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It was a hack to use "unused" control/signalling paths - see https://en.wikipedia.org/wiki/SMS https://en.wikipedia.org/wiki/SMS Effectively came "for free" fo

• lxgr 2y ago
  That’s a common misconception. The signaling channels used aren’t “unused” – they’re literally set up and torn down to deliver SMS!
  • aidenn0 2y ago
    I think you meant to end that with "phone calls" rather than "to deliver SMS?" SS7 was used for that, but had a significant amount of idle time since most phone calls are much longer than the time needed to setup the connection.
    • lxgr 2y ago
      I do indeed mean SMS, but I was focusing more on the air interface. There, SMS definitely consume resources in the same way that calls do (although of course at a very different rate: SDCCH uses 0.8 kbit/s, as opposed to 13 kbps for full rate voice/CSD traffic channels).
      • aidenn0 2y ago
        But the signaling channels weren't used to send SMS before SMS existed (which the original statement of "unused" implied).