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Signaling System 7 (SS7)
- standard developed in the late 1970s by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T)
- The SS7 network provides the intelligence to the existing PSTN
•SS7 is used to perform out-of-band signaling in the Public Switched Telephone Network (PSTN).
•SS7 signaling enhances the PSTN by handling call establishment, exchange of information, routing, operations, billing, and support for Intelligent Network (IN) services.
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The SS7 network is used
to switch information messages to set up, manage, and release telephone calls as well as to maintain the signaling network.
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Signaling Elements
separate the voice network from the signaling network.
contains the source and destination point code address.
route signaling messages and provide access to the SS7 network and to databases.
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3 Types of Signaling Elements
- •SSPs are end office or tandem switches that connect voice circuits and perform the necessary signaling functions to originate and terminate calls.
- Statically or manually configured
- •The STP routes all the signaling messages in the SS7 network.
- •The SCP provides access to databases for additional routing information used in call processing.
- –The key element for delivering IN applications on the telephony network.
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STP
•It is the STP network's job to properly route circuit-based SSP packets to the destination.
•It is the STP network's job to properly route non-circuit based message packets between the SSP and the database interface known as the SCP.
•STPs also measure traffic and usage.
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STP Hierarchy
defines network interconnection and separates capabilities into specific areas of functionality.
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SCP
SCP provides the interface to the database where additional routing information is stored for non-circuit based messages
–The SCP provides the conversion between the SS7 and the X.25 protocol.
x.25 has a lot of overhead
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Signaling Links
•All signaling points in the SS7 network are connected by signaling links.
there backups
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Associated signaling
in that the signaling and voice paths are directly connected between the two signaling endpoints.
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Nonassociated signaling
- uses a separate logical path for signaling and voice.
- is the most common form of signaling in the SS7 network
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Quasi-associated signaling
signaling uses a separate logical path for signaling through the minimal number of transfer points to reach the final destination.
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SS7 Link Types
•A-links are interconnects between signaling endpoints and STPs.
•Bridge Links (B-links) are interconnects between two mated pairs of STPs.
Cross Links (C-links) interconnect an STP with its mate..
C-links are used only when failure or congestion occurs,
•Diagonal Links (D-links) are used to interconnect mated STP pairs of one hierarchical level to mated STP pairs of another hierarchical level.
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E-Links and F-Links
Extended Links (E-links) are used to interconnect an SSP to an alternate STP
F-links are used to directly interconnect two signaling endpoints..
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•Signaling links are grouped together into linksets when the links are connected to the same endpoint.
–Signaling endpoints provide load sharing across all the links in a linkset.
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•SS7 protocol stack has only 4 layers.
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Message Transfer Part (MTP)
L1, L2, and L3 provide the transport protocols for all other SS7 protocols.
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•The MTP2 protocol is used to create reliable point-to-point links between endpoints in a network.
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•The MTP3 protocol is divided into two main functions:
- –Signaling Message Handling (SMH)—Routes SS7 messages during normal conditions.
- –SNM—Reroutes link traffic during network failure conditions.
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•The SMH handles the following during network link failures:
- –Linkset rerouting
- –Alternate link rerouting
- –Traffic flow rerouting to specific endpoints
- •The SNM process handles the rerouting of traffic through alternate links or linksets during network link failures.
- •The SNM process also controls the flow of traffic to specific endpoints during network link failures.
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•The link management function is divided into three parts—Link Activation, Link Restoration, and Link Deactivation
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•The SCCP provides network services on top of MTP3:
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•Connection-Oriented Services
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–SCCP supports connection-oriented services for TCAP and ISUP, however none of these services is used today.
–As such, this section does not cover SCCP connection-oriented capabilities, messages, or services.
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Connectionless Services and Messages
SCCP provides the transport layer for the connectionless services of TCAP
TCAP-based services include 800, 888, 900, calling card, and mobile applications.
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SCCP Management Functions
SCCP management functions maintain the transfer of SCCP messages during failure conditions, including network and subsystem failures. SCCP management processes alert SCCP users, such as TCAP or ISUP, during these failure conditions
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TUP
•North America was first to implement ISUP as opposed to TUP.
•TUP was the first SS7 user part defined when all calls were considered voice calls.
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•ISUP connects, manages, and disconnects all voice and data calls in the PSTN.
•ISUP sets up and tears down the circuits used to connect PSTN voice and data subscribers.
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•TCAP provides the transaction capabilities carried out by non-circuit based messages used to access remote databases and invoke remote feature capabilities in network elements.
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