By

Aditya Singh

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

Types of SignalingSS7 SignalingSS7 Protocol ArchitectureSS7 Network ArchitectureBasic Call SetupSS7 ApplicationsSS7/IP Inter-working

CONTENTS

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

•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)

•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)

SwitchA

SwitchB

Voice Trunks

Signaling Link

Advantage of CCS over CAS

oFaster call setup

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

oGreater 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.

oInformation Transfer: CCS allows the transfer of additional information along with the signaling traffic providing facilities such as caller identification and voice or data identification

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

SS7 Protocol Stack

OSI SS7

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

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

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

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

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

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

SS7 Networks

STP

STP

STP STP

STP

SS7 Components

•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)

•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)

Databases Accessible via SCP

HLRHome Location Register

Used in cellular networks to store subscriber information.

LNPLocal Number Portability

Allows people to change service providers but keep their same telephone number

OSSOperation Support System

Associated with remote maintenance center for monitoring and managing SS7 and voice networks.

VLRVisitor Location Register

Used when a cell phone is not recognized by the mobile switching center (MSC).

SS7 Link Types

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

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

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

Basic Call Setup Example

1

6,10

9

2

513

15

Implementation of SS7 in GSM

Um

Abis

ABSS

radiosubsystem

MS MS

BTSBSC

BTS

BTSBSC

BTS

network and switching subsystem

MSC

MSC

Fixed partner networks

IWF

ISDNPSTN

PDN

SS

7EIR

HLR

VLR

ISDNPSTN

Applications

Prepaid CallingLocal Number Portability (LNP)Global RoamingInternational CallbackVirtual OfficeInternet Call Waiting/ Caller IDLeast Cost RoutingToll Bypass

Unified Messaging800 / Free Phone Services Short Message Service (SMS)Tele-votingLocation-based Services Caller Ring Back Tone (CRBT)

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