Signaling system 7

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The Signaling System # 7 , in German Signaling System Number 7 , ( hereinafter referred to as SS7 ) is a collection of protocols and procedures for signaling in telecommunications networks.

It is used in the public telephone network , in connection with ISDN , landline and cellular networks and, since around 2000, increasingly in VoIP networks. In VoIP networks, SS7 is only used in conjunction with media gateway controllers . The log collection is also available under other names such as Signaling System No. 7 , Signaling System Number 7 , the signaling system no. 7 , central signaling system no. 7 , ZZS7 , CCITT -Zeichengabesystem no. 7 , Central Signaling System # 7 and C7 known.

The ITU-T (formerly CCITT) is developing detailed proposals for the implementation of national and international signaling networks in the Q.600 and Q.700 series under the name “ITU-T Recommendation”. The proposals are converted into binding standards by normative organizations such as ETSI (European Telecommunication Standardization Institute) or ANSI (American National Standardization Institute) as well as by IETF (Internet Engineering Task Force) through RFCs.

Today, SS7 is the most common and often the only signaling system in national and international telecommunications networks. As a result of this popularity, various protocols of the SS7 stack for SS7oIP (Signaling System Over Internet Protocol) have been specified and developed and used.

Telecommunication facilities such as exchanges or gateways work with SS7 protocol stacks that are adapted to the national standards or specifications of the individual service providers. Like most of the ITU-T recommendations, the Q.600 and Q.700 series have a very variable structure and allow a large number of variations. In contrast to IP, for example, there is no uniform SS7 protocol stack, but specific implementations.


SS7 was founded in 1975 by AT & T developed previously in the United States used Signaling System # 5 and Signaling System # 6 to replace. In these versions, the signaling still took place in-band , in that certain tones were used for communication between the exchanges. This led to abuse, known as phreaking . Control signals were introduced into the end device with the aim of being less expensive.

The ITU-T standardized the SS7 in 1981, just as it had accepted the predecessors SS6 and SS5. After that, the SS7 quickly spread around the world.


SS7 is a central signaling system or "Common Channel Signaling System". A dedicated channel in a transmission system (usually a multiplex system ) transmits the signaling information for all user channels (bearer channels) or voice channels. This signaling information can contain information about called or calling number, charges, busy, number unknown, etc., for example.

SS7 is a highly efficient protocol that, compared to other types of communication, manages with comparatively small amounts of data. In most cases, a data rate of 64 kbit per time slot is sufficient to serve several E1 connections. Several 64 kbit time slots can be combined if one time slot is not sufficient for signaling. In the USA (ANSI), T1 connections are used instead of E1 connections. Instead of E1 connections with 2 Mbit / s it is then only 24 × 64 kbit / s, this corresponds to 1544 kbit / s.

In mobile communication networks, the proportion of signaling is very high because of the mobility and the use of SMS. There are systems in the fixed network, but especially in the mobile network, which only have signaling connections, such as an SMS center .

SS7 provides procedures for the fastest possible troubleshooting and for finding alternative paths. The switching times in the event of a fault or failure of a node are usually in the range of a few milliseconds and are therefore far superior to the usual methods in IP networks such as the Internet.

Regulations and standards often require availability rates of 99.9% (maximum downtime 8 hours 45 minutes per year) or 99.999% (maximum downtime 316 seconds per year), which telephone operators and manufacturers must prove using long-term tests and protocols.

Signaling protocols

The most important components of SS7 are recommendations that describe different aspects of the complex communication model:

MTP - Message Transfer Parts

MTP or Message Transfer Part describes how signaling information is transmitted. This includes definitions of the electrical or optical interfaces, details of how individual messages are separated from each other and how individual exchanges or, better in the jargon of ITU-T signaling points , are addressed.

Representation of MTP1 to MTP3
  • MTP Level 1 defines the physical, electrical and functional parameters for a signaling route. This includes specifications such as clock rate, voltages, coding methods and the dimensions and shape of the connectors . Interfaces that correspond to recommendations V.35 or G.703 are common. Level 1 represents the bit transmission layer for a signaling link, which in a digital network usually consists of a 64 kbit / s channel.
  • MTP Level 2 defines the procedures for an error-free exchange of messages over a signaling path. This includes functions to activate or terminate the communication link, to check for errors and to correct them if necessary. The messages are separated from one another by flags. Level 2 is similar in structure to the frequently used HDLC procedure , but with some additional functions.
  • MTP Level 3 defines the interaction of several individual signaling links. All aspects that are common at the logical level for the exchange of messages between two signaling points over several signaling paths are regulated. This includes the forwarding of incoming messages to the desired signaling path. The separation of these functions in a separate level 3 also serves to manage the signaling network: Signaling routes can be added or, in the event of an error, switched to an alternative route without having to change the configuration at higher, more abstract levels.
  • E1 LSL and HSL connections E1 LSL have been used since SS7 was introduced. LSL (Low Speed ​​Link) refers to the connections in which 64 kbit time slots are used. Since only 16 time slots can be switched per linkset, the bandwidth is limited accordingly. This only results in a bandwidth of 1 Mbit / s per linkset. The HSL (High Speed ​​Link) have been specified for some time. The HSL allow a bandwidth of 2 Mbit / s per time slot, which would result in a theoretical bandwidth of 32 Mbit / s for a linkset. HSL are used where the bandwidth with LSL is too small. However, since HSL is very expensive, HSL is only used if, for example, SS7oIP, the cheapest alternative, is not (yet) feasible due to a lack of connection options in the network.
  • Linkset describes the logical connection between two signaling point codes (PCs). Linksets are only used with E1 connections, not with SS7oIP. The restriction to 16 timeslots per link set is due to the missing bits for the SLC in ITU, since the SLC (Signaling Link Code) only has 4 bits. In the ANSI standard, however, 8 bits are available for the SLC, which then enables 256 time slots. However, if more time slots (bandwidth) must be available between two point codes and HSL is not possible, a second link set must be created. So that this can be made possible, a capability point code is set up in the Signaling Transfer Point (STP), which allows a further link set to be defined.

User parts

The functions that are available to a user are described in the User Parts. These functions depend on the service used ( ISDN , analog telephone, mobile communications) and are therefore described separately. The most important user parts are:

  • TUP Telephone User Part is the simplest User Part, which only describes the basic functions. This includes information such as connection establishment (calling), disconnection (hanging up), busy or unknown number.
  • ISUP ISDN User Part describes the functions that are available to ISDN users. The most important element of this is the description of the service or bearer capability . ISDN allows different terminal devices such as telephone, fax or computer to be operated on the same connection. With a connection in ISDN, a description of the service type is always sent so that only the terminal device that supports the desired service answers. This prevents, for example, a fax machine from attempting to accept a voice connection if both terminals are ISDN-capable.
  • DUP Data User part is intended to convey special information for data connections.

SCCP - Signaling Connection Control Part

Signaling Connection Control Part is a layer that is based on MTP Level 3 and allows end-to-end signaling in the signaling network. Four service classes are made available in the SCCP:

  • Class 0: Connectionless basic service: Longer messages can be shared. Higher layers are responsible for the correct composition of these parts
  • Class 1: Connectionless service with sequence numbers: This number (SLS code) is 4 or 8 bits long (ITU-T or ANSI standard). Related messages use the same SLS code. If several links (linkset) are used for a message, the sequence number differs in the least significant bits.
  • Class 2: Connection-oriented basic service: The signaling connection must be established and cleared.
  • Class 3: Connection-oriented basic service with flow control

TCAP - Transaction Capabilities Application Part

TCAP is based on SCCP and enables the overlying protocols, such as INAP, CAP, MAP and OMAP, to communicate worldwide via the SS7 network.

INAP - Intelligent Network Application Part

The functions for intelligent networks ( IN ) are processed via INAP . This includes, among other things, number portability (LNP Local Number Portability) or 0800 numbers, which are forwarded to the nearest exchange depending on the location of the caller.

CAP - CAMEL Application Part

CAP is used in cellular networks and is used for Customized Applications for Mobile networks Enhanced Logic (CAMEL).

MAP - Mobile Application Part

MAP is used for communication between the various components of the cellular network (including HLR , VLR , SMSC ). The standard can also be used for communication between mobile radio networks from different providers and is therefore one of the prerequisites for roaming functionality. A subscriber can use roaming to log into third-party networks (for example, foreign mobile operators with a roaming contract or to be able to make emergency calls, even if the subscriber is not in the service area of ​​their own operator). The billing-relevant components are transferred using the Transferred Account Procedure (TAP).

Short messages (SMS) are transmitted in the MAP in addition to roaming and control of the call connections. Functions for determining the device type and the IMEI are also transferred in the MAP so that mobile phone-specific configurations can be transferred from the mobile operator to the terminal.

OMAP - Operations, Maintenance and Administration Part

The functions for operation, maintenance and administration include, for example, software maintenance, configuration and setting up telephone number blocks for telephone subscribers.

Layer model

SS7 is based on the OSI layer model . Layers 1–3 are referred to as MTP ( Message Transfer Part ) of SS7:

OSI layers SS7
Layer 7 - Application INAP CAP MAP OMAP (ISUP) User Parts: TUP, ISUP
Layer 6 - Presentation
Layer 5 - Session TCAP
Layer 4 - Transportation SCCP
Layer 3 - Network MTP Level 3 or M3UA
Layer 2 - data MTP Level 2 or M2UA
Layer 1 - Physical MTP level 1 or IP

The content of these messages is determined by the highest layer, which is application-specific. The layers below are only viewed as means of transport with different functions and properties. With the SS7oIP, which is now emerging today, the lower 3 layers are being replaced in the most common form, as can be seen in the diagram. The OSI layers are usually not mentioned in the SS7 literature, as this can give rise to confusion. Depending on the use of the SS7oIP technologies, entire layers can then be saved and the layers used can no longer be transferred to the OSI model, since up to 2 layers can be omitted or saved. M3UA is the most common layer in the SS7oIP. Other possibilities would be, for example, to replace the SCCP with SUA, with one shift being saved as an example.

Signaling network

The signaling network describes the devices that exchange signaling information and how they are interconnected. The descriptions for this are very broad and located on a level that is as abstract as possible in order to give telephone network operators as much freedom as possible for design.

Another important function of the signaling network is routing. It describes how individual signaling messages are forwarded from one device to the next and how the optimal path is found in a larger network.

Signaling network components

  • Signaling points
SPs are the individual devices that exchange SS7 information. Except for the STP, all SPs interact with components outside the SS7 network:
  • STP s ( Signaling Transfer Points ) are devices that switch messages between SSPs in more complex networks and can be referred to as "SS7 routers ".
  • SSP s ( Service Switching Points ), local or long-distance exchanges and gateways to other networks (Voice over IP) or to other operators in Germany or abroad.
  • SCPs ( Service Control Points ) are the facilities in the SS7 network that process the functions of the intelligent network (IN).
  • SG s ( Signaling Gateways ) are gateways that can only convert signaling from E1 connections to SS7oIP instead of voice.
  • IPs (Intelligent Peripherals) are components connected to the SSP or SCP that provide additional resources for a call. Mostly these are voice announcements that are played during a call and with which you can also interact (using keys or voice).
  • Signaling links
Describe the logical connections with redundancy and bundling of lines in order to increase availability and performance. The ITU-T defines different connection types:
  • A-Link (Access Link) is used, for example, by SSPs (Service Switching Points) that are coupled to an STP.
  • B-Link (Bridged Link) connects two STPs (Signaling Transfer Points) with each other.
  • C-Link (Cross Link) connects two signaling points in tandem operation. As a rule, telephone exchanges are operated as a pair (mated pair) in order to achieve greater reliability. This pair appears logically as a single telephone exchange.
  • D-Link (Diagonal Link) connects an STP (Signaling Transfer Point) with another STP of another switching level (for example with a node on the national remote level).
  • E-Link (Extended Link) connects an SSP (Service Switching Point) with an additional STP on the same switching level. This is mainly done for redundancy reasons.
  • F-Link (Fully Associated Link) connects two SSPs (Service Switching Points) that do not require any other functions such as “Intelligent Network” (IN) or routing.
  • Signaling modes
There are only two modes (operating modes) in the SS7:
  • Associated Mode
  • Quasi associated mode

Signaling points

Typical SS7 signaling network
  • Originating point
This is the "sender" or the signaling point from where an SS7 message is initialized.
  • Destination point
The destination of an SS7 message or the signaling point where the SS7 message arrives.

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