SS 7 Signalling

7/30/2019 SS 7 Signalling

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Types of Signaling

Signaling in Telecommunications Network

Channel Associated Signaling (CAS)

Common Channel Signaling (CCS)

Signaling System Number (SS7) is a formof Common Channel Signaling.


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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 thesame path that the call setup signals

used.


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Common Channel Signaling

Out of Band signaling

Employs separate, dedicated path for

signaling.

Voice trunks are used only when aconnection is established, not before.

Faster Call Setup.

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 bynetwork and frequency of human speechpattern.

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 caller

identification and voice or data identification


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SS7 History

CCITT developed a digital signaling standard calledSignaling System 6

SS6 was based on Packet-Switched, proprietary datanetwork.

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

SS7 began deployment in 1983, was initially used

for inter office network, but now it is deployed inlocal central offices.

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


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SS7 Principle

Out of band Signaling

Higher Signaling data rates (56Kbps & 64Kbps)

Signaling traffic is bursty and of shortduration, hence operates in connectionlessmode using packet switching

Variable length signal units with maximumsize limitation

Optimum use of bandwidth

Reliability and flexibility


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SS7 Protocol Stack OSI SS7


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Protocols

Message Transfer Part (MTP Level 1) Physical

Provides an interface to the actual physical channelover which communication takes place

CCITT recommends 64Kbps transmission whereas

ANSI recommends 56 Kbps


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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


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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


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Protocols

Signaling Connection Control Part (SCCP)

Provides connectionless and connection-orientednetwork services

Provides global title translation (GTT) capabilitiesabove MTP level 3; translates numbers to DPCs andsubsystem numbers

Provides more detailed addressing information thanMTPs

Used as transport layer for TCAP (Transaction

capabilities applications part) based services


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ProtocolsTransaction 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


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ProtocolsTelephone User Part (TUP)

Basic call setup and tear down

In many countries, ISUP has replaced TUP for callmanagement

ISDN User Part (ISUP)

Necessary messaging for setup and tear down of allcircuits (voice and digital)

Messages are sent from a switch, to the switchwhere the next circuit connection is required

Call circuits are identified using circuit identification

code (CIC)


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SS7 Networks

STP

STP

STPSTP

STP


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SS7 Components


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Signaling transfer point (STP)

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


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STP Levels

SSP

SSP SSP

SSP

STP

STP

STPNATIONAL

INTERNATIONAL

GATEWAY

ITU-TS

ANSI

l h h l k


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National STP exists within the national network

Protocol converters often interconnect a National and an

International 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 use both the International and National protocol

standards.


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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 apoint code, and the address ofthe database it interfaces with is a subsystem number.

The database is an application entity which is accessed viathe 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 title

translation


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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

STP STP

STP

STP

STP

STP

STP

STP


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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 ofB 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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Addressing in SS7 Network

Point Codes are carried in signaling messages and

exchanged between signaling points to identify the

source and destination of each message (24 bit

address).

NETWORK

IDENTITY

MEMBER

IDENTITY

1 1 1

CLUSTER

IDENTITY


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What goes over Signaling Link

Signaling information is passed over thesignaling link in form of messages, whichare called signaling units (SUs)

3 Types of SUs are:

Message signal units(MSUs)

Link status signal units(LSSUs)

Fill-in signal units(FISUs)


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Signaling Units

Message signal unit (MSU)

Carries signaling associated with call setup &

teardown, database query and response and SS7

network input

BSN/

BIB

FLAG Length

Indicator

FSN/

FIB

Check

sum

Signaling

Info. field

Service

Info. octet

1 1 11 1 8-272 1


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Service Information Octet

Bit 1 - 4 Type of Information in Signaling

Information Field

Bit 5 - 6Whether Message is intended for

National or International Network Bit 7 - 8 To identify Message Priority

1 2 3 4 5 6 7 8


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Signaling Information Field

8 272 Octets

The first portion of this field is routing label

Routing Label Identifies Message Originator,Intended destination & Signaling Link Selected.

Routing Label is of 7 Octets

Destination Point Code 3 Octets

Originating Point Code 3 Octets

Signaling Link Selection 1 Octet


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Signaling Units

Link status signaling units (LSSU)

Inform the far end about the changes in status of link

Message length can be 1 or 2 bytes

FLAG BSN/

BIBFSN/

FIB

Length

Indicator

Check

Sum

1 1 1 1 1

Status

Field

1 or 2


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FLAG BSN/

BIBFSN/

FIB

Length

Indicator

Check

Sum

1 1 1 1 1

Signaling Units

Fill-In Signal Units (FISU)

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.


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Basic Call Setup Example

1

6,10

9

2

513

15

S


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ISUP Messages 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)


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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 Phone

Services

Short Message

Service (SMS) Tele-voting

Location-based

Services Caller Ring Back

Tone (CRBT)

Date post:	14-Apr-2018
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SS 7 Signalling

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