3 ms·

> I’ve always found that a weird characterization. It’s sent in the signaling channel, but why is that “leftover”? The channel is still established for the SMS,

> I’ve always found that a weird characterization. It’s sent in the signaling channel, but why is that “leftover”? The channel is still established for the SMS, and it’s not like that channel bandwidth would go unused otherwise. In modern standards, it is indeed not a “leftover” (although RCS exists which is another can of worms), but originally it was transmitted in a best-effort manner using what is essentially a "hack" on SS7 (unlike phone calls which is guaranteed reception - or at least not going through).

lxgr 2y ago
I don't think this was ever true, unless I'm misunderstanding what you mean by "best-effort" or "hack". In GSM, SMS are delivered either over the SDCCH (when no voice call is happening simultaneously) or the SACCH (when a voice call is already in progress). In the latter case, you might argue that they're piggy-backing onto existing resources, but in the former, there are definitely dedicated resources being allocated specifically for SMS delivery. SMS delivery has also always been reliable, both on the lower level (both SDCCH and SACCH are reliable) and the upper one (the phone reports successful delivery back to the sending SMSC), so while there are no timing guarantees (is that what you mean by best-effort?), delivery always eventually succeeds once resources are available. The protocol even goes to significant lengths to ensure timely (re)delivery in case various error scenarios, such as a full inbox on a phone or a phone being out of reception. While 140 bytes aren't much, reliability is actually great, until you add spam filtering, roaming, and inter-network delivery to the mix, when things can quickly go off the rails. (One unexpected consequence of how it's implemented is that for mutual reachability, it doesn't only matter what operators the sender and recipient have a contract with, but also in which network the recipient is currently roaming.)
- TeMPOraL 2y ago
  > SMS delivery has also always been reliable, both on the lower level (both SDCCH and SACCH are reliable) and the upper one (the phone reports successful delivery back to the sending SMSC), so while there are no timing guarantees (is that what you mean by best-effort?), delivery always eventually succeeds once resources are available. That's the opposite of my experience in the early 2000s. I mean, the whole reason you'd always enable delivery reports is because delayed delivery or failed delivery were almost a daily occurrence. Anyone who sent SMS blind would quickly learn to either enable delivery reports, or just start calling people more.

lxgr 2y ago

I don't think this was ever true, unless I'm misunderstanding what you mean by "best-effort" or "hack". In GSM, SMS are delivered either over the SDCCH (when no voice call is happening simultaneously) or the SACCH (when a voice call is already in progress). In the latter case, you might argue that they're piggy-backing onto existing resources, but in the former, there are definitely dedicated resources being allocated specifically for SMS delivery. SMS delivery has also always been reliable, both on the lower level (both SDCCH and SACCH are reliable) and the upper one (the phone reports successful delivery back to the sending SMSC), so while there are no timing guarantees (is that what you mean by best-effort?), delivery always eventually succeeds once resources are available. The protocol even goes to significant lengths to ensure timely (re)delivery in case various error scenarios, such as a full inbox on a phone or a phone being out of reception. While 140 bytes aren't much, reliability is actually great, until you add spam filtering, roaming, and inter-network delivery to the mix, when things can quickly go off the rails. (One unexpected consequence of how it's implemented is that for mutual reachability, it doesn't only matter what operators the sender and recipient have a contract with, but also in which network the recipient is currently roaming.)

TeMPOraL 2y ago
> SMS delivery has also always been reliable, both on the lower level (both SDCCH and SACCH are reliable) and the upper one (the phone reports successful delivery back to the sending SMSC), so while there are no timing guarantees (is that what you mean by best-effort?), delivery always eventually succeeds once resources are available. That's the opposite of my experience in the early 2000s. I mean, the whole reason you'd always enable delivery reports is because delayed delivery or failed delivery were almost a daily occurrence. Anyone who sent SMS blind would quickly learn to either enable delivery reports, or just start calling people more.