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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-exchangefunctions of the public switched telephone

network (PSTN). It identifies

functions to be performed by a signaling-system

network and a protocol to enabletheir performance.

DEFINITION

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Types of SignalingSS7 SignalingSS7 Protocol Architecture

SS7 Network ArchitectureBasic Call SetupSS7 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 ofCommon Channel

Signaling.

TYPES OF SIGNALING

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Used for In-Band Signaling

Signaling is transmitted in the same frequencyband as used by voice.

Voice path is established when the call setup iscomplete, using the same path that the call setupsignals 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 isestablished, not before.Faster Call Setup.

Common Channel Signaling (CCS)

SwitchA

SwitchB

Voice Trunks

Signaling Link

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Advantage of CCS over CAS

oFaster call setup

oNo interference between signaling tones by network andfrequency of human speech pattern.

oGreater Trunking Efficiency: CCS has shorter call set up andtear down times that result in less call holding time, therebyreducing the traffic on the network.

oInformation Transfer: CCS allows the transfer of additional

information along with the signaling traffic providing facilitiessuch 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, henceoperates 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 channelover which communication takes place

CCITT recommends 64Kbps transmission whereasANSI recommends 56 Kbps

Protocols

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MTP Level 2 (Data Link)

Ensures accurate end-to-end transmission of a message acrossa signaling link

Variable Length Packet Messages are defined here

Implements flow control, message sequence validation, errorchecking 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 SS7network

Signaling network management that provides traffic, linksand 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 networkservices

Provides global title translation (GTT) capabilities above MTPlevel 3; translates numbers to DPCs and subsystem numbers

Provides more detailed addressing information than MTPs

Used as transport layer for TCAP (Transaction capabilitiesapplications 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 SwitchingPoint (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 hasreplaced 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

STPSTP

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 messageand 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 PointInternational Signal Transfer PointGateway 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 itinterfaces 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 processingcapabilities

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 NumberPortability

Allows people to change serviceproviders but keep their sametelephone number

OSS

Operation SupportSystem

Associated with remote maintenancecenter for monitoring and managingSS7 and voice networks.

VLR

Visitor LocationRegister

Used when a cell phone is notrecognized by the mobile switchingcenter (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 Blinks interconnect peer (or primary) STPs (STPs from anetwork connect to STPs of another network)

C link (cross) Connects STPs performing identical functions, forminga mated pair (for greater reliability)

D link (diagonal) Connects a secondary (local or regional) STP pair to aprimary (inter-network gateway) STP pair in a quad-link configuration; the distinction between B and D

links is arbitraryE link (extended) Connects an SSP to an alternate STP

F link(fully associated)

Connects two signaling end points (SSPs and SCPs) inthe 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 linkactive-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

513

15

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Implementation of SS7 in GSM

Um

Abis

ABSS

radio

subsystem

MS MS

BTS

BSCBTS

BTS

BSCBTS

network and switching

subsystem

MSC

MSC

Fixed partner networks

IWF

ISDNPSTN

PDN

SS7

EIR

HLR

VLR

ISDNPSTN

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Applications

Prepaid 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 PhoneServices

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_over

view.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

http://www.aws.cit.ie/personnel/dpesch/notes/msc_sw/ss7_protocol_overview.pdfhttp://www.aws.cit.ie/personnel/dpesch/notes/msc_sw/ss7_protocol_overview.pdfhttp://www.techfest.com/networking/wan/ss7.htmhttp://www.mobilein.com/ss7.htmhttp://www.telecomspace.com/ss7.htmlhttp://en.wikipedia.org/wiki/Signalling_System_No._7http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/http://www.cisco.com/univercd/cc/td/doc/product/tel_pswt/vco_prod/ss7_fund/http://en.wikipedia.org/wiki/Signalling_System_No._7http://www.telecomspace.com/ss7.htmlhttp://www.mobilein.com/ss7.htmhttp://www.techfest.com/networking/wan/ss7.htmhttp://www.aws.cit.ie/personnel/dpesch/notes/msc_sw/ss7_protocol_overview.pdfhttp://www.aws.cit.ie/personnel/dpesch/notes/msc_sw/ss7_protocol_overview.pdf

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