Signaling System # 7 / Signaling System # 7
Signaling System # 7 / Signaling System # 7 is a set of network protocols that allow the exchange of service messages between mobile stations (mobile phones) and telephone exchanges, as well as between the telephone exchanges themselves.
Currently, SS # 7 is used as a signaling standard in telephone networks.
This article will describe the structure and principle of operation of SS # 7.
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All telephone calls consist of two integral components. The first and most obvious is the actual content — our voices, fax data, modem data, etc. The second component is information that is exchanged between network devices to organize the connection and delivery of data to the intended destination.
SS # 7 is a protocol stack that describes the methods of communication between telephone switchboards (switches) in open telephone networks. Used by telephone companies for inter-station signaling. In the past, in-band signaling used inter-station highways inside. This method of signaling provided one common channel for using both components of telephone calls. This method was not effective and was soon replaced out of band.
In order to properly understand the Signaling System No. 7, it is first necessary to understand the main drawbacks of the previous signaling methods used in the PSTN (Public Switched Telephone Network). Until the recent past, all telephone connections were carried out by a multitude of technicians based on in-band common channel signaling.
A network using out-of-band common channel signaling is a combination of two networks in one:
SS # 7 is the main inter-office ISDN protocol. But with no less success is used outside of ISDN.
Signaling System No. 7 is an interchangeable set of network elements used for messaging to support telecommunications functions. The SS # 7 protocol is designed to promote these capabilities and maintain the network on which they are provided.
Fig. 1 SS # 7 protocol stack structure
Message Transfer Part
At this level, the functions of electron-optical conversion, providing the necessary signal power transmission. MTP1 is compatible with different interfaces (E1, T1).
It performs the following functions: frame synchronization, checking errors during transmission of one frame, rate matching, organization of retransmission of frames in which errors are detected.
At this level 3 types of frames are formed.
MSU (Message Signaling Unit) is a transmission frame that is used to transmit signaling messages (for organizing, breaking connections, etc.).
Fig. 2 MSU frame structure
Numbers - the number of bits of each field. The purpose of all fields will be described below.
LSSU (Link Status Signal Unit) - a transmission frame that carries information about the status of signaling messages, and the status of the signaling connection.
Fig. 3 LSSU frame structure
FISU (Fill In Signaling Unit) - this type of frame does not carry information and is called “empty”. It is used in case of unidirectional transmission of signaling messages by the receiving node for signaling the transmitting node about the presence of errors and the organization of retransmission.
Fig. 4 FISU frame structure
The MTP2 level forms the transmission frame, complementing the existing fields (Info, SIO, SIF or SI) with the following fields - flags F, control field FCS (Frame Check Sequence), length indicator LI (Lenght Indicator), index bit forward FIB (Forward Indicator Bit) ), the index bit back BIB (Backward Indicator Bit), the sequence number forward FSN (Forward Sequnce Number), the sequence number back BSN (Backward Sequnce Number).
The BSN field of the MSU message in the direction from node A to node B includes the number of the last frame received by A from B. If A received an error from B, then A inserts the frame number in error in the BSN field and inserts "1" in the BIB field. B, having received this message, sends the frame again and writes “1” in the FIB field, which means retransmission.
The FSN field is used to indicate the sequence number of the transmitting side, and the BSN is used to indicate the sequence number of the last frame received. That is, sending the first MSU frame, node A writes “0” to the FSN field. If node B received the frame successfully, it forms a response message and writes the number “0” received in FSN into its FSN field. And, having received the answer from B, he reads the FSN field, makes sure that his first frame has reached successfully, forms the second frame, and writes “0” to the BSN. Thus, when transmitting the second frame from A to B, node B also receives a report that its response to the first frame was received by node A without errors. And so on.
With the BIB bit, you can order a retransmission if an error occurs at the reception. Fits "1", if it was and "0", if everything went well.
With the FIB bit, the transmitting side informs the receiving party of the presence of a retransmission.
The functions of this level coincide with the functions of the network layer of the OSI model. Performs addressing on the SS # 7 network, routing.
On MTP3, SIO, SIF and SI fields are formed.
The SIF (Signaling Information Field) field is used to indicate the ID code of the signal node, and the node code that transmits the message ( OPC - Originating Point Code), as well as the node code to which this message is assigned ( DPC - Destination Point Code).
The CIC (Circuit Identity Code) field is used to specify a time interval (time-slot'a), which is used to transmit signaling messages and is located in one of the E1, T1 streams.
The Service Information Octet SIO is used to identify the type of service. NI (Network Indicator) - a network pointer, used to indicate the type of network (national or international network). Pri (Priority) - this field is usually a reserve, in some cases it can be used to indicate priority. SI (Service Indicator) - indicates to which type of services the signaling message belongs, which is located in the information field.
At the third level, signal connections are formed between the nodes.
SL (Signaling Link) is a connection between two nodes through which signaling messages are exchanged. As a rule, the number of SL is more than 2.
Two SLs connecting two signaling nodes are usually included in the Signal Link Set ( SLS ) signal lines. The SLS set can contain 2, 3 and more SL, depending on the capacity of the connecting line between the PBX.
In the SS # 7 network, there are three types of signal nodes:
SSP (Signaling Switching Point) is a node that performs node switching.
SCP (Signaling Control Point) - controls the operation of the SSP, contains a database, thereby controlling access to the services provided by the SSP.
STP (Signaling Transfer Point) - a node that performs the functions of signaling message routing.
This level contains a set of protocols, allowing the use of SS # 7 in an analog fixed-line telephony network, adapted to a co-channel signaling system used in an analog subscriber line. Currently not used.
A set of protocols that allows the use of SS # 7 in ISDN networks. It supports the principle of operation of all ISDN interfaces, determines the algorithm for forming connections.
Sifnaling Connection Control Part (SCCP)
It performs the function of monitoring the connections on the SS # 7 network. Allows you to organize 4 types of data transfer. Each species is characterized by a class from 0 to 3.
Class 0
Formation of connections without agreement between the terminals.
Class 1
Formation of a connection taking into account the sequence number during transmission. Not focused on the connection.
Class 2
Formation of the connection with prior approval, after the transfer occurs.
Class 3
Formation of a connection with a preliminary agreement, after which data transfer occurs with the control of the transmission rate.
Transanction Capability Application Part (TCAP)
Provides data processing functions for remote access equipment. TCAP is used to provide roaming between networks. In this case, the “global translator” service is used, which translates the signal node code (SIF) into a telephone number format.
TCAP consists of several sublevels.
A set of protocols that allows the use of SS # 7 in a mobile network. In this case, these protocols support all interfaces of the mobile network, define the principle of hand-over, the principles of forming connections.
A set of protocols used to provide roaming between networks of the same standard and between networks of different standards (GSM and CDMA, for example).
This set of protocols is used to apply SS # 7 in intelligent communication networks (IN). Defines the principle of formation of compounds in IN. At the same time, it is possible to use authentication as a subscriber authentication method.
Currently, SS # 7 is used as a signaling standard in telephone networks.
This article will describe the structure and principle of operation of SS # 7.
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Introduction
All telephone calls consist of two integral components. The first and most obvious is the actual content — our voices, fax data, modem data, etc. The second component is information that is exchanged between network devices to organize the connection and delivery of data to the intended destination.
SS # 7 is a protocol stack that describes the methods of communication between telephone switchboards (switches) in open telephone networks. Used by telephone companies for inter-station signaling. In the past, in-band signaling used inter-station highways inside. This method of signaling provided one common channel for using both components of telephone calls. This method was not effective and was soon replaced out of band.
In order to properly understand the Signaling System No. 7, it is first necessary to understand the main drawbacks of the previous signaling methods used in the PSTN (Public Switched Telephone Network). Until the recent past, all telephone connections were carried out by a multitude of technicians based on in-band common channel signaling.
A network using out-of-band common channel signaling is a combination of two networks in one:
- 1. A circuit-switched network that provides voice and data. Carries a physical channel between the sender and receiver.
- 2. Network signaling, provides the transfer of service information that controls the call. Packet switched network using common channel switching protocols.
SS # 7 is the main inter-office ISDN protocol. But with no less success is used outside of ISDN.
SS # 7 protocol levels
Signaling System No. 7 is an interchangeable set of network elements used for messaging to support telecommunications functions. The SS # 7 protocol is designed to promote these capabilities and maintain the network on which they are provided.
Fig. 1 SS # 7 protocol stack structure
Message Transfer Part
Messaging subsystem
MTP1
At this level, the functions of electron-optical conversion, providing the necessary signal power transmission. MTP1 is compatible with different interfaces (E1, T1).
MTP2
It performs the following functions: frame synchronization, checking errors during transmission of one frame, rate matching, organization of retransmission of frames in which errors are detected.
At this level 3 types of frames are formed.
MSU (Message Signaling Unit) is a transmission frame that is used to transmit signaling messages (for organizing, breaking connections, etc.).
Fig. 2 MSU frame structure
Numbers - the number of bits of each field. The purpose of all fields will be described below.
LSSU (Link Status Signal Unit) - a transmission frame that carries information about the status of signaling messages, and the status of the signaling connection.
Fig. 3 LSSU frame structure
FISU (Fill In Signaling Unit) - this type of frame does not carry information and is called “empty”. It is used in case of unidirectional transmission of signaling messages by the receiving node for signaling the transmitting node about the presence of errors and the organization of retransmission.
Fig. 4 FISU frame structure
The MTP2 level forms the transmission frame, complementing the existing fields (Info, SIO, SIF or SI) with the following fields - flags F, control field FCS (Frame Check Sequence), length indicator LI (Lenght Indicator), index bit forward FIB (Forward Indicator Bit) ), the index bit back BIB (Backward Indicator Bit), the sequence number forward FSN (Forward Sequnce Number), the sequence number back BSN (Backward Sequnce Number).
The BSN field of the MSU message in the direction from node A to node B includes the number of the last frame received by A from B. If A received an error from B, then A inserts the frame number in error in the BSN field and inserts "1" in the BIB field. B, having received this message, sends the frame again and writes “1” in the FIB field, which means retransmission.
The FSN field is used to indicate the sequence number of the transmitting side, and the BSN is used to indicate the sequence number of the last frame received. That is, sending the first MSU frame, node A writes “0” to the FSN field. If node B received the frame successfully, it forms a response message and writes the number “0” received in FSN into its FSN field. And, having received the answer from B, he reads the FSN field, makes sure that his first frame has reached successfully, forms the second frame, and writes “0” to the BSN. Thus, when transmitting the second frame from A to B, node B also receives a report that its response to the first frame was received by node A without errors. And so on.
With the BIB bit, you can order a retransmission if an error occurs at the reception. Fits "1", if it was and "0", if everything went well.
With the FIB bit, the transmitting side informs the receiving party of the presence of a retransmission.
MTP3
The functions of this level coincide with the functions of the network layer of the OSI model. Performs addressing on the SS # 7 network, routing.
On MTP3, SIO, SIF and SI fields are formed.
The SIF (Signaling Information Field) field is used to indicate the ID code of the signal node, and the node code that transmits the message ( OPC - Originating Point Code), as well as the node code to which this message is assigned ( DPC - Destination Point Code).
The CIC (Circuit Identity Code) field is used to specify a time interval (time-slot'a), which is used to transmit signaling messages and is located in one of the E1, T1 streams.
The Service Information Octet SIO is used to identify the type of service. NI (Network Indicator) - a network pointer, used to indicate the type of network (national or international network). Pri (Priority) - this field is usually a reserve, in some cases it can be used to indicate priority. SI (Service Indicator) - indicates to which type of services the signaling message belongs, which is located in the information field.
At the third level, signal connections are formed between the nodes.
SL (Signaling Link) is a connection between two nodes through which signaling messages are exchanged. As a rule, the number of SL is more than 2.
Two SLs connecting two signaling nodes are usually included in the Signal Link Set ( SLS ) signal lines. The SLS set can contain 2, 3 and more SL, depending on the capacity of the connecting line between the PBX.
In the SS # 7 network, there are three types of signal nodes:
SSP (Signaling Switching Point) is a node that performs node switching.
SCP (Signaling Control Point) - controls the operation of the SSP, contains a database, thereby controlling access to the services provided by the SSP.
STP (Signaling Transfer Point) - a node that performs the functions of signaling message routing.
Telephony User Part (TUP)
This level contains a set of protocols, allowing the use of SS # 7 in an analog fixed-line telephony network, adapted to a co-channel signaling system used in an analog subscriber line. Currently not used.
ISDN User Part (ISUP)
A set of protocols that allows the use of SS # 7 in ISDN networks. It supports the principle of operation of all ISDN interfaces, determines the algorithm for forming connections.
Sifnaling Connection Control Part (SCCP)
Signaling Channel Connection Management System
It performs the function of monitoring the connections on the SS # 7 network. Allows you to organize 4 types of data transfer. Each species is characterized by a class from 0 to 3.
Class 0
Formation of connections without agreement between the terminals.
Class 1
Formation of a connection taking into account the sequence number during transmission. Not focused on the connection.
Class 2
Formation of the connection with prior approval, after the transfer occurs.
Class 3
Formation of a connection with a preliminary agreement, after which data transfer occurs with the control of the transmission rate.
Transanction Capability Application Part (TCAP)
Applied part of transaction funds
Provides data processing functions for remote access equipment. TCAP is used to provide roaming between networks. In this case, the “global translator” service is used, which translates the signal node code (SIF) into a telephone number format.
TCAP consists of several sublevels.
Mobile Application Part (MAP)
A set of protocols that allows the use of SS # 7 in a mobile network. In this case, these protocols support all interfaces of the mobile network, define the principle of hand-over, the principles of forming connections.
IS 45
A set of protocols used to provide roaming between networks of the same standard and between networks of different standards (GSM and CDMA, for example).
Inteligent Network Application Part (INAP)
This set of protocols is used to apply SS # 7 in intelligent communication networks (IN). Defines the principle of formation of compounds in IN. At the same time, it is possible to use authentication as a subscriber authentication method.
Source: https://habr.com/ru/post/113792/
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