> For the complete documentation index, see [llms.txt](https://prosbcdocs.telcobridges.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://prosbcdocs.telcobridges.com/telecom-references/isdn-and-cas/short-history-of-ss7.md). # Short history of SS7 ### Short history of SS7 Starting in the 1960s, public switched telephone networks (PSTN) were built using analog technologies and multiple in-band signaling methods. The growth of common fixed telephony end-users and the increasing demand for more advance services quickly made old protocols such as [R1 CAS](https://github.com/telcobridges-main/tmedia-wiki/tree/main/reference/protocols/cas/README.md), [R2 CAS](https://github.com/telcobridges-main/tmedia-wiki/tree/main/reference/protocols/cas/README.md) to become less attractive for service providers. The [ISDN](https://github.com/telcobridges-main/tmedia-wiki/tree/main/reference/protocols/isdn/README.md) protocol revolution was a big step in the telecom industry going from an in-band signaling protocol (using tones, stolen bits from voice channel or dedicated channel) to an out-of-band signaling (separated from voice channels) with message based primitives fitted for digital networks. Even if ISDN was fixing many problems of older protocols, its mass deployment in core networks never really occurred thus leaving providers with a complex hierarchy of equipments interworking with each other (or at least trying to). Although ISDN offered an easier way to provide supplementary services (i.e. call hold, call transfer) to the end-users, ISDN progressively migrated toward the edge of the network closer to the end-user. In addition, no equipment provider really implemented the ISDN base specifications thus creating lots of local variants and headaches to system integrators. At that point, international consortia had already started working on a unified way of establishing calls, to provide numerous services (including those already supported by ISDN and older protocols) and to be able to build new applications (services) without having to change the whole network architecture. [SS7](https://telcobridges.com/media-gateways/solutions/signaling/migrating-ss7-sigtran/) (or Signaling System 7) took many years before being deployed all over the world but is now achieving what it was designed to do: offer an overall communication standard providing many services and being able to grow with public’s demands. Even if local variants were still implemented and multiple revisions of the protocol occurred, this signaling protocol currently connects most major telephone networks all over the world. It also added a very critical feature to every network all over the world: robustness and fault tolerance. Being layered in different functional groups, SS7 was built so that different nodes in the network don’t need to implement every portion of the protocol stack if they don’t require to. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://prosbcdocs.telcobridges.com/telecom-references/isdn-and-cas/short-history-of-ss7.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.