04/22/23 1

SS7

By

DEEPAK SACHDEVA

04/22/23 2

CONTENTS• History of network.• Need of SS7.• Main components.

• STP• SSP• SCP• CRP• IP

• Network Links• Network Architecture.• SS7 User part functionality.

• MTP-1• MTP-2• MTP-3• SCCP• TCAP• ISUP

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HISTORY OF NETWORK

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NEED OF SS7• SS7 Defines the Procedures and Protocols by which the NE’s in the PSTN

exchange information over the digital signaling network to effect wireless (cellular) and wire line :

• Call Set – up• Routing & Control• Network Management & Maintenance.

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Main Components:• STP (Signaling Transfer Point)

• Digital Sister of Switch in PSTN.• Paired for Redundancy.• Single STP Engineered to 40% of Capacity.• Capacity = 0.5 * Total Traffic Diverted at a pair / 0.4

•SSP (Service Switching Point) • with capability for Call set – up.• Queries to unknown databases.• Hang up the call processing and make the necessary actions.

•SCP (Service Control Point)• Acts as the front end of the database.

•CRP (Customer Routing Point)• Database maintained by the Corporate/ Large company for itself.

• IP (Intelligent Peripheral)• Database access with Intelligent functions ( frequent Numbers called, voice

recognition calling etc.)

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MSC •Point code in a SS7 Network.

– Maintains link with PSTN and SS7 Network.•HLR

•Information to validate the customer•Billing information.

• VLR•Maintains database for subscribers (roaming).

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Network Links

• Access Links – Link a SP to a local STP pair. •Cross Links – Link two STP’s together to form a pair. •Bridge Links – Link two pairs at the same level (local/local or regional/regional ). •Diagonal Links – Link a STP Pair to higher level STP pair. •Extended Links – Link a SP to a remote STP pair. •Fully Associated Links – Link two associated nodes together.

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NETWORK ARCHITECTURE

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SS7 USER PART FUNCTIONALITY•Features

•Modular approach ( each module performs in sequence and hands over to the next one.)

– Has led to the “layered” Protocol

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APPLICATION TO APPLICATION COMMUNICATION

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Functions of User PartsMTP – 1 (Message transfer part)

•Physical Layer – Deals with hardware and electrical configurations.•Deals with physical issues at the level of

–cards, –processors & multiplexers.

MTP –2•Last to handle outgoing messages and first to handle the incoming.•Monitors Links and reports status to higher part.•Ensures

–message integrity.–Sequence of messages.–Discards bad messages and request copies.–Acknowledge good ones.–Places link in service, takes it out and restores the link .

• Signal Unit Delimitation (placing of flag with MSU).• Signal unit Error Detection (check bit algorithm).

– Discarded upon disparity in check bit.• Signal Unit Error Correction.

– Land based Transmission ( copies sent from Transmission buffer)– Stops sending the copies when receiving side catches up with the sequence.

( Start of Original messages)– Satellite based Transmission

– Limited Copies held ( retransmitted until acknowledgement received)• Signaling link Alignment.

• Invoked on restoring link– Normal Alignment ( 2 secs – 4 states – standard 64kbps links)– Emergency Alignment (0.5 secs – 1 trial )

• Congestion Monitoring – SIB (status indicator busy) to transmitting side.

Functions

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• Signaling message handling• Network management

Network Management•Traffic management

• Use of link sets to determine the message flow on priority basis (0 to 3).

•Assigns SLS.• Use of forced rerouting

•TFP (transfer prohibited)•TCP(transfer cluster prohibited)

• Controlled rerouting•TFR(transfer restricted)•TCR (transfer cluster restricted)

MTP-3

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Link Management•Signaling Link Activation

– At request of an application.•Signaling Link changeover

– Performs and generates COO (Change over Order) & – receives COA (Change over acknowledgment) from adjacent node.

•Signaling Change back – generates CBD ( Change back declaration) on switchback.–Gets CBA ( Change back acknowledgement ).

Routing Management•Flip side of Traffic Management with

–TFP ( transfer restricted )–TCP ( transfer cluster prohibited )–TFR ( transfer restricted )–TCR ( transfer cluster restricted )–TFA ( transfer allowed )–TCA ( transfer cluster allowed ) * Sent by STP to adjacent nodes

(Contd……)

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Congestion Control• Manages on the monitoring done by MTP-2.• Decides by event sensing

– Congestion onset value– Congestion abatement value– Discard onset value ( for 3 priority levels, level four – network

management )

(Contd……)

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•“Flow control” procedures invoked•MTP-2……congestion is detected on receiving end.

•“Congestion control” procedures invoked•MTP-3…….congestion is detected on sending end / transmit / retransmit buffer.

Signal Messaging Handling•Message Discrimination

– Message destination as related to the SP.– DPC ( Destination point code)

•Message Distribution– Intended for which User Part.

– Defined by field in the message.

(Contd……)

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•Services types

•Class 0 connectionless.–Messages are not transferred in reference / delivered without sequence.

•Class 1 connectionless.–Services of MTP-3 asked (SLS stops rotating & remains same).

–Messages delivered in sequence.

•Extended functions• Specialized routing.

– DPC (Identification / addressing of a database)–SSN ( For routing to the appropriate network and database)–GTT ( Global Title Translation )

– helps in access to centralized database.– SP’s access to all databases(universally) without having to maintain them.–Better control of databases ( use of GTT to hide/ restrict access) by organizations.

SCCP

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Subsystem Management

•Deals with database management ( backup / redundancy)

•SCCP interacts by sending following commands•N-coord request – from database(withdrawing) to local SCCP.

•Subsystem – out – of service – from local SCCP redundant SCCP.

•N-coord confirmation – sent to local subsystem if local timer(set by local SCCP) phases out.

•N-coord response – sent by SCCP(redundant) to SCCP(local) if resources ready for load sharing.

•Sub- system prohibited – sent by SCCP(local) to CP(Concerned points).

•Sub- system allowed – sent by SCCP(local) to CP – on return of services.

•Subsystem – status check – sent to SCCP(Prohibited) to check the service status.

(Contd…….)

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TCAP (Transaction capabilities application Protocol)• Offers services to OMAP (Operation , maintenance & administration part), IS-41D (Interim Standard 41, version D) & GSM MAP(Mobile Application Part).

•Allow applications to exchange information using signaling(not circuit switched).

•Used by switching locations to obtain data from databases.–SSP from 800db, MSC from HLR.–Invoke features to another switch ( Number redialing, auto call back)

•Provides procedures for database updates be reference to other databases.

•TCAP messages go in sync with SCCP messages(specialized routing) & have appropriate message structure and parameters to acquire and package data.

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ISUP (Integrated services digital network user part)

•Used throughout PSTN to provide messaging necessary necessary for call set-up and teardown of all circuits…..both voice and digital.•Used by wireless networks to establish connection in PSTN.•Services offered

–Basic•Services involved in setting up and tear down a call.

–Supplementary•Services necessary to maintain / modify the call.

•Functionalities–SPRC(Signaling procedure control) – directly interfaces with services of MTP.–CSC(Circuit supervision control) – maintaining of call.–CPC(Call processing control) – maintaining of call.

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WHAT LIES AHEAD……..• SMSC

–Product Overview–Services & Features.

• SMSC – as related to SS7.

•How does SMS flow.

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THANK YOU