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[Discussion] What is a Modem in a Mobile Phone?

2019-06-26 14:15:19
170 1

Dear Mi Fans!

This is a new series about all parts your smartphones and how it works on device!
Almost all modern smartphones are equipped with Modem. But not all know how it work's! Today you know it and more about this smartphone detail.  We will not talk about how they are produced! And let's talk about the main concepts and technical characteristics. I think many will be interested to find out this information.

What is Modem in mobile phone!?
In this thread we know more about modem in smartphone, which are responsible for wireless communication. In modern phones are many types of communications for different purposes: for talking, the Internet, headset pairing, navigation. And we begin with the fundamental function of all mobile phones - cellular communication.

The first generation of mobile networks (1G) appeared in the 80s of the last century. These were analog voice and voice messaging systems. Already in the 90s there was a transition to digital networks. The birth of well-known 2G cellular standards has taken place. Such as GSM and CDMA. In addition to voice, the opportunity to transmit data. It was allowed to implement the GSM add-ons - GPRS and the next EDGE. Of course, the speed provided by these standards did not allow full use of the Internet, but for web surfing, checking email was quite enough. The third generation of cellular networks, 3G, was a real breakthrough.The UMTS standard and its modifications made it possible to provide a high data transfer rate, sufficient for online viewing of media content, video calls, and just a comfortable stay on the Internet. And while in Ukraine this standard (or rather, the more advanced generation of the 3.5G" HSPA +) is only gaining momentum, in many countries the connection of the 4G generation is already relevant. This includes technologies such as LTE, WiMAX and others. Formally, only the upgraded versions (LTE-Advanced, WiMAX Release 2) are the true representatives of 4th generation communications. These standards are characterized by high data transfer rates and high network bandwidth. Next, you can see a brief summary table of generations of cellular communication.

Today, intensive work is underway to develop and implement communication of the fifth generation 5G. Especially succeeded in this is the Chinese company Huawei.

For communication with the cellular network in the smartphone meets the radio module, in a different way - the modem. For some manufacturers, this module is part of the SoC (system on a chip), while for others it is a separate module. Moreover, the components can be used by different manufacturers. For example, for Apple processors, radio modules are supplied by Quallcomm. Different versions of modems support different communication standards. Flagship devices in most cases can work in all relevant cellular networks. Below is a table of popular SoCs and standards that they support.

Stand alone are devices with support for two or more SIM-cards. As a rule, they use one radio module, that is, when we are talking on one SIM card, the other is not active. There are also solutions on the market with two modems that allow two carrier cards simultaneously. True, such a decision has to pay the worst autonomy. Also do not forget about the software compatibility of the operating system and the radio module. The quality of the cellular network signal will depend on this.

Another important type of wireless connection in a smartphone is Wi-Fi. This standard is primarily used to access the Internet. But it can also be used to exchange data between smartphones and tablets (Wi-Fi Direct), transfer media to a TV (Wireless Display, Miracast), remotely control photos and video cameras, and home appliances. There are several Wi-Fi standards, the main differences of which are throughput.

Almost all modern phones support Wi-Fi standards 802.11a / b / g / n, and the flagships of this year and 802.11ac. Before the advent of Wi-Fi Direct, the only normal way (the infrared port is not considered normal) to transfer files between two phones was a wireless connection according to the Bluetooth standard. With the advent of the ability to do this using Wi-F, the “bluetooth” lost its importance in this direction, as it had a small bandwidth and a range of up to 10 meters. But he found another task - pairing with all sorts of accessories, such as headphones, talking headsets, fitness trackers, smart watches. Actual today is the fourth revision of Bluetooth with increased data transfer speed and low power consumption - Bluetooth 4.0 LE.

Do not get lost in an unfamiliar place will help the GPS module. Its presence in the smartphone is already commonplace. GPS capabilities in the smartphone can be used in geosocial networks, in sports and fitness, for navigation on trips and hikes, for geotagging photos and videos, tracking a lost or stolen phone. The quality and accuracy of the GPS module largely depends on the chip manufacturer. For example, there was a time when solutions in SoC from MediaTek and other Chinese chip makers were losing heavily to competitors. Next, a brief look at several other types of wireless communications that may be present in the smartphone. Infrared communication. Today it is rarely used, although some manufacturers equip their smartphones with an infrared port to control household appliances. NFC is a high-frequency wireless standard with a short range. Its potential has not yet been fully disclosed, but in the future, the presence of an NFC module in a smartphone will allow mobile payments, purchases, use of the device as an electronic key, an identity card.

Hot Knot is an analogue of NFC from Chinese chip maker Mediatek. This feature was first introduced in 2013 as part of SoC MT6592. It is characterized by a high data transfer rate and a small range. To activate this feature you need to touch smartphones. Unlike NFC, Hot Knot does not need radio antennas and chips, which allows a lot to reduce production costs.
A modern smartphone supports almost all current types of wireless communication. It may well replace the Wi-Fi router, GPS navigator, media player for the TV, remote control for home appliances. I am sure that soon the smartphone can be used as a credit card to pay for goods and services (some work has already been done), as an identity document, as a key to the car, apartment. Wireless communication in the phone makes our life more convenient!

I think the essence is clear to all! But we don’t think about how the world works and how it works, I will try to explain very interesting information about the modem and the SIM card.

The question of what a radio module in a phone is, users started asking themselves after the need and desire to use two SIM cards at the same time arose. Up to this point, only lovers of mobile phones have been interested in the radio module in the phone, as well as a master of mobile phone repair.

What does the radio module in the phone give?
The radio module in the phone allows you to receive and transmit network signals, which means that:
  • - you can call;
  • - receiving and sending text and multimedia messages;
  • - and finally - the opportunity to use the mobile Internet to visit your favorite social networks and to work!

The radio module in the phone is a small area on the motherboard that receives radio signals.

The blade of the radio module or its incorrect installation, the installation of an unsuitable radio module board, the presence of metal parts next to it, leads to a decrease in the quality and signal strength until the message appears on the screen “Network not found / Network search”. It is the use of different radio modules, the difference in their installation by manufacturers and suggests that one phone will catch in the underpass, the second phone will fail, and the third just will not show the network signal. In addition to the electronics, the radio module in the phone must be considered and specialized software. These are libraries of drivers that make all the hardware parts of the phone work and “communicate” with each other.

There are a large number of Chinese-made phones for which SIM cards "fly out" and require the installation of additional SIM card patches so that everything works fine. It also happens that when updating the operating system of a smartphone (firmware), the network signal disappears - then you need a newer version of drivers and patches. The radio module in the phone and two SIM cards Programmers and engineers have learned to make one radio module work with two (and even three!) SIM cards, namely, with two different networks and phone numbers. But with active work with one network, the second network (the second SIM card) becomes inaccessible. This problem is solved by the use of two radio modules. Both networks are available at any time and in any place where there is network coverage, but the simultaneous operation of two radio modules greatly shortens the autonomous life time of the phone. In conclusion, we write one thesis definition. What is a radio module in the phone? This is the electronic part of the smartphone associated with the rest of the "hardware" with the help of drivers, allowing you to work with the network of the mobile operator.

But the most interesting is that the radio module has its own operating system as well as the sim card.

Yes, in every smartphone (it doesn't matter whether it is an Android, iOS or WP device) with a radio module providing mobile communication with a cellular network (for example, 3G or LTE), there is not one, but two operating systems. Someone always knew about it, someone heard for the first time, and someone just guessed ... In any case, this is almost never talked about in the media and this fact is not much discussed.

In addition to the main OS, with which the end user interacts (for example, Android or iOS), there is a second, hidden and small operating system serving the part of the phone that works with the radio. Since such functionality requires high efficiency, a real-time operating system is used.

Files of this operating system, of course, are stored separately from the phone data, and the OS itself works on a separate processor, the radio modem chip. As a rule, this second real-time OS is always proprietary, that is, closed, its source codes are not publicly available and nobody has ever seen these source codes except developers and, possibly, certification services (special services?).

For example, the OS that works with the Qualcomm MSM6280 radio modem is called AMSS, it is built on REX’s own closed core and serves simultaneously 69 tasks that control everything from USB to GPS. All this works on a separate mobile ARMv5 processor. The problem is obvious: no one really understood how the radio module and the hidden proprietary operating system work, no one has ever really tried to raise the issue of the security of all this undocumented closed-source software, which is, in fact, "a cat in a bag."

This is at least strange, because the hidden operating system, operating on any modern device with a radio module, is not the last in the management of confidential information and ensuring user security.

Of course, you can, by default, rely on the statement that the same AMSS is safe and one hundred percent reliable. But what if it really isn't? What if having on your phone the world's most secure and most reliable mainstream operating system, you will get a big security breach in the form of a second OS not noticed by anyone?

The insecurity of software serving radio modules is a consequence of erroneous design and standardization. The standards by which radio modems work were developed already in the 80s, and the standards by which the corresponding software works were developed in the 90s. For mobile electronics, this is a very long time.

Imagine for a second that an exploit has appeared that uses the bugs of a hidden operating system. All phones running the OS, to which the exploit was written, will automatically be in the zone of catastrophic risk. To make matters worse, the radio modem automatically trusts everything it receives from the base station (i.e., the cellular tower). Nothing is checked, all received and transmitted information is considered to be 100% reliable by default. In addition, in most smartphones, the radio modem processor is considered to be the main processor, and the application processor (on which the user OS we are accustomed to “spinning”) is secondary. So, we have a full-fledged real-time operating system running on an ARM processor, and no one thinks about the possibility of creating exploits and the consequences, and the radio modem automatically trusts any received information, instructions, or a piece of code.

Approximately with such thoughts, a researcher in the field of information security, Ralph-Philip Weinmann from the University of Luxembourg (yes, this is the Ralph who hacked the iPhone in 20 seconds in 2010), began engineering analysis (also known as reverse engineering) of software Qualcomm and Infineon radio modems.

The researcher easily discovered a large number of bugs and a common software mess in hidden operating systems that can easily lead to the creation of exploits not only for causing smartphones to fail, but also allowing an attacker to remotely execute arbitrary code. And all this through the air, without direct physical impact on the phone. For example, Ralph-Phillip found that it was enough to create an exploit that sent a message of only 73 bytes to be able to execute code on the device remotely.

An attacker who has received such a powerful tool in his own hands may, for example, turn on an invisible and inaudible answering machine using Hayes modem commands. This "language of communication" telephone Dial-UP modems was developed in 1981, but, nevertheless, it is still used in modern smartphones!

Sprint Base Station

Despite the fact that we can at least trust the base stations of large operators, implying their safety, we should understand that base stations are not some kind of super-exclusive top-secret equipment - base stations become cheaper every day, there are even precedents selling those on eBay 'e. In addition, there is already free and open source software that performs the role of BS on a regular computer.

In every sci-fi thriller UbiSoft there is some truth

If you have a big desire, tools and knowledge, it is enough to install a fake base station (for example, in the center of the business part of the city) and you will be able to turn on and turn off microphones, cameras, install and run rootkits, make calls and send SMS messages to any numbers and so on. You can even permanently disable the phone!

This is quite a serious problem, however, not much raised and covered in the media.

Of course, not everyone has the power to develop the mentioned exploit repeatedly, and not everyone has the power to “dive” deep enough into the dark pool of closed software, closed specifications, to engage in reverse engineering and creating fake base stations. But what if someone, nevertheless, will give this time? And what if this someone will be successful in his research (and this is not fantasy - this has already happened quite recently)?

At the CampZer0 conference held in Hungary in 2013 year, the researchers showed the reception, decoding and decoding of the GSM network signal using the simplest amateur radio RTL-SDR, which can be purchased at an online store selling Chinese gadgets for as little as $ 15-20.

It looks like this RTL-SDR

The demonstrated equipment, using a radio receiver, made in the form of a USB-dongle DVB-T and available software, allows you to access voice calls and SMS messages transmitted within GSM networks.

RTL-SDR (Software-defined Radio based on Realtek microcircuit) is assembled on the basis of an “innocuous” microcircuit originally intended for television receivers. Savvy Chinese electronics manufacturers invented to exploit the non-advertised operating mode found on the chip as a digitizer of a certain radio range for the production of similar USB key fobs sold by radio amateurs and enthusiasts.

It should be noted that modern radio amateurs have been using RTL-SDR tuners for quite some time for their needs - with the help of these devices they listen to air traffic controllers, taxi dispatchers, talk of various services on the radio, other radio amateurs, receive the signal from wireless sensors, decode the signal of digital and analog television etc. There are whole open source projects like PowerSDR and GnuRadio.

The GSM international mobile digital cellular standard was developed by a group of European scientists in the late 1980s and replaced the analog cellular service (AMPS). Since then, GSM has been widely used worldwide as a de facto cellular standard. This standard is used by operators located in 210 countries.

Reverse SIM Card Engineering.

P.S. Speaking quite precisely, in smartphones, not two, but three whole operating systems, two of which are hidden, work. In addition to the real-time OS processing the data for the radio modem, a very tiny operating system runs inside the SIM card. Yes, no matter how strange it sounds, inside the SIM card there is a processor of its own and its own memory, which is controlled by a separate OS.

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2019-06-26 14:15:19
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Master Bunny

Wooky63 | from app


Phew that was a long interesting read, thanks.
2019-06-27 05:32:17
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