Netheads vs. bellheads redux: the strange victory of SIP over the telephone network

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2023-01-27 07:00:06

How much do you know about how the PSTN (that is, the public telephone network) works? At some point long long ago, it was signalled using in-band frequencies, which facilitated the infamous phreaking using so-called “blue boxes”. This led to a switch to out of band signalling, ultimately culminating in something known as Signalling System 7 (SS7). SS7 traditionally was carried over things like a T1/E1 or similar. The standards for SS7 are defined by the ITU-T and have basically no “cultural” relation to the Internet or IP. Ultimately, it defines a circuit switched communications network based around separated signalling and time-division-multiplexed (TDM) voice channels.

And then, of course, the internet happened. This led to an interesting conflict between two fundamentally different cultures of networking and associated visions. This has been popularly documented as the “netheads vs. bellheads” phenomenon. The difference between these worldviews is fundamental. The bellhead view saw communications in terms of circuit switching, and viewed applications as being integrated into a smart network and provided by the telco.

It's actually interesting to note just how oblivious the bellhead world was to its ultimate fate. Once upon a time, I flipped through some old books about ISDN out of idle curiosity. One of the striking things was the repeated use of the term “datacom”, a term which now seems basically extinct. The reason I found this striking is because it implies that, to those working in the telco field at the time, “datacom” was not the primary purpose of a network but a kind of strange aside, a peculiar niche addition. From the modern perspective this worldview seems almost comical; ironically, the term datacom is no longer relevant because everything is “datacom”.

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