June 8, 2022
Signaling System 7
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10mincategories
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Basic Overview
A Signaling System (SS7) is basically a protocol for international telecommunication which involves networks - such as how the Public Switched Telephone Networks (PSTNs) exchange information and the signals will be controlled. AT&T developed these protocols in 1975 and they were accepted as the worldwide standard in 1980 by the International Telegraph and Telephone Consultative Committee.
Protocols in SS7 Architecture
Its architecture is designed on an OSI Model; like the Message Transfer Protocol (MTP) level 1 physical layer and MTP level 2 data link, MTP level 3 network layer and others can be considered under its application, presentation, and session layers.
Layers present in SS7 architecture:
ISUP: Integrated Services Digital Network User Part
Provides stream multiplexing using ISUP protocols modules. It will include all the circuit management functions and will be used to manage and release circuits.
TCAP: Transaction Capabilities Application Part
It provides non-circuit related information for transfer capabilities and some standard services to applications which will make it independent of the application. It allows other applications to use SS7.Its services can be used for signaling points or network service centers.
SCCP: Signaling Connection Control Protocol
Under MTP level 3, when two SS7 networks exchange their messages, point codes overlap. SCCP resolves this problem using a higher level of addressing with unique global titles. It will also improve the message holding facilities of MTP. Both connection and connectionless protocol are included under SCCP. Functions performed by SCCP are asynchronous.
The remote signaling points will be shared to the application as receiving indications. The application receiving the signal will reply back using the SCCP service response function. This response application is translated by the SCCP layer into an encoded message and then sent back as a decoded version. The image below shows the communication and operations such as for sending data.
MTP Level 3 (M3UA)
It is a user adaptive layer protocol. It is a part of the SS7 stack, not the SINGTRAN family. It transports the user’s signaling messages over the IP using SCTP. Its connected endpoints don’t have to conform to SS7 topology because each association won’t require a link. Under M3UA there are no 16link/linkset restrictions. All the routing keys will be supported by M3UA protocols for partitioning SS7 traffic using combinations of the originating point code, destination point code, network indication and subsystem network. It provides network management and routing.
Its main services will be the transfer request indication, MTP status indication, MTP resume indication, and MTP pause indication.
MTP Level 2
It will ensure that the signaling transfer is reliable and is responsible for error-free transmission of message packets among two adjacent signaling nodes.
MTP Level 1
It will respond to the physical layer of the OSI model. It defines the electrical and physical characteristics of the signaling link nodes.
Types
FISU: Fill in Signal Units
FISU monitors the link status and error rates constantly over the network. This will make SS7 highly reliable because it can detect the link quality. When no information is getting passed it will be idle.
LSSU: Link Status Signal unit
No information traffic will be carried on the link when LSSU are sent. It uses only single point to point links. It is getting used by level 3 at one node to transmit link status information to adjacent nodes. It will carry the link status information.
MSU: Message Signal Units
It is a carrier of SS7 information. It consists of MTP protocol fields.
Other protocols include:
Service indicator octet: ISUP, TUP and other standards.
Service information field: Carries control information and routing label.
Applications
Prepaid calls, conference calls, call hold, automatic caller ID, and blocking subscriber numbers are the key applications which a normal customer uses.
It can also be used for collecting and filtering unique subscriber information and sending it to the back end processor.
It’s used for collecting call detail records information for billing.
It allows the user to search and create criteria automatically through current session selection.