The 90s were a wonderful time for portable communications devices. Cell phones had mass, real buttons, and thick batteries – everything you want in next year’s flagship phone. Unfortunately, Zach Morris’ phone hasn’t been able to find a tower for the last decade, but that doesn’t mean these phones are dead. This weekend at Shmoocon, [Brandon Creighton] brought these phones back to life. The Motorola DynaTAC lives again.
[Brandon] has a history of building ad-hoc cell phone networks. A few years ago, he was part of Ninja Tel, the group that set up their own cell phone network at DEF CON. That was a GSM network, and brickphones are so much cooler, so for the last few months he’s set his sights on building out a 1G network. All the code is up on GitHub, and the hardware requirements for building a 1G tower are pretty light; you can roll your own 1G network for about $400.
The first step in building a 1G network, properly referred to as an AMPS network, is simply reading the documentation. The entire spec is only 136 pages, it’s simple enough for a single person to wrap their head around, and the concept of a ‘call’ really doesn’t exist. AMPS looks more like a trunking system, and the voice channels are just FM. All of this info was translated into GNU Radio blocks, and [Brandon] could place a call to an old Motorola flip phone.
As far as hardware is concerned, AMPS is pretty lightweight when compared to the capabilities of modern SDR hardware. The live demo setup used an Ettus Research USRP N210, but this is overkill. These phones operate around 824-849 MHz with minimal bandwidth, so a base station could easily be assembled from a single HackRF and an RTL-SDR dongle.
Yes, the phones are old, but there is one great bonus concerning AMPS. Nobody is really using these frequencies anymore in the US. That’s not to say building your own unlicensed 1G tower in the US is legally permissible, but if nobody reports you, you can probably get away with it.
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)”
Remember the early days of cellphones and carphones when they were super-bulky and all the rage? Those early handsets used analog technology for communications in a protocol called Advanced Mobile Phone Service (AMPS). As more customers flocked to wireless providers, networks were transitioned over to digital phones in order to save bandwidth. Some places still support AMPS but it has rapidly gone the way of the Dodo. But a few years back [Mark Atherton] got his hands on some old hardware, including a bag-phone and some test equipment, and set out to build a base station that can control AMPS handsets. In short, he’s creating his own analog cellphone tower. There’s a wealth of information on his page. The writeup comes out as a mix of protocol and electronic resources he scavenged across the net, as well as a work log serving as a testament to his successes and failures. He did his experiments in New Zealand, so if you’re thinking of undertaking this make sure to research your local radio regulations first.
[Jenn’s] family is a single-car household. Because of this, it’s a little more difficult to get a jump start when the headlights run down the battery. Not wanting to ask the neighbors for help, her husband [Richard] decided to come up with his own solution.
Rummaging through the parts on hand, [Richard] went with his old friend Sonic the Hedgehog. He used two 12-volt, 1 amp Sega Genesis power adapters in parallel hooked up to a 12 volt, 3 amp power supply. The end result is a 12-volt 5 amp source hooked to the car’s electrical system and used to get their road machine started.
We have enjoyed some of [Richard’s] offerings in the past, such as Super Nintoaster and the Super Genintari but this is a bit less… eloquent. A few questions do come to mind. First of all, is this the best way to use parts of your 20-year-old gaming system? How many amps does your average car starter pull down? And finally, what kind of issues are we looking at with the lead acid battery under these conditions? Weigh in on the conversation in the comments.