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