Signal Transfer Point
Encyclopedia
A Signal Transfer Point is a router that relays SS7 messages between signaling end-points (SEPs)
and other signaling transfer points (STPs). Typical SEPs include service switching points (SSPs)
and service control points (SCPs)
. The STP is connected to adjacent SEPs and STPs via signaling links. Based on the address fields of the SS7 messages, the STP routes the messages to the appropriate outgoing signaling link. Edge STPs
can also route based upon message body content using deep packet inspection
techniques, and can provide address translations and screen content to limit the transfer of messages with dubious content or sent from unreliable sources. To meet stringent reliability requirements, STPs are typically provisioned in mated pairs.
These 'routers' are connected just by signaling links; they don't have users attached (where a user could be a mobile station
(MS), a PSTN user in case of a public terrestrial network, or a piece of terminal equipment
at the end of an ISDN B channel). SEPs send signaling messages to other SEPs, but the messages are normally routed via the SEP's adjacent STPs. An STP's main function is to identify the best path for two SEPs to communicate. A typical application would be for two SEPs to agree on the use of a shared data path (e.g., using ISUP to initiate a voice call between a user on one SEP and a user on the second SEP). In this way, STPs route signaling messages (for starting, maintaining or finishing any kind of calls originated by the SEPs' attached users) while avoiding disabled intermediary STPs.
A signaling message typically never goes directly from a given SEP to the destination SEP: the message would normally have to pass through the initiating SEP's adjacent STP so that it can be routed to the destination SEP. In some applications, however, SEPs might be directly connected with signaling links; this would typically be done to enhance robustness or performance between two critical SEPs. Such mesh network configurations are also common in Europe, where STPs have not found widespread deployment.
In some cases, signaling messages can be originated by the STP to learn about the state of the signaling network Some examples include:
A given piece of equipment can implement both SEP and STP functionality. This is commonly done in some SSPs
. This is also seen in Signaling Gateways
that also have Application Server (AS)
functionality as defined by the IETF
.
Some UMTS number portability solutions are implemented in STPs. In UMTS, the STP provides Global Title Translation
(GTT), which may be used to route queries from a gateway MSC (GMSC) to the HLR. Note that for every call to an MS, the call is first routed to the MS's Gateway MSC.
Signaling End Point
In telecommunications, a Signaling End Point is an SS7 endpoint. This is to be contrasted with a Signal Transfer Point .Examples include:*Intelligent Network components such as Service Control Points and Service Switching Points...
and other signaling transfer points (STPs). Typical SEPs include service switching points (SSPs)
Service switching point
In telephony, a service switching point is the telephone exchange that initially responds, when a telephone caller dials a number, by sending a query to a central database called a service control point so that the call can be handled...
and service control points (SCPs)
Service Control Point
A service control point is a standard component of the Intelligent Network telephone system which is used to control the service. Standard SCPs in the telecom industry today are deployed using SS7, Sigtran or SIP technologies. The SCP queries the service data point which holds the actual...
. The STP is connected to adjacent SEPs and STPs via signaling links. Based on the address fields of the SS7 messages, the STP routes the messages to the appropriate outgoing signaling link. Edge STPs
Edge STP
Edge STPs are networking hardware devices embedded with software that performs routing, signaling, firewall, and packet conversion functions. Their primary purpose is to unify networks that use various transports and signaling protocols – such as SS7, SIP, SIGTRAN, TDM, IP, etc. – into cohesive...
can also route based upon message body content using deep packet inspection
Deep packet inspection
Deep Packet Inspection is a form of computer network packet filtering that examines the data part of a packet as it passes an inspection point, searching for protocol non-compliance, viruses, spam, intrusions or predefined criteria to decide if the packet can...
techniques, and can provide address translations and screen content to limit the transfer of messages with dubious content or sent from unreliable sources. To meet stringent reliability requirements, STPs are typically provisioned in mated pairs.
These 'routers' are connected just by signaling links; they don't have users attached (where a user could be a mobile station
Mobile Station
The mobile station comprises all user equipment and software needed for communication with a mobile network.The mobile station refers to global system connected to the mobile network, i.e. mobile phone or mobile computer connected using a mobile broadband adapter. This is the terminology of 2G...
(MS), a PSTN user in case of a public terrestrial network, or a piece of terminal equipment
Terminal equipment
In telecommunication, the term terminal equipment has the following meanings:* Communications equipment at either end of a communications link, used to permit the stations involved to accomplish the mission for which the link was established....
at the end of an ISDN B channel). SEPs send signaling messages to other SEPs, but the messages are normally routed via the SEP's adjacent STPs. An STP's main function is to identify the best path for two SEPs to communicate. A typical application would be for two SEPs to agree on the use of a shared data path (e.g., using ISUP to initiate a voice call between a user on one SEP and a user on the second SEP). In this way, STPs route signaling messages (for starting, maintaining or finishing any kind of calls originated by the SEPs' attached users) while avoiding disabled intermediary STPs.
A signaling message typically never goes directly from a given SEP to the destination SEP: the message would normally have to pass through the initiating SEP's adjacent STP so that it can be routed to the destination SEP. In some applications, however, SEPs might be directly connected with signaling links; this would typically be done to enhance robustness or performance between two critical SEPs. Such mesh network configurations are also common in Europe, where STPs have not found widespread deployment.
In some cases, signaling messages can be originated by the STP to learn about the state of the signaling network Some examples include:
- an STP may send route set test messages to probe the availability of a particular SEP;
- it may send low-level MTPMessage Transfer PartThe Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
messages to an adjacent signaling point to check the Bit Error Rate (BER) on a particular signaling link; or - it may let other adjacent signaling points know that it is going to out of service; in this way, the adjacent signaling points will try to avoid this OOSOOSOOS may refer to:* Object-oriented scanning, a measurement method* Occupational overuse syndrome, a human disorder* Oos River, running through Baden-Baden, Germany* Out of specification* Online Operating System...
STP.
A given piece of equipment can implement both SEP and STP functionality. This is commonly done in some SSPs
Service switching point
In telephony, a service switching point is the telephone exchange that initially responds, when a telephone caller dials a number, by sending a query to a central database called a service control point so that the call can be handled...
. This is also seen in Signaling Gateways
Signaling gateway
A Signaling Gateway is a network component responsible for transferring signaling messages between Common Channel Signaling nodes that communicate using different protocols and transports...
that also have Application Server (AS)
Application server
An application server is a software framework that provides an environment in which applications can run, no matter what the applications are or what they do...
functionality as defined by the IETF
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
.
Some UMTS number portability solutions are implemented in STPs. In UMTS, the STP provides Global Title Translation
Global Title
A Global Title is an address used in the SCCP protocol for routing signaling messages on telecommunications networks. In theory, a global title is a unique address which refers to only one destination, though in practice destinations can change over time....
(GTT), which may be used to route queries from a gateway MSC (GMSC) to the HLR. Note that for every call to an MS, the call is first routed to the MS's Gateway MSC.