The Route of a Text Message

ericlott | 627 points

A really good article! The telecoms world is fascinating, it's a whole separate parallel stack that doesn't get talked about anything like as much as the web world.

One section caught my eye, and I wanted to add some detail:

>In order to efficiently send and receive signals, antennas should be no smaller than half the size of the radio waves they’re dealing with. If cell waves are 6 to 14 inches, their antennas need to be 3-7 inches. Now stop and think about the average height of a mobile phone, and why they never seem to get much smaller.

A common neat trick is to take advantage of the fact that this is only true in air, and the wavelength depends on the material around the antenna. Some modern antennas (especially common in GPS and Bluetooth) are built as a metal foil around a ceramic element, which has a much shorter wavelength and allows them to be shrunk[1].

You can also "fold" the antenna a bit, and hence get away with a quarter wavelength in return for reduced performance in other areas. A common example in phones is the Inverted F antenna [2].

Finally, a large "invisible" component of many antennas is the size of the ground plane attached. Shrinking this can affect antenna performance a lot. Generally, this is an internal copper layer of the PCB that is used by other components too, but it's important to realise its an important part of the RF performance. Therefore, you can't judge how good an antenna is from the visible size of it alone, the form of the whole device matters.

[1] https://electronics.stackexchange.com/a/245245

[2] http://www.antenna-theory.com/antennas/aperture/ifa.php

snops | 5 years ago

Interesting, but so much left out. This quote:-

>Through some digital gymnastics that would take entirely too long to explain, suddenly my wife’s phone shoots a 279-byte information packet containing “I love you” at the speed of light in every direction, eventually fizzling into nothing after about 30 miles.

is the biggest understatement in the whole article. It omits the entire baseband!

Within the baseband, even the channel coding process itself is insanely complex, involving convolutional codes, CRCs, weird interleaving schemes... and then there's the modulation, and all the L1 signalling to support it all... I could go on.

3chelon | 5 years ago

Great post. I used to work with SMS for a living, and still remember the first time I visited a customer's data center and watched one of their engineers send an SMS... by telnetting directly into the SMSC and punching out a command in raw EMI:

https://en.wikipedia.org/wiki/EMI_(protocol)

And an old blog post of mine on why MMS failed but still managed to delay actual mobile internet by 10 years:

https://gyrovague.com/2014/06/27/how-sms-set-back-the-mobile...

jpatokal | 5 years ago

Although this wa a little wordy at times and not given proper sectional headers (easily fitting in a hyperlink toc), this us the type of quality content I wish for every time I open HN

Supermancho | 5 years ago

I am a developer in a big company that acts as SMSC. We work with hundreds of operators around the world. The explanation in the article is way better that all internal documentation combined we have in the company.

I wrote algorithms to convert 8bit to 7bit and back as some our customers require that.

One thing that strikes me about SMS is that most people think SMS is secure and offer great privacy. It is not. The messages are not encrypted, and from the technical perspective there is no way to prevent SMSC from reading your messages.

molodec | 5 years ago

Has anyone encountered the interview question similar to "Tell me what happens when you type www.google.com into a browser."

This article reads like the most immense answer to that question might.

VBprogrammer | 5 years ago

This was really well-written and fascinating. For something that I have used all the time and think of as relatively simple, there is a tremendous amount of complexity going on behind the scenes. I suppose that's true for nearly any technology one interacts with these days, but it's fun to see the curtain pulled back.

dgritsko | 5 years ago

Does anyone know if it's possible for the BTS or BTC to notice that the two numbers are on the same tower/BTC and send the message along without notifying the SMSC?

amaccuish | 5 years ago

I find it pretty amazing that I can take my phone overseas, and someone in a third country could send me an SMS and it will find my phone in a matter of seconds.

toomanybeersies | 5 years ago

Incredible article! Brings me back to my EEE days. I have become very interested in Telnyx recently as they move towards being a next generation carrier replacing the old-school PSTN with API oriented connectivity. Would highly recommend checking them out here - https://telnyx.com/products/programmable-sms?utm_source=refe...

eoinclancy1 | 5 years ago

About 20 years ago, before texting really became popular, you used to be able to change your SMSC to one from a specific company and get free texts. I think in the UK I used one from Finland.

gadders | 5 years ago

I wish you'd have dived into software part of rendering text. Fonts, scaling etc.

Great article, thank you very much!

samat | 5 years ago

Are most of the images failing to load for anyone else? It's a delightful post, but so much of it refers to the images that I feel like I'm missing a lot

adamson | 5 years ago

This is delightful!

And this phrase kills me (in a good way):

"The process isn’t entirely frictionless, which is why my phone vibrates lightly upon delivery"

colinprince | 5 years ago

It's a fun read, intentionally glossy on the details (is this "pop tech"?) but I was fascinated.

Except the value judgments about endianness in encoding. All of the decisions in this system were made in a broader context. When you see something weird, it's tempting to say, that is inane, but it is enlightening to ask, what am I missing?

nerdile | 5 years ago

> If cell waves are 6 to 14 inches, their antennas need to be 3-7 inches. Now stop and think about the average height of a mobile phone, and why they never seem to get much smaller.

Is this actually true? I thought this was mostly because this was a convenient size for phones to be, and that antennas were often internally folded anyways.

> You know that every point of light, like the Christian God or Musketeers (minus d’Artagnan), is always a three-for-one sort of deal. Red, green, and blue combine to form white light in a single pixel.

Doesn't Samsung use PenTile displays, which has the normal number of green subpixels and but with fewer red and blue ones?

saagarjha | 5 years ago

One of the best articles I've read in awhile.

I'd love to know more about this line: "There’s also a little flag in the DCS byte that tells the phone whether to self-destruct the message after sending it"

And also, how do towers deconvolute all those signals?

buchanae | 5 years ago

What a fun - and educational - post about the full and complete journey from the moment someone starts typing “I love you” on a phone to when it gets read on the other end. That was great!

danyork | 5 years ago
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| 5 years ago

Brilliant. That was so much more interesting than I expected it to be. Thanks for posting.

zoggjones | 5 years ago

Still common to call it BTS (GSM era) contra RBS in the states?

holmberd | 5 years ago

I love this writing style. Thank you. Would like to see more!

leetbulb | 5 years ago

Delightful article! Thank you!

f055 | 5 years ago

I think he left out the part where the NSA gets the data sent to them.

flareback | 5 years ago

The visible scroll bar is missing on mobile. This prevents us from estimating whether we have time to read this.

Chris_Chambers | 5 years ago