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Comment by monkeyspaw | Hacker News Reader

Thank you for connecting the SMS to the lack-of-dial tone. I thought it was the most interesting part of the article, but wasn't explained very well.

So if I have this correctly: the Push-To-Dial feature necessitated an out of band control mechanism, and this mechanism is what SMS lives on?

Do SMS messages still use the out of band? I'm guessing that iMessage, etc., use the data channel (based on the encryption discussion last week). But when I send a text that's a proper SMS, or an SMS from Twilio/etc., does that use this out of band mechanism?

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SMS does use the out of band but it was more of an accident of the SS7 protocol. you should read up on SS7 - http://en.wikipedia.org/wiki/Signalling_System_No._7

SMS is more of vestigial debug feature of GSM that was brilliantly commercialized than anything that has to do with SS7. And by the way SMS is out of band on the backhaul network, not on the air interface (and the whole SMS protocol on air interface is horribly complex and inefficient, it essentially involves building data call that passes SS7-like messages, typically one per such "call" in each direction, another funny fact of this technology is that address fields in these messages have larger maximum size (256B) than than actual SMS contents (140B)).

But when one wants to take SS7 into this dial tone discussion it is important to note that traditional SS7 call flow (INITIAL ADDRESS/SUBSEQUENT ADDRESS/ADDRESS COMPLETE) is designed for normal incremental dialing and not cell phone style number at once then send style. I'm too lazy to look it up but I believe that GSM's ISUP/DSS1 derived on air signaling protocol is capable of that too (although I've never seen device that allows user to do that). Relatively common user experience problem with VoIP deployments I have seen is that this is effectively impossible with SIP and user has either to explicitly end dialed number with something or wait for some timeout.

Thanks for clarifying! It's like i had all the concepts there - out of band and SS7 - but didn't really get the details. This is why I love HN.

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