Show HN: A USB-C LTE modem that gives my computer a phone line

Posted by abijahkaj 1 day ago

Counter60Comment42OpenOriginal

Comments

Comment by gruez 1 day ago

>Where 2G and 3G are switched off, calls need a module firmware with VoLTE and a carrier that allows it; ask the seller.

This is a huge caveat. OP, does your setup even support VoLTE? Otherwise this might become a brick in a few years as your carrier shuts down 2G/3G, as many carriers around the world have already done

Comment by abijahkaj 1 day ago

Yes, the Quectel EC25-EUX supports VoLTE natively on the modem baseband, and from the board's perspective the audio path is identical: the modem handles the IMS/SIP registration and AMR codec internally and outputs standard 1.8V PCM digital audio to the RP2040 whether the call is over VoLTE or 2G/3G fallback.

I put that caveat in the README because VoLTE support in practice depends on carrier whitelisting and having the right carrier MBN profile loaded on the Quectel module (AT+QMBNCFG), not on the rest of the PCB. Here in Europe, 2G/3G is still up for a few more years, and major carriers have MBN profiles in the standard EC25-EUX firmware, so testing both VoLTE registration and CSFB fallback will be part of bring-up when the board lands.

Comment by Vaushite 1 day ago

Obviously AI vibed, which means the project probably has never been built and wouldn't work if you built it.

Comment by 1 day ago

Comment by abijahkaj 1 day ago

I mentioned in the top comment that Koppar generated the board and that the first batch is about to be ordered, so no detective work needed there.

I'm just as skeptical of blind AI hardware output as you are, which is why I audited every net against the datasheets and put all the flaws in REVIEW.md in the repo before ordering: - The 0.33 mm RF trace came out to ~35 Ω instead of 50 Ω and hopped layers through vias (I included Pi-matching pads to tune it on the bench). - The 3.8 V buck converter was placed too far from its inductor, and the 1.37 mm power trace is narrower than Quectel's 2 mm recommendation (~100 mV drop at 3 A). - The SIM socket (J2) is oriented inward, which is annoying mechanically.

5 bare boards + 2 PCBA units are currently being ordered at JLCPCB, and I'll order the EC25-EUX modules separately to hand-solder onto U8 since JLC was out of stock.

Since the KiCad files are in the repo, if you spot a specific net or pinout error that will brick bring-up, let me know so I can bodge-wire it next week.

Comment by drum55 1 day ago

The PCB looks super weird, what is going on here?

https://files.catbox.moe/nijoin.png

https://files.catbox.moe/suazr4.png

They sort of look sensible until you look closer and then the whole board is fever dream autorouting.

Comment by cowlevel 1 day ago

they answered with AI and got flagged for being AI. You can turn on showdead in settings to see it but there's nothing of value in their comment really.

Comment by 1 day ago

Comment by abijahkaj 1 day ago

yeah it's raw output from the router i'm coding without a trace smoothing pass yet, so it draws ugly jagged steps. looks bad to a human layout engineer, no argument there. either way it won't stop it from working, the electrons don't care how ugly the bend is at 2 MHz. just want to see if it boots when the jlcpcb parcel lands.

edit: idk why my comments are being [flagged], i've written to hn but the issue is apparently still there

Comment by Neywiny 21 hours ago

It's not about your clock speed, it's about your edge rate. Anyone who's had to lower drive strength or slew rate can tell you that even at 0.0000...1 Hz clock speed, if your routing is trash it can ring to the point of detecting multiple edges or interfering with other signals.

Also some of the issues were on USB lines which hopefully aren't running at 2MHz.

Lastly, since you're coding it, maybe make it pull reference layouts+routing for power supplies from the datasheets. That's step 1 for any real eCAD student.

Comment by abijahkaj 20 hours ago

[flagged]

Comment by mh- 1 day ago

Probably because of some of them appear to be LLM-authored.

https://www.pangram.com/history/67234835-58ff-4603-ba17-2234...

Comment by mcculley 1 day ago

This is great. Apple needs to figure out how to fit one into a 16" MacBook Pro.

Comment by DaiPlusPlus 1 day ago

They don’t want to: it helps deuce demand for the iPhone via the macOS/iPhone tethering integration, and for calls taken via FaceTime.

Comment by sellmesoap 1 day ago

If they can hack one into a hardware crypto wallet why not a MacBook https://news.ycombinator.com/item?id=50054092

Comment by wlesieutre 1 day ago

They could if they wanted to, I’m a bit surprised that they didn’t add it with this big new MBP generation.

Especially now that they have their own modem and wouldn’t need to pay whatever royalties Qualcomm would demand, which I’ve read are based on the device price and would get very high on a premium laptop.

Comment by drum55 1 day ago

There was a prototype macbook pro on ebay from 20 years ago with a 3G modem in it, the retail MBP from the time even has the space for a SIM card near the battery door. Presumably they decided against it for a good reason.

https://www.macrumors.com/2011/08/14/photos-of-a-prototype-m...

Comment by kotaKat 1 day ago

They're not fully confident yet on the mmWave to go in the Macbook.

There's a reason they carefully have been bringing their own modem in via "weird" devices. The C1 came in on the 16e, C1X was the Air (phone+iPad), and they're finally confident enough to bring C2 to the mainline iPhones, but it seems like maybe not ready until C3 comes around next fall.

Comment by Vaushite 1 day ago

Just buy a different laptop - not an ultra-thin one. The WiFi card is usually on a separate module that plugs in by a standard connector and can be swapped to a cellular module.

Comment by mycall 1 day ago

Will LTE continue to work when 6G rolls out in 3 years?

Comment by wolrah 1 day ago

> Will LTE continue to work when 6G rolls out in 3 years?

A significant change that came with LTE is the use of OFDM as the base of the air interface, which means that cellular providers no longer need to allocate large segments of spectrum to one protocol or another. Towers that were upgraded from 4G to 5G didn't need to disable 4G or consistently split their spectrum between the two they way they did when upgrading from 2G to 3G or 3G to 4G, they can dynamically allocate subchannels in each mode based on the immediate needs of their users.

It seems unlikely that whatever technologies come in the future will regress in this way, so there shouldn't be a need for a hard cutoff at any point and I'd expect LTE devices to keep working for many generations to come.

Comment by retatop 1 day ago

I don't think your conclusion is correct. 4G -> 5G will result in 5G networks completely consuming the 4G ones. T-Mobile has already been talking about shutting down LTE in a few years. The reason afaik that 4G and 5G are seemingly compatible right now is that most networks are in 5G NSA mode (non standalone) which means that LTE is actually used for negotiation and orchestration stuff, with 5G only used for the actual data pipe. I have been told that most of the benefit of 5G comes with 5G SA (standalone), but that requires not supporting the LTE stuff

Comment by cowlevel 1 day ago

Ironic given that LTE, literally called Long-Term Evolution, was supposed to make future backwards incompatible changes unnecessary.

Comment by DaiPlusPlus 1 day ago

> was supposed to make future backwards incompatible changes unnecessary

I struggle to think of any system/scheme/protocol/spec/etc that promised this to have actually delivered. One can never predict the future.

Comment by mycall 16 hours ago

IPv6?

Comment by DaiPlusPlus 8 hours ago

Thing is, IPv6 has not replaced IPv4 yet ;)

Comment by gruez 1 day ago

It's not just spectrum, though. The core network is totally different than 5G SA or whatever 6G has. Maintaining all of that will eventually be considered not worth the trouble, and it'll be decommissioned. If anything decommissioning will be faster for LTE because unlike 2G/3G (which were needed because VoLTE support was flakely and had device whitelists) there isn't much point in keeping LTE around for legacy reasons.

Comment by cowlevel 1 day ago

What are the differences between 4G and 5G core networks?

Comment by gruez 1 day ago

https://en.wikipedia.org/wiki/5G#Core_network_architecture

Big enough that most carriers can't even launch 5G with its associated core network ("5G standalone"), and had to use the kludge of 5G + 4G core network ("5G non-standalone)

Comment by cowlevel 1 day ago

Doesn't sound incompatible. So far these new architectures have just been different ways of describing mostly the same thing. E.g. the NodeB from 3G becomes the eNodeB in 4G, and it's not actually a different thing, it just describes one that has been upgraded with 4G support. In practice the exact same device is both NodeB and eNodeB in a 3G/4G network.

Comment by OJFord 1 day ago

> they can dynamically allocate subchannels in each mode based on the immediate needs of their users.

Right, so if I'm still rocking 4G only while 'everyone' else in the area is using 6G with some other stragglers on 5G, ...

Comment by cowlevel 1 day ago

They're (mostly) the same channels shared by all of them. It's not like an ethernet address (where everyone on the network speaks the same protocol and just ignores packets not for it), it's a radio frequency timeslice that's allocated to you, and you and your nearby tower can speak any radio modulation they want in that timeslice, even ones that other phones or other towers don't support.

However new technologies sometimes also bring new spectrum blocks like mmwave 5G.

Comment by wolrah 1 day ago

> Right, so if I'm still rocking 4G only while 'everyone' else in the area is using 6G with some other stragglers on 5G, ...

You will keep receiving service until it becomes unprofitable to do so, same as always, but that time will be a lot further down the road than it used to be because there's basically no reason to care outside of congested cells where making the most of every bit of airtime matters.

If you're in a congested area, prepare for newer tech to be prioritized, often legitimately because the newer modes tend to be better at sharing the air and thus more users can be better served by prioritizing them on limited spectrum.

Comment by ChocolateGod 1 day ago

6G will likely replace 5G rather than 4G.

4G has become the new 2G when it comes to IoT etc.

Comment by j45 1 day ago

Probably for a while - lots of IoT uses it, especially with the push away from 3G.

Comment by mycall 16 hours ago

Yeah, LTE is good enough for lower bandwidth applications.

Comment by whalesalad 1 day ago

I’d love one of these as a framework module

Comment by efren6 1 day ago

Er

Comment by abijahkaj 1 day ago

I wanted a real phone line on my computer: calls and texts on a physical SIM, answered through my headset at my desk, or by a script or an AI agent when I'm away. Cloud numbers get treated as VoIP, and old Android phones give you no clean way to get call audio in and out over USB.

So this is a 4-layer USB-C board around a Quectel EC25-EUX. Behind a USB hub, the computer sees an LTE network adapter, the modem's AT serial ports (caller ID, SMS, answer), and an RP2040 acting as a USB sound card. The RP2040 bridges the modem's 1.8 V PCM audio through a level shifter. There's a 3.8 V buck sized for the 2 A GSM bursts, and no battery.

I wrote the spec as one page of text, and Koppar, the tool I'm building, generated the schematic and the board. Then I reviewed it against the datasheets. The review found a 35 Ω antenna feed that hopped layers, a buck 60 mm from its inductor, and a SIM socket facing into the board. The parts matching would have bought an 18 µH inductor for a 4.7 µH slot. REVIEW.md in the repo has all of it with the numbers.

Status: the boards aren't built yet. The repo is the design as I'm ordering it: KiCad 10 project, Gerbers, JLCPCB BOM and placement files. Bring-up and the sound-card firmware come next.

Happy to answer anything about the audio path or the power design, and I'd like to hear what you'd change before the first bring-up.

Comment by j45 1 day ago

I believe there are VOIP phones that are sim card based. Imagine a country that doesn't have land lines but folks want a home phone.

Comment by wolrah 1 day ago

There are SIP to cellular gateways which have historically been very popular in regions where landline to mobile calls cost money but mobile to mobile do not, but they mostly don't exist in the US market other than high-density devices mostly used by SMS spammers.

The only actual VoIP phone with a SIM card I'm aware of is the Yealink T74LTE which for a long time was exclusive to Verizon's business phone service but has recently become available for general purchase, those can support both SIM over LTE data and VoLTE (which is also basically SIP but over a prioritized channel).

Comment by Scoundreller 1 day ago

I'd like this kind of setup because my VoIP provider charges a couple bucks a month for 9-1-1 while sending 9-1-1 over cellular would be free

Comment by j45 1 day ago

I recall seeing some that were offered in Asia but might not be elsewhere.

It looked like an office desk phone but instead of Ethernet, a SIM card goes only. Some came with a cordless handset with it too. seeing

Comment by wolrah 1 day ago

> It looked like an office desk phone but instead of Ethernet, a SIM card goes only.

Eww, I hate that such a thing exists. Everyone involved in its development needs their decision-making privileges revoked for anything more consequential than breakfast.

There is no good reason for an internet connected device to be cellular-only unless it's intended to effectively never be in "home territory". Every other such device should at least have WiFi so it's useful in areas where there is no cell service and doesn't need to use limited cellular spectrum even wheb there is service.

There is also no good reason for an internet connected device that's not expected to move while in operation to not have an ethernet port. It's unfortunately common amongst consumer-targeted devices (e.g. Nintendo Wii and a surprisingly large number of printers) but it's bad nonetheless. If it can reasonably be wired it should be wired, period. If it doesn't move and isn't in a weird spot, you need to have a really good reason why it can't be wired to justify using WiFi and "I don't like wires" has never been and will never be such a reason.

Comment by j45 21 hours ago

Everything doesn’t have to be for everyone.

The reason why this product exists is for homes in places where there is no telephone or cable tv wiring to the house.

This is a massive part of the world and part of why the countries that didn’t have a wired setup in many cars jumped straight to wireless.