Author Topic: How do text messages travel from one phone carrier to another?  (Read 7586 times)

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Offline tooki

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Re: How do text messages travel from one phone carrier to another?
« Reply #25 on: January 07, 2019, 04:01:02 pm »
I did. The quality was highly variable. Sometimes the best course was to "hang up and try again".
Well, that happens even today! :P

Quote
And then to add insult to injury, phones also got worse. The old handsets of the Bell era had nice, big, well-made speakers and microphones in them. The quality of phones post-deregulation plummeted quickly.

Precisely, although that isn't limited to one carrier and one manufacturer.
Who said it was?!? I said “Bell era” to pin down the time, I didn’t mean exclusively “under the Bell system”. ;)

The GSM codecs greatly reduce the data rate by encoding the signal, just as mp3 etc does. The difference is the GSM codecs incorporate a model of the human voice tract, so it would be surprising if they transmitted arbitrary sounds well.
We know they can’t. They’re why hold music sounds so terrible, those codecs are hardcore optimized for voice, and on non-voice they absolutely fall flat and degrade into pure compression artifacts.
 

Offline madires

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Re: How do text messages travel from one phone carrier to another?
« Reply #26 on: January 07, 2019, 04:04:26 pm »
My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

There's G.722 with 7kHz for a higher audio quality. Most marketing departments call it "HD voice".
 

Offline coppice

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Re: How do text messages travel from one phone carrier to another?
« Reply #27 on: January 07, 2019, 04:04:48 pm »
My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).
300-3400Hz is NOT enough. Clear speech requires about 7kHz bandwidth. Without the energy between about 5kHz and 7kHz you cannot distinguish between unvoiced sounds, so something like  F S F S F S sounds like the same character repeating. Its the 3kHz bandwidth limit that has people spelling out things as "foxtrot uniform charlie kilo, this call is bloody awful". The old analogue local lines were not filtered down to 3kHz, but seldom achieved much more. Long distance analogue calls were always limited to about 3kHz. The PSTN has never compressed the hell out of anything. The A-law and u-law codecs used for the digital telephone network apply just about the lightest possible compression there is - they just reduce 13 bit samples to 8 bits in a pseudo-logarithmic manner. Digital cellular started with heavy compression, to keep the bit rate viable, but sounded a lot better than most analogue cellular calls, which were mostly pretty horrible.

I doubt if I have actually ever heard a call made on a true analog phone line from end-to-end. Digital trunk lines were the standard by the time I was born
Most countries only achieved a fully digital telephone network in the late 90s, so unless you are quite young you've probably heard some local calls which were analogue from end to end. They were not too bad, but in the 60s and 70s when most long distance calls were analogue end to end the quality was highly variable, and could be bloody awful. The shift to a digital backbone was the best thing that ever happened to the PSTN.
 

Offline tooki

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Re: How do text messages travel from one phone carrier to another?
« Reply #28 on: January 07, 2019, 04:14:27 pm »
My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

There's G.722 with 7kHz for a higher audio quality. Most marketing departments call it "HD voice".
I know — I already linked to info about that in the post you’re quoting.


My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).
300-3400Hz is NOT enough. Clear speech requires about 7kHz bandwidth. Without the energy between about 5kHz and 7kHz you cannot distinguish between unvoiced sounds, so something like  F S F S F S sounds like the same character repeating. Its the 3kHz bandwidth limit that has people spelling out things as "foxtrot uniform charlie kilo, this call is bloody awful". The old analogue local lines were not filtered down to 3kHz, but seldom achieved much more. Long distance analogue calls were always limited to about 3kHz. The PSTN has never compressed the hell out of anything. The A-law and u-law codecs used for the digital telephone network apply just about the lightest possible compression there is - they just reduce 13 bit samples to 8 bits in a pseudo-logarithmic manner. Digital cellular started with heavy compression, to keep the bit rate viable, but sounded a lot better than most analogue cellular calls, which were mostly pretty horrible.
I don’t think it was quite that bad. (I mean, a bad circuit could be, but a good one was good enough.)


My love for FaceTime aside, one thing telecom veterans have been saying for a long time is that modern phone lines suck. The 300-3400Hz is actually enough — if it’s done well. And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).
300-3400Hz is NOT enough. Clear speech requires about 7kHz bandwidth. Without the energy between about 5kHz and 7kHz you cannot distinguish between unvoiced sounds, so something like  F S F S F S sounds like the same character repeating. Its the 3kHz bandwidth limit that has people spelling out things as "foxtrot uniform charlie kilo, this call is bloody awful". The old analogue local lines were not filtered down to 3kHz, but seldom achieved much more. Long distance analogue calls were always limited to about 3kHz. The PSTN has never compressed the hell out of anything. The A-law and u-law codecs used for the digital telephone network apply just about the lightest possible compression there is - they just reduce 13 bit samples to 8 bits in a pseudo-logarithmic manner. Digital cellular started with heavy compression, to keep the bit rate viable, but sounded a lot better than most analogue cellular calls, which were mostly pretty horrible.

I doubt if I have actually ever heard a call made on a true analog phone line from end-to-end. Digital trunk lines were the standard by the time I was born
Most countries only achieved a fully digital telephone network in the late 90s, so unless you are quite young you've probably heard some local calls which were analogue from end to end. They were not too bad, but in the 60s and 70s when most long distance calls were analogue end to end the quality was highly variable, and could be bloody awful. The shift to a digital backbone was the best thing that ever happened to the PSTN.
Yeah, that’s true, local calls (or at least within the same local exchange) probably were analog.

And yes, no doubt analog long distance was terrible!
« Last Edit: January 07, 2019, 04:37:08 pm by tooki »
 

Online newbrain

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Re: How do text messages travel from one phone carrier to another?
« Reply #29 on: January 07, 2019, 04:27:55 pm »
And in the olden days of genuine analog lines, it was done well. But we switched to digital trunk lines long ago, and those compress the hell out of the audio. And then with cellphones, we moved to even more severe digital compression (not to mention having to handle lost data).

I doubt if I have actually ever heard a call made on a true analog phone line from end-to-end. Digital trunk lines were the standard by the time I was born
[...]
Anyway, the real issue is standards: two cellphones making a call do not create a high-speed point-to-point data link, nor are they just shooting packets at each other (like FaceTime). They’re running a call via the voice network, which has strict standards that are essentially inviolable. So without upgrading the entire voice network infrastructure, you can’t just easily upgrade the voice quality.

Nonetheless, a few carriers have been doing this, allowing calls made within their networks, when using a supported handset on both ends, to have better sound quality. But support between carriers has been slow to roll out. See https://en.wikipedia.org/wiki/Wideband_audio
Old analog lines were, in my personal experience, ranging from horrible to bad.
When the land line was finally switched to a digital exchange (the last mile was still analog, and still is) quality went up quite a bit.

The only compression done on POTS on digital trunks was A-law or u-law (depending on your country), which causes very little loss in quality.

One of the initial phases in a mobile phone call is codec negotiation, where the phones and the network agree on which transcoder to use for compressing voice.
In some lucky case AMR-WB can be selected, but there also cases where the selected codec is worse than POTS quality.
For GSM and 3G, both circuit calls, it is possible that compressed voice is directly exchanged between the user terminals, with obvious advantages, instead of being:
1. compressed in one mobile
2. expanded in the network - where depends on 2G/3G and network architecture
3. transported as A/u-law samples
4. re-compressed before being sent to the other mobile
5. re-expanded in the mobile
Search for "transcoder free operation", but all the planets must be correctly aligned for that to work.

The GSM codecs greatly reduce the data rate by encoding the signal, just as mp3 etc does. The difference is the GSM codecs incorporate a model of the human voice tract, so it would be surprising if they transmitted arbitrary sounds well.
Exactly, so much that DTMF tones needs to be transported out of band. As above, they might or might not be transformed back to samples in the network.

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Offline Lord of nothing

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Re: How do text messages travel from one phone carrier to another?
« Reply #30 on: January 07, 2019, 05:07:18 pm »
Quote
FaceTime
what ever that is.  :scared:
Normally you have "HD Voice" who is mention before. The work perfect.  :-+
There are some good Explanation Video from CCC and Defcon who explain how a 2G,... Network work include how the Voice Codeq work.
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Offline tooki

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Re: How do text messages travel from one phone carrier to another?
« Reply #31 on: January 08, 2019, 12:20:18 am »
FaceTime is like Skype, except with better audio/video quality!

It’s too bad there’s no indication of whether a given phone call is using HD voice or not — I have a hunch we don’t notice when it’s working, but of course we do notice when we have a bad connection.
 

Offline Lord of nothing

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Offline johnh

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Re: How do text messages travel from one phone carrier to another?
« Reply #33 on: January 09, 2019, 06:47:09 am »
With VoLTE over 4G Codec choice is negotiated between devices.   Either a handset or Media Gateway

The network might strip/remove some codec choices by manipulating the SDP, in the INVITE and response in 200OK

The media gateway can used to perform trans-coding, this a last choice because of the performance penalty of trans-coding.
 

Offline Zbig

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Re: How do text messages travel from one phone carrier to another?
« Reply #34 on: January 09, 2019, 12:01:28 pm »
As this thread has already diverted to codec quality, bandwidth and compression, I remember how surprised I was back in the day upon learning at school about the concept of "comfort noise". Apparently, in digital communications devices, there was an audible background noise artificially injected locally by the user's terminal equipment (telephone). Turns out people would get confused when, in the periods of both talking parties going silent, the line would go totally silent as well. They were so used to the poor quality of analogue telephony and its inherent background noise that they would associate the noise with the connection ongoing. In order to save time and/or bandwidth, the equipment was gating the audio signal (passing it through only once its level has exceeded some arbitrary threshold) and inserting the locally-generated artificial static during the pauses. I remember how outraged my teenage geek self was at the idea or "ruining" the clear, digital signal path deliberately so it could sound as bad as POTS. If memory serves, 2G cellphones, DECT phones were all doing that, among others. I'm not sure I'm not imagining this now but I think I even saw an option to disable or enable the comfort noise in a VOIP gateway device.

Fast forward some twenty years, I have both VoLTE and VoWiFi available with my telco and on my phone. While the improvement in quality is apparent and very welcome, I have more than once found myself going like "Hello? You still there?" when the person I was talking to went silent for a while. It was then when it dawned on me that I can no longer hear any "comfort noise" during the pauses in our conversation.

That's why I now wonder: is the comfort noise feature now being omitted on purpose? Have they arrived at the conclusion that the new generations don't need it anymore as they don't remember/weren't born in the era of analogue telephony and it's time to abandon the idea? Or is it the negligence/incompetence on the device manufacturer's part? Are the codecs/standards specific in the topic of whether to use comfort noise or not?
 
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Offline madires

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Re: How do text messages travel from one phone carrier to another?
« Reply #35 on: January 09, 2019, 03:12:49 pm »
Usually VoIP phones have some configuration switches for comfort noise. And there's RFC 3389 (https://tools.ietf.org/html/rfc3389) to help managing the comfort noise feature.
 
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Offline coppice

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Re: How do text messages travel from one phone carrier to another?
« Reply #36 on: January 09, 2019, 04:12:58 pm »
As this thread has already diverted to codec quality, bandwidth and compression, I remember how surprised I was back in the day upon learning at school about the concept of "comfort noise". Apparently, in digital communications devices, there was an audible background noise artificially injected locally by the user's terminal equipment (telephone).
Rubbish. Comfort noise has nothing to do with poor quality lines. Its about silence suppression, and the effect it has on the background mush you normally hear from the far end room in a high quality call. If you are only trying to carry speech, you can stop transmission and save bandwidth, when there is nobody talking. However, the background noise from the room stops along with the voice, and that's confusing. It seems like the call has dropped. A proper comfort noise system estimates the volume and spectral character of the background noise in the room, and sends those parameter to the far end at infrequent intervals. The receiving end then synthesises background noise of a similar character to play to the recipient. Done well, you barely notice the silence suppression cutting in and out, and you can save maybe 30% of the total bits transmitted.
 

Offline Zbig

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Re: How do text messages travel from one phone carrier to another?
« Reply #37 on: January 09, 2019, 07:35:48 pm »
Rubbish.

Sorry, you lost me there. You were saying...?
 

Offline MadScientist

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Re: How do text messages travel from one phone carrier to another?
« Reply #38 on: June 30, 2021, 03:27:18 pm »
Having worked briefly in this area. The whole telco billing system is more complex then the switching system.
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Offline duckduck

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Re: How do text messages travel from one phone carrier to another?
« Reply #39 on: June 30, 2021, 03:52:24 pm »
How do carriers exchange SMS messages? The same way that carriers exchange any other data: at an internet exchange. Since rack space / servers / bandwidth are not free, there are peering agreements between carriers.

https://en.wikipedia.org/wiki/Internet_exchange_point

https://en.wikipedia.org/wiki/Peering (much more info here)


EDIT:

OK, maybe not so much. I'm not sure where SS7 data is exchanged between carriers. Apparently they sell access:

https://www22.verizon.com/wholesale/solutions/solution/SS7.html
« Last Edit: June 30, 2021, 05:08:03 pm by duckduck »
 

Offline MadScientist

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Re: How do text messages travel from one phone carrier to another?
« Reply #40 on: June 30, 2021, 03:53:49 pm »
What the OP is referring to is mobile backhaul , E1/T1 bundles were the mainstay , now largely superseded by fibre and to some extent microwave

In Africa sat based backhaul is used in places.
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Offline MadScientist

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Re: How do text messages travel from one phone carrier to another?
« Reply #41 on: June 30, 2021, 04:02:41 pm »
SMS messaging did not the internet abs still doesn’t  imho.

SMS makes use of resources in the gsm signaling protocol
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Offline esepecesito

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Re: How do text messages travel from one phone carrier to another?
« Reply #42 on: June 30, 2021, 04:46:36 pm »
Wow! So much answers and only a couple right :)
The correct one is "SS7" it is a dinosaur, but still newer as R2, which was replaced by SS7.
 

Online nctnico

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Re: How do text messages travel from one phone carrier to another?
« Reply #43 on: June 30, 2021, 06:59:58 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?

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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Offline RJSV

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Re: How do text messages travel from one phone carrier to another?
« Reply #44 on: July 02, 2021, 03:21:58 am »
   Yes, as mentioned, using digital trunk lines, a 'Teleco'
organization like DSC, in my limited expertise, I think they sell 'giant switches', that essentially use a decimate and re-bundle strategy for handing mass numbers of signal (DS1 type and DS3 type).
  Say you have the DSC 'switch'; a cross-point having 8192 (that's 8K) by 8192. Each of those DS3 streams is containing 28 DS1 streams, ... something approx like that. Each DS1 being one single channel. (I don't have enough TELECOM expertise to discuss full or half duplex, but at any rate the job of this mass switch is to
for example pull out any DS1 signals that need to go to Rome, then assemble a new DS3 stream, containing all those DS1 streams. Then, that gets sent on down to Rome, in this case.
  It's probably very complicated, as you could imagine in a world wide network, with demands for speed, forany many clients, each themselves operating on a mass basis (and with 8K by 8K switch stations peppered throughout.
  So that's my barely-informed guess, as I had some experience at DSC in Texas
(Digital Switch Corporation).
   After re-building one of those larger mass streams, my guess is that 'Trunk Line' often means a high capacity microwave or even optical transmission.
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Offline RJSV

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Re: How do text messages travel from one phone carrier to another?
« Reply #45 on: July 02, 2021, 10:51:56 pm »
   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.
 

Offline esepecesito

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Re: How do text messages travel from one phone carrier to another?
« Reply #46 on: July 06, 2021, 02:12:43 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?

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.

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.
 

Offline madires

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