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CONTENTS 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
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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. 3
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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, hence operates 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
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OSI SS7
PROTOCOLSMessage 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
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PROTOCOLSMTP 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
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PROTOCOLSMTP 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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PROTOCOLSSignaling 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
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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 10
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PROTOCOLSTelephone 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) 11
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SS7 NETWORKS
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STP
STP
STP STP
STP
SS7 COMPONENTS
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SERVICE SWITCHING POINT (SSP)
SSPs are switches that have SS7 software and terminating signaling links
SSPs create packets (signal units) and send those messages to other SSPs, as well as queries to remote shared databases to find out how to route calls
SSPs communicate with the voice switch via the use of primitives and have the ability to send messages using ISUP (call setup and teardown) and TCAP (database lookup) protocols.
The switch can originate, terminate, or switch calls
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SERVICE SWITCHING POINT (SSP) Local exchange in the telecommunication network SSP can be
a combined voice and SS7 switch an adjunct computer connected to a local exchange’s
voice switch SSP communicates with the voice switch via
primitives and creates signal units for communication over SS7 network
SSP converts signaling from voice switch into SS7 format
SSP may send messages for data base queries through SS7 network
SS7 traffic has been mainly circuit-related but is now becoming more non-circuit-related
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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 tableDoes not offer termination servicesSTPs are paired to ensure redundancy There are three levels of STPs.
National Signal Transfer PointInternational Signal Transfer PointGateway Signal Transfer Point
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STP LEVELS
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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 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 the international network.
Depending on its location, the Gateway STP must be able to use both the International and National protocol standards. 18
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SIGNALING TRANSFER POINT (STP)
Other tasks of the STP include traffic measurements usage measurements
Traffic measurements are used for performance monitoring of the SS7 and telecommunication network
Usage measurements are used for billing purposes
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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 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
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DATABASES ACCESSIBLE VIA SCP
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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).
OPERATIONS SUPPORT SYSTEMS
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OSS are remote maintenance centers for monitoring and management of SS7 networks
Monitoring takes place through digital interfaces, on-site personnel is no longer required.
OSS provides maintenance personnel with an interface into the network and allows to view the status of network elements, e.g. SSPs, STPs, etc, on larger screens.
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OPERATIONS SUPPORT SYSTEMS
SS7 LINK TYPES
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STP
STP
STP
STP
STP
STP
SS7 LINK TYPES
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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
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).
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NETWORK IDENTITY
MEMBER IDENTITY
1 1 1
CLUSTERIDENTITY
WHAT GOES OVER SIGNALING LINK
Signaling information is passed over the signaling link in form of messages, which are called signaling units (SUs)
Signal Units are the data packets that are sent in an SS7 network.
SS7 uses three types of signal units SS7 network management uses all three types of
signal units whereas information is sent using only one type of signal unit
SS7 is different to other types of data networks as it does not provide user-to-user data transmission but machine-to-machine data transmission
Signal Units rely on the services of the MTP for routing, link control, and error control 27
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MESSAGE SIGNAL UNIT
MSU carries SS7 information MSU consists of MTP protocol fields and two
additional fields Service indicator octet (SIO)
indicates type of protocol at level 4, e.g. TUP, ISUP, and type of standard, e.g. national, international.
Service information field (SIF) used to carry control information as well as level 3
routing label. SIF can be up to 272 octets and is used by all level 4 protocols
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LINK STATUS SIGNAL UNIT (LSSU)
Used to carry link status information Used by level 3 at one node to transmit link
status information to its adjacent node LSSU only used on single point-to-point links,
never through the network No information traffic is carried on a link
when LSSU are sent
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FILL-IN SIGNAL UNIT (FISU)
FISU is used when no information needs be sent and the network is idle
FISU is used to monitor error rates on links. This allows SS7 to be highly reliable as it can detect link quality even when idle
In addition to FISU transmission the MTP protocol is constantly monitoring the link status
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PRIMITIVES
Primitives are used to provide standard interfaces between the 4 levels of the SS7 protocol architecture
Primitives are commonly used to define interfaces. However, primitives are not seen in the network and are typically software functions at each signalling point
Primitive are not unique to SS7, but types used are unique
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SS7 PRIMITIVE STRUCTURE
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PRIMITIVE FIELDS
X - indicates originator of primitive “MTP” if the MTP is passing information to ISUP
Generic name - indicates the type of information being provided When information regarding address of originator
(calling party address) is sent from ISUP to MTP, generic name is “unitdata”
generic name will different between levels Specific name - describes the action to be
taken request, indication, response, confirmation
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BASIC CALL SETUP EXAMPLE
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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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SS7 APPLICATIONSN
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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
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Um
Abis
ABSS
radiosubsystem
MS MS
BTSBSC
BTS
BTSBSC
BTS
network and switching subsystem
MSC
MSC
Fixed partner networks
IWF
ISDNPSTN
PDN
SS
7
EIR
HLR
VLR
ISDNPSTN
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.
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SIGTRAN PROTOCOL STACK MODELThe SIGTRAN’s protocol specify the means by which SS7 messages can be reliably transported over IP network.
The architecture identifies three componentA standard IPA common signaling transport protocol for SS7 protocol layer being carried.An adaptation module to emulate lower layers of the protocol.
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STREAM CONTROL TRANSMISSION PROTOCOL(SCTP)
Allows the reliable transfer of signaling messages between signaling end points in an IP network
Allows signaling messages to be independently ordered with in multiple streams to ensure in sequence delivery between associated end points.
SIGTRAN recommends SCTP rather than TCP/IP for transmission of signaling messages over IP networks.
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ITU-T SS7 SPECIFICATIONS
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BRAINSTORMING
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What is the full form of SS7 Signalling System 7
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What is the signalling type used in SS7 Common Channel Signalling
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What are the functions of MTP Layer MTP1 (Physical) MTP2 (Data Link) MTP3 (Netwok)
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What are the Components in SS7 Architecture Service Switching Point (SSP) Signal Transfer Point (STP) Service Control Point (SCP)
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Name the Call Handling Layers in SS7 Stack Telephone User Part (TUP) ISDN User Part (ISUP)
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Name the Database Handling Layers in SS7 Stack Transaction and Capabilities Application Part
(TCAP)
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What are the types of Signalling Units Message Signal Unit Link Status Signal Unit Fill In Signal Unit
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What protocol specifies reliable transport of SS7 over IP Sigtran
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What is the addressing schema in SS7 Networks Point Code Sub System Number
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Name an important application of SS7 GSM Networks
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THANK YOU
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