Like many of us, [Zak Kemble] has an indeterminate number of tiny packages coming his way from all over the globe at any given time. Unfortunately, the somewhat unpredictable nature of the postal service where he lives meant he found himself making a lot of wasted trips out to the mailbox to see if any overseas treasures had arrived for him. To solve the problem, he decided to build an Internet-connected mailbox notification system that could work within some fairly specific parameters.
For one thing, the mailbox is too distant to connect directly to it over WiFi. [Zak] mentions that 433 MHz might have been an option, but he decided to skip that entirely and just connect it to the cellular network with an A9G GPRS/GSM module from A.I. Thinker. This device actually has its own SDK that allows you to create a custom firmware for it, but unfortunately the high energy consumption of the radio meant it would chew through batteries too quickly unless it had a little extra help.
Not wanting to have to change the batteries every couple months, [Zak] added a ATtiny402 to handle the notifier’s power management needs. By using a P-MOSFET to completely cut power to the A9G, the notifier can save an incredible amount of energy by only activating the cellular connection once it actually needs to send a notification; which in this case takes the form of an HTTP request that eventually works its way to a Telegram group chat.
To cut a long story short, testing seems to indicate that the notifier can fire off approximately 800 requests before needing its 10440 lithium battery recharged. Given how often [Zak] usually receives mail, he says that should last him around five years.
The A9G module, the ATtiny402, a BME280 environmental sensor (because, why not?), the battery, and all the ancillary support hardware are on a very professional looking PCB. That goes into a relatively rugged enclosure that’s designed to keep the electronics from shorting out on the mailbox’s metal case as well as keeping any particularly weighty parcels from crushing it.
If you’ve got the freedom so mount whatever you want outside, then you can certainly build a more technically impressive mailbox. But considering the limitations [Zak] had to work around, we think he did an excellent job.
SIM cards are all around us, and with the continuing growth of the Internet of Things, spawning technologies like NB-IoT, this might as well be very literal soon. But what do we really know about them, their internal structure, and their communication protocols? And by extension, their security? To shine some light on these questions, open source and mobile device titan [LaForge] gave an introductory talk about SIM card technologies at the 36C3 in Leipzig, Germany.
Starting with a brief history lesson on the early days of cellular networks based on the German C-Netz, and the origin of the SIM card itself, [LaForge] goes through the main specification and technology parts of each following generation from 2G to 5G. Covering the physical basics, I/O interfaces, communication protocols, and the file system located on the SIM card, you’ll get the answer to “what on Earth is PIN2 for?” along the way.
Of course, a talk like this, on a CCC event, wouldn’t be complete without a deep and critical look at the security side as well. Considering how over-the-air updates on both software and — thanks to mostly running Java nowadays — feature side are more and more common, there certainly is something to look at.
Continue reading “36C3: SIM Card Technology From A To Z”
If you’ve been thinking of adding cellular connectivity to a build, here’s a way to try out a new service for free. Hologram.io has just announced a Developer Plan that will give you 1 megabyte of cellular data per month. The company also offers hardware to use with the SIM, but they bill themselves as hardware agnostic. Hologram is about providing a SIM card and the API necessary to use it with the hardware of your choice: any 2G, 3G, 4G, or LTE devices will work with the service.
At 1 MB/month it’s obvious that this is aimed at the burgeoning ranks of Internet of Things developers. If you’re sipping data from a sensor and phoning it home, this will connect you in 200 countries over about 600 networks. We tried to nail them down on exactly which networks but they didn’t take the bait. Apparently any major network in the US should be available through the plan. And they’ve assured us that since this program is aimed at developers, they’re more than happy to field your questions as to which areas you will have service for your specific application.
The catch? The first taste is always free. For additional SIM cards, you’ll have to pay their normal rates. But it’s hard to argue with one free megabyte of cell data every month.
Hologram originally started with a successful Kickstarter campaign under the name Konekt Dash but has since been rebranded while sticking to their cellular-connectivity mission. We always like getting free stuff — like the developer program announced today — but it’s also interesting to see that Hologram is keeping up with the times and has LTE networks available in their service, for which you’ll need an LTE radio of course.
This gem from the AT&T Archive does a good job of explaining the first-generation cellular technology that AT&T called Advanced Mobile Phone Service (AMPS). The hexagon-cellular network design was first conceived at Bell Labs in 1947. After a couple of decades spent pestering the FCC, AT&T was awarded the 850MHz band in the late 1970s. It was this decision coupled with the decades worth of Bell System technical improvements that gave cellular technology the bandwidth and power to really come into its own.
AT&T’s primary goals for the AMPS network were threefold: to provide more service to more people, to improve service quality, and to lower the cost to subscribers. Early mobile network design gave us the Mobile Service Area, or MSA. Each high-elevation transmitter could serve a 20-mile radius of subscribers, a range which constituted one MSA. In the mid-1940s, only 21 channels could be used in the 35MHz and 150MHz band allocations. The 450MHz band was introduced in 1952, provided another 12 channels.
The FCC’s allocation opened a whopping 666 channels in the neighborhood of 850MHz. Bell Labs’ hexagonal innovation sub-divided the MSAs into cells, each with a radius of up to ten miles.
The film explains quite well that in this arrangement, each cell set of seven can utilize all 666 channels. Cells adjacent to each other in the set must use different channels, but any cell at least 100 miles away can use the same channels. Furthermore, cells can be subdivided or split. Duplicate frequencies are dealt with through the FM capture effect in which the weaker signal is suppressed.
Those Bell System technical improvements facilitated the electronic switching that takes place between the Mobile Telephone Switching Office (MTSO) and the POTS landline network. They also realized the automatic control features required of the AMPS project, such as vehicle location and automatic channel assignment. The film concludes its lecture with step-by-step explanations of inbound and outbound call setup where a mobile device is concerned.
Continue reading “Retrotechtacular: Ma Bell’s Advanced Mobile Phone Service (AMPS)”