibirsk.
AX-10- developed by Ericsson (Sweden). Several years ago, a joint venture was created in Yugoslavia with the Nikola-Tesla company to produce AXE-10. Supplies to Russia come mainly from Nikola-Tesla. The maximum capacity of the system is 200,000 numbers. The system is certified for AMTS, UAK, GTS, STS
MD-110 - capacity 20-20000 numbers. Company Nikola-Tesla. Purchased for a departmental network as a UPBX
5ESS(AT&T company). Made in USA. American companies began to develop the Russian market recently, around 1994. The first automatic telephone exchange of type 5 ESS was delivered in Moscow in the Tushinsky district. The maximum capacity of the system is 350,000 numbers. One such station is sufficient for the existing Novosibirsk GTS. This PBX is very expensive. Certified to work on GTS, AMTS, UAK. A joint venture was created in China.
TDX- Samsung company, South Korea. The maximum capacity is 100,000 numbers. The systems are supplied to the Far East. TDX is certified for GTS.
SI-2000 - capacity 20 - 10000 numbers. A joint venture was created in Yekaterinburg with the Yugoslav company Iskra (Slovenia) to produce these stations. Parts are produced in Slovenia, and assembly is carried out in Yekaterinburg. Used for STS and UTS. Advantage - it can work on all types of connecting lines (like Kvant).
UT-100- purchased in Italy. Capacity up to 100,000 numbers. Distributed throughout Russia. Produced by Italtel.
ATS-CA (S-32) very good domestic automatic telephone exchange developed by TsNIIS. Provides for the inclusion of only digital subscriber lines, i.e. A digital stream of 32 kb/s is delivered to the subscriber. The ATS has been developed, there is trial operation, but it has not been put into production. Currently, this station's element base is already outdated.
All 4th generation PBXs are also aimed at creating cellular networks.
All of the named telephone exchanges (except for MT-20/25) are focused on integrated service digital networks (ISDN) with a narrow-band digital stream.
ISDN - TsSIO-U narrowband systems with information transmission speeds of 64-2048 kb/s. Systems with ISDN are not in demand among the population, because... allow switching only telephone channels. In addition to telephone communications, the subscriber may have other types of communications: television, mobile communications, radio communications, etc.
BSDN - TsSIO-Sh broadband systems. The subscriber receives a digital stream with a transmission speed of 150-600 Mbit/s. For such signals, all of the above systems are not suitable, because such digital streams require optical switching, and this is a matter of the future.
In Novosibirsk, in Akademgorodok, an experimental BSDN is being built and a transport network based on fiber optic lines is being built to use BSDN. The broadband signal switching system is very expensive: it requires 5 - 6 billion dollars to bring it to serial production. BSDN are 5th generation switch nodes.
Brief technical information about CSK are given in table 1.1.
Table 1.1–Technical characteristics of digital switching systems
Generalized block diagram of a digital switching system
Figure 1.1 – Generalized block diagram of the CSK
K - hub
OP AL - equipment for connecting subscriber lines
OP SL - equipment for connecting trunk lines
AAL - analogue subscriber line
DSL - digital subscriber line
ASL - analog trunk line
DSL - digital trunk line
TsKP - digital switching field
OTS - tone signal equipment
OSI - alarm equipment
CS - control system
UVV - input/output devices
Purpose:
OP AL - serves to coordinate AAL and DSL with the digital switching field. Includes subscriber interfaces and devices for converting analog signals into PCM signals. The number of OP AL depends on the capacity of the telephone exchange. The minimum number of subscriber lines in OP AL is 64.
SL OP is used to coordinate ASL and DSL with the digital switching field. It must be borne in mind that the DSL and the PCM path are one and the same. OP CO includes trunk interfaces and devices for converting analog signals into PCM signals. The minimum number of ASLs in the SL OP is 32 (i.e. 1 PCM path). Not all PBXs have devices for connecting ASL. There are no such lines abroad, because... It is very difficult to coordinate the ASL OP with the DATS equipment.
OSI - used to organize signaling within the PBX and inter-station communications. The OSI provides reception and transmission of all linear signals, control signals and interprocessor communication signals.
ITS - generate and issue information signals towards the subscriber - station answer, busy, call control.
US - carries out all call servicing processes and technical operation ATS. Provides monitoring of the performance of the automatic telephone exchange and all modes of technical operation.
UVVs are video terminals and printers designed to perform all technical operation processes.
TsKP (OK) - used for switching all temporary channels included in the TsKP. All PBX devices are connected to the central communication center via PCM paths (PCM lines). The primary group of the PCM path is 30/32 time channels, regardless of the transmission system. Channel 0 is used to transmit synchronization signals, channel 16 is used to transmit signaling information, channels 1-15, 17-31 are conversational.
K - used to connect remote subscribers to the CSK. This is part of the CSK equipment, placed in a place where subscribers are concentrated.
Features of building digital switching systems
1. Use of time division of channels and time switching of channels when constructing a digital switching field. Any signal through the switching field of a digital switching system is transmitted in digital form.
2. Using standard channels whose parameters are normalized:
Voice channel with an effectively transmitted frequency band of 0.3-3.4 kHz
Primary digital channel with an information transfer rate of 64 kB/s
3. Connection of digital subscriber lines without additional converters to the PBX. The conversion is carried out in the subscriber installation, which can be any device.
4. Use of reception paths and transmission paths when establishing a connection. The receive and transmit paths are separated, so any connection uses 2 time channels.
5. Use of signaling equipment to receive and transmit signaling via channel 16 and voice channels. CCITT recommended by USC No. 7.
6. The use of concentrators, which can significantly reduce the cost of the subscriber network, because the cost of the hub + the cost of transmission systems is much less than the cost of the subscriber network. (Disadvantage: all connections of one hub are made through the central communication center of the core PBX).
Figure 1.2 – Connecting concentrators to the CSK
Advantages of CSK:
1. Sharp reduction in the cost of linear structures by reducing the cost of the subscriber network when using hubs.
2. Reducing costs for production, installation and operation of the CSK due to the use of a more advanced element base, due to ease of installation, due to a reduction in the number of maintenance personnel, high automation of maintenance work on the central control system, due to the high reliability of the operation of the central control equipment.
Table 1.2
| Production |
Installation |
Exploitation |
|
| ATSKU |
|||
| ATSCE |
30 - 40 |
40 - 50 |
10 - 20 |
| ATSC |
20 - 30 |
10 - 20 |
5 - 10 |
3. Reduction of production space for CSK equipment. To accommodate the equipment, a production area is required that is 4-6 times smaller than for mechanical equipment due to reduced dimensions.
4. Use of technical operation centers of central heating plants, allowing you to remotely manage maintenance work on several digital telephone exchanges and monitor the operation of several telephone exchanges from one center. In this case, no additional equipment is required; all control is carried out using software.
5. Full automation of equipment operation control.
6. Reducing the metal consumption of CSK structures.
7. Improving transmission and switching quality.
8. Increasing the number of VAS for users.
Disadvantages of CATS:
1. High energy costs: 1.2 - 3 watts per output (no less than in analog PBXs). This can be explained by the fact that in mechanical PBXs the control devices operate only when there is a call, while in digital ones they operate continuously.
