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How do text messages travel from one phone carrier to another?
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RJSV:
   Looking at search results, for Digital Multiplex Hierarchy in telephone / texting systems:
  An intro into PCM (pulse code modulation) you can search / read about:
   DS0 represents PCM at 64 kbits/sec. (one signal).
   DS1 can have up to 24 of those DS0,
   (~ 1.5 mbits/second.),
   DS3 has 28 DS1, and so that is 672 individual signal channels.
   I can't supply much info beyond that, but most search engines have lots of description, regarding 'voice data' and regarding 'SMS' and texting in general.
esepecesito:

--- Quote from: nctnico on June 30, 2021, 06:59:58 pm ---
--- Quote from: Beamin on December 01, 2018, 11:47:42 pm ---Say I have Verizon in MD and send it to a att phone in MA. It goes over a data channel in CDMA to my local tower then it ____________________ tower transmits on a voice/data channel to the gsm handset?


--- End quote ---
Nowadays text messages are probably split off at the cell tower and transmitted over an IP network with gateways between operators. Back in the old days the text messages where included in the signalling channel on T1/E1 links (which could be aggregated in links with more bandwidth channels) from the base stations and then get exchanged between telecom operators. The problem with this system is that it was never designed to handle the amount of text messages that users where sending so companies started making products to split the text messages from the signalling channels and aggregate these on seperate links between operators.

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Another change that also lead to change (only partly) to IP was the introduction of number portability: the SMS had to be routed based on the number to a different operator. No Telephony switch was prepared for that massive routing task, so, at least were I worked, it was implemented in a FreeBSD box, that would do the rounting, and because many operators had the same system from the same company, the operator to operator link was also IP. But I guess it is not the norm. The normal thing was and I bet still is SS7. You have to have dedicated SS7 links anyway with other operators, big ones, because each call implies may messages back and forth.
madires:
High level view on how SMS works in LTE: https://blog.3g4g.co.uk/2012/02/high-level-view-on-how-sms-works-in-lte.html
Two Types of SMS in 5G: https://blog.3g4g.co.uk/2020/09/two-types-of-sms-in-5g.html
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