What is OKS 7?. Reference data

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Today, if not in most, then in any case, in many communication systems, information is transmitted in digital form. For the functioning of digital telecommunications switched networks, a common channel signaling system (CSS) is required, which is recommended by the International Telecommunication Union (ITU) and called CCS No. 7, or simply CCS 7. This system is described in the article. Its basic concepts are given, the architecture of the list (stack) of SS 7 protocols is given. The principles of signaling point numbering are outlined and the structure of SS 7 networks of the Interconnected Communications Network of the Russian Federation (VSS RF) is considered. The article is accompanied by a dictionary of Russian and English abbreviations.

The common signaling channel is a discrete communication channel between two software-controlled switching stations, through which the transmission of functional telephone signals (linear and connection control signals) related to a whole bunch of conversational channels, and acoustic signals is organized by the address-group method. In addition, a number of additional functional signals are transmitted over the SS, such as load accounting, cost accounting, network control, operation and maintenance signals. Function signals are intended to perform a certain phase of call processing.

Linear signals determine the main phases of calls: channel occupation (call start), subscriber answer (conversational state), channel busy (call failure), release (clear state), channel disconnection (call end).

Call control signals are transmitted during call setup and contain the called or calling party number and other information needed to establish a call at a particular site.

These signals are fundamental to the switching process.

Acoustic signals in the form of buzzers and mechanical voices (answering machines) are transmitted to the subscribers participating in the connection, and in some cases - to the telephone operator as information about the state of the connection.

The common signaling channel can be used in different parts of the switched network. The advantages of an alarm system with OKS are:

• the speed of establishing a connection, provided by a high rate of information transfer in the SS path;
• the possibility of almost unlimited expansion of the composition of telephone signals (due to the large reserve of codes);
• significant savings on linear sets, which are practically eliminated due to the processing of all signals by the control machine;
• simplicity of technical solutions if it is necessary to use two-way channels and lines, which increases the efficiency of switched channels, especially in small bundles;
• increasing reliability with the introduction of controlled transmission through the organization of bypass routes for the transmission of signaling messages, the use of a reliable element base and group redundancy.

The first implementation of a common channel signaling system was based on the CCITT Recommendations for System No. 6 (CS6). SS 6 protocols were not structured into layers and were intended for the efficient use of analog channels with a transmission rate of 2,4 bit / s.

In the mid-70s, the CCITT began preparing Recommendations for SS7, designed to organize signaling over digital channels. By this time, the layered approach of the International Standards Organization (ISO) in the development of protocols has received significant development and has been adopted as the basis for the implementation of applications in the field of signaling.

The requirements for Signaling System No. 7 (SCS 7) were defined by the CCITT in 1981 in Recommendations Q.701 - Q.741 (Red Book).

The SS 7 signaling system is intended for use at a message rate of 64 kbit/s, the length of messages is a variable multiple of 8. The probability of error detection has been increased. Redundancy of channels and equipment is provided to improve the reliability of the OKS.

Today, the OKS 7 system is a basic tool for ensuring the interaction of digital communication nodes at the national and international levels. The composition of the messages provides for the possibility of interaction between SS 7 and analog signaling systems.

According to ITU-T Recommendation Q.700 of the White Paper, SS7 is an internationally standardized signaling system over a common channel for universal purposes:

• optimized for operation in digital telecommunication networks at stations with program control;
• meets the current and future requirements for the transmission of information during interprocessor exchange in telecommunication networks in order to provide signaling for connection control, remote monitoring and maintenance, and also meets the requirements for signaling for connection control for telephone networks, a digital network with integrated services ( ISDN) and circuit-switched data communications. At the same time, it can be used as a reliable transport system for transferring other types of information between stations and special-purpose centers in telecommunication networks (for example, for the purposes of management, operation and maintenance);
• provides a reliable means of transmitting information without loss of messages, duplication and violation of the order of messages.

Since any switched network (digital, analog) consists of telephone exchanges, automatic switching nodes and channels and lines connecting them, the telephone signaling system in various parts of the network is built on the basis of a specific switching system of stations and nodes, as well as a transmission system in the section under consideration.

The methods of signal transmission between exchanges and individual devices of the exchange have changed as the exchanges themselves have improved. The development of the signaling system was determined by the need to increase its reliability, reduce the volume and cost of equipment, increase the number of transmitted signals and their transmission speed, and ensure the possibility of data transmission directly from subscriber units [1].

Architecture OKS 7

The modern architecture of the list (stack) of SS 7 protocols with mapping to the levels of the OSI model (interconnection of open systems, ITU-T Rec. X200) is shown in fig. 1, in which the functions of the transport, session and presentation layers in the SS7 model do not require implementation, and the application protocols are shown selectively in an illustrative form.

Reliable message transfer between application subsystems of SS 7 is provided by the network services subsystem, which consists of the Message Transfer Part (MTP) and the Signaling Connection Control Part (SCCP).

The subsystem for transmitting messages between signaling points is common to all user subsystems in one signaling point. The users of this subsystem can be: signaling connection control subsystem, telephony user subsystem (Telephone User Part - TUP), data user subsystem (Digital User Part - DUP) and user subsystem (ISDN) (Jntegrated Service Digital Network - JSDN).

The functions of the message passing subsystem are divided into three functional levels (Fig. 2):

• signaling data link (layer 1);
• signaling link (level 2);
• signaling networks (layer 3).

The Signaling Connection Control Part (SCCP) provides additional functions for the network subsystem of the SS7 signaling system and is described in ITU-T Recs Q.711-714, ITU-T 716-T.

Author: V.Efimushkin, Ph.D. Sciences, M. Zharkov, Ph.D. those. Sciences, A. Ivanov, Moscow

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