Signaling System 7 (SS7)

By : Ali Mustafa

Contents

• Types of Signaling

• SS7 Signaling

• SS7 Protocol Architecture

• SS7 Network Architecture

• Basic Call Setup

• SS7 Applications

• SS7/IP Inter-working

• VoIP Network Configuration

• SIGTRAN Protocol Stack Model

• Network Evolution to an All-IP Network

Types of Signaling

Signaling in Telecommunications Network

•Channel Associated Signaling (CAS)

•Common Channel Signaling (CCS)

Signaling System Number (SS7) is a form of Common Channel Signaling.

Channel Associated Signaling (CAS)

• 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.

Common Channel Signaling

• 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.

Switch

A

Switch

B

Voice Trunks

Signaling Link

Advantage of CCS over CAS

• Faster call setup

• No interference between signaling tones bynetwork and frequency of human speech pattern.

• Greater Trunking Efficiency:- CCS has shorter callset up and tear down times that result in less callholding time, thereby reducing the traffic on thenetwork.

• Information Transfer:- CCS allows the transfer ofadditional information along with the signalingtraffic providing facilities such as calleridentification and voice or data identification

SS7 History

• CCITT developed a digital signaling standard called Signaling System 6

• SS6 was based on Packet-Switched, proprietary data network.

– Uses 2.4 Kbps data links to send packets of data to distant switches to request service.

• SS7 began deployment in 1983, was initially used for inter office network, but now it is deployed in local central offices.

• Provide a global standard for call setup, routing, control and database access.

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

Protocols

Message Transfer Part (MTP Level 1) Physical

• Provides an interface to the actual physical channel overwhich communication takes place

• CCITT recommends 64Kbps transmission whereas ANSIrecommends 56 Kbps

Protocols

MTP Level 2 (Data Link)

• Ensures accurate end-to-end transmission of a messageacross 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 SS7network

• Signaling network management that provides traffic, linksand routing management, as well as congestion (flow)control

• Re-routes traffic away from failed links and signalingpoints, controls traffic when congestion occurs

Protocols

Signaling Connection Control Part (SCCP)

• Provides connectionless and connection-orientednetwork services

• Provides global title translation (GTT) capabilitiesabove MTP level 3; translates numbers to DPCsand subsystem numbers

• Provides more detailed addressing informationthan MTPs

• Used as transport layer for TCAP (Transactioncapabilities applications part) based services

Protocols

Transaction Capabilities Applications Part (TCAP)

• Exchange of non-circuit related data

– Between applications across the SS#7 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)

SS7 Networks

STP

STP

STP STP

STP

SS7 Components

Service switching point (SSP)

– SSPs are switches that have SS7 software andterminating signaling links

– SSPs create packets (signal units) and sendthose messages to other SSPs, as well asqueries to remote shared databases to find outhow to route calls

– SSPs communicate with the voice switch viathe use of primitives and have the ability tosend messages using ISUP (call setup andteardown) and TCAP (database lookup)protocols.

– The switch can originate, terminate, or switchcalls

Signaling transfer point (STP)

–STPs are packet switches, and act like routers in theSS7 network.

–Routes each incoming message to an outgoingsignaling link, based on routing information containedin 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

STP Levels

SSP

SSP SSP

SSP

STP

STP

STPNATIONAL

INTERNATIONAL

GATEWAY

ITU-TS

ANSI

National STP exists within the national network

• Protocol converters often interconnect a National and anInternational STP by converting from ANSI to ITU-TS.

International STP functions within an international network.

• All nodes connecting to an International STP must use the ITU-TS protocol standard.

Gateway STP converts signaling data from one protocol to another.

• Gateway STPs are often used as an access point to theinternational network.

• Depending on its location, the Gateway STP must be able to useboth the International and National protocol standards.

Service control point (SCP)– 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 foradvanced call processing capabilities

– Accepts a query for information from a subsystem atanother node

– Used by STP to perform a function called global titletranslation

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).

SS7 Link Types

STP STP

STP

STP

STP

STP

STP

STP

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

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 Phone Services

• Short Message Service (SMS)

• Tele-voting• Location-based

Services

• Caller Ring Back Tone (CRBT)

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

ISDN

PSTN

PDN

SS

7

EIR

HLR

VLR

ISDN

PSTN

SS7/IP Inter-working

• IP offers an economical solution to challengescreated by high volume traffic at network edge.

• Operators off load voice calls from PSTN to VoIPnetworks because it is less costly to carry voicetraffic over IP network than over Switched circuitnetwork.

• In VoIP network, digitized voice data is highlycompressed and carried in packets over IPnetwork, thereby efficiently utilizing thebandwidth and increasing the number of voicecalls carried.

• Saving realized in using VoIP network are passedon to users in the form of lower cost.

Features of SS7 over IP

• Flow Control

• In-Sequence Delivery of Signaling Messages within a single control stream

• Identification of the originating and terminating signaling points

• Identification of voice circuits

• Error detection, re-transmission and other error correcting procedures.

• Controls to avoid congestion on the internet.

• Detection of status of peer entities.

• Support for security mechanism.

Signaling in VoIP networks

VoIP Network carry SS7 over IP using protocols defined bySignaling Transport (SIGTRAN) working group of the InternetEngineering Task Force (IETF).

In IP telephony networks, signaling information is exchangedbetween the following functional elements.

•Media Gateway

•Media Gateway Controller

•Signaling Gateway.

VoIP Network Configuration

• Media Gateway- Terminates voice calls on Inter-switchTrunks from the PSTN, compresses and packetizes the voicedata and delivers compress voice packet to the IP network.For voice calls originating in an IP network, it performs thesefunctions in reverse order.

• Media Gateway Controller- It handles the registration andmanagement of resources at Media Gateway(s), also knowsas Soft Switch.

• Signaling Gateway- It provides Transparent interworking ofsignaling between switched ckt. and IP network. It mayterminate SS7 signaling or translate and relay messages overan IP network to a media gateway controller or anothersignaling gateway.

SIGTRAN Protocol Stack ModelThe SIGTRAN’s protocol specifythe means by which SS7messages can be reliablytransported over IP network.

The architecture identifiesthree component

•A standard IP

•A common signaling transportprotocol for SS7 protocol layerbeing carried.

•An adaptation module toemulate lower layers of theprotocol.

Stream Control Transmission Protocol(SCTP)

• Allows the reliable transfer ofsignaling messages betweensignaling end points in an IPnetwork

• Allows signaling messages tobe independently orderedwith in multiple streams toensure in sequence deliverybetween associated endpoints.

• SIGTRAN recommends SCTPrather than TCP/IP fortransmission of signalingmessages over IP networks asTCP/IP does enforce Head-of-line Blocking.

Network Evolution to an All-IP Network

• Both traditional Circuit switch and IP based services need to be supported by single network infrastructure simultaneously.

• Hybrid architecture may be the best solution.

• Transition to All-IP network will not happen overnight.

IP Network

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