Signaling System #7 (SS7) is a telecommunications protocol that defines high-speed circuit switching for telephone calls and uses out-of-band signaling between service switching points, signal transfer points, and service control points. It has advantages like separation of control information onto logically separate paths, message-oriented call information exchange, and ability of a single signaling channel to carry information about multiple trunks. The SS7 architecture includes service switching points, service control points, and signal transfer points that communicate using protocols like ISUP, TCAP, and SCCP.

1. Signaling System #7
DHRUBAJI MANDAL

2. Introduction
Signaling System 7, a telecommunications protocol defined by the International
Telecommunication Union (ITU)
 is characterized by high-speed circuit switching and out-of-band signaling using
Service Switching (SSPs), Signal Transfer Points (STPs), and Service Control Points
(SCPs) (collectively referred to as signaling points, or SS7 nodes).
Signaling refers to the exchange of information between call components
required to provide and maintain service.

3. SS7 Advantages
Out-Band
Separation of control information from the user information into logically
separate paths.
Message-Oriented
Call information/Signaling information are exchanged in the well formatted
message.
Common Channel
Single Signaling channel can carry information about multiple trunks.
Internationally standardized
optimized for digital telecommunication (64 kbps)
can meet present and future requirements of information transfer.
reliable, sequenced, without loss or duplication, Scalable

4. SS7 Architecture

5. SS7 Signaling
 Service Switching Point (SSP)
A Signaling Point which is capable of switching functions and of
handling signaling functions.
 Service Control Point (SCP)
A Signaling Point which serves as an interface to the central database,
used to store routing information.
 Service Transfer Point (STP)
A Signaling Point which is capable of routing messages between SSPs
or between an SSP and an SCP.

6. Links Types in SS7
Primary STPs
Secondary STPs
An "A" (access) link connects a signaling end point (e.g., an SCP or SSP) to an STP.
A "B" (bridge) link connects an STP to another STP
A "C" (cross) link connects STPs performing identical functions into a mated pair
A "D" (diagonal) link connects a secondary STP pair to a primary STP pair
An "E" (extended) link connects an SSP to an alternate STP
An "F" (fully associated) link connects two signaling end points

7. SS7 Protocol Layers
MTP1 Message Transfer Part Level 1
MTP2 Message Transfer Part Level 2
MTP3 Message Transfer Part Level 3
SCCP Signaling Connection Control Part
ISUP ISDN User Part
TUP : Telephone user Part
TCAP Transaction Capabilities App Part
MAP Mobile Application Part

8.  Message Transfer Part 1
Define physical, electrical, & functional characteristics of the signaling
link.
 Message Transfer Part 2
Ensures reliable exchange of information between two signaling points.
Perform error control, flow control, and link control activities.
It makes sure that messages are free of errors and there is no loss of information
during the transmission

9. Three types of signal units
Message Signal Unit (MSU) - User data
Link Status Signal Unit (LSSU) - Link status messages
Fill-in Signal Unit (FISU) - Link health check
If error is found MSU is discarded NACK is sent to the originating signaling point

10. Message Transfer Part 3
Message Transfer Part, Layer 3, (MTP3), is a Network Layer Protocol, responsible
for routing of messages in the Signaling System Number 7 (SS7) network
MTP3 controls the signaling links and does network reconfigurations, when any
network element (link, node etc.) fails
Message Transfer Part, Layer 3, is responsible for Message Handling and
Network Management functions
This includes procedures for message discrimination and routing, network
management, multiplexing several logical links onto a single link, and congestion
control.

11. SCCP : Signaling Connection Control Part
SCCP Is a higher level protocol than MTP3. Although MTP provides routing
capabilities based upon the Point Code, SCCP allows routing using a Point Code
and Subsystem number or a Global Title.
The overall objectives of SCCP are to provide the means for
Logical signaling connections within the SS7 network
A transfer capability for SDUs with or without the use of logical signaling
connections
Segmentation / Re-assembly mechanisms for large data transfers

12. The SCCP runs above the MTP layers and provides a set of facilities similar
to those provided by the UDP and TCP layers of TCP/IP
Specifically, SCCP provides five classes of service such as connectionless
(like UDP) and connection-oriented (like TCP) with options of error
recovery and flow control.
Class 0: Connectionless
Class 1: Sequenced connection
Class 2: Basic connection oriented
Class 3: Flow control connection oriented
Class 4: Error recovery & flow control connection oriented

13. ISDN User Part (ISUP)
ISDN User Part (ISUP), provides the signaling functions required to
support basic bearer services and supplementary services for voice and
non-voice applications in the ISDN network.
This protocol is used to setup and tear down telephone connections
between 2 parties.
It has added advantage of supporting Intelligent Network (IN)
functions and Integrated Services Digital Network (ISDN) services
It defines a set of messages and a protocol using these messages that allows a
telephone call to be set up and torn down.

14. IMPORTANT MESSAGE
 Initial Address Message (IAM)
 Address Complete Message (ACM)
 Answer Message (ANM)
 Release Message (REL)
 Release Complete Message (RLC)
Signaling Link Test Message (SLTM)
When a link is brought in to service at the MTP2 layer, the MTP3 layer starts the
'signalling link test' process: a signaling link test message (SLTM) is sent by MTP3 on
the link to the adjacent node.
Signaling Link Test Acknowledge (SLTA)
If all of these values match those configured at the adjacent node then it responds
with a Signaling Link Test Acknowledgement (SLTA), sending back the test pattern it
received in the original SLTM message.

15. ISUP Message Flow

16. ISUP Message

17. Transaction Capabilities Application Part (TCAP)
Its primary purpose is to facilitate multiple concurrent dialogs between the
same sub-systems on the same machines, using Transaction IDs to
differentiate these , used to access network databases (HLR / VLR)
TCAP supports the functions required to
connect to an external database
perform a query of the database and
It is used for database services such as calling card, switch-to-switch
services including repeat dialing and call return.
Because TCAP messages must be delivered to individual applications
within the nodes they address, they use the SCCP for transport

18. Message In TCAP
Begin
Continue
End
Abort
Cancel
As TCAP provides messages and protocol used to communicate between applications
(deployed as subsystems) in nodes
Some of Important Message in TCAP are

19. TUP ( Telephone User Part)
Used to set up a telephone call between two SS7 nodes.
It defines a set of messages and a protocol using these messages that allows a
telephone call to be set up and torn down.
TUP messages flow only immediately before a call is established and then
immediately before it is terminated. No TUP messages are exchanged when a
call is in progress.
TUP was one of the first SS7 protocols and designed to support simple analog
phones (POTS).
Its successor is ISUP, which supports both POTS and ISDN.