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Common Channel Signaling System No. 7 (i.e., SS7 or C7) is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T).
The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to effect wireless (cellular) and wireline call setup, routing and control.
The ITU definition of SS7 allows for national variants such as the American National Standards Institute (ANSI) and Bell Communications Research (Bellcore) standards used in North America and the European Telecommunications Standards Institute (ETSI) standard used in Europe.
What is SS7 used for? The SS7 network and protocol are used for:
basic call setup, management, and tear down
wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication
toll-free (800/888) and toll (900) wireline services
enhanced call features such as call forwarding, calling party name/number display, and three-way calling
efficient and secure worldwide telecommunications
Signaling Links SS7 messages are exchanged between network elements over 56 or 64 kilobit per second (kbps) bidirectional channels called signaling links. Signaling occurs out-of-band on dedicated channels rather than in-band on voice channels. Compared to in-band signaling, out-of-band signaling provides:
faster call setup times (compared to in-band signaling using multifrequency (MF) signaling tones)
support for Intelligent Network (IN) services which require signaling to network elements without voice trunks (e.g., database systems)
improved control over fraudulent network usage
Signaling Points Each signaling point in the SS7 network is uniquely identified by a numeric point code. Point codes are carried in signaling messages exchanged between signaling points to identify the source and destination of each message. Each signaling point uses a routing table to select the appropriate signaling path for each message. There are three kinds of signaling points in the SS7 network: SSP (Service Switching Point) STP (Signal Transfer Point) SCP (Service Control Point)
SSP’s are switches that originate, terminate or tandem calls
STP’s route network traffice between signaling points (ie acts like a network hub)
Centralize database which determines how to route a call (ie 1-800/888 services)
F Links SSP SCP Associated Networks
Europe has adopted Associated Network Structure as of Day 1
Backbone network allows for service to be offered across boundaries.
An single pair of STPs could be partitioned to support all or any layers
Using layers to separate networks
Signaling Interconnection RSTP D Links E Links LSTP LSTP SCP C Links A Links B-Link Quad SSP SSP LSTP LSTP Link Types: Access (A-Links): Connects STPs with SSPs, SCPs or MSC (Message Switching Center) Bridge (B-Links): Connects STPs Cross (C-Links): Connects mated STPs Diagonal (D-Links): Connects regional/local STPs to primary STPs Extended Links (E-Links): Connects SSPs , SCPs or MSC to alternate STPs Fully-Associated (F-Links): Connects SSPs
Linksets Combined Linksets Linkset 1 Linkset 3 Linkset 2 Linksets: Group of links between two nodes in the network Links are deployed in numbers to the power of 2 (1, 2, 4, 8) Traffic is loadshared over all link in the linksets Linkset size is limited by the protocol being used Combined linksets between STP pairs allow for loadsharing over both nodes to increase redundancy.
Point Codes Point Codes: Each signaling point in a SS7 network is assigned a unique point code Point code structure: ITU - 14 bit Point code ANSI - 24 bit Point code China - 24 bit Point code Japan - 16 bit Point code ITU Point Code: Basic: 14 International: 3 (Zone) - 8 (Area Network) - 3 (Signaling Point) Austria: 5 (Zone) - 4 (Region) - 5 (Signaling Point) China: 4 (Zone) - 7 (Exchange) - 3 (Signaling Point) Germany: 4 (Num Area) - 3 (HVST) - 4 (KVST) - 3 (Signaling Point) Node Identifiers: Origination Point Code (OPC) Destination Point Code (DPC) 0-1-0 1-1-0 3-3-3
Routesets Routesets Routes are defined between paths to a specific DPC A collection of Routes are defined as a Routeset Multiple Routes in a Routeset ensure that redundancy is build into the product. Routes are costed (Less expensive routing) 0-1-0 0-1-0 1-1-0 1-1-0 3-3-3 3-3-3
Subsystems Numbers/GTT Subsystem Number (SSN) Uniquely identifies an application residing on a destination point; typically an SCP database. Global Title Translation (GTT) Global Title Translation(GTT) provide specific information with regards to the Destination Point Code of the location of the application.
SS7 Protocol Stack The hardware and software functions of the SS7 protocol are divided into functional abstractions called "levels". These levels map loosely to the Open Systems Interconnect (OSI) 7-layer model defined by the International Standards Organization (ISO).
SS7 Protocol SS7 Protocol Layer Message Transfer Part (MTP) Level 1 Message Transfer Part (MTP) Level 2 Message Transfer Part (MTP) Level 3 Signal Connection Control Part (SCCP) Transaction Capabilities Part (TCAP) Key Protocol Specifications ITU Specifications: White Book (1992) Q701-Q709 (MTP), Q710-Q716 (SCCP), Q75x (Measurements) ETSI Specifications: 300 008 (MTP), 300 009 (SCCP) ANSI Specifications: GR-82, GR-246
Message Transfer Point (MTP): Level 1 to 2 MTP Level 1 The lowest level, MTP Level 1, is equivalent to the OSI Physical Layer. MTP Level 1 defines the physical, electrical, and functional characteristics of the digital signaling link. Physical interfaces defined include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 24 64 kp/s channels), V.35 (64 kb/s), DS-0 (64 kb/s), and MTP Level 2
Data link layer “handshaking”
Ensure message are transmitted accurately end to end, in sequence
Fill-In Signal Units (FISUs) are transmitted continuously on a signaling link in both directions unless other signal units (MSUs or LSSUs) are present. FISUs carry basic level 2 information only (e.g., acknowledgment of signal unit receipt by a remote signaling point). Because a CRC checksum is calculated for each FISU, signaling link quality is checked continuously by both signaling points at either end of the link. (Note: In the ITU-T Japan variant, signaling link quality is checked by the continuous transmission of flag octets rather than FISUs; FISUs are sent only at predefined timer intervals (e.g., once every 150 milliseconds).)
Link Status Signal Units (LSSUs) carry one or two octets (8-bit bytes) of link status information between signaling points at either end of a link. The link status is used to control link alignment and to indicate the status of a signaling point (e.g., local processor outage) to the remote signaling point.
Message Signal Units (MSUs) carry all call control, database query and response, network management, and network maintenance data in the signaling information field (SIF). MSUs have a routing label which allow originating signaling point to send information to a destination signaling point across the network.
Message Types FISU LSSU MSU
Message Transfer Point (MTP):Level 3 MTP Level 3 Network level Message routing and network management Most effective message route between two signaling points Rerouting traffic away from failure Notification to all network to ensure minimal impact during failure modes (congestion control).
SCCP Layer Provides connectionless and connection-oriented network services. Where MTP route messages to a signaling point, SCCP will determine an application at that point. Determines network address and relay the information to MTP layer SCCP also provides the means by which an STP can perform global title translation (GTT), a procedure by which the destination signaling point and subsystem number (SSN) is determined from digits (i.e., the global title) present in the signaling message.
The ISDN User Part (ISUP) defines the protocol and procedures used to set-up, manage, and release trunk circuits that carry voice and data calls over the public switched telephone network (PSTN). ISUP is used for both ISDN and non-ISDN calls.
MTP Message Flow STP C STP A MSUs 0-1-0 0-1-2 STP B STP D
Message flow from one Signaling Point to another Signaling Point through the STP.
Providing the links between the STPs are a combined linkset the messages are loadshared across the links.
All message for the same call will follow the same path through the network
STP C STP A MSUs 0-1-0 0-1-2 STP B STP D Signaling Network Management (Restricted state) TFRs
Fault occurs on the link from the STP A to SSP (0-1-2)
STP A notifies it associated links (Directly connected links) to not send message to the SSP if another path is available - Restricted
All messages are now sent from SSP (0-1-0) through STP C to STP B.
Any message that was not acknowledged as being received will be rerouted via STP B
STP C STP A MSUs 0-1-0 0-1-2 STP B STP D Signaling Network Management (Restricted state - C links) TFRs (0-1-2)
Several faults occur in the Network
All messages are now sent from SSP (0-1-0) through STP C to STP A to STP B - C link routing
Any message that was not acknowledged as being received will be rerouted via STP B
STP C STP A MSUs 0-1-0 0-1-2 STP B STP D Signaling Network Management (Prohibited state) TFPs
Fault occurs on the link from the STP B to SSP (0-1-2). Node is now isolated
STP A and B notifies all nodes (associated and quasi-associated) to not send message to the SSP - Prohibited
Nodes send test message every 30 sec (provisionable time) to verify if the node is still down
Signaling Link Selection
Country A NI=2 Country A NI=2 Country C NI=3 Country B NI=2 Network Indicator International Network Indicators (“0” and “1”are not regulated and for use at the International boundaries) National Network Indicators (“2=National”, and “3=National spare”) are left up to National networks to define. Nationally standardized Network Indicator values has started to cause interworking issues. Global Operators that operate in more than one country are facing NI conflicts when trying to connect to different National and International networks each defining their own NI usage requirements. NI 0 International NW 1 Spare (international use) 2National Network 3 Reserved for National use ? Needs are proprietary; Conflicts and needs can be resolved at a gateway STP
Country “ X ” NI=2 NI=2 STP A” NI=2 NI=2 NI=3 Country “ Y ” Country “ Z ” NOTE: NI=2 NI=3 Country “ Z ” sees STP “ A ” as having an NI=3 Network Indicator Interworking Allows the incoming and outgoing NI to be changed for internal STP processing, allowing for:
Interworking between switches in different countries with different regulatory issues
GW Country B Country A GW PC= 2-4-1 NI=2 PC= 2-4-1 NI=2 Point Code Interworking ITU Point code formats are based on the 14-bit format. Each national country specifies its own national specific point code format. Basic: 14 International: 3 (Zone) - 8 (Area Network) - 3 (Signaling Point) Austria: 5 (Zone) - 4 (Region) - 5 (Signaling Point) Germany: 4 (Num Area) - 3 (HVST) - 4 (KVST) - 3(Signaling Point) It is possible for two nodes in different countries to have the same point assigned to the same Network.
Universal Signaling Point - SS7/IP Gateway application: Third Generation application based on Signaling Server Platform Technology - Part of Succession CS2K, CS3K and e-mobility solution
Universal Mediation Platform - Adds ability to control requests from another operator’s networks. Guards against inappropriate and/or illegal use of host network and services. Screens ISUP and TCAP messages and their content.