8.2 Signaling System No 7

8/9/2019 8.2 Signaling System No 7

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SIGNALING SYSTEM NO. 7


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Optimized for operation with digital networks where switches

use stored program control (SPC).

Can meet present and future requirements of information

transfer for , remote control,

and , and .

Provides a reliable means of information transfer in correct

sequence without loss or duplication.

Common-channel signaling system

INTRODUCTION


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Common Channel Signaling System No. 7 (i.e., SS7 or C7)

The SS7 network and protocol are used for:

toll-free (800) services (First Usage of SS7 in 1980 in USA)

basic call setup, management, and tear down

wireless services such as personal communications

services (PCS), wireless roaming, and mobile subscriberauthentication

local number portability (LNP)

enhanced call features such as calling party name/number

display efficient and secure worldwide telecommunications


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Intentionally Left Blank


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SS7 Network Architecture

SS7 network is separate from the voicenetwork.

SS7 is used for the purpose of switching datamessages (relating to connecting telephone

calls and maintaining the signaling network).

Packet Switching is the method used for

transferring messages through network.


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Telephone switches perform dual function Voice related functions( connectivity with other exchanges and

switching of voice circuits)

Signaling functions

Signaling function is accomplished through adjunct Computers, which

are connected through digital links to other computers in the

network.

These Computers are referred to as Signaling Points.

Allnodes in the SS7 networkare calledSignaling Points


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Signaling Points

Provide access to the SS7 network

Provide access to databases

Transfer SS7 messages to other signaling points within thenetwork

Each Signaling Point in the SS7 network is uniquely identified by anumeric Point Code. Point codes are carried in signaling messagesexchanged between signaling points to identify the source anddestination of each message

Three types of Signaling Points in the SS7 network exist SSP (Service Switching Point) STP (Signal Transfer Point)

SCP (Service Control Point)


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Service Switching Point (SSP)

An SSP can be a combination voice switch and SS7 switch

OR An adjunct computer connected to the local exchange's voiceswitch

SSP must convert signaling from the voice switch into SS7signaling messages

In the case of database access, the SSP will be sending database

queries through the SS7 network to the computer systems locatedcentrally to the network. (First usage of the SS7 network)

Called and Calling Party Address must be passed from the voiceswitch to the SSP for transfer across the network


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Signal Transfer Point (STP)

All SS7 packets travel from one SSP to anotherthrough the services of STP

STP serves as a Router (Packet Switch ) in the SS7

network.

Messages are not usually originated by an STP.

The STP switches SS7 messages as received from thevarious SSPs through the network to their appropriatedestinations



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Service Control Point (SCP)

Serves as an interface to telephone companydatabases

These databases are used to store information about

Subscribers services

Routing of special service numbers (such as 800)

Calling card validation and fraud protection

Advanced Intelligent Network (AIN) services

SCP is usually a computer used as a front end to the

database system


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Signaling Data Links

All SS7 Signaling points are interconnected viasignaling data links.

These links are bi-directional, using both a transmit

and receive pair for simultaneous transmission inboth directions.

These data links are 56 Kbps in North America

4.8 Kbps in Japan

64 Kbps in nearly every other portion of the world


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There are three modes of signaling

In associated signaling, the signaling link is directly parallel with

the voice facility for which it is carrying signaling

This is not ideal, because it would require a signaling link from

the end office to every other end office in the network


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Non-associated signaling involves the use of STPs to reach theremote exchange

There are usually multiple nodes involved for signaling messagesto reach the final destination , while the voice may be a directpath to the destination.

Quasi-associated signaling uses a minimal number of nodes toreach the final destination.

InQ

uasi-associated signaling, both SSPs connect to the same ST

P.The signaling path is still through the STP to the adjacent SSP

This is the most favorable method of signaling, because each nodeintroduces additional delay in signaling delivery.

For this reason, SS7 network favour quasi-associated signaling


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SSP SSPSignaling Path

Voice Trunk

Associated Signaling

Signaling Path

STP STP

SSP SSP

Voice Trunk

Non -Associated Signaling

Signaling Path

STP

SSPSSP

Signaling Path

Voice Trunk

Quasi-Associated Signaling


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The SS7 protocol stack consists of only four levels

The OSI model consists ofseven layers

This is due to the fact that SS7 was developed before the OSI

model

SS7 in 1980 & OSI in 1982

CCITT Rec. Q.709 specifies no more than

for 95% of calls.

To accomplish this, a

, called STPs, that can be traversed by a signaling message

and by the


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Message Transfer Part ( MTP )

MTP is divided into three levels

MTP Level 1

MTP Level 2 MTP Level 3


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A

B

C

D

T1156170-93/d06

Level 4 Level 3 Level 2 Level 1

UserParts

TUP

DUP

O thertype

Signalling network functions

Signalling messagehandling

Messagedistribution

Message TransferPart

Signalling link

Signallinglink

functions

Switchingfunctions

Testing and maintenance

Signalling Network ManagementSCCP

Messagediscrimination

Message

routing

Signallingtraffic

management

Signallingroute

management

Signallinglink

management

Signalling data link

Transmissionchannels

Signalling message flow

Controls and indications

Detailed structure of signalling system functions


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It defines the physical, electrical, and functional characteristics

of the signaling data link and the means to access it.

The signaling data link should always be Terrestrial , although

Satellite links are supported in the standards

Layer 2 carries out the signaling link function

It defines the functions and procedures for the transfer of

signaling messages over one individual signaling data link.

A signaling message is transferred over the signaling link in

variable-length signal units.


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1. Delimitation of a signal unit by means of flags.

2. Flag imitation prevention by bit stuffing.

3. Error detection by means of check bits included in each signal

unit.

4. Error control by retransmission and signal unit sequence

control by means ofexplicit sequence numbers in each signal

unit and explicit continuous acknowledgments.

5. Signaling link failure detection by means ofsignal unit error

monitoring and signaling link recovery by means of special

procedures.


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Defines such transport functions and procedures that are common

to and independent of individual signaling links.

These functions direct the message to the proper signaling link or

user part

These control real-time routing, control, and network

reconfiguration, if required.


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Each user part defines the functions and procedures peculiar to the

particular user.

Such as:

Telephone (TUP)

Data (DUP)

ISDN user part (ISUP)


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It contains user information, such as data or call control

signals, management and maintenance information, and

type and format of message.

It also includes a label. The label enables the message to bethrough the signaling network to its destination

and directs the message to the desired user part or circuit.


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On the signaling link, signaling information is contained in themessage signal units (MSUs).

It also include transfer control functions related to layer 2

functions on the link.


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In the standard label, the portion that is used for routing is called

the routing label.

The routing label includes:

Destination and originating points of the message.

A code used for load sharing.


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The goal ofTraffic management is to keep traffic moving

toward its destination, even in the event of network failures

and congestion.

This movement often involves rerouting traffic onto an

alternate network path and, in some situations, mightrequire message retransmission.

For example, when a TFP is received for a destination, traffic

management must determine whether an alternate route is

available and shift traffic to this alternate route.


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1. Signaling traffic management

2. Signaling route management

3. Signaling link management

The three signaling network management functional

blocks are:


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The transfer of signaling traffic is performed in accordance with

specific procedures.

Changeover is the process of diverting traffic to a new link when a

link becomes unavailable.

Changeover to a New Linkset


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Link management can be divided into three processes:

Signaling link management controls the locally connected

signaling link sets.

The signaling link management interacts with the signaling link

function at level 2 by receipt of indications of the status of

signaling links.


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Signaling route management only relates to the quasi-associated

mode of signaling.

Signaling route management communicates the availability

of routes between SS7 nodes.

A signaling transfer point may send

messages indicating inaccessibility of a

particular signaling point via that

signaling transfer point, thus enabling

other signaling points to stop routing

message to an inoperative route.


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A signaling data link is a path for

signaling comprising operating together in

opposite directions at the .

A digital signaling data link is made up of digital

and or their terminating equipmentproviding an

E-1 (2.048 Mb/s; 32 64 kb/s channels)DS-1 (1.544 Mb/s; 24 64 kb/s channels),

V.35 (64 kb/s), for use with high speed modems

DS-0 (64 kb/s), and

DS-0A (56 kb/s)


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Equipment such as , , or

attached to the transmission link

The on a digital bearer is 64 kbits/s.

The for telephone call control

applications is 4.8 kbits/s

The for signaling is time slot 16.

When time slot 16 is not available, any time slot available for

64-kbit/s user transmission rate may be used.


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The signaling link (level 2) functions include:


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A that is coded as separates consecutive signal

units on a signaling data link. The flag octet indicates the beginningor end of an SU.

It is optional whether a single flag is used to mark both the beginning

and end of an SU, or whether a common flag is used for both. The

latter is the most common implementation.

Zero Bit Insertion and Deletion


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Two forms of error correction are provided

1. Basic error correction (BEC):a. Signaling links using means.

where one-way

propagation is less than 15 ms

2. Preventive cyclic retransmission (PCR) method:where the one-way

delay is equal to or greater than 15 ms.

Neither method tries to repair a corrupt MSU; rather, they both

seek .


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MTP Level 2 is equivalent to the OSI Data Link Layer

It ensures that

All transmitted blocks of data are delivered with no losses or

duplications Blocks of data are delivered in the same order in which they

were transmitted

The receiver is capable of exercising flow control over the

sender

Many of the techniques found in well known data link protocols,

such as HDLC, LAPD and LAPB are used in in the SS7 signaling link

level


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Flow control used is SlidingWindow mechanism

Error control used is Go-Back-N ARQ

The blocks of data transmitted at the MTP level 2 ( signaling link

level ) are called Signal Units

Message Signal Unit ( MSU )

Link Status Signal Units ( LSSU )

Fill-In Signal Units ( FISU )


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Fill-in Signal Unit (FISU)When no traffic is being sent and the network is idle,

FISU is sent to provide constant error checking on thelink

Because a CRC checksum is calculated for each FISU,signaling link quality is checked continuously by bothsignaling points at either end of the link

In the event that a link has degraded to a point where

it is causing too many errors, the link can be takenout of service

FISUs are sent only at predefined timer intervals (e.g.,once every 150 milliseconds)


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Link Status Signal Unit (LSSU) Sent between two signaling points to indicate the

status of the signaling link on which it is carried

The link status is used to control link alignment and

to indicate the status of a signaling point (e.g., localprocessor outage) to the remote signaling point

Important concept with LSSU is that the status

reported is that of the transmitting signaling point,

and not ofthe transmission facility.

SS7 Protocol Stack


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Message Signal Unit ( MSU ) Carries user data from level 4

Message Signal Units (MSUs) carry

all call control messages (IAM , SAM etc. )

database query and response

network management, and network maintenance

data

in the Signaling Information Field (SIF). SIF has a Routing label which allows an originating

signaling point to send information to a destination

signaling point across the network

SS7 Protocol Stack


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MTP Level 3 ( Signaling Network Level ) MTP Level 3 is equivalent to the OSI Network Layer

SS7 Protocol Stack


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Length Indicator ( LI ) The 6-bit LI can store values between zero and 63

Indicates signal unit type

Also indicates the length in octets of the upper level

fields

SS7 Protocol Stack

LI value Signal Unit Type0 FISU

1,2 LSSU

3..63 MSU (Variouslengths of MSU


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Service Information Octet ( SIO )

SIO field in MSU is used to

Identifies the user part ( TUP , ISUP etc.)

Identifies version of SS7 protocol / network indicator

( National SS 7, International SS7)

National Call or International Call

Finds application in Gateway STP where Protocol

conversion between National SS7 protocol andInternational SS7 protocol is performed

SIO field contains two sub fields

Service Indicator (4 bits ) Subservice field( 4 bits)

SS7 Protocol Stack


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Subservice Field

Another reason for this field is because of the differencebetween the point codes used between international andnational networks

SS7 Protocol Stack

00XX International Network

10XX National Network


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Service Indicator specifies the MTP userSS7 Protocol Stack

Service

Indicator

MTP User

0 Signaling Network Management Message (SNM)

1 Maintenance Regular Message (MTN)2 Maintenance Special Message (MTNS)

3 Signaling Connection Control Part (SCCP)

4 Telephone User Part (TUP)

5 ISDN User Part (ISUP)

6 Data User Part (call and circuit-related messages)

7 Data User Part (facility registration/cancellation

messages)


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Signaling Information Field ( SIF ) SIF field in MSU contains

Routing Label ( OPC , DPC, SLS )

User data from SS7 user part like TUP, ISUP etc or

Network Management data Actual Message ( IAM , SAM etc. )

SS7 Protocol Stack

Signaling Information Field ( SIF )

8n 4 14 14User Part / Network

Management

SLS OPC DPC

Routing Label


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8.2 Signaling System No 7

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