Signaling System 7

Signaling System 7: 

Signaling System 7 Alok Pandey 108351

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Signaling System 7 (SS7) is an architecture for performing out-of-band signaling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN). It identifies functions to be performed by a signaling-system network and a protocol to enable their performance. DEFINITION

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Types of Signaling SS7 Signaling SS7 Protocol Architecture SS7 Network Architecture Basic Call Setup SS7 Applications SS7/IP Inter-working CONTENTS

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Signaling in Telecommunications Network Channel Associated Signaling (CAS) Common Channel Signaling (CCS) Signaling System Number 7(SS7) is a form of Common Channel Signaling . TYPES OF SIGNALING

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Used for In-Band Signaling Signaling is transmitted in the same frequency band as used by voice. Voice path is established when the call setup is complete, using the same path that the call setup signals used. Channel Associated Signaling (CAS)

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Out of Band signaling Employs separate, dedicated path for signaling. Voice trunks are used only when a connection is established, not before. Faster Call Setup. Common Channel Signaling (CCS) Switch A Switch B Voice Trunks Signaling Link

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Advantage of CCS over CAS Faster call setup No interference between signaling tones by network and frequency of human speech pattern. Greater Trunking Efficiency : CCS has shorter call set up and tear down times that result in less call holding time, thereby reducing the traffic on the network. Information Transfer : CCS allows the transfer of additional information along with the signaling traffic providing facilities such as caller identification and voice or data identification

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SS7 Principle Out of band Signaling Higher Signaling data rates (56Kbps & 64 Kbps) Signaling traffic is bursty and of short duration, hence operates in connectionless mode using packet switching Variable length signal units with maximum size limitation Optimum use of bandwidth Reliability and flexibility

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SS7 Protocol Stack OSI SS7

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Message Transfer Part (MTP Level 1) Physical Provides an interface to the actual physical channel over which communication takes place CCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps Protocols

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MTP Level 2 (Data Link) Ensures accurate end-to-end transmission of a message across a signaling link Variable Length Packet Messages are defined here Implements flow control, message sequence validation, error checking and message retransmission Monitor links and reports their status Test links before allowing their use Provides sequence numbers for outgoing messages Protocols

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MTP Level 3 (Network) Message routing between signaling points in the SS7 network Signaling network management that provides traffic, links and routing management, as well as congestion (flow) control Re-routes traffic away from failed links and signaling points, controls traffic when congestion occurs Protocols

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Signaling Connection Control Part (SCCP) Provides connectionless and connection-oriented network services Provides global title translation (GTT) capabilities above MTP level 3; translates numbers to DPCs and subsystem numbers Provides more detailed addressing information than MTPs Used as transport layer for TCAP (Transaction capabilities applications part) based services Protocols

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Transaction Capabilities Applications Part (TCAP) Exchange of non-circuit related data Between applications across the SS7 network Using the SCCP service Queries and responses sent between Signaling Switching Point (SSPs) and Signaling Control Point (SCPs) Sends and receives database information Credit card validation Routing information Protocols

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Telephone User Part (TUP) Basic call setup and tear down.In many countries, ISUP has replaced TUP for call management ISDN User Part (ISUP) Necessary messaging for setup and tear down of all circuits (voice and digital) Messages are sent from a switch, to the switch where the next circuit connection is required Call circuits are identified using circuit identification code (CIC) Protocols

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SS7 Networks STP STP STP STP STP

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SS7 Components

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STPs are packet switches, and act like routers in the SS7 network. Routes each incoming message to an outgoing signaling link, based on routing information contained in the SS#7 message and a pre-defined route table Does not offer termination services STPs are paired to ensure redundancy There are three levels of STPs: •National Signal Transfer Point •International Signal Transfer Point •Gateway Signal Transfer Point Signaling transfer point (STP)

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An SCP is usually a computer used as a front end to a database system. It is an interface to application-specific databases. The address of an SCP is a point code, and the address of the database it interfaces with is a subsystem number. The database is an application entity which is accessed via the TCAP protocol. Databases that provides information necessary for advanced call processing capabilities Accepts a query for information from a subsystem at another node Used by STP to perform a function called global title translation Service control point (SCP)

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Databases Accessible via SCP HLR Home Location Register Used in cellular networks to store subscriber information. LNP Local Number Portability Allows people to change service providers but keep their same telephone number OSS Operation Support System Associated with remote maintenance center for monitoring and managing SS7 and voice networks. VLR Visitor Location Register Used when a cell phone is not recognized by the mobile switching center (MSC).

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SS7 Link Types

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SS7 Link Types A link (access) Connects signaling end point (SCP or SSP) to STP B link (bridge) Connects an STP to another STP; typically, a quad of B links interconnect peer (or primary) STPs (STPs from a network connect to STPs of another network) C link (cross) Connects STPs performing identical functions, forming a mated pair (for greater reliability) D link (diagonal) Connects a secondary (local or regional) STP pair to a primary (inter-network gateway) STP pair in a quad-link configuration; the distinction between B and D links is arbitrary E link (extended) Connects an SSP to an alternate STP F link (fully associated) Connects two signaling end points (SSPs and SCPs) in the same local network

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3 Types of SUs are : Message signal units(MSUs) -Carries signaling associated with call setup & teardown, database query and response and SS7 network input Link status signal units(LSSUs) -Inform the far end about the changes in status of link -Message length can be 1 or 2 bytes Fill-in signal units(FISUs) -Fill the gaps between MSU and LSSU messages -Sent only when the buffer is empty, to keep the signaling link active -Facilitate in constant monitoring of link quality. What goes over Signaling Link

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Initial address message (IAM) : contains all necessary information for a switch to establish a connection Address complete message (ACM) : acknowledge to IAM; the required circuit is reserved and the “phone is ringing” (ring back tone) Answer message (ANM) : occurs when the called party picks up the phone Release (REL) : sent by the switch sensing that the phone hung up Release complete (RLC) : each exchange that receives REL, sends an RLC message back (this acknowledges receipt of REL) ISUP Messages

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Basic Call Setup Example 1 6,10 9 2 5 13 15

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Implementation of SS7 in GSM U m A bis A BSS radio subsystem MS MS BTS BSC BTS BTS BSC BTS network and switching subsystem MSC MSC Fixed partner networks IWF ISDN PSTN PDN SS7 EIR HLR VLR ISDN PSTN

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Applications P repaid Calling Local Number Portability (LNP) Global Roaming International Callback Virtual Office Internet Call Waiting/ Caller ID Least Cost Routing Toll Bypass Unified Messaging 800 / Free Phone Services Short Message Service (SMS) Tele-voting Location-based Services Caller Ring Back Tone (CRBT)

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References http://www.aws.cit.ie/personnel/dpesch/notes/msc_sw/ss7_protocol_overview.pdf http://www.techfest.com/networking/wan/ss7.htm http://www.mobilein.com/ss7.htm http://www.telecomspace.com/ss7.html http://en.wikipedia.org/wiki/Signalling_System_No._7 http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/ althosbooks.com/sisy7sba3rde.html

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